Tall fescue is a dominant grass across the US
Tall fescue is a deep-rooted, persistent perennial grass that is considered high-quality forage for livestock. Tall fescue originated from Europe and was likely brought here by early European settlers. Kentucky 31 (KY-31), infamous fescue cultivar released in 1942, is well adapted to the eastern states and has become a dominant grass that filled a void in the landscape and a need for farmers. Yet by the early 1950s poor animal performance, with lower-than-expected average daily gains, low conception rates, lower milk production, poor heat tolerance, and vasoconstriction that resulted in loss of hooves and tails had been observed in cattle (Stuedemann and Hoveland 1988). Pregnant mares consuming KY-31 tall fescue have been reported with reduced milk production, prolonged gestation (up to 27 days longer), abortions, and thickened placentas (Cross et al 1995, Blodgett 2001).
Discovering an endophyte and the cause of fescue toxicosis
In the 1970s a USDA scientist Charles Bacon observed an endophyte (endo=inside, phyte=plant) associated in fescue pastures that correlated with fescue toxicosis (Bacon et al 1977). This endophyte is a fungus that remains inconspicuous in its host, colonizing the above ground tissue and is also transmitted to the next generation via the seed (Fig. 1). As our knowledge of this fungus improved it underwent several name changes (Epichloë typhina, Acremonium coenophialum, Neotyphodium coenophialum), but is now known as Epichloë coenophiala (Leutchmann et al 2014). What makes tall fescue toxic to livestock is not simply the endophyte but the fact that the endophyte can produce toxic compounds known as ergot alkaloids, such as ergovaline (Fig. 2A) (Porter et al 1981). These mycotoxins can interfere with the receptors such as those for dopamine and serotonin (Klotz 2015).
How are ergot alkaloids produced?
The endophyte found in Kentucky 31 has a set of genes (genetic machinery) that encode the biosynthetic steps required to generate ergot alkaloids that cause livestock toxicity. Ergovaline (Fig.2A) is an ergot alkaloid that can be detected in KY-31 fescue pastures. Although produced by the fungus, production of ergot alkaloids can also respond to environmental and management triggers. As example, when nitrogen is applied to the field this can increase production of the ergot alkaloids. In addition, the various parts of the plant can also have different concentrations of ergot alkaloids. During the vegetative stage of fescue growth, the endophyte is predominantly in the leaf sheath and pseudostem of the plant and very little endophyte is present in the blade. This is a great reason to make sure E+ KY-31 isn’t overgrazed otherwise the livestock will be eating fescue with the highest amounts of endophyte and the highest concentration of ergot alkaloids, whereas the leaf blades can contain much lower levels (Takach et al 2012). Once the inflorescences start to develop, the endophyte starts to colonize the developing seed heads, which is a peak time for ergot alkaloid production (Smith et al 2009).
Epichloë species produce other compounds that protect their host
It is easy to give tall fescue a bad name due to the production of ergot alkaloids, but these compounds are not the only ones produced by the Epichloë species. Other compounds, peramine (pyrrolopryazine), lolines, and indole diterpenes can also be produced by the fungus, which provide natural protection against some insects (Fig. 2) (Schardl et al 2013). In fact, there are strains of fescue endophytes that can produce some combination of all these compounds, including strains that aren’t able to make ergot alkaloids at all (Table 1). The endophyte strains that can’t produce ergot alkaloids don’t have the genes or genetic machinery required for production of ergot alkaloids (Takach and Young 2014). Identifying endophytes that are compatible with tall fescue but don’t make ergot alkaloids is one way to solve the fescue toxicosis problem (Young et al 2013). Several non-ergot alkaloid endophyte strains (Table 1) have now been identified in tall fescue that grows in the Mediterranean region. We use molecular tools to easily distinguish all of these endophyte strains.
Endophytes were the cause but can also be the solution
The initial solution to prevent fescue toxicosis was to develop endophyte-free (E-) tall fescue by removing the endophyte from the seed with heat and humidity or long-term storage. Unfortunately, these E- pastures were not as persistent as their endophyte-infected (E+) KY-31 counterparts, and within a few short years the E- stands are lost (Bouton et al 1993). It turns out the endophyte provides its host with persistence to insects (see the comments above about other compounds the endophyte makes), pathogens, drought, and some nutrient deficiency (Schardl et al 2023).
Endophyte-free (E-) fescue wasn’t the solution but a group in New Zealand was able to create cultivars with naturally occurring endophytes that don’t produce ergot alkaloids (Table 1) but still retain host persistence (Johnson et al 2013). When tested on cattle, the animal performance was far superior to that of the E+ KY-31, where cattle grazing the new novel endophyte infected (NE+) cultivars, Jesup MaxQ and Texoma MaxQ II, had almost double the average daily gains than those on E+ KY-31 (Fig. 3) (Parish et al 2013). NE+ tall fescue is also a suitable forage for pregnant mares, as they performed as well as those grazing orchardgrass/Kentucky blue grass mix, showing no signs of fescue toxicity typically seen with E+ KY-31 (McDowell et al 2019). In addition, the NE+ cultivar Texoma MaxQ II has been shown to be more desirable for beneficial dung beetle performance, which provides nutrients back into the soil (Shymanovich et al 2020).
Several non-ergot alkaloid producing endophytes have now been deployed in tall fescue cultivars that can be used for pasture renovation to combat fescue toxicosis. Information on the available NE+ tall fescue cultivars has been developed by Phipps et al 2021. Our understanding of fescue toxicosis and the cause has come a long way since it was first observed in the 1950s. An endophyte that was once considered the bane of fescue production has now been replaced with better endophyte strains (NE+ cultivars) and these new cultivars can provide all the benefits for retaining host persistence, without ergot alkaloid toxicity to grazing livestock.
~ Dr. Carolyn Young, Department of Entomology and Plant Pathology, North Carolina State University, Raleigh, NC | Email: email@example.com | Twitter: @KiwiYoung. Proceedings written for the Equines and Endophytes Workshop, Lexington, KY January 25, 2023
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- Phipps K, Talbott C, Newsome M, Harmon D, Poore M. 2021. https://content.ces.ncsu.edu/comparison-of-commercially-available-novel-endophyte-tall-fescue-forage-varieties
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- Takach JE, Mittal S, Swoboda GA, Bright SK, Trammell MA, Hopkins AA, Young CA. 2012. Genotypic and chemotypic diversity of Neotyphodium endophytes in tall fescue from Greece. Applied and Environmental Microbiology. 78:5501-10.
- Takach JE, Young CA. 2014. Alkaloid genotype diversity of tall fescue endophytes. Crop Science. 54:667-78.
- Young CA, Hume DE, McCulley RL. 2013. Forages and pastures symposium: fungal endophytes of tall fescue and perennial ryegrass: pasture friend or foe? Journal of Animal Science. 91:2379-94.
The Alliance for Grassland Renewal is a national organization focused on enhancing the appropriate adoption of novel endophyte tall fescue technology through education, incentives, self-regulation and promotion. For more resources or to learn more about the Alliance for Grassland Renewal, go to www.grasslandrenewal.org