LAUSR

Objectives were to 1) characterize fixed-time AI (FTAI) pregnancy rates using the 5-Day CO-Synch + CIDR protocol in mature, suckled Bos indicus-influenced beef cows, 2) compare FTAI pregnancy rates in the latter to a modified version (5-Day Bee Synch + CIDR; Bee Synch I) that included treatment with prostaglandin F2α (PGF2α) at CIDR insertion on Day 0, and 3) test the hypothesis that elimination of both GnRH-1 at the onset of synchronization and the double dose of PGF on Day 5 (Bee Synch II) would not reduce FTAI pregnancy rates compared to Bee Synch I. For Experiment 1-trial 1, Brahman x Hereford (F-1) cows (n = 168) at least 40 d postpartum (PP; r = 40-92 d) at the time of CIDR insertion were administered the 5-Day CO-Synch + CIDR protocol with FTAI at 72 h after CIDR removal. Pregnancy rates to FTAI averaged 34.9 ± 1.9%. In Experiment 1-trial 2, fall- and spring-breeding Brahman x Hereford (F-1) beef cows (n = 269) were stratified by days PP and assigned randomly to receive either the 5-Day CO-Synch + CIDR (n = 136) or Bee Synch I (n = 133) protocol, with FTAI at 66 h after CIDR removal. Pregnancy rate to FTAI was greater (P < 0.05) in Bee Synch I (52.6 ± 0.9%) than in the 5-Day CO-Synch + CIDR procedure (40.4 ± 5.7%). For Experiment 2, 422 mature Braford, Brangus, Nelore x Brahman, and Brahman crossbred cows (Bos indicus proportion unknown) at 4 locations were treated with Bee Synch I, with FTAI at 66 h. Overall FTAI pregnancy rate averaged 51.7 ± 2.1%. Finally, from 2013 through spring 2018, we used a switchback design using fall- and spring-breeding herds to compare Bee Synch I (402 observations) to Bee Synch II (393 observations). Overall frequency of detected estrus at 66 h using ESTROTECT™ breeding indicator patches was 57.2 ± 2.4%, conception rates of those detected in estrus was 64.4 ± 3.5%, and FTAI pregnancy rates averaged 52.3 ± 2.4%, none of which differed between treatments. Moreover, pregnancy rates to FTAI in both treatments did not differ in cows synchronized between 40 and 80 d PP but increased after 80 d PP (P < 0.05). Bee Synch II, which eliminates GnRH-1 and the double dose of PGF2α on Day 5, results in FTAI pregnancy rates essentially identical to Bee Synch I but reduces synchronization costs and avoids the need for off-label (double dose PGF2α) drug use.