Development of a national genetic evaluation for cow fertility.

A national fertility evaluation was developed based on pregnancy rate, which measures the percentage of nonpregnant cows becoming pregnant within each 21-d opportunity period. Data for evaluation are days open, which are calculated as date pregnant minus previous calving date. Date pregnant is determined from last reported breeding or from subsequent calving minus expected gestation length. Success or failure of last breeding can be confirmed by veterinary diagnosis or a report that the cow was sold because of infertility. Data are adjusted for parity and calving season within geographic region and time period and evaluated. Fertility records are considered complete at 250 d in milk, and lower and upper limits of 50 and 250 d are applied to days open. For calculation of genetic evaluations, days open are converted to pregnancy rate by the linear formula pregnancy rate = 0.25 (233 - days open). Evaluations are expressed as predicted transmitting ability for daughter pregnancy rate, and calculation is done with an animal model. Genetic correlations among several fertility measures and other evaluated traits were estimated from 3 large data sets. Correlation with days open was less for nonreturn rate than for days to first breeding, probably because nonreturn rate had lower heritability. Cow fertility was negatively correlated with yield but is a major component of longevity. Thus, recent selection for longevity may have slowed the long-term decline in fertility. Direct selection for fertility could halt or reverse the decline.