Dairy Bull Fertility

Francisco Peñagaricano University of Florida Assistant Professor of Dairy Cattle Genetics & Genomics

Dairy bull fertility receives in general scarce attention. However, the fertility of the bull is critical in determining the reproductive performance of the herd. Indeed, some studies have shown that a significant percentage of reproductive failure is attributable to bull subfertility.

Since August 2008, the US dairy industry has access to a national phenotypic evaluation of service sire fertility called Sire Conception Rate (SCR). This bull fertility evaluation is exclusively based on field data. The current model includes both factors related to the service sire under evaluation (including age of the bull and AI organization) and also factors (nuisance variables) associated with the cow that receives the unit of semen (including herd-year-season, cow age, parity, and milk yield).

The trait SCR is defined as the expected difference in conception rate of a given bull compared to the mean of all other evaluated bulls; in other words, a bull with an SCR value of +5.0% is expected to achieve a conception rate of 36% in a herd that normally averages 31% and uses average SCR bulls. Contrary to evaluations for other traits such as production or cow fertility, SCR is designed as a phenotypic rather than a genetic evaluation, because the published estimates include not only genetic but also non-genetic effects.

Currently, there are about 12,500 Holstein bulls and 1,500 Jersey bulls with official SCR evaluations. Figure 1 shows the distribution of SCR values in these two breeds. Interestingly, there is a remarkable variation in SCR both in Holsteins and in Jerseys; indeed, there are more than 10 points (10% conception rate difference) between the two extremes, i.e., between high-fertility and low-fertility bulls. Our group is investigating potential genetic factors underlying this variation.

Recently, we evaluated the feasibility of predicting SCR using genomic data. Notably, our findings suggest that genomic prediction of service sire fertility is feasible. This could be the foundation for the development of genomic tools that help the dairy industry to make accurate genome-guided decisions, such as early culling of predicted subfertile bull calves.

Figure 1. Distribution of Sire Conception Rate (SCR) values in Holstein (A) and Jersey (B) bull population.

For more information, contact Dr. Peñagaricano at <[email protected]> or call (352) 392-1981 ext. 231.