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Introduction
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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