Energy Requirments for Plant Growth How Plants Manufacture Food Factors Affecting a Plant's Response to Grazing Management for Sustainability Integrating Forage Resources Literature Cited

by Chuck Coffey

Factors Affecting a Plant's Response to Grazing (page 3 of 6)
Most forages used by livestock have evolved under the influence of grazing. Therefore, proper grazing by livestock can be a useful tool in maintaining productive, healthy plants. Two major factors that will determine a plant's response to grazing are frequency of grazing and intensity of grazing.

Frequency of grazing refers to "how often" a plant is grazed. Livestock are very selective in their grazing habits. These grazing preferences can be seen between plant species as well as within a single plant species. If livestock are allowed to be too selective, many plants will be overgrazed while others will be undergrazed. When grazing frequency is too often, this disproportionate use of plants can cause dramatic shifts in plant species composition. Plants need time to recover after grazing. The proper recovery period will vary greatly depending on the plant species and its stage of growth.

Grazing intensity refers to the amount of plant material removed from a single grazing event. Generally, it is advisable to take half and leave half, especially when grazing native plant communities. The table below supports this rule of thumb:

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The removal of 50 percent or less of top growth has little or no effect on root growth; however, as we begin to remove more than 50 percent of top growth, root growth stoppage becomes increasingly evident.

Another factor to consider regarding the "take half, leave half" rule is the regrowth potential of the above ground portion of the plant. The most important factor of a plant's regrowth potential is the amount of leaf area remaining after grazing has occurred. If very little above ground forage is present, the plant will draw upon carbohydrates located in the crown. Repeated use of these carbohydrates may ultimately result in a plant's inability to overwinter, reduced vigor or very slow regrowth. It was once believed a plant could mobilize energy from its root system for use in topgrowth. Research, however, indicates that carbohydrates allocated to the root system are not capable of being used for above ground growth (Davidson and Milthorpe 1966). Thus, the most important consideration for a plant's regrowth potential is the rate and amount of photosynthesis from the existing leaf area (Sambo 1983, Zarrough, et al 1984, Volenec and Nelson 1984). Moreover, the term "reserve energy" may well be a misconception of a plant's ability to regrow (Deregibus 1982).

Grazing frequency and intensity must be considered simultaneously in an overall grazing plan to maintain the integrity of forages. Remember to take half and leave half and allow plants time to regrow before removing the next half.