Advertisement

Effects of increasing air temperature on physiological and productive responses of dairy cows at different relative humidity and air velocity levels

Open AccessPublished:November 16, 2021DOI:https://doi.org/10.3168/jds.2021-21164

      ABSTRACT

      This study determined the effects of increasing ambient temperature (T) at different relative humidity (RH) and air velocity (AV) levels on the physiological and productive responses of dairy cows. Twenty Holstein dairy cows were housed inside climate-controlled respiration chambers, in which the climate was programmed to follow a daily pattern of lower night and higher day T with a 9°C difference, excluding effects from sun radiation. Within our 8-d data collection period, T was gradually increased from 7 to 21°C during the night (12 h) and 16 to 30°C during the day (12 h), with an incremental change of 2°C per day for both nighttime and daytime T. During each research period, RH and AV were kept constant at 1 of 5 treatment levels. A diurnal pattern for RH was created, with lower levels during the day and higher levels during the night: low (RH_l: 30–50%), medium (RH_m: 45–70%), and high (RH_h: 60–90%). The effects of AV were studied during the day at 3 levels: no fan (AV_l: 0.1 m/s), fan at medium speed (AV_m: 1.0 m/s), and fan at high speed (AV_h: 1.5 m/s). Effects of short and long exposure time to increasing T were evaluated by collecting data 2 times a day: in the morning [short: 1 h (or less) − exposure time] and afternoon (long: 8 h − exposure time). The animals had free access to feed and water and both were ad libitum. Respiration rate (RR), rectal temperature (RT), skin temperature (ST), dry matter intake, water intake, milk yield, and composition were measured. The inflection point temperatures (IPt) at which a certain variable started to change were determined for the different RH and AV levels and different exposure times. Results showed that IPt under long exposure time for RR (first indicator) varied between 18.9 and 25.5°C but was between 20.1 and 25.9°C for RT (a delayed indicator). The IPt for both RR and RT decreased with higher RH levels, whereas IPt increased with higher AV for RR but gave a minor change for RT. The ST was positively correlated with ambient T and ST was not affected by RH but significantly affected by AV. For RR, all IPt was lower under long exposure time than under short exposure time. The combination of higher RH levels and low AV level negatively affected dry matter intake. Water intake increased under all treatments except RH_l-AV_l. Treatment RH_h-AV_l negatively affected milk protein and fat yield, whereas treatments RH_m-AV_m and RH_m-AV_h reduced milk fat yield. We concluded that RH and AV significantly affected the responses of RR, RT, ST, and productive performance of high-producing Holstein cows. These responses already occurred at moderate ambient T of 19 to 26°C.

      Key words

      INTRODUCTION

      Once thought to be limited to (sub)tropical areas, the effects of high ambient temperatures on dairy cows have now become relevant in temperate climate areas due to rising global temperatures (T;
      • Polsky L.
      • von Keyserlingk M.A.G.
      Invited review: Effects of heat stress on dairy cattle welfare.
      ;
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Amon T.
      Critical THI thresholds based on the physiological parameters of lactating dairy cows.
      ). In addition, the intensive genetic selection of milk production has resulted in dairy cows that are more susceptible to heat stress than they were in the past (
      • Ravagnolo O.
      • Misztal I.
      • Hoogenboom G.
      Genetic component of heat stress in dairy cattle, development of heat index function.
      ), rendering the problem of heat stress during the summer increasingly prominent, along with all the accompanying negative effects on dairy cows' health (
      • Kadzere C.
      • Murphy M.
      • Silanikove N.
      • Maltz E.
      Heat stress in lactating dairy cows: A review.
      ;
      • de Andrade Ferrazza R.
      • Mogollón Garcia H.D.
      • Vallejo Aristizábal V.H.
      • de Souza Nogueira C.
      • Veríssimo C.J.
      • Sartori J.R.
      • Sartori R.
      • Pinheiro Ferreira J.C.
      Thermoregulatory responses of Holstein cows exposed to experimentally induced heat stress.
      ), production (
      • Hill D.L.
      • Wall E.
      Dairy cattle in a temperate climate: The effects of weather on milk yield and composition depend on management.
      ), and reproduction (
      • García-Ispierto I.
      • López-Gatius F.
      • Bech-Sabat G.
      • Santolaria P.
      • Yániz J.L.
      • Nogareda C.
      • De Rensis F.
      • López-Béjar M.
      Climate factors affecting conception rate of high producing dairy cows in northeastern Spain.
      ;
      • Schüller L.K.
      • Burfeind O.
      • Heuwieser W.
      Impact of heat stress on conception rate of dairy cows in the moderate climate considering different temperature–humidity index thresholds, periods relative to breeding, and heat load indices.
      ) as well as increased mortality risk (
      • Vitali A.
      • Segnalini M.
      • Bertocchi L.
      • Bernabucci U.
      • Nardone A.
      • Lacetera N.
      Seasonal pattern of mortality and relationships between mortality and temperature-humidity index in dairy cows.
      ).
      Dairy cows are particularly sensitive to high ambient T (
      • Kadzere C.
      • Murphy M.
      • Silanikove N.
      • Maltz E.
      Heat stress in lactating dairy cows: A review.
      ). In the thermal comfort zone, cows can easily balance heat loss with heat production independent from ambient T (
      • Mount L.E.
      Adaptation to Thermal Environment. Man and His Productive Animals.
      ). However, when ambient T rises above this (thermal comfort) zone, the cows must recruit extra physiological responses to get rid of the produced heat. The physiological responses could include increased respiration rate (RR), increased blood flow from the core body to the skin surface, increased sweating rate, increased water consumption, reduced rumination activities, increased heart rate, reduced heat production, and increased core body T (
      • Burfeind O.
      • Suthar V.S.
      • Heuwieser W.
      Effect of heat stress on body temperature in healthy early postpartum dairy cows.
      ;
      • Hill D.L.
      • Wall E.
      Dairy cattle in a temperate climate: The effects of weather on milk yield and composition depend on management.
      ;
      • Galán E.
      • Llonch P.
      • Villagrá A.
      • Levit H.
      • Pinto S.
      • del Prado A.
      A systematic review of non-productivity-related animal-based indicators of heat stress resilience in dairy cattle.
      ;
      • Amamou H.
      • Beckers Y.
      • Mahouachi M.
      • Hammami H.
      Thermotolerance indicators related to production and physiological responses to heat stress of Holstein cows.
      ).
      With regard to the response of increasing RR,
      • Kadzere C.
      • Murphy M.
      • Silanikove N.
      • Maltz E.
      Heat stress in lactating dairy cows: A review.
      reported that about 15% of accumulated metabolic heat is dissipated by the respiratory tract for evaporative heat loss under hot conditions. Heat stress occurs when a cow is exposed to T that exceed her biological thermal comfort zone and has to make a lot of effort (and may even fail to) dissipate enough heat to maintain her body thermal balance (
      • Majkić M.
      • Cincović M.R.
      • Belić B.
      • Plavša N.
      • Lakić I.
      • Radinović M.
      Relationship between milk production and metabolic adaptation in dairy cows during heat stress.
      ).
      • Li G.
      • Chen S.
      • Chen J.
      • Peng D.
      • Gu X.
      Predicting rectal temperature and respiration rate responses in lactating dairy cows exposed to heat stress.
      reported that the rectal temperature (RT) of a high-producing cow started to increase when the ambient T was above 20.4°C. The models produced by
      • McArthur A.J.
      Thermal interaction between animal and microclimate: A comprehensive model.
      showed that metabolic heat production began to decline above the T threshold of 23°C, in response to elevated body T. Recognizing the importance of ambient air humidity, the temperature-humidity index (THI) is frequently used to assess heat stress magnitude in dairy cows. For example,
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Amon T.
      Critical THI thresholds based on the physiological parameters of lactating dairy cows.
      determined the THI thresholds for RR, heart rate, and RT of high-producing cows and suggested heat mitigation actions should be taken when THI rises above 65. In addition, the activation of the thermoregulatory mechanisms also varies with the duration of heat exposure (
      • de Andrade Ferrazza R.
      • Mogollón Garcia H.D.
      • Vallejo Aristizábal V.H.
      • de Souza Nogueira C.
      • Veríssimo C.J.
      • Sartori J.R.
      • Sartori R.
      • Pinheiro Ferreira J.C.
      Thermoregulatory responses of Holstein cows exposed to experimentally induced heat stress.
      ;
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Amon T.
      Critical THI thresholds based on the physiological parameters of lactating dairy cows.
      ).
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Amon T.
      Critical THI thresholds based on the physiological parameters of lactating dairy cows.
      demonstrated an increase in the RR of the cows by prolonging time beyond a THI of 65.
      Still, little research has been done involving the T at which cows start to show symptoms of heat stress, as these T depend on relative humidity (RH) and air velocity (AV) levels. In this paper we examined what order can be detected in the cows' responses to these changes, to determine whether the animals have a strategy or a certain method of coping. Other relevant questions remain: (1) under which ambient conditions will cows obviously react through physiological changes, and (2) could additional ventilation support the cow to remain within the thermal comfort zone? The objectives of this study were (1) to quantify the effects of increasing ambient T on physiological and productive parameters of Holstein dairy cows; (2) to determine the inflection point temperatures (IPt) for activation of adaptive mechanisms at different RH and AV levels; and (3) to assess the effect of different exposure times to increasing T conditions on physiological responses.

      MATERIALS AND METHODS

      The experiment was conducted in 2021 at the Carus animal research facilities of Wageningen University and Research, the Netherlands. The experimental procedures were approved by the Institutional Animal Care and Use Committee of Wageningen University and were conducted under the Dutch Law on Animal Experiments (Project No. 2019.D-0032).

