Horse Owners Cautioned about Hardy Toxic Plant

With cases reported already this year, horse owners are being cautioned about a toxic plant that flourishes during drought and in overgrazed conditions.

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 up to 3 feet tall. Grayish green "hairs" cover the stems, leaves and seed pods. It is well-adapted to dry conditions and grows in drought-stressed and overgrazed pastures.

"We have had cases of hoary alyssum toxicity in horses already this year, as a result of hay cut last year," said Karen Waite, Michigan State University (MSU) Extension equine specialist.

Horse owners need to learn to identify hoary alyssum and the signs of its toxicity in their animals. Waite warns that those who might 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 also graze on hoary alyssum if they find it in their fields. Although horses don't seem to prefer it, they will nibble on it when other plants are not growing because of overgrazing or drought.

Signs of toxicity in horses range from depression to stocking up (swelling in the lower legs), fever, and diarrhea. Some horses are extremely susceptible to the toxin. In these animals, consumption of hoary alyssum could prove fatal. As little as 10% hoary alyssum in one bale of hay might cause problems for some horses.

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 safe plants in the pasture, removing the animals for the remainder of the growing season should be considered.

Hay should also be thoroughly examined for the presence of hoary alyssum.

MSU 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." It can be found online or is available at any MSU Extension county office.

Images and other information about hoary alyssum can also be found on the MSU Diagnostic Services weed and plant identification Web site.  

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