Minimizing Livestock Plant Poisoning On Western Nebraska Rangelands

Annually, Nebraska producers lose livestock to plant poisonings. Understanding which plants are toxic and strategically managing grazing can help minimize losses.

Scott E. Cotton, Extension Educator

A review of scientific research, including 37 grazing studies, indicates that between 2 and 5 percent of livestock losses in the United States each year are attributed to plant poisonings. According to the data, Nebraska experiences similar losses.

Poisonous plants, whether a native or introduced species, are a component of most Nebraska plant communities. Poisonings vary according to management approaches, climatic conditions, and location, but livestock producers should be aware of significant impacts and correlations that exist.

Although the most profound impact of plant poisoning is livestock death, research shows that other impacts can have a more widespread economic effect on livestock operations. These impacts include reduced fertility, reduced conception rates, weight loss, and reduced milk production. In addition, “ghost poisonings” lead to undocumented abortions. The Colorado State University Diagnostic Laboratory reported in 2005 that over 72 percent of infertility and abortion evaluated there were directly related to plant poisonings, most by the locoweed (Oxytropis spp.) and crazyweed (Astragalus spp.) plants.

In western Nebraska, a number of poisonous plants are found on native rangelands, including larkspur, milkvetch, licorice, crazyweed, lupine, deathcamas, nightshade, and hemlock. Considering that livestock in this region are constantly exposed to these plants, why aren’t poisonings consistent?

Factors Affecting Toxicity to Livestock

Livestock have a number of physiological reactions to toxic and poisonous plants that may help condition them to avoid such poisoning. In other cases, animals die because toxicity is acute (requires extremely small amounts of plant matter) or the animal ingests the plant as a large percentage of its diet in a brief period.

Plant toxicity can vary depending on the animal species, animal condition, growth form of the plant, plant subspecies, plant age, whether the toxin accumulates as a residue, inherent toxin potency, what percentage of the animal’s diet was made up of the plant, and whether other toxins were also consumed.

Other factors that play into whether livestock are poisoned include novelty, forage availability, water availability, grazing intensity, and introduced feeds. When understood, these factors will allow managers to implement Best Management Practices to minimize poisonings in most situations.

Location and Plant Novelty

Livestock adjust to specific regions and plant communities, often developing grazing preferences that minimize plant poisonings under “normal” conditions.

When livestock are abruptly moved to a grazing area with plant communities significantly different than what they are accustomed to, there is the potential that they may consume a higher level of toxic plants due to unfamiliarity and the lack of time for their “negative feedback” functions to work.

Poisonings following moves to new locations can be worsened if there are plants with acute toxin levels, the livestock are hungry when moved, or forage is limited at the new site. In one instance in southwestern Wyoming in 1987, a flock of 10,000 sheep was unexpectedly moved to a new grazing allotment on federal rangelands. Within two weeks the death loss was over 3,000 head. Investigations of the site documented a heavy percentage of Halogeton which had not been an issue when a prior grazer had pastured cattle.

Forage Shortages

Livestock usually develop and maintain forage preferences that make them productive, healthy, and safe. However, when the desirable, healthy forages are not available in the amounts they want, livestock start to consume higher percentages of less desirable forages and begin to experiment with eating plants they find undesirable. For these reasons the likelihood of poisoning can increase when grazing occurs prior to significant growth of desirable plants, on overutilized segments of pasture or overgrazed plant communities, and during periods of drought. It’s common to see documented incidents of poisoning increase during early spring and during drought.

Water Shortages

When livestock have limited access to water they will seek out succulent plants but will specifically search areas where water was present at one time, such as dried reservoirs and streambeds, and will tend to consume more plants near those areas. One of the plants often in these areas is poison suckleya, which is toxic and usually lethal when ingested in small amounts.

Economic Stress Periods

Drought results in less forage on rangeland and often pushes rangeland managers toward using the resources in a manner that reduces the plant selection available to livestock. During drought, the economic feasibility of a landowner conducting selective weed control (chemical or mechanical) of poisonous plants or other undesirable plants is often diminished.

If a manager does not adjust stocking rates to reflect the inhibited forage production during drought, forage shortages, water shortages, and grazing intensity can compound the risk of poisoning.

Intensive Grazing

Intensive grazing is often defined as grazing more livestock per acre for shorter than normal grazing. Research shows if stocking rates are comparable, neither rotational grazing nor season-long grazing changes the poisoning rates. However, the presence of acutely toxic plants such as some milkweeds, locoweeds, and larkspurs may significantly increase losses during intense grazing because very little consumption is required.

