Effects of crude protein and lysine contents of the diet on growth and body composition of slow-growing commercial broilers from 42 to 77 days of age

The effects of two dietary crude protein levels (H = 22.5 and L = 17% on average) and four lysine levels (0.56, 0.66, 0.76 and 0.86%) were compared in a 2 x 4 factorial design, on growth and carcass composition of slow growing broilers (Label) during the finishing period (42-77 days). There were six pens of 28 male chickens per treatment. In each pen, 14 chicks received a stimulating starter diet (13 MJ.kg(-1), 1.4% lysine) and the other 14 chicks a poor starter diet (12.1 MJ.kg(-1), 1.1% lysine) from 0 to 6 days of age. The starter regime had no significant effect on subsequent performances and no interaction with the other traits. During the finishing period, lysine deficiency at 0.56% induced feather pecking and cannibalism resulting in more severe mortality with the H (33.9%) than the L diet (5.4%). Growth, feed conversion and breast meat yield were significantly improved by lysine supplementation up to 0.76% in the diet whatever the protein level. However, the deterioration of growth and feed conversion for lower lysine levels was more severe for the H diet compared to the L diet (interaction P < 0.001). The H fed chickens had less abdominal fat (3.3%) than the L fed chickens (4.3%) without a significant effect of the lysine level. The lysine requirement measured by regression (eponential and ascending line with a plateau) was close to 0.68%. High dietary protein levels associated with subdeficiency in lysine might favour cannibalism in label chickens during the finishing stage.