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ARS Home » Pacific West Area » Dubois, Idaho » Range Sheep Production Efficiency Research » Research » Publications at this Location » Publication #124926
Title: EFFECT OF DURATION OF FEEDING ON VARIANCE COMPONENT ESTIMATION FOR LAMB GROWTH

Author
item SNOWDER, GARY
item VAN VLECK, LLOYD

Submitted to: Journal of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: 2/24/2001
Publication Date: 3/1/2001
Citation: Snowder, G.D., Van Vleck, L.D. 2001. Effect of duration of feeding on variance component estimation for lamb growth [abstract]. Journal of Animal Science (Suppl. 1):71.

Interpretive Summary: In the United States, rams are often placed in performance testing programs to measure traits of general interest. The most common trait of interest is growth rate as measured by average daily gain. The duration of a performance test is not standardized in the United States. There has been some question about the effect of length of a performance test for average daily gain on estimating the breeding value of rams. Therefore, the objective of this study was to determine if altering the length of time the rams are adjusted to the performance test and the actual length of the performance test period biases the ranking of rams for genetic merit. The results indicate that for the particular performance test program examined the length of the adjustment period and the performance test can indeed influence the genetic parameters being estimated. It was shown that an adjustment period of over 4 weeks is unnecessary and that a performance test for average daily gain should extend for a minimum of 8 weeks. Also, those conducting performance tests for average daily gain may consider the cost savings of reducing a long term performance test to a shorter term while still providing an accurate measure of genetic differences.

Technical Abstract: 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 pellet 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 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 g at 4 and 14 wk on test, respectively. Estimates of direct heritability increased with extended time eon 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 infers that all measures of ADG were genetically similar. Environmental correlations among duration of feeding groups ranged from 0.41 to 1.00 with the smaller environmental correlations occurring between 6 or 4 wk with 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.