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June 20, 2002

Report from Jimmy Henning, Extension Forage Specialist
Department of Agronomy
University of Kentucky College of Agriculture

Copper Fertilization and MRLS

Horse farm managers should not apply copper sulfate in an attempt to minimize MRLS or the ergot fungus in pastures.

In a recent letter to the editor of The Bloodhorse, a Virginia horse owner indicated that applying copper sulfate to their pastures seemed to alleviate an abortion problem, and strongly advocated that horse farm managers check their pastures/soils for copper. They recommended to ‘make sure that the copper levels are high enough so this lethal fungus cannot grow.’

The letter also implied that fertilization with copper sulfate will prevent problems arising from the ergot fungus. Ergot refers to a type of fungus and its chemical products, alkaloids, that are known to cause problems in horses. Ergot toxicity arises from fungal growth in the mature seedheads of grasses, and the ergot appears as visible black growths. This fungal growth appears later in the season, only in grasses with mature seedheads and then only under moist conditions. Mowing pastures to minimize seedheads should take care of this problem. Applications of copper sulfate as fertilizer will not prevent the growth of the ergot fungi in the seedheads of grasses.

Tall fescue infected with the endophyte can also produce ergot- type alkaloids, but these are always internal to the plant and are not eliminated by mowing pastures. These compounds have been measured in pastures as part of the UK MRLS Monitoring program since late February. These are either low or at seasonally expected levels. Some fields have had elevated levels of ergot alkaloids originating from tall fescue, but this pasture trait has not been correlated to MRLS.

Copper levels in many Kentucky forages can be low. Diets are commonly supplemented accordingly. Applying copper in fertilizer is not recommended because it will be tightly bound in the soil and therefore will not effectively increase copper in the diet of grazing animals. Applying large amounts can also be toxic, especially to sheep. In addition, horses that had early or late abortions in 2002 consistent with MRLS had normal copper levels in blood.

Making sure that copper requirements are met in the overall diet of horses is prudent. However, there is no real need to test soils for copper, and certainly no evidence that fertilizing with copper sulfate will minimize the occurrence of the ergot fungus or MRLS.

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