Perennial ryegrass toxicosis can be a serious and widespread problem in livestock grazing perennial ryegrass-dominant pastures during summer and autumn.
The toxin consists of a combination of chemicals:
- some of these chemicals affect the central nervous system, causing muscle tremors and incoordination or ’Ryegrass staggers’
- other chemicals affect blood supply to the skin, causing animals to overheat and rush into streams/dams in an effort to cool off.
Outbreaks of perennial ryegrass toxicity occur annually. They are most common in southern Victoria and Tasmania.
Conditions when perennial ryegrass toxicosis is likely to occur
Perennial ryegrass (Lolium perenne) is the most commonly sown pasture grass in Australia. About 90% of established perennial ryegrass plants are infected with the endophyte fungus Neotyphodium lolii. Commercially bred varieties of perennial ryegrass have their seed inoculated with endophyte fungus.
The fungus produces chemicals that are insecticidal, thus ensuring better pasture persistence. It is not harmful to the grass, but can produce toxic effects in livestock when eaten in sufficient quantity. Ryegrass breeders continually search for ryegrass variety/endophyte combinations which balance their palatability, persistence, and livestock safety.
Conditions under which perennial ryegrass toxicosis is more likely to occur include:
- summer and autumn seasons
- when abundant pasture growth due to late season rainfall is followed by hot spells during the summer and autumn dry period
when the main legume in mixed pastures has dried off, been trampled and decomposed, leaving the pasture as a pure stand of perennial ryegrass.
Identifying and diagnosing perennial ryegrass toxicosis
The most commonly recognised sign of perennial ryegrass toxicosis is staggering.
Other clinical signs include:
- tremors exaggerated by external stimuli such as mustering, humans, dogs, vehicles and other sources of noise
- loss of coordination and control over direction of movement, a stiff gait and arched back.
- recumbency, inability to rise, convulsions
- deaths resulting from mishaps due to a lack of coordination such as drowning in dams.
Strategies to prevent perennial ryegrass toxicosis
A risk management plan should be prepared on properties with perennial ryegrass dominant pastures in winter rainfall regions. This is especially important if perennial ryegrass toxicosis has been problematic in the past.
Pastures can be oversown with vigorous cultivars of legumes and/or other non-toxic grass species to dilute the toxin. The optimum solution to perennial ryegrass toxicosis may be to renovate pastures after first eliminating old perennial ryegrass plants and seeds that contain the 'wild type' endophyte fungus from the environment.