Grazing

Grazing generally describes a type of feeding, in which a herbivore feeds on plants (such as grasses), and also on other multicellular autotrophs (such as algae). Grazing differs from true predation because the organism being eaten from is not generally killed, and it differs from parasitism as the two organisms do not live together, nor is the grazer necessarily so limited in what it can eat (see generalist and specialist species).

Many small selective herbivores follow larger grazers, who skim off the highest, tough growth of plants, exposing tender shoots. For terrestrial animals, grazing is normally distinguished from browsing in that grazing is eating grass or forbs, and browsing is eating woody twigs and leaves from trees and shrubs.

Grazing is important in agriculture, in which domestic livestock are used to convert grass and other forage into meat, milk and other products.

The word graze derives from the Old English (OE) grasian, "graze", itself related to OE graes, "grass".

Water animals that feed for example on algae on stones are called grazers-scrapers. Grazers-scrapers feed also on microorganism and dead organic matter on various substrates.

United States
The use of livestock grazing can be dated back to the Civil War. During this time, land ownership was not common, and ranchers grazed their cattle on the surrounding, often federal, land. Not having a permanent home, these cowboys would frequently graze an area down, and then continue on their way. More commonly, however, cattle were rotated between summer and winter ranges. Soon, the public saw how profitable cattle could be and many tried to get into the cattle business. With the appearance of free, unlimited grass and feed, the land became overcrowded and the forage rapidly depleted. Ranchers tried to put a stop to this by using barbed wire fences to barricade their land, water sources, and cattle. After failed attempts, the Taylor Grazing Act was enacted in 1934. This act was put into place to help regulate the use of public land for grazing purposes and allotted ranchers certain paddocks of land. Additionally, “fees collected for grazing livestock on public lands was returned to the appropriate grazing district to be used for range improvements”. The Taylor Grazing Act helped to stabilize rancher’s operations and allow them to continue raising their livestock.

Grazing systems
In the 19th century, grazing techniques were virtually non-existent. Pastures would be grazed for long periods of time, with no rest in between. This led to overgrazing and it was detrimental to the land, wildlife, and livestock producers. Today, ranchers have developed grazing systems to help improve the forage production for livestock, while still being beneficial to the land.

Seasonal
Seasonal grazing incorporates “grazing animals on a particular area for only part of the year”. This allows the land that is not being grazed to rest and allow for new forage to grow.

Rotational
Rotational grazing “involves dividing the range into several pastures and then grazing each in sequence throughout the grazing period”. Utilizing rotational grazing can improve livestock distribution while incorporating rest period for new forage.

Rest rotation
Rest rotation grazing "divides the range into at least four pastures. One pasture remains rested throughout the year and grazing is rotated amongst the residual pastures." This grazing system can be especially beneficial when using sensitive grass that requires time for rest and regrowth.

Deferred rotation
Deferred rotation “involves at least two pastures with one not grazed until after seed-set”. By using deferred rotation, grasses can achieve maximum growth during the period when no grazing occurs.

Patch-burn grazing
Patch-burn grazing burns a third of a pasture each year, no matter the size of the pasture. This burned patch attracts the grazers (cattle or bison) which graze the area heavily because of the fresh grasses that grow in. The other patches receive little to no grazing. During the next two years the next two patches burn consecutively and then the cycle begins anew. In this way, patches receive two years of rest and recovery from the heavy grazing. All this results in a diversity of habitats that different prairie plants and birds can utilize, mimicking the effects of the pre-historical bison/fire relationship where bison heavily graze one area and other areas have opportunity to rest.

The Tallgrass Prairie Preserve in Northeast Oklahoma is within the Flint Hills ecosystem, and they have been patch-burn grazing with bison herds for over ten years now. Their efforts have effectively restored the bison/fire relationship on a large landscape scale of 30000 acre.

Riparian area grazing management
Riparian area grazing is used more towards improving wildlife and their habitats. It uses fencing to keep livestock off ranges near streams or water areas until after wildlife or waterfowl periods, or limiting the amount of grazing to a short period of time.

