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The objective was to determine the effect of periods of adjustment and duration of performance test on estimating genetic variance parameters for ADG. Variance components were estimated from ADG data collected from 1978 to 1984 on a total of 1,047 Targhee ewe and ram lambs at the U.S. Sheep Experiment Station (Dubois, ID). Across all years, lambs averaged 84 ± 9 d of age and 25 ± 5.4 kg of weight when placed on test. Lambs were provided ad libitum access to a commercial pellet of barley grain and ground alfalfa for 14 wk. Every 2 wk, ADG was recorded. Overall mean ADG for the entire 14-wk period across all years was 249.3 ± 56.5 g. Variance components were estimated from a single-trait animal model using REML for cumulative combinations of time on feed in 2-wk intervals from 4 to 14 wk and varying the adjustment period from 2 to 6 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 and weight at beginning of performance test). As the duration of the performance test increased, phenotypic variances for ADG decreased: 54 and 13 g2 at 4 and 14 wk on test, respectively. Also, estimates of direct heritability increased with extended duration on test: 0.20 ± 0.06 and 0.35 ± 0.07 at 4 and 14 wk on test, respectively. Heritability estimates increased little 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 durations of the performance test were estimated from two-trait models. All genetic correlations among durations of performance test were greater than 0.88 which suggests that all measures of ADG were genetically similar. However, environmental correlations among duration of performance test ranged from 0.31 to 1.00 with the smaller environmental correlations occurring between 4 to 6 wk with 12 to 14 wk on feed. These results indicate that a period of 8 wk or greater was sufficient to observe differences among animals for ADG due to direct genetic effects under this environment.