      Animals and Feed

      Twenty Holstein Friesian dairy cows were used with an average milk yield (±SD) of 30.0 ± 4.7 kg/d at 206 ± 39 DIM, 687 ± 46 kg of BW, and a parity of 2.0 ± 0.7 lactations. Nineteen cows were pregnant and the average number of days they carried their calves was 105 ± 38. Cows were grouped in 4 blocks of 5 cows based on parity and expected milk yield. Each cow within a block was randomly assigned to 1 of the 5 treatments. The BW, milk yield, parity, DIM, and pregnant days of cows in different treatment groups are described in Table 1. The cows received ad libitum feed via a feed trough fixed in front of the cubicle and water via a drinking bowl.
      Table 1Body weight, annual average milk yield, parity, DIM, and pregnant days of cows in 5 treatment groups (means ± SD)
      ItemTreatment
      RH = relative humidity; AV = air velocity; l = low; m = medium; h = high.
      I (RH_l-AV_l)II (RH_m-AV_l)III (RH_h-AV_l)IV (RH_m-AV_m)V (RH_m-AV_h)
      BW, kg695 ± 54671 ± 52667 ± 41721 ± 50680 ± 29
      Milk yield, kg/d27.2 ± 7.230.8 ± 3.929.0 ± 6.932.0 ± 1.930.9 ± 2.0
      Parity2.3 ± 0.52.3 ± 0.52.5 ± 1.02.8 ± 1.02.5 ± 0.6
      DIM212 ± 35192 ± 40182 ± 54227 ± 31215 ± 35
      Pregnant days100 ± 27116 ± 20
      In treatment II, there was 1 nonpregnant cow.
      85 ± 60104 ± 30120 ± 48
      1 RH = relative humidity; AV = air velocity; l = low; m = medium; h = high.
      2 In treatment II, there was 1 nonpregnant cow.
      All cows were subjected to the same feeding scheme. The cows were fed (Table 2) twice daily at 0500 and 1530 h, and the diet was formulated to meet or exceed the nutritional requirements of lactating Holstein cows according to the Dutch System (
      • CVB
      CVB Table Ruminants 2008, series nr. 43.
      ). The amount of feed offered to each cow was adjusted daily to yield an excess (uneaten feed) of at least 5%.
      Table 2Ingredients and chemical composition of the mixed diet fed to the cows in the experiment
      Concentrate was fed at the rate of 6 kg/d. Roughage was adjusted to maintain ad libitum intake. Concentrate was ground to decrease the size of the particles, making it easier to mix it with the roughage.
      ItemAmount
      Roughage ingredient, g/kg
       Corn silage623.2
       Grass silage376.8
      Roughage chemical composition, % of DM
       CP9.0
       NDF42.4
       ADF23.5
      Concentrate chemical composition, % of DM
       CP18.6
       NDF23.0
       ADF13.7
      1 Concentrate was fed at the rate of 6 kg/d. Roughage was adjusted to maintain ad libitum intake. Concentrate was ground to decrease the size of the particles, making it easier to mix it with the roughage.

      Acclimatization and Adaptation Period

      To acclimate to the experimental conditions, cows were housed in tiestalls 7 d before entering the climate-controlled respiration chambers (CRC), located approximately 2 km from Carus. Like in CRC, the cows were then placed in individual tiestalls, wore halters, were in frequent contact with animal caretakers, and received the experimental diet.
      Every single day for the first 3 d after entering the CRC, in addition to receiving feeding and milking visits, the cows were visited 2 times daily by a researcher. During each visit a simulation of data collection action was performed on the cows to learn about their individual temperaments and to allow cows to become familiar with the actual data collection activity. In the chambers, cows could also see and hear other cows through transparent windows.

      Climate-Controlled Respiration Chamber

      In this study, 2 identical CRC were used. Each chamber was split into 2 individual airtight compartments with thin walls equipped with transparent windows to allow audio and visual contact between 2 cows and thereby minimize the effects of social isolation on their behavior. Each compartment measured 12.8 m2 and had a volume of 34.5 m3 as described in detail by
      • Gerrits W.
      • Labussière E.
      Indirect Calorimetry: Techniques, Computations and Applications.
      . For each compartment the RH was monitored by one RH sensor (Novasina Hygrodat100, Novasina AG), and the air T was monitored by 5 PT100 temperature sensors (Sensor Data BV) evenly distributed over the room at animal height as shown in Figure 1. For climate control the median value of all T sensors was used to rule out disproportional effects of potential deviating values. Experimental RH was achieved by means of a humidifier (ENS-4800-P, Stulz), a dehumidifier (koeltechniek, Nijssen), or both. The circulating air was heated or cooled depending on the deviation from set point T. High AV was achieved using Professional Fans (500 mm diameter, model 8879, HBM Machines BV) fixed on the ceiling of the chamber (at a height of 2.5 m above the floor) as described in Figure 1, in such a way that the wind was blowing on the axial body length of the cow from back to front. The chambers were artificially lit (390–440 lx) for 16 h per day (0500 to 2100 h for CRC 1 and 0600 to 2200 h for CRC 2), and during the night (2100 to 0500 h in CRC 1 and 2200 to 0600 h in CRC 2) the light was dimmed significantly (35–40 lx).
      Figure thumbnail gr1
      Figure 1Schematic diagram and overview photo of the climate-controlled respiration chamber. There are 2 temperature sensors hanging on each side wall (left and right), and 1 temperature sensor and 1 relative humidity sensor hanging on the wall in front of the cow. The material of the solid floor is rubber mat and the slatted floor is rubber-covered metal grills (
      • Gerrits W.
      • Labussière E.
      Indirect Calorimetry: Techniques, Computations and Applications.
      ). The cow inside the chamber was tied up loosely so that she could easily move forward and backward as well as lie down.

      Research Design

      The diurnal patterns of the climatic condition were simulated from retrospective data obtained from the Dutch National Weather Service (
      • KNMI
      Hourly data for the weather in the Netherlands.
      ), which is a typical diurnal pattern for Dutch weather during the summertime. The T and RH for day and night conditions were then applied in the treatments (Table 3, Figure 2).
      Table 3Temperature, relative humidity (RH), and air velocity (AV) treatment parameters (l = low; m = medium; h = high) used in climate-controlled respiration chambers (CRC)
      TreatmentTemperature, °CRelative humidity, %Air velocity
      2100–0700 h
      2100–0700 h indicates the duration of the night from 2100 h until next d 0700 h, for the first CRC.
      1000–1900 h
      1000–1900 h indicates the duration of the day from 1000 h until 1900 h on the same day, for the first CRC.
      2100–0700 h1000–1900 h0900–2100 h
      0900–2100 h indicates the duration of air velocity treatment from 0900 h until 2100 h on the same day, for the first CRC. There was a 1-h delay for all the controlling parameters for the second CRC.
      I (RH_l-AV_l)7–21
      7–21 (or 9–23) marks the air temperature on d 1 as 7°C (or 9) and on d 8 as 21°C (or 23) at night, and the temperature was increased by 2°C for the following day within the period; 16–30 (or 18–32) marks the air temperature on d 1 as 16°C (or 18) and d 8 as 30°C (or 32) during the day, and the temperature was increased by 2°C for the following day within the period.
      16–305030Fan off
      II (RH_m-AV_l)7–2116–307045Fan off
      III (RH_h-AV_l)7–2116–309060Fan off
      IV (RH_m-AV_m)9–2318–327045Fan on, speed 1
      V (RH_m-AV_h)9–2318–327045Fan on, speed 2
      1 2100–0700 h indicates the duration of the night from 2100 h until next d 0700 h, for the first CRC.
      2 1000–1900 h indicates the duration of the day from 1000 h until 1900 h on the same day, for the first CRC.
      3 0900–2100 h indicates the duration of air velocity treatment from 0900 h until 2100 h on the same day, for the first CRC. There was a 1-h delay for all the controlling parameters for the second CRC.
      4 7–21 (or 9–23) marks the air temperature on d 1 as 7°C (or 9) and on d 8 as 21°C (or 23) at night, and the temperature was increased by 2°C for the following day within the period; 16–30 (or 18–32) marks the air temperature on d 1 as 16°C (or 18) and d 8 as 30°C (or 32) during the day, and the temperature was increased by 2°C for the following day within the period.
      Figure thumbnail gr2
      Figure 2(a) Schematic temperature and relative humidity (RH) patterns during the 8-d research period. Between 0700 and 1000 h, the temperature and RH rose gradually to daytime levels and stayed constant until 1900 h. Between 1900 and 2200 h, the temperature and RH gradually decreased to nighttime levels and stayed constant again until the next d 0700 h; (b) An example of temperature and RH patterns on d 2 with 45 to 70% RH.
      Each cow was subjected to an 8-d research period in the CRC with a specific treatment consisting of combinations of T, RH, and AV. Air T inside the chambers were gradually increased from 7 to 21°C at night and 16 to 30°C during the day within 8 d (by steps of 2°C per day for both nighttime and daytime T) as shown in Figure 2. The experimental treatments consisted of 3 RH levels and 3 AV levels as described in Table 3. Three levels of RH during the day (d) and night (n) were RH_l (low) 30% (d) and 50% (n); RH_m (medium): 45% (d) and 70% (n); and RH_h (high) 60% (d) and 90% (n). However, the capacity of the cooling system in the CRC (for the dehumidification of air) was not sufficient to reach the intended values of all treatment combinations. In particular, it proved not to be possible to achieve combinations of low air T with low RH conditions during the first days of the research period. During the day (0900 to 2100 h), 3 AV levels were created: AV_l (low): fan off (0.1 m/s); AV_m (medium): fan speed level 1 (1.0 m/s); AV_h (high): fan speed level 2 (1.5 m/s). The fan was off during the night. For AV_m and AV_h the starting T was increased by 2°C, making the T range from 18 to 32°C during the day within 8 d. The AV_m and AV_h were only combined with RH_m. The air T, RH, and AV conditions for the 3-d adaptation period in the CRC were set and controlled the same as the first day of the corresponding experimental period. There were 4 replications per treatment.
      Technically, the T and RH inside the CRC required a time span of 3 h to adjust to new levels. The daytime T and RH were adjusted from 0700 to 1000 h, whereas the nighttime T and RH were adjusted from 1900 to 2200 h. The measurements for both chambers were conducted consecutively by the researchers, accounting for a 1-h time lag between the 2 chambers regarding changes in T, RH, AV, light setting, feeding, and milking. The exposure times (the time points when data were collected) were defined as short when the cows were exposed to the new stable T levels within 1 h and the exposure time was defined as long when the cows were exposed to the new T levels for more than 8 h.

