Targeted Grazing to Manage Weedy Brush and Trees

Targeted livestock grazing to control brush has been applied in all regions of the United States. In Texas, goats have been used to slow juniper encroachment. Goats in Arizona and California have strategically browsed in the chaparral region to reduce fire risk created by volatile brush species. Sheep and goats have been applied in the Pacific Northwest to control invasive shrubs like blackberries and gorse. Sheep and goats have been used in the Intermountain region to manage sagebrush and oak brush. In the Eastern United States, sheep and goats have been used to control multiflora rose.

In their evolutionary struggle to survive, woody plants have developed defense mechanisms to reduce their probability of being grazed or browsed. To develop an effective browsing plan for shrub and tree management, these physical and chemical defenses need to be addressed. Some shrubs defend against herbivores with structural features like spines, thorns, and thatched branching patterns. Others contain chemicals that cause animals to avoid eating them. Among the most prevalent aversive phytochemicals, also called secondary chemicals, are terpenoids found in juniper and sagebrush, tannins found in oak and blackbrush, and alkaloids found in acacias and mesquite. Browsing animals generally avoid an otherwise nutritious plant that contains significant amounts of aversive chemicals.

Sheep, goats, and cattle vary in how readily they will consume woody plants. Goats are particularly well suited for managing woody plants. They consume more browse than either cattle or sheep. They consume fewer forbs than sheep and less grass than sheep or cattle. Their narrow muzzles and prehensile tongues allow them to efficiently remove leaves and young stems. Their digestive systems are well adapted for extracting nutrients from woody tissue and detoxifying secondary compounds like tannins and terpenes. Goats have larger livers (relative to body size) than sheep or cattle, and the detoxification capacity of their digestive organs is generally greater than in other livestock species. Research shows that detoxification in the liver is more active and effective in goats than in sheep or cattle.

It is important to browse the target plant when it is relatively palatable either because it is more nutritious than alternative forages or has a low level of secondary chemicals. Effective control of woody plants requires browsing when animals are likely to consume the target plant. Diet studies provide information on when the use of a target species is greatest during the year. For example, consumption of juniper trees is generally highest during the winter months (November to February) when other forage is dormant. Winter is also the season when aversive phytochemicals in juniper foliage are at their lowest levels. Sagebrush consumption is greater in the fall and winter, perhaps because of the seasonally low concentration of monoterpenes.

The age of a plant or branch may present another period of vulnerability to browsing. Some chemically defended plants, like juniper, have lower concentrations of aversive chemicals in early growth stages, such as the seedling stage and initial regrowth following a topkill. Palatability studies of juniper seedlings indicate that immature seedlings are a preferred forage.4 In other plants, aversive chemicals are in greatest amounts in the new annual stems. For example, the stems of blackbrush that are older than one year have less tannins than the new year's branches.

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