Toxic plants

Many pastures grazed by cattle, sheep and goats contain potentially toxic plants.

In small amounts, some of these plants are tolerated well by livestock. However, if grazed to excess or under particular conditions, poisonings can occur.

Plants may be toxic in small amounts, or as plant toxins have a cumulative effect with damage to internal organs developing over time.

It is important to be able to recognise the plants in the area that are toxic to livestock.

Examples of potentially toxic plants grazed by livestock in Australia include:

• Paterson's curse
• Perennial ryegrass
• Pimelea
• St John's wort

Some poisonings by toxic plants have highly visible consequences, while others remain unapparent for weeks or months. In the absence of a drought or seasonal feed shortage, any dramatic change in animal condition should be investigated as a possible disease. This includes poisoning by toxic plants. If this is the case a veterinary investigation should be carried out.

Although there are numerous poisonous plants with a large variety of toxins, there are a relatively small number of effects they can have. Examples are:

• Liver damage, which usually leads to secondary light sensitivity and sunburn can be caused by toxins in plants such as lantana, fireweed and St John’s wort.
• Central nervous system effects can be caused by toxins in plants such as marshmallow, bracken fern and ryegrass (staggers).
• Heart disease can be caused by toxins in plants such as gidgee and
• Kidney damage can be caused by toxins in plants such as the nitrates in pigweed.

With all diseases, nutritional deficiencies and poisonings by toxic plants, assess the risk based on previous local district history.

Managing poisoning risks from toxic plants

Knowledge and management are key elements in reducing the risks posed to livestock by plant toxins. Here are some tips:

• Toxic plants may include pastures species at certain growth stages, native species and garden plants.
• Weeds that spring up first in disturbed or overgrazed soil, after drought-breaking rains or bushfire are attractive to grazing livestock but are often poisonous.
• The relative toxicity of plants may vary according to season and the stage of plant growth:
• wilting in dry conditions and rapid growth after rain can increase the toxicity of some plants
• applying fertiliser to promote lush growth may increase toxicity
• some plants may only be toxic when growing in particular soil types
• stressful growth conditions, such as drought or insect attack, may cause toxins to concentrate in a plant
• plant parts can vary in their relative toxicity
• herbicide treatments can increase the palatability of plants
• When livestock are hungry, they may gorge themselves on things that they would not normally eat. Do not introduce hungry livestock to areas when toxic plants are known to be growing.
• Livestock grazing in a particular area for extended periods may become accustomed to eating small amounts of toxic plant material. New mobs introduced to the same area will not have the same tolerance.