Parameter estimates for number of lambs born at different ages and for 18-month body weight of Rambouillet sheep

Genetic parameters were estimated using REML with animal models for number of lambs born and 18-mo body weight in Rambouillet sheep. Number of lambs b orn was modeled either as repeated measurements on the same trait or as dif ferent traits at different ages. The original data for number of lambs born were separated according to age of the ewe into two classes: 2 and 3 yr, a nd older than 3 yr. Numbers of ewes with lambing records for the age classe s were 653 and 466 with 1,106 and 1,118 records, respectively. For the data set that included all ages, the number of ewes was 684 with 2,224 records, and for 18-mo body weight the number of ewes measured was 557. For number of lambs born, the animal model included random genetic, permanent environm ental, and residual environmental effects and fixed effects for age of ewe, year of lambing, and month of year of lambing. Lambing day within season w as used as a covariate. For 18-mo body weight, year of birth of ewe was use d as a fixed effect. Actual age in days when the ewe was weighed was used a s a covariate. Estimates of heritability for number of lambs born by age gr oup were .04, for 2- and 3-yr old ewes, and .06, for ewes greater than 3 yr old, from the two-trait (two age of ewe classes) analyses and .06 when all ages were included. Estimates of heritability for number of lambs born fro m the single-trait analyses were somewhat less than estimates from two-trai t analyses. Estimate of genetic correlation between number of lambs born fo r the 2 and 3 yr and the >3 yr classes was near unity (1.00), which suggest s that a repeated measures model for number of lambs born is adequate for m aking selection decisions. Estimate of genetic correlation between number o f lambs born and 18-mo body weight was .35 with a heritability estimate of .48 for 18-mo body weight. The estimate of genetic correlation suggests tha t selection for increased number of lambs born would result in increased 18 -mo body weight.