Trends and seasonality of reproductive performance in Florida and Georgia dairy herds from 1976 to 2002.

Trends in reproductive performance from 1976 to 2002 were studied for dairy farms located in Florida and Georgia using 2,897,517 Dairy Herd Improvement Association lactation records of Holstein cows. One-half of the 1552 herds in the final edited records had measures for at least 8 yr. Measures of reproductive performance changed significantly over time. Days to first service increased from a low of 84 d in 1983 to 104 d in 2001. Cows that calved during spring had 9.2 (1983) to 33.2 (1999) more days to first service than cows that calved during fall. Annual pregnancy rates (PR) for 71 to 364 d since last calving (DSC; PR(71-364)) decreased from 21.6% in 1977 to 1979, to 12% in 2000 to 2002. The greatest PR(71-364) was observed during winter and the lowest during summer (15.8 vs. 5.6% in 2002, respectively). The absolute difference between PR(71-364) during winter and summer remained similar over time at 11 percentage units. Pregnancy rates in the early stages since calving (71 to 133 d) showed greater decreases over time than PR in the later stages since calving. From 1998 to 2002, PR in the later stages since calving (134 to 364 d) was on average 11.5%. Pregnancy rate from 71 to 133 DSC remained greater (13.4%). In the winter, the decrease in PR(71-364) was primarily due to a large decrease in PR(71-91). Average days to conception increased from a low of 121 in 1982 to a high of 167 in 1998. The average difference between cows that calved during spring and fall increased from 22 d in 1976 to 47.5 d in 1986, but remained constant at 39.1 d from 1985. Average calving interval increased from 399 d in 1976 to 429 d in 2000. Average days dry between 1976 and 2001 remained similar at 69 d. Days to culling of nonpregnant cows after 182 DSC increased from 341 in 1983 to 415 in 1998. Season of calving had no clear association with average days to culling. The last milk yield recorded less than 1 mo before culling of nonpregnant cows after 182 DSC decreased by DSC to approximately 1 yr since calving, after which it remained constant at an average of 12.3 kg/d. The last known milk yield of cows culled during spring was 1.6 kg/d greater than those culled during fall. This difference did not significantly change over time. Increases in actual 305-d herd milk production were associated with increased days to first service, days to conception, and calving interval, but also with increased PR(71-364). Herd size did not have a clear association with reproductive efficiency.