Hoary Alyssum Toxicity in Horses

Hoary alyssum up close. Notice the tall, stiff stems.

Hoary alyssum is a weed species that typically grows in grazed pastures and along trails or roads.  In pastures, it typically grows along the fence line as opposed to the center of the grazing space, but it’s not picky about where it plants itself.  With this in mind, it is important to note that this weed can be poisonous to horses and appears to adversely affect some horses more than others.  Keeping this in mind, horse owners should become familiar with identifying this plant in their own pastures and in the hay they are purchasing or harvesting for themselves.

Horses that ingest hoary alyssum can experience depression and “stocking up”, or lower leg swelling.  Occasionally, fever and diarrhea may present themselves within 12-24 hours of consumption and symptoms normally subside within 2-4 days after access to the plant is removed.  More severe symptoms can include severe swelling in the lower legs, joint stiffness, apparent founder, and sometimes death.  It is important to note that these symptoms are often a result of high levels (30-70%) of hoary alyssum in hay.

Hoary alyssum plants can be identified by a rosette of leaves (figure 1 below) with a single upright stem ranging 1 to 3 feet tall.  Stems are very stiff and will have many branches near the top of the plant.  The leaves on the stem are simple with a shape that is narrow and oblong.  The stems, leaves, and seed pods are covered in a rough grayish-green “hair” giving the plant its name.  Hoary alyssum will also grow numerous white flowers (figure 2 below) with four notched petals.

Figure 1. Hoary alyssum rosette
Figure 2. Flower produced by hoary alyssum

Aside from being toxic to horses, this plant can also reduce the quality and yield of your fields and thrives on a stressed field or one that has poor soil fertility. Good pasture management and herbicide application can help control this weed, but be sure you are using one that is safe to use on forage grasses. Pursuit and Sinbar are some of the most effective herbicides for controlling hoary alyssum, but be sure to read the product labels to ensure each is appropriate for your particular management as well as the timing of application.

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