Protein supplementation for winter-calving beef cows between calving and turnout to pasture was studied in two trials. Changes in cow body weight (BW) and condition and calf BW gains were recorded from calving to weaning in the fall. Reproductive performance was monitored, and in trial 2, milk production was determined. In trial 1, multiparous silage-fed cows were allocated to one of five levels of corn gluten meal (CGM): 0, 200, 400, 600 and 800 g d−1. In the second trial, cows were allocated to either restricted or ad libitum silage feeding in combination with three protein supplements: no protein, 400 g d−1 CGM or 475 g d−1 soybean meal (SBM). In trial 1, cows gained weight during supplementation, but on pasture they lost weight (linear effect, P = 0.10). Calf gains and weights at turnout and weaning showed a quadratic response to protein supplementation (P < 0.05). There was a trend toward a linear decline in days to first service with increasing levels of CGM (P < 0.10). In trial 2, cows on restricted silage feeding lost weight (P < 0.05) and body condition (P < 0.10), whereas those fed ad libitum silage gained weight and condition. On pasture, this pattern of weight change was reversed (P < 0.05). Protein supplementation did not influence BW change but tended to cause a lower body condition score at turnout (P = 0.10). Restricting silage intake to the cow did not affect calf performance, but protein supplementation increased gains both during supplementation and on pasture, thus increasing weaning weights (P < 0.05). When silage was fed ad libitum, SBM was more effective than CGM for increasing calf gain. However, when silage was restricted, CGM was more effective. Restricting silage intake did not impair milk production, but feeding CGM increased milk production (P < 0.05). For ad libitum-fed cows, protein supplementation increased pregnancy rate; restricting feed had the opposite effect. Our data suggest that protein supplementation to silage-fed beef cows in good body condition can increase calf performance; however, the level of supplementation is critical, and possible adverse effects on breeding have to be taken into account, particularly with cows in negative energy balance after calving. Key words: Beef cow, protein, milk production, calf gains, silage
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