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United States Department of Agriculture

Agricultural Research Service

Title: Estimated Heterogeneity of Phenotypic Variance of Test-Day Yield with a Structural Variance Model

Authors
item Gengler, N - GEMBLOUX AGRIC UNIV
item Wiggans, George
item Gillon, A - GELBLOUX AGRIC UNIV

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 20, 2003
Publication Date: June 1, 2004
Citation: Gengler, N., Wiggans, G.R., Gillon, A. 2004. Estimated heterogeneity of phenotypic variance of test-day yield with a structural variance model. Journal of Dairy Science. 87(6):1908-1916.

Interpretive Summary: Although a common assumption of genetic evaluation models is that the variability within a population is similar, this assumption is often incorrect. This study documented the existence and nature of unequal phenotypic variation of test-day yields of dairy cattle by modeling heterogeneity of variance through a structural variance model jointly with (co)variance estimation for milk, fat, and protein. Maximum heritability for test-day milk yield was estimated to be about 20% around 200 to 250 days in milk; heritabilities were slightly lower for test-day fat and protein yields. Herd-time effects explained 12 to 20% of phenotypic variance and had the greatest impact at start of lactation. Variances of test-day yields increased with time, subclass size, and milking frequency. Test month had limited influence on variance. Variance increased for herds with low and high milk yields and for early and late lactation stages. A method was developed to adjust phenotypic variance for routine genetic evaluation, thereby reducing bias in rankings of animals and providing dairy producers with more accurate information for making breeding decisions.

Technical Abstract: First-lactation test-day milk, fat, and protein yields from New York, Wisconsin, and California herds from 1990 through 2000 were adjusted additively for age and lactation stage. A random regression model with third-order Legendre polynomials for permanent environmental and genetic effects was used. The model included a random effect with the same polynomial regressions for 2 years of calvings within herd (herd-time effect). (Co)variance components were estimated using expectation-maximization restricted maximum likelihood simultaneously with phenotypic variances that were modeled using a structural variance model. Maximum heritability for test-day milk yield was estimated to be about 20% around 200 to 250 days in milk; heritabilities were slightly lower for test-day fat and protein yields. Herd-time effects explained 12 to 20% of phenotypic variance and had the greatest impact at start of lactation. Variances of test-day yields increased with time, subclass size, and milking frequency. Test month had limited influence on variance. Variance increased for herds with low and high milk yields and for early and late lactation stages. Repeatabilities of variances observed for a given class of herd, test-day,and milking frequency were 14 to 17% across nested variance subclasses based on lactation stage.

Last Modified: 12/19/2014
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