Genetic variance components of birth weight in a herd of unselected cattle

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 (sigma 2A), additive maternal effects (sigma 2M) and a covariance between transmitted and maternal effects (sigma 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, h2M and rAM) were .36, .82 and -.51, respectively, in the overall data set but varied considerably when estimated in the subsets.(ABSTRACT TRUNCATED AT 250 WORDS)