Several horses were recently reported to have been poisoned after being fed a new batch of hay. The signs of the poisoning were extreme swelling of the horse's legs. After inspecting the hay, we found large quantities of hoary alyssum, which has been associated with toxicity to horses. Because of this incidence, it is worthwhile to briefly review hoary alyssum toxicity to horses and understand the potential risk of infested hay.
Hoary alyssum is a perennial in the mustard family and is common on sandier soils.
It grows a couple feet tall, has small white flowers, and has hairy, grayish-green leaves and stems. At lower percentages, the signs are more mild. Signs of poisoning are not seen in all horses. Hoary alyssum toxicity to ruminants has not been reported, but ruminants may avoid or reject infested hay. Considering that hoary alyssum is not highly toxic like some other weeds and is fairly common, it is probably not practical to completely avoid hoary alyssum.