Controlling annual ryegrass toxicity (ARGT) through management of ryegrass pasture

Page last updated: Thursday, 25 June 2020 - 1:03pm

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Annual ryegrass (Lolium rigidum) is the main host for the seed-gall nematode Anguina funesta. This nematode carries the toxin-producing bacterium, Rathayibacter toxicus, into ryegrass seed-heads. The toxins produced cause annual ryegrass toxicity (ARGT) in grazing animals when consumed in sufficient quantity. Controlling annual ryegrass removes the nematode’s breeding site, and without the nematode the bacteria are unable to enter the plant, which minimises the risk of ARGT.

Broadscale effective control of annual ryegrass has not been able to eliminate the nematode completely, and numbers can quickly build up if the ryegrass returns.

In recent times ryegrass populations have developed resistance to a range of commonly used herbicides. This may have led to increased nematode numbers, and hence increased populations of toxic bacteria, particularly where there has been a high intensity of cropping. To manage herbicide resistance in annual ryegrass, many farmers are adopting an integrated approach to ryegrass control.

When an increase in the relative profitability of livestock occurs, many wheatbelt farmers return to mixed farming and use livestock to help control the ryegrass. There is considerable potential for many of these animals to be exposed to toxic annual ryegrass.

Below are some options to consider to assist in the control of ARGT.

Chemical manipulation

Major resistance in annual ryegrass is to the in-crop herbicides from group A (fops and dims) or group B (chlorsulfuron, triasulfuron). Recent surveys showed 100% of the Western Australian ryegrass populations tested had some level of group A or B resistance.

Control of ryegrass seedset in the pasture phase, using hay-freezing and spraytopping, is also popular for preparing paddocks for cropping. These techniques reduce the number of ryegrass plants in the following year. However, because of the late timing of application of the chemicals (generally at the watery-dough stage), both nematode and toxic bacteria numbers can remain high in the year of application. To control the ARGT organisms and reduce the risk of ARGT in the year of application, spraytopping must be done earlier (see table below). By seedset the galls are formed and rapidly developing toxicity.

Spraying earlier will cause more damage to desirable legume components of pastures, but is necessary to prevent any further development of the toxic galls. The earlier application time also increases the likelihood that rain will occur after spraying. This often results in regrowth of additional heads from the base of the plants, particularly where paraquat has been used. To prevent having to re-spray, graze sheep at a stocking rate of more than 10 per hectare to remove this regrowth before it becomes toxic. Generally the regrowth tillers are not as infested with bacteria as the original heads.

Time of spraying and chemical rates for spraytop grazing control of ARGT
Herbicide Rate Time Grazing witholding period

(250 grams per litre)


400 millilitres per hectare No later than 10 days after first head emergence Check label

(450 grams per litre)


360 millilitres per hectare At first head emergence Check label

Mechanical manipulation

Mechanically topping annual ryegrass pastures up to 10 days after the first heads emerge will reduce production of toxic bacterial galls. As with spraytopping, control any regrowth with sheep to prevent toxicity developing. This technique can be useful on smaller holdings, but is usually impractical for managing large areas on bigger properties, where spraying is more practical.

Heavy grazing

Heavy grazing during winter can be used to synchronise when the ryegrass starts running to head. At the end of winter, take livestock off the paddock for a short period and leave it ungrazed. This will result in the ryegrass producing mostly erect seed-heads, which livestock generally prefer and easily graze off. This is particularly the case in pastures that have other components competing with ryegrass. Once head emergence begins, return stock to the pasture.

The toxicity in the seed-heads increases rapidly as the seeds set and mature, so the grazing pressure has to be sufficient to remove the heads before they become toxic. This technique is more risky and requires careful management to ensure stock safety.

Safeguard ryegrass

Safeguard is a variety of ryegrass resistant to gall production. It flowers earlier than the previously available Guard ryegrass and is more suited to WA conditions. It is susceptible to herbicides, has resistance to cereal root diseases and greater herbage production. The resistance to nematode gall formation is dominant, and when planted in the correct proportion to background ryegrass (3:1), the variety will carry this resistance across to the progeny providing flowering times are similar. This option is not likely to be taken up by those already trying to tackle herbicide-resistant ryegrass.

Twist fungus

Twist fungus (Dilophospora alopecuri) is carried into ryegrass by attaching itself to the same nematode that carries the bacterium. The fungus hinders the movement of the nematode and reduces its ability to invade ryegrass and restricts the growth and reproduction of nematodes and bacterium in ryegrass.

Twist fungus is no longer commercially available as the demand for it declined to a point where it was no longer viable to produce. Ryegrass seed-heads infected with twist fungus can look very similar to those infected with ARGT.