Questionable Methods

Gary Jones

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Published: March 25, 2002

The methods used in the experiment by Gutman et al. (2001) do not reflect real world conditions and thus are of limited value. Clipping grasses is not at all the same as grazing grasses. Grazing ruminants do not defoliate the sward evenly, they deposit wastes as they graze, and their hooves affect the character of the soil in several ways. They do not neatly clip grasses, they grasp grasses with their tongues and saw the grasses against their teeth. This affects the plant roots and can even uproot poorly attached individuals. Grazers pick and choose which individuals to eat based on sensory information. For example, they avoid grazing near dung pats, urine patches, and infestations of less palatable grasses. This lack of uniformity allows variable maturity across the sward and, thus, variable seed production and tillering.

Ruminants are not the only grazers. There as many species of fungi, bacteria, and insects grazing the sward as well. Other types of soil life affect the sward in many ways. The Gutman et al. (2001) study yields abstract information about biomass partitioning in idealized situations, but should not be equated to grazing or used as the basis of grazing policy decisions.

The swards used by Gutman et al. as control/comparison that were grazed considered only stocking rate variation. The most important variable in grazing is the time during which grasses are under pressure. When grazers are kept in a sward continuously, they “high-grade” the select bits, often regrazing tender young growth before enough foliage regrows to replenish root reserves. This greatly affects the vigor of the plants and biomass partitioning.

Managed grazing limits the time when grazers have access to a paddock by moving them to fresh, mature paddocks regularly. This emulates normal herd behavior without fences and enclosures, and matches the ruminants’ needs to the grasses’ needs.

Studies such as Gutman et al. (2001) that do not reflect realistic conditions and do not make it very clear that the data are biased and incomplete do us harm. Others will cite this study to draw incorrect conclusions and establish destructive policies.

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Gutman, M., I. Noy-Meir, D. Pluda, N. A. Seligman, S. Rothman, and M. Sternberg. 2001. Biomass partitioning following defoliation of annual and perennial Mediterranean grasses. Conservation Ecology 5(2): 1. [online] URL: http://www.consecol.org/Journal/vol5/iss2/art1