|Van Vleck, Lloyd|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/10/2000
Publication Date: N/A
Interpretive Summary: Estimates of genetic correlation between number of lambs born to ewes at two and three years of age and older than three years of age was near unity. Therefore, number of lambs born could be modeled as a single repeated measures trait for genetic evaluations although the phenotypic variance is greater for the older age class. The estimate of genetic correlation between number of lambs born and 18 mo body weight was moderat suggesting that selection for either increased number of lambs born or increased 18 mo body weight would result in an increase in the other trait.
Technical Abstract: Genetic parameters were estimated using REML with animal models for number of lambs born and for 18 mo body weight in Rambouillet sheep. Number of lambs born was modeled either as repeated measurements on the same trait or as different traits at different ages. Data were separated according to the age of the ewe into two classes; 2 and 3 yr, and older than 3 yr. Numbers of ewes with records for the different age classes were 653 and 466. For 1 mo body weight, number of ewes measured was 557. For number of lambs born, the model included random genetic, permanent environmental, and residual environmental effects and fixed effects for age, year and month. Lambing day within season was used as a covariate. For 18 mo body weight, year of birth was a fixed effect with age in days when weighed as a covariate. Estimates of heritability for number of lambs born by age group were .04 and .06 from two-trait (two age of ewe classes) analyses and for the repeated measures model the estimate was .05. Estimates of heritability fo number of lambs born from the single-trait analyses were somewhat less than estimates from two-trait analyses. Estimate of genetic correlation between number of lambs born for the 2 and 3 yr and the >3 yr classes was near unity (1.00) which suggests that a repeated measures model for number of lambs born may be adequate for making selection decisions. Estimate of genetic correlation with repeated measures model between number of lambs born and 18 mo body weight was .35 with heritability estimate of .48 for 18 mo body weight. The estimate of genetic correlation suggests that selection for increased number of lambs born would result in increased 18 mo body weight.