Estrous synchronization and artificial insemination have potential to increase genetic improvement, calf uniformity and improve calf weaning weight in beef cattle operations. Synchronization programs utilizing exogenous progestins (CIDR® or MGA®), gonadotropin releasing hormone (GnRH) and prostaglandin (PG) have been shown to be effective for estrous synchronization in beef heifers and can help induce some prepubertal heifers to cycle. While many options exist for synchronization of estrus, a common method utilized in heifers is known as MGA Select. The MGA Select protocol involves feeding MGA for 14 days followed by an injection of GnRH (Cystorelin®, Factrel®, OvaCyst® or Fertagyl®) on day 26 and an injection of PG (Lytalyse®, Estrumate®, estroPLAN® ProstaMate® or InSynch®) on day 33 (Figure 1).
Figure 1. MGA Select synchronization protocol.
Reports1 have shown this method to be effective for heifers with AI pregnancy rates near 71% and seasonal pregnancy rates over 90%. However, advanced planning is needed, as this protocol extends for a 33-day period before PG injection. Recently, a study2 was conducted at the University of Arkansas to evaluate synchronization methods similar to MGA® Select but using two different progestin sources (MGA and CIDRs) as well as shortening the entire length of the protocol.
Beef heifers were sorted into two treatment groups. Treatment one (T1) received a progesterone insert (CIDR) from day 0 to 14, GnRH on day 16 and PG on day 23. Heifers assigned to a treatment two (T2) were fed MGA (0.5 mg/head/day) from day 1 to 14, received GnRH on day 16 and PG on day 23 (Figure 2). At the time of PG dosing, each cow or heifer was equipped with a heat detection patch to improve determination of the onset of standing estrus. During a 96-hour period following PGF treatment, all heifers were observed at least twice daily for onset of standing estrus. Time of onset of standing estrus and AI were recorded for each heifer. Artificial insemination was performed by one experienced technician at ~12 hours after onset of standing estrus was observed. Then fertile bulls were placed into breeding pastures for “clean-up” service. Conception rate to AI was evaluated 70 days after the end of the synchronization period by ultrasonography. Differences in fetal crown-rump length were used to identify pregnancies resulting from AI service or from pasture bulls. A second ultrasonography was conducted 30 to 45 days later to confirm overall seasonal pregnancy rate.