RESTRICTED MAXIMUM-LIKELIHOOD-ESTIMATES OF GENETIC-PARAMETERS AND GENETIC TRENDS OF CHICKENS DIVERGENTLY SELECTED FOR EGGSHELL STRENGTH

A White Leghorn population was used for two-way selection to increase (weak line) or decrease (strong line) nondestructive deformation over 10 generations. Nondestructive deformation (NDD) from the first to the 10th generation was changed from 64.7 to 100.6 mu m/kg in the weak li ne and from 59.9 to 51.9 mu m/kg in the strong line. Breaking strength was changed from 2.77 to 2.17 kg in the weak line and from 2.95 to 3. 76 kg in the strong line. Egg production was changed from 0.840 to 0.8 80 eggs/day in the weak and from 0.838 to 0.730 eggs/day in the strong line. Breeding values and genetic parameters were estimated by the be st linear unbiased prediction method and the restricted maximum likeli hood method (REML) under a multiple traits animal model. In the weak l ine, regression coefficients of predicted breeding values on generatio ns were 2.19 mu m/kg per generation for NDD, -0.06 kg for breaking str ength and 0.005 eggs/day for egg production. These notes were -0.90 mu m/kg for NDD, 0.03 kg for breaking strength and -0.005 eggs/day for e gg production in the strong line. All regressions were significant at the 1% level. Cumulative selection differential and response for NDD w ere 92.5 and 35.9 mu m/kg, respectively, in the weak line and only 56. 9 and 8.0 mu m/kg, respectively, in the strong line. Realized heritabi lities were 0.388 in the weak line and 0.140 in the strong line, altho ugh the realized heritabilities in the two directions were not signifi cantly different. Estimated heritabilities using multiple traits REML were 0.373 for NDD, 0.386 for breaking strength and 0.184 for egg prod uction in the weak line; 0.364, 0.372 and 0.301 in the strong line. Ge netic correlations were -0.785 between NDD and breaking strength, 0.20 4 between NDD and egg production and -0.305 between breaking strength and egg production in the weak line; -0.818, 0.318 and -0.333, respect ively in the strong line. The selection for lower NDD can increase she ll breaking strength but reduces egg numbers.