2013-08-23 / Columnists

Nature Notes

If Ingested, Hoary Alyssum Can Cause Health Problems for Horses
By Patricia Martin

When most people hear the word alyssum, they think of sweet alyssum (Lobularia martinima) a low-growing garden plant (six inches or less), an annual that seeds in from year to year. This European plant decorates the edges of beds and walkways, and is commonly used in rock gardens. It is a many branched and spreading mustard with flower colors ranging from white to pink to purple.

Recently, however, another plant called hoary alyssum (Berteroa incana) has made news on Mackinac Island. It is again a native of Europe, and in the mustard family (Brassicaceae). The reason this plant was brought to the attention of people here is that a horse that came to the Island for a few days apparently became ill after ingesting hoary alyssum.

Hoary alyssum stands about one to two feet tall. It has tiny white flowers in elongated clusters at the tip of the stem. The four petals of each flower are deeply notched and the sepals and flower stalks are hairy, as are the leaves and stems, or as the botanical books put it, their “dense pubescence gives the plants a distinctive gray-green aspect.” The leaves are much longer than broad (0.5 to 1.5 inches long) and they’re wider in the middle with tapering ends and the leaves are smooth. There is usually a rosette of leaves at the base of the stalk, and those on the stem are alternately arranged. There may be multiple stems. The stalk is rigidly upright, and the seedpods are usually present below the flowers. The pods are oval with a pointed tip. They are typically found flowering in summer and early fall. In North America, hoary alyssum was brought in as a contaminate of alfalfa and clover seed and has naturalized in. It has been found in Michigan since about 1900 and has spread throughout the state. It is found in meadows, but more commonly in waste places along roadsides, railroads, sandy fields, on shores and banks, and even invading woodlands. It is often found on poor soils, often sandy and gravely soils, and those that have a limestone base. I noted its presence in the collection of plants that I found for my thesis in 1995, although it certainly could have been here much longer. The first place it was noted was at British Landing near the beach. Hoary alyssum is considered to be an annual to a short-lived perennial herb.

Hoary alyssum is not uncommon in hay. It will seed into a field, and do particularly well if there is a dry summer, as it’s a drought-tolerant plant. While the grass or alfalfa is struggling, hoary alyssum does well if a field is stressed because of a lack of rain or nutrients. Hoary alyssum will get a toehold and then spread. If the field is healthy and the grass is flourishing, hoary alyssum doesn’t do well. It does not compete well with a healthy field. So one of the main ways of controlling it in hay fields is to keep them healthy, properly fertilized, seeded, and watered. With the drought of last year in many parts of the country, it’s not surprising that in some areas there may be an increase of hoary alyssum appearing in some fields.

Studies done on hoary alyssum show that the only domesticated grazing animal that has any problem with it is a horse, and very few of them seem to find it that toxic. In field cases, where the field had 30% to 70% hoary alyssum, only 50% of the animals that ingested it showed any clinical signs, and all of those were mild. If desirable foliage is available, they will not eat the alyssum and will reject it out of hand. The other day I stopped in a barnyard, where there were horses. The grass and weeds around the yard were pretty evenly chewed down except for a few foot-tall stalks of hoary alyssum. Generally horses won’t eat it.

Even in hay, most horses will eat around the hoary alyssum and leave it alone (sort of what many do with bird’s foot trefoil, which my horse leaves in little nests around the turnout). The cases of hoary alyssum poisoning in horses that were serious have all been in hay that has more than 30% to 70% hoary alyssum in it, so to even have any type of reaction to alyssum, the horse has to ingest large quantities of food of a type that doesn’t usually appeal to them.

Hoary alyssum might effect a horse in the following ways: Usually stocking up (swelling of the legs) occurs 12 to 24 hours after ingestion. Fever and short-term diarrhea may occur. Within two to four days, the clinical signs subside after the alyssum source is removed. In more severe cases, founder and stiff joints may occur and the horse may have a reluctance to move. Usually the horses do recover, although in a few cases death has occurred. Dryer states than Michigan usually have more problems with hoary alyssum. In Montana in 2008, it was determined by law to be a noxious weed. Hoary alyssum may also have some detrimental effects on the environment when it becomes widely established. One study indicated that there was a decrease in pollinator diversity in areas where native plants were replaced by hoary alyssum.

You may wonder how the horse in question got hoary alyssum poisoning. Well, it wasn’t from Mackinac Island or any of the hay that is regularly brought in. Apparently the owner of the horse, who was originally from Australia, bought hay at an earlier stop out of state and brought it with him to the Island.

If you happen to see hoary alyssum on your property or near a barn, it should be removed and usually can be hand pulled, but be sure to remove as much of the root as you can. If the plant breaks off above the rosette of leaves, it will regrow. Do not compost in your own pile, but carefully burn it or bag it for the landfill; otherwise you run the risk of spreading the seeds.

Patricia Martin is a yeararound resident of Mackinac Island, has earned a master’s degree in botany from Central Michigan University, and owns Bogan Lane Inn.

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