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Biological Mechanisms in Grassland Plants can be Beneficially affected by Grazing
Llewellyn L. Manske PhD
Associate Range Scientist
North Dakota State University
Dickinson Research Extension Center
Grassland ecosystems are diverse and complex, which makes developing management recommendations difficult. However, increasing knowledge of ecological principles and the intricacies of the numerous mechanisms that function in the grassland ecosystem have allowed for improvements in management strategies.
Several greenhouse and laboratory studies have been conducted within the last 10 to 12 years that have enabled scientists to begin to understand how grassland plants react when defoliated. Grassland plants and herbivores have evolved together for 20 million years. During this long period of coevolution, grassland plants have developed mechanisms to compensate for defoliation from herbivores and fire. These adaptive tolerance mechanisms can be separated into two main categories but they do not function independently. The first mechanism involves numerous changes in the physiological growth processes within the grassland plant itself and the second involves numerous changes in the activity levels of the symbiotic soil organisms in the rhizosphere, which is the narrow zone of soil around perennial plant roots.
The physiological responses within the plant caused by defoliation have been reviewed and grouped into nine categories by McNaughton (1983). Physiological responses to defoliation do not occur at all times, and the intensity of the response varies. Grass plants have different physiological responses at various stages of growth. The key to ecological management by defoliation is to match the timing of defoliation events to the appropriate stage of growth that triggers the desired outcome.
All possible combinations of relationships between the physiological responses and the application of the defoliation management treatment have not been quantitatively evaluated with scientific research yet. One of the main physiological effects of defoliation is the temporary reduction in the production of the blockage hormone, auxin, in young developing leaves and within the meristem (the growth point where tissue is formed by cell division). This reduction of plant auxin in the lead tiller allows either for the synthesis of cytokinin (a growth hormone) in the roots or crown or its utilization in axillary buds, which are growth points with potential to develop into vegetative tillers resulting in the production of new plants (Murphy and Briske 1992). Partial defoliation of young leaf material reduces the hormonal effects of apical dominance, which is the hormonal suppression of growth of other buds by the lead tiller, and allows secondary tillers to develop from the previous year's axillary buds. Secondary tillers can develop without defoliation manipulation after the lead tiller has reached the flowering growth stage. Usually, only one secondary tiller develops from the potential of five to eight buds because this secondary tiller also suppresses additional axillary bud development hormonally. When the lead tiller is partially defoliated between the third leaf stage and flowering, several axillary buds can develop subsequently into secondary tillers. No single secondary tiller is apparently capable of developing complete hormonal apical dominance following defoliation of the lead tiller at that time. Some level of hormonal control from the older axillary buds still suppresses development of some of the younger axillary buds. This mechanism is not completely understood and we have not been able to manipulate the hormone levels so that all of the axillary buds develop into secondary tillers.
Besides encouraging grassland plants to tiller, defoliation also stimulates soil organism activity levels in the rhizosphere. The rhizosphere is that narrow zone of soil around living roots of perennial grassland plants where the exudation of materials like sugars, amino acids, glycosides, and other compounds affects microorganism activity. Bacterial growth in the rhizosphere is stimulated by the presence of carbon from the exuded material (Elliott 1978, Anderson et al. 1981). Protozoa and nematodes graze increasingly on the increased bacteria, and accelerate the overall nutrient cycling process through the "fast" pathway of substrate decomposition as proposed by Coleman et al. (1983). The activity of the microbes in the rhizosphere increases the amount of nitrogen available for plant growth (Ingham et al. 1985, Clarholm 1985). The presence of specialized fungi enhances the absorption of ammonia, phosphorus, other mineral nutrients, and water. Rhizosphere activity can be manipulated by defoliation at early growth stages, when a higher percentage of the total nitrogen of the plant is in the aboveground parts and a higher percentage of the total carbon of the plant is in the belowground parts. At that time, partial defoliation disrupts the plant's carbon to nitrogen ratio, leaving a relatively high level of carbon in the remaining plant. Some of this carbon is exuded through the roots into the rhizosphere in order to readjust the carbon-nitrogen ratio.
Bacteria in the rhizosphere are restricted in growth and activity levels because of limited access to simple carbon chains under conditions when defoliation is absent. When defoliation management is used, rhizosphere bacteria increase in activity in response to the increase in exuded carbon. The increases in activity by the bacteria trigger increases in activity levels in the other micro organisms that make up the nutritional food chain of the rhizosphere. These increases in activity levels ultimately increase available nutrients for the defoliated grass plant. The relationship between grassland plants and organisms in the rhizosphere is truly symbiotic with both entities receiving benefits from their association.
Rhizosphere activity can be stimulated by disrupting the carbon-nitrogen ratio through plant defoliation at early growth stages. During middle and late growth, the carbon and nitrogen are distributed fairly evenly throughout the plant and at these stages defoliation does not remove a disproportionate amount of nitrogen, and very little or no carbon is exuded into the rhizosphere. Also, water levels in the soil generally decrease during the middle and late portions of the grazing season and limit the activity levels of rhizosphere organisms.
The adaptive tolerance mechanisms that pertain to the changes in physiological growth processes within grassland plants, and to the changes in activity levels of the symbiotic organisms in the rhizosphere following defoliation are the key to understanding the manipulation of beneficial effects from defoliation management under field conditions. Manipulation of these mechanisms by defoliation are also key to the development of ecologically sound recommendations for management of North America's grassland natural resources.
The period that defoliation of grass plants showed beneficial effects on the increases in vegetative tillers and symbiotic rhizosphere organism activity occurred between the third leaf stage and the flowering period.
The grassland plant community can be changed beneficially when grazing defoliation is properly timed to coincide with the appropriate growth stage of the grass plants. Grass plant density is increased, and total herbage production is increased when defoliation by grazing is timed to occur between the third leaf stage and the flowering stage. A greater amount of vegetation can remain at the end of the grazing season, which causes a noticeable change in the vegetation canopy cover. There is a decrease in the amount of bare ground present in the pastures.
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Clarholm, M. 1985. Interactions of bacteria, protozoa, and plants leading to mineralization of soil nitrogen. Soil Biology and Biochemistry 17:181-187.
Coleman, C.D., C.P.P. Reid, and C.V. Cole. 1983. Biological strategies of nutrient cycling in soil ecosystems. Advancements in Ecological Research 13:1-55.
Elliot, E.T. 1978. Carbon, nitrogen, and phosphorus transformations in gnotobiotic soil microcosms. M.S. Thesis. Colorado State University. Ft. Collins, Colorado.Ingham, R.E., J.A. Trofymow, E.R. Ingham, and D.C. Coleman. 1985. Interactions of bacteria, fungi, and their nematode grazers: effects on nutrient cycling and plant growth. Ecological Monographs 55:119-140.
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Murphy, J.S. and D.D. Briske. 1992. Regulation of tillering by apical dominance: chronology, interpretive value, and current perspectives. Journal of Range Management 45:419-429.