GRAZING OF SPOTTED LOCOWEED (ASTRAGALUS LENTIGINOSUS)BY CATTLE AND HORSES IN ARIZONA

Authors: Pfister, James; Stegelmeier, Bryan; Gardner, Dale; James, Lynn

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2003
Publication Date: 8/1/2003
Citation: PFISTER, J.A., STEGELMEIER, B.L., GARDNER, D.R., JAMES, L.F. GRAZING OF SPOTTED LOCOWEED (ASTRAGALUS LENTIGINOSUS)BY CATTLE AND HORSES IN ARIZONA. JOURNAL OF ANIMAL SCIENCE 81:2285-2293, 2003.

Interpretive Summary: Locoweeds are toxic plants of the genera Astragalus and Oxytropis, and they may be the most serious poisonous plant problem on western U.S. rangelands. The objective of this study was to investigate locoweed consumption patterns in cattle and horses in relation to locoweed growth and toxicity. Five cow-calf pairs and 4 horses grazed a 50 acre pasture about 10 miles SE of St. Johns, Arizona during spring, 1998. Locoweed dominated the available forage, far exceeding the amounts of other forage. Spotted locoweed was a nutritious feed, averaging 30.4% NDF and 18.4% crude protein. Horses ate more locoweed than did cows (0=15.4 and 5.1%, respectively). Horses generally increased spotted locoweed consumption over time, whereas cattle ate little locoweed until later in the grazing season. Cows ate more dry grass (36.6 vs. 7.8%), other forbs (7.7 vs. 0.7%) than did horses, whereas horses selected more green grass (75.9 vs. 48.6%). Horses exhibited depression and began losing weight after they had been eating locoweed for about two weeks. At the end of the 8-week study all horses were severely poisoned. Our observations suggest that horses were selecting green locoweed instead of dormant grasses precisely because locoweed was green. Horses avidly selected the small quantities of green grass that were available, and it appeared that their propensity to eat scarce green forage in

Technical Abstract: Spotted locoweed (Astragalus lentiginosus var. diphysus) is a toxic, perennial plant that may, if sufficient precipitation occurs, dominate the herbaceous vegetation of pinyon-juniper woodlands on the Colorado Plateau. Five cow/calf pairs and four horses grazed a 20 ha pasture with dense patches of locoweed about 17 km from St. Johns, Arizona during spring, 1998. Locoweed density was 0.7 plants m-2 in the pasture. Locoweed averaged 30.4% NDF and 18.4% crude protein. Concentrations of the locoweed toxin, swainsonine, fluctuated from 1.25 to 2 mg g-1. Horses ate more (P<0.01) locoweed than did cows (15.4 and 5.1%, respectively). Horses generally increased locoweed consumption over time, whereas cattle ate little locoweed until later in the grazing season. Horses were very avid in selecting the small quantities of green grass, and it appeared that their propensity to eat scarce green forage influenced their locoweed consumption as well. Horses ate relatively little dry grass, even when it was abundant, whereas cattle ate large amounts of dry grass until green grasses became more abundant. Calves began eating locoweed on the same day as their dams, and ate about 20% of their bites as locoweed. Serum concentrations of swainsonine were higher (P<0.05) in horses than cattle (433 vs. 170 ng ml-1, respectively). Baseline swainsonine was zero in all animals, but swainsonine was rapidly elevated to above 800 ng ml-1 in horse serum as they ate locoweed. Horses exhibited depression after eating locoweed for about 2 weeks, and after 5 weeks of exposure, horses became anoretic and behaviorally unstable. Although limited in scope, this study indicates that horses should not be exposed to spotted locoweed.