Annual Ryegrass Toxicity
Annual ryegrass toxicity is an acute and often fatal neurological disease caused by the consumption Annual Ryegrass seed that has been infected by the Rathayibacter toxicus bacteria. The bacteria live in the soil and are carried to the Ryegrass by a parasitic nematode, Anguina funesta. The nematodes create galls in the seed head which are then colonised by the bacteria to produce a bacterial gall. The bacteria produce toxic corynetoxins from the end of flowering until seed maturity in the galls. The galls remain toxic over summer and autumn or for long periods if they are kept dry in grain or hay.
The toxic components are corynetoxins that affect the nervous system and are toxic to all vertebrates. It is a cumulative toxin. The lethal dose for sheep by injection is 20-40 micrograms per kg body weight. Less than 5% of the toxin is absorbed from ingested plant matter and this results in oral lethal doses of 1-5 milligrams per kilogram body weight.
Symptoms include stumbling, tremors, convulsions and a characteristic "rocking horse" gait. Dead stock is often the first indicator that the farmer reports. Symptoms appear about 4 days after exposure to the toxin. Symptoms may continue to develop and stock deaths may continue to occur for 10 days after the animals have been removed from the infested paddock. Some animals are more sensitive and show symptoms well before the mob. Weaners (especially on stubble paddocks) often show symptoms more quickly than other types.
There are few economic forms of treatment.
Remove stock from the source of toxin as soon as possible.
There are some drugs that are useful for valuable animals.
Provide ample water.
Introduce the Twist fungus.
Plant Annual Ryegrass cultivars such as Safeguard and Guard that have Annual Ryegrass toxicity resistance (as well as take-all and cereal cyst nematode resistance).
Test paddocks - 600 galls per kg and mature ryegrass is high risk.
Test hay and grain.
Reduce Annual Ryegrass in paddocks by winter spraying, heavy grazing before flowering, spray topping, cutting for silage, green manure or early hay and burning crop residues. Paddocks that are cut need to be grazed to prevent Annual Ryegrass regrowth that may become toxic.
Cobalt has been used as a partial preventative measure for low toxin fodder and paddocks. It increases the level of toxin the stock can tolerate.
Vaccines may be useful in some situations
It is difficult to predict risk as it varies from season to season and not all infected paddocks are toxic.
Inspect stock daily and remove at the first sign of symptoms.
It originated from South Australia and the first sheep deaths in WA were officially recorded in 1970 but was probably present before 1968. The number of affected farms doubled every three years over the 20 year period to 1988 and appeared to plateau around 2000. It is still spreading north to Binnu and east through Lake Grace and Esperance. It appears to be only very slowly spreading south and west.
Deaths in the field occur from October to April with most occurring from mid October to mid December in WA. Autumn deaths are usually associated with introduced feed or moving sheep onto low feed or ungrazed paddocks. Outbreaks often occur after rain and the reasons are probably a change in grazing pattern where stock graze Annual Ryegrass in shelterbelts or the standing toxic heads dry faster after rain and may be more attractive to stock. Deaths from infected fodder or grain can occur at any time.
The annual cost in WA is around $26 million per year.
Average stock losses are around 20,000-30,000 sheep and 230 cattle per year. In 1989, 900 farms were affected in WA and 187,000 sheep and 500 cattle had died over the previous 20 years. Stock losses vary from year to year with 1991 and 2000 being high loss years with around 44,000 sheep deaths per year.
In addition to stock deaths, sub lethal doses reduce wool fibre diameter, staple strength, weight gain and lambing percentages. Hay containing any infected Annual Ryegrass cannot be exported.
The Twist fungus is carried by the nematode and grows on the plant reducing nematode and bacterial numbers. It is slow in action. About 250,000 ha have been inoculated in WA.