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Plant Factors Contributing to Plant
Toxicity
There is no one particular compound or single factor that
creates a poisonous plant. “The degree of danger a poisonous weed represents
is a function of the plant’s prevalence, toxicity, and desirability”.
The general toxicity of a plant is directly related to the toxin or
toxins it contains, the organ or system it affects, and species of animal which
ingests it. There
are also plant attributes that affect the specific toxicity of a plant or group
of plants.
The growth stage of plants often has a significant effect on the
amount of toxins an equine will ingest when grazing the plant.
In some plants, the immature, new growth may be the most poisonous, while
in others the mature growth contains the highest amount of toxin.
Many plants such as larkspur (Delphinium spp.) and death camas are
far more toxic when young, which is also when they are at their most succulent
and likely to be selected. The
leaves of the red maple (Acer rubrum), however, are not toxic when green,
but are poisonous when dried or shed. Many plants have equal toxicity throughout, with little
variability between stems, leaves, and fruits.
The toxic effects of some plants, such as the black walnut
tree (Juglans
nigra), do not even require ingestion, but are the result of contact with
its tissues or inhalation of its oils or pollen.
Plants vary in palatability and their level of appeal to grazing
animals. There is an order of
palatability in pastures that changes with the seasons.
Horses will selectively graze for less mature and more tender forages.
As plants mature, palatability decreases, which may result in selective
grazing and overgrazing by horses. After the desirable young forage is grazed
out, mature grasses and other less palatable forage, including poisonous plants,
may become more acceptable and more likely to be eaten by equines.
In addition, in equines in particular, selectivity results in
spot grazing, which tends to create spaces for less desirable species, including
weeds, to establish.
Animals tend to select for less toxic plants if given a choice,
and plants containing certain alkaloids will be consistently avoided.
However, grazing animals will readily graze more unpalatable and possibly
toxic forage if forced by hunger to do so, and toxic effects may not alter
intake.
Most forage grasses have been found to be toxin-free and of
little concern. If the pasture
consists of largely forage grasses and is kept in relatively good condition,
chances are there will be few, if any, toxic events.
However, in one study, forbs were found to constitute 40% of the diet of
the equines, while grasses and sedges contributed 39%.
Studies have shown that some toxic plants are highly palatable,
and horses will seek them out in preference to other forages.
These preferences are sometimes referred to as picas, the
addiction to abnormal or unusual foods.
Addictive
plants to which equines may develop a preference include yellow starthistle (Centaurea
solstitialis), locoweeds, and trailing or creeping indigo (Indigofera
spicata). This may occur in as
little as four days.
Fruits and sweetness contribute to plants’ palatability, which
vary with season and moisture content.
Plants
sprayed with certain herbicides such as 2,4-D have higher sugar content, and the
resulting increase in sweetness raises their palatability level to equines.
Some palatable toxic plants such as white snakeroot (Ageratina
altissima) are much more toxic as very mature plants, a condition that
persists when dried, resulting in possible poisoning problems if included in
hay.
Still other plants are highly palatable and toxic in hay though
not generally palatable or eaten as green plants in the pasture.
These plants include houndstongue (Hieracium cynoglossoides),
buckwheat (Fagopyron esculentum), common St. Johnswort or Klamath weed (Hypericum
perforatum), and western brackenfern (Pteridium aquilinum).
However, some poisonous plants become less dangerous if the
plant material is eaten dried in hay.
Examples
of this are plants containing cyanogenic glycosides such as oleander (Nerium
oleander) and buttercups (Ranunculus spp.), and others containing
certain alkaloids.
The amount and part of the poisonous plant ingested have a
direct effect on the duration and severity of plant poisoning effects. The figure most often cited as potentially lethal is five to
ten percent of the animal’s body weight over a period of two to ten weeks.
However, grave or critical episodes may occur in as little as
very few hours or as long as several months after ingestion of the toxic plant
material. Toxins often accumulate
in the animal’s organs and tissue, which may result in symptoms appearing in
winter when the animal is on uncontaminated hay rather than the previous summer
when the green plant was grazed.
Toxic amounts may range from as little as a few ounces, as for
Japanese yew or western water hemlock (Cicuta douglasii) to as much as 86
to 200% of the animal’s body weight, as has been estimated for yellow
starthistle. In theory, nearly
anything can be toxic to an animal if it consumes enough of the material, though
fortunately most equines will not gorge themselves on less palatable foods.
In addition to the weight or amount of plant matter eaten, the
plant part an animal consumes may have a significant effect on the amount of
actual toxic compound it ingests. For
instance, the seeds of the common fiddleneck (Amsinckia menziesii v.
intermedia), trailing or creeping indigo, and rattlebox or rattlepod (Crotalaria
spp.) contain the highest concentrations within the plant of an alkaloid toxic
to horses.
The direct digestive process is the most frequent method by
which toxins affect animals negatively.
However,
an additional problem is the tendency of certain toxins to reduce the
digestibility of other forages eaten with the toxic plant, resulting in reduced
nutritional value of the overall forage to the animal.
These compounds may tie up nutrients, kill digestive microbes or enzymes,
or reduce preference. Therefore, even if the compound does not cause direct
toxicity to occur in the animal, it may still indirectly cause immediate or
eventual systemic or digestive damage.
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