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1 Department of Dairy and Poultry Sciences, University of Florida, Gainesville, FL 32611-0920
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 breeding. 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 location x month of breeding. The summer drop in 90-d NRR was of lower magnitude 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 milk yield class increased, the summer depression in 90-d NRR was more pronounced. 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°C). The 90-d NRR for cows having average temperatures >20°C on d –10 was less than 90 d NRR for cows with average temperatures 
20°C on d –10 (60.1 vs. 36.5%). Similar results were found on d 0 (59.6 vs. 41.4%) and d 10 (56.9 vs. 41.1%). Thus, heat stress before and after breeding, and on the day of breeding, is associated with low 90-d NRR.
Key Words: nonreturn rate • cattle • heat stress • fertility
Submitted on March 25, 1999
Accepted on July 2, 1999
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