Successful Heifer Synchronization Using MGA
Effective estrous synchronization protocols are one of the keys to successful AI projects. Synchronization allows for groups of cattle to be bred at a set time, resulting in more calves born earlier in the calving season.
Most of the recommended protocols include administering some form of progesterone. Progesterone serves to inhibit estrus (heats). Progesterone “sets up” the cow or heifer so that she will respond to the prostaglandin or GnRH that will be used later in the protocol.
Melengestrol acetate, or MGA, is labelled for use in heifers to suppress estrus and was one of the first products used for synchronization. The MGA protocol for heifers involves feeding MGA at a rate of 0.5 mg per head per day for 14 days, followed by a prostaglandin injection 19 days later. The heifers would then be inseminated based on observation of estrus; or inseminated 72 to 84 hours after prostaglandin (along with an injection of GnRH) for fixed-time AI.
The primary advantages to using MGA are cost and simplicity. MGA is considerably less expensive than using a controlled internal drug release (CIDR) insert. Using MGA requires only two trips through the chute; once to give the prostaglandin and once to inseminate the heifer. Three or four trips are required when using CIDRs, depending on the protocol.
There are some disadvantages for using MGA. Using MGA to synchronize estrus requires 36 days from starting the protocol until the heifers are bred, compared to as few as 7 to 9 days for some protocols using CIDRs. The cost of supplemental MGA feeding (including labor) could be an additional expense, especially if heifers are not in a drylot setting. MGA is only labelled for use in heifers, while CIDRs can be used to synchronize estrus in both heifers and cows. Finally, using MGA successfully requires that each heifer consumes the proper amount of feed each day so that the dosage is correct, which is not a concern when using CIDRs.
If heifers do not consume enough of the MGA, there is a greater likelihood that heifers will show estrus during the 14-day feeding period. Over-consumption of MGA affects how well the progesterone clears the heifer’s system, which may delay a heifer’s response to the protocol. Either condition results in an inconsistent response to the synchronization protocol and reduced AI pregnancy rates.
Because achieving the proper dosage is so critical, there are some key factors that should be followed to achieve the best results when using MGA to synchronize estrus. These factors include:
- Bunk Management: Poor bunk management leads to inconsistent intakes and consequently inconsistent results. This is especially important if the MGA is included in a mixed ration as opposed to a hand-fed supplement.
- Make sure that bunk space is not limiting access. Allowing two feet per head would make sure that every heifer has an equal opportunity to eat.
- The supplement containing MGA should be fed at the same time every day.
- Protocols need to be followed precisely. Feeding MGA longer than 14 days or increasing the dosage affects the timing of estrus and can result in poorer pregnancy rates.
- Over-conditioned heifers may show inconsistent responses to MGA.
- Heifers will begin showing signs of estrus 2 to 3 days after MGA feeding is stopped. DO NOT BREED at that time. Conception rates will be very poor if breeding on this heat. Waiting 19 days, giving prostaglandin and breeding will be much more successful.
Source: Warren Rusche, South Dakota State University