Annual Ryegrass Toxicity: What You Need To Know

Annual ryegrass toxicity (ARGT) represents one of the most significant neurological threats to livestock in Australia, costing producers an estimated $40 million annually. This potentially fatal condition affects sheep, cattle, horses, and other grazing animals when they consume infected annual ryegrass, making it crucial for producers to understand its mechanisms, distribution, and management strategies.

What is Annual Ryegrass Toxicity?

Annual ryegrass toxicity is a complex neurological poisoning of livestock caused by consuming annual ryegrass (Lolium rigidum) infected with the bacterium Rathayibacter toxicus (formerly known as Clavibacter toxicus). The disease is not a direct result of the grass itself, but rather stems from a sophisticated biological interaction involving the grass, a nematode vector, and toxin-producing bacteria.

The process begins with a migratory nematode called Anguina funesta, which invades the flowering parts of developing ryegrass plants. These nematodes colonise the seed heads, eventually taking them over to form distinctive nematode galls. When the bacterium R. toxicus is present in the same soil, it hitchhikes into the plant via the nematode. Once inside, the bacterium produces neurotoxic compounds called corynetoxins - complex glycolipid toxins virtually identical to the antibiotic tunicamycin. These toxins accumulate in the infected seed heads as the grass matures and dries off.

The toxins produced are cumulative in nature, meaning they build up in an animal's system over time. It can take up to six months for all effects of the toxin to clear from an affected animal, and toxins consumed up to nine weeks apart will accumulate until they cause clinical disease.

Geographic Distribution in Australia

ARGT was first recorded near Black Springs, South Australia, in the 1950s, followed by identification near Gnowangerup, Western Australia, in the 1960s. Since then, the disease has spread rapidly and extensively across southern Australia. Currently, approximately 40,000 to 60,000 square kilometres of farmland in Western Australia are infested with ARGT-causing organisms, with similar areas affected in South Australia.

Originally confined to Western Australia's wheatbelt region, ARGT has expanded its geographic reach. Cases now occur in the South West region of Western Australia and along the coastal plain. The distribution of ARGT is intrinsically linked to the presence of Anguina nematodes, which serve as the essential vector for the toxic bacteria. While the disease has also been reported in South Africa, Australia remains the primary global hotspot for this condition.

The geographic spread continues to expand, with the affected areas likely much greater than recorded in earlier surveys from 2008. This expansion is facilitated by various means of seed dispersal, including contaminated hay and grain, infected ryegrass seed, uncleaned machinery and vehicles, animals carrying galls in their fleece, and even surface runoff water and creeks that can transport the organisms considerable distances.

Why ARGT Exists and How It Develops

The existence of ARGT is fundamentally tied to the ecological relationship between the annual ryegrass host, the nematode vector, and the bacterial pathogen. This three-way interaction creates the perfect conditions for toxin production and subsequent livestock poisoning.

The nematode Anguina funesta has evolved specifically to parasitise annual grasses, particularly Lolium rigidum. During their life cycle, second-stage juveniles of A. funesta become anhydrobiotic - entering a dormant state where they lose water content and survive as desiccated galls with minimal metabolism until sufficient rainfall triggers their reactivation. This survival strategy allows the nematodes to persist through harsh conditions and continue their cycle when environmental conditions improve.

The bacterium R. toxicus has developed a symbiotic relationship with these nematodes, using them as a vector to gain entry into the ryegrass plant. Once inside the developing seed heads, the bacteria often outcompete the nematodes, taking over the galls and producing their characteristic toxins. The timing of this process is critical - the bacteria must establish themselves during the plant's reproductive phase when seed heads are forming.

Environmental conditions play a crucial role in ARGT development. The disease flourishes in Mediterranean-type climates with wet winters followed by hot, dry summers. Seasonal conditions that promote both ryegrass growth and the survival of nematode-bacteria complexes create ideal circumstances for ARGT outbreaks. Years with late spring rainfall followed by hot spells during summer are particularly conducive to the development of toxic seed heads.

Livestock Species Affected

All grazing animals are susceptible to ARGT, including sheep, cattle, horses, goats, and pigs. However, the disease most commonly affects sheep and cattle, which are the primary grazing livestock in affected regions. The susceptibility is universal among domestic livestock species because the corynetoxins affect fundamental cellular processes related to protein glycosylation that are conserved across mammalian species.

