Understanding Plant Response to Grazing
Grazing is a natural process that has influenced the evolution of plants for millennia. Along with fire, it was the first vegetation management tool ever applied by humans. Grazing, or herbivory, is a constant influence on all natural plant communities. Every plant species varies in its ability to survive and prosper in a grazed ecosystem. Most plants are not killed with a single grazing event that removes its foliage, flowers, and stems. Rather, plants have evolved mechanisms that reduce their likelihood of being grazed or promote their regrowth after grazing.
Plants low in forage value or containing potentially toxic compounds have lower palatability, and herbivores usually avoid them. Palatability is a collective term for the plant characteristics that influence whether an herbivore will prefer or avoid a plant. Many weeds have an acrid or bitter taste or “noxious” smell, at least to humans. Yet, sheep and goats readily consume the bittertasting spotted knapweed and leafy spurge. The high fiber and lignin in some weeds make it difficult for herbivores to tear full bites of foliage, reducing the plant’s palatability. Still, most weeds are quite palatable and have good forage value during some point in the growing season. Many weeds are similar in structure and digestibility to native grasses and forbs. In fact, some weeds, like leafy spurge, remain greener, more succulent, and more nutritious longer into the summer than neighboring native plants.
All plants possess a variety of compounds that can reduce forage value or deter grazing. Some are innocuous and some have the potential to harm livestock. These plant chemicals, called secondary compounds, include tannins, terpenes, alkaloids, oxalates, and glycosides. Levels of these compounds vary seasonally in plants and among plant parts. They can deter grazing by reducing plant digestibility, producing toxic effects, or causing illness. Animals reduce their intake of chemical- laden plants by selecting among different species or grazing specific plant parts like leaves or flowers that may have lower concentrations of these compounds.
Plants have traits that increase their ability to regrow after grazing. Some are simply better than others at replacing leaves or stems lost to grazing and producing new shoots to sustain growth and reproduction. A plant’s ability to recover after grazing depends largely on its ability to reestablish leaves and renew photosynthesis. Plants do not maintain large stores of energy and nutrients, so they need carbohydrates gained from photosynthesis to survive, grow, and reproduce.
Early in the growing season, plants need fewer nutrients because they are smaller with fewer leaves and stems. Losing leaves and reducing the ability to capture sunlight early in the season is less damaging than later in the growing season when energy demands are higher. For these reasons, grazing early in the season may have little effect on the plant community. However, many perennial plants have large root systems to support. Spring may be a poor time for controlling invasive herbaceous plants unless they grow and mature early in spring. The effects of early-spring browsing on shrubs are less well researched than for grasses and forbs. As with herbaceous plants, shrubs often tolerate earlyseason grazing because water and nutrients needed for regrowth are readily available.
Plants are most likely to be damaged by grazing at specific stages of development. Generally, a plant has the most difficulty recovering if it is grazed or browsed between the time when the flowerhead is ready to emerge (boot or bud stage) and full bloom. Grasses are most susceptible to grazing in the boot stage when the developing, elongating flowerhead is causing the stem to swell, often bulging where the flowerhead is forming. For example, wheatgrasses grazed after the stem starts elongating and the flowerhead begins to emerge produced fewer new shoots the following year than when grazed earlier in the season, although the exact time when grazing is most detrimental varies by species. Likewise, forbs are most susceptible to grazing when stems are elongating and exposing the developing flowerhead – called the bolting stage.
To be effective, grazing must be applied with the right species at the right time to suppress the target plant and leave the desired or native plants relatively intact. For example, Kentucky bluegrass is invading wetter sites in the Northern Great Plains. Because it starts growing relatively early in the season, Kentucky bluegrass may be suppressed by grazing early in spring when the native grasses are dormant. Annual grasses like cheatgrass are among the first plants to start growing in the spring. They begin flowering and elevating their seed-stalk when native grasses are still in the vegetative stage, for example, they have not started producing flowers. That opens an opportunity to graze such grasses early in the season to suppress them and favor growth of perennial grasses. In August 2005, sheep grazing a foothill bench in Montana avidly consumed flowerheads of spotted knapweed and avoided the native perennial grasses, most likely because the relatively green spotted knapweed had greater nutritive value than the dormant perennial grasses. Grasses were hardly used because they were dormant.