ASAS / ADSA Extension Education and PSA Extension and Instruction : Dairy , Swine , and Poultry 297

Variance components were estimated from Targhee data collected from 1978 to 1984 on a total of 1,047 ewe and ram lambs at the U.S. Sheep Experiment Station, Dubois, ID. Lambs were fed for 14 wk with ADG recorded every 2 wk. Lambs were provided ad libitum access to a commercial pelleted feed of barley grain and ground alfalfa. ADG for the entire 14-wk period across all years was 249.3 ± 56.5 g. Variance components for ADG were estimated from a single trait animal model using REML for sequential combinations of duration of time on feed (n = 9) with a range of 4 to 14 wk. The model included fixed effects for year (1978 to 1984), sex of lamb (ewe or ram), and genetic line (selected or control), and two covariates (age on test and initial weight at beginning of feeding). Phenotypic variances for ADG decreased with extended time on feed: 0.54 and 0.14 g2 at 4 and 14 wk on test, respectively. Estimates of direct heritability increased with extended time on feed; 0.20 ± 0.06 and 0.35 ± 0.07 at 4 and 14 wk on test, respectively. There was little increase of direct heritability estimates after 8 wk on feed (0.33, 0.33, 0.38, and 0.35 for 8, 10, 12, and 14 wk, respectively.) Genetic and environmental correlations among duration of feeding combinations (n = 16) were estimated from a two-trait model. All genetic correlations among duration of feed groups were greater than 0.82 and infer that all measures of ADG were genetically similar. However, environmental correlations among duration of feeding groups ranged from 0.41 to 1.00 with the smaller environmental correlations occurring between 4 or 6 wk and 12 to 14 wk on feed. These results indicate that a period of 8 wk or greater is sufficient to observe differences among animals for ADG due to direct genetic effects under this environment.