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.

39National Forests and most problems can be solved by grazing management.

False hellebore (Veratrum spp.) is f ound in moist habitats in the Pacific Northwest and Rocky Mountain states. It is grazed by sheep and goats and causes birth defects (i.e., monkey-faced lambs). Livestock management to avoid losses t o false hellebore i s r elatively simple because the window of toxicity when false hellebore poisons t he fetus i s r elatively narr ow (i.e., 14 to 33 days gestation). Pregnant animals, particularly sheep, should not be allowed access t o veratrum-infested pastures during this period. Cattle rarely ea t the plant, therefore no special management is needed. For sheep, false hellebore i s quite palatable, and herders must keep bred sheep from ingesting false hellebore f or about one month after the rams are removed. This is not difficult to accomplish because false hellebore is limited in distribution to moist mountain habitats and grows in easy to identify dense patches. Although effective herbicidal control is available, it may not be practical because the major populations grow in

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.

Acute t oxicity problems are l es s common now, but lar ge sheep losses occurred frequently 100 years ago. Deaths occ ur when livestock, usually sheep, ingest a large amount of seed pods in a short time. This can occur from contaminated hay or from hungry animals gaining access t o lupine-dominated forage, and can be prevented by using lupine-free hay and avoiding lupine-dominated ranges when other forage is scarce. During some years, lupine populations may temporarily increase on rangelands not normally problematic. Livestock producers need to be aware of lupine populations and be sufficiently alert to alter grazing or breeding programs when these eruptions occur. Lupine populations increased dramatically during 1997 in Washington, Oregon, Idaho, and Montana, causing severe l osses. For example, producers in Adams County, Washington lost over 30% of their calves ( >4000 calves) from lupine- caused birth defects.

38of rotational grazing scheme. Herbicidal control of lupines is feasible, but is usually more expensive than altering a grazing management program.

Lupines (Lupinus spp.) are widely distributed throughout the western U.S. Cattle eating lupine may have deformed or "crooked" calves, and sheep may be poisoned outright by lupine. Livestock losses from lupine poisoning can largely be prevented by understanding two interrelated aspects. First, the highest concentrations of toxic alkaloids tend to occur in immature lupine plants and seed pods. Second, pregnant cattle are susceptible to the effects of alkaloids that cause birth defects during a window from 40 to 70 days of gestation, occasionally extending to 100 days. Birth defects in cattle can be prevented by using breeding or grazing programs that avoid placing pregnant cattle in lupine-dominated pastures in the first trimester of gestation. Alternatively, risk can be reduced by allowing only short-term access to lupines by pregnant cattle in some form

Houndstongue (Cynoglossum officinale) is not only a toxic plant that contains alkaloids, but also a noxious weed that is increasing over much of North America. The plant spreads from bur-like seeds that cling to wildlife, livestock and humans, and invades disturbed areas. Houndstongue is generally unpalat- able when growing on rangelands, but lactating cows and horses may eat green houndstongue at times. When houndstongue contaminates hay, it is readily eaten by cattle and horses, and is quite toxic.

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

Nitrogen is a basic building block of proteins and an essential nutrient for animal and plant growth. However, nitrogen in the form of nitrate (NO3) and in excessive amounts can cause l ivestock to ea t less, loose weight, abort fetuses, or even die. Nearly all plants are capable of accumulating toxic l evels of nitrate. However, excessive nitrate concentrations usually only occur when plants are growing in moist nitrogen-rich soils or in cloudy overcast weather. Some plants ar e more l ikely to accumulate nitrates t han others. The key to reducing livestock risk of nitrate toxicity is t o understand the conditions that lead to nitrate accumulation in plants.

Animals possess several mechanisms to negate or restrict the toxic or negative effects of plant compounds once i ngested. If a t oxin is eaten, it is in the animal's best interest to quickly get r id of it. Sheep, goats, a nd cattle can and will vomit in response to eating toxins, but it is rarely observed. Horses probably do not vomit except when near death, but commonly experience diarrhea. Diarrhea aids in rapid elimina- tion of toxins from the gut which can reduce absorption. In some episodes of diarrhea, there is a decrease in intestinal motility, further reducing t he absorption of toxins.

