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Department of Dairy Science, University of Wisconsin, Madison 53706U.S. Dairy Forage Research Center, USDA-Agricultural Research Service, Madison 53706
* Trade names and the names of commercial companies are used in this report to provide specific information. Mention of a trade name or manufacturer does not constitute a guarantee or warranty of the product by the USDA or an endorsement over products not mentioned.
The objective of this study was to measure cow response to feeding of two dietary concentrations of P, one of which was close to recent National Research Council requirements, and the other of which was well in excess of the requirement. Diets containing 0.37 or 0.57% P (dry basis) were fed to Holstein cows for the first 165 d of lactation, and occasionally longer until cows were confirmed pregnant approximately 60 d after insemination. At calving, cows were randomly assigned to experimental diets. The number of cows completing a minimum of 165 d of lactation was 123 for the 0.37 and 124 for the 0.57% P groups. Cows were housed in a stanchion barn and fed one of two transition diets, each formulated to contain one of the P treatments for the first 3 wk of lactation, and then cows were moved to a free-stall barn where the experimental diets were group fed. Milk production, milk fat, and milk protein averaged 35.1 kg/d, 3.92%, and 2.90% for the 0.37% P diet, and 34.9 kg/d, 3.98%, and 2.91% for the 0.57% P diet. None of these measures were different between treatments. Blood serum P concentrations on d 50 and 100 of lactation averaged 6.1 and 6.2 mg/dL for the 0.37% P diet, and 6.8 and 6.9 mg/dL for the 0.57% P diet. No treatment differences were detected in milk production, cow health, or body condition score.
). This excess P may accumulate in the environment through recycling of manure to land as fertilizer for crop production. Surface runoff from soils high in P content can contain sufficient P to stimulate algae growth and eutrophication of surface waters (
). Since fecal P excretion increases linearly as P intake is increased above the requirement, close monitoring of dietary P is required to minimize risk of environmental damage caused by excessive P excretion in the feces (
requirements. Eliminating the excess supplemental P from dairy diets may result in as much as 25 to 30% reduction of P content of manure, and a savings of $10 to 15 per cow per year in P supplementation costs (
Anecdotal evidence suggests that dairy producers have reduced dietary P a modest amount since 1999, but there continues to be resistance to lowering of dietary P down to
requirements. Part of the resistance is due to uncertainty about how much of a safety margin should be provided above the minimum needed to avoid deficiency symptoms. Evidence is now available (
) from long-term lactation studies suggesting that moderate to high-producing dairy cows need a minimum of approximately 0.30% dietary P because deficiency symptoms may begin to appear at this dietary concentration. If
requirements are used as a feeding guideline (0.32 to 0.38% dietary P, depending on level of milk production) then a 10 to 20% margin of safety would be assured.
Perhaps a more often cited reason for not reducing P to
) that feeding of extremely low dietary P can impair reproductive performance, there is no suggestion in the literature that feeding P in excess of current
recommendations will improve reproductive performance.
The majority of studies dealing with P feeding and reproductive performance have lacked power of test to detect treatment responses. Large numbers of animals are needed to draw conclusions regarding reproductive performance. The objective of this study was to measure milk production, blood serum P concentrations, and incidence of health problems with a large number of cows fed P either in amounts close to the
requirement or well in excess of the requirement. This paper reports results pertaining to milk production and animal health, and an accompanying paper (
) reports results related to reproductive performance.
Materials and Methods
A total of 267 lactating Holstein cows started the experiment. They were assigned at calving to diets containing either 0.37 (recommended) or 0.57% P (excess) on a DM basis. Since milk production is the focus of this report, cows not completing at least 165 DIM were deleted from this part of the overall study. A total of 20 cows (11 and 9 cows for the recommended and excess P groups, respectively) were removed from the experiment due to significant health problems or to routine culling from the herd before completing 165 DIM. A total of 128 primiparous and 119 multiparous cows completed this phase of the study. With only rare exceptions, primiparous and multiparous cows were fed the same diet for the 4 wk preceding calving.
requirement. During the first 3 wk of lactation cows were fed transition diets containing either 0.37 or 0.57% P (DM basis). Starting at the beginning of wk 4 and continuing until the end of the trial, cows were fed lactation diets containing the corresponding recommended (0.37%) or excess P (0.57%). Treatment diets were nearly identical, except that the 0.37% P diet was formulated without supplemental P. The 0.57% P diet was obtained by adding monosodium phosphate. Several forage sources were used throughout the 18 mo of this study, and, in response to changes in nutrient composition of forages, appropriate changes were made in the ration formulation to maintain diet energy and protein levels. Most concentrate ingredients (soybean meal, vit/min mix, salt, limestone, sodium bicarbonate, magnesium oxide, and monosodium phosphate) were incorporated into the ration as premixes for 9 of the 12 mo that transition diets were fed and for 12 of the 17.5 mo that lactation diets were fed. For the rest of the experimental period, rations were mixed daily using individual ingredients. The diet ingredients and the minimum and maximum amount used during the study, expressed as a percentage of diet DM, are shown in Table 1.
Table 1Formulation of TMR containing recommended (0.37%) or excess (0.57%) dietary P.
