Grazing management of H. mantegazzianum should be timed to catch the early foliage, i.e. during April or at the latest early May, depending on regional differences in seasons. The grazing should exhaust the root resources by either continuous or recurrent periods of biomass removal, and can be discontinued in early autumn when re-sprouting has stopped.
The management strategy for grazing control of H. mantegazzianum depends on the stand area and density. Two main management strategies can be applied: strike-force grazing and summer grazing. Strike-force grazing consists of grazing with a very high stocking rate for several short periods distributed over the growing season. Summer grazing in this context is continuous, normal pastureland grazing from late spring to mid autumn with a stocking rate that is balanced in accordance with the site productivity and other possible aims of the management, e.g. patchy, uneven structured vegetation to promote specific flora or fauna species. Tether grazing and strip-grazing systems have the same effect as strike-force grazing.
In dense stands of H. mantegazzianum, it is necessary to start with a series of heavy grazing interventions over short periods, three or four times per year. Short, intensive grazing periods have an effect similar to cutting, as productive foliage is completely eaten and regrowth draws substantial resources from the root. This type of grazing can be implemented using a strike-force herd of animals, which are moved from site to site according to a planned timetable, with regular return to all target sites during the growing season. Heavy grazing will leave the area open to light and there is a side effect of heavy poaching. Poached soil with sufficient supply of light and increased nutrient mineralization (Buttensch0n et al., 2001) will provide good conditions for germination of H. mantegazzianum seeds. The effect of the strike-force management should be monitored and discontinued once the stands of H. mantegazzianum have been reduced to a minor contribution to the vegetation of the site, because the secondary aim of the control management normally is to re-establish a dense sward of semi-natural grassland vegetation, and this cannot be accomplished on a site heavily disturbed by grazers. Development of a dense sward also prevents or slows down the reinvasion of the site by H. mantegazzianum (see Chapters 14 and 17). As an alternative, the strike-force concept may be continued, but with reduced stocking rate - this could be used in combination with other control measures on numerous, but small sites, e.g. in urban environments where small patches of land at several sites are prone to H. mantegazzianum invasion.
Where the stand of H. mantegazzianum is open and less dense, summer grazing is preferred to strike-force grazing, because from the beginning it supports the development of a dense, wear-resistant pastureland sward. Summer grazing can only be performed on areas a few hectares or more in size and reduces the man-hour input as the livestock need not be handled and moved around. If there is semi-natural pastureland adjacent to the controlled site, its inclusion into the grazing regime should be considered as it can promote revegetation of the controlled site and development towards a stable pasture-land.
Grazing control should include monitoring for inflorescence development and, if necessary, cutting of umbels to prevent seed production on the site.
Herds used to graze H. mantegazzianum should include some individuals familiar with the plant. The importance of the social contact and learning process that takes place in an animal herd with mother-offspring contact is crucial. Animals not acquainted with the vegetation in question tend to overeat plant species that they acquire a taste for, and thus can be subject to poisoning or digestive disorders. In the case of H. mantegazzianum, bloating and diarrhoea is a latent problem, as noted above.
In Denmark several different models are used to organize control of H. mantegazzianum based on grazing (S0rensen, 2002):
1. The local authority provides its own livestock. In Denmark control using a herd owned by the local authority is often carried out on public land, as part of job training activities or projects involving handicapped people. This approach is expensive, but funds may be obtained from other public bodies engaged in addressing social problems.
2. A private nature management society owns the herd of livestock and provides it on a member partnership basis. Such partnerships often emerge in larger townships, or their vicinity, and relate to common land for larger development complexes or public amenity lands, such as parks, green connection corridors or riverside lawns. The society finances the herd and the members manage it. There are examples of such societies operating with success over more than a decade (Hansen, 1997). This solution normally involves no public expenses, but fencing subsidies may be available.
3. Control on public land based on a grazing contract between a public authority and a farmer. In Denmark the farmer typically gets the grazing use of the land free of charge, while the authority provides fencing.
4. Grazing by contractors, where a local livestock breeding society or a group of farmers offer solutions for nature managers or control of alien species using mobile livestock herds. The contractor is usually paid for this service. Herds are moved from site to site during the season. This solution is frequently used for 'strike-force' grazing. In Denmark contractor grazing is partly funded through European Community (EC) area subsidies, but additional public funds are usually necessary.
Within the EC the basis for granting area subsidies varies from one member state to another, so that national applications of contract grazing must be adapted accordingly. In practice, mainly sheep have been used for the control of H. mantegazzianum by contract grazing, but goats, cattle or pigs may be used instead.
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