Beware the Buttercups: Pretty Yellow Poison to Horses

University of Kentucky forage specialists have fielded several calls on the safety of yellow buttercups in Kentucky pastures and asked Megan Romano, DVM, DABVT, toxicologist, UK Veterinary Diagnostic Laboratory, to comment on the potential risks to horses.

According to the current USDA PLANTS database, nearly 30 different species of Ranunculus, or buttercups, are found in Kentucky [Ed. and all across Canada, even in the Arctic]. Leaves, flowers and stems of buttercup have a sharp, pungent taste and the plants are generally avoided by grazing livestock.

Some Ranunculus species contain ranunculin, a compound hydrolyzed to protoanemonin when the plants are damaged – for example, when they are chewed. Protoanemonin is a vesicant, meaning it causes blistering of the skin, mouth and digestive system.

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Ranunculus species with the highest ranunculin concentrations are the most toxic. Damage to plant cells also occurs when buttercups are cut and dried in hay. Hydrolysis of ranunculin to protoanemonin likely occurs as the plants dry. Protoanemonin then forms anemonin, which is not a vesicant. Dried Ranunculus plants are therefore expected to lose toxic potential fairly rapidly, although specific research has not been published to confirm this. The risk posed by Ranunculus species in Kentucky is minimal if there are plenty of other forages present – animals avoid grazing the unpalatable fresh plants, and the dried plants appear to be much less toxic.

Buttercups can cause mouth pain and blisters, drooling, oral and gastric ulcers, colic and diarrhea. Horses are probably the most sensitive species to the gastrointestinal effects of Ranunculus species. These effects can be severe if buttercups are ingested in large quantities, but their acrid taste usually deters further grazing. Clinical signs are typically seen only in animals forced to consume buttercups when they have nothing else to eat.

A few anecdotal reports have suggested an association between the presence of Ranunculus species in the pastures and abortions in cattle and horses; these reports are unconfirmed, and attempts to reproduce the disease have been unsuccessful. Bur buttercup (Ceratocephalus testiculatus) can cause significant illness, but this plant occurs primarily in the Western U.S. and is not a true buttercup, as it belongs to a different genus.

A review of UKVDL records over the last 13 years found no cases of livestock deaths attributable to Ranunculus. It is possible, however, that cases of colic or diarrhea have unknowingly been caused by ingestion of Ranunculus species and were never attributed to the plant. Buttercup toxicosis poses the greatest risk to starving animals with nothing else to eat; it can be easily prevented by providing animals with adequate forage. Because animals avoid grazing Ranunculus, it proliferates in overgrazed pastures. Overgrazing can be prevented by maintaining appropriate stocking rates.

According to the UK publication, “Broadleaf Weeds of KY Pastures, AGR-207,” late February or March is the time of the year to spray for buttercup control. Maintaining good grass cover prevents many weeds, including buttercup, from germinating in fall or winter. Resting pastures and not overgrazing are key to improving pasture health. Thin stands with bare areas or that contain summer annual grasses like crabgrass can be overseeded with a pasture mix in September. Be sure to soil test every two to three years and apply amendments based on soil test recommendations. In most horse pastures, nitrogen is most beneficial in the fall to improve root density and thicken stands.

For more information, check out Establishing Horse Pastures or Soil Sampling and Nutrient Management of Horse Pastures. Additional information on buttercup in pastures and control methods can be found here.

~ Ray Smith, PhD, Megan Romano, DVM, DABVT, and Krista Lea, MS, University of Kentucky.