|Van Vleck, Lloyd|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/1997
Publication Date: N/A
Interpretive Summary: Lines of livestock can be developed that have high relationships to a founder animal. Animals in a line area related so that matings within a line result in inbreeding. Build-up of inbreeding within a line due to matings of related animals usually leads to decreased performance. Matings between animals of different lines may result in heterosis (if the cross- line average exceeds the average of the parent lines). Theory suggests that heterosis is recovery of effects of accumulated inbreeding depression. This study involved records of 8,065 animals of five lines of Hereford cattle and crosses among lines after line development. Records came from the USDA-ARS Fort Keogh Livestock and Range Laboratory in Montana. The results showed that 1) inbreeding of animal and dam resulted in substantial decreases in birth and weaning weight, 2) crossing of lines recovered almost completely the accumulated effects of inbreeding depression, and 3) gain in genetic value due to selection overcame depression in performance due to inbreeding.
Technical Abstract: Five lines founded by single bulls at the USDA-ARS Fort Keogh Livestock and Range Research Laboratory, Miles City, Montana, were used to evaluate effects of inbreeding and heterozygosity. Members of each line were all 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, parity of dam, with covariates for direct and maternal line fractions, inbreeding and heterozy- gosity coefficients. Heterozygosity coefficients were computed for crosses between lines. Random effects were direct and maternal genetic and un- correlated maternal permanent environment. Individual inbreeding and heterozygosity coefficients averaged 9.8 and 34.3, and maternal inbreeding and heterozygosity 7.5 and 29.4%. Birth weight was reduced by 5.8 plus/ minus 1.1 and 4.7 plus/minus 1.3, preweaning daily gain by .19 plus/minus .03 and .25 plus/minus .04 and weaning weight by 44.5 plus/minus 6.6 and 56.1 plus/minus 8.4 kg for a change from zero to 100% of direct and mater- nal inbreeding, respectively. Coefficients of partial regression on heterozygosity were near zero, holding inbreeding constant. Estimates for direct heritability, maternal heritability, and direct-maternal genetic correlation were .37, .12, -.01 for birth weight; .16, .25, -.27 and .17, .26, -.21 for daily gain and weaning weight. Results suggest that heterosis represents recovery of accumulated inbreeding depression. Results also indicate selection can overcome inbreeding depression and that antagonism exits between direct and maternal genetic effects for preweaning traits.