Rangeland plants have evolved with a long history of grazing and browsing, although there are considerable differences in resistance to grazing among species. Plants have either positive or negative responses to grazing depending on the intensity of defoliation. Studies have indicated that degree and season of defoliation are the primary indicators that determine the level of damage.
Contents
Positive Impacts of Grazing
Proper grazing management can stimulate plant growth, enhance nutritive value, remove excessive litter, and accelerate nutrient cycling. Positive impacts are typically associated with light to moderate grazing. Grasses that have been lightly defoliated usually show increased tillering and leaf growth. They tend to be shorter but leafier and cover more ground surface than ungrazed grasses. The new leaves also have higher nitrogen and nutritional content than those of ungrazed grasses. The effect of defoliation on forage plants also depends on the season of grazing. Generally, plants are little affected if defoliation occurs during dormancy because photosynthesis has already ceased. Plants may also quickly recover from defoliation early in the growing season because there is still time for the plant to produce new growth while moisture and temperature levels remain favourable.
Browsing of shrubs removes the terminal bud causing an increase in the growth of lateral buds. This leads to increased sprouting and bushier shrub growth. Antelope-brush that has not been browsed shows a decline in annual branch and leaf growth. The amount of forage and nutrient content of forage plants can often be enhanced with certain levels of grazing.
back to top
Negetive Impacts of Over Grazing
Heavy grazing negatively impacts range plants by causing decreased photosynthesis, reduced root growth and seed production, and a reduced ability to compete with plants that are ungrazed. Plants are considered to be most vulnerable to grazing damage when carbohydrates in the roots are not sufficient enough to initiate regrowth. With intense defoliation, the entire root system becomes smaller, shallower, and less branched. This affects the plant’s ability to absorb water and nutrients from the soil. Eventually, with continued heavy grazing, the plant’s health declines to the point where death may occur.
The most critical time for defoliation of grasses usually occurs just before the plants begin flowering and setting seeds. This is because carbohydrate stores are still recovering from winter dormancy, and the rapid part of the growing season is coming to an end because of declining soil moisture. If heavy defoliation occurs every year during early bloom, bluebunch wheatgrass decreases as a proportion of the plant cover, and may virtually disappear from the plant community after only a few years.
This illustrates how changes in the plant community will occur if heavily grazed in spring months year after year. Grazed plants are replaced by weedy species that are more resistant to grazing. These weedy plants are generally less productive and palatable than those with low grazing resistance. This overgrazed state generally reduces the value of grasslands for forage, livestock, and wildlife production. Overgrazed plant communities also bring about changes in the physical micro-environment of forage plants. As there is less plant leaf material to intercept rang and trap snow, more moisture is lost to the system by runoff and erosion.
Effects on Succession
Succession is the replacement of one plant community by another. The changes may come about as a result of the natural development of a community or as a result of disturbance caused by animals or man, or climate and soil changes. Overgrazing can force the grassland to shift the successional pattern and move down in productivity. The goal of the grassland manager is to maintain or restore the desirable plant species.
Proper Range Management
BC’s grasslands are valuable spring and fall forage grazing areas. Proper range management commonly focuses on 2 objectives: leaving sufficient leaf area for plants to photosynthesize; and maintaining carbohydrate reserves of perennial forage plants. These goals can be achieved with a variety of techniques:
To learn more about ecological systems, click a link below.
Grazing Animals
Plant Communities
Ecosystem Responses to Humans
Grassland Monitoring
Print This Page