Be aware of toxic plants

Be aware of toxic plants and their impact on livestock

Top Stories
POISONOUS: Mother of Millions Bryophyllum delagoense flowers. Photo by John Hosking.

POISONOUS: Mother of Millions Bryophyllum delagoense flowers. Photo by John Hosking.


While plants play an integral role in the planet's sustainability, under certain conditions even the most innocuous can be detrimental to livestock health.


Plants are vital to life on Earth.

Their ability to take up carbon dioxide and release oxygen into the atmosphere is critical to our continued survival.

Plants remarkable ability to convert sunlight through photosynthesis and store that energy provides the foundations of the food web which supports all life.

There is incredible diversity in plants which humans have utilised as food, fibre, medicine and tools

We have also learnt through hard experience and scientific studies that many plants can be toxic to our health.

Some of these plants are commonly found in our gardens and even our pantries.

Two or three teaspoons of nutmeg can kill an adult and potatoes turning green from exposure to sunlight produce the toxin solanine which can be fatal.

TOXIC: Green cestrum buds, flowers and fruit. Photo: G. Wisemantel.

TOXIC: Green cestrum buds, flowers and fruit. Photo: G. Wisemantel.

Oleander, a common garden plant (a popular planting in primary schools around the 1960s and 1970s) is highly toxic to humans and livestock.  

In 2010, a group of boy scouts in America used oleander sticks to roast their marshmallows on the campfire.

Seven died and five were very ill.

Oleander cuttings thrown over the fence have been responsible in the death of livestock.

While it is true to say more animals die from not getting enough food than die from what they eat, as for humans, plants can be toxic to livestock and it pays to be aware of plants in your region that can be potentially toxic to animals on your property.

Due to the fact that there are many causes of death in livestock and often multiple factors involved, it can be difficult to determine plant poisoning as the cause.

Some plants can be safe to stock and then become toxic  with changes in conditions.

Stock will often ignore toxic plants until feed becomes scarce or they are moved into a new paddock.

Newly introduced animals may be naïve to suspect plants and fall victim to poisoning while the resident herd have learnt to avoid them.

Some plant poisoning of stock can be sudden, common with green cestrum, but in many cases the animals will exhibit symptoms including:

  • weight loss;
  • diarrhoea;
  • staggers
  • photosensitisation (skin becomes very sensitive to sunburn or causes liver damage);
  • irritation;
  • loss of production (wool contamination etc);
  • congestive heart failure – oedema; and
  • red water (bacterial disease which can result from high nitrate plants).            


Primary photosensitisation – a severe inflammatory reaction in the deep skin layers, caused by a reaction between UV radiation from the sun, and plant-derived pigments.

Secondary photosensitisation – a similar reaction, but with chemicals which have built up in the blood stream because of impaired liver function.

Primary photosensitisation is not seen very commonly, but can be caused by plants such as St John’s Wort.

Secondary pphotosensitisation is more common.

Chronic grazing of plants such as heliotrope or Pattersons Curse irreversibly damages the animal’s liver.

The liver is the chief organ responsible for processing toxins in the blood stream.

Without liver function, the animal cannot process a toxin called chlorophyll which is present in all green plants.

This chlorophyll builds up in the animal’s blood stream and reacts with UV light to cause the clinical signs of secondary photosensitisation.

HARMFUL: Stock grazing in Pattersons Curse.

HARMFUL: Stock grazing in Pattersons Curse.

Monitor livestock to avoid nitrate poisoning

During a drought, soil levels of nitrogen increase because of reduced leaching, increased organic  matter breakdown and reduced uptake of nitrogen by plants.

When it rains and the drought breaks, nitrogen is taken up by plants in large quantities.

If hungry animals are allowed free access to these plants, stock losses can be disastrous.

Nitrates are caustic to the gut lining and can cause excess salivation, diarrhoea and abdominal pain.

Nitrites are much more toxic. They can be ingested directly or converted in the rumen from nitrate.

When found in feed, it is usually in stored hay that has been allowed to heat up, or attacked by bacteria or fungi.

Nitrites change haemoglobin in the blood so that it can no longer carry oxygen. This causes death.

Much of the world’s soils are deficient in nitrogen.

Newer farming practices aim to increase it’s concentration in the soil by planting legumes or applying fertilisers like urea.

This can increase the risk of poisonings.

Other factors that increase the risk of poisonings include drought, cloudy weather, application of herbicides and  wilting.

Nitrate levels are usually higher in young, fast growing plants.

Hay made from plants in drought conditions can contain toxic levels when they heat up, especially oaten hay.

So even plants that are normally perfectly good stock feed can become toxic under certain conditions.

Pigs are the most susceptible, then cattle, then sheep, then horses.

To avoid livestock poisoning, landholders need to be diligent in monitoring the plants on their property.

They should be aware of conditions and factors that increase the likelihood  of stock’s vulnerability to toxic plants.      


From the front page

Sponsored by