EFFECTS OF SEX, HEAT-STRESS, BODY-WEIGHT, AND GENETIC STRAIN ON THE DIETARY LYSINE REQUIREMENT OF BROILER CHICKS

Experiments were carried out to investigate the effects of sex, heat stress (37 C), body weight (heavy and light within strain), and strain of chicks on the dietary lysine requirement of chicks during 8 to 22 days posthatching. A lysine-deficient basal diet (.64% total lysine, 23% CP, 3,200 kcal ME(n)/kg) containing com, feather meal, and soybean meal was supplemented with graded levels of L-lysine.HCl to produce growth response curves. The lysine-deficient diet contained .52% true digestible lysine as determined with a precision-fed cecectomized adult cockerel assay (Experiment 1). Hubbard x Hubbard chicks were used in Experiment 2 and New Hampshire x Columbian crossbred chicks were used in Experiments 3 and 4. Experiment 2 compared lysine requirements of male and female chicks. Weight gains between sexes were similar when diets were deficient in lysine, but males grew faster than females when lysine-adequate diets were fed. Male chicks required a higher level of dietary lysine than females for both maximal weight gain and feed efficiency. Also, regardless of sex, the lysine requirement (percentage of diet) for maximal feed efficiency was higher than that for maximal weight gain. In Experiment 3, heat stress reduced weight gain and feed intake of both males and females by about 22%, and it increased the lysine requirement of female but not male chicks. In Experiment 4, light and heavy chicks were selected from male and female populations. Heavy and light chicks exhibited the same dietary lysine requirement for maximal growth. However, the lysine requirement for maximal feed efficiency was higher for heavy birds than for light birds. There was no strain effect on the lysine requirement.