Impact of initiating a fixed-time AI program on herd calving date and weaning weight

Abstract Text: Improved reproductive efficiency is critical for profitability in cow-calf operations. Cows that give birth early wean more pounds of calf and stay in the herd longer. Thus the objective of this study was to determine the impact of implementing a fixed time AI protocol in a herd that had a defined 60-d breeding season, on the calving distribution and reproductive efficiency. Individual performance data (calving date, birth weight, and weaning weight) were collected annually (yr 1 and 2 no reproductive technology was used; yr 3, 4, and 5 fixed-time AI was used; n = 225 to 296 cows annually). These data were analyzed using the glimmix and mixed procedures of SAS. There was no effect of year (P = 0.34) on calving date during these years (103.6 ± 1.1, 101.7 ± 1.0, 102.8 ± 1.0, 101.0 ± 1.1, and 101.0 ± 1.1 for each year).  However, weaning weights were influenced by year and calf sex (P < 0.01).  Weaning weights were similar between yr 2 and 3 (P = 0.50), but increased each year after fixed-time AI was implemented (212.6 ± 2.2, 220.1 ± 1.9, 222.0 ± 2.0, 242.0 ± 2.1, 265.2 ± 2.2 kg for each year).  Males weaned heavier than females (239.4 ± 1.3 and 225.3 ± 1.4 kg).  In yr 3, calving distribution shifted (P < 0.01) from 1% and 4% (for yr 1 and 2) to 60% calving within the first 21 d.  Over all years, there was a negative correlation between calving date and weaning weight (‑0.53 kg; P < 0.01).  Animals were retrospectively assigned to a group based on if they did or did not conceive to fixed-time AI in yr 3.  Cows that conceived to AI tended (P = 0.08) to calve earlier in both yr 1 (102.0 ± 1.4 and 105.0 ± 1.1) and yr 2 (100.1 ± 1.4 and 102.9 ± 1.1).  Furthermore, cows that conceived to fixed-time AI in yr 3 also calved earlier in yr 4 (P < 0.01; 99.8 ± 1.3 and 104.9 ± 1.4).  Implementing a fixed-time AI program in a herd with a short defined breeding season did not impact Julian calving date, but did increase the proportion of cows giving birth early and pounds weaned per cow.  Cows that conceived to AI during the first year of implementation tended to be the earlier calving cows the previous years.

Keywords: Fixed-time AI; Reproductive efficiency;Calving distribution