      Data Collection

      All the data collection procedures are described in Table 4. Throughout the 8-d research period, the T and RH of the chamber compartments were continuously and automatically recorded at 30-s intervals. Using a hand-held anemometer (Testo 5–412–983, Testo SE and Co. KGaA), the actual AV was manually measured 2 times a day at 5 locations from a distance of about 5 cm around the cows' body surface for 30 s each time: neck, middle trunk, and rump, and both lateral sides. The dependent variables RR, skin temperature (ST), and RT were measured 2 times a day at 1000 and 1800 h. The RR was measured by observing the movement of the flank quietly and the time needed for counting 10 breaths was recorded with a stopwatch. The ST was measured on 4 different parts of the body (heart, back, belly, and rump) with a thermometer probe (Testo 0602 0393, Testo SE and Co. KGaA) and a handheld datalogger (Testo 435–4, Testo SE and Co. KGaA) by directly touching the surface of the skin under the hair. The RT was measured by inserting a digital thermometer (VT 1831, Microlife AG) to the depth of approximately 3 cm into the rectum of the cow and the result was read once the beep sounded. During the measurement, the cows did not seem to respond noticeably to the measurements. Feed refusals were collected and weighed before the morning feeding and were analyzed for DM and chemical composition. The cows were milked inside the CRC twice daily at 0500 and 1530 h and the milk yield was recorded at each milking for each cow. Milk samples were collected at each milking for an analysis of fat, protein, and lactose composition by Veluwe IJsselstreek (Nunspeet, the Netherlands). Individual water intake was measured by reading the water meter (Unimag Cyble UT4 BH-A, Itron) 2 times daily just before milking.
      Table 4Data collection and measurement time in 2 climate-controlled respiration chambers (CRC)
      ItemsMeasurement device and methodMeasurement time
      CRC 1CRC 2
      Climatic parameter
       Air temperatureAir temperature is continuously measured by 5 temperature sensors.Continuously, every 30 sContinuously, every 30 s
       Relative humidityRelative humidity is continuously measured by a relative humidity sensor.Continuously, every 30 sContinuously, every 30 s
       Air velocityMeasure 5 locations around the cow's body surface: neck, middle trunk, and rump, and both lateral sides by using a handheld anemometer.1000 h; 1800 h1100 h; 1900 h
      Animal parameter
       Respiration rateObserve the movement of the flank quietly and the time needed for counting 10 breaths; record using a stopwatch.1000 h; 1800 h1100 h; 1900 h
       Rectal temperatureInsert a digital thermometer to the depth of approximately 3 cm into the rectum of the cow and read the result once the beep sounds.1000 h; 1800 h1100 h; 1900 h
       Skin temperatureMeasure 4 different areas by directly touching the skin surface under the hair (heart, back, belly, and rump) using a thermometer probe.1000 h; 1800 h1100 h; 1900 h
       DMIFeed refusals are collected and weighed before the morning feeding; samples were collected and analyzed for DM content.0500 h0600 h
       Milk yieldMilk yield is recorded at each milking; milk samples for fat, protein, and lactose analysis are collected in tubes at each milking and stored at 4°C until sent to the company for analysis.0500 h; 1530 h0600 h; 1630 h
       Water intakeWater intake is measured by reading the water meter.0500 h; 1530 h0600 h; 1630 h

      Statistical Analysis

      The cow was considered as the experimental unit for all parameters. All statistical analyses were performed in SAS 9.4 (SAS Institute Inc.). One cow (receiving treatment IV: RH_m-AV_m) was excluded from the experiment because of mastitis.
      Data were first analyzed to investigate the distribution, outliers, and to determine which statistical model (linear or nonlinear regression) would best fit the cows' responses to the treatments. To evaluate the effects of the treatment T, RH, AV on the response variables, data were first analyzed with a nonlinear model.
      yijk = Ci + ai · z + cowijk + εijk,


      where yijk is the observed response variable; Ci is a constant over a range of temperature at each treatment level (i = 1...5); ai is the regression coefficient for z and the interaction between z with the ith treatment; z is the structural part that creates a broken-line regression of T with an IPt; cowijk is the random effect of the jth cow for the kth research day; and εijk is the residual error.
      z = (T > IPt) × (TIPt),


      where (TIPt) is defined as zero if (TIPt). The broken-line regression model was fit and IPt was determined for RR and RT by meeting fit statistics criterion, using SAS NLMixed procedure including cow random effects (
      • Robbins K.R.
      • Saxton A.
      • Southern L.
      Estimation of nutrient requirements using broken-line regression analysis.
      ). The starting values for a nonlinear model were first chosen by visually observing the data distribution and then were changed using the output from the model. The best fit model was determined by comparing Akaike information criterion (AIC; smaller is better). The χ2 test was used to test the significance of the model, and t-test was applied to pairwise compare the differences between treatments and between 2 exposure times (short and long).
      If the model failed to converge, a linear regression model was used. The MIXED procedure was used to investigate the influence of increasing T for each treatment at different exposure times. Although multiple measurements per animal cannot be regarded as independent units of observations, repeated measures was considered in the model including cow and experimental day as random effects. Different covariance structures were tested for each analysis, and the covariance structure with the smallest AIC values was selected. The linear regression model was as follows:
      yijk = µi + (a + bi) · T + cowijk + εijk,


      where yijk is the observed response variables; µi is the intercept for each treatment level (i = 1...5); a and bi are regression coefficients for T and the interaction between T with the ith treatment, respectively; cowijk is the random effect of the jth cow for the kth research day; and εijk is the random residual error. The χ2 test was used to test the significance of the model, and the adjusted Tukey t-test was applied using the PDIFF statement to pairwise compare the differences between treatments and between 2 exposure times (short and long).
      For the production parameters, the average baselines for feed intake, drinking water intake, milk yield, protein yield, and fat yield were calculated for the data from the first 2 research days. The MIXED procedure was applied to investigate the effect of increasing T on these parameters.
      Model assumptions were evaluated for both the broken-line model and the linear model by examining the distribution of residuals (homogeneity of variance and normality) using the UNIVARIATE procedure. Significance was declared when P ≤ 0.05 unless otherwise indicated.

      RESULTS

      Climate-Controlled Chambers and Environmental Conditions

      The realized (measured) T and RH for the different RH treatments are shown in Figure 3. The daily cyclical T were kept strictly constant according to set points with a deviation smaller than ±0.50°C. The RH_l failed to reach the set point (30–50%) within the first 5 d, but it got close from d 6 to 8. Similarly, RH_m could not reach the intended values (45–70%) within the first 3 d. The AV was calculated by taking the average of each cow's 5 measurement points. The AV_l ranged from 0.05 to 0.11 (mean 0.08 ± 0.01 m/s), AV_m ranged from 0.48 to 1.74 (mean 1.14 ± 0.30 m/s), and AV_h ranged from 0.72 to 1.98 (mean 1.35 ± 0.29 m/s).
      Figure thumbnail gr3
      Figure 3Average measured hourly temperature and relative humidity (RH) during the 8-d research period for 3 RH levels.

