Winter Rye and Ryegrass are not the same thing, although both have some safety issues. Winter rye is a cereal crop, normally grown to harvest the grain. It may also be planted as a ”green fertilizer,” plowed under before spring planting. Rye is also sometimes used for straw.
We’ve heard some horse owners in drought-stricken areas are considering planting winter rye to be harvested early in the spring as hay. Dr. Robert Wright of the Ontario Ministry of Agriculture, Food and Agricultural Affairs, their equine expert, advises against this.
Dr. Wright reports that winter rye can be infested with Claviceps mold. ”There is a major issue if pregnant mares consume infected rye. Claviceps produces an array of ergot alkaloids, like erovaline associated with fescue toxicity. The case I was involved with resulted in seven dead foals of the first eight mares to foal, with mares in second-stage foaling for up to 30 hours.” High doses can also cause hoof changes resembling laminitis, tremors, dry gangrene.
Ryegrass, on the other hand, is a pasture grass, also used for hay. Seed is either annual (must be sewn every year) or perennial. Ryegrass prefers moist soils with high fertility. It grows rapidly and tolerates cool temperatures so is often included in pasture mixes to allow for early season grazing, or even planted in the fall in warmer areas of the country.
Although it’s uncommon in this country, annual ryegrass infected with a specific nematode (worm) that is in turn infected with a bacteria and bacteriophage can contain corynetoxins. This can produce a syndrome called ryegrass staggers, with abnormal gait, convulsions, extensive brain damage or death. Control measures revolve around mowing before seed heads form and controlling the nematode. Rotating fields sewn with annual rye, burning fields and plowing under the grass are nematode control measures.
Perennial ryegrass is hardier, and often used in pasture mixes, but it too can become infected with endophytic fungi that may cause problems for pregnant mares. These fungi are of value to the grass, often making it more resistant to insect attacks. ”Endophyte-free” seed is available for sale, but Dr. Wright reports that designation isn’t always reliable. While fescue is usually incriminated in the problems with pregnant mares, endophytes on perennial fescue might also be involved. The safest course is to avoid pastures or hays with this grass for mares in late pregnancy.