Effect of Grazing on Heracleum mantegazzianum and its Management

In a grazing study on a mesotrophic meadow, H. mantegazzianum was significantly reduced within the first 2-3 years of sheep grazing (Andersen, 1994) and the species disappeared almost completely from the grazed area by the seventh year (Andersen and Calov, 1996). By this time, the cover of H. mantegazzianum in the grazed area was below 1% and the species was only present in one of two initially dense stands, while in two ungrazed control areas it had 40-50% cover. During the first 3 years of the experiment, the number of other plant species increased from below 30 to about 70, but by the seventh year had dropped back to just over 30. There was some initial increase in the number of grass species, followed by a non-significant decrease, but the trend in species number was mostly accounted for by herbs and, more locally, woody species (Andersen and Calov, 1996).

Andersen and Calov (1996) also sampled the seed bank in the grazed and ungrazed areas in the sixth and seventh year of the experiment. Seedlings of

- Root-cutting/digging out

- - Above-ground cutting once

- Sheep grazing

Duration of management (years)

Fig. 15.2. Extinction-rate trend for H. mantegazzianum under different control measures. The trends are based on an assumption of constant extinction rate from year to year. The extinction rate by root cutting and above-ground cutting is taken from the results of Nielsen (2005), while the extinction rate for sheep grazing is from the results of Andersen (1994), Andersen and Calov (1996) and Sorensen (2002). Unfortunately Andersen made no count of H. mantegazzianum individuals during the first 3 years of grazing, but states that no mature individuals were found after the third grazing season. In the same study, no vegetation observations were made during the fourth to sixth grazing season, and then no H. mantegazzianum individuals were found in observations after the sixth grazing season.

- Root-cutting/digging out

- - Above-ground cutting once

- Sheep grazing o

Duration of management (years)

Fig. 15.2. Extinction-rate trend for H. mantegazzianum under different control measures. The trends are based on an assumption of constant extinction rate from year to year. The extinction rate by root cutting and above-ground cutting is taken from the results of Nielsen (2005), while the extinction rate for sheep grazing is from the results of Andersen (1994), Andersen and Calov (1996) and Sorensen (2002). Unfortunately Andersen made no count of H. mantegazzianum individuals during the first 3 years of grazing, but states that no mature individuals were found after the third grazing season. In the same study, no vegetation observations were made during the fourth to sixth grazing season, and then no H. mantegazzianum individuals were found in observations after the sixth grazing season.

large herbs including H. mantegazzianum emerged in large quantities in trials using the soil from ungrazed areas, but no germination of H. mantegazzianum was obtained from the soil taken from grazed plots (for details on seed bank in H. mantegazzianum, see Chapter 5).

In his short-term comparative study of control methods, S0rensen (2002) found a marginally significant reduction in the number of H. mantegazzianum seedlings (P = 0.1), an increase in small vegetative plants (P < 0.05) and a reduction in large flowering individuals (P < 0.001) in grazed compared to ungrazed plots at a waste site. This comparison did not consider cover or biomass changes, just numbers of individuals, and only refers to changes in the first season after the first 4-5 months of management. Yet it provides solid evidence that grazing prevented plants of H. mantegazzianum from flowering (for details on the effect of grazing on delayed flowering in H. mantegazzianum, see Perglov√° et al., Chapter 4, this volume).

The above studies show that sheep grazing is an efficient control measure even in the first season. However, to achieve an appreciable level of control, grazing must be continued until H. mantegazzianum disappears from the pasture sward. The seed bank will normally be depleted within 2-4 years (Chapter 5; Krinke et al., 2005; Nielsen, 2005), whereas storage roots may persist at least twice that period (R.M. Buttensch0n and C. Nielsen, unpublished observations). In initially dense stands the management strategy may be changed from intensive to extensive measures after 2-3 years of grazing

Table 15.2. Suitability-criteria of livestock species in relation to H. mantegazzianum control. The scale used is: ++ = very suitable, + = suitable, (+) = suitable, but not good, - = not suitable. The fencing costs are mentioned in more detail in connection with the subheading 'economics'. The minimum area demand is based on estimates on minimal number of individuals to sustain a herd with in-herd fodder utilization learning: two or three adults with offspring, and the carrying capacity of a 'typical' H. mantegazzianum site.

Table 15.2. Suitability-criteria of livestock species in relation to H. mantegazzianum control. The scale used is: ++ = very suitable, + = suitable, (+) = suitable, but not good, - = not suitable. The fencing costs are mentioned in more detail in connection with the subheading 'economics'. The minimum area demand is based on estimates on minimal number of individuals to sustain a herd with in-herd fodder utilization learning: two or three adults with offspring, and the carrying capacity of a 'typical' H. mantegazzianum site.

Sheep

Goat

Cattle

Horse

Pig

Consumption of H. mantegazzianum

++

++

++

(+)

++

Suitability for strike-force grazing

++

++

(+)

-

(+)

Dry pastureland

++

++

++

++

+

Moist pastureland

+

+

++

+

+

Wet pastureland

-

-

++

+

++

Rich grass-sedge flora

+

+

++

++

Rich herb flora

-

+

++

++

Rich moss-lichen flora

+

+

++

+

Minimum strike force area unit (ha)

0.05

0.05

Minimum area (ha) needed to sustain

0.5

0.5

2

2

0.5

a herd

Fencing costs

High

High

Low

Intermediate

High

Amenity-user conflicts - comfort of

Low

Low

Intermediate

High

Intermediate

public with animal species, potential

injury by animals

Dog conflicts

High

Intermediate

Intermediate

Low

Intermediate

(Andersen and Calov, 1996). In general, eradication of H. mantegazzianum from a site takes 5-10 years of grazing management (Fig. 15.2).

The impacts of grazing on other vegetation, and the likely vegetation mix after grazing, are relevant when considering management options. Several years of sheep grazing may result in rather species-poor, nitrogen-demanding vegetation of disturbed sites with a few dominant species such as Urtica dioica L. or Juncus effusus L. This is because a typical site invaded by H. mantegazzianum is on humus- and nutrient-rich, moist soils where species less preferred from a conservation management viewpoint tend to increase. Therefore, the choice of animal species needs to be related to site-specific management goals (Table 15.2).

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