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  Behavioral Education for Human, Animal,
Vegetation & Ecosystem Management

Stories of Applied Animal Behavior
Created by members of a graduate Foraging Ecology Class
     at the University of Idaho and Washington State University
     under the direction of Drs. Karen Launchbaugh and Lisa Shipley

Does Drought Contribute to Livestock Losses
from Poisonous Plants?

By Rachel Frost 


Drought is a serious threat to the livelihood of livestock producers everywhere.  When a rancher loses livestock to poisonous plants during a drought, it only serves to compound the economic hardship he or she is facing due to increased supplementation costs or reduced stocking rate.  It is generally accepted that most poisonous plant losses occur when all non-poisonous forage has been exhausted and poisonous plants are the only available food source. The main challenge of drought is a decrease in forage production, that coupled with a lack of planning, puts livestock in the ultimate foraging predicament.  Do they consume plants that they know to be poisonous or face starvation?

Drought's Contribution

Ironically, the initial response of most plants to moisture stress is increased palatability due to delayed maturity.  Therefore, while there is less forage overall, it is generally of higher quality and greater palatability. On the other hand, some plants experiencing drought increase their concentrations of qualitative defensive compounds (those that are effective at small doses) and decrease their concentrations of expensive, quantitative compounds (those that require large doses to cause poisoning).  Periods of drought can also stimulate plants to form toxic compounds like nitrate and prussic acid that interfere with the herbivore's ability to transport oxygen in the
blood.  In these situations drought acts to increase the toxicity of the plants, but this only affects the herbivores if they choose to consume the plant at that time.

Learned or Inherited Behavior of Herbivores

Herbivores can learn how to differentiate between nutritious and toxic foods based on how they feel after eating the food.  They cautiously include small amounts of new foods in their diet in attempts to determine the value of a food without experiencing any harmful effects of the toxins.  Herbivores must continuously sample foods, even familiar foods in familiar environments, as the nutrient content and toxicity of plants change frequently.  To further complicate the problem, many poisonous plants are quite nutritious.  In order to reap a portion of the nutritional benefits of a poisonous plant, herbivores can learn to regulate their intake below the toxic threshold of a plant.

Herbivores gather information on what plants are palatable by observing  their mothers and peers.  While social learning is not effective in teaching animals to avoid foods, it is an extremely strong force encouraging animals to sample foods they see others eating.  If the food does not cause the animal to feel ill, then they will continue eating the plant.  Despite the undeniable influence of social facilitation, ultimately the decision of whether or not to consume a plant is made by the individual.

To Eat or Not to Eat?

There is no evidence that drought, in and of itself, is the reason for livestock choosing to consume poisonous plants.  Rather several factors about the animal, the vegetation, and the climate interact to produce foraging situations that result in livestock dying from poisonous plant consumption.

(1). Herbivores grazing rangelands suffering from extended drought are likely to be experiencing shortages of needed nutrients, and be in poor body condition.  The decline in palatable forage availability creates a conflict between their desire to alleviate nutrient deficiencies and their instinctive avoidance of any forage that gives negative feedback.  Intense hunger from forage deprivation may induce animals to increase intake of toxic plants despite their potential  negative feedback.  To further add to the herbivore's problems, animals under
nutritional stress may be less able to detoxify plant secondary metabolites.  This occurs primarily because of the nutritional "cost" of metabolizing a toxic compound. Detoxification most often occurs in the liver and requires additional nutrients to alter toxins and allow the body to maintain acid-base equilibrium.  Prolonged periods of nutritional stress can result in a loss of liver mass, and low protein diets decrease the amount of activity of the cytochrome P450 enzyme system of the liver further hampering the animal's detoxification abilities  For chemically defended plant to be contained in the diet, it must be quite nutritious itself, or there must be sufficient other
nutritious forage available to help meet energy requirements.

     (2). Herbivores are dependent on how they feel after eating to "tell" them if a plant is potentially toxic.  However, some poisonous compounds found in plants do not stimulate the emetic system (i.e., cause nausea), so no connection between the taste of the food and the illness is ever formed.  Furthermore, compounds that result in allergies, bloating, and lower intestinal discomfort generally do not form food aversions.  Therefore, animals do not recognize the plant as harmful and will increase consumption of the plant until a lethal dose is consumed.

     (3). Strange as it may seem, an animal's surroundings can help determine if it survives an encounter with a poisonous plant.  It is possible that herbivores may die from ingesting what has always been harmless amounts of plant toxins if they are in a different location.  Perhaps this is a contributor to livestock consuming lethal amounts of known poisonous plants when moved to different pastures.  Managers that are forced to alter their grazing patterns during drought may experience greater death loss because the animals were in different pastures at the time the plants were most toxic.

     (4). Herbivores face the added difficulty of multiple types of toxins being present in rangeland forages, and , multiple plants containing the same toxin.  In a situation of limited forage availability, animals may be more likely to consume small amounts of many poisonous plants within a short time period that contain synergistic compounds that increase the effects of the toxins.

      (5). Animals have the ability to learn which foods ameliorate toxicity, and to combine those foods in the proportions that minimize toxic effects.  Once again, in a limited forage situation, one of the necessary plants may not be available, so the herbivore is unable to counteract the toxic effects of a plant.  Furthermore, if the "medicinal" plant is less tolerant of drought, it may decline to the extent that animals can no longer harvest enough of the plant to be effective.

Grazing Management to Minimize Loss

The main reason ranchers experience livestock losses to poisonous plants appears to be because of  management errors, primarily failure to provide dequate forage for animals at all times of the year.  Drought often serves  to bring these management problems to the forefront and compound their negative affects. Managers should consider implementing a grazing plan that will provide for emergency "forage reserves" and reduce stocking density to avoid causing animals to consume poisonous plants out of hunger.  Simultaneous grazing of multiple species of livestock can prevent selective grazing of one species from moving the plant community toward one dominated by poisonous plants. Careful planning and conservative stocking rate appear to have the greatest impact on reducing losses to poisonous plants regardless of the weather. 

Additional References

Holechek, J.L. 2002.  Do most livestock losses to poisonous plants result from "poor" range
     management? J. Range Manage. 55:270-276.

Pfister, J.A., F.D. Provenza, K.W. Panter, B.L. Stegelmeier, and K.L. Launchbaugh. 2002. Risk
     management to reduce livestock losses from toxic plants. J. Range Manage. 55:291-300.

Provenza, F.D., J.A. Pfister, and C.D. Cheney.  1992.  Mechanisms of learning in diet selection
     with reference to phytotoxicosis in herbivores.  J. Range Manage. 45:36-45.

Taylor, C.A., Jr., and M.H. Ralphs. 1992. Reducing livestock losses from poisonous plants
     through grazing management. J. Range Manage. 45:9-12.

Thurow, T.L., and C.A. Taylor, Jr. 1999. Viewpoint: the role of drought in range management. J.
     Range Manage. 52:413-419.

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Learn more about the Foraging Ecology Class by visiting http://www.cnr.uidaho.edu/range556/