Heritability estimates of calving date in Hereford cattle maintained on range under natural mating.

Abstract

Data consisting of 1,650 calving dates over 16 yr on an Ozark upland range were used to estimate heritability of calving date in Hereford cattle. Exposure classes were: 1, a heifer's first breeding season; 2, a cow's second breeding season; 3, a cow's third breeding season; and 4, all breeding seasons over three. Variance components were estimated in two stages from mixed models including year and service sire or sire of dam. Variance components were estimated for each exposure class from data excluding open cows (£) and from data including open cows (I). Components of variance for service sire and sire of dam were obtained from separate analyses. Service sire accounted for the largest percentage of variation in each exposure class and in both data sets (15.64 to 59.90%). Sire of dam accounted for only a small portion (<10%) of the variation. Paternal half-sib estimates of heritability were calculated for exposure classes 1, 2 and 4. Heritability estimates (h2E) and h2I) were low for exposure classes 2 and 4; h2E and h2I for exposure class 1 were .20 ± .16 and .39 ± .21, respectively. By including open cows in the evaluation, more genetic variation appeared to exist. More variation also was present in first calving date than in calving date of older cows. These data suggest that selection for calving date among first-calf heifers will be effective.