Selenium. Gumweeds accumulate selenium when growing in seleniferous soils. Selenuim, rich soils are found in the central United States, typically in the dry, alkaline soils of the western states. Excess selenium in the diet causes abnormal hair and hoof formation as a result of selenium replacing sulfur in keratin, the primary protein in hair and hoof.
Biennial or perennial herbaceous plants growing 2-3 feet in height. Leaves are alternate, simple serrate and more or less dotted with resin. The flowers have yellow ray flowers (some are rayless) and are surrounded by several layers of pointed bracts that are recurved. The flower buds often contain abundant white gum-like resin.
Initially circular ridges forms in all feet. As the hoof wall grows out, the ridges may crack, causing severe lameness due to lamimitis.The hoof wall distil to the crack may slough off exposing the pedal bone. The fore feet are often most severely affected, but all four feet are generally affected.
Affected animals should be removed from any food or water source containing more than 5 ppm. of selenium. Diets containing adequate amounts of sulfur and copper have a protective effect against chronic selenium poisoning. Alfalfa has sulfur-containing amino acids and is a useful food source in areas with selenium-rich soils.
Chronic selenium poisoning causes loss of the long hairs of the mane and tail.
A diagnosis of selenium poisoning is best confirmed by submitting samples of hay or forages for analysis. Selenium levels greater than 5 ppm should be considered potentially toxic. Blood levels of 1-4 ppm are typical of chronic selenium poisoning, whereas serum levels up to 25 ppm have been reported in acute poisoning. Liver and kidney levels greater than 4 ppm are indicative of selenium toxicosis. Hoof wall containing 8-20 ppm is indicative of chronic selenosis. Hair samples containing in excess of 10 ppm selenium indicate excessive selenium intake capable of causing toxicity.