Effectiveness of zinc bacitracin on production traits and energy metabolism of heat-stressed hens compared with hens kept under moderate temperature

Two experiments with a total of 100 White Leghorn hens (26 to 59 wk of age) and 120 Rhode Island Red chickens (4 to 12 wk of age) were conducted to study the effect of 100 mg zinc bacitracin (ZBA)/kg of feed on hens and chickens acclimatized to 20 or 34 C environments. Layers were provided ad libitum access to a diet containing 11.12 MJ ME/kg (1 MJ=.239 Mcal) and 174 g CP/kg supplemented with 0 and 100 mg ZBA/kg. Body weight gain, egg number, total egg mass, and feed efficiency of ZBA-treated hens kept at a moderate temperature (20 C) were not significantly improved by 3.5, 2.6. 3.3, and 3.1%, respectively. However ZBA supplementation induced a more pronounced increase in performance in heat-stressed hens. The respective traits were significantly improved by 66.3, 15.4, 16.9, and 5.9%. Additionally, treated hens maintained at 34 C consumed significantly more feed than untreated hens. Eggshell breaking strength of treated hens held under normal conditions was not significantly improved. Feeding ZBA to heat-stressed hens led to a statistically higher breaking strength. Supplementation with ZBA for hens acclimatized to 20 and 34 C reduced fasting heat production by 4.1 and 7.6%, respectively. However, in the 20 C environment the differences were significant at only one of seven ages. The upper critical temperature (UCT) of ZBA-treated hens increased significantly at both temperatures from 34 to 42 wk of age relative to the control group. No significant effects of ZBA on UCT were observed at 24 to 32 wk and 40 to 52 wk of age. Heat tolerance of chickens 4 to 12 wk of age fed ZBA was significantly improved by 23.7% (20 C) and 51.2% (34 C). Results indicated a clear response of ZBA on improving performance of hens acclimatized to 34 C, which was due to overall heat production, especially for hens kept at constant high temperature. The effect of ZBA on hens housed under thermoneutral (20 C) conditions was not statistically significant.