      Physiological Responses to Treatments

      From the model selection, the broken-line model fitted for RR and RT and the linear model fitted for ST. The results of the effects of different treatments and exposure times on the coefficients of the broken-line or linear model for different physiological parameters are given in Table 5.
      Table 5Coefficients (means ± SEM) from broken-line or linear regression with increasing temperature (T) on physiological parameters at different relative humidity (RH) and air velocity (AV)
      Dependent variableExposure time
      Exposure time with “short” means the cows stayed in the condition within 1 h and with “long” means the cows stayed in the condition for about 8 h.
      Adjusted R2Regression model component
      Constant = basal values; IPt = inflection point temperature; b = regression coefficient for interaction between temperature and treatments.
      Treatment
      Treatment levels: RH_l: 30%; RH_m: 45%; RH_h: 60%; AV_l: 0.1 m/s; AV_m: 1.0 m/s; AV_h: 1.5 m/s. l = low; m = medium; h = high.
      I (RH_l-AV_l) n = 4II (RH_m-AV_l) n = 4III (RH_h-AV_l) n = 4IV (RH_m-AV_m) n = 3V (RH_m-AV_h) n = 4
      Respiration rate, breaths/minShort0.733Constant36.0 ± 2.35
      Values within a row with different superscripts differ, P < 0.05.
      31.3 ± 2.91
      Values within a row with different superscripts differ, P < 0.05.
      36.3 ± 5.01
      Values within a row with different superscripts differ, P < 0.05.
      33.6 ± 2.26
      Values within a row with different superscripts differ, P < 0.05.
      32.7 ± 2.35
      Values within a row with different superscripts differ, P < 0.05.
      IPt25.8 ± 0.50
      Values within a row with different superscripts differ, P < 0.05.
      21.9 ± 0.82
      Values within a row with different superscripts differ, P < 0.05.
      20.9 ± 0.69
      Values within a row with different superscripts differ, P < 0.05.
      22.7 ± 1.12
      Values within a row with different superscripts differ, P < 0.05.
      24.2 ± 0.84
      Values within a row with different superscripts differ, P < 0.05.
      Slope9.4 ± 2.28
      Values within a row with different superscripts differ, P < 0.05.
      5.9 ± 1.04
      Values within a row with different superscripts differ, P < 0.05.
      6.4 ± 0.64
      Values within a row with different superscripts differ, P < 0.05.
      4.1 ± 1.17
      Values within a row with different superscripts differ, P < 0.05.
      5.3 ± 1.23
      Values within a row with different superscripts differ, P < 0.05.
      Long0.725Constant36.5 ± 3.23
      Values within a row with different superscripts differ, P < 0.05.
      30.1 ± 3.70
      Values within a row with different superscripts differ, P < 0.05.
      28.4 ± 6.93
      Values within a row with different superscripts differ, P < 0.05.
      24.8 ± 2.88
      Values within a row with different superscripts differ, P < 0.05.
      30.6 ± 3.69
      Values within a row with different superscripts differ, P < 0.05.
      IPt25.5 ± 0.42
      Values within a row with different superscripts differ, P < 0.05.
      21.0 ± 0.90
      Values within a row with different superscripts differ, P < 0.05.
      18.9 ± 1.04
      Values within a row with different superscripts differ, P < 0.05.
      21.0 ± 0.97
      Values within a row with different superscripts differ, P < 0.05.
      22.8 ± 0.88
      Values within a row with different superscripts differ, P < 0.05.
      Slope9.5 ± 1.34
      Values within a row with different superscripts differ, P < 0.05.
      5.1 ± 0.88
      Values within a row with different superscripts differ, P < 0.05.
      5.3 ± 0.63
      Values within a row with different superscripts differ, P < 0.05.
      4.2 ± 0.54
      Values within a row with different superscripts differ, P < 0.05.
      5.6 ± 0.93
      Values within a row with different superscripts differ, P < 0.05.
      P-value
      P-value for statistical difference between 2 exposure times.
      Constant NS
      NS: P ≥ 0.10.
      NS0.0660.002NS
      IPtNSNS0.0110.0510.039
      SlopeNSNS0.035NSNS
      Rectal temperature, °CLong
      The broken-line model could not be fitted for rectal temperature with short exposure time.
      0.581Constant38.6 ± 0.13
      Values within a row with different superscripts differ, P < 0.05.
      38.7 ± 0.07
      Values within a row with different superscripts differ, P < 0.05.
      38.4 ± 0.08
      Values within a row with different superscripts differ, P < 0.05.
      38.1 ± 0.15
      Values within a row with different superscripts differ, P < 0.05.
      38.4 ± 0.10
      Values within a row with different superscripts differ, P < 0.05.
      IPt25.3 ± 0.92
      Values within a row with different superscripts differ, P < 0.05.
      25.9 ± 0.45
      Values within a row with different superscripts differ, P < 0.05.
      20.1 ± 1.2
      Values within a row with different superscripts differ, P < 0.05.
      21.0 ± 1.19
      Values within a row with different superscripts differ, P < 0.05.
      25.3 ± 1.32
      Values within a row with different superscripts differ, P < 0.05.
      Slope0.08 ± 0.03
      Values within a row with different superscripts differ, P < 0.05.
      0.14 ± 0.05
      Values within a row with different superscripts differ, P < 0.05.
      0.10 ± 0.02
      Values within a row with different superscripts differ, P < 0.05.
      0.07 ± 0.02
      Values within a row with different superscripts differ, P < 0.05.
      0.07 ± 0.02
      Values within a row with different superscripts differ, P < 0.05.
      Skin temperature, °CShort0.804Intercept27.4 ± 1.34
      Values within a row with different superscripts differ, P < 0.05.
      27.3 ± 0.75
      Values within a row with different superscripts differ, P < 0.05.
      29.4 ± 1.10
      Values within a row with different superscripts differ, P < 0.05.
      21.8 ± 0.68
      Values within a row with different superscripts differ, P < 0.05.
      23.8 ± 0.82
      Values within a row with different superscripts differ, P < 0.05.
      Slope0.28 ± 0.06
      Values within a row with different superscripts differ, P < 0.05.
      0.28 ± 0.03
      Values within a row with different superscripts differ, P < 0.05.
      0.23 ± 0.05
      Values within a row with different superscripts differ, P < 0.05.
      0.44 ± 0.03
      Values within a row with different superscripts differ, P < 0.05.
      0.38 ± 0.03
      Values within a row with different superscripts differ, P < 0.05.
      bNSNSNS0.22 ± 0.06
      Values within a row with different superscripts differ, P < 0.05.
      0.16 ± 0.06
      Values within a row with different superscripts differ, P < 0.05.
      Long0.811Intercept30.1 ± 0.76
      Values within a row with different superscripts differ, P < 0.05.
      30.4 ± 0.83
      Values within a row with different superscripts differ, P < 0.05.
      31.3 ± 0.83
      Values within a row with different superscripts differ, P < 0.05.
      25.1 ± 0.92
      Values within a row with different superscripts differ, P < 0.05.
      25.3 ± 0.66
      Values within a row with different superscripts differ, P < 0.05.
      Slope0.21 ± 0.03
      Values within a row with different superscripts differ, P < 0.05.
      0.19 ± 0.04
      Values within a row with different superscripts differ, P < 0.05.
      0.18 ± 0.04
      Values within a row with different superscripts differ, P < 0.05.
      0.36 ± 0.04
      Values within a row with different superscripts differ, P < 0.05.
      0.35 ± 0.03
      Values within a row with different superscripts differ, P < 0.05.
      bNSNSNS0.19 ± 0.05
      Values within a row with different superscripts differ, P < 0.05.
      0.17 ± 0.05
      Values within a row with different superscripts differ, P < 0.05.
      P-value0.0140.0050.0220.047NS
      a–d Values within a row with different superscripts differ, P < 0.05.
      1 Exposure time with “short” means the cows stayed in the condition within 1 h and with “long” means the cows stayed in the condition for about 8 h.
      2 Constant = basal values; IPt = inflection point temperature; b = regression coefficient for interaction between temperature and treatments.
      3 Treatment levels: RH_l: 30%; RH_m: 45%; RH_h: 60%; AV_l: 0.1 m/s; AV_m: 1.0 m/s; AV_h: 1.5 m/s. l = low; m = medium; h = high.
      4 P-value for statistical difference between 2 exposure times.
      5 NS: P ≥ 0.10.
      6 The broken-line model could not be fitted for rectal temperature with short exposure time.
      The constants are reflecting the basal levels of RR at the lower ambient T. They varied between 31.3 and 36.3 breaths/min for short exposure and between 24.8 to 36.5 breaths/min for long exposure. There were some differences between the constants of the different treatments, but these could not be directly linked to RH or AV levels. In addition, constant RR was different depending on exposure times in 2 treatments RH_h-AV_l and RH_m-AV_m (P = 0.066 and P = 0.002, respectively), the RR was lower in long exposure as compared with short exposure. Given the variation in basal RR between individual cows, milk yield was considered as a covariable for normal RR (data from first 2 research days) but no effect of milk yield was observed on basal RR. The IPt varied between 20.9 and 25.8°C for short exposure and between 18.9 and 25.5°C for long exposure. Generally, IPt increased with decreasing RH and increasing AV. Under the long exposure, IPt for RR decreased (P < 0.05) as RH increased: 25.5, 21.0, and 18.9°C for RH_l, RH_m, and RH_h, respectively. The increased exposure time decreased IPt for RR for a combined treatment of RH_h-AV_l (P < 0.05) and RH_m-AV_h (P < 0.05), and there was a tendency of an effect of RH_m-AV_m (P < 0.10). Under AV_h, the IPt for RR was higher than the IPt at AV_l (21.0, 22.8°C for AV_l and AV_h, respectively; P < 0.05). The IPt for RR under AV_l or AV_m were not different. The slope (regression coefficient “a” in the broken-line model) varied between 4.1 to 9.4 breaths/°C for short exposure and between 4.2 to 9.5 breaths/°C for long exposure. For low RH the slope was much higher as compared with the other treatments (P < 0.05), which means that under low RH level, although the cows could maintain a basal RR for a wider ambient T range, they had to increase RR rapidly above IPt.
      Rectal T was affected by increasing T in combination with different levels of RH and AV (Table 5). The RT during short exposure is not shown in Table 5 because it could not be fitted by a broken-line model. The average RT for short and long exposure times were 38.4 ± 0.3 and 38.7 ± 0.4, respectively (P < 0.01). For long exposure, constant RT remained within the range of 38.1 to 38.7°C for the 5 treatments. Under low AV conditions, the basal RT at high RH level was lower than the RT at other RH levels (P < 0.05). Treatment RH_m-AV_m had the lowest basal RT among the 5 treatments (P < 0.05). The IPt for RT varied between 20.1 to 25.9°C. Under low AV, the IPt for RT under RH_h was much lower (P < 0.05) compared with RH_l and RH_m (20.1°C vs. 25.3 and 25.9). The effects of AV on IPt for RR were not consistent: at a medium AV level, the IPt appeared smaller than at a high AV level (P < 0.05). The slope varied between 0.07 to 0.14°C/°C. The effect of AV levels on the slope was evident: the slope was significantly lower at medium or high AV levels than at the low AV level in combination with medium or high RH (P < 0.05).
      Skin T increased linearly with increasing T (P < 0.001). For the 5 treatments, the intercept for ST varied between 21.8 to 29.4°C for short exposure and between 25.1 to 31.3°C for long exposure. The intercept for ST was larger in high RH than in RH_l and RH_m treatments with low AV under short exposure (P < 0.05), whereas under long exposure there was no difference in intercepts between the 3 RH levels with low AV. The intercept for ST was higher in the AV_h group than in the AV_m group under short exposure (P < 0.05), whereas under long exposure there was no difference. Generally, cows had lower ST intercepts at higher AV levels for both short and long exposure times. The slope [coefficients (a + b) in the linear model] varied between 0.23 to 0.44°C/°C for short exposure and between 0.18 to 0.36°C/°C for long exposure. There was no interaction effect between RH with T on the increasing rate of ST with low AV level, whereas there was an interaction (P < 0.05) of AV and T to an increasing rate of ST for both exposure times. The interactions of AV and T on ST were obvious, especially at AV_l compared with AV_m and AV_h (Figure 4, c2). Under higher AV levels, the increase in ST slope was higher than that for AV_l. The average ST was higher (P < 0.05) for long exposure compared with short exposure except for RH_m-AV_h, whereas under the short exposure the increase in ST per degree Celsius increase in T was more pronounced.
      Figure thumbnail gr4
      Figure 4Relationships for the different treatments between ambient temperature and (a1) respiration rate, (b1) rectal temperature, (c1) skin temperature under treatment I, II, and III (RH_l, RH_m, and RH_h: 30, 45, and 60%; AV_l: 0.1 m/s); (a2) respiration rate, (b2) rectal temperature, (c2) skin temperature under treatment II, IV, and V (AV_l, AV_m, and AV_h: 0.1, 1.0, and 1.5 m/s; RH_m: 45%). RH = relative humidity, AV = air velocity; l = low, m = medium, h = high.

