Overgrazing

Overgrazed area, by native fauna in western New South Wales, in the upper right corner.
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Overgrazing occurs when plants are exposed to intensive grazing for extended periods of time, or without sufficient recovery periods. It can be caused by either livestock in poorly managed agricultural applications, or by overpopulations of native or non-native wild animals.

Overgrazing reduces the usefulness, productivity, and biodiversity of the land and is one cause of desertification and erosion. Overgrazing is also seen as a cause of the spread of invasive species of non-native plants and of weeds.

Overgrazing is used as the canonical example of the Tragedy of the commons.^[1] Sustainable grassland production is based on grass and grassland management, land management, animal management, and livestock marketing. Grazing management, with sustainable agriculture and agroecology practices, is the foundation of grassland-based livestock production since it affects both animal and plant health and productivity.^[2]

Process

Overgrazing can occur under continuous or rotational grazing. It can be caused by having too many animals on the farm or by not properly controlling their grazing activity. Overgrazing reduces palatable plant leaf areas, which reduces interception of sunlight and plant growth. Plants become weakened and have reduced root length, and potentially the pasture sod can be weakened although in many locales overgrazing results in an increased sod vigour dominated by unpalatable grasses. The reduced root length makes the plants more susceptible to death during dry weather. A weakened sod allows weed seeds to germinate and grow.^{[citation needed]}

Indicators

One indicator of overgrazing is that the animals run short of pasture. In some regions of the United States under continuous grazing, overgrazed pastures are predominated by short-grass species such as bluegrass and will be less than 2-3 inches tall in the grazed areas. In other parts of the world, overgrazed pasture is typically taller than sustainably grazed pasture, with grass heights typically over 1 meter and dominated by unpalatable species such as Aristida or Imperata. In all cases, palatable tall grasses such as orchard grass are sparse or non-existent. In such cases of overgrazing, soil may be visible between plants in the stand, allowing erosion to occur, though in many circumstances overgrazed pastures have a greater sward cover than sustainably grazed pastures.

Rotational grazing

Under rotational grazing, overgrazed plants do not have enough time to grow to the proper height between grazing events. The animals are turned into a paddock before the plants have restored carbohydrate reserves and grown back roots lost after the last defoliation. The result is the same as under continuous grazing: in some parts of the United States tall-growing species die and short-growing species that are more subject to drought injury predominate the pasture, while in most other parts of the world tall, drought tolerant, unpalatable species such as Imperata or Aristida come to dominate. As the sod thins, weeds encroach into the pasture in some parts of the United States, whereas in most other parts of the worlds overgrazing can promote thick swards of native unpalatable grasses that hamper the spread of weeds.

Another indicator of overgrazing in some parts of North America is that livestock run out of pasture, and hay needs to be fed early in the fall. In contrast, most areas of the world do not experience the same climatic regime as the continental United States and hay feeding is rarely conducted.

Overgrazing is also indicated in livestock performance and condition. Cows having inadequate pasture immediately following their calf's weaning may have poor body condition the following season. This may reduce the health and vigor of cows and calves at calving. Also, cows in poor body condition do not cycle as soon after calving, which can result in delayed breeding. This can result in a long calving season. With good cow genetics, nutrition, ideal seasons and controlled breeding 55% to 75% of the calves should come in the first 21 days of the calving season. Poor weaning weights of calves can be caused by insufficient pasture, when cows give less milk and the calves need pasture to maintain weight gain.

Ecological impacts

Overgrazing typically increases soil erosion.^[3] Reduction in soil depth, soil organic matter and soil fertility impair the land's future natural and agricultural productivity. Soil fertility can sometimes be mitigated by applying the appropriate lime and organic fertilizers. However, the loss of soil depth and organic matter takes centuries to correct. Their loss is critical in determining the soil's water-holding capacity and how well pasture plants do during dry weather.

Native plant grass species, both individual bunch grasses and in grasslands, are especially vulnerable.

North America - United States

In the continental United States, to prevent overgrazing, match the forage supplement to the herd's requirement. This means that a buffer needs to be in the system to adjust for the fastest growth of forages.

Another potential buffer is to plant warm-season perennial grasses such as switchgrass, which do not grow early in the season. This reduces the area that the livestock can use early in the season, making it easier for them to keep up with the cool-season grasses. The animals then use the warm-season grasses during the heat of the summer, and the cool-season grasses recover for fall grazing.

The grazing guidelines in the table are for rotationally grazed, cool-season forages. When using continuous grazing, manage pasture height at one-half the recommended turn-in height for rotational grazing to optimize plant health. The growth habit of some forage species, such as alfalfa, does not permit their survival under continuous grazing. When managing for legumes in the stand, it is beneficial to use rotational grazing and graze the stand close and then give adequate rest to stimulate the legumes' growth.

Africa - Sahel region

Main article: Sahel

• overgrazing consequences in region: 2010 Sahel famine

See also

• Overexploitation

References

1. ^ Garrett Hardin, "The Tragedy of the Commons", Science, Vol. 162, No. 3859 (December 13, 1968), pp. 1243-1248. Also available here and here.
2. ^ West Virginia University Extension Service Overgrazing Can Hurt Environment, Your Pocketbook Ed Rayburn. 2000.
3. ^ C.Michael Hogan. 2009. Overgrazing. Encyclopedia of Earth. Sidney Draggan, topic ed.; Cutler J. Cleveland, ed., National council for Science and the Environment, Washington DC

External links

• Western Watersheds Project