Ryegrass Staggers – The Mycotoxin Challenge of Summer & Autumn

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  5. Ryegrass Staggers – The Mycotoxin Challenge of Summer & Autumn

The condition commonly known as “Ryegrass Staggers”, or more accurately as Perennial Ryegrass Toxicosis (PRGT), can be a serious and widespread problem in livestock grazing perennial ryegrass dominant pastures during summer and autumn months.  It affects sheep, cattle, horses, dear and alpacas.

The most commonly recognised sign of PRGT is staggers, which usually develops 7 – 14 days after stock graze infected pastures (or hay or silage).  Mildly affected stock develop tremors which are exaggerated by physical stress such as mustering, and external stimuli such as humans, dogs, vehicles and other sources of noise, animal husbandry.  As the toxicosis worsens, animals lose coordination, develop a stiff gait and lose control of their direction of movement.  They may collapse, have convulsions, and be unable to rise, leaving them susceptible to dehydration, starvation and attack by predators.  Deaths also result from mishaps due to lack of co-ordination, such as drowning in creeks and dams.

Usually only a small proportion of the herd or flock show signs of staggers. Less obvious problems also occur, even when toxin concentrations are too low to cause staggering.

Less obvious signs of PRGT:

  • Ill-thrift and reduced liveweight gain is the most common problem, and young stock are most susceptible.
  • Toxins reduce blood flow to the skin and extremities. This reduces the animal’s ability to regulate body temperature, leading to heat stress.  Animals may seek shade for longer periods, reducing grazing time, and may crowd into dams, troughs and streams in an attempt to cool down, sometimes resulting in mass drownings.  Reduced blood flow to the extremities may also aggravate foot problems.
  • Toxins may disrupt digestion, leading to scouring, dags and fly problems.
  • Lowered fertility has been reported in both male and female animals.
  • Milk yields and milk fat and protein levels may be reduced.

Perennial Ryegrass Toxicosis (PRGT) is caused by two ergot alkaloids, Ergovaline and Lolitrem B.  These toxins are produced by an endophyte fungus that lives in the plant.  The fungus is not harmful to the grass, and in fact benefits the plant by enhancing seedling vigor, tillering, seed production and resistance to drought and insect pests.  However, the toxins produced by the fungus can cause considerable problems in the livestock grazing infected pastures.

The Endophyte Lifecycle. Image courtesy of Aus West Stephen Seeds.

Cow exhibiting signs of Grass Tetany.

Lolitrem B is a tremorgenic neurotoxin and is responsible for the staggering. It can be found concentrated at the base of the ryegrass plant in the leaf sheath and also the seed head in reproductive tillers.  Ergovaline acts as a vasoconstrictor and is responsible for heat stress, ill thrift and reproductive problems.  Concentrations of these toxins increases when the plant is stressed for moisture or when soil nitrogen is high.  Significant concentrations of this substance will only develop during summer and autumn, and only in some years.  Perennial ryegrass hay or silage harvested during these periods could potentially contain significant amounts of this toxin.

Prevention and Treatment

Ideally the toxic feed source should be removed from the diet, by taking animals off the affected pastures or stop feeding out affected hay or silage.  It is advisable to replace the affected fibre sources with less rye grass-dominant pasture, hay or silage.  Adding a good quality mycotoxin binder, such as TOXO® MX (from Trouw Nutrition) to the diet can help speed up the recovery.  They bind with the mycotoxin, preventing their passage through the wall of the gut and into the bloodstream.  In the end, the mycotoxin and binding agent harmlessly pass through the animal, exiting with the manure.  Depending on the severity of the staggers syndrome, animals may recover within a few days or it can take up to a month.

Article contributed by Geoff Irish of Auspac Ingredients.

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