Thermal comfort and ventilation preferences of dairy calves raised in paired outdoor hutches during summertime

Dairy calves are social creatures who are highly motivated for access to a companion. Additionally, heat stress negatively affects the welfare and productivity of calves housed in outdoor hutches. However, no studies have examined the potential tradeoffs pair-housed calves face between competing motivations for social contact and thermal comfort. We evaluated the effects of hutch ventilation on thermoregulatory and behavioral responses of pair-housed calves in outdoor hutches during a Wisconsin summer. Fifty Holstein-Friesian heifer calves were pair housed (n = 25 pairs) in adjacent hutches with a shared outdoor area. In each pair of hutches, 1 was ventilated ( V ) with 2 windows at the rear base and the rear bedding door propped open; the other had no rear windows and a closed bedding door (non-ventilated, NV ). Calves were exposed to 4 conditions for 1 h each (1100 to 1200 and 1230 to 1330 h on 2 consecutive d during wk 4, 6, and 9 of life) in a 2 × 2 factorial design in a balanced order: individually or in pairs in the NV or V hutch. Immediately before and after the 1 h hutch restriction period, respiration rate ( RR ) and rectal temperature ( RT ) were recorded while calves were outside. On the subsequent 3 d in those weeks, the locations of each calf (outdoors or inside a hutch) were recorded at 15-min intervals us-ing timelapse cameras. Linear mixed models were used to evaluate the fixed effects of ventilation, number of calves inside the hutch, week of life, and their interactions, on change in THI, RR, and RT after 1 h; pair of calves was the subject of the repeated statement. Within weeks, the proportion of time calves spent in each hutch and together were averaged across the 3 d of observation. One-sample t -tests were used to evaluate preferences compared with 50% (chance, no preference): (1) for the V (vs. NV) hutch and (2) to be together (or separate) in either the V or NV hutch as well as overall. The THI gain inside the V hutch after 1 h with calves present was lower relative to the NV hutch (0.90 vs. 1.79 units, respectively, SEM = 0.16). Calves in wk 9 of life increased the hutch THI more than in wk 6 of life (1.81 vs. 0.72 units respectively, SEM = 0.16). After 1 h, RR decreased vs. was unchanged, respectively, when calves were in the V vs. NV hutch (−14.4 vs. −0.9 breaths/min, respectively, SEM = 1.4 breaths/min). No differences were detected in RT. Calves chose to be together most of the time regardless of location (wk 4, 6, and 9, respectively: 83.1 ± 2.4%, 80.3 ± 2.1%, and 78.0 ± 3.1%). Calves had no hutch preference during wk 4 but developed a preference for the V hutch as they aged (wk 4, 6, and 9, respectively: 47.3 ± 4.5%, 61.2 ± 5.1%, and 72.8 ± 4.3%). This is the first study to demonstrate passive ventilation improves animal welfare by reducing heat stress in pair-housed dairy calves in outdoor hutches.


ABSTRACT
Dairy calves are social creatures who are highly motivated for access to a companion.Additionally, heat stress negatively affects the welfare and productivity of calves housed in outdoor hutches.However, no studies have examined the potential tradeoffs pair-housed calves face between competing motivations for social contact and thermal comfort.We evaluated the effects of hutch ventilation on thermoregulatory and behavioral responses of pair-housed calves in outdoor hutches during a Wisconsin summer.Fifty Holstein-Friesian heifer calves were pair housed (n = 25 pairs) in adjacent hutches with a shared outdoor area.In each pair of hutches, 1 was ventilated (V) with 2 windows at the rear base and the rear bedding door propped open; the other had no rear windows and a closed bedding door (non-ventilated, NV).Calves were exposed to 4 conditions for 1 h each (1100 to 1200 and 1230 to 1330 h on 2 consecutive d during wk 4, 6, and 9 of life) in a 2 × 2 factorial design in a balanced order: individually or in pairs in the NV or V hutch.Immediately before and after the 1 h hutch restriction period, respiration rate (RR) and rectal temperature (RT) were recorded while calves were outside.On the subsequent 3 d in those weeks, the locations of each calf (outdoors or inside a hutch) were recorded at 15-min intervals using timelapse cameras.Linear mixed models were used to evaluate the fixed effects of ventilation, number of calves inside the hutch, week of life, and their interactions, on change in THI, RR, and RT after 1 h; pair of calves was the subject of the repeated statement.Within weeks, the proportion of time calves spent in each hutch and together were averaged across the 3 d of observation.One-sample t-tests were used to evaluate preferences compared with 50% (chance, no preference): (1) for the V (vs.NV) hutch and (2) to be together (or separate) in either the V or NV hutch as well as overall.The THI gain inside the V hutch after 1 h with calves present was lower relative to the NV hutch (0.90 vs.

