Genetic Evaluations for Cow Fertility
by Kathy Lee, Michigan State University
Genetic evaluations for daughter pregnancy rate (DPR) allow the measurement of genetic differences between bulls in fertility of their daughters.
Genetic selection for dairy cow fertility can be achieved by incorporating PTAs (predicted transmitting abilities) for daughter pregnancy rate (DPR) into sire selection decisions. Daughter pregnancy rate is defined as the percentage of cows eligible to become pregnant in a 21-day period that actually become pregnant. PTAs for DPR have been available since 2003.
Dairy Herd Information Association (DHIA) records for days open are used to calculate PTAs for DPR. For each 1 percentage unit increase in PTA DPR, approximately 4 fewer days open are expected. In a given herd, daughters of a bull with a PTA DPR of +1.0 are predicted to conceive 4 days earlier, on average, than daughters of a bull at +0.0 PTA DPR.
There is significant variation among the U.S. Active AI proven sires for DPR. Using the April 2013 USDA-CDCB (United States Department of Agriculture-Council on Dairy Cattle Breeding) genetic evaluations for Holsteins, the top 10 percent of the bulls averaged +2.4 for PTA DPR. The bottom 10 percent of the Active AI proven Holstein sires had an average PTA DPR of -2.6. This difference of 5.0 DPR percentage units represents 20 days open.
The heritability of DPR is 4 percent which indicates the proportion of observed variation in the trait that is due to genetics. Environmental and management factors have a major influence on reproduction in dairy cows. However, given the significant economic importance of reproduction efficiency on dairy herd profitability and the availability of genetic evaluations for daughter pregnancy rate, sire selection criteria should include the use of bulls that will improve cow fertility. An effective method of selecting for improved cow fertility is to incorporate DPR with other economically important traits into a selection index such as Net Merit (NM$), Total Performance Index (TPI) or Jersey Performance Index (JPI). Based on information presented at Michigan State University Extension workshops in February, the relative weighting for DPR in NM$ is 11 percent.
Dairy cow fertility can be improved through genetic selection, although the rate of progress for daughter pregnancy rate will be relatively slow due to its lower heritability. Dairy producers and herd managers should use an economic index as the primary tool for selecting high-ranking sires. PTAs for DPR can be used as a secondary selection tool if additional emphasis on the trait is desired (e.g. to screen low fertility bulls).
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