Exploring the effects of dietary lysine and tryptophan on the social behavior of pigs.

Negative social behaviors between pigs can cause stress, which can compromise welfare. There has been significant interest in exploring the effect of diet on negative social behaviors and the wider social behavior repertoire of pigs. The aim of this study was to determine the effects of dietary Lysine (Lys) and Tryptophan (Trp) levels on the social behavior of commercially housed pigs. A total of 2,293 PIC Camborough barrows with a mean starting weight of 11.87 ± 1.35 kg were used in a randomized complete block design with a 2 × 3 factorial arrangement, with 16 replicates per treatment, of the following factors: 1) Standardized ileal digestible Lys levels: 100% Lys = diets with 100% PIC requirement at the midpoint of the growth phase (Lys 100) vs 80% Lys = diets with 80% PIC requirement at the midpoint of the growth phase (Lys 80); and 2) Trp to Lys ratio of 0.210, 0.185, or 0.160. Pigs were randomly allocated across the 6 treatments over 2 starting dates. Behavior and lesion data were collected. There was an effect of Lys (P = 0.032) on ear-biting behavior, with pigs on the Lys 80 treatments showing a higher level of ear-biting behavior. We also found an effect of the Lys score week interaction on the proportion of pens showing ear lesions (P < 0.001) and an effect of the Lys Trp interaction (P = 0.030) and the Lys score week interaction (P = 0.0104) on the proportion of pens showing severe ear lesions. In conclusion, the lysine content of feed can affect the social behavior of pigs, specifically ear biting, in commercial conditions.