INTRODUCTION
Ambient temperatures continue to rise globally, with the 9 warmest years on record within the 1880 to 2022 database occurring over the last 9 years (2014-2022; NOAA National Centers for Environmental Information, 2023).Pre-weaned calves are at a high risk for heat stress, as they cannot properly thermoregulate for the first few weeks of their lives (Hill et al., 2011).Recent findings determined thresholds for when respiration rate (RR), an early thermoregulatory indicator of heat stress, begins to rise in dairy calves: at a Temperature Humidity Index (THI) value of 65 and 69, respectively, in sub-tropical and continental climate zones in the US (Dado-Senn et al., 2020a, 2023a).For calves, rectal temperatures above 39.4°C are considered hyperthermic (McGuirk, 2008;McGuirk and Peek, 2014) and have been recorded in heat stressed calves (Kovacs et al., 2018).
In the United States, approximately 38% of operations house pre-weaned dairy calves in outdoor hutches, (USDA, 2021).Previous research on heat abatement in outdoor hutches has included providing additional shade (Coleman et al., 1996;Spain and Spiers, 1996;Kovács et al., 2018) or reflective covers over the hutch (Manriquez et al., 2018), increasing airflow either by elevation of the rear of the hutch or additional ventilation ports (Moore et al., 2012;Reuscher et al., 2019), and providing fans (Dado-Senn et al., 2023b).These studies have focused on the effects of modifying the hutch and its surrounding environment on the hutch microclimate, but without evaluating the responses of the calf herself, nor the contributions of the body heat she produces on the microclimate.Additionally, past studies did not evaluate how much a calf utilizes -and thus receives the potential benefits of -the heat abatement provided.
A further knowledge gap is how thermal comfort and heat abatement interact with social contact in calves.There has been a great deal of literature over the past few decades which reports the benefits of housing calves in social groups (pairs or groups; reviewed by Costa et al., 2016).Social housing of calves has resulted in improvements in welfare such as greater behavioral flexibility (Meagher et al., 2015;Horvath et al., 2017) and reduced neophobia to unfamiliar calves or feed (Jensen et al., 1997;De Paula Vieira et al., 2012).Furthermore, socially housed calves showed greater DMI and ADG than individually housed counterparts in several studies (e.g., De Paula Vieira et al., 2010;Jensen et al., 2015).
Importantly, social housing addresses calves' motivation for social contact.Indeed, calves are willing to continue to work to have physical contact with another calf even as the required amount of effort increases (Holm et al., 2002;Ede et al., 2022).Within social housing, calves often maintain proximity to one another.In studies with calves pair-housed in connected outdoor hutches, they were observed inside the same hutch approximately 80% of their total time inside the hutches (Pempek et al., 2013;Wormsbecher et al., 2017).This type of pair housing creates an environment where 2 calves are often within the same hutch; however, it is unknown how the body heat of either 1 or 2 calves interacts with the microclimate of the hutches.An unintended consequence of pair housing in hutch settings during warm weather is that this sharing of the hutch could exacerbate the risk of heat stress.Therefore, it is possible that pair-housed calves may face a tradeoff between competing motivations for social contact vs. thermal comfort and have to decide between accessing a cooler microclimate (in a hutch alone) or social contact (sharing a hutch).However, it is also unknown if modifications to the hutch, such as additional ventilation, could alleviate the potential heat gain contributed by the calves' heat production when sharing the hutch.
Therefore, the objectives of this study were to evaluate the interactive effects of pair housing and hutch ventilation on the hutch microclimate, thermoregulatory responses, and preferences of dairy calves in outdoor hutches during a Wisconsin (continental) summer.We hypothesized that hutch ventilation and the number of calves in a hutch would interact to affect the hutch microclimate and calves' thermoregulatory responses.Specifically, we predicted that 2 calves inside a nonventilated hutch would result in the warmest hutch microclimate and the greatest heat-stress responses by the calves, whereas 1 calf inside a ventilated hutch would result in the opposite.We expected intermediate responses in the 2 remaining combinations.Regarding calf preferences, we hypothesized they would prefer to spend the majority of their time together in ventilated hutches.

MATERIALS AND METHODS
The study was conducted during the late spring to fall (May to September 2019) at the University of Wisconsin-Madison (UW-Madison) Emmons Blaine Dairy Cattle Research Center located in Arlington, WI, with all procedures approved by the Institutional Animal Care and Use Committee (protocol #A006133).

Animals, Housing, and Management
Fifty Holstein-Friesian dairy heifer calves were pair housed in (n = 25 pairs) in outdoor hutches.Hutch ventilation was preassigned randomly to one hutch per pair of connected hutches, with the side (left or right) within a pair balanced among pairs, before calf enrollment.Calves were assigned to pairs of hutches sequentially by birth order.

