Perennial Ryegrass Toxicosis
Old or naturalised perennial ryegrass pastures contain fungal endophytes, which can produce high levels of alkaloids that can be toxic to livestock.
Ryegrass staggers is the most visual and problematic symptom. Serious outbreaks across high-rainfall Victoria have been associated with big springs, followed by a hot and dry autumn. In 2002, it was estimated that 90 000 sheep and 500 cattle died from direct effects of these alkaloids, and a similar number from indirect causes (e.g. inability to muster and treat sheep for worms, flystrike and so on); other productivity losses also occurred. Although the forecast for the coming autumn is not currently for hotter and/or drier conditions than usual, much of Victoria has experienced abundant spring growth. So, if you experienced severe livestock issues in the past, it is worth considering now what you might do to reduce the impacts this time.
Perennial ryegrass is a widely sown grass in high-rainfall areas of southern Australia, including Tasmania. It is estimated that more than 80% of old, naturalised perennial ryegrass pastures may contain endophytes. Endophytes are fungi that are found in many pasture plants; the 'wild type' in many of these old or naturalised pastures produces levels of alkaloids—lolitrum B and ergovaline—that can be toxic to livestock over the summer and early autumn period. These alkaloids provide benefits to the plants that improve persistence, such as improved drought and pest resistance. Many perennial ryegrass cultivars are now available that have less toxic levels of the alkaloids, different alkaloid types or no alkaloids. However, many pastures still contain the old varieties, and in some years the endophytes can cause serious problems for livestock.
The symptoms, collectively called perennial ryegrass toxicosis (PRGT), cover a range of nervous disorders, including staggers, ill-thrift, heat stress, digestion problems leading to scours, and possibly lower fertility. Heat stress, increased flystrike and lower growth rates may occur on these pastures over summer in years when staggers is not evident.
Significant episodes of PRGT occurred in Victoria in 1986, 1999, 2002 and 2005. Indirect costs other than sheep losses included reduced wool growth, greater incidence of disease (e.g. worms), lower lamb growth rates and higher labour costs associated with management and treatment.
The key indicators of high risk to animals (sheep, cattle and alpaca) were identified as:
- a dominance of perennial ryegrass in the pasture with wild-type endophyte—that is, naturalised or older sown pastures
- a big spring with high rainfall in spring–summer, which prolonged the length of the growing season and increased pasture growth
- subsequent dry conditions in March, and high average maximum temperatures in March (23 °C) and April (20 °C).
Much of Victoria has had very good spring pasture growth, which is one of the precursors to PRGT in summer and autumn. There are no clear indicators at this stage that hot, dry conditions will occur in late autumn, but, if you have old sown or naturalised pastures with perennial ryegrass that are infected with wild-type endophyte (you therefore probably experienced the effects in 2002), consider developing a management plan now, to try to reduce the impacts if the season does develop to favour PRGT.
This would include the following:
- Assess each paddock on the farm for the risk of PRGT (knowledge of past effects).
- Identify which paddocks should not be grazed by susceptible classes of stock (young sheep or cattle, or breeding ewes) in the high-risk period. If this is a high proportion of paddocks, consider using stock containment areas for joining ewes if spring lambing (this ensures that both rams and ewes are off high-risk pastures). Also consider using stock containment areas for weaner stock (particularly merino weaners) because they are already under higher stresses of lower production and higher mortality during the autumn period.
- Limit time spent grazing high-risk paddocks.
- Minimise seed head production and access to seed heads.
- Access to water and assess the risk of drowning. Although sheep affected with ryegrass staggers are at risk of drowning, the increased heat stress associated with ergovaline increases the need to drink; this means that provision of water to affected stock and the risk of dehydration may need to be planned. For example, one trial at Smeaton showed a 25% increase in water intake by sheep grazing pastures with wild-type endophyte over summer, in a year when no staggers occurred.
Diluting the toxins with supplementary feed on high-risk pastures is unlikely to reduce the problem because only small amounts of the plant need to be digested to be toxic; and just the act of running out feed trails can cause muscle tremors and for stock to fall over.
At least for staggers, not all breeds of sheep appear to be equally susceptible. If you have changed stock types, you may increase (or reduce) susceptibility to a serious outbreak.
In the long term, consider resowing pastures with other pasture species (e.g. phalaris or fescue) or other ryegrass cultivars that have 'animal-safe' endophytes but still have persistence attributes for the plant.
Source: the information in this article has been sourced largely from Management of perennial ryegrass toxicosis by Kevin Reed and John Webb Ware, published by Meat & Livestock Australia.