For many, toxic tall fescue is a nagging concern this time of year, but with little idea about how to manage it, we often ignore it and simply hope for the best. But we can do better. There are several steps we can take to protect mares this year and to prevent this issue from challenging us again.

The Basics

First, let’s start with a quick review of tall fescue toxicity. Tall fescue is a cool season (growing in spring and fall), perennial grass that is naturalized throughout the southeastern U.S. Most tall fescue is infected with an endophyte or fungus that lives in between the cell walls of the plant. The plant and the endophyte have a mutualistic relationship. The plant provides the endophyte with a place to live, nutrients to feed on and a method to reproduce via its seed. In exchange, the endophyte produces a series of compounds, some that give the plant increased drought tolerance, pest deterrence and overall resiliency. However, some other compounds, such as ergot alkaloids, are also produced and are toxic to livestock, including horses.

We know that ergot alkaloids, most notably ergovaline, have acute adverse effects on late-term pregnant mares. Mares grazing toxic tall fescue will often experience prolonged gestation (14-28 days longer), thickened, retained placenta (sometimes presents as a redbag delivery) and agalactia, or reduced milk production, that often requires medical or supplemental intervention. Mares can also suffer from late-term abortions in severe cases. In early pregnancy, mares are less sensitive to tall fescue and ergovaline, but can experience prolonged estrus cycles, reduced pregnancy rates and early term losses as well.

However, non-pregnant horses, including growing, working and breeding stallions, seem to adapt to grazing tall fescue well with minimal impact on health, so the remainder of our discussion will focus on reducing tall fescue toxicity in broodmares.

Time of Year

Ergovaline production, and therefore the risk of grazing tall fescue, are not consistent throughout the year. Typical years will see two peaks in production – in the spring when seedheads are forming around late May and again in the fall once the summer heat breaks, around September. Because mares are most sensitive in the last 30 days of pregnancy, we can reduce some of the risk by considering our foaling dates. For example, a mare due to foal in early March will be in her last 30 days of pregnancy in February, when ergovaline levels are typically quite low. On the flip side, a mare due to foal in July will be in her last 30 days in June, when ergovaline levels are the highest of the year. This mare would require additional steps to protect.

Recently, mild winters have made managing mares on tall fescue more complicated because we often don’t see the reduced ergovaline levels until much later in winter. Historically, January foaling mares were quite safe, but in the last two years, the UK Veterinary Diagnostic Lab has observed toxic levels of ergovaline well into January, putting those early-term mares at risk.

Fig. 1: Ergovaline (vertical axis) concentration changes throughout the year and typically has two main peaks in spring and fall. Historically, levels have been low in summer and winter, as shown in 2007 (green), but more recently, mild winters have delayed the fall decreases and resulted in prolong elevated levels of ergovaline into the winter months, as seen in 2022 (orange).

Pasture Height

The endophyte lives in the base of the plant but must infect the seeds during plant reproduction to ensure the next generation of endophytes survives. In these areas of importance for the endophyte, we will also see concentrations of ergovaline found in the stem and seedhead and in the bottom 2-3 inches of the plant base. Dealing with the top is quite easy; just mow them. By keeping seedheads mowed down, we greatly reduce ergovaline in the horse’s diet and improve the overall forage quality of the pasture.

The bottom is a bit harder to manage. Horses are spot grazers and prefer to graze the same areas again and again rather than grazing consistently across the pasture. Those frequently grazed areas will often contain more ergovaline, making rotational grazing important for tall fescue management. Small paddocks where overgrazing is almost guaranteed is where the highest ergovaline concentrations are often found and therefore should be avoided for late-term mares as much as possible.

Good Hay

Another common option for tall fescue mitigation is to provide good quality hay and grain to dilute the tall fescue portion of the diet. But horses much prefer green grass over hay, so this is only effective if the hay is of high enough quality that horses choose to eat plenty of it.

High-quality alfalfa, alfalfa/mixed hay or even early maturity grass hay can be used and doesn’t have to be of Western U.S. origin. Kentucky hay producers can put up very high-quality hay, though you may have to search for it a bit more. Forage testing can speak to the quality of the hay, but ultimately it all depends on if they eat it or not. 

Fig. 2: Feeding high quality hay will encourage horses to consume more hay and less toxic tall fescue.

Restricting Pasture Access

Particularly in late spring, no quality of hay will entice a horse as much as green grass, and therefore we must restrict pasture access to protect these mares. This could include keeping mares up in stalls for part of the day, establishing a dry lot where hay is fed or turning mares out with grazing muzzles to restrict intake during the day and continue feeding hay at night in the stall. Be careful to consider the source of hay or grass bedding, as ergovaline does occasionally persist in these as well, though this is not common.

Medical intervention

When all else fails, the use of domperidone has been shown to eliminate many of the effects of toxic tall fescue but has its own financial and physiological impacts that should be discussed with a veterinarian before use.

Eliminating Tall Fescue

With several variables contributing to tall fescue toxicity in broodmares, and their very low tolerance to ergovaline, the only way to truly ensure safety for mares is to eliminate toxic tall fescue from the diet. Removing it isn’t all that difficult, and several options do exist, but what to replace it with is the challenge. Endophyte-infected tall fescue is the strongest, toughest, most durable plant found in most horse pastures. Removing it leaves pastures open to many problematic plants, including nimblewill, foxtail and just about any broadleaf weed. Orchardgrass and bluegrass are not likely to take over or even maintain cover alone.

Novel endophyte tall fescue varieties are likely the best replacement for toxic tall fescue and are both safe and productive for horses. Novel endophyte tall fescue varieties are commercially available tall fescues that contain a different endophyte – one that still provides the plant with durability and toughness, but doesn’t produce toxic compounds. These grasses survive well under heavy grazing by horses, tolerate drought, heat and pests and can compete with weeds and undesirable grasses. These grasses are best established in a completely killed pasture rather than overseeded. Details on the renovation process and on different novel endophyte varieties can be found in this UK publication.

During this and all foaling seasons, managers should track pregnancies for symptoms of tall fescue toxicity, regularly walk pastures to evaluate tall fescue presence and test pastures when needed. While complete re-establishment of pastures sounds scary, this is actually the best way to not only remove toxic tall fescue from pasture, but also greatly improve the overall quality of pastures and eliminate other challenges. More information regarding tall fescue toxicity, ergovaline testing and pasture renovation can be found on the UK Forages website, www.forage.ca.uky.edu