The effect of different dietary protein levels in the rearing and laying periods on performance of white Leghorn chickens.

An experiment was conducted to determine the effects of three rearing and four laying protein regimens on growth and subsequent performance of a commercial strain of White Leghorn chickens. In the rearing period, the pullets of the control group received a sequence of 18, 15, and 12% protein for 0 to 6, 6 to 14, and 14 to 20 weeks of age, respectively. The pullets of the low-protein rearing regimens received either 18 and 12% protein for 0 to 6 and 6 to 20 weeks of age, respectively, or 18, 12, and 15% protein for 0 to 6, 6 to 18, and 18 to 20 weeks of age, respectively. Birds of each rearing group received four laying regimens from 20 to 72 weeks of age. These consisted of a diet containing 16.5% protein, a diet containing 14.5% protein, a moderate step-down protein regimen (MSP) of 18, 16.5, and 15.5% protein for 20 to 36, 36 to 60, and 60 to 72 weeks of age, respectively, and a low step-down protein regimen (LSP) of 16.5, 15.5, 14.5, and 13.5% protein for 20 to 36, 36 to 48, 48 to 60, and 60 to 72 weeks of age, respectively. The level of total sulfur amino acids were maintained at .59% and the minimum level of lysine was .68% in the laying rations. Birds fed the low-protein rearing regimens were significantly lighter at 20 weeks of age, consumed significantly less feed, energy, and protein in the rearing period, and their age at 50% production was delayed by 2 to 3 days compared to control fed birds. Birds fed the low-protein rearing regimens produced fewer eggs and slightly lighter weight eggs during the first laying phase. However, overall egg production, egg size, and feed consumption were not influenced by the protein levels in the rearing period. Pullets receiving the 14.5% protein diet in the laying period produced significantly fewer and lighter weight eggs and consumed significantly less protein and lysine than those fed the 16.5% protein diet. Overall egg production, egg weight, and nutrient intake of birds of the MSP groups were not significantly different from those fed the 16.5% protein diet. Egg production was similar for the birds of the LSP groups compared to those receiving the 16.5% protein diet, whereas egg weight, protein, and lysine intake were significantly lower for the birds of the LSP group.(ABSTRACT TRUNCATED AT 400 WORDS)