Knapweed Toxicity in Horses

An abundant plant in the horse pasture was positively identified as Brown knapweed (Centaurea jacea) by the Morris Arboretum at the University of Pennsylvania. Brown knapweed is a perennial that grows 20 to 48 inches tall and is native to Eurasia and is scattered throughout the western US and Northeast. From our experience, it is only an occasional weed in our region. The Plants of Pennsylvania by Rhoads and Block lists seven species of Centaurea in Pennsylvania with spotted knapweed (Centaurea stoebe or maculosa) being quite common mostly along roadsides, but rarely consumed by livestock. We were not familiar with brown knapweed prior to this report. The knapweeds are a member of the Aster family that also includes the thistles. The potential toxicity of the knapweeds seems to vary. Russian knapweed (Acroptilon repens) and yellow starthistle (Centaurea solstitialis) are plants that are known to be toxic to horses—causing “chewing disease”—but cattle and sheep appear to consume the plants without any apparent signs of toxicity. The toxicity of brown knapweed is less known, but we assume that it may be similar to Russian knapweed or yellow starthistle. The knapweeds are similar in appearance and can sometimes be difficult to distinguish from one another. Brown knapweed is in general a larger plant than spotted knapweed with flowers that are more than double the size (see accompanying image). The flower bracts are often one of the most distinguishing characteristics for the knapweeds, but close examination is still necessary. Brown knapweed flower bracts are broad, thin, with papery margins that appear more or less irregularly torn. The bracts on spotted have obvious vertical veins below the black triangular spot on the tip. The tip and upper margin have a soft spine-like fringe in which the center spine is shorter than the others. Are you confused yet? A Pacific Northwest publication includes identification information for many of the knapweeds including brown and a North Dakota State publication does a very nice job discussing spotted vs. diffuse vs. Russian.

Spotted Knapweed, Montana Weed Control Association

The affected horse in this incident was head tossing, trying to stretch her jaw, sticking her tongue out and having trouble eating. She kept doing a chewing motion. According to the literature, initial symptoms of the disease include impaired ability to eat or drink, as well as anxious or confused behavior. Classic “chewing disease” symptoms include hypertonicity (sustained contraction) of the muscles of the muzzle, lips, and tongue. The mouth may be held open or closed with the tongue hanging out in a curled manner. This is accompanied by constant chewing-like motions of the mouth. During this stage of chewing disease, horses are unable to eat pasture or hay, but their ability to swallow is not compromised. Muscle paralysis means that they are unable to drink water in a normal fashion, and horses may learn to submerge their muzzles deeply so that water will flow to the esophagus, allowing it to be swallowed. Other abnormal behaviors observed include yawning, violent head tossing, drowsiness, and other locomotive impairments. If left untreated, horses normally die of starvation, dehydration, or inhalation pneumonia. Symptoms can become lessened if removed from the infested pastures and treated by a vet.

Horses generally do not like to eat knapweed unless suitable forage is unavailable. Horse owners should monitor their pastures and if they notice horses or other livestock consuming toxic plants, immediately remove the animals from the infected area and provide alternative forage. Fall is an excellent time to manage many perennial weeds in pasture and hay fields, so inspect your pastures this fall and provide appropriate control measures.

Photo credits –
Brown knapweed, Alicia Spangler and Spotted knapweed, Montana Weed Control Association