Power Calculation
As preference for hutch type was our primary measure, the number of calves needed was based on the data from a previous heat abatement study (Chen et al., 2013) which examined preference between 2 options in adult cows, compared with chance (0.50, no preference).Preferences for the heat abatement (compared with the control) ranged from 0.64 to 0.85 (SD = 0.08 to 0.29) translating to large to huge effect sizes and requiring sample sizes of n = 1 to 20 experimental units (i.e., pairs of calves) per treatment.Therefore, to confidently achieve a power of 0.8 or higher, we chose to use 25 experimental units to account for potential unforeseen data loss or exclusion.

Enrollment Criteria
Calves were separated from their dam, weighed, had their navels dipped with iodine solution, and fed 3.8 L of colostrum within 6 h of birth (d 1 of life).They also received clostridium perfringen types C & D antitoxin.Calves weighing < 30 or > 48 kg were excluded from the trial (±2 SD from the mean weights of the farm's calves from the 12 mo preceding the start of the trial).If weight criterion was met, calves were then temporarily housed in individual plastic hutches (Calf-Tel Deluxe II; Hampel Corp., Germantown, WI) to await results from the passive transfer test.To test for transfer of passive immunity, blood samples were collected from the jugular vein within 48 h after the first colostrum feeding using 10.0 mL 16 × 125 mm BD vacutainer venous blood collection tubes, with clot additive.After collection, tubes were stored at room temperature for 30 min to allow for clotting.Tubes were then centrifuged for 15 min at 1,545 × g.Serum was analyzed using a Premiere RHC-200-ATC refractometer (C & A Scientific Co. Inc., Sterling, VA).Only calves with serum protein > 5.1 g/dl (Elsohaby et al., 2019) were enrolled in the trial.To be assigned within a pair, calves' birth weights had to be within 10 kg of each other.Once enrollment criteria were met, calves were moved to their treatment housing.

Experimental Housing
Each pair of calves had 2 south-facing adjacent hutches (Calf-Tel Deluxe II, 1.1-m-wide × 2.1-m-long × 1.2-m-high inside dimensions) connected with a 0.6-m-wide × 1.2-m-high, 9.5-mm-thick white plastic board (High Density Polyethylene Sheet; Hemisphere Design Works, Stockbridge, GA) and a 2.7-m-wide × 1.8-m-long outdoor area enclosed with 1.2-m-high wire fencing.Within the pair, one hutch was non-ventilated (NV) with the front door as the only opening, while the other hutch was ventilated (V) with 2 25.4-cmdiameter adjustable windows at the rear base and the rear bedding door (63.5 cm wide × 39.4 cm long) held open 17.8 cm with the built-in wire prop.All hutches and the outdoor enclosure were deep bedded with sand, with additional sand added according to weather conditions.Pair-housed calves were initially separated with a wire fencing panel dividing the outdoor enclosure.Once both calves assigned to the pair were determined by farm staff to be drinking milk strongly (i.e., not needing assistance), calves were paired by removing the dividing panel by the end of wk 1 (d 6 ± 1 of life, mean ± SD, range: d 4 to 9 of life).Within a given pair, the difference in age was 1 ± 1 d (range: 0 to 3 d).The ventilation windows were not opened until habituation began during wk 2 of life.

Feeding
For all calves, starting from birth, drinking water was offered on the fence outside of the hutches in a black plastic pail (C18933; Nasco, Fort Atkinson, WI).All calves had ad libitum access to starter beginning on d 3 of life provided in one bucket inside each hutch.Starter was topped up 2 × /d (0600 and 1300 h) and fully replaced 2 × /week; to ensure ad libitum availability, the amount of starter offered was increased daily based on consumption (>5% refusals by weight).All calves were fed pasteurized whole milk outside of the hutch twice daily between 0400 and 0530 h and 1500-1600 h.They were initially hand fed 1.9 L 2 × /d by bottle.Once calves were determined by farm staff to be drinking sufficiently (d 5 ± 1 of life, mean ± SD), milk volume was increased to 2.8 L 2 × /d, fed in bottles hung on the fence.When calves reached d 14 ± 1 of life (mean ± SD), milk allowance was increased to 3.8 L 2 × /d.The quantities of milk and starter were consistent among calves, but the delivery method beginning at this age varied for the purposes of a separate study to evaluate strategies to mitigate cross-sucking (Salter et al., 2021).All calves were weaned in a stepdown fashion, decreasing milk to 1.9 L 2 × /d at d 42 ± 1 of life (mean ± SD), then to 1.9 L 1 × /d during the afternoon feeding at d 49 ± 1 of life.Calves were completely weaned by d 53 ± 1 of life and remained in their hutches for at least 6 d (9 ± 2 d).

