J. M. Thomas

Delayed insemination of nonestrous cows improves pregnancy rates when using sex-sorted semen in timed artificial insemination of suckled beef cows.

This experiment was designed to test the hypothesis that delayed insemination of nonestrous cows would increase pregnancy rates when using sex-sorted semen in conjunction with fixed-time artificial insemination (FTAI). Estrus was synchronized for 656 suckled beef cows with the 7-d CO-Synch + controlled internal drug release (CIDR) protocol (100 μg GnRH + CIDR [1.38 g progesterone] on d 0, 25 mg PGF2α at CIDR removal on d 7, and 100 μg GnRH on d 10, 66 h after CIDR removal). Estrus detection aids (Estrotect) were applied at PGF2α and CIDR removal on d 7, and estrous expression was recorded at GnRH on d 10. Cows were assigned to 1 of 3 treatments: 1) FTAI (concurrent with GnRH, 66 h after CIDR removal) with conventional semen regardless of estrous expression, 2) FTAI with sex-sorted semen regardless of estrous expression, or 3) FTAI with sex-sorted semen for cows having expressed estrus and delayed AI 20 h after final GnRH for cows failing to express estrus. A treatment × estrous expression interaction was found (P < 0.0001). Higher pregnancy rates (P < 0.0001) were achieved with conventional semen (Treatment 1; 77%) than with sex-sorted semen (Treatments 2 and 3; 51 and 42%, respectively) among cows that expressed estrus. However, among cows that failed to express estrus, delayed insemination with sex-sorted semen yielded higher (P < 0.0001) pregnancy rates than with sex-sorted semen at the standard time (Treatments 2 and 3; 3 versus 36%, respectively). Furthermore, among cows that failed to express estrus, FTAI pregnancy rates when using sex-sorted semen at the delayed time (36%) were comparable (P = 0.9) to those achieved using conventional semen at the standard time (Treatment 1; 37%). These results indicate that delaying AI of nonestrous cows by 20 h from the standard FTAI improves pregnancy rates when sex-sorted semen is used with FTAI.

Control of Estrus and Ovulation in Beef Heifers

Expanded use of artificial insemination (AI) and/or adoption of emerging reproductive technologies for beef heifers and cows require precise methods of estrous-cycle control. New protocols for inducing and synchronizing a fertile estrus in replacement beef heifers and postpartum beef cows in which progestins are used provide new opportunities for beef producers to synchronize estrus and ovulation and to facilitate fixed-time AI. This article reviews the various estrous synchronization protocols currently available for use in replacement beef heifers. New methods of inducing and synchronizing estrus now create the opportunity to significantly expand the use of AI in the United States cowherd.

Comparison of a 16- versus a 19-day interval between controlled internal drug release removal and prostaglandin F2α following a 14-day controlled internal drug releasetreatment and fixed-time artificial insemination in postpartum beef cows1

This experiment compared 2 long-term controlled internal drug release (CIDR)-based protocols to synchronize estrus before fixed-time artificial insemination (FTAI) in postpartum beef cows. Cows were assigned to treatments by age, BCS, and days postpartum. Cows assigned to the 14- to 19-d CIDR-PGF2α protocol (n = 196) received CIDR inserts (1.38 g progesterone [P4]) from d 0 to 14 and PGF2α (25 mg, i.m.) 19 d after CIDR removal on d 33. Cows assigned to the 14-to-16-d CIDR-PGF2α protocol (n = 195) received CIDR inserts from d 3 to 17 and PGF2α 16 d after CIDR removal on d 33. Cows were artificially inseminated on d 36, 72 h after PGF2α, with GnRH (100 μg, i.m.) at FTAI. Cows were exposed for natural service 14 d after FTAI for 75 d. Blood samples for P4 were collected at d -10 and 0 to determine pretreatment estrous cyclicity status and again at PGF2α. Blood samples for estradiol (E2) were collected at PGF2α and FTAI. HeatWatch estrus detection transmitters were used from CIDR removal until FTAI to determine onset of estrus after CIDR removal and PGF2α. Dominant follicle diameter was determined at PGF2α and FTAI. Pregnancy diagnosis was performed 70 d after FTAI and confirmed at d 140 of gestation. Estrous response after CIDR removal was similar between treatments. Cows in both treatments had similar size dominant follicles on d 33 at PGF2α and d 36 at FTAI. Progesterone at PGF2α was greater (P = 0.03) for 14-to-16-d compared to 14-to-19-d treated cows. Mean concentrations of E2 at PGF2α were similar between treatments but were greater (P = 0.01) at FTAI for 14-to-16-d compared to 14-to-19-d treated cows. Estrous response after PGF2α was greater (P < 0.01) for 14-to-19-d compared to 14-to-16-d treated cows (47.4 vs. 29.7%, respectively). Pregnancy rate resulting from FTAI was affected by the treatment × age group interaction (P = 0.08). Pregnancy rate after FTAI among cows ≥ 4 yr tended to be greater (P = 0.06) for 14-to-19-d compared to the 14-to-16-d treated cows, suggesting that the 14-to-19-d schedule works better for older age cows compared with the 14-to-16-d schedule. Final pregnancy rates were similar between the 2 treatments. In summary, these data indicate that a range in intervals from CIDR removal to PGF2α may be feasible when using long-term CIDR-based protocols in cows and raise questions that warrant further study regarding the benefits of extending this interval based on cow age.