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Environmental Factors Contributing to Plant Toxicity
There are no common denominators that occur in
every plant poisoning of an equine. Rather, there are a number of factors that
may influence the occurrence and/or severity of these events.
Grazing stress, heat, drought, frost, and other physical influences all
have an effect on the toxicity of the plants themselves.
Drought may significantly affect the toxicity
of an individual
plant and the composition and herbivory of the entire plant communities, as in
the case of Senecio species. Some plants in which toxin levels are altered by
drought include nightshades, sorghum grasses (Sorghum spp.), wild cherry
or chokecherry (Prunus spp.), and pigweeds (Amaranthus spp.).
On the other hand, excessive moisture can also be a contributing
factor to toxicity. In the case of
certain sweetclovers (Melilotus spp.), hay or haylage containing these
plants may be toxic if put up under wet conditions or not properly cured, due to
the presence of coumarin.
The occurrence of a killing frost (or hard freeze)
is sufficient
to convert certain glycosides in plants to prussic acid, also known as hydrogen
cyanide. This is most likely to take place in the forage Sudangrasses (Sorghum
bicolor.) but the potential for it also exists in white sweetclover, vetches
(Astragalus and Vicia spp.) and chokecherry (Prunus virginiana).
The lush regrowth of sorghum species after frost also may accumulate high levels
of prussic acid.
Soil mineral content and balance may in certain cases have an
effect on plant toxicity. One
mineral that livestock producers and equine owners in many areas have difficulty
regulating in pastures and hay is selenium.
Soils high in selenium readily supply plants with the mineral, though all
selenium in soil is not available to plants. There are two types of selenium
accumulators: those that grow only on soils high in selenium are called obligate
accumulators, and those that do not need soil with a high selenium content to
concentrate it in their tissues are called facultative accumulators. Obligate
accumulators are often highly unpalatable and possess an odor of garlic and
sulfur. Examples of these are
milkvetches (Astragalus spp.), prince’s plume (Stanleya pinnata),
some woody asters (Xylorrhiza glabriscula), and goldenweeds (Oonopsis
spp.).
Facultative accumulators are usually more palatable and readily eaten by
horses but they become less so with higher soil selenium levels.
These include saltbush (Atriplex spp.), curlycup gumweed (Grindelia
squarrosa), and broom snakeweed (Gutierrezia sarothrae).
Other potentially toxic minerals include aluminum, cadmium,
mercury, molybdenum, and arsenic, though poisonings due to these are quite rare.
Soils that are deficient in phosphorus or sulfur may also contribute to
the accumulation of nitrates by plants.
Applied herbicides and added fertilizers may also
impact the
levels of toxins. Herbicides can
increase the palatability of some plants, but can also affect their level of
toxicity. For example, herbicide
application can increase levels of cyanogenic glycosides in plants that contain
these compounds. The herbicide
metsulfuron has also been found to increase the toxicity of larkspur.
Fertilizers may also raise the toxic levels of cyanogenic
glycosides in certain plants. Horse
manure contains an average of .05% phosphorus; therefore if heavily used horse
pastures are fertilized with phosphorus, the soils may exceed optimum levels of
the element.
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