Mythbuster Monday: Sweet Vernal Grass Is Dangerous for Horses

On Mythbuster Monday, we tackle a variety of equestrian myths to either bust or confirm. Today’s discussion: Is Sweet Vernal grass dangerous for horses to consume?

It’s Mythbuster Monday, where Horse Nation dives into different equestrian myths and provides research-based evidence to either bust or confirm those myths. Today’s topic: Is sweet vernal grass dangerous for horses to consume? How much can they eat? What makes it dangerous? What does it look like? Read further to find out!

Myth: Sweet Vernal grass is dangerous for horses

Myth or Fact: Both

Sweet vernal is a grass variety that is native to and mostly found in Europe, but is now found widely across North America — especially since it is a more drought tolerant grass. When dried for hay, it produces a sweet smell that farmers thought was enticing to horses. However, it was discovered that the substance that creates the sweet smell, coumarin, actually has a bitter taste that horses tend to stray away from.

Before getting into the risks of sweet vernal, it’s important to note that several other grasses tend to be confused for it. Cocksfoot grass, timothy grass, Yorkshire fog grass, prairie grass, and Kentucky blue grass are similar looking enough to be mistaken for sweet vernal. Here’s what the other types of grasses look like so you can make better judgement of what’s in your pastures and hay.

Cocksfoot grass, also known as orchard grass, cockspur or barnyard grass, is a horse-friendly grass that grows over dry summer months. It has good levels of protein and makes for excellent hay.

Timothy grass, also known as meadow’s cat’s tail, is another horse friendly grass that grows in late spring and early summer. This grass provides high quality pastures and hay.

Yorkshire fog grass is a horse-friendly grass that can grow in acidic soils. It’s dense and hairy and good for horses who tend to put on weight quickly as it is less nutritious than other types of grass.

Prairie Grass is in the brome family of grasses. Studies have concluded that this type of grass is preferred over all others for many horses. This is because it is reasonably higher in sugar than other grasses.

Kentucky blue grass is often referred to as weed grass because it is low quality. However, it is horse-friendly and grows well in low-fertility soil. The quality of it can be improved with fertilizer and it is preferred because it can densely cover the ground.

Now that we can identify the slight differences in the different types of grasses, we’ll dive into sweet vernal grass and whether or not it is okay for horses to ingest.

According to an article by Calm Healthy Horses, horses eat sweet vernal grass frequently without issues. It’s when the grass is cut into hay and the hay molds that causes the issues. This is because the substance that creates the sweet smell, coumarin, converts into dicoumarol when the hay molds. If the moldy hay is consumed excessively, it can inhibit normal blood clotting.

Richmond Times-Dispatch put out an article discussing how to avoid risk factors of toxicity from Sweet Vernal. They state that only under certain conditions the plant can be an issue to horses. This is when coumarin converts into dicoumarol. Only when sweet vernal gets moldy does it make the switch. Ensuring that hay is properly dried before baling and keeping the hay dry reduces the formation of dicoumarol.

In cases where deaths were associated with sweet vernal, the hay was often comprised of 80-90% of it. They state that small amounts of sweet vernal grass in hay is not typically a problem and that the horses can usually sense when it is no good for them and will eat around it.


The University of Minnesota Extension also stated in their article that sweet vernal is only an issue when it becomes moldy. However, it is not the only plant that has undesirable effects when moldy. Clover also becomes potentially harmful to horses. When clover becomes moldy it can cause liver damage and bleeding in horses. The moral of the article was to keep hay dry to prevent health issues due to an array of different plants becoming unhealthy.


Overall, sweet vernal only becomes toxic to horses when it gets moldy because the coumarin changes to dicoumarol. However, research is limited. There are more research studies on the effects on cattle rather than equines. Also, this isn’t the only plant that becomes toxic to horses when moldy. Clover has a similar effect. In cases of severe toxicity and death, hay was compromised with 80-90% sweet vernal. Most often horses will eat around the parts of the hay that are potentially harmful to them but storing hay in a dry area and preventing mold is the best way to decrease the risk of undesirable issues.

Do you have an equine myth you’d like us to tackle? If so, send it our way! Email your suggestions to [email protected]. Put Mythbuster Monday in your subject line.