Influence of limited fall protein supplementation on performance and forage utilization by beef cattle grazing low-quality native grass pastures

摘要

Two studies were conducted to evaluate the impact of feeding limited quantities of a high-crude protein (CP) supplement during the fall on beef cow performance and forage utilization. The supplement used in both experiments contained 455 g/kg CP (drymatter [DM] basis; rumen degradable protein [RDP] = 652 g/kg of CP) and was fed 3 days/week with the amount fed pro-rated to deliver the designated daily allotments. In Study 1, 136 pregnant, Hereford x Angus cows (body weight [BW] = 490 +/- 46.7 kg) andtheir nursing calves were assigned to supplementation treatments. Control cows received no fall supplementation, while supplemented cows received 0.61 kg/day of supplement pre- and post-weaning (PRPO; 15/8 to 14/12; weaning = 15/10) or only post-weaning(POST; 15/10 to 14/12). During the winter, all cows received 1.62 kg supplement/day (14/12 to calving). Through weaning, PRPO cows gained more (P=0.03) BW. After weaning, PRPO and POST had higher (P=0.02) BW gains and lost less (P=0.02) body condition score (BCS) than did control cows. Cumulative BW gains were higher (P=0.05) for supplemented cows through calving, although BCS change did not differ among treatments. Calf birth weights did not differ among treatments, but the gain of calves from PRPO and POST was faster (P=0.03) than that of controls. Fall supplementation did not affect the proportion of cows cycling before the breeding season or pregnancy rates. In Study 2, 16 ruminally fistulated Hereford x Angus steers (initial BW = 259 +/- 22.7 kg)received either fall supplementation (FS) or no fall supplementation (NFS) during early (September) and late (November) fall. Steers were individually fed the same amount of supplement, relative to BW, as the cows in Study 1. The quality of the diet selected decreased with advancing season (i.e., the CP decreased and fiber increased) and, as a result, digestible organic matter intake and digestion were less (P=0.04 and 0.02, respectively) during November. Fall supplementation did not influence diet selection or forage intake, but did tend (P=0.06) to increase extent of digestion. Feeding beef cows a limited quantity of a high CP supplement during fall can positively affect BW and BCS. Under the conditions of this experiment, supplementation's effect on forage intake and digestion was less important than was provision of additional nutrients in eliciting treatment effects.