LAUSR

Cryopreservation allows for long-term storage of semen; however, it leads to damage of sperm that may result in complete loss of viability or changes that possibly decrease sperm functionality. Liquid semen is not exposed to these stressors and may result in a longer lifespan in the female reproductive tract, thus increasing the range in timing of insemination without affecting fertility. The objective of this study was to compare fertility of liquid and frozen semen when varying the interval from CIDR removal to AI using the 7-day CO-Synch+CIDR protocol for synchronization of time of estrus. Within age group, crossbred cows (n=389) were randomly assigned to insemination at 36 or 60h after CIDR removal with either liquid or frozen semen (36L, 60L, 36F, and 60F) from one of two Angus bulls. Cows were monitored for estrous activity from CIDR removal until 60h thereafter. Cows that failed to exhibit estrus received GnRH (100μg, i.m.), and a blood sample was collected for analysis of estradiol concentration. There was no difference in pregnancy rates when liquid or frozen semen (53% and 52%) was used, but cows inseminated at 60h had a greater (P<0.01) pregnancy rate than those inseminated at 36h (72% and 31%). There was no time of AI by semen type interaction (P=0.57). Estrus was detected in 63%, 61%, 56%, and 62% of 36F, 36L, 60F, and 60L, respectively (only 5% and 1% of 36F and 36L were detected in estrus before insemination). Overall cows that exhibited estrus had a greater pregnancy rate compared with cows that did not (P<0.01; 79% compared with 24%). Among cows that did not exhibit estrus, those inseminated with liquid semen tended to have greater pregnancy rates than those inseminated with frozen semen (P=0.06). Cows that became pregnant had greater (P<0.01) concentrations of estradiol at 60h than those that did not (10.7±0.55 compared with 7.9±0.26pg/mL). In summary, there was no difference in pregnancy success between liquid and frozen semen. However, cows that exhibited estrus and were inseminated at 60h after CIDR removal had greater pregnancy success compared to cows that did not exhibit estrus.