Dr Lou Baskind, the District Veterinarian from South East Local Land Services in Braidwood, discussed weeds, livestock and how pasture plants can be poisonous to ruminant livestock in some conditions. Temperature, rainfall and the plant’s lifecycle can influence the amount of poisonous compounds in a plant. Other plants have inherent physical and chemical properties that are poisonous.
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The three common problems that weeds can cause in livestock are:
- Physical impediments such as wool contamination and physical damage to the animal
- Animal species are affected by different weeds, for example, alpacas can tolerate some weeds that sheep and cattle can’t.
- Sharp weed seeds can cause damage to the face, eyes and hide of animals. The affected animal is then predisposed to other health problems including secondary infections such as pink eye in cattle and scabby mouth in sheep.
- Weeds often out compete other more nutritious pasture species and this can affect the animal’s ability to consume enough feed to meet its nutritional requirements. The fibrous nature of some grass weeds with low digestibility creates gut fill, limiting the intake of nutritious feed. Most weed species are too low in nutrition to maintain body condition. Serrated tussock has the same digestibility as cardboard.
Over 200 plant species are potentially poisonous to ruminants. The three main factors that contribute to plant poisonings are:
- Plant factors such as toxic chemicals in the plant that are there as a defense mechanism, the stage of growth and the part of the plant
- Environmental factors – temperature, water stress and drought
- Animal factors – age, species, prior learning, hunger, malnutrition and confinement.
- Fodder oats can cause nitrate toxicity in cattle. When grazing new paddocks be careful with monocultures of the same grass type. Where possible avoid sudden changes of diet because the microbes in the rumen don’t have time to adjust to the new feed which can cause poisoning. Feeding roughage (such as oaten hay) is protective against nitrate poisoning.
- If you have multiple deaths in your flock or herd, call your local vet or the district veterinarian. Don’t dispose of the animal carcasses because they can be used for autopsy and blood testing.
- First aid if you suspect plant poisoning: remove the animals quietly from the pasture they are grazing, don’t stress or overwork the animals. Provide clean (not green) oaten hay, shade and access to water.
- Plant poisonings can be chronic (sudden onset) and cumulative.
St John’s Wort poisioning
Bracken fern poisoning
Australia’s poisonous plants, McKenzie, R 2012
NSW DPI Weeds Wise App