Effect of nicotinic acid on microbial protein synthesis in vitro and on dairy cattle growth and milk production.

The effect of nicotinic acid (niacin) on microbial fermentation was tested in vitro. Microbial protein synthesis was greater with niacin and soybean meal than with niacin and urea. Otherwise, in most instances niacin decreased synthesis with urea. These responses to niacin with soybean meal occurred regardless of roughage type or ratio of roughage to concentration, except when the substrate contained 50% roughage from alfalfa; then the opposite was true. Adding niacin to urea-containing rations of heifers weighing 375 or 114 g failed to improve the heifers' weight gain or feed efficiency. In one of two lactation studies with cows in midlactation fed urea-containing rations, a slight increase in milk production was attributable to niacin; in the other, a slight improvement in milk protein production was attributable to niacin. In a third lactation study, but with fresh cows, milk production increased in cows receiving niacin and soybean meal but not in those receiving niacin and urea. In a fourth lactation study with fresh cows fed soybean meal, cows receiving niacin gave slightly more milk than did those receiving none. Response to niacin is greater in fresh cows than in those in midlactation and is greater in cows fed natural protein than in those fed urea.