Fireweed, Tannins And Alkaloids

Fireweed, Senecio madagascariensis, is an toxic, invasive plant that occurs in Australia, South America, Africa and Hawaii.  In Hawaii it is especially problematic as there are no natural enemies, such as insects or other disease. With year-round growing conditions, it behaves more as a perennial than an annual.  It has a wide geographical range and can occur in low rainfall areas near sea level, higher rainfall areas and harsh climates. It grows from sea level up to 11,500 ft affecting virtually all grazing lands on the islands of Hawaii and Maui.  Fireweed can make up 30% of the vegetation and can reduce forage production as much as 40% through competition and reduced grazing because cattle avoid plants growing near fireweed.

Fireweed contains pyrrolizidine alkaloids (PA) that cause permanent liver damage; cattle and horses are especially susceptible. Fireweed is considered unpalatable to cattle but reduced production due to chronic exposure to fireweed and deaths due to over-ingestion of fireweed are suspected. Currently, we don’t know why what conditions lead to increased consumption cattle eat of fireweed by cattle, or how much fireweed is toxic.

Tannins are compounds that bind to protein and other compounds in plants found in many woody plants and some forbs. Eating forages plants containing tannins enables livestock to eat more forage plants with high levels of saponins or alkaloids. Tannins may have the same effect on PA enabling cattle to safely eat fireweed.

Carolyn Wong, a USU graduate student, is studying how cattle graze fireweed infested pastures and the effects of tannins on the toxicity of fireweed in cattle. Her objectives are to determine: 1) how amount of fireweed in pastures affects how much fireweed cattle eat; 2) if tannins enable cattle to eat more fireweed without toxic effects;  3) The degree to which tannins bind PAs in the rumen 4) whether or not the binding in the rumen reduces liver damage in cattle, and 5) recommendations for cattle grazing fireweed infested pastures to control its spread and minimize PA poisoning. This information will be useful in developing recommendations for cattle grazing fireweed infested pastures in order to protect the health and productivity of the animals as well as provide additional alternatives for grazing management strategies to improve the condition of those pastures.