Phalaris and ryegrass poisoning in livestock

Colin Trengove, Pro Ag Consulting

Phalaris poisoning is a risk when this common perennial pasture starts growing after rain on warm soils. This most often happens in autumn in southern Australia, particularly in areas receiving more than 500 mm of rainfall, but it can also occur in winter or spring when conditions favour new growth.

The main pasture species involved are Phalaris aquatica and Phalaris tuberosa. These grasses can cause poisoning in sheep and cattle, showing up as either phalaris staggers or, less commonly, sudden death.

The problem occurs because toxic tryptamine alkaloids build up in the young shoots of the plant. As the pasture grows and the tillers reach more than about 10 cm, the toxins become diluted and the risk decreases.

Phalaris staggers

The most common form of poisoning is phalaris staggers. Affected animals may show signs days to months after grazing the pasture.

Typical signs include:

• tremors and twitching
• head shaking
• rapid eye movements
• muscle weakness
• falling over or difficulty standing.

Animals often appear normal when left alone, but signs become obvious when they are disturbed or moved. Unfortunately, the toxins can cause permanent brain damage, so recovery is rare. If animals are still able to travel, the best option is usually to sell them for slaughter.

Prevention using cobalt

Providing high levels of cobalt can help prevent staggers. Cobalt encourages rumen microbes to break down the toxic alkaloids before they are absorbed by the animal.

Options include:

• 2 ruminal cobalt bullets before grazing risky pasture
• spraying 50 g cobalt sulphate per hectare every 2 to 4 weeks during high-risk periods.

It’s important to note that cobalt does not prevent the sudden death form of phalaris poisoning.

Sudden death syndrome

A less common but more serious form is phalaris sudden death.

Animals may show signs such as:

• disorientation and wandering
• falling over
• mouth champing
• convulsions.

Death usually occurs within 24 to 72 hours.

This form is often linked to high-nitrogen pasture. Diagnosis can be confirmed by testing ammonia levels in eye fluid within 24 hours of death. Unfortunately, there is no effective treatment.

Key risk factors

The risk of phalaris poisoning increases when:

• rain follows a dry period
• pastures grow rapidly
• soils are basalt or high calcium, which can reduce cobalt availability
• high rates of nitrogen fertiliser are used
• pastures are grazed very short
• livestock are not used to grazing.

Practical ways to reduce risk

Farmers can reduce the chances of poisoning by:

• feeding hay before introducing stock to phalaris pasture
• avoiding sudden introduction to fresh autumn growth
• preventing overgrazing of short phalaris-based pastures
• using lower-risk cultivars such as Sirolan or Sirosa.

Perennial ryegrass poisoning (ryegrass staggers)

Perennial ryegrass poisoning, commonly called ryegrass staggers, can look very similar to phalaris poisoning, so it is important to tell the two apart. Both conditions tend to occur at similar times of the year. Ryegrass staggers usually appear in summer or early autumn, when ryegrass starts growing again after rain or when soil moisture improves. Some older varieties of perennial ryegrass (Lolium perenne)—particularly types originally from Victoria—contain high levels of a fungal endophyte. This fungus helps protect the plant from pests such as the Argentine stem weevil, but it can also cause problems for livestock grazing the pasture. The fungus lives in the leaf, stem, sheath and seed of the ryegrass plant and produces toxins that affect the nervous system of grazing animals.

Signs in livestock

Animals grazing ryegrass-dominant pastures may start to show signs within 7 to 14 days. Typical signs include:

• head nodding or shaking
• jerky or uncoordinated movement
• muscle tremors
• animals lying down or unable to stand properly
• arched backs
• severe muscle spasms.

Affected animals are easily stressed and the signs often become worse if they are forced to move quickly or are disturbed. In most cases animals recover if they are left quiet and undisturbed. Deaths are uncommon if they are well managed and usually only happen due to misadventure such as falling in a dam.

Diagnosis

Ryegrass staggers does not cause obvious internal damage, so it can be difficult to confirm without testing. Laboratory tests can detect the ergot alkaloid toxins produced by the endophyte. These toxins can be measured in pasture samples and/or animal urine samples. Pasture samples can also be tested using an ELISA test to determine the level of toxin risk in a paddock.

Management and prevention

If livestock show signs of ryegrass staggers, the first step is to remove them from ryegrass-dominant paddocks. Most animals will recover once they are off the toxic pasture.

To reduce the future risk of ryegrass staggers, farmers can:

• sow newer ryegrass varieties with low or novel endophytes
• avoid hard grazing early in the season
• provide hay or silage to reduce heavy grazing of ryegrass
• introduce other pasture species into the mix
• use toxin-binding feed additives such as Elitox or Mycofix in high-risk situations.

Managing pasture composition and grazing pressure is the best long-term strategy for reducing ryegrass staggers in livestock.