b'Changing the Hormone BalanceManipulating estrus is essentially a game of altering hormonal balance to trick a mares reproductive system into staying longer in a nonestrous phase or to bring on estrus faster and shorten its duration, our sources say.Delicate balances of primarily proges-terone and estrogens induce the various phases of a mares estrous cycle, explains Aurich. Estrogen is the predominant hormone during estrus. In diestrusthe period between estrous cycles progesterone takes the lead. Its the pro-SUSAN KORDISH/COWGIRL PHOTOGRAPHYgestation hormone, meaning it promotes pregnancy by encouraging the uterus to accept and hold on to an embryo. Very basically, estrus makes a mare sexually excited, whereas progesterone keeps a mare calm and maternal.Scientists have developed drugs to ma-nipulate estrus, using equine hormones, hormones from other species, and syn-thetic variations of these hormones. They force a shift in the hormonal balance,Estrus manipulation is often necessary in mares that become difficult to work with when in heat.changing the estrous cycle. The reproductive system can also beAn injectable form reduces contamina-according to where she is in her cycle.encouraged to alter that balance with- tion risk for drug tests (more on this in aDepending on that cycle state and the out drugs, our sources say. By changingbit) but increases the risks of skin reac- objective of estrus manipulation (syn-aspects of the mares environment tions, Ellerbrock says.chronizing donor and recipient mares, including her uterine environmentatA new, though not yet FDA-approved,inducing ovulation for insemination, different points in the estrous cycle, heralternative is a long-term oxytocin injec- etc.), veterinarians and owners can treat body believes it needs to be releasingtion, which is a cheaper option thatwith various hormones and/or synthetic different proportions of hormones than itdoesnt risk hormonal exposure for theanalogues (lab-made variants), says Alva-normally would.barn staff, she explains, potentially inter- renga. Some of these include:fere with a womans reproductive systemProstaglandin F2 (dinoprost trometh-Altrenogest: Still the Gold Standardif absorbed through the skin.amine), a naturally produced com-for Performance Horses Oxytocin injections, given daily forpound that regulates hormones.Oral altrenogest (Regu-Mate) is todaysa week, can cause cramping and dontProstaglandin analogues (sodium gold standard in estrus manipulationwork in all mares. When they do, they cancloprostenol). for performance horses in the U.S. andprolong diestrus by up to 60 days, says Progesterone, a naturally produced many other countries, says Ellerbrock.Donnelly.hormone. This is the safest (for the mare) and mostProgestins, synthetic versions of natu-effective option, with the most predict- A Custom Blend for Breeding Stock ral progesterone.able outcome, she says. Its also the onlyManipulating estrus in breeding maresEstrogens (estrone, estradiol, and es-FDA-approved product for suppressingis a much less straightforward process,triol), naturally produced hormones.estrus, she adds.because youre fine-tuning a mares indi- Human chorionic gonadotropin (hCG), Altrenogest is a synthetic hormone thatvidual cycle by working with her uniquea hormone produced by placental cells shifts the hormone balance to becomephysiology. Estrus manipulation shouldin human embryos.progesterone-dominant by interactingalways be performed based on the condi- Gonadotropin-releasing hormone with the mares progesterone receptors,tion of individual mares, Aurich says.(GnRH) agonists (deslorelin acetate, says Callum G. Donnelly, BVSc (HonsThe mare has to be examined and thehistrelin), drugs that target the pituitary I), Dipl. ACT, ACVIM (LA), of the Finnoreproductive stage clearly determined be- glands to regulate hormone release.Laboratory in the Department of Popula- fore any action is taken. Otherwise, drugAlvarenga says new studies suggest that tion Health and Reproduction at theadministration may be without effect. using prostaglandin, estradiol, and pro-University of California, Davis, School ofThats in part due to the mares relative- gestins alone, in evolving doses to mimic Veterinary Medicine. Given orally as a liq- ly long estrous cycle, she explains. Plus,a natural cycle, can synchronize mares for uid every day during the breeding season,sometimes ovulation occurs at the endembryo transfer. However, it only works altrenogest generally holds off estrus untilof estrus, sometimes after. So veterinar- well in mares that are already cycling and about five days after treatment stops. ians must manipulate each mares estrusjust need adjustments, he adds.TheHorse.com|The HorseMay 202025'