510 Impact of estrus synchronization and fixed-time artificial insemination on calving distribution in Bos indicus influenced beef heifers

Abstract

To determine the effects of estrus synchronization (ES) and fixed-time artificial insemination (TAI) on calving distribution in Bos indicus influenced heifers, 751 Bos taurus × Bos indicus beef heifers were enrolled in a complete randomized design at 2 locations from January to May of 2016. Within location, all heifers were randomly assigned to one of two treatments: 1) SYNCH (n = 371); heifers were exposed to the 5-day CO-Synch + CIDR protocol where they received a 100-µg injection of GnRH (Factrel; gonadorelin hydrochloride; Zoetis Animal Health), 25 mg of PGF2α (Lutalyse; dinoprost tromethamine; Zoetis Animal Health), and a controlled internal drug releasing (EAZI-BREED CIDR; 1.38 g of progesterone; Zoetis Animal Health) insert on d 0, heifers received 50 mg of PGF2α (Lutalyse; dinoprost tromethamine; Zoetis Animal Health) at CIDR removal on d 5, followed by a 100-µg injection of GnRH and TAI 66 ± 2 h later on d 8; or 2) CONTROL (n = 380); heifers were exposed to natural service without any ES or TAI. On d 9, all heifers were exposed to bulls for the remainder of the breeding season at each location. Blood samples were collected on d -9 and on d 0 to determine pretreatment estrous cyclicity (progesterone ≥ 1.0 ng/mL). Pregnancy was diagnosed via transrectal ultrasonography 54 d after TAI by determining the presence of a viable fetus. Fetal age was estimated based on fetal size and structural features at the time of pregnancy diagnosis. Pregnancy rates on d 54 differed (P < 0.001) between locations, but did not differ (P = 0.777) between CONTROL and SYNCH treatments (64.5 vs 65.7%, respectively). Pregnancy rates were greater (P < 0.001) in cycling compared with non-cycling heifers (63.9 vs 42.4%). A greater (P < 0.01) proportion of SYNCH heifers became pregnant in the first 21 d of the breeding season compared with CONTROL heifers (52.2 vs 46.4%). Overall breeding season pregnancy rates did not differ (P = 0.982) between treatments. In summary, ES and TAI increased the percentage of heifers that conceived in the first 21 d of the breeding season, and therefore, potentially altered the calving distribution by ensuring that more heifers calve early during the subsequent calving season.