Evidence Concerning Genetic Improvement in Commercial Stocks of Layers

Genetic interpretation of time trends in Random Sample Test results was studied by comparing laying performance of six major commercial stocks (A) with that of the Cornell Randombred Leghorn controls (B) under low floor (F) and high cage (C3 and C5) densities of housing: 1860 vs. 1390 cm2. during floor rearing to 20 weeks and 2230 on floor vs. 622 and 372 cm2 in cages to 72 weeks of age. Each commercial stock and each of two replicates of controls had 180 pullets per brooding density and six floor laying pens of 20 birds each plus five blocks of 9 and five of 15 birds each in 41 × 46 cm. cages. Other management was identical for all birds. Ratio of cage to floor means (C̄/F̄ × 100) for A vs. B stocks was 98 vs. 91 for egg mass/hen housed, 88 vs. 90 for feed/hen-day, 108 vs. 100 for egg/feed weight, 95 vs. 88 for eggs/live hen-day, 97 vs. 89 for eggs/hen housed, 101 vs. 102 for weight/egg laid, 104 at 20 weeks and 103 vs. 107 at 71 weeks for body weight and 97.5 for albumen height; sexual maturity and adult mortality were unaffected but 0 to 20 week % mortality was 5.1/2.0 vs. 6.6/3.7. Ratio of commercial to control stocks (Ā/B̄ × 100) under F, C3 and C5 environments was 116, 125 and 124 for egg mass/hen; 102, 102 and 97 for feed intake; 118, 128 and 129 for egg/feed weight, comparison with RST trends suggests genetic changes from 1958 to 1970 in commercial stocks under F and C environments of about 8 and 14% in both egg number and egg mass/hen housed, none in feed intake, 9 and 16% gain in both eggs/live hen-day and feed conversion, 3 and 8% lighter final body weight, 8% earlier 1st egg, 4% greater albumen height but little change in mortality, egg size or shell strength.