Toxic Weed Hoary Alyssum Found in Drought-Affected Pastures

The 2007 growing season's drought conditions are causing challenges for anyone who handles plants or animals. Horse owners are no exception, and to a list of concerns including a shortage of hay and lack of pasture growth they need to add awareness of hoary alyssum, a weed that is poisonous to horses.

Hoary alyssum is found across the northeastern and north central United States and Canada. A member of the mustard family, it produces small white flowers and can grow one to three feet tall. Grayish-green "hairs" cover the stems, leaves, and seed pods. It is well-adapted to dry conditions and grows in drought-stressed, over-grazed pastures.

"This season's drought conditions are definitely favoring certain plants over other plants," said Ben Bartlett, Michigan State University (MSU) Extension U.P. dairy and livestock educator. "This year, plants that can get by on less moisture are doing great because they are getting enough moisture and there's less competition."

Horse owners need to learn to identify hoary alyssum and the signs of its toxicity in their animals. Bartlett warns that those who may feel that they don't have to worry because they feed supplemental hay and grain should think again.

Horses on pasture, even if they receive hay and grain, might graze on hoary alyssum if they find it in their fields. Though it is not preferred by most horses, they will nibble on it when other plants are not abundant due to overgrazing or drought.

"People need to keep in mind that animals being fed hay still like green things to chew on," Bartlett said. "You may be feeding supplemental feed and think they are getting enough, but animals will walk around and nibble on green things."

Toxic symptoms in horses range from depression to stocking up (swelling in the lower legs), fever, and diarrhea.

Minimizing problems with hoary alyssum and other weeds begins with good pasture establishment and controlled grazing. When the weed is found in an established pasture, the herbicides that are labeled for its control will also affect forage legumes. If drought conditions have left few other plants in the pasture, removing the animals for the remainder of the growing season is an option. But what about next year?

"If the pasture has not been overgrazed, then the other plants should be able to outcompete hoary alyssum next year, as they did in the past," Bartlett says.

Michigan State University and MSU Extension offer a number of drought-related resources. For more information about hoary alyssum, horse owners can refer to a fact sheet titled "Toxicity, Identification and Control of Hoary Alyssum in Forages," found online at This publication is also available at any MSU Extension county office.

Information about managing pastures and forages under drought conditions is available online from the MSU Extension Beef Team at, and help for field crop farmers can be found in the MSU Integrated Pest Management program's Crop Advisory Team Alert at

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