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Genetic Variance Components of Birth Weight in a Herd of Unselected Cattle

US Department of Agriculture¹ and Montana Agricultural Experiment Station, Miles City, MT 59301

Abstract

Two replicates of a randomly selected herd of Hereford cattle have been developed at the Livestock and Range Research Station in Miles City, Montana. Dams were first bred to a random selection of bulls in 1976. For this study, birth weights of 1,012 calves born in 1977 through 1983 were adjusted for sex, age of dam and year effects. Analyses were on the entire data set and a series of subsets. Subsets were determined by parental birth weights, either above (H) or below (L) the overall mean for parents of that sex. Subsets were: H sires x random dams; L sires x random dams; random sires x H dams; random sires x L dams; H sires x H dams; H sires x L dams; L sires x H dams and L sires x L dams. Two additional subsets were termed assortative (H x H and L x L) and disassortative (H x L and L x H) matings. Genetic variance components including additive direct effects (²_A), additive maternal effects (²_M) and a covariance between transmitted and maternal effects (_AM) were estimated by solving simultaneous equations after estimating coefficients for offspring-sire, offspring-dam and offspring-midparent mean regressions and a paternal half-sib covariance. Estimates for heritability of direct effects, heritability of maternal effects and the genetic correlation between transmitted and maternal effects (h²_A, h²_M and r_AM) were .36, .82 and –.51, respectively, in the overall data set but varied considerably when estimated in the subsets. Deviations from normality in the distribution of progeny birth weights in the different data subsets were examined and interpreted as indicating nonadditive gene action. Realized heritabilities measured by both sire (H vs L) and dam (H vs L) differences were less than the regression and paternal half-sib heritabilities, but differences were interpreted as further evidence of nonadditivity. Overall results were interpreted as indicating a relatively constant ²_A effect, but that ²_M, r_AM and other effects were influenced by an interaction of which parent was selected and its relative birth weight in combination with the mating system used.

¹ Fort Keogh Livestock and Range Research Laboratory.