Montana State Univ. workers used 53 first-calf suckled beef cows to determine if long-term bull exposure before estrus synchronization (ES) or short-term exposure during and after ES affects AI pregnancy rate. One-half of the cows were exposed to bulls for 40 days (BE) and half were not (NE). After 40 days, each group was further subdivided into groups that were either exposed to bulls for 20 days during and after ES or not exposed for this 20-day period.

The percentage of cows cycling after 40 days did not differ between BE and NE treatments; however, significantly more BE than NE cows began cycling before the start of the ES protocol.

AI pregnancy rate was significantly greater for long-term exposed cows than for short-term (80.8% vs. 53.9%) exposed females.

The authors concluded that short-term bull exposure during and after estrous synchronization did not affect AI pregnancy rate, but breeding performance was enhanced by exposing cows to bulls for 40 days prior to the initiation of the synchronization protocol.