A University of Nebraska study shows fixed-time AI can provides a viable alternative to modified heat detection in beef heifers. Synchronization with fixed-time insemination can reduce labor and animal handling when compared with AI systems that use estrus detection, but some tests have shown reductions in AI conception rates.
In a head-to-head comparison, a group of University of Nebraska researchers recently conducted a trial with 971 Angus-cross heifers managed in three groups, including drylot and pasture development. They synchronized all the heifers by feeding MGA for 14 days followed by a prostaglandin (PG) injection 19 days later on day 33. At the time of the PG injection, the researchers applied estrus-detection patches to all the heifers, and at breeding, scored each animal for estrus. Heifers with 0 to 50% of the patch worn off received scores of 1 or 2 and were not considered in estrus. Heifers with 50 to 100% of the patch worn off received scores of 3 or 4 and were considered to have expressed estrus. At 72 hours after the PG injection, the researchers inseminated half the heifers, randomly selected, regardless of their heat status based on the patch scores. For the other half, the researchers evaluated heifers for estrus at 58 and 70 hours after PG injection and inseminated them in the following order:
- Heifers in estrus at 58 hours post- PG.
- Heifers in estrus at 70 hours post- PG.
- Heifers not expressing estrus at either observation. These received GnRH at AI.
Thirteen days following AI, the researchers placed clean-up bulls with all the heifers at a bull to heifer ratio of 1:50 for a 42-day breeding season. A minimum of 51 days after AI, the researchers weighed the heifers and diagnosed pregnancy using transrectal ultrasonography. Heifers not pregnant by AI were diagnosed for pregnancy again 45 days following bull removal.
In this trial, both the timed-AI and the estrus-detection groups had an AI pregnancy rate of 62%. Total pregnancy rates after exposure to bulls were 96% for fixed-time AI and 97% for estrus detection, with the difference not statistically significant.
Noting that added labor has limited adoption of synchronization and AI among beef producers, the researchers conclude that fixed-time AI could limit cattle handling and eliminate estrus detection without compromising conception rates.
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