Ecological effects
A number of ecological effects derive from grazing, and these may be either positive or negative. Negative effects of grazing (or more usually over-grazing) include increased soil erosion, adverse water quality impacts from increased runoff and loss of biodiversity. For example historical grazing, along with other land consversion, in Northern and Central California has reduced native chaparral and forest lands by approximately 70 percent. Ongoing grazing expansion {and land conversion} driven by human population growth in this region threatens the remaining integrity of California chaparral and woodlands habitat in this region.

In some habitats, appropriate levels of grazing may be effective in restoring or maintaining native grass and herb diversity in rangeland that has been disturbed by overgrazing, lack of grazing (such as by the removal of wild grazing animals), or by other human disturbance. Conservation grazing is the use of domestic livestock to manage such habitats, often to replicate the ecological effects of the wild relatives of livestock, or those of other species now absent or extinct. For example, heathland in Europe requires grazing by cattle, sheep or other grazers to maintain its structure and diversity.

Much grazing land has resulted from a process of clearance or drainage of other habitats such as woodland or wetland.

Production
By utilizing grazing systems, livestock production has the potential to be maximized. “Approximately 85 percent of U.S. grazing lands are unsuitable for producing crops. Grazing animals on this land more than doubles the area that can be used to produce food. Cattle serve a valuable role in the ecosystem by converting the forages humans cannot consume into a nutrient-dense food”. Some could say ranchers themselves are conservationists. They work to use natural resources and often perform tests on their land to analyze soils, control weeds, and utilize smart grazing practices. Through this, they benefit themselves by using profitable land to maximize their livestock production in turn for a profit.

Environmental
Grazing is claimed by vested interests to be very beneficial to the ecosystem. It is advantageous towards the soil and grasses, promoting nutrient dense soil and stimulating the growth of plant varieties. Through grazing, livestock encourages plant growth, consequently increasing forage production. Furthermore, the animal’s urine and feces "recycle nitrogen, phosphorus, potassium and other plant nutrients and return them to the soil". It also acts as rations for insects and organisms found within the soil. These organisms “aid in carbon sequestration and water filtration”. Nutrients and organisms, all of which are necessary for soil to be prosperous and capable for production.

Grazing also helps to promote the growth of native plants and grasses. Often, these indigenous plants are not able to compete with the surrounding plants that utilize the majority of water and nutrients. By livestock grazing, the non-native grasses are controlled and the native plants can redevelop. As well as using grazing to increase plant growth, the actual hoof action of the livestock also promotes growth. The trampling helps to imbed the seeds into the soil so that the plants and grasses can continue to germinate. Additionally, management in many parks makes use of grazing to help lower fire hazards by reducing the amount of potential fuel, such as large buildups of forage. When the land is not grazed, dead grasses accumulate. These dead grasses are often a large fire hazard in the summer months. On the other hand, grazing can also allow for "accumulation of litter (horizontal residue)" helping to eliminate soil erosion. Soil erosion is important to minimize because with the soil erosion comes a loss of nutrients and the topsoil. All of which are important in the regrowth of vegetation.

Biodiversity
Grazing may also promote biodiversity. Many species are dependent on ranch lands and grazing animals to maintain their habitat. The grasses that are stimulated through grazing provide a habitat for many species. When the land is left unattended or is not grazed, grasses will die with the seasons and accumulate as litter on the ground. For many birds, this is not attractive and they avoid making a nesting area of it. However, when the grass is grazed, the dead litter grass is reduced and allows for the birds to utilize it, while at the same time the livestock benefit. Just as importantly, it increases species richness. When grazing is not used, many of the same grasses grow, for example, brome and bluegrass, consequently creating a monoculture.

"In North American tallgrass prairies, diversity and productivity are controlled to a large extent by nitrogen availability…Nitrogen availability in prairies was driven by interactions between frequency of fires and grazing by large herbivores…Spring fires enhance growth of certain grasses, and herbivores such as bison preferentially graze these grasses, keeping a system of checks and balances working properly, and allowing many plant species to flourish."

