Kamada, T., Kimata, M., Notsuki, I. 1993. The effect of providing pigs with toy on the welfare and performance. Journal of Animal Science 71(Supplement 1), 115 (Abstract).

These data suggest that providing pigs with toy [suspended bicycle tire tubes, baseballs] would improve welfare situation [decreased aggression between pen mates] at every developing stage but would have little effect on pig performance.