EFFECTS OF INBREEDING AND HETEROZYGOSITY ON PREWEANING TRAITS IN A CLOSED POPULATION OF HEREFORDS UNDER SELECTION

Records of five inbred lines at the Livestock and Range Research Labor atory were used to evaluate effects of inbreeding and heterozygosity o n preweaning traits. Members of each line were descendants of a single founder Hereford bull. A total of 8,065 records of birth weight and 7 ,380 records of preweaning daily gain and weaning weight were analyzed by derivative-free REML using a model that included fixed effects of sex, combination of year and month of birth and parity of dam, with co variates for direct and maternal genetic fractions of inheritance from the genetic groups, inbreeding, and heterozygosity fractions. Heteroz ygosity fractions were computed for crosses between lines. The random model effects were direct and maternal genetic and uncorrelated matern al permanent environmental and temporary environmental. Direct inbreed ing and heterozygosity fractions averaged .098 and .343, and maternal inbreeding and heterozygosity fractions averaged .075 and .294. Regres sion coefficients of traits on direct and maternal inbreeding fraction s were -5.8 +/- 1.1 and -4.7 +/- 1.3 for birth weight, -.189 +/- .031 and -.252 +/- .039 for preweaning daily gain, and -44.5 +/- 6.6 and -5 6.1 +/- 8.4 kg for weaning weight. Estimates for direct heritability, maternal heritability, and direct-maternal genetic correlations were . 37, .12, and -.01 for birth weight; .16, .25, and -.27 for daily gain; and .17, .26, and -.21 for weaning weight. Results suggest that heter osis represents recovery of accumulated inbreeding depression. Results also indicate that selection can overcome inbreeding depression and a ntagonism exists between direct and maternal genetic effects for prewe aning traits.