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MGA (melengestrol acetate) was approved 30 years ago by the FDA for use as a feed additive in heifers. It was designed to suppress estrus, increase average daily gain, and improve feed efficiency. In 1994 the FDA cleared MGA for a zero pre-slaughter withdrawal time. Feedyards are still finding it an effective tool in feeding out heifers. Dwayne Thompson, manager of Texas Beef Producers Feedyard in Dumas, Texas, started using it about five years ago. "In the cattle-feeding industry, the real money maker is feed conversion," he said. "Until we began feeding MGA, our heifer performance wasn't as competitive as we would have liked." MGA has also been tested in recent university trials to help test different estrus synchronization methods. Kansas State University recently released a study comparing different products used to synchronize estrus in replacement beef heifers. According to the published report, over 72 percent of the heifers were detected in heat during the target-breeding week, with the MGA+PGF2 treatment having the greatest percentage of heifers in heat. The treatment costs for MGA were also lower than the other products used in the study. 
Normally, a heifer comes into heat every 21 days (Figure 1) when a mature follicle on her ovary ruptures (ovulation). Prior to ovulation, the mature follicle produces natural estrogen in large amounts, which is primarily responsible for bringing on signs of estrus. After the heifer has been in heat for nearly 12 hours, the mature follicle ovulates on the command of the luteinizing hormone (LH). This hormone is released into the bloodstream by the anterior pituitary gland located at the base of the brain.MGA helps maintain a level amount of estrogen throughout the heifer's reproductive cycle (Figure 2). As a result, rate of gain and feed efficiency improve because heifers do not come into heat--typically marked by riding and inconsistent feed consumption. Many feedlot managers find that feeding heifers is an attractive alternative to steers, even though heifers do not gain as rapidly and do not convert feed as efficiently as steers. However, their feed intake is lower and they generally move faster through the feedlot. When used in open feedlot heifers, MGA has been shown to increase weight gain by 10.3 percent and feed efficiency by 6.5 percent. This can mean an $8 to $10 profit per head. To ensure optimum results, MGA must be carefully incorporated into the heifer ration, providing a daily intake of .25 to .5 mg per head for heifers intended for slaughter and a daily intake of .5 mg per head for heifers intended for breeding. "MGA is one of the best products ever introduced to the cattle business," said Jim Biehl of Biehl Cattle Co. in Lexington, Neb. Biehl was introduced to MGA by his feed supplier and nutritionist when the product went on the market in 1968. Since his feedlot was nearly 100 percent heifers, Biehl thought MGA was worth a try. "I'm thankful we didn't miss this opportunity," he says. "Feed efficiency improved and it kept the heifers a lot calmer--which meant they also gained better." The decision to include MGA in the heifer ration was reinforced when Biehl's feed mill burned to the ground a decade ago. "We were without it for a few days while we were making other arrangements for feed," he explains. "Right away, we noticed agitation and aggressiveness. We were glad we got the heifers back on MGA." |
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