Individual study: A review of the effectiveness of livestock grazing for control of giant hogweed Heracleum mantegazzianum in Europe
Nielsen C., Ravn H.P., Nentwig W. & Wade M. (2005) The giant hogweed best practice manual. Guidelines for the management and control of an invasive weed in Europe. Forest & Landscape Denmark, Hoersholm.
Giant hogweed Heracleum mantegazzianum (native to eastern temperate Asia) is a tall perennial plant that is now widespread in many temperate areas of Europe. It has significantly increased its geographical range in recent years, being especially abundant along stream and river banks. Because of its tall height and that it often forms dense mono-species stands, giant hogweed often stifles out native plant communities.
A European Commission funded project was undertaken to provide scientifically-based guidelines for management and control in Europe of an invasive plant, giant hogweed Heracleum mantegazzianum, and two closely related species, H.sosnowski and H.persicum. As part of this project, the effectiveness of grazing with domestic livestock was reviewed.
The studies reviewed showed that grazing with domestic livestock is a very effective method of control of large stands of invasive hogweed. Evidence for the effects of grazing came mostly from the use of sheep, but giant hogweed is also very palatable to cattle. These grazers eat most above ground plant parts, thus greatly reducing photosynthesis and depleting energy resources stored in the root. Sheep and cattle prefer young and fresh plants, and the most efficient control is obtained by commencing grazing in the spring when the plants are small. Studies indicate that usually livestock require a familiarization period before they regularly eat hogweed but that they soon develop a preference for it. In areas with dense stands of hogweed, a single cut is recommended to encourage fresh growth and allow establishment of other plant species as livestock are less likely to be detrimentally affected by eating hogweed if the diet is mixed.
Giant hogweed contains toxins (furanocoumarins) whose toxicity is enhanced in the presence of ultraviolet radiation. These cause inflammation of the skin and mucus-secreting membranes e.g. lips and nostrils, when exposed to light. Bare and unpigmented skin is particularly susceptible whereas darker and hairy skin is more resistant. Choosing livestock with dark pigmentation of the bare skin, e.g. blackfaced sheep, can reduce potential inflammation of mucus-secreting membranes. Clinical studies have shown reduced fecundity after oral application of furanocoumarins to livestock but this has not been reported so far for animals grazing on hogweed in the field.
Grazing can be a cheap method of control where large areas can be fenced, and it may be possible to incorporate smaller stands into adjoining areas if these are already grazed. Where possible, grazing should be undertaken in areas surrounding existing hogweed stands where seeds may have dispersed. Evidence indicates that over time (several years), grazing promotes a dense sward of grazing-tolerant species which limits the amount of suitable substrate in which hogweed seeds can germinate and become established.
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