Changes in body composition and energetic efficiency in response to growth curve and dietary energy-to-protein ratio in broiler breeders

Body composition plays an important role in reproduction in broiler breeders. The aim of this study was to evaluate the dynamics in body composition and energetic efficiency in broiler breeders, using differ-ent dietary strategies. About 1,536-day-old pullets were randomly allotted to 24 pens in a 2 pound 4 factorial design with 2 growth curves (standard or elevated (+15%)) and 4 diets, with a step-wise increment in energy (96, 100, 104, and 108% apparent metabolizable energy nitrogen corrected [AMEn]) fed on a pair-gain basis. Body composition was determined at 10 time points from 0 to 60 wk of age. Body protein mass was linearly related to body weight (BW) in growing breeders, which can be expressed as-6.4 + 0.184 * BW (R2 = 0.99; P < 0.001). Body fat mass was exponentially related to BW in growing breeders, which can be expressed as-42.2 + 50.8 * 1.0006BW (R2 = 0.98; P < 0.001). A higher energy-to-protein ratio resulted in higher body fat mass at the same BW (P < 0.001). Sex-ual maturation was related to body protein mass at 21 wk of age, where each 100 g of body protein mass extra advanced sexual maturation by 5.4 d (R2 = 0.83). Esti-mates of energetic efficiency for growth (kg) and egg pro-duction (ke) appeared not constant, but varied with age in a quadratic manner between 0.27 and 0.54 for kg and between 0.28 and 0.56 for ke. The quadratic relationship could be expressed as kg = 0.408 -0.0319 * Age + 0.00181 * Age2 (R2 = 0.72; P < 0.001) and ke =-0.211 +0.034 * Age -0.00042 * Age2 (R2 = 0.46; P < 0.001). Body protein mass in broiler breeders is tightly regu-lated and mainly depended on BW and seems to be the main determinant for sexual maturation. Body fat mass is exponentially related to BW, where an increase in die-tary energy-to-protein ratio results in a higher body fat mass. Treatments had minimal effects on estimated energetic efficiencies in breeders.