Introduced Feeds

During “normal” years, livestock producers usually buy feedstuffs to supplement forage demands of livestock. During years when factors such as drought, competition, forage shortages, or extended winter occur, producers may reach farther afield for supplemental forage. The risk of poisoning increases simply to due to the potential for introducing unknown or unexpected poisonous plants that have been harvested with the purchased feed.

A common scenario is to buy large bales of hay which have picked up showy or whorled milkweed. Whorled milkweed, which has an acute toxin that retains potency when dry, requires as little as 2 to 3 percent of diet composition to cause death. In one instance the plants were consumed in hay by horses which had no familiarity and no aversion to the milkweed plants in hay. After postmortem toxin screening identified the poison, the supplier admitted that the milkweed was common around his hay field but it was of no concern because he fed only cattle which were accustomed to and avoided the milkweed plants.

Toxic Habits (Shared — Herd Effect)

Livestock tend to follow experienced lead animals to the choicest forage locations. In the same manner, lead animals compensating for forage shortage, poor management techniques, or drought conditions can lead less experienced animals into higher levels of poisonous plant consumption.

Suggested Best Management Practices to Reduce Poisonings

Identify Poisonous Plants

Learn Which Toxins Are Your Risks

Manage Your Livestock to Reduce Risk

Table I. Common poisonous plants in western Nebraska. (This list does not include all poisonous plants in western Nebraska.)
Poisonous Plant
Symptoms Mode of Action Location Found

Wooly or Purple Locoweed
Photo copyright Al Schneider

Woolly or Purple Locoweed
(Astragalus mollisimus)
Weakness, convulsions, frequent urination, lack of coordination, abortions, labored breathing, coma Swainsonine alkaloid and indolizidine alkaloids tie up cellular hemoglobin, starving the animal for oxygen Nebraska Panhandle and western Sandhills
White Locoweed
Photo by W.L. Wagner
courtesy of Smithsonian Institution
White Locoweed (Oxytropis sericea) See Woolly or Purple Locoweed See Woolly or Purple Locoweed Western edge of Nebraska
Broom Snakeweed
Photo by Clarence A. Rechenthin
Broom Snakeweed (Gutierrezia sarothrae) Anorexia, rough hair, weight loss and abortions in cattle Strong detergent foaming action in rumen disrupts digestive process and diet Various western Nebraska range sites
Riddell's Groundsel
Photo by Clarence A. Rechenthin
Riddell’s Groundsel (Senecio riddellii) Lethargy, listlessness, abdomen fills with fluid, wobbling, dragging hind feet, some death Pyrrolidizine alkaloid destroys liver, toxins buildup in body Southern and central Nebraska Panhandle
Common Cocklebur
Photo by Clarence A. Rechenthin
Common Cocklebur (Xanthium strumarium) Liver damage; lethal if 0.75 of body weight is consumed Various toxins damage liver Drainages and disturbed areas
Geyer's Larkspur
Photo by Jack Dykinga
Geyer’s Larkspur (Delphinium geyeri) Uneasiness, bloating, face downhill, death Polycyclic diterpene alkaloids block neuromuscular signals Shortgrass prairie, brush, sagebrush southwest Panhandle
Ponderosa Pine
Photo by Susan McDougall
Ponderosa Pine (Pinus ponderosa) Abortions of fetus 14-16 days after consumption Labdane resin acids (ICA) decrease placental blood and oxygen flow Western Nebraska and windbreaks
Silver Lupine
Photo by Tom Barnes,
University of Kentucky
Silver Lupine (Lupinus argentus) Respiratory difficulty and paralysis in sheep Quinolizidine alkaloid inhibits cellular respiration Western Nebraska and Sandhills
Silky Lupine
Photo by Susan McDougall
Silky Lupine (Lupinus sericeus) Crooked calf syndrome in cattle when consumed 40-70 days during pregnancy Anagyrene alkaloid in seed pods deforms fetus Western Nebraska rangelands
Lambert Crazyweed
Photo by Larry Allain
Lambert Crazyweed (Oxytropis lambertii) Weakness, convulsions, frequent urination, labored breathing, and coma Indolizidine alkaloids tie up cellular hemoglobin, starving the animal for oxygen and other factors Dense clay areas like the White River and Toadstool regions of western Nebraska

This publication has been peer reviewed.

Visit the University of Nebraska–Lincoln Extension Publications Web site for more publications.
Index: Animals, General
Issued July 2009