Factors affecting seasonal variation in 90-day nonreturn rate to first service in lactating Holstein cows in a hot climate

The objective was to determine factors controlling seasonal variation in 90 -d nonreturn rate to first service (90-d NRR) including effects of location , milk yield, and weather variables on specific days before and after breed ing. Dairy Herd Improvement Association records on first services from 8124 Holstein cows in south Georgia (GA, n = 7 herds), north Florida (NF, n = 5 ), and south Florida (SF, n = 5) were used. The 90-d NRR was affected by lo cation x month of breeding. The summer drop in 90-d NRR was of lower magnit ude and duration in GA than in NF or SF and of lower magnitude and duration for NF than SF. When cows were grouped according to mature equivalent milk yield, there was a milk yield class x month of breeding interaction. As mi lk yield class increased, the summer depression in 90-d NRR was more pronou nced. In a second series of analyses, effects of average air temperature at d -10, 0, and 10 relative to breeding were evaluated with subsets of cows in which average air temperature on the 10 d before the reference day were cool (<25 degrees C). The 90-d NRR for cows having average temperatures >20 degrees C on d -10 was less than 90 d NRR for cows with average temperatur es less than or equal to 20 degrees C on d -10 (60.1 vs. 36.5%). Similar re sults were found on d 0 (59.6 vs. 41.4%) and d 10 (56.9 vs. 41.1%). Thus, h eat stress before and after breeding, and on the day of breeding, is associ ated with low 90-d NRR.