Decreases in aggression in tryptophan-supplemented broiler breeder males are not due to increases in blood niacin levels.

In previous studies with feed-restricted broiler breeders, it was found that supplementation with dietary Trp decreases aggressive activity more in socially dominant males than in socially subordinate males. Although it is probable that this effect is mediated centrally by serotonin, an alternative possibility is that it is associated with other metabolic products of Trp such as niacin. The objectives of this study were to determine whether 1) supplemental dietary niacinamide decreases aggression in broiler breeder males, 2) elevated blood niacin levels are associated with a decrease in aggression, and 3) social status influences any of these effects on behavior. Using a randomized complete block design, a control (0.19% Trp, 22 mg niacin/kg), Trp (1.5% Trp, 22 mg niacin/kg), or niacinamide (0.19% Trp, supplemented with 140 mg niacinamide/kg) diet was assigned to each pen. There were 16 birds per pen and three pens per treatment. Birds were maintained on skip-a-day feed restriction throughout, and behavioral observations were conducted until the social hierarchy in each pen was determined. Treatment diets were then fed from 15 through 18 wk of age, and the number of aggressive pecks and threats per pen recorded in three 20-min observation periods/wk. Blood samples were collected from dominant and subordinate birds and assayed for niacin using Tetrahymena pyriformis. Supplementing the diet with Trp significantly (P < 0.05) decreased aggression. Niacinamide-treated birds did not differ from either Trp-treated or control-treated birds, and no decreases in aggression were seen in birds with elevated blood niacin levels. Blood levels of niacin were, however, higher in dominant niacin-treated birds than in dominant control birds (P < 0.05), although there were no differences among subordinate or Trp-treated birds. Thus, the modulatory effect of Trp on aggression does not appear to be mediated by increased niacin synthesis.