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.

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In five experiments an attempt was made to reduce the effects of heat stress on broiler chicks by lowering the heat increment of the diet. Percent of energy derived from fat was increased by replacing corn starch with fat, calorie for calorie, which reduced formula weight and increased diet density. In one treatment total protein level was reduced also while maintaining adequate essential amino acid levels. The effect of caloric and nutrient density was separated from that of "percent fat calories" by the use of a non-nutrient filler in one diet. Experimental rations were fed to male broiler chicks from 4 to 7 weeks of age in hot (31.1 ± 2 C) vs. cool (20.0 ± 2 C) controlled temperature chambers.In all experiments, metabolizable energy (ME) intake was increased when fat calories replaced carbohydrate (CHO) calories. In the high fat treatments, chicks gained significantly more body weight than did the high CHO control groups. This increase, however, was similar at both temperatures, indicating that the beneficial effect of dietary fat was independent of temperature. A portion of the improvement in ME intake and weight gains resulting from increased ratio of fat to non-fat calories was retained when diet density was equalized. Heat production measured by energy balance studies was numerically lower in chicks fed diets with the high ratio of fat to non-fat calories.

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The levels of plasma carotenoids were markedly reduced in broiler cockerels infected with Eimeria acervulina or E. tenella. The mechanisms of this depigmentation differed between the two species, being primarily associated with interference of absorption of xanthophyll (carotenoids) from the intestinal lumen with E. acervulina infection and with leakage through the damaged wall of the cecum with E. tenella infection. Chicks reared on an essentially carotenoid-free diet and inoculated with E. acervulina showed no detectable levels of carotenoids in the blood 48 hours after being changed to a diet containing 30 mg of xanthophyll/kg. Conversely, uninoculated chicks and chicks inoculated with E. tenella showed significant and similar increases in plasma levels of carotenoids. Chicks reared on a diet containing xanthophyll and inoculated with E. tenella showed a more rapid decrease in plasma carotenoids than did uninoculated controls when changed to a xanthophyll-free diet. In chicks fed high xanthophyll diets containing chromic oxide, no indication of malabsorption was seen in chicks infected with E. tenella compared with uninoculated controls, whereas chicks inoculated with E. acervulina showed significantly less xanthophyll absorption. Conversely, a marked increase in the xanthophyll : Cr2O3 ratio was observed in the cecal contents of chicks inoculated with E. tenella compared with uninuoculated controls or those inoculated with E. acervulina. Studies of uninoculated chicks pair-fed with chicks inoculated with E. acervulina or E. tenella indicated that the decrease in plasma carotenoids and increases in intestinal pH are not associated with the reduced intake of feed associated with infection. The studies involving uninoculated birds with reciprocal chagnes between high and low xanthophyll diets indicated that plasma carotenoids are a more rapid and sensitive means of measuring changes in pigmentation levels than are visual skin scores carotenoid levels from the skin.