Fireweed (Senecio madagascarensis), an opportunistic weed, is a native of South Africa. It was first recorded in Australia in the Hunter Valley in 1918 and is now established along the entire NSW coast, extending into Victoria to the south and as far north as central Queensland.
It has been declared a Weed of National Significance because of its invasiveness, potential for spread and economic and environmental impacts. It is able to establish and spread quickly where ground has been cultivated or disturbed, and/or where there is reduced groundcover competition because of drought or overgrazing. It is highly adaptable to environmental conditions, able to behave as a short-lived perennial or an annual according to the season. In coastal districts the most common form of fireweed is a low, heavily branched, short-lived perennial bush.
One plant is capable of producing 5,000–30,000 seeds in one season depending on conditions. Most stages of the plant can be seen at almost any time of the year, while most seeds will germinate in autumn.1
Fireweed is one of approximately thirty Senecio species reported to be responsible for livestock and human toxicity around the world. The toxic potential of Senecio species is due to the presence of pyrrolizidine alkaloids (PAs) in all above-ground parts of the plant, at all stages of growth. PAs primarily cause liver damage. Poisoning may not be evident for weeks to months or even years after the plants are eaten. The liver damage caused by fireweed is irreversible and untreatable.
Infestation of fireweed can incur high control costs, reduce the total grazing capacity of a property, decrease pasture production, and affect animal health.
Grazing is a recognised control measure often recommended to be incorporated into a management plan for fireweed. Sheep are promoted as a tool of fireweed control, as they are more inclined to eat fireweed and are significantly more resistant to its toxic effects than other livestock species. Where pigs are given the rating of 1, in being the most susceptible of domestic livestock to poisoning by PAs, poultry are rated 5, cattle and horses at 15, with sheep and goats each being rated at 150.2
An example of sheep being suggested as a fireweed control tool is on the NSW DPI WeedWise website : “Sheep and goats will eat fireweed: they preferentially graze the plant, and display a high tolerance to its poison, and have proven to be a simple, cheap and effective management method. To avoid stock health issues when fireweed is present, land managers should use a new group of animals each year, and to limit the cumulative effects of fireweed poisoning, not graze the same group on infested areas for more than two consecutive seasons. Do not use breeding stock for fireweed control. Either Merino or goat wethers are the ideal control group, or, if this is not possible, British breeds or crossbred sheep.” 1
While the suggestion that sheep can be a valuable tool in fireweed control has a high uptake in some regions, the qualifications to this are not necessarily so well acknowledged. Many producers in the Bega Valley have incorporated sheep into their farming enterprise within the last 5-10 years to assist with control of fireweed without adhering to the guidelines regarding class of sheep used and duration of use as fireweed controllers.1 2
In 2015, South East LLS assisted Jade Moxey, a Year 10 student at Sapphire Coast Anglican College, Bega, with a science project, “The Spread of Seeds Through Cattle”. This project led to Jade being awarded the NSW Young Scientist of the Year 2015, being placed second overall in the Scientific Investigations category at the BHP Biliton National Science and Engineering Awards and being placed 4th in the world for Plant Sciences at the 2016 Intel International Science and Engineering Fair (Intel ISEF) held in the U.S. Among other findings, her research indicated that S. madagascariensis was capable of germinating after passing though the digestive tract of a bovine. With this knowledge, her experience living on her family sheep/cattle property in the Bega Valley, NSW, and her observations of the use of sheep to aid the control of fireweed, Jade embarked on a new science project in 2016, “Grazing Sheep on Fireweed”.3 South East LLS and DPI collaborated with Jade on this project to investigate three questions:
Q 1. Do sheep spread fireweed?
The project definitively showed that sheep are capable of spreading fireweed through their manure. Of the total plants that germinated from the sheep manure, over 28% were fireweed, close to 60% were pasture grasses/clover, and 12% were broadleaf weeds. These results are in contrast to the pasture content of over 95% of desirable plant species. Given that the property on which this trial was done had an average of 6.5 fireweed plants/m2, considered a low-moderate density, this result suggests that sheep may not be as effective a control of fireweed as generally accepted, and may aid spread. Further research is required to compare the quantity of seeds consumed with the number of viable seeds excreted.
Q 2. Do the PAs in fireweed affect sheep health?
