There is a wide variation in the response of ruminant livestock to supplemental niacin, in part due to variations in (1) endogenous niacin synthesis from tryptophan, (2) niacin supply and bioavailability in common feedstuffs and (3) rumen niacin synthesis and degradation. Young, preruminant calves and lambs would be expected to have a dietary requirement for niacin. However, early studies with lambs and calves failed to produce a deficiency by feeding niacin-free diets. The development of a low-tryptophan milk diet enabled researchers to study the situation more clearly. From these studies, it was concluded that a dietary niacin requirement does not exist as long as dietary tryptophan is maintained near 0.2% of dry matter (Hopper and Johnson, 1955). In the absence of dietary tryptophan, 2.5 mg per liter of milk offered ad libitum was required to prevent deficiency symptoms. The minimum niacin level suggested for calf milk replacers is 2.6 mg per kg (1.2 mg per lb) (NRC, 1989). Niacin supplementation of milk replacer would be of more concern when alternative, non-milk protein sources are used as the primary protein source in milk replacer, due to low tryptophan content.
