Sire Distribution of Calves in a Herd with use of Fixed Time Artificial Insemination Followed by Immediate Bull Exposure for Natural Service

Abstract Use of fixed time 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 as well as increase pregnancy rates earlier in the breeding season. Considering potential influence of factors such as 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. We hypothesized that of calves born in the first 14 d of the calving season, less than 5% would be sired by natural service bulls exposed to females immediately following fixed time AI. Heifers (n = 191) and cows (n = 156) were synchronized and inseminated using the 7-d CO-Synch + CIDR fixed time AI protocol. All females were inseminated by the same AI technician using a single sire for heifers and a different single sire for cows. All females were exposed to bulls immediately following insemination. DNA was collected from a random subset of calves (Calves born from heifers n = 59; Calves born from cows n = 89) born in the first 14 d of the calving season for parentage analysis to estimate the overall percentage of calves sired by AI and natural service. Chi-Square tests were used to determine if the percentage of calves actually sired by natural service differed from the hypothesized 5% or less. Among calves born from heifers, the actual percentage sired by natural service (5.1%; n = 3/59) was similar (P = 0.98) to our hypothesized percentage. Among calves born from cows, the actual percentage sired by natural service (14.6%; n = 13/89) was greater (P < 0.001) than our hypothesized percentage of 5%. If commercial producers use fixed time AI followed by immediate bull exposure in cows, natural service bulls may sire more calves early in the calving season than expected.