Not all weeds are created equal and hoary alyssum is a particularly nasty weed for horses – potentially deadly.

Found across the northeastern and north central United States and Canada, hoary alyssum, is a member of the mustard family that produces small white flowers and can grow 1 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," says Karen Waite, Michigan State University 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 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, may also graze on hoary alyssum if they find it in their fields. Though it is not preferred horse forage, horses will nibble on it when other plants are not growing because of overgrazing or drought.

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

Minimizing problems with hoary alyssum and other weeds begins with good pasture establishment and controlled grazing, Waite advises. 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.

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

MSU and MSU Extension offer a number of drought-related resources. For more information about hoary alyssum, horse owners may 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 at pestid.anr.msu.edu/WeedsPlantIdentification/Hoaryalyssum/tabid/139/Default.aspx.