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

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