Effect of diet composition on feed intake and growth of chicks under heat stress. II. Constant vs. cycling temperatures.

Broiler chicks 5 to 7 weeks of age were subjected to a constant cool (14 +/- 1 C) or hot (31 +/- 1 tc) environment and fed diets varying in fat level and energy and nutrient density. Chicks gained significantly (P less than or equal to .01) more body weight in the cool than in the hot environment. In both environments chicks fed high fat or high fat-high density diets gained more weight than those fed diets low in fat, and there was no diet X temperature interaction. When temperatures were cycled diurnally (cool, 14 to 22 C; hot, 22 to 33 C), as would occur under natural conditions, the growth depression due to heat stress was less in chicks fed the diets high in fat. The special benefit of fat at high temperature was probably the result of reduced heat increment. Under constant temperature conditions, chicks in the cool environment, pair-fed at the same level of feed intake as those in the hot environment, gained significantly (P less than or equal to .05) more weight than the latter. These data indicated that factors other than reduced feed intake contributed to the growth depression associated with high temperature. Thyroid size was significantly (P less than or equal to .05) reduced in chicks held at constant high temperature and slightly but not significantly reduced in chicks subjected to cyclic heat stress. This decrease in thyroid size suggested there is a practical limit on overcoming by nutritional means the deleterious effects of heat stress in broilers.