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Plan Ahead For Heat Synchronization Of Replacement Heifers

published: March 8th 2009
by: Glenn Selk
source: Oklahoma State University Extension

Producers that plan to use artificial insemination as part or all of this upcoming spring breeding season should start their preparations immediately.  A popular synchronization protocol for heifers involves the feeding of an additive, and the feed must be ordered and delivered and the proper time.  Melengestrol acetate (MGA®; Pfizer Animal Health) is a feed additive commonly used in heifer feedlot rations to block the cycling activity of heifers.  Melengestrol acetate is a synthetic progestin that has "progesterone-like" activity.  When fed for a short period of time and then removed from the diet, the sudden absence of progestin tends to allow a large percentage of cattle to exhibit heat together.  Compared to normal heats, fertility at this first heat after MGA® removal has been reduced.  Subsequent heats have returned to normal fertility.
    Armed with this knowledge, researchers have outlined an estrous synchronization protocol using MGA® and a prostaglandin injection.  This program calls for the feeding of melengestrol acetate at the rate of 0.5 milligram per head per day for 14 days.  After 14 days, MGA® is removed from the feed.  Most of the heifers will then exhibit heat over the next 8 days.  Nineteen days after the MGA® feeding has stopped, each female is injected with prostaglandin to interrupt the next cycle.  One to six 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, thereby increasing the percentage of those cattle bred early in the insemination season.
    An important consideration is that the MGA® /prostaglandin synchronization program must be started 35 days before 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 five weeks before the date of the first breeding.  This synchronization method is designed to work best with replacement heifers.