      Productive Responses to Treatments

      Treatment effects on DMI, water intake, and milk yield are presented in Table 6. At the beginning of the research periods (the first 2 d), the basal DMI for cows in different treatments varied between 17.9 to 21.1 kg/d and the water intake varied between 23.6 to 32.9 kg/d. The DMI decreased 0.003 to 0.14 kg/d per °C increase across the 5 treatments. The most severe decrease in DMI was observed for treatment III (RH_h-AV_l; P < 0.01). There was a tendency (P = 0.079) for decreased DMI under treatment II (RH_m-AV_l) as well. No difference was observed for the other 3 treatments. Water intake was positively related to increasing T (P < 0.05) for all treatments except for treatment I (RH_l-AV_l).
      Table 6Linear regression between DMI (kg/d), water intake (kg/d), milk yield (kg/milking), protein yield (kg/milking), fat yield (kg/milking), and ambient temperature (means ± SEM)
      Dependent variableMilking timeItem
      Baseline represents the values calculated from first 2 research days; slope represents the regression coefficient for different variables with relationship of ambient temperature; slope P-value shows the significance level that the slope differs from zero.
      Treatment
      Treatment levels: RH_l: 30%; RH_m: 45%; RH_h: 60%; AV_l: 0.1 m/s; AV_m: 1.0 m/s; AV_h: 1.5 m/s. RH = relative humidity; AV = air velocity; l = low; m = medium; h = high.
      I (RH_l-AV_l) n = 4II (RH_m-AV_l) n = 4III (RH_h-AV_l) n = 4IV (RH_m-AV_m) n = 3V (RH_m-AV_h) n = 4
      DMIBaseline17.9 ± 0.5320.5 ± 0.3718.8 ± 0.2921.1 ± 0.6418.3 ± 0.39
      Slope−0.003 ± 0.065−0.14 ± 0.076−0.14 ± 0.045−0.10 ± 0.072−0.023 ± 0.065
      Slope P-value NS
      NS: P ≥ 0.10.
      0.079<0.01NSNS
      Water intakeBaseline26.3 ± 1.9232.8 ± 2.2623.6 ± 1.9326.1 ± 3.2832.9 ± 3.48
      Slope0.38 ± 0.581.61 ± 0.710.63 ± 0.262.24 ± 1.041.25 ± 0.31
      Slope P-valueNS<0.05<0.05<0.05<0.001
      Milk yieldamBaseline11.6 ± 0.4815.1 ± 0.7512.8 ± 0.2614.2 ± 0.2113.1 ± 0.35
      Slope0.018 ± 0.0460.041 ± 0.0530.039 ± 0.049−0.008 ± 0.0400.076 ± 0.039
      Slope P-valueNSNSNSNS0.058
      pmBaseline9.3 ± 0.4612.3 ± 0.6210.4 ± 0.1911.2 ± 0.1610.7 ± 0.38
      Slope0.021 ± 0.063−0.016 ± 0.060−0.020 ± 0.0380.025 ± 0.035−0.004 ± 0.037
      Slope P-valueNSNSNSNSNS
      Protein yieldamBaseline0.45 ± 0.0150.52 ± 0.0190.50 ± 0.0090.52 ± 0.0070.48 ± 0.009
      Slope−0.001 ± 0.002−0.001 ± 0.002−0.001 ± 0.002−0.002 ± 0.0010.001 ± 0.001
      Slope P-valueNSNSNSNSNS
      pmBaseline0.35 ± 0.0150.42 ± 0.0160.39 ± 0.0060.40 ± 0.0060.37 ± 0.011
      Slope−0.000 ± 0.002−0.003 ± 0.002−0.003 ± 0.001−0.001 ± 0.001−0.002 ± 0.001
      Slope P-valueNSNS<0.05NSNS
      Fat yieldamBaseline0.60 ± 0.0130.70 ± 0.0300.68 ± 0.0250.75 ± 0.0100.62 ± 0.030
      Slope−0.003 ± 0.003−0.004 ± 0.005−0.003 ± 0.004−0.004 ± 0.0010.001 ± 0.003
      Slope P-valueNSNSNS<0.05NS
      pmBaseline0.54 ± 0.0180.65 ± 0.0340.62 ± 0.0260.65 ± 0.0110.57 ± 0.024
      Slope−0.002 ± 0.003−0.005 ± 0.004−0.006 ± 0.003−0.003 ± 0.002−0.005 ± 0.002
      Slope P-valueNSNS<0.050.0550.066
      1 Baseline represents the values calculated from first 2 research days; slope represents the regression coefficient for different variables with relationship of ambient temperature; slope P-value shows the significance level that the slope differs from zero.
      2 Treatment levels: RH_l: 30%; RH_m: 45%; RH_h: 60%; AV_l: 0.1 m/s; AV_m: 1.0 m/s; AV_h: 1.5 m/s. RH = relative humidity; AV = air velocity; l = low; m = medium; h = high.
      3 NS: P ≥ 0.10.
      Increasing T had no effect on morning or afternoon milk yields for all the treatments (NS; P > 0.10) except for treatment V (RH_m-AV_h) where the morning milk yield increased (P = 0.058). However, under treatment III (RH_h-AV_l) the protein and fat yield for afternoon milking decreased with increasing T (P < 0.05). Decreased fat yield was also a tendency under treatment IV and V (RH_m-AV_m, RH_m-AV_h; P = 0.055 and 0.066, respectively), which showed that although the milk yield was not affected by increasing T the component yield could be affected.

      DISCUSSION

      This study evaluated the effects of increasing T at different RH and AV levels on thermoregulatory responses of Holstein Friesian dairy cows. The results of this study may contribute to the development of new strategies for heat stress mitigation that take into account the physiological adaptations of the animals.

