Choosing Livestock for Control by Grazing

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Large mammal grazers have evolved three basic principles for dealing with cellwall breakdown:

1. Ruminants (e.g. sheep, cattle) have pre-stomach fermentation and an emol-liating tank, where micro-organisms break down cell walls. The food bulk is emolliated in fluid, regurgitated, re-chewed and finally sent to the animal's proper digestive system, resulting in a long gastro-intestinal passage time.

2. Hindgut fermentators (e.g. horses) pass food through the normal digestive system first, i.e. acid-enzyme breakdown and absorption of metabolites, then microorganisms ferment the food in the wide hindgut where further absorption takes place, leading to a medium gastro-intestinal passage time.

3. Omnivores (e.g. pigs) largely depend on their own digestive effort and select for low cell-wall content in fodder, resulting in a short gastro-intestinal passage time.

The different digestion strategies have implications for diet selection of the livestock (Table 15.1), as has the size of the animals.

Large herbivores have been described over a gradient from grazers to browsers (Hofmann, 1989) according to the degree that they rely on field-layer vegetation (grazers) and foliage and twigs of woody species (browsers) as their nutrition source. This approach, however, is rather rigid and does not allow for actual habitat differences, adaptation thereto by the animals, or offspring learning from adults, which appears to be vital for good performance in a given habitat (Illius and Gordon, 1993; Provenza and Cincotta, 1993). This means that the grazing characteristics of livestock should be seen in the context of evolved genotypes and phenotypic adaptation to the actual grazing scenario. We have summarized some main characters of livestock in Table 15.1. Other traits, which determine the utilization of different plant species by livestock, are smell and taste. H. mantegazzianum has a bitter taste (Terney, 1993). Horses in particular, and to a lesser extent cattle, avoid many bitter-tasting plants. To what extent this influences the uptake of H. mantegazzianum needs to be further assessed.

In choosing livestock for control of H. mantegazzianum, we need to assess the suitability of the livestock in relation to the typical H. mantegazzianum site - moist, but not wet, with near neutral pH, nutrient- and humus-rich soils and a high mineralization rate (see Thiele et al., Chapter 8, this volume). In practice, this excludes none of the livestock in question. The critical question is do all the livestock species eat the plant in sufficient amounts to be efficient in its control?

Sheep and goats seek out H. mantegazzianum and prefer it to rough grasses and sedges (Terney, 1993; Andersen, 1994; Caffrey, 1994; Dodd et al., 1994; Lundstrom and Darby, 1994; Andersen and Calov, 1996; Tiley et al., 1996; S0rensen, 2002). Several authors mention cattle and pig grazing as effective control measures (Terney, 1993; Dodd et al., 1994; Tiley and Philp, 1994; Tiley et al., 1996). Our practical experiences in Denmark with a strike-force cattle herd or sheep and cattle herd substantiate this. Terney (1993) mentions horse grazing as a possible control measure of H. mantegazzianum, but reports from practice suggest that horses do not eat substantial amounts of the plant.

Sheep prefer young lush foliage of H. mantegazzianum to older leaves with higher fibre content and less energy and protein, but older leaves, stem and flowers are also eaten, and their stems broken to reach them (Terney, 1993). Seedlings, however, are reported to be generally avoided by sheep in Danish grazing control practice (K. M0ller, Municipality of Vejle, 2003, personal communication). The sheep's preference ranking, young foliage > old foliage > stems, may apply to other livestock species that eat H. mantegazzianum, because it optimizes the energy input and nutritive value of the diet.

Table 15.1. Grazing characteristics of the main husbandry animal grazers (after Buttenschon and Buttenschon, 1982; Tolhurst and Oates, 2001).

Heracleum

Food preferences Grazing pattern Comments mantegazzianum

Cattle, grazers: Graze a broad spectrum of plant species and communities. Prefer fresh vegetation in vegetative growth, but include variable amounts of tall, coarser vegetation with stems, inflorescences; browse, senescent leaves and litter. Preference ranking sweet

Horses, grazers: Graze a broad spectrum of plant species and communities. Prefer fresh vegetation in vegetative growth, but include variable amounts of tall, coarser vegetation with stems, inflorescences; browse, senescent leaves and litter. Preference ranking sweet

> neutral > acidic >> acrid. Selectively avoid several poisonous species.

