BODY-WEIGHT AND EGG WEIGHT DYNAMICS IN LAYERS

The association between body weight-age and egg weight-age patterns wa s studied in a segregating population of laying hens belonging to the Fg generation of a Rhode Island Red x White Leghorn reciprocal cross. Body weight and egg weight were expressed as a function of time using the model developed by Weatherup and Foster. Each hen was characterize d in terms of its asymptotic body weight (ABW), maturing rate for body weight (MBW), asymptotic egg weight (AEW), and maturing rate for egg weight (MEW) values. Four groups of hens were distinguished by means o f a principal component analysis. Birds belonging to Groups 1 and 3 we re discriminated for their egg weight-age pattern. Group 1 included he ns laying the heaviest eggs (AEW = 66.1 g) at the lowest maturing rate (MEW = 0.922), the inverse being true for birds in Group 3 (AEW = 55. 7 g and MEW 0.737). Birds belonging to Groups 2 and 4 were distinguish ed for their body weight-age pattern. Hens in Group 2 showed the lowes t ABW (1,893 g) and MBW (0.764) whereas the heaviest (ABW = 2,802 g) a nd less mature (MBW = 0.929) birds were found in Group 4. The results confirm the partial pleiotropic basis of the body weight-egg weight co rrelation, evincing the feasibility of applying selective pressure not only on each character separately but also on maturing rate independe ntly of asymptotic weight within each trait. This strategy could be im plemented using a biological selection index based on principal compon ent analysis equations.