Among breeds of dairy cows, the intense selection for higher milk production during the last 20 to 30 years has accentuated the problem of negative energy balance in early lactation. Consequently, as milk production has increased, reproductive efficiency has decreased. Although a conception rate of 50% is considered to be a poor level of reproductive performance today, it is probably above average in the U. S. dairy industry.
Some believe that the decrease in reproductive efficiency is due to genetic selection for milk production. However, research indicates that the conception rate of heifers has remained unchanged for the last 25 years, suggesting that genetic selection for higher milk production is not the cause of lower fertility. The heritability of reproductive traits, such as days open, is very low.
Thus improving reproduction through selection would be very inefficient. It is likely that cows selected for higher milk yield were also selected (indirect selection) for their ability to mobilize body reserve and to ingest more feed. Cows with higher intake in early lactation are likely to have fewer reproductive problems than cows that mobilize large amounts of body reserves. Thus it is probable that selection of cows with higher intake capacity in early lactation may allow for higher milk production with minimal negative effects on reproduction
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