Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 411

220-42). The grazers, which include cattle, horses, and bison, principally
consume grasses, although they may shift to forbs and shrubs low in volatile
oils when grass is less available. Bison and horses are even less flexible than
cattle in their grazing preferences and rarely consume more than 15% forbs.
Goats consume a high percentages of forbs and shrubs and use plant materials
high in volatile oils more effectively than grazers. Sheep are intermediate
feeders. They use grasses, forbs, and shrubs, depending on availability. Fox &
Seaney (1984) observed goats, cattle, and sheep grazing together in northeast-
ern USA. They found that goats consumed a greater number of plant species
and obtained only 34% of their diet from grasses, whereas sheep and cattle
obtained 78% and 90%, respectively, from grasses.
A second important characteristic distinguishing different groups of
grazing animals is their susceptibility to plant chemical compounds. A plant
that is poisonous for one herbivore may not be poisonous for another species
(Launchbaugh, 1996). Centaurea solstitialis is poisonous to horses, but the pre-
spiny stages can be consumed by sheep and cattle (Thomsen et al .,1993).Cattle
avoid Euphorbia esula and may avoid palatable plants within E. esula infesta-
tions,whereas sheep can be managed to control this weed with minimal detri-
mental effects (Lorenz & Dewey, 1988). A lethal dose of Senecio jacobaea for
cattle or horses is 3% to 7% of body weight, whereas for sheep and goats a
lethal dose is 200% to 300% of body weight (Sharrow, Ueckert & Johnson,
1988).
Lastly,treading impacts vary among animal species,aresult of differences in
size relative to total hoof area. Sheep range from 0.7 to 0.9 kg of body weight
per square centimeter of hoof area, whereas cattle, with 1.3 to 2.8 kg cm 2 of
hoof area, have a much greater impact (Spedding, 1971, p. 115). The treading
impact of geese, with their large foot area and low body weight, is much lower.
Treading impact can be increased by the deliberate movement of large groups
of animals, preferably cattle due to their size, in an agitated fashion to break
down unused standing forage for faster decomposition, to trample more
fragile weed species,and to open up areas of dense shrub growth for grazing by
smaller species (Savory, 1988, pp.263-72; Harris, 1990).
The role of plant palatability,architecture,and life cycle in
herbivory
Whether grazing can be used to reduce a weed problem depends on
the ability of each plant species in the vegetation complex to avoid herbivory
and to recover afterwards.Briske (1996) referred to these as avoidance and tol-
erance mechanisms that together determine a plant's ability to grow and
reproduce under grazing.