B-vitamin supplementation of diets for feedlot calves.

B-vitamin supplementation of diets for 144 shipping-stressed crossbred calves (116 kg) at levels up to 10 times that recommended for growing pigs did not influence (P greater than .20) weight gain or feed conversion during a 56-d receiving trial. However, vitamin supplementation tended (P less than .10) to reduce morbidity. In a second trial, supplemental B-vitamins had no effect (P greater than .20) on efficiency of microbial growth or site and extent of digestion of organic matter, acid detergent fiber and N. Supplemental dietary riboflavin, niacin, folic acid, B12 and ascorbic acid were largely metabolized [degraded and (or) absorbed] anterior to the small intestine, with escape values of 1, 3, 10 and 0% of added vitamins, respectively, while dietary B6 and biotin largely escaped the rumen. Thiamine and pantothenic acid were intermediate in ruminal escape (52 and 22%, respectively). Small intestinal absorption of thiamine, niacin, riboflavin, B6 and B12 averaged 75, 79, 25, 79 and 48%, respectively. Coefficients for ruminal escape and microbial synthesis for each B-vitamin were calculated using the slope-ratio technique. Measured flows for thiamine, riboflavin and B12 in a third experiment were similar to flows predicted from escape and synthesis equations, though flow of niacin and B6 were under-predicted by 37 and 44%, respectively. Results are interpreted to indicate that intestinal B-vitamin supply can be predicted based on dietary composition and intake.