Effect of timing of insemination following CO-Synch protocol with or without progesterone on pregnancy in beef heifers

Abstract

The objective of the study was to determine the effect of 56 or 72 hr insemination following COSynch protocol with or without progesterone on pregnancy per AI (P/AI) in beef heifers. Angus cross beef heifers (n=1,314) were randomly assigned to CIDR-CO-Synch (n=633) and CO-Synch (n=681) synchronization protocol groups. All heifers were given a body condition score (BCS: 1, emaciated; 9, obese) and a temperament score (0, calm, slow chute exit; walk, 1, excited, fast chute exit; jump, trot or run). Briefly, heifers in CIDR-CO-Synch group received a 1.3 g progesterone intravaginal insert (CIDR) and 100µg of gonadorelin diacetate tetrahydrate (GnRH) im, on day 0. On day 7, the CIDRs were removed and a single dose of 25 mg dinoprost tromethamine (PGF2α) was administered im. Heifers in CO-Synch were given 100µg of GnRH im on Day 0 and 25 mg of PGF2α im on Day 7. Heifers in both synchronization groups were randomly assigned to AI56 (n=657) or AI72(n=657) groups and were inseminated at 56 h or 72 h after PGF2α administration, respectively. All heifers received 100µg of GnRH im concurrently at the time of AI. On Day 7 at the time of CIDR removal, all heifers were fitted with estrus detection aids and were observed for estrus, twice daily, from PGF2α administration to AI. Heifers were examined for pregnancy status between 50 and 70 d after AI using ultrasonography. A partial farm budget-type analysis for projected economic outcome was calculated based on mean P/AI differences for each protocol. Accounting for BCS (P<0.01), estrus expression (P<0.01), and estrus expression by synchronization treatment (P<0.05) and time of insemination by synchronization treatment interactions (P<0.001), the P/AI differed between the time of insemination (P<0.01). The P/AI for heifers inseminated at 56 h after PGF2α administration was greater [59.7%, (378/633)] compared to heifers inseminated at 72 h after PGF2α administration [52.1%, (355/681)]. The P/AI did not differ between COSynch and CO-Synch+CIDR groups, 54.8 (373/681) and 56.7% (359/633), respectively (P>0.1). The P/AI for CO-Synch-56, CIDR-CO-Synch-56, CO-Synch-72 and CIDR-CO-Synch-72 were 56.9, 61.3, 50.1 and 55.3%, respectively. Given the mean P/AI differences among different protocols from this study, the economic sensitivity analysis indicated that benefit for implementing the CO-Synch synchronization protocol could be as great as $979 (Table 3) compared to implementing CIDR-CO-Synch protocol. In conclusion, the P/AI were greater for CIDR-CO-Synch-56 compared to CO-Synch-72. However, the economic analysis revealed CO-Synch protocols without CIDR were cost effective.