In this section you'll find information about:
- Principles of Grazing Management
- How Plants Respond to Grazing
- Effects of Grazing (Defoliation)
Written by Rachel Frost and Jeff Mosley, Montana State University; Updated by Beth Burritt, Utah State University 4-12-16
Managing rangelands is both art and science. Scientific knowledge is combined with practical local experience to find solutions to specific management problems. This section will focus on how science can be used to manage grazing animals
Grazing management entails managing how grazing animals, forage plants, and soils interact to meet specific ecological and economic objectives. A successful grazing manager needs to know how plants grow and reproduce if he wants to understand how grazing animals affect plants by the amount of plant material removed, hoof action, and other aspects of grazing. In addition, managers must understand the grazing animal, specifically its nutritional needs at critical points of the production cycle and its behavior including diet selection.
Managers need to consider the following:
- When should grazing occur? (timing)
- How often should grazing occur? (frequency)
- How much forage should be removed, or how much residual plant material should remain after grazing? (intensity)
How plants respond to grazing is determined by the timing, intensity, and frequency of grazing as well as the physiological and morphological characteristics of plants and how these characteristics affect plant growth before and after grazing.
Timing of grazing (When should I graze). Grazing managers should avoid grazing an area at the same stage of plant growth year after year. Changing the timing of grazing prevents repeated defoliation during critical periods of plant growth (links to Range Ecology), which benefits the plant. The most critical growth stages are when plants are beginning to grow in the spring or fall and particularly when plants are initiating regrowth after grazing. The growth of additional leaves and/or the regrowth of leaves require(s) energy from the plant. The plant also needs to store energy for future growth. The first two or three leaves that appear on a plant after a dormant period uses energy stored from the previous growing season. Plants need adequate leaf tissue to produce enough energy to meet both growth and storage needs. If grazing managers are unable to change the season of grazing so plants have enough leaf area to meet their energy needs for plant growth and storage, then managers may need to reduce the intensity or frequency of defoliation (grazing).
Grazing during winter, when plants are dormant, has few if any adverse physiological effects on plants unless grazing intensity and/or trampling are so severe they remove or damage the basal buds at the base of the plant that initiate growth the following spring. Moderate grazing during the dormant season may help reduce the buildup of dead plant material above the buds on plant crowns. This can benefit plant growth because more or higher quality sunlight reaches the buds and activates them to grow. Finally, managers should avoid grazing when soil moisture is excessively high for long periods because the soil can be easily compacted by trampling and become more susceptible to erosion.
Frequency of grazing (How often should I graze?) Grazing managers should avoid grazing plants too frequently during a single growing season. If plants are given an opportunity to regrow and replenish their stored energy reserves after grazing, they can be grazed again or more during a single growing season. Regrowth can be abundant under following conditions:
- when soil moisture is adequate for plant growth,
- soil and air temperatures are optimum for plant growth,
- soil fertility is good, and
- grazed plants can produce new leaves after being grazed.
However, in arid areas where soil moisture for plant growth is inadequate most of the year, grazing more than once a year may be harmful to plants.
If grazing occurs too infrequently, the accumulation of too much dead material will hinder plant growth. The plant's growth potential is compromised because an insufficient amount of sunlight reaches plant buds to initiate tiller growth, and/or many of the leaves do not receive the maximum amount of sunlight. Long periods of no or limited grazing also can cause the nutritional quality of the plant to decline.
Intensity of grazing (How much of the plant should be grazed?) Grazing managers should avoid removing too much of a plant's leaf area. Leaf blades are the main sites of energy production for the plant. If the leaf area that remains after grazing is very small or the growing points located at the base of the leaf blade is removed, the plant may be unable to regrow and replenish its energy reserves. This is likely to occur if soil moisture levels are low and not replenished shortly after grazing. Also, grazing managers should leave enough residual dry matter to prevent soil erosion and to protect the plant's roots and stem bases from excessive cold or heat. Intense defoliation of plants, particularly if it occurs frequently, can reduce a plant's leaf area for a long enough period that the plant cannot store enough energy to form the buds needed for next year's growth and/or sustain the buds through long dormant periods.
