Sire Distribution of Calves in a Beef Herd with Use of Fixed Time Artificial Insemination Followed by Immediate Bull Exposure for Natural Service in Cows and Heifers

Use of fixed time (FT) artificial insemination (AI) followed by immediate exposure of females to bulls for natural service can be a useful management strategy for commercial cow-calf producers to limit labor and time related to bull turnout and increase pregnancy rates earlier in the breeding season. Considering influence of bull fertility and time to and length of estrus in females, expectations for outcomes in natural service sire versus AI sire parentage is relatively unknown. Our objective was to determine the relative percentages of calves sired by either a natural service or FTAI sire within the same estrous period. In two consecutive years, heifers and cows were synchronized and inseminated using the 7-day CO-Synch + controlled internal drug release (CIDR) FTAI protocol. All females were inseminated by one AI technician using one sire for heifers and a different sire for cows. Females were exposed to natural service bulls immediately after insemination. After calving, DNA was collected from a random subset of calves born in the first 21 days of the calving season for parentage analysis (calves born from heifers in Year 1 = 59 and in Year 2 = 82; calves born from cows in Year 1 = 89, Year 2 = 102). The percentage of calves sired by AI and natural service was determined following parentage verification. In Year 1 , for calves born from heifers in the first 21 days of the calving season, 5.1% (n = 3/59) were sired by natural service. For calves born from cows, 14.6% (n = 13/89) were sired by natural service. In Year 2, for calves born from heifers, 9.8% (n =