Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 424

Figure 9.5 Predicted effects oftimber tree age and planting density on livestock
carrying capacity.(Adapted from Knowles,1991.)
( Larix occidentalis ) 61% and 38%, respectively. In addition to reducing moisture
stress,grazing accelerates nutrient cycling.Spring and summer sheep grazing
increased tree growth equivalent to the application of 100 and 50 kg ha 1 of
ammonium sulfate respectively in the Coast Range of Oregon (Sharrow &
Leininger, 1983).
Two separate economic comparisons demonstrated the effectiveness of
grazing for vegetation management in needleleaf tree plantations (Krueger,
1985).In one case,grazing was found to be 25%-33% of the cost of chemical or
manual control. In the other case in which grazing was being used on 240000
hectares, chemical grass or brush control cost $75-150 ha 1 , whereas grazing
produced a slight profit of $2.50 ha 1 . In both studies plantation managers
thought trees grew better when grazing was used for vegetation management
than when herbicides were used, although no data were presented.
Damage to trees from browsing or treading can be highly variable depend-
ing on available forage,livestock management,and age of the trees.Under the
management described below, tree mortality was 1% to 5% (Currie, Edminster
& Knott, 1978; Krueger, 1985; Thomas, 1985), which is usually only a small
part of total mortality caused by factors such as wild herbivores and droughts.
Damage levels to young plants from herbicide use were similar to or greater
than the levels due to grazing (Krueger,1985).Young plants can tolerate some
defoliation, loss of lateral branches, and bark scraping (Hughes, 1976; Lewis,
1980). Trampling damage is more likely with cattle, whereas browsing
damage occurs more with sheep (Sharrow & Leininger, 1983).
Grazing for vegetation control varies in effectiveness depending on the pal-
atability of available foliage. On more favorable sites, improved forages may
be planted. Krueger (1985) found that seeded grasses reduced the presence of