Written by Rachel Frost and Jeff Mosley, Montana State University
The effects of grazing or any other form of defoliation on a grass plant are directly related to how grasses grow. These effects depend upon the following:
Grazing (defoliation, trampling, or other losses) intensity is the amount (percentage) of plant tissue removed by grazing animals. Frequency refers to how often a plant is grazed. Most perennial grass plants can tolerate either heavy grazing or frequent grazing but not both simultaneously. When grazing removes most of the leaf tissue, and when it occurs repeatedly, plant health is threatened and plants are more likely to suffer stress or even death. Frequent heavy grazing results in a plant with little leaf area for most of the growing season. This has two adverse effects if it happens repeatedly across several years. First, when the leaf area is repeatedly grazed 50% or more, the root system will decrease in size. Fewer roots means the plant extracts less water and nutrients from the soil and has less growth potential. Second, the smaller leaf area is unable to produce enough soluble carbohydrates (energy reserves) to develop buds capable of surviving the winter dormant period and produce the first one to three leaves the following spring. Without sufficient stored energy to keep the basal buds alive and produce the initial green leaves in spring, the plant will die.
The time of year when animals graze a plant affects the plant's ability to tolerate defoliation. Grazing when plants are still in the vegetative stage, particularly early- to mid-growth, is less harmful because the growing points are rarely removed and there is normally enough soil moisture for regrowth. The plant can complete its growth cycle (unless it is repeatedly grazed) and store enough energy reserves to survive the winter and produce the first few leaves the next spring. However, grazing when plants are in their early reproductive or "boot stage" of growth removes the apical and intercalary meristems (growing points) responsible for plant growth. When these meristems are removed, regrowth must initiate from the axillary buds at the base of the plant. This is a much slower process that requires moisture and nutrients, generally at a time when soil moisture is rapidly disappearing. Growing conditions also influence response to defoliation. Plants withstand defoliation best when soil moisture and fertility are high. Drought or other environmental stress will decrease the amount of new leaf and tiller production following defoliation, which can hasten plant death.
Competition from neighboring plants for moisture, nutrients, and light can intensify the effects of grazing. Plants can tolerate grazing better when neighboring plants are also defoliated. Herbivores, however, graze selectively, often defoliating one plant and leaving others nearby ungrazed. Repeated, selective grazing of a community's more palatable plants can result in their decline and an increase of less palatable and/or less productive species. The replacement species are likely to be weeds, some of which are toxic to livestock.
Properly managed grazing can benefit plants and ecosystems in several ways. A lack of disturbance or defoliation can result in a buildup of dead plant material that "chokes" new plant growth. The result is overgrown decadent plants that produce less biomass (forage), have fewer seeds, provide less nutritional value to herbivores, and are less resilient to disturbance, disease, and insects.