Graze preferred vegetation to 3-6 cm high lawns in a mosaic pattern. Eat substantial amounts of coarse vegetation. Graze most woody species as an integrated part of grazing. Have coprophobic behaviour -dung pats are avoided for months.

Graze down to 2-3 cm height. Develop large lawns with short vegetation and leave other areas practically ungrazed. Eat large amounts of coarse vegetation. Limited browsing of woody species, many of which are avoided. Have coprophobic behaviour - dung pats are avoided for months and much dung is excreted in latrine areas.

Perform well on wet ground, but may cause poaching of soil. Broad face limits selectivity. Slashing and ripping off vegetation, rather than biting it off. Limited effect on woody species encroachment - fragmented woodland develops over time. Low-cost fencing.

Perform well on wet ground but poaching may be a problem -poaching is more likely with shod horses. Vegetation is bitten off. May accelerate development of woodland fragments. Are susceptible to a variety of poisonous plants. Low-cost fencing.

H. mantegazzianum lies within the grazing choice of cattle. The relatively high grazing height may prolong the vegetative survival of the plant. The acrid taste/smell has not been reported to reduce uptake. Coprophobic behaviour provides grazing-free areas.

Reports on H. man-tegazzianum herbivory by horses vary. The smell and taste appear to repel substantial uptake. Toxic substances may be ingested in substantial amounts before the hindgut fermentation breaks them down. Coprophobic behaviour provides larger grazingfree areas.

Sheep, grazers-browsers: Graze selectively on the plant species or plant parts. Avoid plants or plant parts with high fibre content. Eat inflorescences preferentially. Leave high tussocks and coarse vegetation ungrazed. Preference ranking: neutral = sweet > acrid > acidic.

Graze down to 1-2 cm height. Graze in small-scale areas and over time develop a small-scale pattern lawn mosaic frequently in a net of neglected vegetation. Eat large amounts of browse and young herbs selectively. Do not avoid own dung.

Do not perform well on wet ground. The narrow face allows sheep to graze deep into scrub. Have a large effect on woody species and may conditionally control woodland fragment development. Eat inflorescences of many herbs and limit seeding. High-cost fencing.

Sheep eat H. mantegazzianum by preference. Eat inflorescences. The very close grazing will enhance root-resource exhaustion.

Continued

Table 15.1. Continued.

Food preferences

Grazing pattern

Comments

Heracleum mantegazz-ianum

Goats, browsers-grazers: Graze selectively on the plant species or plant parts. Graze high vegetation - tall lush grass, high herbs; browse, but not senescent leaves and litter. Preference ranking: neutral = sweet > acrid > acidic.

Pigs, grazers-omni-vores: Graze selectively on plants or plant parts with high-energy content - fresh leaves, fruits, acorn, seeds and roots. May de-root high and coarse vegetation structures with attractive roots. Preference ranking not known.

Can graze close to ground, but prefer to graze in all vegetation horizons that are present. Eat large amounts of browse selectively. Do not avoid own dung.

Graze and root at and below soil level. Do not avoid own dung.

Do not perform well on wet ground and are susceptible to damp and cold weather. The narrow face allows goats to graze deep into scrub. Effective browsers that can control woodland development. High browse consumption and resting under scrub may enhance local development of nitrophilous flora. High-cost fencing.

Perform well on wet ground. Pigs are well equipped to root deep into the ground and eat roots. Extensive rooting may disturb the ecosystem undesirably. Also eat elements of soil fauna.

Goats eat H. mantegazz-ianum by preference. Eat inflorescences. The differentiated grazing height is a draw-back at low stocking rates.

Young H. mantegazz-ianum leaves are eaten by pigs, which also eat the roots.

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