Seasonal grazing of Columbia milkvetch by cattle on rangelands in British Columbia.

Abstract

There is a dearth of knowledge on the selection and utilization of Columbia milkvetch (Astragalus miser Dougl. ex Hook. var. serotinus) by grazing livestock on rangelands in British Columbia. Four grazing trials were conducted with cattle on Columbia milkvetch range in southern interior British Columbia. In the first 2 trials during 1990 and 1991 cattle grazed an upper grassland site in late spring. In 1992 and 1993, the animals grazed a lodgepole pine forest site during early summer. The density of Columbia milkvetch and its basal area were similar at both locations. The Columbia milkvetch was not a preferred species on the grassland site as indicated by the bite count technique that determined its percentage in the diet. Consumption of Columbia milkvetch increased gradually as other forage species were preferentially selected and depleted. On the grasslands, consumption of Columbia milkvetch by individual animals did not show an addictive pattern. At the forest site, utilization of Columbia milkvetch was determined on a weekly basis during 1992 and on a biweekly basis during 1993 by paired plots. In contrast to the grassland site, Columbia milkvetch was a preferred species at the forest site where it was utilized to a greater extent than grasses or other forbs. Approximately 80% of the Columbia milkvetch was utilized during 1992 and 60% during 1993, which was significantly greater than the utilization of grasses or other forbs. Forage nutrient analysis at the forest site indicated Columbia milkvetch had higher crude protein and lower ADF content than other forages but it caused livestock poisoning in 1993.