Close observers of sire summaries have noticed, and have been frustrated by, seasonal fluctuations in fertility traits. Scientists at both USDA and the Council on Dairy Cattle Breeding have been frustrated as well, but believe they have both found and fixed the problem.
In the past, a large percentage of young bulls saw their fertility evaluations change, depending on the season of the triannual sire run (April, August or December). The geneticists say the primary reason “was that the seasonal grouping was derived previously from the heifers’ breeding dates instead of cows’ breeding dates.”
The models estimating fertility have been adjusted, and with these adjustments, the seasonal variation no longer occurs. As a result of the changes, the Predicted Transmitting Abilities for both active and genomic bulls will decline slightly for daughter pregnancy rate (DPR), cow conception rate (CCR) and heifer conception rate.
For example, DPRs will decline on average 0.68 for Active AI Holstein bulls, 0.99 for Genomic Holstein bulls, 0.50 for Active AI Jersey bulls and 0.61 for Genomic Jersey bulls. The declines will be even larger for CCR, with Active AI Holstein bulls dropping 1.71, Genomic Holstein bulls declining 2.36, Active AI Jersey bulls declining 1.63 and Genomic Jersey bulls down 1.72. HCR will also be down, but those declines will only be marginal.
You can read more on the changes here.