Applying Targeted Grazing to Coniferous Forest Management in Western North America
Targeted livestock grazing offers many opportunities for managing coniferous forests including pines, firs, spruce, hemlock, and larch. Grazing applications include removing biomass from grasses, forbs, and shrubs to prepare a site for planting tree seedlings; to reduce competition with young trees; to reduce snow press from tall grasses and forbs; as a pre-thinning treatment to remove shrubs and make thinning easier; as a post-thinning treatment to reduce slash; and to remove forest floor and ladder fuels to reduce fire risk or to create firebreaks. Successes in using targeted grazing on forestlands are widely reported.
When selecting animals for grazing in forests and plantations, one must consider the type, breed, and class of livestock and the size and topography of the area to be managed with targeted grazing. Of particular importance is whether the plant community between and under coniferous trees is predominantly herbaceous grasses and forbs or woody shrubs. Cattle have been used in some open forest plantations to reduce biomass of grasses and forbs between plants. However, cattle generally cause greater trampling damage than sheep or goats. Sheep tend to avoid browsing coniferous trees in favor of forbs and grasses. Sheep also travel frequently while grazing, so tree browsing is generally spread fairly evenly among trees in grazed areas. Goats are more likely than sheep to strip bark from woody plants. While this can damage trees, it also provides an opportunity to use goats to girdle and kill target brush and hardwood tree species, even after the vegetation has grown quite large. Although cattle can damage young conifers by browsing and trampling, sheep and goats impact trees predominantly by browsing and, to a lesser extent, by stripping bark.
The breed of sheep appears to make little difference in the risk of grazing damage to young trees. However, breed selection may be important because breeds differ in their herding tendencies. Merino or Rambouillet crossbreeds like Columbia are easier to herd because of their greater tendency to form a tight flock. Farm sheep breeds like Suffolk, Romney, and Hampshire have been used successfully for fenced agroforest grazing39 and for open-herded forest grazing. However, their tendency to form numerous small groups makes controlling large numbers of them a challenge in steep, brushy country.
Tree species vary in palatability. Generally, sheep and goats prefer to browse hardwoods over conifers. Phelps (1979) reported little browsing on trees in a mixed stand of Pacific silver fir, Douglas fir, and western hemlock in which herded sheep consumed about 47% of the understory vegetation. Among conifers, spruce is unlikely to be browsed even under high grazing pressure while Douglas fir, ponderosa pine, western hemlock, western white pine, and western larch are frequently grazed. Ellen (1990) listed pine, Douglas fir, and spruce in order of decreasing susceptibility to sheep browsing. White fir has been reported to be more readily browsed than Douglas fir, ponderosa pine, or sugar pine. Western red cedar is more palatable to browsing than Douglas fir.
Season strongly affects the levels of browsing on conifers. The palatability of conifer foliage declines rapidly as it matures. Sheep and goats are more likely to browse trees shortly after bud break in the spring when new light-green needles are present. Mature needles (fully expanded and dark green) are much less attractive to browsing animals than immature needles, and old needles from previous years' growth are seldom consumed. Spring bud burst in conifers often coincides with initiation of spring growth of associated grasses and forbs, both of which are more palatable than young conifers. By the time grasses and forbs have matured, conifer foliage has also matured. During the summer, forest shrubs and young hardwood trees generally are more palatable to sheep and goats than conifers. So, while palatability of conifer foliage varies substantially throughout the season, sheep seldom seek it over other available forage.