Journal of Dairy Science, Vol 77, Issue 9 2679-2686, Copyright
© 1994 by American Dairy Science Association
Factors affecting calculation and use of conversion equations for genetic
merit of dairy bulls
R. L. Powell, G. R. Wiggans and P. M. VanRaden
Animal Improvement Programs Laboratory, Agricultural Research Service,
USDA Beltsville, MD 20705-2350.
Factors affecting calculation and use of conversion equations were
reviewed. Methods of expressing reliability of converted evaluations
were surveyed. Of 16 countries responding, 6 did not calculate
reliability for converted evaluations, 5 accepted reliability from
the exporting country, and 5 assumed genetic correlations of .6 to
1.0 with the US. Genetic correlations between the US and 8 other
countries were estimated and generally were > or = .9; estimated
correlations between the US and Canada were 1.0. Estimated
correlations averaged .93 for milk, .89 for fat, and .92 for protein
yields. Correlation estimates were lowest for countries differing
most from the US in management conditions (Australia, New Zealand) or
trait definition (Germany), which suggests that correlation estimates
< 1.0 indicate differences in trait measurement as well as
differences in biological expression. Conversion equations were
computed from data of US and Canadian Holstein bulls with and against
the gene flow. Equations against the gene flow generally had
regression coefficients and intercepts lower than those calculated
with the gene flow. Lower regression coefficients were explained by
selection on the dependent variable. Lower intercepts were attributed
to preferential treatment of daughters from imported semen, which
would lower intercepts for equations against the gene flow and
inflate intercepts with the gene flow.
