Influence of protein nutrition and virginiamycin supplementation on feedlot growth performance and digestive function of calf-fed Holstein steers

Two experiments were conducted to examine the influence of protein and virginiamycin (VM) supplementation on feedlot growth performance, digestion, and metabolizable AA (MAA) supply of calf-fed Holstein steers. Growth performance and dietary energetics were evaluated in 120 Holstein steers (127 +/- 9 kg). During the initial 112-d feeding period, a steam-flaked corn-based diet was balanced to meet either 100% (MAB) or 87% (UREA) of MAA requirements. Diets were supplemented with or without 22.5 mg/kg VM in a 2 x 2 factorial arrangement. Subsequently (d 112 to 308), all steers received the UREA diet with or without VM. During the initial 112-d, MAB increased ADG, G:F, and dietary NE (P < 0.01). Thereafter, when all steers received the UREA diet, ADG, G:F, and dietary NE were not different (P > 0.10) across initial supplementation treatments. Overall (d 1 to 308), MAB did not affect ADG (P > 0.10) but enhanced G: F efficiency (P = 0.03) and dietary NE (P = 0.05). During the initial 112-d period and through the remainder of the experiment, VM increased G: F (P < 0.01) and dietary NE (P < 0.01). Four Holstein steers (146 +/- 4 kg) with cannulas in the rumen and proximal duodenum were used in a 4 x 4 Latin square design to evaluate initial 112-d treatment effects on digestive function. There were no treatment effects (P > 0.10) on ruminal digestion of OM, NDF, starch, microbial efficiency, or total tract digestion of OM and NDF. The MAB increased indispensable AA flow to the small intestine (P < 0.01) and total tract digestion of N (P < 0.01) and starch (P = 0.04). Observed AA supply to small intestine was in agreement with expected supply (r(2) = 0.96). Virginiamycin decreased (P = 0.04) nonammonia N flow to the small intestine and did not affect (P > 0.10) total tract N digestion. Extrapolating from AA supplies in the metabolism study, MAB satisfied indispensable AA requirements during the initial 112-d period, whereas the UREA diet met 73.5% and 79.2% of methionine and lysine requirements, respectively. During the subsequent periods (d 112 to 308) indispensable AA supplies exceeded theoretical requirements. We conclude that enhancements in energy utilization when diets are balanced to meet MAA requirements of calf-fed Holstein steers during the initial 112-d feedlot period remain appreciable throughout time on feed. Virginiamycin enhanced efficiency of energy utilization throughout the feedlot growing-finishing period.