Divergent selection for yolk cholesterol in laying hens

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Abstract
Lines were selected for divergence in yolk-cholesterol concentration for 3 and 4 generations in the Cornell Control (Leghorn) and Athens-Canadian (AC) (meat-type) randombred populations, respectively. Selection was based on both individual and family records. Cholesterol values were obtained from 2 consecutively laid eggs when birds were 28 wk of age. The yolks of eggs from individual hens were pooled and analyzed for cholesterol content by the colorimetric assay of Zlatkis et al. (1953) on the fat extract obtained by a modification of the procedure of Folch et al. (1956). The largest differences between high and low lines (1.27 and 1.38 mg cholesterol/g yolk) were observed in the last generation of selection. There was evidence of a lack of response in the low lines. Realized heritability estimates obtained by dividing the difference between high and low lines by half the cumulative selection differential ranged from 0.11-0.25 in the AC population and from 0.21-0.25 in the Leghorn population. Realized estimates on a within-line basis suggest that selection for yolk cholesterol is effective only in the upward direction.