April 2006
Plant Competition Affects Pasture Composition

by Don Ball A pasture plant faces adversity created by numerous factors, one of the most important of which is other plants growing in association with it. Pests, climatic conditions, and soil factors can create challenges, but survival is made even more difficult when neigh-boring plants are competing for the same resources needed for survival and growth. Nutrients- Pasture plants do best when adequate nutrients are available, and a given plant may have difficulty getting its share of soil nutrients due to uptake by nearby plants, especially if neighboring plants have a more extensive or more favorably placed root system. The timing of fertilizer applications may also favor certain pasture species over others. For example, applications of nitrogen fertilizer only in early spring and early autumn favor cool season grasses, while applications only during summer favor warm season grasses. Furthermore, the type of fertilizer applied may favor certain pasture species over others. Fertilizing a grass/legume mixture with nitrogen tends to favor the grass, because this practice stimulates competition against the legume. The reverse may be true if only phosphate and potash fertilizer is applied. Proper fertilization, especially with potash, also has an extremely important influence on the susceptibility of many forage species to winterkill. Soil Moisture- Competition for soil moisture is obviously often a key reason for persistence of some plant species and not others in a certain area. This may be associated with some species having deeper and more extensive root systems, a protective mechanism for reducing water loss during drought periods, or other factors. Competition for moisture (without providing good quantity and/or quality of forage) is one of the primary reasons why weeds are undesirable in pastures. Conversely, some plants are more tolerant of high levels of soil moisture than others and thus more competitive in moist areas. Light- Plants cannot compete well or even survive without adequate light, but there is great variation in the amount of shade tolerance of various plant species. The greater the extent to which a forage plant is shaded by plants growing in association with it, the more difficult it will be for that forage plant to survive. One of the benefits of proper grazing management is that it helps ensure that desirable plants are receiving enough light. Allelopathy- This term refers to the secretion of chemical com-pounds by a plant that has an adverse effect on the establishment and/or growth of other plants. The inhibitory effects can be against plants of a different species or even of the same species. An example of same-species allelopathy is that established alfalfa plants can inhibit germination of alfalfa seed and growth of seedling alfalfa plants. Allelopathy is probably more common and important than is presently generally realized. Summary- Competition among plants in a pasture is fierce and never-ending. Forage management provided by a livestock producer should normally be aimed at maintaining the weakest of the desirable species present in a pasture. Furthermore, it is usually easier to provide management that will keep undesirable plants out (or at least keep their populations low), than it is to eliminate them once they have become established. Don Ball is an Extension Forage Crop Agronomist with Auburn University.