Alkaloid-containing plants exact a heavy economic toll on livestock produc-tion in rangelands of western North America. Losses to these plants can be reduced or eliminated by recognizing plants containing alkaloids, understand-ing when livestock graze specific toxic plants, and knowing signs of potential toxicity. Grazing schemes can then be developed based on knowledge of the temporal and spatial dynamics of alkaloid concentration and consumption by livestock.Lossescanbereducedbyensuringthatlivestockarenotexposedorhave limitedexposureduringperiodsofgreatestrisk(i.e.,highesttoxinconcentration)or whenlivestockaremostlikelytoeattoxicplantsinsufficientamountstoproduce toxicity.

Death camas (Zigadenus spp.) grows on foothill ranges in much of the Rocky Mountain area. Animals eating death camas die from reduced blood pressure and heart failure. Death camas is one of the first plants available during spring, and animals may graze the plant if other forage is lacking. Generally, recognizing the presence of death camas and understanding the acutely toxic nature of the plant will aid in avoiding problems. Hungry animals should not bedriventhroughadeathcamas-infestedpasture.Sheepinparticularshouldnotbe beddednearlargepatchesofdeathcamas,andsheepherdersshouldavoidstressing sheep by rapidly driving them if they do eat death camas. Death camas can be controlledbyphenoxyherbicides.

Poison hemlock (Conium maculatum) grows throughout the United States in areas with abundant moisture (i.e., creeks, ditches etc.). Animals eating poison hemlock die from acute respiratory failure or have deformed offspring. The most critical season to avoid poison hemlock is spring because the plant often appears before other forage has emerged. Green seed pods may be eaten in mid-to-late summer. Furthermore, poison hemlock may regrow in fall after seeds shatter. Ingestion during fall may coincide with birth defects in pregnant cattle, if they are i n the firs t trimester of gestation (days 30-75). If poison hemlock invades hay fields, the contaminated hay can poison livestock. Even though toxicity decreases upon drying, sufficient toxins may be r etained to poison livestock. Cattle appear to be particularly susceptible because of their acceptance of the plant and their sensitivity to the alkaloids t hat c ause birth defects. Poison hemlock can be easily controlled with phenoxy herbicides.

37These herbicides do not reduce toxic alkaloid concentrations in treated larkspur plants, and metsulfuron may increase toxicity. Therefore, sprayed areas should not be grazed until larkspur has withered and decomposed.

Senecio or groundsel species (Senecio spp.) and houndstongue (Cynoglossum officinale) contain highly toxic pyrrolizidine alkaloids. These alkaloids are potent liver toxins that cause wasting and photosensitization. Senecios and houndstongue occur on many western U.S. rangelands. Only seven of more than 112 senecio species are known to be toxic so correct identification is essential. Managing rangelands so that plant communities are in good condi- tion and adequate forage is available is crucial to reduce losses to senecio. Generally, senecios are not very palatable, and are avoided by grazing livestock if other forage is available. Drought stress and overgrazing can increase populations of threadlleaf groundsel, as the plant is an aggressive invader. Drought is an especially dangerous time because other forage may be lacking and the toxic alkaloid concentration in senecio plants increases during drought, so grazing animals may ingest higher quantities of more toxic forage. Senecio species are also most toxic when plants are reproducing, thus avoiding pastures when these plants are in bud, flower, or seed is prudent. Proper grazing management must consider stocking rates, as excessive stocking may increase the amount of toxic plant consumed when alternative forages become limited. Excessive stocking may lead to degradation of the desirable plant community allowing senecio species to increase. Herbicidal control may alleviate some problems if incorporated into an overall management program