The neurological syndrome produced by corynetoxins primarily affects the vestibulocerebellar system, which controls balance, coordination, and fine motor control. Clinical signs typically appear between four days and several weeks after animals begin consuming toxic feed. If signs appear sooner than four days after stock are moved to a new paddock, it indicates the animals had already consumed toxin before the move.

Symptoms in affected animals include fine muscle tremors (fasciculations) particularly visible in the shoulders, chest, and front limbs, frequent blinking, ear twitching, head nodding, muzzle tremors, loss of coordination and difficulty controlling direction of movement, a characteristic stiff gait with arched back, and inability to rise in severe cases, often followed by convulsions and death.

The signs are notably exacerbated by stress or physical activity, which is why the disease is often first detected when animals are moved or mustered. Peak deaths in ruminants typically occur four days after removal from the affected feed source, though animals may continue showing clinical signs for up to 10 days after being moved to safety.

Risk Mitigation Strategies

Preventing ARGT requires a multi-faceted approach combining pasture management, livestock monitoring, and feed testing protocols. The key principle is that prevention is always better and more cost-effective than managing the consequences of an outbreak.

Pasture Management

Effective ryegrass control is fundamental to ARGT prevention. Since annual ryegrass serves as the primary host for Anguina funesta, removing or reducing ryegrass populations eliminates the nematode's breeding site. Without the nematode vector, the bacteria cannot enter the plant, significantly reducing ARGT risk.

Several strategies can be employed for ryegrass control:

Early intervention: Ryegrass should be controlled at or prior to first head emergence, no later than 10 days after head emergence begins. This timing is critical because toxic galls develop rapidly once seed heads mature.

Spraytopping: Chemical control using herbicides during the reproductive phase can effectively reduce seed head production. However, timing is crucial - applications must occur before galls are fully formed to prevent toxicity in the current season.

Mechanical control: Topping annual ryegrass pastures up to 10 days after first heads emerge will reduce toxic gall production. Any regrowth must be controlled through grazing to prevent subsequent toxicity development.

Strategic grazing: Heavy winter grazing can synchronise ryegrass heading, followed by a brief removal of livestock to allow uniform head emergence. Stock can then be returned to graze off the heads before they become toxic.

Livestock Monitoring

In districts where ARGT is known to occur, stock grazing paddocks containing annual ryegrass should be inspected at least once daily from October through December. When stock are placed on stubbles containing ryegrass during summer, daily inspection is essential for the first two weeks.

Effective monitoring involves moving the mob briskly over 100-200 metres during inspection, as the stress of movement will reveal early signs of toxicity in mildly affected animals. This approach allows for early detection and prompt intervention, minimising losses.

Feed Testing and Certification

All hay, grain, and chaff containing annual ryegrass should be tested for ARGT risk before feeding to livestock. When purchasing feed, producers should request a commodity vendor declaration stating that the feed has been tested and classified as 'low risk' for ARGT.

Feed testing involves sampling protocols that account for the variable distribution of bacterial galls in feed materials. For hay, it's recommended that 15% of bales (or 12 bales, whichever is larger) should be sampled using a hay bale corer. A typical test sample requires 100 grams from each of 10 bales to create one kilogram sample.

Resistant Varieties

The development of ryegrass varieties resistant to nematode gall formation offers another management tool. Ryegrass cv. Safeguard is a variety resistant to gall production that is well-suited to Western Australian conditions. The resistance gene is dominant, and when planted in the correct ratio to background susceptible ryegrass (3:1), it can pass resistance to progeny, provided flowering times are synchronised.

Testing Services through DPIRD Western Australia

The Department of Primary Industries and Regional Development (DPIRD) in Western Australia provides comprehensive testing services for ARGT diagnosis and prevention. The department offers both diagnostic testing for affected animals and preventive testing for feed materials.

Feed Testing Services

DPIRD conducts ELISA testing for the presence of Rathayibacter toxicus bacteria in feed samples. Results are reported as bacterial galls per kilogram of feed, with risk ratings categorised as zero, low, medium, or high risk. The current cost for feed testing is $66.50 per kilogram sample.

Testing protocols are well-established and detailed on the DPIRD website, including specific instructions for proper sample collection from various feed types. The main challenge with testing is that bacterial seed head galls may be variably distributed throughout pasture and hay samples, making representative sampling crucial for accurate results.

