Heifers Heat Sync

Heat Synchronization in Replacement Heifers Using MGA
The relatively new heat synchronization technique being used in replacement heifers is the combined use of MGA and prostaglandin. Melengestrol acetate (MGA; Pharmacia-UpJohn Co.) is a feed additive commonly used in feedlot heifer rations to block cycling activity of the heifers. At this printing, MGA is FDA approved also for estrous control in heifers on pasture. ~

     When fed for a short period of time, and removed from the diet, the removal of MGA tends to allow a large percentage of the cattle to exhibit heat together.

     Fertility at the first heat after MGA removal has been reduced compared to normal heat. Subsequent heats have been normal.

     This program calls for feeding 0.5 milligrams of MGA per head per day for 14 days. At this point MGA is removed from the feed. Most of the females will then exhibit heat.

     Seventeen days after the MGA feeding has stopped, each female is injected with prostaglandin to interrupt the next cycle.

     Two to five days later, females are bred following detected standing heat.

     Research at several experiment stations has indicated that this method has induced some non-cycling replacement heifers to begin cycling therefore increasing the percentage that were bred early in the insemination season.

     Normal conception rates were achieved when cattle were inseminated on the second heat after MGA removal.

     Kansas State University researchers have reported that heifers or cows could be inseminated after detected heat for 72 hours following the prostaglandin injection and then all remaining females be inseminated by appointment 72 hours following the prostaglandin injection. This would reduce the heat detection time to three days. Data reported on this method of synchronization has shown more consistently favorable results with heifers than with post-calving cows. Cows may be less consistent because of greater variation in body condition at calving, milk production, age, suckling intensity, and post- calving nutrition.

     An important consideration is that the MGA/prostaglandin synchronization program must be started precisely 35 days prior to the start of the breeding season. This means that prior planning must be done to assure that the feed containing MGA is prepared and ready to feed 5 weeks before the breeding season.

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