      Physiological Responses to Treatments

      The increase in RR was the first reaction of cows under warm conditions attempting to maintain a constant body T by increasing evaporative heat loss from the respiratory tract (
      • Silanikove N.
      Effects of heat stress on the welfare of extensively managed domestic ruminants.
      ).
      • Berman A.
      • Folman Y.
      • Kaim M.
      • Mamen M.
      • Herz Z.
      • Wolfenson D.
      • Arieli A.
      • Graber Y.
      Upper critical temperatures and forced ventilation effects for high-yielding dairy cows in a subtropical climate.
      found that RR in the lactating dairy cow started to rise when ambient T surpassed 25°C. They suggested that respiratory evaporative heat loss is extremely important for maintenance of thermal stability in large cattle due to their large body size. In our study, under a combination of high RH and low AV, RR had increased slightly (IPt) just at 19°C. At lower RH levels the determined IPt was higher. The difference in IPt between low (30%) and high RH (60%) was approximately 5°C for short (1 h) exposure and 6.5°C for long (8 h) exposure, which was consistent with the results from other studies (
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Amon T.
      Critical THI thresholds based on the physiological parameters of lactating dairy cows.
      ). The IPt increased with increasing AV, whereas the first step increase of IPt from low AV (0.1 m/s) to medium AV (1.0 m/s) seemed to be smaller than the second step increase from medium AV to high AV (1.5 m/s). According to
      • Spiers D.E.
      • Spain J.N.
      • Ellersieck M.R.
      • Lucy M.C.
      Strategic application of convective cooling to maximize the thermal gradient and reduce heat stress response in dairy cows.
      , the benefit of fan cooling is highly dependent on T and because of the small difference between T and ST, the medium AV did not contribute much to dissipating the heat through convection. Additionally, the medium AV might not have been high enough to overcome the thermal resistance of the hair coat, which resulted in a small difference in RR between 2 AV levels. In addition, if sweat can be fully evaporated without being limited by the ambient potential evaporation rate of the air at RH of 45% (
      • Gash J.H.
      • Shuttleworth W.J.
      Evaporation.
      ), the AV will not provide much help for the cutaneous evaporative heat loss. To assess heat stress magnitude,
      • Gaughan J.B.
      • Mader T.L.
      • Holt S.M.
      • Lisle A.
      A new heat load index for feedlot cattle.
      developed a heat load index based on panting responses incorporating air T, RH, black-globe T, and AV; when air T and solar radiance were fixed, RH increased from 20% to 80%, and no chilling effects of AV increment (1 to 2 m/s) were found (
      • Wang X.
      • Bjerg B.S.
      • Choi C.Y.
      • Zong C.
      • Zhang G.
      A review and quantitative assessment of cattle-related thermal indices.
      ). In future research, a combination of higher RH levels (higher than 60%) with higher AV levels could be helpful for ascertaining the significant effect of AV, because at high RH, increasing AV could help to raise the potential evaporation rate and further increase cutaneous latent heat dissipation.
      • Gebremedhin K.G.
      • Hillman P.E.
      • Lee C.N.
      • Collier R.J.
      • Willard S.T.
      • Arthington J.D.
      • Brown-Brandl T.M.
      Sweating rates of dairy cows and beef heifers in hot conditions.
      reported that increased RH negatively affected the cutaneous latent heat loss in cows, which implies that RH conditions need to be monitored when implementing evaporative cooling using nebulizers (
      • Berman A.
      Predicted limits for evaporative cooling in heat stress relief of cattle in warm conditions.
      ). With a drop of RH levels from high (60%) to low (30%), the IPt could be raised by approximately 5°C for short exposure and 6.5°C for long exposure, respectively. In Figure 5, the evaporative cooling process was simulated based on thermodynamics and psychrometrics (the T of the air could be reduced by transferring heat from air to evaporating water), the chamber's air T started at 30°C, and moisture was added at different RH levels (
      • ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)
      2009 ASHRAE Handbook: Fundamentals.
      ;
      • Silva R.G.
      • Maia A.S.C.
      Evaporative cooling and cutaneous surface temperature of Holstein cows in tropical conditions.
      ). The simulation showed that RH rose from 30% to 60% when moisture was added, implying that the chamber's air T could be lowered 6.8°C. From this simulation, in any environment where air T is high and there is no AV, applying evaporative spraying could cause an increase in RH, which might prevent the evaporation of sweat from the skin because of the damp air's insufficient evaporation potential. This implies that an evaporative cooling of the air in combination with a higher AV might not reduce the evaporation potential of the air, but further research is needed to confirm this hypothesis.
      Figure thumbnail gr5
      Figure 5Relationships between the drop of the temperature of ambient air (having an initial temperature of 30°C) and the initial relative humidity using evaporative cooling to reach 60% relative humidity (
      • ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)
      2009 ASHRAE Handbook: Fundamentals.
      ).
      Limited research has been reported on the effect of exposure time on physiological responses. Our study was the first one to investigate the exposure time effect and we found that IPt for RR was decreased under long exposure time. In our study the ambient conditions were controlled such that we could clearly see the effect of exposure time on RR.
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Amon T.
      Critical THI thresholds based on the physiological parameters of lactating dairy cows.
      , however, performing a study in a conventional barn during the summer period, found it difficult to say whether the increased RR (2.9 breaths/min increase after the critical threshold was exceeded) was due to the increase of heat load magnitude or whether it was because of increasing exposure duration.
      When the cow fails to dissipate heat, the RT will rise.
      • Brown-Brandl T.M.
      • Nienaber J.A.
      • Eigenberg R.A.
      • Hahn G.L.
      • Freetly H.
      Thermoregulatory responses of feeder cattle.
      continuously recorded RR and detected a delayed increase of RT compared with RR. The relationship between RT and T has been studied by many researchers. A recent study by
      • Li G.
      • Chen S.
      • Chen J.
      • Peng D.
      • Gu X.
      Predicting rectal temperature and respiration rate responses in lactating dairy cows exposed to heat stress.
      stated that the RT started to rise at T of 20.4°C without mentioning the conditions of RH or AV, the IPt of which was only comparable to that under treatment RH of 60% with low AV in this study.
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Amon T.
      Critical THI thresholds based on the physiological parameters of lactating dairy cows.
      and
      • Yan G.
      • Liu K.
      • Hao Z.
      • Shi Z.
      • Li H.
      The effects of cow-related factors on rectal temperature, respiration rate, and temperature-humidity index thresholds for lactating cows exposed to heat stress.
      reported that the critical THI threshold for RT was 70, which was in line with our results from treatment I (RH_l-AV_l and 25°C; Figure 6). In our study, we found that the average RT from short exposure was lower than that from long exposure. According to
      • McGovern R.E.
      • Bruce J.M.
      AP—Animal production technology: A model of the thermal balance for cattle in hot conditions.
      , the heat increment that was not dissipated by all cooling methods would be stored in the core body, causing the body T to rise. In agreement, we could present that the effects of the exposure time on RT is an important factor when assessing the heat tolerance of cows under high ambient T, especially under high RH without the intervention of AV. This observation indicated that during warm days with high RH, evaporative cooling could not be effective. However, when conducting the experiment in commercial farms,
      • Mullick D.N.
      Effect of humidity and exposure to sun on the pulse rate, respiration rate, rectal temperature and haemoglobin level in different sexes of cattle and buffalo.
      found that a high RH had a tendency to lower the RT, which could be due to the fluctuation of air T and RH. When the ambient RH was raised from 30 to 60% by evaporative cooling, the T could be decreased from 30 to 23.2°C (Figure 5) and there was no positive effect for the RT. The most commonly practiced heat stress intervention on conventional farms, at least in the Netherlands, is to provide high air speed around cows using either head-level or ceiling-level fans. The AV in the practice is comparable with the AV_m in this study and RH levels in the barns are generally around 45 to 60% (RH_m in our design;
      • André G.
      • Engel B.
      • Berentsen P.
      • Vellinga T.V.
      • Oude Lansink A.G.J.M.
      Quantifying the effect of heat stress on daily milk yield and monitoring dynamic changes using an adaptive dynamic model.
      ). Presumably, given the mentioned RH, the AV_m in the practice could be effective enough for the cows to maintain the RT at normal range. In addition, the difference in individual cows' characteristics, such as body condition (fatness) or genetics, can influence their responses to heat stress (
      • Gaughan J.
      • Holt S.
      • Hahn G.
      • Mader T.
      • Eigenberg R.
      Respiration rate: Is it a good measure of heat stress in cattle? Asian-Australas.
      ;
      • Berman A.
      Estimates of heat stress relief needs for Holstein dairy cows.
      ). However, those cow factors were not included in our analysis. In addition, the way we installed fans in the CRC (Figure 1) was different from how it is usually done on farms. We measured AV at 5 points and found the highest AV around the rump and the lowest AV around the lateral sides, which might explain the limited effects of higher AV on IPt for RT. In addition, one cow was excluded from treatment IV because of mastitis, which led to lower estimation accuracy for the IPt for RT. The importance of exposure time when identifying heat stress responses has been recognized by other researchers (
      • Kaufman J.D.
      • Saxton A.M.
      • Ríus A.G.
      Short communication: Relationships among temperature-humidity index with rectal, udder surface, and vaginal temperatures in lactating dairy cows experiencing heat stress.
      ;
      • Peng D.
      • Chen S.
      • Li G.
      • Chen J.
      • Wang J.
      • Gu X.
      Infrared thermography measured body surface temperature and its relationship with rectal temperature in dairy cows under different temperature-humidity indexes.
      ). One example that should be considered when planning cooling schemes is that once T exceeds the IPt, the shorter exposure time is the more stable RT that the cow can maintain.
      Figure thumbnail gr6
      Figure 6Temperature-humidity index calculated according to
      • NRC
      A Guide to Environmental Research on Animals.
      . The colors indicate the severity levels of the heat stress (
      • Zimbelman R.
      • Rhoads R.
      • Rhoads M.
      • Duff G.
      • Baumgard L.
      • Collier R.
      A re-evaluation of the impact of temperature humidity index (THI) and black globe humidity index (BGHI) on milk production in high producing dairy cows.
      ): green (≤67) is no heat stress; yellow (68–71) is mild heat stress; orange (72–79) is moderate heat stress; and red (≥80) is severe heat stress. The temperature-humidity index values in 3 white blocks were temperature and relative humidity conditions in our experiment.
      The cows' ST increases with an increase in the ambient T if there is no cooling facility. We observed that when providing high AV, the cows' ST was much lower than under low AV, especially when the ambient T was low. When there was less need for heat transfer (under low ambient T) there was no extra blood vessel dilation (
      • McGovern R.E.
      • Bruce J.M.
      AP—Animal production technology: A model of the thermal balance for cattle in hot conditions.
      ;
      • Silanikove N.
      Effects of heat stress on the welfare of extensively managed domestic ruminants.
      ). According to
      • Collier R.J.
      • Dahl G.E.
      • VanBaale M.J.
      Major advances associated with environmental effects on dairy cattle.
      , who did not provide RH or AV information, as ST rose above 35°C, the cows gradually began to store heat as indicated by increased RT. We agree with the authors as the linear lines in Figure 4 b1 and c1 presented asymptotic patterns except at treatment RH_m-AV_m, which could be due to individual variation in different cows. It was difficult to compare the results with previous studies without sufficient information on the RH and AV conditions. As said, except for treatment RH_m-AV_m, under the intervention of AV (Figure 4, b2 and c2) the behavior of the linear lines for ST and RT were different. The starting ST in treatment II was already approximately 3°C higher than the other 2 treatments (Figure 4, c2); consequently, the basal RT in treatment II was obviously higher than the other 2 treatments (Figure 4, b2). This confirmed that the different responses in individual cows were significant. The starting ST in the high RH treatment (Figure 4, c1) was already higher compared with other groups, and whether this high starting ST led to the lowest IPt for RR and RT (Figure 4, a1 and b1) was an interesting finding. In theory, convective heat transfer occurs between T gradients; in this case, as ST was already high, the capacity to transport surface heat to the ambient air was limited. Therefore, it led to early increases in both RR and RT.
      Although the Holstein Friesian cows in this study were not at the peak of their production level (were entering late lactation), results still showed that the cows responded physiologically to T even a bit lower than 20°C. The first response to increased T was an increase in RR, whereas RT did not increase until T was above 20°C.
      • Yan G.
      • Liu K.
      • Hao Z.
      • Shi Z.
      • Li H.
      The effects of cow-related factors on rectal temperature, respiration rate, and temperature-humidity index thresholds for lactating cows exposed to heat stress.
      reported that THI threshold at which RT and RR began to increase in China was lower for early-lactation cows compared with late-lactation cows. This meant that for high-producing cows near peak production, the IPt for RR and RT could be even lower. In addition, this study only mimicked a gradual rise in T over the 8-d period rather than a heat wave exposure over several days, which is common in the subtropics and tropics (
      • Pinto S.
      • Hoffmann G.
      • Ammon C.
      • Heuwieser W.
      • Levit H.
      • Halachmi I.
      • Amon T.
      Effect of two cooling frequencies on respiration rate in lactating dairy cows under hot and humid climate conditions.
      ). The adverse effect of heat can be underestimated if animals do not have a recovery period of lower T during the night (
      • Gaughan J.B.
      • Mader T.L.
      • Holt S.M.
      • Lisle A.
      A new heat load index for feedlot cattle.
      ).