Diagnostic Services

For livestock showing suspected ARGT symptoms, DPIRD provides diagnostic testing of rumen contents to confirm the presence of toxigenic bacteria. There is also an ELISA test available that can quantify bacterial toxins directly, though this is more expensive and takes longer to complete than the standard bacterial detection test.

Emergency Response

DPIRD operates an Emergency Animal Disease hotline (1800 675 888) for producers who observe unusual disease signs in their stock. Given that ARGT symptoms can mimic some exotic diseases, veterinary investigations may qualify for rebates under the Significant Disease Investigation Program, which helps rule out exotic diseases while ensuring correct diagnosis.

Twist Fungus as a Biological Control Agent

The twist fungus (Dilophospora alopecuri) represents a fascinating biological control approach that was developed to combat ARGT without the need to eliminate ryegrass entirely. This fungus operates by parasitising the same nematode vectors that carry the toxic bacteria, effectively disrupting the ARGT disease cycle.

Mechanism of Action

The twist fungus attaches itself to Anguina funesta nematodes and travels with them into ryegrass seed heads. Once established, the fungus physically hinders nematode movement and reduces their ability to invade ryegrass plants successfully. More importantly, the fungus restricts the growth and reproduction of both nematodes and bacteria within infected seed heads, significantly reducing the production of toxic galls.

The fungus competes directly with both the nematode and bacterial components of the ARGT complex, often displacing them entirely from infected seed heads. Seed heads infected with twist fungus can appear visually similar to those infected with ARGT-causing organisms, making field identification challenging without laboratory testing.

Effectiveness and Limitations

Research has demonstrated that twist fungus can be highly effective under optimal conditions. At extremely high application rates, the fungus was capable of eliminating all ARGT-causing organisms present in a single growing season. The biological control agent proved effective in reducing ARGT risk without requiring the complete elimination of annual ryegrass, which was a significant advantage over chemical control methods.

However, real-world applications revealed several limitations. Unlike laboratory conditions, field-scale evaluations in Western Australia's wheatbelt showed more variable results. The establishment and persistence of twist fungus proved dependent on numerous environmental factors, including moisture levels, temperature, and the presence of competing microorganisms.

Normal crop management practices, including fungicide treatments, do not significantly inhibit twist fungus establishment. However, spray-topping herbicide treatments commonly used in ryegrass pastures may reduce establishment levels, though they won't prevent it entirely.

Current Availability

Unfortunately, twist fungus is no longer commercially available as a biological control agent. The product was withdrawn from the market when demand declined to levels that made commercial production economically unviable. This decline in demand was partly due to inconsistent field performance compared to initial laboratory results and the availability of alternative management strategies.

The discontinuation of commercial twist fungus production represents a setback for biological control approaches to ARGT management. However, research continues into other biological control agents, including non-toxigenic Rathayibacter species that could potentially compete with and displace the toxic R. toxicus strains.

Conclusion

Annual ryegrass toxicity remains a significant challenge for Australian livestock producers, particularly in the expanding affected regions of Western Australia and South Australia. The complex interaction between ryegrass, nematodes, and bacteria that creates this condition requires equally sophisticated management approaches.

Success in preventing ARGT lies in implementing integrated management strategies that combine pasture management, regular livestock monitoring, feed testing protocols, and where possible, the use of resistant grass varieties. While biological control options like twist fungus showed promise, their current unavailability underscores the importance of conventional prevention methods.

The ongoing expansion of ARGT-affected areas highlights the need for continued vigilance, particularly as climate patterns and agricultural practices evolve. Producers in previously unaffected regions should remain alert to the possibility of introduction through contaminated feed or seed, while those in known affected areas must maintain consistent prevention practices.

Early detection through proper livestock monitoring, combined with prompt veterinary intervention when symptoms appear, remains crucial for minimising losses. The availability of testing services through DPIRD Western Australia provides producers with essential tools for both prevention and diagnosis, supporting informed decision-making in ARGT management.

As research continues into new biological control agents and improved grass varieties, the long-term outlook for ARGT management continues to evolve. However, the fundamental principles of prevention, monitoring, and rapid response remain the cornerstone of successful ARGT management in Australian livestock operations.