Common Name
Two-grooved milk vetch
Botanic Name
Astragalus bisulcatus (Hook.) A. Gray
Plant Family
Dry alkaline soils, containing selenium. Two-grooved milk velch is an obligate selenium accumulator, it requires selenium for growth and therefore is an indicator of selenium-rich soils.
Animals Affected
Cattle, horses, and sheep
Toxic Principle
Requires selenium-rich soils for growth, and accumulates significant selenium when growing in seleniferous soils. Excess selenium in the diet of animals causes abnormal hair and hoof formations as a result of selenium replacing sulfur in keratin, the primary protein in hair and hoof. Two-grooved milk vetch also contains swainsonine the aklaloid responsible for causing locoism. See Oxytropis sericea Nutt. for complete description of swainsonine poisoning.
Leafy stemmed perennial often growing in large clumps. The leaves are pinnate with numerous leaflets. White to pink flowers are produced in dense racemes. The seed pods have 2 grooves running lengthwise which give the plant its name.
Horses, in particular, may become severely lame due to the cracking of the hoof wall. Initially circular ridges forms in all feet. As the hoof wall grows out, the ridges may crack and lames can be severe due to lamimitis and even sloughing of the hoof wall.
Congenital Defects
Deformed bones of the legs (crooked legs).
Animals with chronic selenium poisoning should be fed diets with less than 5 ppm selenium. Adequate levels of sulfur and copper in the diet have a protective effect against high levels of selenium. There is no specific treatment for locoism. Complete recovery from locoism is unlikely becuase of residual brain damage.
Nervous System
Abnormal behavior including sudden changes in temperament, aggressiveness, ataxia, falling over unexpectedly, violent reaction to routine mangement practices such as putting a halter on, and cattle refusing to go through a chute, are typical of locoed animals. Some horses become very depressed and sleepy. Horses often show more severe neurological effects of locoweed poisoning than cattle and sheep. The unpredictable behavior makes the animals dangerous to work around or ride.
Reproductive System
Abortions, decreased prgnancy rates can result from the swainsonine alkaloid in the plant.
Integumentary System
Chronic selenium poisoning causes breaking-off of the long hairs of the mane and tail.
Samples of hay or forages containing 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.
Special Notes
Livestock rarely if ever eat two-grooved milk vetch because it is unpalatable due to the high levels of selenium it accumultaes. Its presence in a pasture however indicates that the soil is high in selenium, and therefore other forages in the area will accumulate selenium and pose a risk to livestock.
Astragalus bisulcatus flowers
Two-groved milkvetch
Astragalus bisulcatus pods with 2-grooves.
Circular hoof cracks- chronic selenosis