Estrus synchronization and fixed-time artificial insemination alter calving distribution in Bos indicus influenced beef heifers.
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To determine the effects of estrus synchronization (ES) and fixed-time artificial insemination (TAI) on calving distribution in Bos indicus influenced heifers, 751 Bos taurusBos indicus beef heifers were enrolled in a completely 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 were treated with 100g of GnRH, 25mg of PGF2, and a controlled internal drug releasing insert (CIDR) on d 0; heifers received 50mg of PGF2 at CIDR removal on d 5, and were treated with 100g of GnRH and TAI 662h later on d 8; or 2) CONTROL (n=380); heifers were exposed to natural service without 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 (progesterone1.0ng/mL). Pregnancy was diagnosed via transrectal ultrasonography 54d 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. Pregnancy rates on d 54 were greater (P<0.001) in cycling compared with non-cycling heifers (63.9 vs 42.4%). A greater (P<0.05) proportion of SYNCH heifers became pregnant in the first 19d 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 19d of the breeding season, and therefore, potentially altered the calving distribution by ensuring that more heifers calve early during the subsequent calving season.
