Two hatches involving 11,158 pullets from 22 Single Comb White Leghorn strain crosses were used in these experiments: 13 from rapid-feathering (k+/-) and 9 from slow-feathering (K/-) dams, some of which shared common genetic backgrounds; and all from 5 homozygous k+ sire types. Birds were grown intermingled within hatch, and were subsequently caged in two laying houses, one with 30.5 cm x 40.6-cm cages and the other with 40.6 cm x 30.5-cm cages, at densities of three or four birds/cage. Data were collected by cage on hen-day rate of lay from 50% production to 52 wk of age; (HD%), number of eggs produced per hen housed (EHH), percentage mortality (%M) to 52 wk of age; 35-wk BW (BW35); egg weight (EW34), specific gravity (SG34), and percentage bloodspots (BS%34) at 34 wk of age. The effects of hatches or house (H), density (D), maternal feathering type (MFT), genetic strain within MFT (GS/MFT), bank level within house (BL/H), and all possible interactions were included in the analysis model. Hatch, house, and cage type were completely confounded, so no general conclusion can be made concerning the effects of the reverse and standard cages on performance. The H effects were significant for all traits except HD%. Cage D significantly affected all traits except SG34 and BS%34. Pullets housed at three/cage weighed 26 g less, produced 5.5 more eggs that were .3 g/egg lighter, laid at a 1.7% higher hen-day rate, and had 3.6 less %M from 20 to 52 wk of age than those housed at four/cage. The MFT significantly affected all traits except %M, SG34, and BS%34. The k+/- daughters from K/- dams weighed 34 g less, produced at a 2.8% lower HD%, laid 6.9 fewer eggs, which eggs averaged .6 g/egg less than the eggs from the k+/- daughters of the k+/- dams. Interactions were in general small and nonsignificant. However, the H x GS/MFT interactions were significant for HD%, EHH, BW35, %M, and EW34 (P less than .05); their significance is suggestive of the presence of genotype x environment interactions.
Heritabilities, genetic and phenotypic correlations and sire x location interaction were estimated for 14 traits from hierarchial covariance analysis within each of four 5-week hatching periods in 9 years. Numbers averaged 93 sires, each with 61 pullets from 11 or 12 dams of two lines distributed among 5 to 14 single-cage field locations, in each of 36 period-years and totalled 204,235 pullets from 3,354 sires. Inbreedings increased .4 to .1% per year but was uncorrelated with reproductive rate or selection applied. Heritability estimates were very stable over time and between seasons, with empirical SE for means of .4 to 1.6%. Lower heritabilities across than within locations, justify multiple-location progeny tests, especially for egg production, sexual maturity, flightiness, shell defects and blood spots.Genetic correlation (r G ) changed with time only for body weight with production rate (.03 ± .01 per year) and between seasons only for production rate with egg size and shape, with empirical SE of means for r G of .02 to .04. Important antagonistic genetic correlations were those of egg production and sexual maturity with egg size and specific gravity (-.2 to -.4) and of body size with egg size (.4). Corresponding phenotypic correlations were made less antagonistic by favorable environmental correlation. Dam component correlations were made less antagonistic than sire correlations by more favorable correlations for dominance and maternal effects. Favorable genetic correlations increased expected gains from 12-trait index selection less than the unfavorable correlations reduced it. Predicted response in 12-trait breeding value from sire selection was 96% of maximum when only adult mortality, % production, egg size, body size and sexual maturity of progeny were used; addition of excitability and six egg quality items should improve all except specific gravity but reduce progress in egg size.
SUMMARYThe influence of endogenous viral (ev) genes on avian leukosis virus (ALV) infection was studied in ALV-free white leghorn chickens exposed to chicks from ALV shedding dams. The study included four lines, each segregating for one ev gene, one line free of ev genes, and four commercial stocks segregating for a number of ev genes. Genes ev12 and ev21 that produce the complete endogenous virus were associated with significant reductions in antibody response to ALV. In commercial stocks with ev21 in all birds, both antibody response and ALV in oviducts were significantly influenced by the genomic background (stock). In one commercial stock, 37.6% of 133 birds with ev21 and only 5.8% of 120 birds without the gene were test-positive for virus in their oviducts at 180 days of age. No such differences were associated with ev1, ev3 or ev6 that do not produce complete endogenous virus. Thus, virus-producing ev genes were a predisposing factor for shedding ALV.
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