Impact of days in milk at the initiation of ovulation synchronization protocols on the efficiency of first AI in multiparous Holstein cows

The objective of this study was to evaluate the impact of days in milk (DIM) at the initiation of ovulation synchronization protocols [spontaneous estrus (n = 899), Presynch (n = 1277), and CIDRsynch (n = 978)] on fertility indices of the first AI in multiparous Holstein cows. The days from calving to the initiation of timed-Al (TAI) were categorized into: < 50 days (D-1), 51-65 days (D-2) and 66-80 days (D-3). Pregnancy rate (P/AI at day 75) of cows at D-2 period was significantly greater than that in the D1 period [OR (odds ratio) = 1.26; P = 0.050). Additionally, embryonic loss rate was significantly reduced in D-2 and D-3 groups compared to the reference group (OR = 0.68 and 0.63; P = 0.012 and 0.001, respectively). Conception (P/AI at day 28) and pregnancy rates in the Presynch group were significantly increased from 26.5 and 19.3% at D-1 period to 35.9 and 28.9% at D-2 period, respectively (OR = 1.59 and 1.72; P = 0.009 and 0.001, respectively). In contrast, conception and pregnancy rates using the CIDRsynch protocol did not differ at the different DIM at the initiation of TAI. The results of the Cox regression model reported significant associations for parity and season of calving with the hazard of embryonic loss (P = 0.029 and 0.036, respectively). The current results suggest that using the Presynch regimen may achieve satisfactory conception and pregnancy rates at 51-65 days in milk; however, the use of CIDRsynch regimen may provide a stable and homogenous conception and pregnancy rates at the different DIM at initiation of TAI.