      Productive Responses to Treatments

      One of the primary measurements of productivity in farm animals in general and dairy cows in particular is DMI (
      • Spiers D.E.
      • Spain J.N.
      • Ellersieck M.R.
      • Lucy M.C.
      Strategic application of convective cooling to maximize the thermal gradient and reduce heat stress response in dairy cows.
      ). According to
      • Mount L.E.
      Adaptation to Thermal Environment. Man and His Productive Animals.
      , DMI decreases with high ambient T to decrease the animal's heat production, which compensates for the lowered heat dissipation. In this study, the cows maintained their DMI when they were under low RH or in the case of high RH combined with high AV. We also reported that effects of medium and high RH without fan on DMI were equally significant, which was comparable with other studies (
      • Hill D.L.
      • Wall E.
      Weather influences feed intake and feed efficiency in a temperate climate.
      ;
      • Herbut P.
      • Hoffmann G.
      • Angrecka S.
      • Godyń D.
      • Vieira F.
      • Adamczyk K.
      • Kupczyński R.
      The effects of heat stress on the behaviour of dairy cows-A review.
      ).
      • West J.W.
      • Mullinix B.
      • Bernard J.
      Effects of hot, humid weather on milk temperature, dry matter intake, and milk yield of lactating dairy cows.
      found a decrease in daily DMI of 0.51 kg for each unit increase in THI between 73 and 82, whereas
      • Hill D.L.
      • Wall E.
      Weather influences feed intake and feed efficiency in a temperate climate.
      reported that the decrease rate of DMI was 0.03 kg for every increased unit of THI. Our results showed that under medium RH, increasing AV had a positive effect on DMI.
      Water is one of the most important nutrients for the dairy cow (
      • West J.W.
      Effects of heat-stress on production in dairy cattle.
      ). In our study, despite some water spilling at the beginning when the cows were adapting to the CRC, there was a very obvious increase in water intake with increasing T. The cow is bred to maintain a milk yield consisting of approximately 87% water, and under heat stress there is evaporative heat dissipation through respiration and sweating. Water intake can and should be increased to compensate for this. In our study, treatment group IV (45% RH and 1.0 m/s AV) had the greatest increase in water intake, whereas treatment I (30% RH and 0.1 m/s AV) had the lowest. The reason for this could be that (1) cows in treatment I randomly had the lowest milk yield or (2) there was not much evaporative heat dissipation. According to
      • West J.W.
      Effects of heat-stress on production in dairy cattle.
      , water intake increased by 1.2 kg per °C increase in ambient T, which is within the range of increase we observed in our study.
      There were no obvious changes in milk yield with increasing T across the 5 treatments. This is interesting because
      • Zimbelman R.
      • Rhoads R.
      • Rhoads M.
      • Duff G.
      • Baumgard L.
      • Collier R.
      A re-evaluation of the impact of temperature humidity index (THI) and black globe humidity index (BGHI) on milk production in high producing dairy cows.
      ,
      • Gauly M.
      • Bollwein H.
      • Breves G.
      • Brügemann K.
      • Dänicke S.
      • Daş G.
      • Demeler J.
      • Hansen H.
      • Isselstein J.
      • König S.
      • Lohölter M.
      • Martinsohn M.
      • Meyer U.
      • Potthoff M.
      • Sanker C.
      • Schröder B.
      • Wrage N.
      • Meibaum B.
      • von Samson-Himmelstjerna G.
      • Stinshoff H.
      • Wrenzycki C.
      Future consequences and challenges for dairy cow production systems arising from climate change in Central Europe – A review.
      , and
      • Hill D.L.
      • Wall E.
      Dairy cattle in a temperate climate: The effects of weather on milk yield and composition depend on management.
      all reported that milk yield began to decline at an average THI of 68 and the decreasing rate could be 2.2 kg/d. A THI of 68 is similar to a combination of 45% RH at 22°C or 30% RH at 24°C in this study (Figure 6). However,
      • Linvill D.E.
      • Pardue F.E.
      Heat stress and milk production in the South Carolina coastal plains.
      found that milk yield only started to decrease after 4 d with a THI above 74. Given the complex design with a diurnal pattern of T, RH, and AV in this study, several explanations could be given for this contradiction: (1) the cows were able to recover overnight in the cooler T: according to
      • Igono M.
      • Johnson H.
      Physiologic stress index of lactating dairy cows based on diurnal pattern of rectal temperature.
      , milk yield only declined when the cow's RT exceeded 39°C for more than 16 h, which was never reached in our study because of the lower T during the night; (2) the total daily yield of nutritional components of the milk declined but we could not directly see this aspect from milk yield only; (3) the reduced milk yield would appear later as a delayed response to heat stress (
      • Linvill D.E.
      • Pardue F.E.
      Heat stress and milk production in the South Carolina coastal plains.
      ;
      • Polsky L.
      • von Keyserlingk M.A.G.
      Invited review: Effects of heat stress on dairy cattle welfare.
      ), which we were unable to observe due to the current research setup and time span.
      The statistical analysis of DMI and milk yield showed that at low RH (30%) the cows were able to sustain DMI and milk, protein, and fat yield across the range of T used in the trial (16 to 30°C). The AV did not show a positive effect on milk components. As mentioned before, once the gradient between ST and T became asymptotic, the function of high AV was weaker in terms of dissipating heat when sweat evaporation is not limited, despite many studies reporting a positive effect of fan cooling on milk production (
      • Calegari F.
      • Calamari L.
      • Frazzi E.
      Fan cooling of the resting area in a free stalls dairy barn.
      ;
      • Sunagawa K.
      • Nagamine I.
      • Kamata Y.
      • Niino N.
      • Taniyama Y.
      • Kinjo K.
      • Matayoshi A.
      Nighttime cooling is an effective method for improving milk production in lactating goats exposed to hot and humid environment.
      ;
      • Wu B.
      • Gooch C.
      • Wright P.
      Verification and Recommendations for Cooling Fans in Freestall Dairy Barns.
      ). This finding is again of great interest for the industry, given most available cooling systems focus on increasing AV with or without nebulizing water (
      • Avendaño-Reyes L.
      • Álvarez-Valenzuela F.D.
      • Correa-Calderón A.
      • Algándar-Sandoval A.
      • Rodríguez-González E.
      • Pérez-Velázquez R.
      • Macías-Cruz U.
      • Díaz-Molina R.
      • Robinson P.H.
      • Fadel J.G.
      Comparison of three cooling management systems to reduce heat stress in lactating Holstein cows during hot and dry ambient conditions.
      ;
      • Fournel S.
      • Ouellet V.
      • Charbonneau É.
      Practices for alleviating heat stress of dairy cows in humid continental climates: a literature review.
      ;
      • Spiers D.E.
      • Spain J.N.
      • Ellersieck M.R.
      • Lucy M.C.
      Strategic application of convective cooling to maximize the thermal gradient and reduce heat stress response in dairy cows.
      ). However, in our study we did not investigate whether higher AV might have an effect combined with high RH level. In any case, it is important for dairy farmers to consider the RH inside the barn when designing evaporative cooling systems.
      This study was simulated in CRC to avoid confounding effects, which allowed us to study the relationship between physiological responses and ambient T and duration of high T. Although the cows were housed in an unnatural situation, they could still respond behaviorally, for example by decreasing their lying time or increasing their water intake. However, cows in this study were not free to walk around or play with others like on a real farm, which would have certain effects on the physiological responses to heat load. It is important to remember that this study offers information about cows housed indoors, whereas cows at pasture also need to deal with radiation from the sun. When there is no shade, the IPt could be significantly lower. Inside a barn with a group of cows, cows have more behavioral options to react to changing indoor climates. In rising T, however, cows will tend to move away from the heat radiating from other cows, which would render the interaction effects between cows rather low in heat stress situations. However, we recommend validating these results found under semi-laboratory conditions in practical circumstances.

      CONCLUSIONS

      Above IPt, significant changes can be observed in dairy cows' thermal physiological responses. Increased RR was the first indicator showing that the cow was reacting to high ambient T. The IPt for RR increased with decreasing RH and with increasing AV. The decrease in IPt for RR from low to high RH almost compensated for the decrease in T with an evaporative cooling of the ambient air. Rectal T increased above an ambient T ranging from 20.1 to 25.9°C. The increase in RT was an indicator that ambient T was above the upper limit of the thermal neutral zone. The IPt for RT was lowest when the highest RH level was combined with the lowest AV level. Generally, the effects of AV on RR and RT are relatively small at medium RH levels. This might be different at higher RH levels, when the evaporation potential of the ambient air is limited. Higher AV levels lowered ST, but this difference became smaller with increasing T. The effects of exposure time (1 or 8 h) on RR, RT, and ST of increased T were significant. This means that cows respond with physiological changes at lower ambient T if forced to remain in hot conditions for a long time.

      ACKNOWLEDGMENTS

      This research was funded by the China Scholarship Council (Beijing, China) and the Sino-Dutch Dairy Development Center (Beijing, China). The authors gratefully acknowledge technical assistance from Marcel Heetkamp, Sven Alferink, Tamme Zandstra, and the animal caretakers of the research facilities of “Carus” (Wageningen University and Research, the Netherlands). The authors have not stated any conflicts of interest.

