Plan ahead for heat synchronization methods for replacement heifers
Producers who 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 at 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 eight days. Nineteen days after the MGA® feeding has stopped, each female is injected with prostaglandin to interrupt the next cycle. Injectable prostaglandin products are available from licensed veterinarians. One to six days later, females are bred following detected standing heat. (See time line below). 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. Other successful synchronization protocols are available for heifers and mature cows. Some protocols are suggested with short time heat detection and others are recommended for timed AI with little or no heat detection. The Beef Reproduction Task Force has available the recommended 2012 synchronization protocols at The Applied Reproductive Strategies in Beef Cattle website under “Resources”: http://beef repro.unl.edu/resources. html. — Glenn Selk, Oklahoma State University Emeritus Extension Animal Scientist