Targeted Grazing of White Locoweed: Short-Term Effects of Herbivory Regime on Vegetation and Sheep

White locoweed (Oxytropis sericea Nuttall) and nontarget vegetation response to 2 yr of targeted grazing by sheep, one treatment of picloram plus 2, 4-D (HER) or no treatment (CON) were compared. Serum of sheep that grazed locoweed intermittently (IGZ, 5 d on locoweed followed by 3 d off locoweed) vs. counterparts that grazed locoweed continuously for 24 d (CGZ) was also examined. Alkaloid toxicity was inferred by serum levels of thyroxine (T4), triiodothyronine (T3), alkaline phosphatase (ALKP), aspartate aminotransferase (AST), and swainsonine, as well as behavior and body weight gains. Three sites were used in a randomized complete block design. IGZ, CGZ, and HER treatments reduced locoweed density (P ,0.01), canopy cover (P ,0.01), number of flower stalks (IGZ: P¼0.02, CGZ and HER: P¼0.01), and plant size (P ,0.01). White locoweed seed density in the soil seed bank was not reduced with grazing, and nontarget vegetation was mostly unaffected by treatments. Grass canopy cover increased in grazed and herbicide plots throughout the study (IGZ: P¼0.03, CGZ and HER: P ,0.01). Percentage bare ground was unchanged (IGZ: P¼0.46, CGZ: P¼0.44) in grazed plots but decreased (P¼0.03) in HER plots. After 24 d, ewes in the IGZ treatment had lower levels of serum ALKP (P ,0.01) and AST (P¼0.02) and marginally lower swainsonine levels (P ,0.07) than CGZ ewes that tended to exhibit lower serum T3 (P ,0.07) and similar serum T4 (P¼0.25) levels. Time spent feeding on locoweed tended to differ (P¼0.06) between treatments. Body weight gain was the same (P¼0.19) regardless of treatment. IGZ of locoweed-infested rangeland with sheep may be a viable short-term means of reducing locoweed density without detrimentally affecting animal health.