Fertility Issues in High Producing Cows

The stress of high milk production, along with increasing herd size and changes in facilities and management, has made fertility one of the main focuses of genetic improvement today. In the US, the reduction in calving rate to first service and the impact of this decrease in fertility on the Holstein breed, mainly in the last 20 years, is of major concern [27]. Figure 1 shows the clear trend for increases in milk yield and reduced conception rates in Jersey and Holstein breeds over time. Milk production per cow has nearly doubled in the past 40 years, and an increase in 40 days open per lactation occurred in the same period (Figure 1). Because there is a positive correlation (roughly +0.35) between milk yield and days open, the task of improving fertility while selecting for high milk production is challenging An important aspect of fertility traits is that they are influenced by both, direct (service sire) and maternal (sire of the cow) effects. In genetic evaluations, the direct effect is regarded as male fertility and the maternal effect as female fertility. Male fertility measures the ability of semen from a service bull to generate pregnancies, while female fertility measures the