GnRH-PGF Based Synchronization Protocols Numerous new synchronization protocols currently recommended for cows use gonadotropin-releasing hormone (GnRH) in conjunction with PGF. Each GnRH-based protocol uses the same basic framework, which involves an injection of GnRH followed 7 days later with an injection of PGF. The way animals are subsequently handled for heat detection and breeding is where the protocols begin to vary. It is important to understand the concept of follicular waves in cattle to understand the benefits of GnRH-based synchronization protocols and how they work. Select Synch With the Select Synch System, cows are injected with GnRH and PGF 7 days apart. Heat detection begins 24-48 hours before the PGF injection and continues for the next 5-7 days. The PGF injection is excluded for cows detected in estrus on day 6 or 7. Animals are inseminated 8 to 12 hours after observed in standing estrus. Alternatively, heat detect and A.I. until 48 to 60 hours after PGF and then mass-AI the rest of the herd at 72 hours and give GnRH to those cows that have not exhibited estrus. Major benefits of the Select Synch system are simplicity and tighter synchrony of estrus. Most animals will display standing estrus 2 to 4 days after the PGF injection. Overall, estrus response rates in well-managed beef herds average ~70 to 75% with noadverse effect on conception rates (60 to 70%), resulting in synchronized pregnancy rates that average between 45 and 50%. Select Synch followed by heat detection and 72 hour fixed time A.I. allows producers to maximize potential pregnancy rates while minimizing labor requirements for estrus detection (7,8). Heat detection is used to catch the early cows and to breed the majority of the herd (60 to 70%) to standing heats. Estrous detection can be terminated at 48 to 60 hours after PGF followed by mass-AI of the non-responders at 72 hours with GnRH. This option gives all cows an opportunity to conceive and, compared to strict fixed-time AI options such as Ovsynch and Cosynch, drug costs are reduced as only 30 to 40% of the herd will receive the second GnRH injection. Additionally, if less than 40 to 50% of the herd is detected in estrus by 72 hours, the mass mating can be aborted, saving drugs, money and semen that might otherwise be wasted on anestrous cows. Select Synch resulted in more cows in standing estrus, equal or better conception rates and ultimately more cows pregnant during the synchronized breeding period. These benefits were particularly evident in the anestrous cows where estrous response rates were improved by 25% and conception rates (66%) were comparable to those of cycling cows. The Select Synch system more than doubled the percentage of anestrous cows that became pregnant during the synchronized breeding period. Ovsynch Ovsynch is a fixed-time AI synchronization protocol that has been developed, tested and used extensively in dairy cattle . The protocol builds on the basic GnRH-PGF format by adding a second GnRH injection 48 hours after the PGF injection. This second GnRH injection induces ovulation of the dominant follicle recruited after the first GnRH injection. All cows are mass inseminated without estrous detection at 8 to 18 hours after the second GnRH injection. Across large numbers of dairy cattle, pregnancy rates to Ovsynch generally average in the 30 to 40% range. Although these numbers may not appear impressive at first, it is important to understand them in terms of an applied reproductive management program.Ovsynch pregnancy rates in dairy herds can be significantly improved if cows are set-up or “pre-synchronized” to be in the early luteal phase of the estrous cycle at the time of the first GnRH injection. This can be accomplished with 2 injections of PGF given at 14-day intervals with the last injection administered 12 to 14 days prior to starting Ovsynch. Although Ovsynch allows for acceptable pregnancy rates with no heat detection, it does not eliminate the need for heat detection. Ovsynch treated animals should be observed closely for returns to estrus 18 to 24 days later. Additionally, natural heats can occur on any given day and as many as 20% of cows will display standing estrus between days 6 and 9 of the Ovsynch protocol. Conception rates in these animals will be compromised if bred strictly on a timed AI basis. Cosynch Although Ovsynch has proven to be a reliable timed AI program for beef cows as well, Ovsynch requires four trips through the working chute. Research at Colorado State University demonstrated that comparable pregnancy rates can be achieved with only animal handlings by breeding all cow coinsiding with the second GnRH injection. Thus, the name Cosynch. As with any fixed time AI protocol, results to Cosynch can be variable, but in general range from 40 to 50%. As with Ovsynch, pregnancy rates are maximized if the early heats are visually detected and bred using the AM/PM rule. MGA - PGF System The MGA-PGF system is a time tested, proven method for synchronizing estrus in beef and dairy heifers. Melengestrol Acetate (MGA) is a synthetic form of the naturally occurring hormone, progesterone. For best results, mix MGA with 3 to 5 lbs of a grain supplement and feed at a rate of 0.5 mg/ head/day for 14 days. Top dressing or mixing MGA in a TMR can work, but intake (and thus results) tends to be more variable. Within 3 to 5 days after MGA feeding, most heifers will display standing heat. DO NOT BREED at this heat as conception rates are reduced. Wait 17 to 19 days after the last day of MGA feeding, and inject all heifers with a single dose of PGF. For the next 5 to 7days, inseminate animals 8 to 12 hours after detected estrus. Success of the MGA system depends on adequate bunk space and proper feeding rates so the appropriate dosage is consumed by each heifer on a daily basis. With good heat detection of well-managed heifers at the proper age, weight and body condition, you can expect to achieve synchronized pregnancy rates of 50 to 70%.Because the synchrony of heats following the MGA-PGF protocol can be variable, pregnancy rates to single, fixed time inseminations are also variable. However, very acceptable pregnancy rates (45 to 55%) have been achieved to a single insemination at 72 hours or by double inseminating at 60 and 96 hours following the PGF injection. 1. Heat detect & AI for 3 to 5 days after removal 2. Fixed-time AI & GnRH at 50 to 60 hours after removal 3. Heat detect & AI 72 hours and fixed-time AI of non-responders with GnRH at 72 to 80 hours after removal