Disbudding and Health Monitoring
Calves were hot-iron disbudded, in conjunction with administration of a lidocaine cornual nerve block and oral meloxicam, at d 17 ± 1 of life.
Calf health was evaluated weekly by recording rectal temperature (RT) with a digital thermometer, along with cough, nasal discharge, and eye, ear, and fecal scores using the UW-Madison School of Veterinary Medicine Calf Health Scorer application (http: / / www .vetmed.wisc.edu/dms/ fapm/ apps/ chs .htm).Sick calves were treated following the farm's standard operating procedures.Eighteen total calves were treated for health conditions: scours (13 calves, 9 unique pairs), jaw abscess (1 calf), and infected ear tag (4 calves, 4 unique pairs).One calf was treated at various time points for different conditions: navel infection, infected ear tag, respiratory cough, and infected horn bud.In addition, when calves drank < 1/2 of their milk meal for any reason including illness or heat stress, electrolytes were provided to both calves within pairs (12 pairs, 24 calves).

Measurements
Environmental Measures.Air temperature (T, °C) and relative humidity (RH, %) were recorded at 5-min intervals by loggers (Kestrel DROP D2AG; Kestrel Meters; Boothwyn, PA) placed in suet cages (EZ Fill Suet Basket Bird Feeder; C and S Products, Fort Dodge, IA) and hung inside the hutches at 1.1 m high for each data collection period in wk 4, 6, and 9 of life.A portable weather station (HOBO U30-NRC, ONSET Computer Corporation, Bourne, MA) was placed in the vicinity of the hutches to record ambient conditions at 5-min intervals: T, RH, solar radiation (W/m 2 ), wind speed (m/s), and precipitation (mm).The 24-h weather conditions are summarized in Table 1.Temperature Humidity Index, both in the hutches and outdoors, was calculated by the following equation from Kelly and Bond (1971): Due to logistical constraints with hutch availability during the study, descriptive air speeds in the hutches were not collected until the following summer.Air speeds in empty hutches of both types were evaluated concurrently with propeller anemometers (Kestrel 5500AG, Nielson-Kellerman Company, Boothwyn, PA) set to record at 1-min intervals and mounted to tripods (0.5 m high, calf resting height) using rotating vane mounts (Kestrel Rotating Vane Mount 5000, Nielson-Kellerman Company, Boothwyn, PA).The hutches used to evaluate air speeds were set up in the same location as the experimental ones the previous summer, and with the same pair-housing system; the ventilated side was switched daily by adjusting the ventilation windows and rear bedding door.The daily average (mean ± SD) airspeed in the ventilated vs. non-ventilated hutch was 0.224 ± 0.365 vs. 0.002 ± 0.003 m/s.Thermoregulatory Responses and Hutch Use.Calves were habituated to temporary restriction within their hutches on 6 consecutive d (beginning on d 15 ± 1 of life; mean ± SD) between 0800 and 1030 h by affixing a 2 × 1 m wire panel over the entrance to the hutch with a bungee cord (Figure 1).This allowed for air exchange while standardizing the amount of time calves were inside the hutch.To ensure familiarity with each of 4 conditions for later behavioral monitoring, calves were restricted in a 2 × 2 factorial design in a balanced order: individually or in pairs in the ventilated or nonventilated hutch (2 conditions/d, with approximately 10 min spent outside between consecutive sessions; on a given day, calves were either separate or in pairs for both conditions).The initial restriction periods lasted 10 min on d 1 and 2, 20 min on d 3 and 4, and 30 min on d 5 and 6.
For data collection, calves were restricted inside a hutch for 1 h per session (1100-1200 and 1230-1330 h) on 2 consecutive d during wk 4, 6, and 9 of life (d 23 ± 2, 38 ± 1, and 55 ± 1 of life, respectively, mean ± SD) in the same 2 × 2 factorial arrangement, in a balanced order.These 3 wks of life (4, 6, and 9) were chosen to represent different time points in the calves' development: by wk 4 of life, calves had received peak milk allowance for at least a week, wk 6 of life was before weaning started, and wk 9 of life was post-weaning, when calves were fed only starter.To avoid additional stress during the weaning period, we chose not to restrict calves during the step-down weaning process.Calves were restricted outside the hutches for 30 min before each restriction period.This was done to reset the microclimate to baseline conditions as well as to allow calves to accumulate heat from solar gain so their starting measurements reflected heat stress conditions (based on the procedure for adult cows in Chen et al., 2016a).
Immediately before and after each 1 h restriction period, RR, RT, skin temperature (ST), and ocular temperature (OT) were recorded while calves were outside the hutch.Due to space constraints inside the hutches, these measures could only be collected outdoors.All measures were collected by one trained researcher Figure 1.Photos of calves being restricted inside the hutches using wire panels and bungee cords for 1 h, twice daily on 2 consecutive d in wk 4, 6, and 9 of life.A) Example of the calves undergoing a pair restriction where they were placed in the same hutch together (in this example, the ventilated hutch).After the allotted time, the pair would be switched to the other hutch in the housing unit.B) Example of the calves undergoing an individual restriction where they were placed separately in each hutch.After the allotted time, the calves would be taken out and would switch hutches.
(K.J.R.).RR was collected by counting 10 flank movements and converting to breaths/min.RT was collected using a digital thermometer (Rectal Temp; Pavia, Plymouth, MN) with disposable sheaths (Probe Covers; Pavia, Plymouth, MN) used between each reading for biosecurity.The area used for ST was a 5-cm 2 shaved site on the right side of the calf's rump.Both ST and OT were collected using an infrared camera (FLIR E6; FLIR Systems, Wilsonville, OR) held 20-cm away from the respective body part.The infrared pictures were analyzed for maximum temperature within the selected area using FLIR Tools software (FLIR Systems, Wilsonville, OR) by 1 trained observer.Inter-observer reliability was determined on a subsample of 44 pictures of each body part using linear regression relative to the trainer (R 2 = 0.97, with slope and intercept not significantly different from 0 and 1, respectively, P > 0.05, indicating a lack of systematic bias).
Time-lapse cameras (recording in 5-s intervals; Day 6 PlotWatcher Pro Time-Lapse HD Video Camera; Day 6 Outdoors) were placed 8 m from the front of the hutch fencing panels (2 hutches/camera) at a height of 1.5 m for the 3 d following restriction in wk 4, 6, and 9 of life.Solar powered lights (SL404 2-in-1 Solar Spotlight; InnoGear, Morgan Hill, CA) with red gel filters (106 Primary Red Gel Filter Sheet; Times Square Stage Lighting, New York City, NY) were placed under the cameras and pointed toward the hutches to add light for the cameras to take photos during the night.Locations of each calf (outdoors or inside a hutch, the left vs. right hutch was noted) were recorded at 15-min intervals by 2 observers in Microsoft Excel spreadsheets.Inter-observer reliability was determined on a subsample of 297 pictures using linear regression relative to the trainer (R 2 ≥ 0.98, with slope and intercept not significantly different from 0 and 1, respectively, P > 0.05).The observers were blinded to hutch ventilation, as the rear of the hutch was not visible from the camera angle.To prevent potential bias, the data were translated from left vs. right hutch into ventilated vs. non-ventilated based on the records of the housing units after the video observations were completed.
The 15-min interval had been determined to accurately represent the calves' locations compared with the originally recorded 5-s intervals using a subset of data from 3 d for 11 pairs.Following the methods in Chen et al. (2016b), we extrapolated instantaneous samples for 7 time intervals (30 s, and 1, 3, 5, 10, 15, and 30 min) and converted these to the total duration spent in each location (both hutches and outside) for each calf in the pair.We then conducted pairwise comparisons between the original values (5-s intervals) and those generated from the 30-s to 30-min sampling intervals using linear regressions.The 15-min interval was determined to be accurate with a coefficient of determination (R 2 ) of > 0.99 and with slope and intercept not significantly different from 1 and 0, respectively (P ≥ 0.18).The proportion of time calves within each pair were in the same location was determined by dividing the number of times calves were coded in the same location by the total number of times calves were observed.