Disadvantages
The negative impacts of cattle grazing on the habitat of native species are readily acknowledged by the agencies and organizations most knowledgeable about these species. Federal wildlife agencies have identified cattle grazing as a factor leading to the listing under the Endangered Species Act (“ESA”) of multiple species that occur on or have been extirpated from rangelands

Environmental
The Center for Biological Diversity(“CBD”) has provided extensive documentation and scientific research papers as part of a review of  grazing. These included a reference list of over 150 scientific, peer-reviewed research papers and articles detailing the detrimental environmental impacts of livestock grazing in the western U. S., and a 13 page summary of livestock grazing impacts onsoil, stream, wildlife, and ecosystemfunctionfrompeer-reviewed,scientific studies,with references. Also included was a comprehensive Survey of Livestock Influences on Stream and Riparian Ecosystems in the WesternUnited States(Belskyet al. 1999), a survey of over 140 peer-reviewed studies on the biological and physical effects of livestock on western rivers, streams, and riparian areas. Belsky et al. (1999) conducted a systematic literature review which specifically searched for peer-reviewed experimental studies showing the positive environmental impacts of grazing, but none could be found.

Studies overwhelmingly show that livestock grazing negatively affects water quality and seasonal quantity, stream channel morphology, hydrology, riparian zone soils, instream and streambank vegetation, and aquatic and riparian wildlife. Cattle grazing in general has severely degraded all aspects of creek function, processes and ecology. These impacts obviously have significant cumulative negative effects for special-status species dependent on aquatic and riparian habitat. The general impacts of cattle grazing (extensive documentation and references can be found in Belsky et al. 1999) can be summarized as follows: - Water quality - Cattle grazing increases nutrient concentrations and bacteria and protozoa levels. Sediment load and turbidity are increased, as well as water temperature. Dissolved oxygen levels often decline. - Stream channel morphology - Channel width increases, and water depth decreases with cattle grazing. Gravels in the channel bed tend to be lost in the erosional environment and fine sediments increased in the depositional environment. Streambank stability is reduced, streambank undercuts are reduced in quality and quantity, and pools decrease in number and quality. - Hydrology (stream flow patterns)- With cattle grazing, overland flow from runoff increases, and peak flow and flood water velocity also increase. Summer and late-season flows decrease, and the water table is lowered. - Riparian zone soils - Grazing increases erosion, the amount of bare ground, and soil compaction. Infiltration of water decreases. Litter layer decreases and soil fertility declines. - Instream vegetation - Algae growth increases with grazing, but higher plants (submerged and emergent) often decline in abundance. - Streambank vegetation - Herbaceous cover, biomass, productivity, and native plant diversity decline due to grazing. Overhanging vegetation and tree and shrub biomass and cover decline. Plant species composition is altered and plant structure (horizontaland vertical) issimplified. Plant age structure becomes even-aged and plant succession is impeded.

Non-grass grazing
Although the word grazing is typically associated with mammals feeding on grasslands, or more specifically livestock in a pasture, ecologists sometimes use the word by extension in a broader sense, to include any organism that feeds on any other species without ending the life of the prey organism. Use of the term varies even more than this, for example a marine biologist may describe herbivorous sea urchins that feed on kelp as grazers, even when they kill the organism by cutting the plant at the base. Malacologists sometimes will apply the word to aquatic snails that feed by consuming the microscopic film of algae, diatoms and detritus, a biofilm, that covers the substrate and other surfaces underwater.

An example of a grazer that may seem counterintuitive is a mosquito, which is not a parasite in that it does not form any lasting association with its prey, and is not a true predator in that it does not kill them by this process (although they can act as a vector for fatal diseases such as malaria). In this sense it is the antithesis of parasitoidism, in which an organism (typically the larval stage of a wasp) feeds on another by eating it from within. In that case, the prey is inevitably killed by predation, and has an intimate association with its predator, such that its premature death would also see the parasitoid die as well.