Responses of laying hens to diets differing in isoleucine content

Abstract

The responses of Hyline (WLxNH) and Tegel (WLxAO) strain laying hens offered six 'response' diets ranging in isoleucine content from 2.8 to 8.6 g kg-1 were examined. The required range of isoleucine content was obtained by formulating a summit diet containing a surplus of all essential amino acids, but a lesser surplus of isoleucine, and blending it in appropriate proportions with a basal diet, structured similarly to the summit diet, but deficient in all essential amino acids and protein. To test that isoleucine was the first limiting nutrient in the 'response' diets, an additional six 'test' diets were given. These were identical to the 'response' diets except for the addition of L-isoleucine which raised the total isoleucine content of each by an amount equivalent to the difference between two adjacent 'response' treatments. The 12 diets were given from 30 to 46 weeks of age (phase 1) and again from 54 to 70 weeks of age (phase 2) following an intervening recovery period. Responses to the diets, determined on data from the final 4 weeks of each phase, were attributed to differences in isoleucine intake. The apparent efficiencies of utilization of isoleucine for egg synthesis and for maintenance were similar for both strains. The daily isoleucine requirement of individual laying hens was estimated to be 9.91 mg g-l egg output plus 33.56 mg kg-1 liveweight per day. Curves describing the egg output response to isoleucine intake were calculated using the Reading model and optimum isoleucine intakes were derived for various liveweights, egg outputs and ratios of input costs and output values. For example, for a flock of hens with an average weight of 2 kg and producing an average of 50 g daily egg mass, the optimum isoleucine intake varied between 581 and 684 mg d-l.