Genetic Aspects of Milk Yield and Reproductive Performance

The genetic relationship between milk yield and reproductive performance was investigated in 72,187 records with breeding information from 201 California dairy herds. Traits characterizing reproductive performance were days to first and last breeding, days open, and number of services; their heritabilities in primaparous cows were 4, 4, 2, and 1. Genetic correlations between reproductive performance and 60-day, 180-day, and 305-day fat-corrected-milk-yields were positive, indicating that higher producing cows were bred later, took longer to conceive, and required more services per conception. Genetic correlations were highest between measures of reproductive performance and 305-day yield but were less for the 180-day and 60-day yields which were unaffected by pregnancy.