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.

For questions, contact the UK Horse Pasture Evaluation Program at UKForageExtension@uky.edu.