Oversupplying metabolizable protein in late gestation for beef cattle: Effects on postpartum ruminal fermentation, blood metabolites, skeletal muscle catabolism, colostrum composition, milk yield and composition, and calf growth performance.

The objective of the study was to determine whether oversupplying MP prepartum affects postpartum cow BW, colostrum composition, milk production and composition, protein catabolism in the dam, and calf growth. Crossbred Hereford heifers were individually fed a control treatment designed to meet MP requirements (CON; n = 10) or 133% of the MP requirement (HMP; n = 11) from d -55 ± 4 until parturition. All cows were provided a common postpartum diet. Cow BW was measured on d 7 ± 1, 14 ± 2, 28 ± 3, 57 ± 4, 82 ± 5, and 111 ± 3 d relative to parturition. Dry matter intake and ruminal pH were measured daily and summarized by wk until d 33. Milk yield was estimated based on a 12-h 2-quarter milk yield on d 7 ± 1, 12 ± 1, 28 ± 3, 33 ± 3, 70 ± 3, and 112 ± 3. Urine samples were collected from cows over a 6-d period starting on d 7 ± 1 and 28 ± 3 and the composited samples were analyzed for 3-methylhistidine (3-MH) and creatinine. Muscle samples were collected from cows on d 13 ± 1 while calf muscle samples were collected on d 2 and d 111 ± 3 of age. Muscle samples from cows were analyzed for markers of protein catabolism and calf muscle samples were analyzed for genes regulating cell growth and differentiation. Data were analyzed as a randomized complete block design using the MIXED procedure of SAS accounting for repeated measures when necessary. Postpartum BW did not differ (P 0.30) by treatment, day, or the interaction of treatment and day (T×D), 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 was not affected by treatment. Milk yield was greatest from d 7 to 33 and decreased thereafter (P < 0.01). Urinary 3-MH and the 3-MH:creatinine ratio did not differ by treatment, day, or the T×D (P 0.22) interaction, nor was there a difference (P 0.13) in the abundance of catabolic proteins. Calf growth was not affected by treatment, but HMP calves had greater expression (T × D, P = 0.05) of PPARG while PKM expression increased for CON calves (T × D, P = 0.04) at d 111 compared to their expression at d 2. Overfeeding MP during late gestation does not improve postpartum indicators of N balance or maternal muscle turnover but may alter colostrum composition and calf gene expression at weaning.