Statistical Analysis.
As pair of calves was the experimental unit, dependent variables from the hutch restriction periods were averaged within pairs before analysis.To evaluate hutch use, the proportion of time calves spent in each hutch, divided by time inside both hutches, was calculated across the 3 d of observation in each week (4, 6, and 9) for each calf in the pair and then averaged between calves within the pair.To evaluate the preference of pair-housed calves to be together, the proportion of time they were observed in the same location (vs.alone) was calculated across the 3 d of observation in each week (4, 6, and 9).
Missing and Excluded Data.After reviewing the ST data, this measure was not analyzed due to solar glare on skin when calves were removed from the hutches to collect the measurements, resulting in nonbiologically possible values.Video footage from 2200 to 0400 h was not available for one pair of calves during wk 4 and another pair of calves during wk 6 (28 images each) due to insufficient overnight illumination resulting from malfunction of the solar-powered lights.
Statistical Models.All analyses were conducted using SAS software (Version 9.4; SAS Institute Inc., 2014).The assumptions of normality and equal variance were evaluated by visually examining plots of the residuals.
To evaluate the effects of 1 calf vs. 2 calves inside the ventilated vs. non-ventilated hutch on the hutch microclimate (change in THI after 1 h of restriction) and calves' thermoregulatory responses (change in RR, RT, and OT after 1 h), linear mixed models (PROC MIXED) were used.Each model included the fixed effects of hutch type, number of calves, wk of life (4, 6, and 9), and all 2-and 3-way interactions, as well as day of restriction (1 or 2) nested within wk of life.The models included a repeated statement with the subject of housing unit (i.e., pair of hutches).Based on the lowest Akaike and Bayesian information criterion (AIC and BIC, respectively) values, a compound symmetry covariance structure was selected.
To evaluate calves' preferences 1) for the ventilated hutch as well as 2) to be together (separately for each hutch type, as well as overall), one-sample t-tests were used to compare the proportion of time they were observed in those locations against 50% (chance, no preference), separately for each week of life (4, 6, or 9).
Significance was defined as P < 0.05 and tendences as 0.05 ≤ P < 0.10.Values in the text, tables, and figures are reported as least squares means and standard error of the mean (SEM).When tendencies were found, 95% confidence intervals (CI) are reported in addition to SEM.
Ocular Temperature.There was tendency for a main effect of number of calves in the hutch (F 1,24 = 3.2, P = 0.088): when 2 calves came out of the hutch, they had a greater reduction in OT (−0.57°C,SEM = 0.11°C; 95% CI: −0.80 to −0.34°C) after the 1 h of restriction relative to when they were restricted individually (−0.30°C; 95% CI: −0.53 to −0.07°C; Figure 2C).There was a 3-way interaction for ventilation treatment × number of calves in the hutch × week of life (F 2,39 = 5.7, P = 0.007), where there was a tendency for OT to increase when calves were restricted individually, but decrease when 2 calves were together in the nonventilated hutch during wk 4 of life (+0.13 vs. −1.0°C,respectively, SEM = 0.24°C; P = 0.088).There was no main effect of ventilation treatment (F 1,24 = 0.9) or week of life (F 2,46 = 0.3), nor interactions for ventilation treatment × number of calves in the hutch (F 1,24 = 0.5), number of calves in the hutch × week of life (F 2,41 = 0.5), or ventilation treatment × week of life (F 2,46 = 0.0; P > 0.36).

