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MGA CAN IMPROVE A.I. AND ESTRUS SYNCHRONIZATION RESULTS

by: Eric Grant

Looking for a way to improve results of your artificial insemination program? Then you may want to evaluate -- and then incorporate -- one of the three following heat synchronization systems that animal scientists have developed in recent years.

At the foundation of these systems is the use of a progestin called MGA -- also known as melengestrol acetate. When fed to cows and heifers, it suppresses heat and prevents ovulation. Although other types of progestin treatments can be substituted in estrus synchronization systems, researchers at the University of Missouri believe MGA works well for several reasons:

First, MGA is economical to use; it costs about two cents per animal per day to feed.

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

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

Fourth, MGA is easily mixed into feed and does not require that animals be handled or restrained during administration.

And fifth, perhaps more importantly, until recently MGA was the only progestin approved for use and available in this country, 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 Dr. David Patterson: Anestrous, or "non-cycling" cows, that experience a short "false heat" after MGA feeding will need a second PG injection. Some cattle fed MGA have an increased incidence of twinning, although Patterson believes that these problems are being overcome by further research.

Feeding MGA

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

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

To get their cattle accustomed to eating this ration, producers should pre-feed their cattle with the carrier without MGA for several days to get cattle accustomed to consuming feedstuff in this manner. After a couple of days, add the MGA.

For best results, producers should provide roughly two linear feet of bunk space per animal during the feeding period. Animals should be observed for signs of behavioral estrus each day of the feeding period. This may be accomplished as animals approach the feeding area and prior to the time feed is distributed, 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.

Preparing cows and heifers for breeding

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

First, adequate time should have elapsed between calving and the time of synchronization. "We suggest a minimum time of 40 days between calving and synchronization," says UM's Dr. Freddie Kojima.

Second, cows need to be in average to above average body condition.

Third, candidate females need to have experienced minimal calving difficulties.

Fourth, candidate heifers need to be at least 65 percent of their projected mature body weight.

And finally, reproductive scores for heifers need to be assigned no more than two weeks prior to synchronization. Heifers need to have reproductive scores of at least three or higher, and at least 50 percent of the heifers need to have a score of four or better.

Now, let's take a look at the three synchronization systems currently in use.

System 1: MGA and prostaglandin

In this system, suggested for use in heifers and cows, producers should administer prostaglandin 19 days after the last day of feeding MGA. Currently, there are four prostaglandin 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 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 provides flexibility for extenuating circumstances," says Kojima.

Cattle should come into heat up to 148 hours after the injection.

System 2: MGA Select

The MGA Select system, which is suggested for use in large groups of cows (100 head or more), is useful for maximizing estrus response and reproductive performance in beef cows. It's also simple and easy to do: simply feed MGA for 14 days and then administer an injection of GnRH (Cystorelin, Factrel or Fertagyl) on Day 26 and an injection of prostaglandin on Day 33.

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

Cattle should come into heat up to 148 hours after the injection.

System 3: 7-11 Synch

This program, which is suggested for use in smaller groups of cows (100 head or less), involves the feeding of MGA for seven - not 14 - days and the administration of prostaglandin on Day 7, the last day of MGA feeding. Cows then receive an injection of GnRH four days after the prostglandin injection and a second injection of prostaglandin seven days after GnRH.

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

Cattle should come into heat up to 148 after the injection.

"We believe that feeding MGA prior to the GnRH-PG estrus synchronization protocol will successfully induce ovulation in anestrous (non-cycling) 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 finally increase the likelihood of successful fixed-time insemination," explains Smith.

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

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

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

"A producer can make substantial improvements in the quality of their cowherd by doing so. And, producers can mass-breed their heifers to bulls selected for low BW-EPD with high accuracy. This practice minimizes the incidence and severity of calving difficulty and decreases 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."

This informational article was made possible by the National Association of Animal Breeders. You can find out more about NAAB's at its website, www.NAAB-CSS.org, or by calling 573-445-4406.

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