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- Plants are not easily classified as poisonous or non-poisonous. Many plants that contain toxins are nutritious, and correct management can allow moderate use. Most are dangerous only when consumed in large quantities, but a few are lethal even in small quantities.
- Plant toxins can produce both short-term and permanent effects on health. Some damage is untreatable.
- Rangelands typically contain several toxic native species, but animals do not usually consume them to a hazardous degree. Stocking practices that maintain a good stand of forage species will reduce risk. It is critical to know which toxic species are present and when they are most hazardous.
- Hungry animals are less selective in what they eat. Also, a lack of salt or minerals can cause them to seek out toxic plants to fulfill their craving.
- Trailing livestock through populations of toxic plants can be hazardous. While on the move, animals tend to graze any green plant along the way. Sometimes roadsides are colonized by weeds which may be toxic.
- Plant selection is very low in the spring after cattle have been on many winter months of hay.
- Fungi can invade and produce mycotoxins in a few forage species. Usually the fungi are invisible to the naked eye.
- Ruminants (such as cattle) and monogastrics (such as horses) differ in their susceptibility to poisonous plants.
- It is important to distinguish highly toxic species from related species that are non-toxic to avoid livestock losses and effort wasted on dealing with patches of non-toxic plants.
- Water sources can be contaminated with toxic blue-green algae (cyanobacteria). High sulfur content in water can result in a toxic level of this element in a diet already high in sulfur.
- Rangeland plant communities can change over time. Overgrazing can lead to increases in toxic species.
- Road-building equipment can introduce toxic weeds to your range.
Canadian Poisonous Plants Information System
Cornell University - Animal Science - Plants Poisonous to Livestock
Identifying Water Hemlock - video
Natural Resources Conservation Service - Plants Database
Plants Poisonous to Livestock in the Western States - available as PDF
Stock-poisoning Plants of Western Canada - available in PDF format only