|Van Vleck, Lloyd|
Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2000
Publication Date: N/A
Citation: N/A Interpretive Summary: Are fleece weights at 1 year of age controlled by the same genes as fleece weights at later ages? A quantitative answer to this question is contained in the genetic correlation between fleece weights of an animal's genotype at different ages. This study of about 1,500 fleece weights of Rambouillet ewes found genetic correlations among yearling, two and three year old, and dgreater than three year old fleece weights to be from .88 to .97. These correlations are large enough to conclude that fleece weights at different ages can be considered to be one and not different traits. Savings in computational costs for genetic evaluations would be substantial with little loss in genetic progress by using a repeated measures model rather than a multiple (age group) trait genetic model. These results confirm those previously found from USDA data for Rambouillet and from Australian data for Merino sheep.
Technical Abstract: Greasy fleece weight of Rambouillet sheep was modeled either as repeated measurements on the same trait or as different traits when measured at different ages. The original data were separated according to the age of the ewe at shearing into three classes; 1 yr, 2 and 3 yr, and older than 3 yr. An animal model was used to obtain estimates of genetic parameters with a REML algorithm. Total numbers of animals in pedigrees for the different age classes were 696, 729, and 573, respectively and 822 for the repeated measures model across ages. The animal model included direct genetic, permanent environmental, and residual environmental random effects and fixed effects for age of ewe, shearing date as contemporary group, and number of lambs born. Days between shearing was used as a covariate. Single-trait analyses were initially done to obtain starting values for multiple-trait analyses. A repeated measures model across ages was also used. Estimates of heritability by age group were .42, .50, and .58 from three-trait (age class) analyses and for the repeated measures model the estimate was .57. Estimates of genetic correlations between fleece yields for 1 yr and 2 and 3 yr, 1 yr and >3 yr, and 2 and 3 yr and >3 yr classes were .88, .89, and .97, respectively. These estimates of genetic correlations suggest that a repeated measures model for fleece weight is adequate for making selection decisions.