Is the Time of Insemination Linked with the Reproductive Response of the Animal? Assessment of Follicular Size at Ftai and Behavioral Patterns in Beef Cattle Following 7-Day CO-Synch + Cidr Estrus Synchronization Protocol.

The objective of the present study was to evaluate follicular size at insemination, pregnancy rate and sexual behavioral patterns in beef heifers and cows synchronized with a fixed time artificial insemination (FTAI) protocol. Heifers (n=38) and cows (n=117) were synchronized using a 7 day CO-Synch + CIDR. At d -7 an administration of 100 µg of GnRH and an intravaginal controlled internal drug release (CIDR) were given to all animals. At d 0, the CIDR was withdrawn and one dose of PGF2a (25 mg) was administered. Animals were observed continuously for behavioral signs of heat after CIDR removal until either 64 hours (FTAI for heifers) or 66 hours (FTAI for cows). Ovarian follicle size was determined at FTAI via ultrasonography. Estrus was classified as passive (animal receiving a behavior) and active (animal conducting the behavior) and characterized by display of mounting, attempted mounting, sniffing or licking genitalia, head butting, chin resting, flehmen, and following with sexual intent. Peak of estrus was determined when behaviors per hour (#behaviors/hr) were the highest in comparison with the animal’s own behavioral patterns. Pregnancy rates were 47.4% (18/38) for the heifers and 58.9% (69/117) for the cows. Estrus expression was 86.8% (33/38) of heifers and 59% (69/117) for cows. On average, heifers exhibited a quicker (P< 0.001) onset of passive (43.7± 9.2 hrs) and active estrus (37.7 ± 6.4 hrs) after CIDR removal compared to cows (passive 52.9 ± 7.1 hrs and active 49.7± 6.8 hrs). Heifers peaked faster (P<0.001) (52.2 ± 7.2 hrs.) compared to cows (57.7 ± 5.8 hrs) and had fewer (P<0.001) mounts/hr (0.99 ± 0.8) compared to cows (3.43 ± 2.4). It was observed that heifers exhibited longer (P<0.001) sexual behavior (active 20.0± 9.4 hrs or passive 14.6 ± 7.2 hrs) compared to cows (active 12.1 ± 6.2 hrs or passive 8.9 ± 5.0 hrs). Average ovulatory follicle size was also different (P=0.02) for heifers (12.01 ± 2.5 mm) compared to cows (13.1 ± 2.6 mm). Based on these results it is concluded that under the influence of FTAI protocol the type of animal greatly influences the timing of the onset of heat, estrus response, intensity of estrus, and ovulatory follicle size at the time of insemination. For mature cows the time of insemination appears to be appropriate, however for heifers it remains to be determined if altering the time of AI relative to the onset estrus can impact their fertility.