Calf Preferences.
Hutch Ventilation Descriptive data for the percentage of time calves spent in the ventilated vs. nonventilated hutches and outside, by week of life, are presented in Figure 3.No preference for hutch type was detected in wk 4 of life (47.3% of hutch time spent in the ventilated one; SEM = 4.5%; P = 0.56).In wk 6 and 9 of life, calves preferred to spend the majority of their hutch time in the ventilated one (61.1 and 72.8%, respectively, SEM = 5.1% and 4.3%, respectively; P < 0.038; Figure 4A).

DISCUSSION
Our study evaluated the interactive effects of social contact and ventilation on hutch microclimate and calf thermoregulatory responses.Our results provide the first demonstrated effects of calf body heat warming the microclimate of the hutch during continental summertime, with their age playing a role.As calves developed from wk 6 to 9 of life, the hutch THI increased further.Additional passive ventilation mitigated the THI gain, regardless of age or the number of calves inside, and also reduced calves' respiration rates.Calves preferred to be together in all 3 wk of life evaluated, regardless of location, and in wk 6 and 9 of life, they preferred the ventilated hutch.Overall, additional ventilation is beneficial for reducing both the heat load of the microclimate within outdoor hutches and the calves' heat stress responses, and calves prefer this form of heat abatement over none.
Our results show that the body heat of calves impacts the hutch microclimate, increasing the THI inside after 1 h.The magnitude of this effect increased as calves aged.In wk 9 of life, calves warmed the hutch microclimate by approximately 1 THI unit greater (a 2-fold  difference) than in wk 6 of life overall.Ours is the first study to investigate the change in hutch microclimate dependent on the number of calves inside or at different ages, and the first to standardize the amount of time calves spent inside the hutches to obtain measurements compared with baseline.Previous studies that examined modifications of hutches in summer did not record, let alone standardize, when calves were present inside the hutches (Moore et al., 2012;Kovács et al., 2018;Reuscher et al., 2019) Regardless of the week of life or number of calves inside the hutch, we found that passive ventilation mitigated the rise in THI, which increased by about half as much (about 1 THI unit) as when there was no ventilation.Furthermore, before the calves were restricted inside the hutch, baseline THI in the ventilated hutch was numerically 2.7 THI units lower (on average, 1.7°C) than in the non-ventilated one.A previous study comparing different methods of passive hutch ventilation found the difference in THI between the most and least ventilated hutches was only 1.1 units (Reuscher et al., 2019); this study did not compare directly to a non-ventilated hutch.Previous studies comparing non-ventilated to passively ventilated hutches reported mixed results, with internal hutch temperature either cooler by approximately 0.6°C in the latter type (Moore et al., 2012) or no differences detected (Lammers et al., 1996).These mixed findings may be due to either differences in the type of polyethylene hutch used or in the method, size, or location of the additional openings (elevated 20 cm at the base in Moore et al., 2012; a small opening on the roof of the hutch in Lammers et al., 1996), or because previous studies did not standardize whether and how long the calves were inside the hutch.
In our study, hutch ventilation not only mitigated the rise in internal THI, but also reduced calves' respiration rates, a sensitive thermoregulatory response.Baseline respiration rate after calves spent 30 min outside of the hutches was, on average, > 40 breaths/min, a breakpoint recently identified as indicative of heat stress responses in pre-weaned calves in this Wisconsin continental climate (Dado-Senn et al., 2022).After 1 h in the non-ventilated hutch, calves' respiration rates decreased by an average of 1 breath/min, suggesting the shade of the hutch prevented further heat gain from solar radiation, but did not provide further cooling.On average, respiration rate was 14 breaths/min lower in calves coming out of the ventilated hutch relative to the non-ventilated one, consistent with previous work, which observed elevated hutches reduced respiration rate by up to 14 breaths/min (Moore et al., 2012); this demonstrated that passive ventilation provided cooling.Furthermore, the ventilated hutch was successful at reducing respiration rate, on average, to below the breakpoint of 40 breaths/min (Dado-Senn et al., 2022).Similar results have been seen from other heat abatement methods, with lower respiration rates in calves exposed to fans (Hill et al., 2011), shade (Lammers et al., 1996;Kovács et al., 2018), or shade and fans (Dado-Senn et al., 2020b;Montevecchio et al., 2022), ranging from 5 to 60 breaths/min difference relative to the control calves.
Similar to our calf age-dependent findings for hutch THI, we found a tendency for calves in wk 6 of life to have a 4-fold greater reduction in respiration rate (difference of −10 breaths/min) after 1 h of restriction compared with in wk 9 of life, when both calves were inside the hutch together.A previous study comparing calves provided with fans vs. none no longer detected treatment differences in respiration rate once they reached wk 6 of life (Hill et al., 2011).By wk 6 of life, depending on the diet provided, calves undergo shifts in development, including in their ruminal microbiota, as the rumen becomes functional (Rey et al., 2012(Rey et al., , 2014)).These changes in the metabolism and energy (heat) production could explain the lesser reductions in respiration rate, and greater increase in hutch THI, we observed in wk 9 compared with wk 6.
