Performance and physiological profiles of high dietary zinc and fasting as methods of inducing a forced rest: a direct comparison.

Equal periods of fasting or high zinc feeding as a means of inducing a forced rest were examined. Single Comb White Leghorn hens (60 weeks of age) were fasted (F) or fed a layer diet containing 20,000 ppm zinc (Zn) for either 4 or 10 days in a 2 X 2 factorial arrangement of treatments. Feed intake was markedly depressed in the first day of high zinc feeding and averaged 10.7 and 6.1 g/hen-day during the 4 or 10 days of Zn feeding. Body weight losses of hens fasted or fed high dietary zinc were not significantly different. Egg production rate was highest for hens treated 10 days (both F and Zn) in the immediate weeks following the rest. However, over the entire 6-month experimental period, there were no significant differences among the four treatments. Ovary and oviduct regressed similarly in F and Zn hens. Tissue zinc concentration greatly increased during zinc feeding, especially in the pancreas, and rapidly returned to normal upon refeeding a normal layer diet. Fasting also caused a substantial increase in tissue zinc. The results show that the effectiveness of zinc as a forced-resting agent for hens is related to changes in feed intake.