Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 17, 1994
Publication Date: N/A
Interpretive Summary: Cows that remain productive over a longer life are more profitable, but selection programs for dairy cattle seldom have included traits that directly measure productive life because its heritability is low and it is expressed at a later age than the traits in current selection programs. This study evaluated national productive life data with the statistical model currently used for yield traits and examined properties of the evaluations. Average productive life was found to have decreased by approximately 2 months for cows born during the 1980's compared with previous decades. However, average genetic merit for productive life has increased steadily since 1980 by about .3 months/year. Effect of adjusting productive life data for milk yield also was investigated, but genetic rankings of bulls were similar regardless of the adjustment. Routine USDA calculation of genetic evaluations for productive life was implemented in January 1994. An economic index that combines evaluations for yield and productive life with relative weights of 2.5:1 can increase economic progress by up to 4%. The net merit index was developed to combine productive life evaluations with those for yield and somatic cell score. Selection for productive life complements current selection for high-yielding cows with desirable type. Genetic gains for productive life will increase cow health and allow breeders to increase culling standards further.
A preliminary national evaluation of productive life was computed from records of 11 million Holsteins. Data were a mixture of projected and completed months in milk by 7 years of age. Data were analyzed with the animal model programs currently used for yield traits. Genetic trend for productive life was positive as a correlated response to past selection for yield and type traits. Phenotypic trend was slightly negative. Active artificial-insemination (AI) bulls had predicted transmitting abilities (PTA's) that ranged from -1.7 to +4.3 months and mean reliability of 56%; 3-year-old cows had mean reliability of 29%. Yield and type PTA's were almost uncorrelated with productive life PTA for active AI bulls but had positive correlations (.25 to .36) for all bulls born since 1980. An index that combines yield and productive life PTA's with relative weights 2.5:1 can increase economic progress by up to 4%. Nearly identical progress can result from an index that combines yield and productive life adjusted for yield. Multitrait evaluations might produce higher reliabilities for productive life by inclusion of correlated traits measured earlier in life such as yield, type, and somatic cell score.