Two of the most widely cultivated cool-season forage grasses in mesic, temperate, perennial grasslands are tall fescue (Festuca arundinacea syn. Lolium arundinaceum) and perennial ryegrass (Lolium perenne) (Easton et al., 1994; Jung et al., 1996; Fribourg et al., 2009; Ball et al., 2019). These grasses are adapted to a range of climatic conditions and various soils, tolerate a wide range of grazing managements and cutting regimes, combine well with legumes and herbs in sown mixed-species pastures and in general provide nutritious feed for livestock such as dairy cows, beef cattle, sheep and horses. They became of particular importance in large areas of the New World, including countries of North and South America, and Australia and New Zealand, as European settlers began to farm livestock globally. However, in these regions over time, these grasses became associated with poor animal performance and health, in some seasons of the year (Hume et al., 2016; Ball et al., 2019). These detrimental effects on farmed animals were seldom apparent in the regions of Europe, North Africa and Asia where these grasses originated.