Utilization of White Locoweed (Oxytropis sericea Nutt.) by Range Cattle

Abstract

Utilization studies conducted on a high mountain range determined the quantity and timing of white locoweed (Oxytropis sericeu Nutt.) consumption by cattle. Paired plots (one caged and one grazed) were clipped at the end of the grazing season to determine seasonal utilization. Biweekly visual appraisals were used to estimate percentage leaf grazed and number of reproductive stalks grazed and thus determine utilization patterns as the season progressed. Loco comprised 26% of the standing crop. Thirty-four percent of the available loco was utilized during the grazing season. Loco flower and pods (heads) were preferred to leaves. Utilization of loco heads increased linearly as the season progressed. LOCO leaves were not consumed until the last 3 weeks of the grazing season. Loco heads also contained the highest concentration of the toxic alkaloid, swainsonhre. Locoweed poisoning of livestock has been the most widespread poisonous plant problem in the western United States (James et al. 1981). Marsh (1909) reviewed reports of locoweed poisoning dating back to 1873 and conducted feeding trials that showed conclusively that white locoweed (Astragalus lambertiinow considered to be Oxytropis sericea Nutt.) and wooly loco (A. mollissimus Torr.) caused locoism in cattle, sheep, and horses. Mathews (1932) conducted further studies to determine quantities of Earle loco (A. mollissimus var earlei Green ex Rydb.) and Garbancillo (A. wootoni Sheld.) required to produce clinical signs of toxicity in domestic livestock. The toxin in locoweeds has been identified as the indolizidine alkaloid swainsonine (Molyneux and James 1982). Desert and semidesert locoweed species (A. lentiginosus Dougl., A. pubentissimus Torr. and Gray, A. mollissimus Torr.) characteristically germinate and become established during a warm, wet fall, remain green over winter, and accelerate growth in early spring. Most livestock losses occur during the winter and early spring when other forage is scarce, or the existing forage is dry and the Authors are with USDA, Agricultural Research Service, Poisonous Plant Research Laboratory, 115OEast 14OONorth, Logan, Utah8432l;and withTexasTechUniversity, Lubbock. The assistance of Steve Fitzwater for his assistance in data collection and Russ Molyneux for alkaloid determination is gratefully acknowledged. This is Journal Paper Number 3103, Utah Agr. Exp. Sta. Manuscript accepted 21 October 1985. 344 locoweed is green (James et al. 1968, James et al. 1969). Due to the habituating properties of loco, livestock that begin eating it will continue to consume it even when good quality green feed is available (Peters and Sturdevent 1908, Marsh 1909, James et al. 1969). White locoweed differs somewhat from other locoweed species. It occurs on the western plains and foothills of the Rocky mountains and up to 3,353 m on some Intermountain ranges (Barneby 1952). It is palatable and livestock graze it throughout the growing season (Marsh 1909). Its population cycles on high mountain ranges apparently are not influenced as much by weather conditions as other locoweed species (James personal observation). Payne (1957) classified it as an increaser species. Recently it has been identified as a predisposing factor in high mountain brisket disease, or congestive right-sided heart failure, in cattle (James et al. 1983). Economic loss occurs annually from the interaction of these two diseases (Barnard 1984). Little is known about why livestock graze locoweeds. Marsh (1909) made some astute observations of livestock grazing white locoweed and made some broad general recommendations to reduce losses (Marsh 1913). However, until we know when cattle eat white locoweed, how much is required to produce intoxication, and what environmental or management factors influence consumption, little can be done to reduce losses other than keeping livestock off infested ranges. The purpose of this study was to describe utilization patterns of white locoweed by cattle on a high mountain range and identify factors that may influence consumption. Once we understand the conditions under which cattle consume white locoweed, management strategies can be developed to reduce losses.