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Creeping Indigo: Poison Lurks Low in Horse PasturesBy Kentucky Equine Research Staff · August 15, 2017

When’s the last time you took an inventory of what was growing in your horse’s pasture? If you’re like many, daresay most, horse owners, you probably don’t think too much about the weeds that grow amid the pasture grasses. One low-lying plant that can play havoc with your horse’s health is creeping indigo, a legume that causes liver and neurological disease.

Like many noxious weeds, creeping indigo (species include Indigofera spicata and Indigofera hendecaphylla) thrives in pastures that have been overgrazed, trampled, or otherwise left barren, especially in tropical or subtropical regions like Florida. The plant fares poorly with competition and is often, though not always, minimized by a healthy and well-tended stand of grass. Unlike other poisonous plants, horses seem to find the plant palatable, so they may be tempted to eat creeping indigo even when plenty of alternative forage is available to them.

Creeping indigo is considered a prostrate plant, meaning the stems grow directly on the top of the soil, lying flat on the ground with very little height. Branched runners, some a meter or more long, radiate from a central taproot. Stems are usually pale green or yellowish-green, and the flowers range in color from pink to dark red. The signature feature of the plant is the firm, sharp-tipped seed pods. Because the plant produces many seeds, it is capable of spreading quickly.

The leaves and seeds of creeping indigo contain the amino acid indospicine, which causes liver disease in horses and other species. Indospicine interferes with arginine, an essential amino acid.

A little-known disease known as “grove poisoning” was once thought to be caused by chemicals routinely used by citrus growers, but veterinarians and researchers now know the real culprit of the disease: creeping indigo. Grove poisoning, recognized currently as creeping indigo toxicity, is characterized by ataxia, difficulty turning, and failure to walk in a straight line. Horses often show abnormal movement, including a crablike, sideways gait or goose-stepping. Horses may be depressed and anorexic, with related weight loss. Vision and respiratory problems might also occur, including corneal opacity and ulceration. Many horses become so weak that they are unable to rise once down.

Usually, horses have to graze creeping indigo for weeks before health problems arise. If caught in its early stages, toxicity in affected horses can be managed by removal of horses from the offending pasture and appropriate supportive therapy supervised by a veterinarian. Supportive therapy may include supplemental vitamin E, preferably a natural, liquid form such as that found in Nano•E.  Some horses have no long-lasting effects of toxicity, while gait changes linger in many survivors.

In Queensland, Australia, a similar disease of horses and other livestock, called “Birdsville disease,” is caused by the consumption of a different species of indigo.

Creeping indigo is difficult to eradicate in pastures. A pasture or weed specialist should be contacted to determine the best way to eliminate the plant from pastures.