Breeding With MGA

Research shows MGA can improve estrous synchronization and AI results.

Looking for a way to improve results of your AI program? You may want to evaluate one of the three following heat synchronization systems that animal scientists have developed in recent years.

At the foundation of all of them is the use of a progestin called melengestrol acetate — also known as MGA. When fed to cows and heifers, it suppresses heat and prevents ovulation.

Although other types of progestin treatments can be substituted in estrous synchronization systems, researchers at the University of Missouri (UM) believe MGA works well for several reasons:

  • It's economical to use, costing about 2¢/animal/day to feed.

  • MGA was recently cleared for use in reproductive classes of beef and dairy cattle.

  • The methodology and understanding of the use of MGA is documented in extensive research dating back to the early 1960s.

  • MGA is easily mixed into feed and doesn't require animals to be handled or restrained during administration.

Until recently, MGA was the only progestin approved for use and available in the U.S., making research of methods to improve and broaden the scope of its use all the more significant.

There are disadvantages, too, and producers need to be aware of these, adds UM's David Patterson. For one, anestrous (“non-cycling” cows) that experience a short “false heat” after MGA feeding will need a second prostaglandin (PG) injection. In addition, some cattle fed MGA have an increased incidence of twinning.

Feeding MGA

Producers should feed MGA for at least 14 days with a grain or protein supplement. Either top-dress it onto other feed or batch-mix it with larger quantities of feed, Patterson says.

UM researchers recommend administering MGA in a 3- to 5-lb. carrier of feed. MGA should be fed at a rate of 0.5 mg/animal/day for the recommended feeding period. The carrier containing MGA should be fed one time per day at approximately the same time each day.

Producers should pre-feed their cattle using the carrier without MGA for several days to get cattle accustomed to consuming feedstuff in this manner.

For best results, producers should provide roughly 2 ft. of linear bunk space/animal during the feeding period. Animals should be observed for signs of behavioral estrus each day of the feeding period — as animals approach the feeding area and prior to feed distribution, or after the animals have consumed the carrier containing MGA. Keep in mind that animals exhibiting estrus during the feeding period have not consumed adequate levels of MGA on a daily basis.

Prepare Them For Breeding

To avoid problems when using estrous synchronization systems, producers should select candidate females based on the following criteria:

  • Allow adequate time between calving and the time of synchronization. UM's Freddie Kojima suggests a minimum time of 40 days.

  • Cows must be in average to above-average body condition.

  • Candidate females need to have experienced minimal calving difficulties.

  • Candidate heifers must weigh at least 65% of their projected mature body weight.

  • Reproductive scores for heifers must be assigned no more than two weeks prior to synchronization. Heifers must have reproductive scores of at least three or higher, while at least 50% of heifers need a score of four or better.

Following are the three synchronization systems currently in use. In each system, cattle should come into heat up to 148 hours after the injection.

MGA And Prostaglandin

In this system, suggested for use in heifers and cows, producers should administer PG 19 days after the last day of feeding MGA. Currently, there are four PG products available in the marketplace: Lutalyse, ProstaMate, InSynch and Estrumate.

This treatment places all animals in the late luteal stage of the estrous cycle at the time of PG injection, which shortens the synchronized period and maximizes the conception rate.

“Although a 19-day interval appears to be optimal, 17- to 19-day intervals produce acceptable results and provide flexibility for extenuating circumstances,” says Kojima.

MGA Select

The MGA Select system is suggested for use in cow groups of 100 head or more. Simply feed MGA for 14 days, then administer an injection of GnRH (Cystorelin, Factrel or Fertagyl) on Day 26 and an injection of PG on Day 33.

“The addition of GnRH to the 14- to 19-day MGA plus PG system improves synchrony of estrus, while maintaining high fertility in postpartum beef cows,” Patterson explains.

7-11 Synch

This program is suggested for use in groups of cows of 100 head or less. It involves feeding MGA for seven, rather than 14 days. The administration of PG comes on Day 7, the last day of MGA feeding. Cows then receive an injection of GnRH four days after the PG injection and a second injection of PG seven days after GnRH.

“This recently developed system shortens the feeding period of MGA without compromising fertility,” says UM's Mike Smith. “It improves synchrony of estrus by synchronizing development and ovulation of follicles from the first wave of development.”

“We believe feeding MGA prior to the GnRH-PG estrous synchronization protocol will successfully induce ovulation in anestrous postpartum beef cows and peripubertal beef heifers; reduce the incidence of a short luteal phase among anestrous cows induced to ovulate; increase estrus response, synchronized conception and pregnancy rate; and increase the likelihood of successful fixed-time insemination,” Smith explains.

Patterson adds that if cows fail to exhibit estrus after any of the above systems, producers should re-administer PG 11-14 days after the last PG injection. Then, observe these females for signs of behavioral estrus for an additional seven days. This will help producers maximize breeding results over a two-week period.

Cows inseminated during the first synchronized period should not be re-injected with PG, however.

Benefits Of Ai

“Using AI through one of these systems provides the opportunity to breed mature cows to the best, most proven sires in the business,” Patterson says.

“Producers can make substantial improvements in the quality of their cow herd by doing so,” he adds. “They can also mass-breed heifers to bulls selected for low birthweight EPDs with high accuracy, minimizing the incidence and severity of calving difficulty and decreasing calf loss that results from dystocia. In addition, heifers that conceive during a synchronized period typically wean calves that are older and heavier at weaning time.

“Finally, heifer calves that result from AI can be an excellent source of future replacements facilitating more rapid improvement in the genetic makeup of an entire herd. The advantages by far outweigh other considerations,” he adds.

This article was produced by the National Association of Animal Breeders. Find out more at: or call 573-445-4406.