Warmer Kentucky Winter Could Change Fescue Toxicosis In Broodmare Pastures

Horse farm managers with broodmares should carefully consider how recent weather conditions might affect tall fescue in pastures, potentially contributing to tall fescue toxicosis in early foaling mares.

According to Ray Smith, researcher and forage extension specialist in the University of Kentucky College of Agriculture, Food and Environment's Department of Plant and Soil Sciences, above-average temperatures and rainfall in the past several weeks have resulted in good pasture growth on horse farms across the region. While this is great news for many horse owners because it will prolong grazing and delay hay feeding, they should monitor the situation for early foaling broodmares.

“Typically, we don't worry about early foaling mares because the cold weather takes care of ergovaline concentrations. But the cold snaps we have had so far have been brief and quickly rebounded to above-average temperatures, keeping tall fescue green and growing into the winter months,” Smith said.

Generally, ergovaline, the toxin produced by the endophyte commonly found in tall fescue, decreases rapidly once temperatures fall into the teens and grass growth is effectively stopped for the year. But because pasture grasses are growing deeper into winter than normal, this also means ergovaline production may continue. At the same time, other common pasture grasses like Kentucky bluegrass and orchardgrass are now dormant and therefore horses are less likely to graze them. Horses on pasture are likely consuming more tall fescue than they normally would in the spring and fall.

“Predicting when ergovaline concentrations will rise and fall is very difficult, so regular testing is still the best method we have. Managers should consider testing the tall fescue in pastures where early foaling mares are currently grazing,” said Krista Lea, coordinator of the UK Horse Pasture Evaluation Program. “Pastures with less than 200 parts per billion ergovaline are likely safe for those mares.”

According to Smith and Lea, broodmares are most affected by ergovaline in the last 60-90 days of pregnancy, so mares expected to foal before the first of March could be impacted by this unusual weather pattern and subsequent pasture growth. Pastures known to be higher in ergovaline in the spring and summer are more likely to be high now as well.

Mares negatively affected by ergovaline can have prolonged gestation, thickened placenta, red bag, poor milk production, dystocia and mare and foal mortality.

To reduce the risk to broodmares, horse owners and horse farm managers should feed hay and grain to reduce ergovaline concentration in the total diet or remove horses altogether from pastures containing tall fescue in the last 90 days of pregnancy.

Local county extension agents can help with pasture testing and submitting samples to the UK Veterinary Diagnostic Laboratory for ergovaline quantification. Pastures under 200 parts per billion are unlikely to cause significant issues in broodmares and will likely remain low until the spring green-up. Levels observed in early December are unlikely to affect other classes of horses or cattle. Sample handling is key, so be sure to read this publication on how to correctly sample for ergovaline.

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What’s The Deal With Tall Fescue And Broodmares?

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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