With continued consumption of a plant containing a specific phytotoxin, the animal may gain an ability to overcome its negative effects because enzyme systems in animal tissue can increase their detoxification capacity and efficiency. Rumen microbes may also facilitate the ability of animals to adapt to diets high in phytotoxins. Microbial populations can change rapidly depending on th e substrates available fo r degradation. These "inducible defenses" could explain why herbivores often appear less sensitive to toxic or low quality plants with continued exposure. Nonetheless, adaptation does not develop to all toxins. The effects of many toxins are cumulative and animals may get progressively more poisoned as they continue to ingest plant material containing these toxins

Once plant toxins are absorbed fr om the gut into the blood, they are often trans- ported to the liver. T he liver primarily, and secondarily the kidney, intestinal mucosa, lungs, a nd skin contain enzyme systems that metabolize or alter toxic compounds, rendering them inert. Ability to metabolize or reduce sensitivity to specific phytotoxins varies by herbivore species a nd individuals. For example, sheep can tolerate and detoxify more pyrrolizidine alkaloids than cattle, therefore it takes five times more tall larkspur (Delphinium occidentalis) to poison sheep compared to cattle

Chemical reactions during ingestion may provide protection against the effects of some plant toxins. The ruminant's large forestomach is generally well adapted to bind, sequester, degrade, or detoxify plant toxins. The neutral p H of the rumen environment may modify a plant toxin or the toxin may be quickly diluted in the large volume of the rumen (e.g., ar ound 60 gallons for cattle). Of great significance for ingesting toxic plants is the massive number of rumen microbes that transform most phytotoxins into inert or less-detrimental compounds. For example, leucaena (Leucaena leucocephala) is a tropical forage legume that contains mimosine, a toxic amino acid. Mimosine i s detoxified by a group of rumen microbes and animals susceptible to mimosine toxicity can be cured by receiving a dose of the "mimosine- metabolizing" microbes. Rumen microbes usually reduce the toxic effects of plant compounds. However, in some cases, such as nitrates or cyanogenic glycosides, the rumen microbes convert a harmless compound into a deadly toxin.

This idea that plants become desirable or aversive depending on their digestive consequences is simple. But, how do grazing animals figure out exactly which plants made them fee l good or ill? One way herbivores apparently accomplish this task is by regarding unfamiliar plants with caution. Animals associate positive or negative effects of nutrients or toxins with novel foods when offered meals that contain novel and familiar foods. When foraging bouts include several novel plants, plants that dominate the diet are probably 'weighted' more than less-consumed plants, even if the minor foods were primarily responsible for the positive or negative feedback. Furthermore, digestive feedback begins within 10 to 15 minutes of consumption which could help animals attribute digestive benefits or liabilities to specific plants. Finally, livestock grazing on rangelands usually become familiar with the forage resource and may seldom encounter truly novel plants. This allows greater opportunity to 'sort out' feedback from individual or similar groups of plants

When a grazing animal smells and tastes a plant, the flavor is either pleasing or distasteful depending on the animal's previous grazing experiences. When a plant is eaten, it provides feedback during digestion. If consumption of a plant improves the nutrient or energy status of the animal, the plant flavor becomes more desirable or pleasing. If eating of the plant yields illness, the flavor becomes aversive and distasteful (Fig. 2). These flavor-consequence relationships form the basis for dietary likes and dislikes, and the animal then seeks highly palatable foods and avoids aversive foods. The resulting behavior patterns generally lead to increased consump- tion of nutritious foods and limited consumption of toxic or low quality plants.

The digestive consequences of forage consumption are determined by plant forage quality and animal digestive and detoxification abilities. This interaction, in turn, affects the nutrients and energy available for animal growth and maintenance.

Successfully navigating the seasonal and spatial variation of forage quality in grazing environments can be accomplished by knowing how much to eat, when to eat, and what else to eat. Grazing animals have a strong natural tendency to select diets composedof several plant species and sample available plants on a regular basis. This behavior may increase the likelihood of ingesting necessary nutrients and reduce the potential of over-ingesting toxins. The toxic effects of a plant are determined largely bythe amount eaten, but the ingestion rate may also be important. Grazing animals can avoid toxicosis by limiting their consumption of a specific toxic plant each day to allow sufficient time for detoxification, and to limit potential cumulative effects ofspecifictoxins

When an animal eats a plant, it receives digestive feedback in the form of energy, nutrients, illness, or toxicosis. If the feedback is positive, preferences are formed to the plant and if the feedback is negative, aversions are formed. The strength of the preference or aversion is determined by the magnitude, nature, and timing of digestive feedback