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White (Oxytropis sericea Nutt. in T. G.) and Lambert (O. lambertii var. biglovii Pursh) locoweed grow adjacent to each other on the foothills of the Rocky Mountains from southeastern Wyoming to northeastern New Mexico. Lambert locoweed matures later and flowers about 3-4 weeks after white locoweed, thus potentially increasing the critical period of poisoning when livestock graze areas infested by both species. The objective of this study was to evaluate cattle consumption of these 2 species as they progress phenologically. In 1998, 15 Hereford cows grazed a 32 ha pasture infested with both species from the time white locoweed flowered in mid June until both species were mature and senesced in August. In 1999, 4 cows were placed in a 5 ha pasture infested with both species for 4 days in each of the following periods: (1) flower stage of white locoweed, (2) flower stage of Lambert locoweed, immature pod at white locoweed, (3) immature pod stage of Lambert locoweed, mature pod while (4) mature pod and seed shatter stage respectively. Diets were estimated by bite-count. Lambert locoweed was preferred over white locoweed in the season-long grazing trial in 1998, and in each of the 4 intensive grazing trials in 1999. The cows consumed white locoweed as availability of Lambert locoweed declined in 1998, but little white locoweed was consumed in the 4 intensive grazing trials in 1999. The toxic locoweed alkaloid swainsonine ranged from 0.04 to 0.06% in white locoweed, but was not detected in Lambert locoweed in this study. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020