After the 1 h of restriction, we found no differences in rectal temperature regardless of ventilation, number of calves present, or age of the calves.Similarly, in previous studies, no differences in rectal temperature were found in calves in elevated vs. non-elevated hutches (Pezzanite et al., 2010) or in hutches with reflective covers vs. none (Moore et al., 2012).In contrast, studies that evaluated shade (Coleman et al., 1996;Kovács et al., 2018), fans (Hill et al., 2011), or fans and shade (Dado-Senn et al., 2020b;Montevecchio et al., 2022) reported RT ranging from 0.1 to 1.1°C lower than in the control treatments.None of these studies standardized, nor reported, how much time calves spent directly using the heat abatement.The variable RT outcomes across studies could be due to several factors including the type of heat abatement, amount of time calves used it, climate, and the type of housing (in a barn vs. outdoor hutches).In the current study, the average baseline rectal temperatures were within normal body temperature range (<39.4°C;McGuirk, 2008;McGuirk and Peek, 2014).The calves' respiration rates were elevated at baseline, and that early thermoregulatory coping mechanism appeared to be successful in maintaining homeothermy; because the calves were not hypothermic, no reduction in rectal temperature would be expected.
The effects of hutch ventilation on internal hutch THI and calf respiration rate, combined with the effects of calf age on these variables, likely explains why calves preferred this hutch type in wk 6 and 9 of life.
Ours is the first study to examine the preference of dairy calves for heat abatement.In a previous study in cold weather, calves between 0 to 3 d of age spent more than half of their time in the area directly under heat lamps compared with further away (Borderas et al., 2009), suggesting they sought out warm microclimates in winter conditions.Other studies demonstrated that older cattle, such as lactating dairy cows (e.g., Chen et al., 2013;Schütz et al., 2011) and feedlot heifers (Parloa et al., 2012), prefer heat abatement resources such as shade, water soakers, or a combination of those resources in summer.We found that calves showed no preference for either hutch type in wk 4 of life but had developed a preference for the ventilated hutch by wk 6 of life, which remained in wk 9 of life.The reason the preference was shown after wk 4 of life could perhaps be related to their developing ability to thermoregulate.Earlier in life, calves may have had less need for heat abatement, and thus had no preference between the hutch types; they could still be learning the difference between the microclimates; or, conversely, they could have at certain times sought out a warmer environment deliberately.As the calves developed a more functional rumen (Rey et al., 2012(Rey et al., , 2014) ) and therefore generated more body heat, they may have shifted their need for heat abatement, resulting in a preference for the ventilated hutch.Another reason the calves may have developed a preference for the ventilated hutch could potentially be due to lower ammonia levels or microbial build up compared with in the non-ventilated hutch.Previous research reported that elevating the back of the hutch by 4 cm can notably reduce the concentration of airborne bacteria inside the hutch (Hill et al., 2011).Future research could explicitly evaluate whether environmental air quality affects calves' hutch preferences.
Regardless of location, calves preferred to be together 80% of the time, which is consistent with other papers using a similar paired-hutch system, which reported calves choosing to be in the same hutch at least 80% of the time (Pempek et al., 2013;Wormsbecher et al., 2017).Calves preferred to be together even inside the non-ventilated hutch.Therefore, we speculate that in terms of the tradeoff between social contact and thermal comfort, the calves may have been willing to sacrifice thermal comfort to be with their companion, even if in a warmer microclimate.Calves have demonstrated developing pair bonds (Lindner et al., 2022) as well as being motivated to access and maintain physical contact with another calf (Holm et al., 2002;Ede et al., 2022).This is an important consideration for calf raisers when implementing social housing; because calves prefer proximity, the housing should provide enough room for both calves to be together comfortably.However, calves still spent about 20% of their in-hutch time in separate hutches, so it is recommended to give them the same per-calf area as in individual housing to allow them to socially isolate at times (Gingerich et al., 2020).
Our findings about calves' preferences for social proximity may explain the counterintuitive results for ocular temperature.This measure decreased by about twice as much (0.3°C) when pairs were restricted together compared with when they were individually restricted.This is opposite from what we expected when we measured ocular temperature as a thermoregulatory indicator (as previously used in pre-weaned dairy calves; Scoley et al., 2019) and had hypothesized that 2 calves sharing a hutch would produce more body heat and thus show a greater increase, or lesser reduction, in heat stress after 1 h of restriction.The phenomenon we observed may be the result of capturing a stress response other than heat stress, such as social isolation stress.In a study with adult cows, social isolation was reported to cause a sharp dip in ocular temperature followed by an increase, before leveling out at baseline after 1 h of isolation (Stewart et al., 2007).In our study, the individual restriction may have caused an unintended additional stress of visual isolation (Boissy and Le Neindre, 1990) from their bonded peer.Additionally, when both calves were in the same hutch, this may have helped them cope with the stress of the handling or restriction process due to social buffering, or the improved ability to cope with stress and reduce fear responses in the presence of a social companion (De Paula Vieira et al., 2010;Overvest et al., 2018).

