Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 423

Grazing for weed control in tree crops
Many tree crops, including coconuts, oil palm, and most fruit trees,
are palatable to domestic herbivores. Unless the young trees are protected,
orchards and plantations cannot be grazed until the tree foliage is out of the
reach of grazing animals. This occurs a year or two earlier for sheep than for
cattle. For needleleaf timber trees, however, only buds and succulent new
growth are browsed, and grazing can begin shortly after tree planting. With
adequate management, needleleaf timber plantations can be grazed each year
even before new growth has hardened off (Krueger, 1985).
After the first few years, the options for employing herbivory for weed
control depend on differences in canopy shape and size, planting density, and
planting arrangement. In oil palm the major opportunity for grazing is from
years three to eight (Payne, 1985), since weed growth is minimal under the
dense canopy of mature plantations. In fruit tree plantings, individual tree
canopies do not overlap.Shade is heavy at the base of individual trees with full
sun outside the canopy. In established coconut, canopy cover is complete, but
light transmission in mature plantations may be as high as 50% to 60%
(Watson & Whiteman, 1981). For a silvopastoral timber plantation in New
Zealand,after year three once trees are established,grazing can be increased to
reduce vegetation competition with trees and to increase animal production
(Figure 9.5). The carrying capacity declines as the tree canopy increases and
reduces herbage growth, but then increases again when the plantation is
thinned in year six. From year eight onward, depending on initial planting
density, the carrying capacity for grazing herbivores declines.
While initially livestock may have been viewed as occasional weed or brush
consumers in tree plantations,grazing for weed control has evolved into dual-
purpose silvopastoral management. The land is managed routinely for both
tree and animal products.
Weed control in evergreen plantations
Some form of weed control such as grazing is necessary in young nee-
dleleaf plantations to reduce shading in more humid environments and mois-
ture stress in dry environments. In southwestern Oregon either grazing or
herbicides to control planted forages resulted in sufficient moisture for three
additional weeks of Douglas fir ( Pseudotsuga menziesii ) growth compared with
plots with no understory vegetation control (Krueger,1985).In another study,
a mixed planting had greater height and diameter at breast height with cattle
grazing than without (Krueger, 1985). Cattle grazing increased height and
diameter of ponderosa pine ( Pinus ponderosa ) 9% and 13% and of western larch