The effect of dietary tryptophan on aggressive behavior in developing and mature broiler breeder males

Broiler breeders display high levels of aggressive activity as a result of feed restriction. The objective of this study was to determine if aggressive activity in developing and mature broiler breeder males is influenced by increased dietary L-tryptophan (Trp). Broiler breeder males were raised using a skip-a-day feed restriction regimen. In Experiment 1, three levels of dietary Trp, .19 (control), .75, and 1.5%, were fed from the start of feed restriction at 4 wk of age and throughout development. All occurrences of aggressive behavior were recorded during three 20-min observation sessions per week on mornings on which feed was not delivered through Week 20. The Trp significantly decreased (P less than .05) aggressive pecking in birds fed supplemental Trp as compared to controls. No differences in threatening behavior were found. In the second part of Experiment 1, the effect of continued Trp supplementation on mature flocks of male broiler breeders and time-of-day effects on aggression were investigated. A depressing effect of supplemental Trp on pecking and threatening was observed. Maximum aggressive activity in all treatment groups occurred on mornings when feed was delivered. In Experiment 2, four levels of Trp, .19 (control), .38, .75, and 1.5%, were fed to developing birds as in Experiment 1. Birds fed supplemental Trp pecked less (P less than .05) than controls and .38% Trp was as effective in decreasing pecking as was 1.5%. It was concluded that increased levels of dietary Trp decrease aggression in both developing and mature, socially stable flocks of broiler breeder males.