CONCLUSIONS
Our study is the first to evaluate the interactive effects of social contact and heat abatement in calves and provides the first demonstrated effect of calves' body heat warming the microclimate of the hutch.As calves developed, the hutch THI increased further.Across all weeks of life evaluated, calves showed a strong preference to spend the majority of their time together, including inside the hutches, regardless of hutch ventilation.Adding passive ventilation to the hutch mitigated the THI gain inside the hutch, regardless of the number of calves inside or week of life, and reduced the heat stress responses shown by the calves.Furthermore, calves developed and maintained a preference for the ventilated hutch as they aged.Overall, passive ventilation is beneficial for mitigating heat stress in pairhoused calves in outdoor hutches.
Reuscher et al.: Hutch Ventilation for Paired Calves Reuscher et al.: Hutch Ventilation for Paired Calves Reuscher et al.: Hutch Ventilation for Paired Calves

Figure 2 .
Figure 2. Dependent variable responses after calves (n = 25 pairs) were restricted inside hutches (ventilated vs. non-ventilated, separately vs. together) for 1 h in wk 4, 6, and 9 of life.The dependent variables were: (A) hutch Temperature Humidity Index, (B) calf respiration rate, (C) ocular temperature, and (D) rectal temperature.Baseline values are shown for reference; treatment differences were analyzed based on the change from baseline after 1 h.
Figure 3. Descriptive graph of the mean ± SD for the percentage of time pair-housed calves (n = 25 pairs) spent in the different locations (inside the ventilated or non-ventilated hutch per pair of hutches, or outside) in wk 4, 6, and 9 of life.

Table 1 .
Reuscher et al.: Hutch Ventilation for Paired Calves Summary of daily weather conditions 1 in May to September 2019