Vit/min mix = VIT TM PAK, Professional Products, Inc. Prairie du Sac, WI. (Ca 18.4–20.4%, Se>320ppm, Vit A>7,084,000 IU/kg, Vit D-3>2,200,000 IU/kg, Vit E>17,600 IU/kg, and S, 5.51% ; Co, 0.04% ; Fe, 2.40% ; Zn, 6.19% ; Cu, 1.33% ; Mn, 5.10% ; I, 0.10%).
3 Vit/min mix = VIT TM PAK, Professional Products, Inc. Prairie du Sac, WI. (Ca 18.4–20.4%, Se > 320 ppm, Vit A > 7,084,000 IU/kg, Vit D-3 > 2,200,000 IU/kg, Vit E > 17,600 IU/kg, and S, 5.51% ; Co, 0.04% ; Fe, 2.40% ; Zn, 6.19% ; Cu, 1.33% ; Mn, 5.10% ; I, 0.10%).
4 Yeast = Diamond V XP Yeast Culture, Diamond V Mills Inc., Cedar Rapids, IA.
Cows were housed in a tie-stall barn during the first 3 wk of lactation and offered a TMR ad libitum (5 to 10% refusal) and approximately 1.7 kg of alfalfa hay (as fed) once daily. Actual amounts of TMR offered and refused by individual animals were recorded daily to monitor feed intake during the first 3 wk of lactation. Cows were moved to a free-stall barn after wk 3 of lactation and fed as 2 groups once daily. Fed consumption was not measured following the first 3 wk of lactation. A trace mineral/salt mix containing 94 to 96% salt and at least 5500 mg/kg of Zn, 5500 mg/kg of Mn, 1400 mg/kg of Cu, 3450 mg/kg of Fe, 80 mg/kg of I, 20 mg/kg of Co, and 60 mg/kg of Se (Vita Plus Corp., Madison, WI) was available ad libitum at all times in both treatment pens. All cows in the study were scored for body condition once per month for the duration of their tenure on the experiment, since cow reproductive measurements continued beyond 165 DIM for some cows (
), and the mean of the 3 scores was recorded. Body condition scoring was done for all cows on a given day, so scores do not coincide with a selected day or week of lactation. Body weight measurements were not made in this study. Health problems were considered as conditions requiring medical treatment and were recorded as days on which treatment was given. Treatment days separated by more than 6 d were considered separate occurrences. Alfalfa silage, corn silage, TMR and orts were sampled daily, frozen, and composited weekly for chemical analysis. Once weekly samples of alfalfa silage, corn silage, and high moisture shelled corn were used for DM determination by oven drying at 60°C for 48 h. These DM measurements were used for weekly adjustments in diet formulations to accommodate changes in DM content of the silages. Concentrates (roasted soybeans, molasses, yeast, soybean meal, and premixes), alfalfa hay, and minerals were sampled monthly.
All dried feed samples were ground through a Wiley mill with a 1-mm screen (Arthur H. Thomas, Philadelphia, PA). Monthly samples of ground concentrate, alfalfa hay, premixes, minerals, and selected weekly composite silage samples (1 to 7 per silo) were analyzed for P. The monthly concentrate samples were ground and composited every 3 mo. These composite samples, ground 4-wk composite premix samples, and ground weekly silage samples were analyzed for DM (105°C), CP (LECO FP-2000 Nitrogen Analyzer, Leco Instruments, Inc., St. Joseph, MI), NDF (heat-stable amylase and Na2SO3 were used), and ADF (
, but the amount of concentrated HCl was increased from 10 to 15 ml. Samples were analyzed for P content by direct current plasma emission spectroscopy by adapting the procedure described by
. A certified commercial P solution (VHG Labs, Inc., Manchester, NH) was used as a calibration standard. Accuracy of the analysis was assured by referring to additional commercial standards (Standard Reference Material 1570a, spinach leaves, and 8436, durum wheat flour; National Institute of Standards and Technology, Gaithersburg, MD).
Results of chemical analyses of feed are reported based on DM measurements made at 105°C. Weekly composite alfalfa silage and corn silage samples were selected for analysis for the period of time that a particular silage was fed. Eleven different alfalfa silages and nine different corn silages were used during the course of this experiment. Each silo was sampled different times (from 1 to 7 samples) and means of these samples were used to calculate nutrient composition of the diet ingredients. Nutrient content of the TMR was computed from the average nutrient content of the individual diet ingredients analyzed as indicated above.
Cows were milked twice daily (0500 and 1700 h), and milk yields were recorded at each milking until 165 DIM. All cows were administered bST (Posilac; Monsanto Co., St. Louis, MO) every 2 wk, beginning at 63 to 70 DIM. Milk samples were collected monthly from a.m. and p.m. consecutive milkings, and sent to AgSource Cooperative Services (Menomonie, WI) for analyses of fat and protein by near-infrared spectroscopy (Foss MilkoScan 4000; Foss Technology, Eden Prairie, MN) and SCC by fluorescence (Fossmatic 5000; Foss Technology).
