The effect of cattle grazing on the growth and miserotoxin content of Columbia milkvetch

Abstract

The growth and miserotoxin content of Columbia milkvetch (Astragalus miser Dougl. var. serotinus (Gray) Barneby) were examined following grazing of early growth by cows at a grassland site in southern British Columbia. Grazing behavior and forage consumption of cows were observed. Growth of Columbia milkvetch was determined by measuring the freeze-dried weight of each plant and miserotoxin levels were determined by a rapid screening method. Cows had a tendency to either avoid Columbia milkvetch or to consume it incidentally with other forage so long as there was adequate grass available. As grass became scarce the use of Columbia milkvetch increased. After being grazed, the rate of growth and the toxicity of Columbia milkvetch were substantially reduced. In comparison to ungrazed plants, the aboveground biomass of grazed plants was reduced by more than 50% and the average miserotoxin content per plant was reduced by more than 75% during a 6-week period of regrowth. While early grazing may reduce the potential hazard of Columbia milkvetch to livestock, the plant is not a preferred species and may not be consumed by cattle until other forage becomes scarce. Heavy grazing intensity may, in turn, result in low vigor of bunchgrasses and a deterioration of range condition which may result in more weeds in the plant community. Clearly these aspects of management require further study.