How to avoid poisoning of livestock by blue-green algae

Page last updated: Wednesday, 13 March 2019 - 9:21am

Blue-green algae are a group of algae including Nodularia spumigena, Microcystis aeruginosa and Anabaena circinalis. They can produce spectacular blooms appearing like iridescent green paint or curdled greenish milk on water surfaces. Some blue-green algae can produce toxins that cause poisoning in livestock.

Causes of a blue-green algal bloom

Blue-green algae growth is triggered by:

  • excessive phosphorus and nitrogen, which promote rapid algal growth and multiplication. Sources of these nutrients include inflow of water from arable land, animal excreta, decaying organic matter and industrial or sewerage waste.
  • warm water with blooms more likely to occur in calm, shallow water during summer and autumn where temperatures exceed 18 degrees Celsius in surface water.
  • lack of microscopic organisms that feed on algae and keep algal levels under control.

Signs of a blue-green algal bloom

Signs of a blue-green algal bloom include:

  • formation of a scum that appears like green paint on downwind surfaces of water; algae are vulnerable to the effects of winds or currents so the scum may disappear and reappear on subsequent days
  • an unpleasant odour, like foul sewerage.

Livestock water supplies should be checked daily in summer and autumn for algal blooms. Treat all algal blooms as possibly toxic to livestock and prevent stock access until the algae are identified and the level of toxin determined. For water testing, contact the Department of Primary Industries and Regional Development's Animal Health Laboratories.

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Prevention of blue-green algal blooms

Barley straw

Note: only barley straw releases chemicals that inhibit blue-green algae. Barley straw treatment is not harmful to plants, fish or crustaceans.

Adding barley straw to contaminated or potentially contaminated water has 2 effects:

  • Nutrient stripping – the high carbon-to-nitrogen ratio of barley straw means that when it breaks down it uses available nitrogen in the water. This reduces conditions favourable to algae growth.
  • Release of chemicals that reduce the growth of harmful blue-green algae – Salcolin A and Salcolin B (both flavonolignans) have been identified as active agents against Microcystis sp.


We recommend adding barley straw to a dam at 50 grams per square metre of surface area of the dam water. This rate releases enough chemicals to last for up to 6 months.


Add the barley straw in late spring to early summer, before water conditions favour blue-green algae growth. Barley straw is likely to be more available after harvest, in early summer, and the straw is more effective in warmer water.

When the surface layer of water is greater than 21°C, adding barley straw to an existing bloom may be effective within 2 weeks.

Adding the straw

Place the straw in coarse-weave bags (such as onion bags) and suspend the bags from floats, such as sealed large drink bottles, drums or a pontoon. The floating straw bags will sink as the straw rots. Remove the straw at this stage, and spread it on paddocks away from the dam.

If there is an incoming flow of water, the straw net should be placed where there is a continuous flow of water over and through the straw. This will help to keep the straw oxygenated and spread the active chemicals throughout the water surface.

Ferric alum

Ferric alum can also be used to prevent algal growth and works by removing phosphorus from the water. A rate of 50g of ferric alum per 1000L of water is used and administered by placing a block in a porous bag attached to a float. It is recommended that water supplies be treated before the summer months when algal growth is most likely.

If there is a high risk of erosion due to reduced groundcover around dams, a sediment trap can be constructed using straw bales, netting or corrugated iron to help limit inflows of nutrients in the short term.

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Long-term strategies for control

Long-term control strategies for blue-green algae include:

  • preventing livestock camping near water supplies
  • planting trees and perennial grasses at inflow areas into water supplies to help stop nutrients and eroded soil entering
  • controlling soil erosion as phosphorus particles stick to eroded soil particles
  • avoiding excessive use of fertilisers.

Treatment of blue-green algal blooms

Several chemicals can be used to treat blue-green algae growth. Chemical treatment of algae can cause a sudden release of toxin, which can persist for several days.

Follow withholding periods for re-grazing with livestock to prevent residues and any harmful effects from the chemical. Livestock should be excluded from water supplies for the length of the withholding period or 14 days, whichever is longer.

Some options for chemical treatment are listed below.


Only simazine products that are registered for blue-green algae control should be used and the label rates and directions followed. The most convenient method of application of simazine is to premix it at the appropriate rate in a fire-fighting unit, then jet it onto the surface of the dam.

Since simazine is a herbicide, water treated with the product should not be used on plants for at least 14 days.


Cupricide is a chelated copper product that kills algae with fewer toxic effects than copper sulfate and does not precipitate out when there is a high concentration of carbonate. Water treated with cupricide can be used on plants. Despite its reduced toxic effects, it should not be used in hard water containing trout, native fish or crustaceans, or in water used for livestock where the livestock are grazing plants that may cause liver damage (e.g. caltrop, lupins, heliotrope or ragwort).

Copper sulfate

Copper sulfate is no longer recommended for treatment of water in dams because it can kill crustaceans, fish and aquatic life. Blocks may be used to treat water in troughs, but these can give the water a copper taste that livestock will refuse to drink. It may also cause toxicity in livestock, especially if they have pre-existing liver damage.

Calcium hypochlorite

Calcium hypochlorite is no longer recommended for farm dams because large amounts of organic material interfere with the treatment and it can kill crustaceans and fish.

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Blue-green algae poisoning in livestock

Blue-green algae can produce neurotoxins, liver toxins and skin allergens.

Livestock can be poisoned by ingestion of algae either in the water or as dried mats on the shore, or from drinking water where algal death has released considerable toxins into the water. Poisoning will depend on toxin levels present and ingested, the susceptibility of the livestock and the amount of food in the animal’s gut, which will help counter the effect of the toxin.

All animals are at risk of poisoning from blue-green algae. Sheep are more likely to be affected than cattle because they tend to drink from the dam edges, while cattle often wade into the dam beyond the toxic area.

Toxicity can change rapidly and can increase as a bloom ages or starts to die. Some toxins persist for more than 3 months before being degraded by sunlight and microbial activity. Water can be toxic to stock even after the bloom has disappeared. Sun-dried algal scum can remain toxic to animals for up to 5 months.

Signs of blue-green algae poisoning when neurotoxins are involved

Signs can include:

  • livestock found dead close to water supply
  • muscle tremors, staggers and convulsions
  • death within 24 hours.

Signs of blue-green algae poisoning when liver toxins are involved

Signs can include:

  • ill-thrift and scouring
  • jaundice
  • photosensitisation – pale, bare areas of skin, particularly around the head, become swollen and red, followed by erosions and scab formation
  • deaths can occur 1–2 weeks after ingestion.

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Treatment of poisoned livestock

When blue-green algae poisoning is suspected, livestock should be removed from the contaminated water supply immediately. There is no specific treatment for livestock showing signs of blue-green algae poisoning. Activated charcoal or bentonite can be used before signs develop to prevent further absorption of the toxin, but are expensive and only an option for valuable livestock.

Diseases which may look like blue-green algae poisoning

Call a veterinarian when high stock losses occur

The signs of blue-green algae poisoning can be similar to other livestock diseases that are reportable. Always ask your veterinarian to investigate whenever sudden death and high death rates occur in livestock. Blue-green algae poisoning can have similar signs to anthrax, which is a reportable disease with human health risks and potential to impact some export markets if not contained rapidly.

If you see unusual disease signs in your stock, call your private veterinarian, a veterinary officer at the Department of Primary Industries and Regional Development (see the Livestock biosecurity contacts page), or the Emergency Animal Disease hotline on 1800 675 888.

Contact information

Anna Erickson
+61 (0)8 9881 0211