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Journal of Dairy Science, Vol 74, Issue 12 4344-4349, Copyright 1991 by American Dairy Science Association


JOURNAL ARTICLE

Expansion of projected lactation yield to stabilize genetic variance

P. M. VanRaden, G. R. Wiggans and C. A. Ernst
Animal Improvement Programs Laboratory, USDA, Beltsville, MD 20705-2350.

Information on partial lactations often is included in genetic evaluations by predicting the cow's eventual 305-d yield. Such projected yields have less phenotypic and genetic variation than completed yields but were modeled as having greater or equal variation in evaluations. Analysis of first lactations from 48,424 daughters of 844 Holstein sires indicated that yields predicted early (46 to 75 d) in lactation had less than one-half as much additive genetic variance as completed yields. Multiple-trait REML estimates of genetic correlations of predicted and completed yields were all above .92, indicating that early lactation information is valuable if modeled appropriately. Expanded records with genetic variances equal to those of completed yields and new lactation length weights were derived. Expanded records have larger error variances than either projected or completed yields and, thus, are given less weight when included in animal model evaluations. Genetic gains are expected to increase only .2 to .3%, but more stable genetic evaluations should result from use of expanded records, particularly for animals evaluated primarily from first lactation records in progress.




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Copyright 1991 by the American Dairy Science Association.