MGA In Different Synchronization Strategies

Melengestrol acetate (MGA) is now officially approved for suppression of estrus (Figure 4) in breeding heifers. Previously MGA was only approved for the suppression of estrus in feedlot heifers. Now it is approved to use MGA for supression of estrus in reproducing heifers... something that has been going on for some time now. This is a great tool for producers to shorten the breeding season and tighten the calving period. This will also allow producers to decrease labor costs during both the breeding and calving seasons, especially in artificial insemination (AI) programs.

One thing to remember when attempting any estrous synchronization program is that cows need to be in a minimum body condition score (BCS) of four (4), and heifers should be at a BCS six (6). Additionally females must be on an increasing plane of nutrition (gaining weight) and the program should not start prior to 45 days post calving (it takes 30 - 45 days for uterine involution).


There are three common methods of estrus synchronization using MGA.

1.  The simplest method involves using bulls to breed synchronized groups of females. This method is most useful in helping producers make the jump from natural service to AI. The process involves heifers receiving MGA at 0.5 mg/animal/day for 14 days. MGA only treatment calendar Bulls are then exposed to treated groups of females at 15 to 18 days after MGA withdrawal. No more than 15 to 20 synchronized females should be exposed to a single bull. Producers should always consider  age and breeding condition of the bull and results of breeding soundness examinations (BSE) when deciding the female:bull ration.

2.  The second method is a more sophisticated means of synchronizing heifers using MGA in combination with prostaglandin (PGF). Prostaglandins such as Lutylase® are luteolytic (involved in regressing the corpus luteum) compounds which are normally secreted by the uterus of the cow. Prostaglandins are capable of inducing luteal regression, but cannot inhibit ovulation. When prostaglandins are administered in the presence of a functional corpus luteum (CL) they begin premature regression of the CL and a return to estrus by the heifer, thereby allowing breeding.

In short prostaglandins speed the return to estrus. In this system prostaglandins should be administered 17 days after MGA withdrawal. This treatment places all heifers in the late luteal stage of the estrous cycle MGA followed by single Prostaglandin injection calendar at the time of injection, which should shorten the synchronization period and allow for maximum conception rate.

    A very high portion of females placed on the second method will exhibit estrus during the synchronization period. However in some cases up to 20% of the heifers will not respond to the prostaglandin injection. Therefore a third system involving a second prostaglandin injection may be used.
     
3.  The third method involves a second injection of prostaglandin administered 11 days after the first injection to heifers that fail to respond to the initial injection. Heifers that have already been inseminated should not be re-injected. The MGA double-injection prostaglandin treatment is useful in further tightening the group of breeding females. This is logical, since some portion of the females that fail to respond to the first injection will respond to the second injection, allowing us to increase the total MGA followed by two (2) prostaglandin injections, calendar number of heifers that can be inseminated during the breeding period.


Estrus synchronization programs can range from the fairly simple and inexpensive (feed MGA for 14 days) to the fairly complex and costly (feed MGA 14 days followed by up to two prostaglandin shots or implants). The goal of any synchronization program is to tighten the breeding season resulting in a tight calving season. The payoff is in decreased labor costs at calving and a more uniform set of calves when it comes time to sell them. The bottom line is more profit returned to the producer.

To read more about MGA and it's mode of action, you can go to this website: http://www.beefheifers.com/ .

Land O'Lakes Farmland can help you with the synchronization of your heifers.  Rangeland Synchronizer and SuperMix H 0.8 are products designed specifically to get the right amount of MGA into your heifers for heat synchronization.  Rangland Synchronizer is fed at the rate of 2#/hd/day and SuperMix H 0.8 is fed at the rate of .625#/hd/day.  Contact your local coop or your Land O'Lakes Farmland Beef Specialist for more information.

 

Figure 4--Physiology of the estrous cycle
Link to larger image of cow cycle


Article adapted from publishing authored by
D.J. Patterson, Ph.D
Department of Animal Sciences
Lexington, Kentucky

At the time of publication Dr. Patterson was a faculty member at the Department of Animal Sciences at the Unversity of Kentucky. Dr. Patterson is now at the University of Missouri.