Effect of high ambient temperature on feed digestibility in broilers

The effect of chronic heat exposure on feed digestibility of broilers was investigated. Eighty 4-wk-old male chickens were brooded in individual battery cages in two controlled-environment rooms at a constant ambient temperature (22 or 32 C) until 6 wk of age. They were equally distributed into three treatments: 22 C, ad libitum feed consumption (22AL); 32 C, ad libitum feed consumption (32AL), and 22 C, pair-feeding on the daily feed intake of heat-exposed chickens (22PF). Broilers were fed either a standard corn-soybean meal diet (control diet) or a practical seasonal diet containing several ingredients including wheat, spring pea, and animal fat (summer diet). Digestibility of energy, dry matter, protein, fat, starch, and nitrogen, and total mineral balances were measured between 38 and 42 d of age. Apparent metabolizable energy content of summer diet was significantly decreased in 32AL compared to 22AL, whereas AME of the control diet did not change. Nitrogen retention was significantly reduced in 32AL birds compared to 22AL and 22PF birds, irrespective of the diet. Taking into account these differences in nitrogen balance, AME, was reduced under hot exposure: -72 and -155 kcal for control and summer diets respectively, in 32AL compared to 22PF chickens. This reduction could be explained by a significant decrease of nutrient digestibility:protein: -4.2 percentage units irrespective of the diet, fat: -1.7 and -5.2 percentage units for control and summer diets respectively, and starch: -4.2 percentage units for summer diet. It thus appears worthwhile to take into account such reduction in digestibility to formulate practical diets for brooding under hot conditions. High quality oil and protein sources should also be used instead of low quality feedstuffs, like animal sources, in such conditions.