Skip to main content
ARS Home » Pacific West Area » Dubois, Idaho » Range Sheep Production Efficiency Research » Research » Publications at this Location » Publication #123292
Title: EFFECT OF DURATION OF PERFORMANCE TEST ON VARIANCE COMPONENT ESTIMATES FOR LAMB GROWTH

Author
item Snowder, Gary
item Van Vleck, Lloyd

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/19/2002
Publication Date: 11/5/2002
Citation: Snowder, G.D., Van Vleck, L.D. 2002. Effect of duration of performance test on variance component estimates for lamb growth. Journal of Animal Science 80:2078-2084.

Interpretive Summary: The American sheep industry commonly measures the growth rate of selected rams by measuring their average daily gain in a central performance test. These performance tests have varying lengths of time to measure growth rate of rams. Previous research has shown that too short of a time on test will result in inaccurate estimates of an animal's genetic value to increase the growth rate of his offspring. Therefore, the objective of this project was to determine an appropriate duration of performance test to compare lambs for genetic differences in growth rate. This was achieved by considering different durations of time for adjusting a lamb to a performance test and also different durations of the actual performance test. The results indicated that lambs should be adjusted to the performance test for 2 to 4 weeks and that the actual performance test should range from 8 to 12 weeks for improved comparison of lambs based on genetic differences. This implies that some performance tests can be shortened to reduce expenses and still have accurate estimates of genetic differences between lambs. Conversely, performance tests shorted than 8 weeks may benefit by extending the performance test period.

Technical Abstract: 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. 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). Phenotypic variances for ADG decreased with extended time on feed: 54 and 13 g 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. 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 wks with 12 to 14 wks on feed.