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Annual Ryegrass Toxicity

Key points:

  • Beware of ARGT risk in 350-500m rainfall zone
  • Wimmera ryegrass most toxic in "dry or hay" stage
  • If stock exhibit signs, remove from paddock
  • Stock losses minimise through preventative measures

 

Annual Ryegrass Toxicity (ARGT) has been a significant issue in some districts in the past. Despite a decline in the incidence, producers should not be complacent and should monitor stock carefully when they are being placed in 'fresh' paddocks &/or stubbles that may contain ryegrass.

Wimmera ryegrass infected with corynetoxin producing bacteria becomes its most toxic to stock as it begins to dry or "hay" off. ARGT outbreaks generally occur in areas receiving between 350-500mm of rainfall. If an ARGT outbreak has occurred in your district previously, preventative measures such as topping or spraying may need to be incorporated in to your management to reduce the risk of ARGT to stock.

ARGT is caused by toxins which when consumed causes severe damage to the animal’s brain. The toxins are produced by a bacteria that adheres to one species of nematode which infests annual ryegrass plants. The bacterium starts to produce a toxin around flowering time and the level increases just prior to the grass drying off. The toxin remains until the plant material has been weathered away and new growth appears, although hay containing toxic seed heads will remain toxic and may infect stock when fed out years later.

Sheep grazing an infected paddock may not show any signs for the first 2-6 days. After 2-6 days in a toxic paddock signs of ARGT will easily be observed by driving the mob of sheep for 100-200 metres. Mildly affected animals will be unable to keep up with the flock and will exhibit a high-stepping gait. More severely affected animals may lose co-ordination in the hind legs and fall over with convulsions. If left quietly they will often regain their feet and return to the mob. Severely affected animals go down and are either unable to get up or fall over repeatably. If no action is taken death can occur from within a few hours of the first ARGT signs.

Affected animals should be moved quietly into a clean paddock. Affected animals may still die and it is possible more stock will exhibit signs of ARGT within a couple days of being put into a clean paddock.

There is no commercial treatment available for sheep with ARGT but stock losses can be kept to a minimum through implementing some of the following preventative measures:

  • Heavy grazing in early spring to reduce the amount of ryegrass reaching maturity and ensure that the pasture is utilised before coming toxic
  • Cutting hay or topping before the seed heads reach "dry" stage (hay which contains infected seed heads will remain toxic)
  • Topping or spray topping with an herbicide before flowering
  • Stock grazing known infected paddocks should be inspected regularly for signs of poisoning

Some ryegrass varieties are resistant to the infection by the nematode carrying ARGT bacteria such as Safeguard annual ryegrass. A biological control known as Twist Fungus is also available. Once established it spreads without further management although it can be difficult to establish.

Testing can also be conducted to indicate safe paddocks; however the test is unable to indicate the degree of infection. The ARGT test requires 200g of pre-flowering ryegrass or 1 kg of mature ryegrass seed-heads. These tests are available for approximately $79 from SARDI Plant and Soil Health, Urrbrae.

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