There’s a lot to worry about when you’re caring for a pregnant mare in the spring. And one risk you won’t want to underestimate is fescue toxicosis, caused by ingestion of tall fescue infected with the endophytic fungus Acremonium coenophialum.
Experienced breeders are well aware of the dangers posed by fescue toxicosis for mares late in their gestational period. These include a prolonged gestation that leads to larger foals that are difficult to deliver; a “red bag” delivery, in which the placenta detaches from the uterus early, depriving the foal of oxygen; and a poor to nonexistent milk supply. Early in a mare’s pregnancy, fescue toxicosis can lead to embryo loss.
Keeping pregnant mares in their final trimester away from fescue—both in pasture and hay—is the best course of action but isn’t always feasible. A bite or two of the grass isn’t dire, but grazing on endophyte-infested pasture certainly can cause serious problems. Fescue is invasive, and in some parts of the country the grass is present in virtually every field, even if other species have been cultivated. Keeping a mare on a dry lot for the last 60 to 90 days of her pregnancy is one option for avoiding fescue in pastures during this critical gestational window, but you’ll also need to check your hay supply carefully to ensure it doesn’t contain the grass.
In addition, the drug domperidone can help protect against the effects of fescue toxicosis. Given daily starting 10 to 15 days before the anticipated due date, domperidone prevents the inhibition of prolactin secretion, encouraging udder development and lactation. The drug can be given for five days after foaling if your mare is still not producing adequate milk. Your veterinarian can provide guidance in administering domperidone in the correct window of time to ensure that the foal receives sufficient colostrum, even with adequate milk production.
This article first appeared in EQUUS issue #463, April 2016.