Hundreds of PAs have been identified in over 6000 plant species. Around half of these PAs are hepatotoxic esters of dehydropyrrolizidine alkaloids (DHPAs). The toxicity arises from their metabolism by the liver into toxic pyrrolic metabolites. These metabolites kill some liver cells directly, and cause inhibition of mitosis of surviving liver cells, resulting in a greatly reduced number of very large cells and much fibrotic tissue. While acute deaths can occur from the grazing of a large number of plants over a short period of time, it is more usual for the toxic effects of PA ingestion to occur over a period of weeks or several years, possibly resulting from a series of sublethal episodes of PA toxicity.4
It was demonstrated in 2006 and 2008 that S.madagascariensis plants in the Bega Valley contain hepatotoxic DHPAs.5
The consumption of excessive PAs by sheep can lead to hepatic insufficiency which can progress over months and years to chronic liver failure. Affected sheep also tend to accumulate copper in their livers, which can lead to toxaemic jaundice.
Early hepatic damage can be detected via measurement of serum liver enzymes. Histopathology of affected livers shows enlarged hepatocytes forming megalocytes. Levels of copper may be increased. Post mortem examination in more advanced chronic cases reveal small, fibrotic livers.
In this project, fifty homebred sheep, apparently in good health, over five different age groups (1-6 years of age) and two breeds (merinos and XB) were blood sampled. Liver biochemistry was performed measuring GGT, GLDH, and AST. CK was also measured to indicate whether the origin of any elevations in AST was liver or muscle. Based on the liver enzyme results, twelve sheep were selected for slaughter for home butchering. The ten sheep with the highest GGT levels were selected plus two sheep with no abnormality in their biochemistry (figure 4). Liver samples were assessed for evidence of hepatic damage due to PA toxicity via histopathology and measurement of copper levels.
The blood results, liver histopathology and liver copper levels revealed that there was no clear evidence that the consumption of fireweed had a negative impact on the health of sheep grazing fireweed plants for up to 6 years.
These results do not guarantee that sheep used to graze higher densities of fireweed under different management practices would not be negatively impacted upon. Further research is required to identify the relationship between fireweed density, years of grazing fireweed, sheep age and liver damage to enable informed best practice guidelines for the grazing of fireweed with sheep in different situations.
Q 3. Are PAs entering the human food chain via consumption of lamb, mutton, and sheep offal?
Specimens of the fireweed plants on this property were sent to Dr Mary Fletcher FRACI CChem, University of Queensland, for identification of the PAs. Eleven alkaloids were found to be present, with five major alkaloids being identified: senkirkine, otosenine, desacetyldorinine, florosenine and doronine. The florosenine alkaloid registered at the highest level.
Samples of liver and muscle were taken from each carcase at the time of slaughter of the twelve sheep and qualitatively analysed to see if the same PAs were present in those tissues as were in the plant material. There were no other PA containing plants identified on the property.
Very low levels of one PA (the major fireweed alkaloid – florosenine) were detected in three of the meat samples. While the levels detected were below the limits of quantitation, the levels were best estimated to be between 0.1 and 0.5ug/kg. No PAs were present in any of the liver samples or in the other 9 muscle samples. There was no correlation between those sheep that had PAs detected in their muscle with their age, breed, or liver enzyme results as shown in the table below (figure 4).
The potential PA dietary intake by an 85kg adult human male from the consumption of meat with a PA content of 0.5ug/kg, compared to the Food Standards Australia New Zealand (FSANZ) and the European Food Safety Authority (EFSA) identified safe PA daily intake, is indicated in the table below (figure 5).
Based on these guidelines, the average Australian male consuming meat with the level of PA contamination detected in three of the twelve samples referred to above, would be consuming 0.0005% of the FSANZ safe daily intake and the average European male would be consuming 6.2% of the EFSA safe daily intake.
While these results indicate that sheep are capable of accumulating PAs in muscle, the identified levels are well below the safe intake levels as recommended by both the FSANZ and EFSA, even if all the red meat consumed contained PAs. There was no evidence of accumulation of PAs in the liver in this trial.
This project has highlighted the need for education of producers who use sheep to aid the control of fireweed, and the need for further research. Further areas of research include investigating the connection between various paddock densities of fireweed, levels of and duration of consumption by sheep, and liver damage and PA contamination of muscle and liver. Such research would enable the informed development of best practice guidelines for producers and industry for the management of the grazing of fireweed by sheep for the benefit of all stakeholders, including the sheep and the consumer.
Rory Arthur, NSW DPI – provision of the funds for the laboratory testing;
Andrew Thompson, Patrick Staples, Erika Bunker, EMAI. NSW DPI – time spent discussing the testing, and their expertise in pathology;
Dr.Mary Fletcher, University of Qld – time spent discussing the testing, willingness to work on the project, her expertise in PAs.
Jade has continued her success of 2015/16 with this project. She has been named the Australian Young Scientific Investigator of the Year at the 2017 BHP Biliton National Science and Engineering Awards and will be heading to the 2017 INTEL ISEF in the U.S in May. For a copy of the project “Grazing Sheep on Deadly fireweed: A Baaaad Situation?" see Reference 6.6