      REFERENCES

        • Amamou H.
        • Beckers Y.
        • Mahouachi M.
        • Hammami H.
        Thermotolerance indicators related to production and physiological responses to heat stress of Holstein cows.
        J. Therm. Biol. 2019; 82 (31128664): 90-98
        • André G.
        • Engel B.
        • Berentsen P.
        • Vellinga T.V.
        • Oude Lansink A.G.J.M.
        Quantifying the effect of heat stress on daily milk yield and monitoring dynamic changes using an adaptive dynamic model.
        J. Dairy Sci. 2011; 94 (21854922): 4502-4513
        • ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers)
        2009 ASHRAE Handbook: Fundamentals.
        American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc, 2009
        • Avendaño-Reyes L.
        • Álvarez-Valenzuela F.D.
        • Correa-Calderón A.
        • Algándar-Sandoval A.
        • Rodríguez-González E.
        • Pérez-Velázquez R.
        • Macías-Cruz U.
        • Díaz-Molina R.
        • Robinson P.H.
        • Fadel J.G.
        Comparison of three cooling management systems to reduce heat stress in lactating Holstein cows during hot and dry ambient conditions.
        Livest. Sci. 2010; 132: 48-52
        • Berman A.
        Estimates of heat stress relief needs for Holstein dairy cows.
        J. Anim. Sci. 2005; 83 (15890815): 1377-1384
        • Berman A.
        Predicted limits for evaporative cooling in heat stress relief of cattle in warm conditions.
        J. Anim. Sci. 2009; 87 (19574571): 3413-3417
        • Berman A.
        • Folman Y.
        • Kaim M.
        • Mamen M.
        • Herz Z.
        • Wolfenson D.
        • Arieli A.
        • Graber Y.
        Upper critical temperatures and forced ventilation effects for high-yielding dairy cows in a subtropical climate.
        J. Dairy Sci. 1985; 68 (4019887): 1488-1495
        • Brown-Brandl T.M.
        • Nienaber J.A.
        • Eigenberg R.A.
        • Hahn G.L.
        • Freetly H.
        Thermoregulatory responses of feeder cattle.
        J. Therm. Biol. 2003; 28: 149-157
        • Burfeind O.
        • Suthar V.S.
        • Heuwieser W.
        Effect of heat stress on body temperature in healthy early postpartum dairy cows.
        Theriogenology. 2012; 78 (23043945): 2031-2038
        • Calegari F.
        • Calamari L.
        • Frazzi E.
        Fan cooling of the resting area in a free stalls dairy barn.
        Int. J. Biometeorol. 2014; 58 (24122338): 1225-1236
        • Collier R.J.
        • Dahl G.E.
        • VanBaale M.J.
        Major advances associated with environmental effects on dairy cattle.
        J. Dairy Sci. 2006; 89 (16537957): 1244-1253
        • CVB
        CVB Table Ruminants 2008, series nr. 43.
        CVB, The Hague, the Netherlands2008
        • de Andrade Ferrazza R.
        • Mogollón Garcia H.D.
        • Vallejo Aristizábal V.H.
        • de Souza Nogueira C.
        • Veríssimo C.J.
        • Sartori J.R.
        • Sartori R.
        • Pinheiro Ferreira J.C.
        Thermoregulatory responses of Holstein cows exposed to experimentally induced heat stress.
        J. Therm. Biol. 2017; 66 (28477912): 68-80
        • Fournel S.
        • Ouellet V.
        • Charbonneau É.
        Practices for alleviating heat stress of dairy cows in humid continental climates: a literature review.
        Animals (Basel). 2017; 7 (28468329): 37
        • Galán E.
        • Llonch P.
        • Villagrá A.
        • Levit H.
        • Pinto S.
        • del Prado A.
        A systematic review of non-productivity-related animal-based indicators of heat stress resilience in dairy cattle.
        PLoS One. 2018; 13 (30383843)e0206520
        • García-Ispierto I.
        • López-Gatius F.
        • Bech-Sabat G.
        • Santolaria P.
        • Yániz J.L.
        • Nogareda C.
        • De Rensis F.
        • López-Béjar M.
        Climate factors affecting conception rate of high producing dairy cows in northeastern Spain.
        Theriogenology. 2007; 67 (17412409): 1379-1385
        • Gash J.H.
        • Shuttleworth W.J.
        Evaporation.
        IAHS Press, 2007
        • Gaughan J.
        • Holt S.
        • Hahn G.
        • Mader T.
        • Eigenberg R.
        Respiration rate: Is it a good measure of heat stress in cattle? Asian-Australas.
        J. Anim. Sci. 2000; 13: 329-332
        • Gaughan J.B.
        • Mader T.L.
        • Holt S.M.
        • Lisle A.
        A new heat load index for feedlot cattle.
        J. Anim. Sci. 2008; 86 (17911236): 226-234
        • Gauly M.
        • Bollwein H.
        • Breves G.
        • Brügemann K.
        • Dänicke S.
        • Daş G.
        • Demeler J.
        • Hansen H.
        • Isselstein J.
        • König S.
        • Lohölter M.
        • Martinsohn M.
        • Meyer U.
        • Potthoff M.
        • Sanker C.
        • Schröder B.
        • Wrage N.
        • Meibaum B.
        • von Samson-Himmelstjerna G.
        • Stinshoff H.
        • Wrenzycki C.
        Future consequences and challenges for dairy cow production systems arising from climate change in Central Europe – A review.
        Animal. 2013; 7: 843-859
        • Gebremedhin K.G.
        • Hillman P.E.
        • Lee C.N.
        • Collier R.J.
        • Willard S.T.
        • Arthington J.D.
        • Brown-Brandl T.M.
        Sweating rates of dairy cows and beef heifers in hot conditions.
        Trans. ASABE. 2008; 51: 2167-2178
        • Gerrits W.
        • Labussière E.
        Indirect Calorimetry: Techniques, Computations and Applications.
        Wageningen Academic Publishers, 2015
        • Herbut P.
        • Hoffmann G.
        • Angrecka S.
        • Godyń D.
        • Vieira F.
        • Adamczyk K.
        • Kupczyński R.
        The effects of heat stress on the behaviour of dairy cows-A review.
        Ann. Anim. Sci. 2021; 21: 385-402
        • Hill D.L.
        • Wall E.
        Dairy cattle in a temperate climate: The effects of weather on milk yield and composition depend on management.
        Animal. 2015; 9 (25315451): 138-149
        • Hill D.L.
        • Wall E.
        Weather influences feed intake and feed efficiency in a temperate climate.
        J. Dairy Sci. 2017; 100 (28109597): 2240-2257
        • Igono M.
        • Johnson H.
        Physiologic stress index of lactating dairy cows based on diurnal pattern of rectal temperature.
        J. Interdiscipl. Cycle Res. 1990; 21: 303-320
        • Kadzere C.
        • Murphy M.
        • Silanikove N.
        • Maltz E.
        Heat stress in lactating dairy cows: A review.
        Livest. Prod. Sci. 2002; 77: 59-91
        • Kaufman J.D.
        • Saxton A.M.
        • Ríus A.G.
        Short communication: Relationships among temperature-humidity index with rectal, udder surface, and vaginal temperatures in lactating dairy cows experiencing heat stress.
        J. Dairy Sci. 2018; 101 (29605321): 6424-6429
        • KNMI
        Hourly data for the weather in the Netherlands.
        https://www.knmi.nl/nederland-nu/klimatologie/uurgegevens
        Date: 2019
        Date accessed: October 1, 2019
        • Li G.
        • Chen S.
        • Chen J.
        • Peng D.
        • Gu X.
        Predicting rectal temperature and respiration rate responses in lactating dairy cows exposed to heat stress.
        J. Dairy Sci. 2020; 103 (32278558): 5466-5484
        • Linvill D.E.
        • Pardue F.E.
        Heat stress and milk production in the South Carolina coastal plains.
        J. Dairy Sci. 1992; 75 (1452860): 2598-2604
        • Majkić M.
        • Cincović M.R.
        • Belić B.
        • Plavša N.
        • Lakić I.
        • Radinović M.
        Relationship between milk production and metabolic adaptation in dairy cows during heat stress.
        Acta Agric. Serb. 2017; 22: 123-131
        • McArthur A.J.
        Thermal interaction between animal and microclimate: A comprehensive model.
        J. Theor. Biol. 1987; 126 (3657231): 203-238
        • McGovern R.E.
        • Bruce J.M.
        AP—Animal production technology: A model of the thermal balance for cattle in hot conditions.
        J. Agric. Eng. Res. 2000; 77: 81-92
        • Mount L.E.
        Adaptation to Thermal Environment. Man and His Productive Animals.
        Edward Arnold (Publishers) Ltd, 1979
        • Mullick D.N.
        Effect of humidity and exposure to sun on the pulse rate, respiration rate, rectal temperature and haemoglobin level in different sexes of cattle and buffalo.
        J. Agric. Sci. 1960; 54: 391-394
        • NRC
        A Guide to Environmental Research on Animals.
        National Academies, 1971
        • Peng D.
        • Chen S.
        • Li G.
        • Chen J.
        • Wang J.
        • Gu X.
        Infrared thermography measured body surface temperature and its relationship with rectal temperature in dairy cows under different temperature-humidity indexes.
        Int. J. Biometeorol. 2019; 63 (30680628): 327-336
        • Pinto S.
        • Hoffmann G.
        • Ammon C.
        • Amon T.
        Critical THI thresholds based on the physiological parameters of lactating dairy cows.
        J. Therm. Biol. 2020; 88 (32125999)102523
        • Pinto S.
        • Hoffmann G.
        • Ammon C.
        • Heuwieser W.
        • Levit H.
        • Halachmi I.
        • Amon T.
        Effect of two cooling frequencies on respiration rate in lactating dairy cows under hot and humid climate conditions.
        Ann. Anim. Sci. 2019; 19: 821-834
        • Polsky L.
        • von Keyserlingk M.A.G.
        Invited review: Effects of heat stress on dairy cattle welfare.
        J. Dairy Sci. 2017; 100 (28918147): 8645-8657
        • Ravagnolo O.
        • Misztal I.
        • Hoogenboom G.
        Genetic component of heat stress in dairy cattle, development of heat index function.
        J. Dairy Sci. 2000; 83 (11003246): 2120-2125
        • Robbins K.R.
        • Saxton A.
        • Southern L.
        Estimation of nutrient requirements using broken-line regression analysis.
        J. Anim. Sci. 2006; 84 (16582088): E155-E165
        • Schüller L.K.
        • Burfeind O.
        • Heuwieser W.
        Impact of heat stress on conception rate of dairy cows in the moderate climate considering different temperature–humidity index thresholds, periods relative to breeding, and heat load indices.
        Theriogenology. 2014; 81 (24612695): 1050-1057
        • Silanikove N.
        Effects of heat stress on the welfare of extensively managed domestic ruminants.
        Livest. Prod. Sci. 2000; 67: 1-18
        • Silva R.G.
        • Maia A.S.C.
        Evaporative cooling and cutaneous surface temperature of Holstein cows in tropical conditions.
        Rev. Bras. Zootec. 2011; 40: 1143-1147
        • Spiers D.E.
        • Spain J.N.
        • Ellersieck M.R.
        • Lucy M.C.
        Strategic application of convective cooling to maximize the thermal gradient and reduce heat stress response in dairy cows.
        J. Dairy Sci. 2018; 101 (29935820): 8269-8283
        • Sunagawa K.
        • Nagamine I.
        • Kamata Y.
        • Niino N.
        • Taniyama Y.
        • Kinjo K.
        • Matayoshi A.
        Nighttime cooling is an effective method for improving milk production in lactating goats exposed to hot and humid environment.
        Asian-Australas. J. Anim. Sci. 2015; 28 (26104401): 966-975
        • Vitali A.
        • Segnalini M.
        • Bertocchi L.
        • Bernabucci U.
        • Nardone A.
        • Lacetera N.
        Seasonal pattern of mortality and relationships between mortality and temperature-humidity index in dairy cows.
        J. Dairy Sci. 2009; 92 (19620660): 3781-3790
        • Wang X.
        • Bjerg B.S.
        • Choi C.Y.
        • Zong C.
        • Zhang G.
        A review and quantitative assessment of cattle-related thermal indices.
        J. Therm. Biol. 2018; 77 (30196896): 24-37
        • West J.W.
        Effects of heat-stress on production in dairy cattle.
        J. Dairy Sci. 2003; 86 (12836950): 2131-2144
        • West J.W.
        • Mullinix B.
        • Bernard J.
        Effects of hot, humid weather on milk temperature, dry matter intake, and milk yield of lactating dairy cows.
        J. Dairy Sci. 2003; 86 (12613867): 232-242
        • Wu B.
        • Gooch C.
        • Wright P.
        Verification and Recommendations for Cooling Fans in Freestall Dairy Barns.
        in: 2016 ASABE Annual International Meeting. ASABE, 2016: 1
        • Yan G.
        • Liu K.
        • Hao Z.
        • Shi Z.
        • Li H.
        The effects of cow-related factors on rectal temperature, respiration rate, and temperature-humidity index thresholds for lactating cows exposed to heat stress.
        J. Therm. Biol. 2021; 100 (34503788)103041
        • Zimbelman R.
        • Rhoads R.
        • Rhoads M.
        • Duff G.
        • Baumgard L.
        • Collier R.
        A re-evaluation of the impact of temperature humidity index (THI) and black globe humidity index (BGHI) on milk production in high producing dairy cows.
        in: Collier R.J. Proc. of the Southwest Nutrition Conference. University of Arizona, Tucson2009: 158-169

      Linked Article