Blood samples (∼10 ml) were collected via coccygeal venipuncture using evacuated tubes (Vacutainer; Becton-Dickinson, Rutherford, NJ) on approximately 50 and 100 DIM, and allowed to clot before chilling. Samples were centrifuged at 1600 × g for 15 min and serum was collected and stored refrigerated at 4°C in 10-ml plastic scintillation vials until analyzed. Serum was analyzed for inorganic P (Pi) by the Marshfield Laboratories (Marshfield, WI) using the molybdovanadate colorimetric procedure (
). The protocol used in this study was approved by the Animal Care Committee of the College of Agricultural and Life Sciences, University of Wisconsin, Madison.
Daily milk yield was reduced to weekly means. These means, data on BCS, and data on milk component percentages were analyzed by the MIXED procedure of SAS using both random and repeat measures statements with autoregressive-1 (ar-1) as the covariate structure for repeated measurements and the model Y = treatment time treatment × time (
). Data on serum P concentrations were compared by Student's t-test.
Results and Discussion
The chemical composition (DM basis; means and SEM) of the TMR throughout the 18-mo duration of the experiment is in Table 2. The P content of the recommended diet corresponded closely to what the National Research Council (
) considers to be the requirement for lactating cows with milk production similar to that of cows used in this experiment. The current NRC recommendations for early to midlactation (90 DIM) diets are 0.36% P (DM basis) for cows milking 45 kg/d and 0.35% P for cows milking 35 kg/d. The
suggests feeding a slightly higher dietary P concentration during the first several weeks of lactation compared with later lactation in order to compensate for the lag in feed intake relative to milk production in the first weeks following parturition. However, the
) suggest that a dairy cow weighing approximately 600 kg could mobilize 600 to 1000 g of P during the first few weeks of lactation. The supply of mobilized bone P could easily replace the need for an elevated concentration of P in early lactation diets.
Table 2Chemical composition of TMR containing recommended (0.37%) or excess (0.57%) dietary P
; Pfeffer, personal communication, 2003) suggest that the true digestibility and availability of P in concentrates is higher (72 to >90%) than the value of 70% used by
) with average- to high-producing cows also suggest that P deficiency symptoms are not likely to be observed until dietary P concentration is reduced to approximately 0.31% of diet DM. Therefore, a dietary P concentration of 0.35 to 0.37%, assuming DMI approximating
values, should provide sufficient P to meet the requirement and to provide a reasonable margin of safety.
Table 3 has the chemical composition of the diet ingredients. Alfalfa silage and corn silage were supplied from different silos during the experiment. The mean and SEM of DM, CP, NDF, ADF, and P are reported for silages and alfalfa hay.
Table 3Chemical composition of forages used in TMR.
The lactation curves for the 2 treatments are shown in Figure 1. The 2 treatment means essentially overlap each other. Table 4 contains milk production and milk composition information. Table 5 contains a listing of published studies where milk production was measured when cows were fed different amounts of dietary P. The overall means for milk production for the low (0.33%) and high P (0.46%) treatment groups were both 29.2 kg/d.
Figure 1Lactation curves for cows fed either 0.37% or 0.57% dietary phosphorus.
Milk composition was not affected by dietary P content. There is no evidence in the literature suggesting that fat content of milk is influenced by feeding P in excess of
), blood serum P concentration could influence milk protein content. However, in this study, milk protein content for the 2 treatments was virtually identical.
Mean BCS from monthly measurements while cows were on the experiment averaged 3.0 for both treatments groups. The monthly BCS are shown in Figure 2. Mean concentrations of P in serum are shown in Table 6. Serum inorganic P (Pi) averaged 6.1 and 6.2 mg/dL at 50 and 100 DIM for the 0.37% P treatment and 6.8 and 6.9 mg/dL at comparable sampling times for the 0.57% P treatment. These values are within the normal range (4.4 to 9.2 mg/dL) typically seen in lactating cows (
Table 6Mean inorganic P content (mg/dl) of blood serum from lactating dairy cows at 50 or 100 d postpartum while receiving recommended (0.37%) or excess (0.57%) dietary P.
Serum P (mg/dl)
0.37% P
0.57% P
P
Average
SEM
Average
SEM
50 d postpartum
6.1
0.1
6.8
0.1
<0.001
100 d postpartum
6.2
0.1
6.9
0.1
<0.001
*Normal range for bovine serum inorganic P = 4.4 to 9.2 mg/dl.
The recorded health problems indicated no apparent association with dietary P content (Table 7). Most studies of dietary P requirements of dairy cows have not had sufficient animal numbers to draw conclusions about dietary P and animal health. There is no evidence in this experiment that feeding P in excess of the
P requirements, did not affect milk production, milk composition, or animal health. This study, along with other studies reported in the literature, provides abundant evidence that feeding P in excess of
The authors thank employees at the US Dairy Forage Research Center farm at Prairie du Sac, WI, for feed preparation and animal care; and Mary Becker, Matias Aguerre, Hendrick Henselmeyer, Zachary Schott, Kathleen Herbert, and Amber Rew for technical support. Appreciation is extended to USDA-CREES National Research Initiative, Agricultural Systems Research Program (Grant # 9703968) for partial funding of this study.
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