Effect of Dietary Protein Concentration on Performance of Hens Selected on Low Protein Diet and Grouped According to Their Early Production

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Abstract
Two pure-bred lines, White Leghorn (WL) and Rhode Island Red (RIR) laying strains, which had been selected for several generations to perform well on a low protein and energy diet, were used in this experiment. A total of 120 birds of each strain were grouped into three production groups, high, medium or low, according to their egg output between 26–29 and 27–29 weeks of age for the WL and RIR strains respectively. From 32–72 weeks of age 10 birds in each group were fed one of three isocaloric (11.30 MJ ME/kg) experimental diets (A, C and E) with calculated crude protein and total lysine contents of 12.4, 15.0 and 17.6% and 0.46, 0.66 and 0.87% respectively. In addition, a fourth diet (control, N) was given, being the one on which both strains were selected, and containing 11.05 MJ ME/kg, 12.9% crude protein and 0.55% total lysine (calculated). The response of the birds, measured as egg production, feed consumption, final body weight, mortality, plumage condition and frequency of damaged feet was studied. Rate of lay, egg output and feed conversion ratio (FCR) of the RIR hens were significantly higher than those of the WL when fed the low protein diets. Both strains responded positively to the high protein diets. Maximum rate of lay and egg output were essentially the same for the two strains and was obtained from birds receiving diet C. However, the WL hens had better FCR than the RIR hens when fed the high protein diets. The superiority of the WL over the RIR strain for FCR on the adequate protein diet was related to observable differences in body size and food intake, as egg output was the same. However, the superiority of the RIR over the WL strain for FCR when fed the low protein diets was related to strain differences in the efficiency of dietary protein utilization. The three production groups within each strain responded similarly to the different dietary protein levels. The high-producing groups maintained their superior egg output throughout the experimental period, the difference being significant (pp<0.001) for the RIR and WL groups respectively. It is concluded that it should be possible to select laying hens on the basis of their utilization of low protein diets, and that emphasis should be placed on medium-sized breeds.