Over-Feeding Metabolizable Protein Supply in Late Gestation Beef Cattle: Effects on Colostrum Composition, Milk and Milk Component Yield, and Pre-Weaning Growth of Calves

The objective was to determine the effect of oversupplying metabolizable protein (MP) pre-partum on colostrum composition, milk and component yield, blood metabolites, and on pre-weaning growth of calves. Twenty-four primiparous crossbred Hereford cows were assigned to either a high protein (HP; 140% of predicted MP requirement, n = 12) or control (CON; 100% of predicted MP requirement, n = 12) treatment. Cows were individually fed their treatment diet from d -56 until parturition and then were switched to a common diet post-partum. Colostrum samples were collected, via hand stripping, immediately after calving. The 24-h milk yield was estimated by measuring a 2-quarter 12-h milk yield (MY) on d 7, 13, 28, 33, 70, and 112. Milk and colostrum samples were analyzed for fat, CP, lactose, urea-N (MUN), and somatic cell count (SCC). Blood samples were collected from cows via jugular venipuncture on d -35, -17, 7, and 28 and analyzed for plasma glucose and serum NEFA. Calf BW and frame measurements were conducted at birth and on d 7, 28, 57, 82, and 112. Data were analyzed as a completely randomized block design using the MIXED procedure of SAS. Fixed effects included treatment, time, and the two-way interaction and the random effects were cow and block. Day was analyzed as a repeated measure. Heifers consuming the HP diet pre-partum had less fat in colostrum (3.63 vs. 7.01%; P < 0.01) compared to the CON but, CP, lactose, and MUN were not affected. There was no effect of pre-partum treatment on MY or composition. Day relative to parturition affected MY (kg/d) with a progressive decrease from 6.91 on d 7 to 5.06 kg on d 112 (P < 0.01). Milk fat decreased from d 7 (3.59%) to d 28 (3.43%) and then gradually increased to d 112 (3.85%, P = 0.01). Milk CP (P < 0.01) and MUN (P < 0.01) followed a similar pattern as reported for fat. Lactose concentration increased from d 7 (4.55%) to d 28 (4.80%) and then decreased to d 112 (4.51%, P < 0.01). Plasma glucose and serum NEFA did not differ by treatment or week. There was no effect of pre-partum treatment on calf BW or frame measurements. Oversupplying MP pre-partum may decrease colostrum fat without affecting MY, milk composition, and calf growth.