Annual Ryegrass Toxicity (ARGT)
Annual ryegrass toxicity (ARGT) is a potentially fatal condition affecting livestock, caused by the consumption of ryegrass which has been affected by Rathayibacter toxicus bacterium.
The bacterium is transferred to the ryegrass plants through nematodes, which occurs as the nematodes become more active after the break in season.
The nematodes move up the stem through the water film, then produces toxins within the seed galls from the end of flowering to seed set. Toxicity develops at flowering and at seed set. Once ingested by a grazing animal, the accumulation of the toxins can cause disease, even with toxin intake up to nine weeks apart. It takes six months to clear the animal of all impacts of the toxins.
Signs of affected ryegrass
The signs of bacterium in ryegrass can sometimes be seen on the plant as it runs up to and while in head. Things to look for include a yellow bacterial slime on seed head which can harden and become an orange- brown colour (this can be hard to see in hot, dry conditions). Other symptoms include spikelets stuck into stem, distortion of head and/or stunted tillers.
Visual assessment is not reliable however and toxicity testing is the only way to know for sure. The tests search for the presence of bacterium in pasture, hay and grain which can determine the risk of ARGT.
Diagnosis in livestock
There are many signs of ARGT which can include physiological and neurological impacts.
Often the first clue is when livestock are being moved, as infected animals can fall behind the mob. If an infected animal has fallen over, it may convulse with stiff legs and throw its head back. The appearance of symptoms intermittently is a clinical sign of ARGT.
Other than observing stock for symptoms, the clear way to know for sure is to test the contents of the rumen for the presence of the bacterium.
Treatment of livestock
There is no treatment for ARGT in livestock. Animals may recover in time but if unable to stand within 12 hours they should be euthanised humanely.
When impacted animals are discovered, they should immediately be moved to a safe paddock to increase chances of recovery. Deaths are most likely to occur four days after being removed from toxic feed, however livestock may continue to show signs of infection up to 10 days after being moved so continual monitoring is required.
Management of ARGT
There are limited options in controlling ARGT, so the best option is to adopt a preventative approach through the management of ryegrass. This can be achieved through mechanical or chemical methods both of which can reduce the risk of ARGT.
Chemical methods are effective at reducing standing ryegrass population but also reducing seed set if application is timed correctly. While generally effective, some areas show resistance which can make it difficult to get a solid result.
Mechanical methods can be effective at removing standing ryegrass which can also help reduce seed set. It should be noted that this can cause disturbance to pastures already present. Heavy grazing during winter (especially when rye is running to head) used alongside mechanical control can significantly increase control of ryegrass and seed set.
In the past twist fungus has been used as a biological control option but it is no longer commercially available.
Safeguard ryegrass is a viable alternative as the variety is resistant to gall production. By eliminating the seed galls it eliminates the location where the bacterium can produce the toxins. By sowing safeguard rye in pasture, it reduces the chances of ARGT for generations as passes the resistance onto the progeny if flowering windows are similar.
For expert knowledge and trusted advice about animal health issues talk to your local Elders livestock advisor.