Be aware of Sweet Vernal Grass in your tall fescue pastures

Sweet Vernal Grass is widely distributed across the tall fescue belt, but it is poorly understood by most farmers. It helps make up a green sod, but when present at a significant level it suggests that pastures are in low state of productivity and may be in need of renovation.

Sweet Vernal Grass, Anthoxanthum odoratum, came to the US from Europe as a forage crop. It is native to acidic pastures in southern Europe and northern Africa. It is a cool season perennial grass that puts up a seedhead very early, about three weeks or so earlier than tall fescue. Forage nutritive value is high, but it is very low yielding, so it is not desirable as a pasture crop in our environment.

Sweet vernal grass is on the increase because it is more tolerant of low soil fertility and pH than other common cool season grasses, and it does well in a mix with those grasses. Across the region hay fields that have been managed with low fertilizer and lime inputs show high levels of sweet vernal in the first cutting hay. This is the one time sweet vernal is often noticed, and the hay yields in these fields are often disappointing, earning it a local name of “cheat”.

Later in the year the plants “hide” among the tall fescue plants which it resembles, so that many farmers forget about it after that disappointing spring hay cutting. However, these pastures and hay fields also don’t produce nearly as much forage as they could if the stand was mostly more productive species.

Sweet vernal grass can often be detected in hay by it’s unique sweet smell. This is often described as “vanilla-like”, a smell that comes from a compound called cumerin. When cumarin is present in moldy hay it is converted to dicumarol which inhibits the action of Vitamin K in the blood clotting system, resulting in slow clotting times. This scenario is more commonly associated with “sweet clover poisoning”, a malady that causes uncontrolled hemorrhaging and death when animals consume moldy sweet clover hay.

Dairy Science researchers at the University of Wisconsin working with sweet clover poisoning discovered dicumarol, and created the rat poison “warfarin” from this compound. This work also led to the first human “blood thinner”, Coumadin. These are important products, but of course are not something you want in your hay! Sweet vernal grass is one of the only other plants that has the potential to cause this malady when it is present in moldy hay. Cases of slow clotting time in cattle have been investigated and traced to moldy hay containing sweet vernal grass.

As I write this on April 14, 2024, sweet vernal grass is in full head and is the dominant grass in many of the low management hay fields and pastures across the northern central Piedmont of North Carolina. These same fields will be dominated by broom sedge (broom straw) in the fall. These pasture that receive a low level of management could be much more productive if renovated to productive species, like tall fescue.

Scout your pastures and make sure you understand the main species you are growing. If you find a lot of sweet vernal grass or other unproductive species, take steps to improve the productivity of your land.

~ Matt Poore for the Alliance for Grassland Renewal

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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 http://www.grasslandrenewal.org