Herbivory, or grazing, is a powerful ecological process that can influence the amount and kinds of vegetation present on the landscape. This occurs primarily because herbivores graze selectively, choosing some plants or parts of plants over others, which alters the competitive ability of plants. Continued heavy grazing can decrease palatable, preferred plants while allowing unpalatable, poisonous, or invasive plants to increase in the community. By controlling the species of herbivore and the timing, frequency and intensity of grazing, managers can shift a forb-dominated system to a grass-dominated system and vice versa. "Targeted grazing" is the use of grazing animals to accomplish specific vegetation management goals through strict control of the species of grazing animal, timing of grazing, and intensity or frequency of grazing.
The absence of herbivory can also be a valuable vegetation management tool. The absence of herbivory, whether through delayed grazing or complete rest periods, is designed to improve the forage stand. Non-grazing periods can be assigned to specific pastures or worked into a planned rotation system. The benefits of planned, non-grazing depend upon the time of year it is implemented:
- Early spring — enhance leaf production by plants
- Spring — enhance plant re-growth when conditions are optimal
- Summer — allow for seed production
- Autumn — improve carbohydrate production and storage, particularly if summer dormant grasses had fall re-growth
- Yearlong — enable seedlings to establish, increase vigor of preferred species, accumulate fine fuels for prescribed burning.
A regularly scheduled or occasional deferment can help range types such as mid-grass, semi-desert bunchgrass, sagebrush-grass, and mountain grasslands to increase forage plant vigor, plant reproduction, and general range condition.
The term "rest" is used in grazing management to denote a full year (12 months) of no grazing. This allows the plants to undergo a complete growth cycle without being grazed. The benefits of rest are best realized in special management situations such as:
- severe drought
- following reseeding
- providing fuel for prescribed burns
- when critical site rehabilitation is required.
While strategies to cope with grazing vary greatly between plant species, plants in general either avoid grazing or tolerate grazing. Plants avoid grazing by diminishing their accessibility or palatability to herbivores. Thorns, hairs, or secondary compounds in some plants are examples of avoidance mechanisms. Plants that tolerate grazing have effective mechanisms to facilitate regrowth following defoliation. Grasses are usually considered to have the highest grazing tolerance.
Plants that withstand grazing generally have one or more of the following characteristics:
- Growing points are low, or elevation of growing points is delayed.
- Plants have a high ratio of vegetative-to-reproductive shoots.
- Apical meristems are activated, and new root growth is initiated following defoliation.
Range plants can be classified by how they respond to grazing pressure. The amount of grazing pressure that a plant can withstand depends on how much it is preferred by grazing animals. The species of grazing animal may also influence how a plant responds to grazing because different species have different dietary preferences and eating behaviors.
Decreaser plants are the first plants to die out under continued heavy grazing. These plants decrease because they are either sought out by grazing animals due to their high palatability or they lack physiological attributes that help them recover from grazing. Highly preferred plants are the first to be grazed, and animals may repeatedly graze these plants throughout the growing season.
Increaser plants generally increase their number as decreaser plants decline. Many increaser plants can avoid grazing damage because they grow close to the ground or are less palatable than decreasers. Increasers often also possess physiological mechanisms that help them recover from grazing. These plants should be monitored because they are a sign of high grazing pressure and can increase in number and abundance beyond what is desirable. If overuse continues, even increaser plants may decline in the community.
Invader plants are commonly weedy plants that become established because more desirable plants have declined due to excessive grazing. A high proportion of invader plants in a community is usually a sign of overgrazing. However, some noxious rangeland weeds, such as leafy spurge and spotted knapweed, are capable of invading healthy rangeland plant communities.
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:
Intensity and Frequency of Grazing: 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 mean 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.
Season of Grazing: 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 Other Plants 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.
When Is Grazing Good for Plants? 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.
Adapted from: Ruyle, G. B., and D. J. Young. Arizona Range Grasses; Grass Morphology, Growth and Response to Defoliation. 2 June 2010.