Abstract
Within seasonal dairy systems, cows that calve late in the calving season are less likely to become pregnant and maintain a yearly calving interval. Very few studies have examined effective strategies for reproductive management of these late-calving cows. The objectives were to evaluate the reproductive performance of early- and late-calving dairy cows that were either inseminated after observed estrus (control) or enrolled in a timed AI and resynchronization protocol [progesterone (P4) Ovsynch Resynch)]. Early-calving cows calved during the first week of the calving season, whereas late-calving cows calved after 6 wk but were at least 10 d in milk at study commencement. Three dairy herds participated in the study with 391 cows total. Within each calving group, cows were randomly assigned to P4 Ovsynch Resynch or control (no treatment) in a 2 × 2 experimental design. Artificial insemination continued for 6 wk after mating start date (MSD) and was followed by 6 wk of natural service. The interval from MSD to AI was shorter (11.7 vs. 14.7 d) and the 3-wk pregnancy rate (49.5 vs. 21.2%) and the 6-wk pregnancy rate (60.8 vs. 42.4%) were greater in the early-calving compared with the late-calving control cows. By design, synchronized cows received timed AI on MSD and were not included in the statistical analysis of submission rate and interval from MSD to AI. The proportion of cows that received a second AI was not increased by the progesterone-based resynchronization strategy but was greater in early-calving compared with late-calving cows. The 12-wk pregnancy rate was greater (64.5 vs. 45.0%) in the early-calving synchronized cows compared with the late-calving synchronized cows. The interval from MSD to pregnancy was 6 and 12.6 d shorter for synchronized compared with control cows in the early- and late-calving groups, respectively. The results demonstrated reduced reproductive performance of late-calving cows compared with early-calving cows. Nonetheless, a major improvement to reproductive performance was achieved by targeting late-calving cows with a synchronization program, even when cows were only 20 to 50 DIM at first AI. Resynchronization of estrus with a progesterone device only, however, was not sufficient to increase the proportion of nonpregnant cows that received a second AI.