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