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A peculiar volatile oil - oil of Erechtites - transparent and yellow, obtained by distilling the plant with water, taste bitter and burning, odour foetid, slightly aromatic, somewhat resembling oil of Erigeron, but not soluble as that is in an equal volume of alcohol. The specific gravity of the oil is variously given as 0.927 and 0.838-0.855, and its rotation 1 to 2. According to Bielstein and Wiegand, it consists almost wholly of terpenes boiling between 175 and 310 degrees F.
Fireweed is responsible for many cases of poisoning resulting in ill-thrift in livestock. This is because it contains pyrrolizidine alkaloids that are toxic. These chemicals produce a characteristic type of liver damage. All growth stages and parts of the plant contain these chemicals. Hay or silage contaminated with fireweed can be toxic, as can stands of the plant that have dried off in summer.
There are three sets of circumstances where ingestion of fireweed by livestock is difficult to avoid:
1. where there is a severe shortage of other feed (often in winter on the coast when fireweed is prolific) and fireweed is the only feed available.
2. where the pasture is so heavily infested with young fireweed plants that they cannot be avoided by grazing livestock;
3. where a paddock with a thick stand of fireweed is slashed and then grazed by stock immediately afterwards.
Cattle and horses are most susceptible to the toxic liver damage from fireweed. Young, hungry stock are in the highest risk category. Normally, cattle and horses avoid fireweed when adequate pasture is available but may eat the weed when the quality of pasture is low.
Sheep and goats readily eat fireweed. They find the plant highly palatable and often eat fireweed in preference to other plants. Paddocks that are grazed with sheep or goats are kept relatively free of fireweed.
Sheep and goats are 20 times more tolerant of pyrrolizidine alkaloids than are either cattle or horses. This is because they have a specific bacterium in their rumen that enables them to detoxify much of the alkaloids.
Although fireweed is much less toxic to sheep and goats, it can cause some liver damage in them if large quantities are eaten over long periods, e.g. consecutive seasons or years.
Pastures contaminated with fireweed should not be baled or made into silage. A program to remove the existing fireweed plants should be carried out prior to any hay making or silage operations.
SYMPTOMS OF POISONING
Symptoms of poisoning with fireweed in cattle and horses primarily are loss of appetite and ill-thrift. Sometimes other liver related signs will include: aimless wandering, loss of muscular co-ordination, apparent blindness, photosensitisation, jaundice, abdominal straining, dullness and chronic scouring. Severe liver damage due to the pyrrolizidine alkaloids can result in death.
The most common effect attributed to fireweed in cattle is ill-thrift in young stock.
A condition in cattle on the Central Coast of New South Wales, commonly called coastal ill-thrift, is probably due to a combination of mineral deficiencies, internal parasites and fireweed toxicity. However, on fireweed-infested properties where mineral deficiencies and internal parasites are not a problem, young stock can still fail to thrive. Varying degrees of chronic liver damage are normally seen in these animals.
There is no effective treatment for fireweed poisoning. The control measure that most benefits livestock is a reduction in the quantity of fireweed in grazing pastures.