If you don't manage an active breeding farm in Central Kentucky, chances are pretty good you've heard of problems with tall fescue grass, but you may not be aware of when or why it can be bad for certain horses.
At its recent client education seminar, Rood + Riddle Equine Hospital's Dr. Maria Schnobrich gave the basics for those of us vaguely wondering whether or not we should be afraid of fescue.
Firstly, Schnobrich said, it's important to understand what it is about the grass that causes problems. The University of Kentucky's Horse Pasture Evaluation Program has yet to find a single pasture that doesn't contain tall fescue; the question is only how much of it shows up in a given field. On average, Schnobrich said UK's data finds it's about 20 percent of a given field, but that can swing wildly depending on the location and management.
“You may have a mixed seeding that you do initially but if the pasture is stressed, the fescue may outgrow the other grasses,” she said. “So while you think initially you may be doing a mixed proportion and have it under control, as you start to see some of the reproductive effects or other things … it's important to think that these pastures are dynamic.”
Fescue can become infected with a fungus called an endophyte which has proven a useful relationship for the grass over time. The presence of the endophyte helps the tall fescue become resistant to drought and pests. Unfortunately, the endophyte will also produce a toxin called ergovaline, which causes decreases in progestin and prolactin in mares. Much is still unknown about the way ergovaline impacts horses, but research indicates its levels are low between January and March in Central Kentucky and may rise astronomically between April and June. There can sometimes be a second spike in the fall. Somewhere between 80 and 100 percent of fescue in the wild is infected with this fungus, but having it on your property doesn't necessarily mean you've got the toxin in your horses' diet.
Most adult horses in work and breeding stallions seem unaffected by ergovaline. Yearlings may demonstrate inhibited growth and more problems with physitis, or growth plate swelling. The biggest problems seem to be in pregnant or nursing mares.
A pregnant mare encountering ergovaline may have a longer gestation (sometimes as long as 13 months – as if pregnancy isn't tough enough already) and then deliver larger than average foals who are not well-muscled. The mares can then produce smaller amounts of colostrum than average, and what they do produce may be of poor quality. The mares may also take longer to come back into heat after foaling than expected. Foals may show symptoms of hyperthyroidism and poor immunity, possibly due to the inferior colostrum quality.
“If you start going through the list and you have a few of these symptoms on your farm, I think it's important to think of fescue,” said Schnobrich.
Pastures can be so dynamic that Schnobrich says sometimes the symptoms may be limited to one field or one barn's worth of mares who happen to be ingesting more tall fescue than horses elsewhere on the same property.
Once a pasture has reached a tipping point with too much fescue, it's not practical to try removing all of it and starting over. Instead, pasture experts suggest trying to dilute the amount of dangerous fescue with something else. There are two varieties of fescue which cannot become infected with the fungus, or managers could add clover or alfalfa seed into their mix. Keeping pastures short and mowing before seed heads form can also help reduce the amount of ergovaline a mare might encounter.
Schnobrich also suggested managers feed extra grass or alfalfa hay during the times of the year when ergovaline is most likely to spike as a way to dilute how much fresh grass the horses will eat. If you can take a pregnant mare off a field with tall fescue and have her graze elsewhere, Schnobrich suggests doing it somewhere between 30 and 90 days prior to foaling.
If you're worried you're already stuck with too much tall fescue, Schnobrich also said administering domperidone beginning 30 days from foaling may also help combat the impacts of the ergovaline.
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