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Does providing metabolizable protein in excess of requirement prior to calving improve nitrogen balance and cow-calf performance?



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The objective of this experiment was to evaluate the effect of over-feeding metabolizable protein (MP) during late gestation on prepartum N balance, and postpartum cow BW and skeletal muscle catabolism, colostrum composition, and milk yield (MY) and composition. Twenty-four (14 cannulated, 10 non-cannulated) crossbred Hereford cows were assigned to a control treatment designed to meet (CON = 100% MP, n =12) or exceed (HMP = 133% MP, n = 12) MP requirements. Cows consumed a common lactation (103% MP) diet postpartum. One HMP and 1 CON cow-calf pair were removed due to dystocia, while 1 CON cow was removed due to aggression at parturition. Heifers were housed and fed individually from d -55 to d 33 relative to parturition and then group-housed until 112 d post-partum. Dry matter intake was measured throughout the study and summarized by week. Cow BW was recorded on d -55 ± 3.7, -41 ± 3.7, -28 ± 3.6, -8 ± 3.9, 7 ± 1.0, 13 ± 2.0, 28 ± 2.9, 57 ± 3.6, 82 ± 5.1, and 112 ± 2.9 relative to parturition. Total fecal and urinary collections were conducted over a period of 6 d starting on d -33 and -15 to measure N balance, and total tract digestibility. Urine samples from the prepartum collections and postpartum spot samples (starting d 7 and 28) were composited and analyzed for 3-methylhistidine (3-MH) and creatinine as indicators of muscle turnover. Digesta samples were collected and ruminal ammonia-N concentration was measured on d -33 ± 5.3, -15 ± 4.4, 7 ± 0.9, and 28 ± 2.9. Blood samples were collected from cows via jugular venipuncture on d -34 ± 5.3, -17 ± 4.4, 7 ± 0.9, 28 ± 2.9, 70 ± 3.2 and 112 ± 2.9 and analyzed for plasma glucose, urea-N, and insulin, and serum NEFA and BHBA. Calf BW and frame measurements were conducted at birth and on d 7 ± 0.9, 28 ± 2.9, 57 ± 3.6, 82 ± 5.1, and 112 ± 2.9. At parturition, colostrum samples were collected and analyzed for nutrient composition and IgG concentration. Milk yield was measured on d 7 ± 0.9, 12 ± 0.9, 28 ± 2.9, 33 ± 2.9, 70 ± 3.2, and 112 ± 2.9 relative to parturition. Milk samples were analyzed for the concentration of fat, crude protein, lactose, urea-N (MUN) and somatic cell count (SCC). Data for the prepartum and postpartum periods were analyzed separately. All data were analyzed as a randomized complete block design using the MIXED procedure of SAS. Fixed effects included treatment, time, and the two-way interaction of treatment and time, while cow nested in block was considered as the random effect. Time (day or week) was analyzed as a repeated measure when appropriate. Prepartum BW did not differ among days for CON. Whereas, HMP cows increased BW as parturition approached (treatment × day, P < 0.01). While not differing by treatment, dry matter intake (% BW) increased (P < 0.01) by 18% on week -2 compared to -8, but then decreased (P <0.01) by 8.0% during week -1. Nitrogen intake, apparent digestion, excretion, and retention (g/d) were all greater (P < 0.01) for HMP heifers than CON. Heifers fed HMP had improved (P < 0.01) DM, OM, and NDF digestibility relative to CON heifers. Concentration of urinary 3-MH and the 3-MH:creatinine ratio did not differ (P ≥ 0.23) between treatments. Maximum ruminal pH and the total concentration of short-chain fatty acids were not affected by treatment. Prepartum ruminal ammonia-N decreased (treatment × day, P < 0.01) as parturition approached for HMP (10.1 to 8.7 mg/dL) whereas, ammonia-N was not affected for CON (1.0 to 1.3 mg/dL). Plasma urea-N was greater (P < 0.01) for HMP heifers (15.0 vs. 7.5 mg/dL). Postpartum BW did not differ (P ≥ 0.30) by treatment, day, or the interaction of treatment and day, but rump fat decreased (P = 0.011) as lactation progressed. Dry matter intake decreased during wk 2 and 3 compared to 1 and 4, whereas ruminal pH was less during wk 2, 3, and 4 relative to wk 1. Colostrum fat concentration was less (P = 0.003) for HMP than CON, but milk production and milk component concentrations were not affected by treatment. Milk yield was greatest from d 7 to 33 and decreased thereafter (P < 0.001). Plasma and serum metabolites were not affected by treatment, but NEFA was greater (P < 0.001) on d 7 and 28 relative to d 70 and 112. Urinary 3-MH and the 3-MH:creatinine ratio did not differ by treatment, day, or the interaction of treatment and day (P ≥ 0.22). Calf growth was not affected by treatment. Over-feeding MP prepartum may improve prepartum heifer BW, NDF and OM digestibility and N balance, but might decrease colostrum fat concentration without affecting lactation or postpartum metabolic indicators of energy balance or protein turnover.



Metabolizable protein, beef cow, transition period, nitrogen balance, ruminal fermentation, milk production



Master of Science (M.Sc.)


Animal and Poultry Science


Animal Science


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