Estimates of genetic parameters for a two-trait twinning and ovulation rate model with genetic groups were: heritabilities of .03 for twinning and .07 for ovulation with a genetic correlation of nearly 1.00 and fractional permanent environmental variances of .06 for twinning and .05 for ovulation rate. Corresponding estimates when group effects were ignored were: heritabilities of .08 and .08 and fractional permanent environmental variances of .02 and .04 for twinning and ovulation, respectively. Twinning rate (%) in the project at the US Meat Animal Research Center has increased in all cows bom in the project by year of calving from 3.4% in 1982 to 28.5% in 1993, a phenotypic increase of 25.1%. The estimated genetic change in twinning of cows by year of calving using the groups model has been 15.2%. The increase in average genetic value by year of birth has been 18.2% in twinning and 15.0% in ovulation rate from 1980 through 1991. Effects for seven selected groups of foundation animals ranged from -6.0% to 33.1% and influenced genetic trend.

L. D van Vleck, K. E Gregory

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 19. Selection and quantitative genetics; growth; reproduction; lactation; fish; fiber; meat., , 28–31, 1994
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