PERENNIAL ryegrass staggers (PRGS) is one aspect of a disease complex called perennial ryegrass toxicity (PRGT) affecting ruminants, and occasionally horses, generally in late summer and early autumn.
PRGT is caused by alkaloids (mycotoxins) produced by an endophytic fungus (Neotyphodium lolii) that has a symbiotic relationship with perennial ryegrasses.
This means the fungus lives its entire life cycle within the ryegrass and gains its nutrients and spread (via seeds) from the ryegrass.
In return, it confers disease and pest resistance to the ryegrass plant.
The endophyte lives between the plant cells in the grass and tends to be concentrated in the leaf sheath (close to ground level) and the flowers and seeds.
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Although endophytes produces a range of mycotoxins that have significant negative effects on grazing animals, these same mycotoxins have many beneficial effects for the ryegrass plant that include disease and pest resistance and also deterrence from overgrazing by stock and drought resistance.
The primary mycotoxin associated with perennial ryegrass staggers (PRGS) is Lolitreme–B.
Once ingested, Lolitreme–B binds to potassium channels within the nerve cells of affected animal causing exaggeration of the normal nerve impulses that results in the classic signs of PRGS of head and muscle tremors, stiff or stilted gait, recumbency and potentially convulsions.
Clinical signs of PRGS are exacerbated if stock are stressed or forced to move.
Stock that become recumbent will usually stand again if left quietly alone.
PRGS tends not to be directly fatal but stock can die from misadventure.
Recovery is fairly quick in mildly affected stock but can take up to several weeks in severely affected animals.
There is no treatment for PRGS once animals are showing clinical signs.
A 2014 MLA report estimated the cost of PRGT to the Australian sheep industry to be approximately $100 million per year.
Prevention of PRGT revolves around reducing the intake of the endophyte mycotoxins by removing stock from affected paddocks and / or supplementary feeding to reduce the intake of pasture.
Provision of ample water, that is easily accessible without the risk of drowning, will ensure affected stock do not become dehydrated.
Feeding mycotoxin binders will assist with prevention if stock have to graze known toxic pastures.
Magnesium supplementation does not prevent PRGT.
Longer term control strategies include slashing of pastures to remove seed heads, establishment of mixed pastures, avoiding close grazing during late summer and autumn and use of perennial rye grasses with novel endophytes.
Most cultivars of perennial ryegrass are available with novel endophytes (AR1, AR5, AR37, NEA2, NEA4) that have varying mycotoxin profiles that tend to be lower in Lolitreme–B while still producing some of the alkaloids that confer disease and pest resistance to the plant (such as Peramine and Ergovaline).
Selecting the best endophyte for your area will depend on pest and disease challenges present meaning good local advice is important.
Nil endophyte perennial ryegrass is commercially available but these cultivars tend to have lower persistence than cultivars that contain endophytes and are not suitable in many areas.
Careful management of pastures with novel endophyte rye grass is needed to avoid overgrazing, re–infection with a wild–type endophyte and re–establishment of rye grass containing wild–type endophytes.
The survival of all endophytes in seed declines over time, particularly with high temperatures and humidity.
Using any purchased seed promptly is important and will help ensure the novel endophyte you just paid good money for is viable at the time of sowing.
