Fixed time artificial insemination (AI) protocols allow for concentrating labor and time requirements into a few days. Management practices that improve pregnancy rates for fixed time AI are valuable to producers. One standard estrus synchronization protocol that includes the use of timed AI for non-estrus expressing heifers is the melengestrol acetate (MGA) – prostaglandin (PG) program.

Under this protocol, heifers expressing estrus in the first 72 hours following the prostaglandin injection are identified, removed from the herd, and artificially inseminated 12 hours later. At 72 hours, heifers not expressing estrus are given an injection of Gonadotropin Releasing Hormone (GnRH) and immediately artificially inseminated. This study compared the fixed time AI treatment at 72 hours with a GnRH injection to a 19 hour delayed AI following GnRH injection treatment. Previous research has indicated that there may be a benefit to pregnancy rates for delayed insemination following the GnRH injection in non-estrus expressing heifers. See Figure 1 for a visual explanation of the treatments. heifers in this study were exposed for 42 days to “clean-up” bulls 13 days after the timed AI. Heifers expressing estrus and artificially inseminated had a 70% pregnancy rate to AI and an overall 92% final pregnancy rate. Heifers not expressing estrus and artificially inseminated at 72 hours with a GnRH injection had a 56% pregnancy rate to AI and an 89% pregnancy rate overall. Heifers not expressing estrus that were given a GnRH injection at 72 hours and then artificially inseminated 19 hours later had a 47% pregnancy rate to AI and a 91% pregnancy rate overall. See Table 1  for results comparing the three treatments for these heifers.

In summary, delaying artificial insemination for 19 hours after GnRH injection did not improve pregnancy rates. The standard treatment of giving non-estrus expressing heifers at 72 hours an injection of GnRH and immediately artificially inseminating them was statistically similar to the delayed artificial insemination treatment. The standard treatment reduces labor when compared to the delayed treatment as heifers can be given the GnRH injection and artificially inseminated at the same time. For more information on recommended estrus synchronization protocols visit the Resources page at the Applied Reproductive Strategies in Beef Cattle website.

In an attempt to find AI protocols that yield acceptable pregnancy rates with less labor and cattle handling as AI protocols using estrus detection, a study was conducted the following year at the same location. Timed AI was compared with a combination of time AI and modified estrus detection. For more information on the study, please read “To Estrus Detect or Not Was the Question” In the May Issue of BeefWatch.