Delaying fixed-time artificial insemination on the 7-Day CO-Synch + CIDR protocol to incorporate utilization of sex sorted semen in beef cows

Abstract

The utilization of sex sorted semen allows beef producers to strategically alter the sex ratio of their calf crop and to selectively mate females to generate terminally crossed males or replacement females. This study aimed to evaluate the effect of insemination timing on pregnancy rates in mature beef cows subjected to the 7-Day CO-Synch + CIDR fixed-time artificial insemination (TAI) protocol using either sex-sorted semen for cows in estrus or conventional semen for non-estrus cows, and to evaluate subsequent calf performance. We hypothesized that delaying TAI from 66 hours to 72 hours after prostaglandin administration and CIDR removal in the 7-Day CO-Synch + CIDR protocol would increase conception rate of cows in estrus inseminated with sexed semen and non-estrus cows inseminated with conventional semen. A total of 1782 multiparous suckled beef cows across 12 locations encompassing 15 breeding groups were stratified by age and days postpartum (DPP) and randomly assigned to one of four treatments based on estrus alert patch status (Estrotect; Rockway Inc., Spring Valley, WI): 1) TAI at 66 h with sex sorted semen for cows in estrus (S66, n = 527); 2) TAI at 72 h with sex sorted semen for cows in estrus (S72, n = 487); 3) TAI at 66 h with conventional semen for non-estrus cows (C66, n = 409); or 4) TAI at 72 h with conventional semen for non-estrus cows (C72, n = 359). Estrus was defined as removal of > 50% of the rub-off coating on the patch. Pregnancy was diagnosed via transrectal ultrasonography between 60 and 74 days after TAI, and season long pregnancy rates were diagnosed between 120 and 130 days after TAI. Data were analyzed using the GLIMMIX procedure of SAS for binomial data with cow as experimental unit and the MIXED procedure of SAS for calf performance data. Estrus differed across treatments, with sex sorted treatments achieving 100% estrus attainment as designed, C72 achieving 20.2% and C66 achieving 0%. Cow body condition score differed (P = 0.02) across treatments with C72 cows having a lesser BCS than S66, and C66 tending (P = 0.08) to differ from S66. Days postpartum were similar (P = 0.98) among treatments. Conception rate to TAI was similar (P = 0.70) among treatments with 40.4 ± 5.3% for C66, 42.7 ± 5.4% for C72, 44.4 ± 4.5% for S66, and 46.3 ± 4.6% for S72. Final pregnancy rates differed (P = 0.03) across treatments with the C72 treatment exhibiting a lesser (90.6 ± 1.6%) final pregnancy rate than S66 (95.5 ± 1.4%) but not differing from other treatments. When calf performance was analyzed, calves from dams inseminated with sex sorted semen were heavier (P < 0.01) at weaning (276 vs 259 ± 10.8 kg), had heavier (P < 0.01) 205-Day adjusted weaning weights (250 vs 236 ± 5.1 kg ), gained more weight (P < 0.01) from birth to weaning (243 vs 226 ± 11.4 kg, ), had increased (P < 0.01) average daily gain (1.02 vs 0.94 ± 0.03 kg), larger (P = 0.02) weaning ratios (100.9 vs 97.9 ± 0.82) and tended to be younger at weaning than calves from dams inseminated with conventional semen, respectively. We conclude that delaying insemination time from 66 to 72 hours in the 7-Day CO-Synch + CIDR TAI protocol did not improve TAI pregnancy rates of estrus beef cows inseminated with sex sorted or non-estrus cows inseminated with conventional semen and calves from dams inseminated with sex sorted semen gained more weight than their conventional counterparts, resulting in a greater average weaning weight.