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The Effects of Grazing Management on Plant Community Composition and Structure, Soil Hydrology, and Root Morphology and Decomposition in the Missouri Coteau Region

Guojie Wang, Kevin Sedivec, Paul Nyren, and Anne Nyren, NDSU Central Grassland Research Extension Center


Introduction  

Specialized grazing systems have been the focus of research on management of rangelands since the1950s. Deferred-rotation (with a seasonal delay) and rest-rotation (with a year-long rest) grazing systems have been applied on the public lands in the western United States since the 1970s. Traditionally, range managers have maintained that stocking rate was the major factor that affected productivity. Today, range managers suggest that the distribution of animals may play a more important role in influencing range condition than stocking rate. A review of the literature on stocking rate and grazing systems in the Great Plains and the western United States shows that rotation grazing systems, when implemented at moderate stocking rates, increase plant production by an average of 13% over season-long grazing, and a greater increase in plant production occurs (an average of 35%) when stocking rate is reduced from heavy to moderate. As a result of these and other findings, the present direction in grazing management is to create natural deferment combined with high grazing intensity for short periods of time with prolonged periods of rest. Three of the most common grazing systems developed along these lines have been high-intensity, low-frequency grazing (HILF), short-duration grazing, and modified deferred rotational grazing. In the HILF grazing system, grazing periods of 15-20 days with rest periods of greater than 60 days are used, while in the short-duration grazing systems, the periods of grazing are reduced to 1-8 days with resting periods that average 30 days. Deferred rotational grazing can vary widely between regions and environments, with seasonal deferment ranging from days to months, and a grazing period of up to several months. Grazing management strategies have been developed in an effort to sustain efficient use of the forage resource by domestic livestock and wildlife. However, these management practices affect many ecosystem components besides livestock and forage production. Grazing can also influence plant community structure, soil physical and hydraulic properties, and the distribution, morphology, and decomposition of below-ground plant material within the plant-soil system.

Soil Hydrology

Predicting the impact of land management on the soil hydrology of agricultural and natural ecosystems will become essential as public demand for water increases. Grazing animals can alter the hydrology and drainage pathways by compacting the soil profile, which is indicated by increased bulk density and decreased macroporosity. This compaction can result in decreased infiltration capacity of the soil and consequently, increased occurrence of overland flow. By consuming above-ground plant biomass, livestock reduce the amount of available biomass and may increase the proportion of bare ground, which may in turn increase soil erosion due to wind and water. However, manure and urine deposition could improve water infiltration rates and reduce soil erosion. With proper grazing management, better manure distribution may reduce bare ground patches and soil erosion.

Grazing systems are designed to meet a diverse set of personal and/or economic goals, the most pervasive of which is the maximization of livestock production or profitability on a sustainable basis. “Sustainability” is considered to be a fundamental goal of rangeland management and sustainability depends primarily on conservation of the soil.

NDSU Graduate student Guojie Wang with Dr. Jimmie Richardson, Professor of Soil Science, conducting research at CGREC, summer 2007.

Roots

Substantial effort has been dedicated to understanding the structural and physiological properties of below-ground plant tissue because of its role in the water and nutrient cycle. Grazing has been shown to influence the rates of litter accumulation and decomposition, canopy biomass (by depressing plant vigor), herbage production, and soil hydrology. Research has demonstrated that considerable plasticity occurs in root morphology and biomass allocated to root development, which is dependent, at least in part, on plant species composition, plant functional type, soil dynamics, and environmental factors. For example, plasticity in root architecture is linked to variation in soil water and defoliation. Defoliation of grasses may reduce, stimulate, or have no effect on root biomass and below-ground allocation. The literature is unclear on the driving factors of soil hydraulic properties. Whether changes in root biomass or morphology cause changes in soil hydraulic properties, or changes in the soil affect the roots, is unknown. The influence of herbivores on root properties may also relate to defoliation and soil texture, and may help to explain previously observed differences in soil hydraulic properties.

Milchunas and Lauenroth (1993) reviewed a worldwide 236-site data set and found no clear relationship in species composition, root biomass, and soil organic carbon of grazed versus ungrazed grasslands. The variance noted in these studies results from soil variations within the studies, differences in the depth of the soil profile being evaluated, and a lack of thorough evaluation of the soil hydrology within the system. We feel that a careful evaluation of the effects of grazing on soil hydrology and balance is important and can be a useful indicator of the effects of grazing management on rangeland health.

The objectives of this study are to evaluate the effects of four grazing treatments on the mixed-grass prairie of the Missouri Coteau Region. These treatments are: rotational grazing in a twice-over grazing system, extremely heavy and moderately stocked season-long grazing, and light to ungrazed management, with each treatment imposed on similar rangeland and soil types for 19 or more years. The variables studied include: 1) above-ground plant community species composition, basal cover, frequency, density, and biomass, 2) soil physical and hydraulic properties, and 3) below-ground plant material morphology and rates of litter and root decomposition. By measuring these factors related to the plant community and soil health, we hope to find an optimum grazing management strategy in terms of sustainability, plant productivity, species diversity, and soil health.

Experimental Design

Four treatments will be studied in this project: long-term light to non-use grazing, rotational grazing, season-long moderate grazing, and season-long extreme grazing. Each treatment has three replicates, one in each of three pastures. In 2006, a 328-foot permanent transect was established in each replicate pasture on the same soil type/ecological site with a slope of 6 to 9 percent. (In one season-long moderate grazing pasture, one transect is only 164-feet long). The topographic locations of each transect included the summit, backslope, and toe.

Methodology

Objective 1: Aboveground Plant Community
A standard 10-pin point frame was used to measure basal density of grass and grass-like species, bare ground, and litter. Species composition, frequency, and forb density were estimated using 50 1-square-foot quadrats per transect, one located every 6.6 feet along the transect (or every 3.3 feet along the 164-foot transect). Above-ground biomass and litter were estimated as peak standing crop in mid- to late-July 2006 by clipping 2.7 square-foot quadrats.

Objective 2: Soil Physical and Hydraulic Properties
After digging a six-foot deep pit in the field, the site and soil profile were described. A single-ring infiltrometer was used to measure the soil infiltration rate. Aggregate stability was determined by the wet-sieving method. Particle-size analysis was used to evaluate soil texture. Soil organic matter was estimated by measuring loss after ignition. Calcium carbonate equivalent was analyzed by the pressure-calcimeter method. Soil cores were used to measure soil bulk density. A soil water characteristic curve was calculated using results from three techniques: a tension table, a pressure plate, and a pressure cooker.

Objective 3: Below-ground Plant Material Morphology, Distribution, and Rates of Litter and Root Decomposition
Below-ground biomass was estimated by taking a 2-inch diameter soil core. After a screen separation, each sample was further divided into live and dead matter by texture. Then below-ground live structures were manually classified under a dissecting microscope into four components according to their average root diameter and function: 1) fine roots, less than 1.0 mm; 2) thick roots, greater than 1.0 mm; 3) rhizomes; and 4) tap roots. After these steps were completed, the root material was oven-dried for 12 hours at 140°F and weighed. In situ litter and root decomposition will be evaluated by burying litter and roots in nylon bags and retrieving them at a later date.

Results

The results will be reported in the 2008 Annual Report.●


NDSU Central Grasslands Research Extension Center
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