Research observation: Cattle preference for Lambert locoweed over white locoweed

White (Oxytropis sericea Nutt, in T.& G.) and Lambert (O. lambertii var. bi glovii Pursh) locoweed grow adjacent to each other on the foothills of the Rocky Mountains from southeastern Wyoming to northeastern New Mexico. Lambe rt locoweed matures later and flowers about 3-4 weeks after white Locoweed, thus potentially increasing the critical period of poisoning when livestoc k graze areas infested by both species. The objective of this study was to evaluate cattle consumption of these 2 species as they progress phenologica lly. In 1998, 15 Hereford cows grazed a 32 ha pasture infested with both sp ecies 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 p asture infested with both species for 4 days in each of the following perio ds: (1) flower stage of white locoweed, (2) flower stage of Lambert locowee d, immature pod at white locoweed, (3) immature pod stage of Lambert locowe ed, mature pod while (4) mature pod and seed shatter stage respectively. Di ets 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 in tensive grazing trials in 1999. The cows consumed white locoweed as availab ility of Lambert locoweed declined in 1998, but little white locoweed was c onsumed in the 4 intensive grazing trials in 1999, The toxic locoweed alkal oid swainsonine ranged from 0.04 to 0.06% in white locoweed, but was not de tected in Lambert locoweed in this study.