TY - JOUR
AU - Bisinger, J. J.
AB - Abstract Beyond grazing, managed grasslands provide ecological services that may offer economic incentives for multifunctional use. Increasing biodiversity of plant communities may maximize net primary production by optimizing utilization of available light, water, and nutrient resources; enhance production stability in response to climatic stress; reduce invasion of exotic species; increase soil OM; reduce nutrient leaching or loading in surface runoff; and provide wildlife habitat. Strategically managed grazing may increase biodiversity of cool-season pastures by creating disturbance in plant communities through herbivory, treading, nutrient cycling, and plant seed dispersal. Soil OM will increase carbon and nutrient sequestration and water-holding capacity of soils and is greater in grazed pastures than nongrazed grasslands or land used for row crop or hay production. However, results of studies evaluating the effects of different grazing management systems on soil OM are limited and inconsistent. Although roots and organic residues of pasture forages create soil macropores that reduce soil compaction, grazing has increased soil bulk density or penetration resistance regardless of stocking rates or systems. But the effects of the duration of grazing and rest periods on soil compaction need further evaluation. Because vegetative cover dissipates the energy of falling raindrops and plant stems and tillers reduce the rate of surface water flow, managing grazing to maintain adequate vegetative cover will minimize the effects of treading on water infiltration in both upland and riparian locations. Through increased diversity of the plant community with alterations of habitat structure, grazing systems can be developed that enhance habitat for wildlife and insect pollinators. Although grazing management may enhance the ecological services provided by grasslands, environmental responses are controlled by variations in climate, soil, landscape position, and plant community resulting in considerable spatial and temporal variation in the responses. Furthermore, a single grazing management system may not maximize livestock productivity and each of the potential ecological services provided by grasslands. Therefore, production and ecological goals must be integrated to identify the optimal grazing management system. INTRODUCTION Pasture forage is a major dietary component for beef cow–calf and stocker production systems (Capper, 2011). However, the area of grazing lands in the United States decreased by approximately 25% from 1945 to 2002, or 0.44%/yr (Lubowski et al., 2006), making land availability a major factor limiting expansion of beef cow herds (Lawrence and Schuknecht, 2005). From 2002 to 2012, the decrease in grazing lands in the United States accelerated to 6.0%, or 0.60%/yr (USDA-NASS, 2004, 2014). Loss of grazing lands during this decade varied from 3.5% in the southern Great Plains to 21.3% in the Midwest and was primarily associated with loss of cropland pasture (Fig. 1). Simultaneous to the loss of grazing lands, the amount of grasslands in government contract programs such as the Conservation Reserve Program, Wetlands Reserve Program, and Conservation Reserve Enhancement Program decreased by 15.7% nationally from 2002 to 2012 with the greatest losses (31.7%) in the Northeast region. Therefore, the total amount of grasslands lost in the Northeast, Southeast, Midwest, northern Great Plains, southern Great Plains, and West were 15.1, 12.5, 20.4, 7.5, 3.5, and 6.0%, respectively, from 2002 to 2012. Although grasslands in some regions may be readily rotated between crop production and pasture, reestablishment of pasture, cow herds, fencing, and water systems and continued competition of land for grain production limit the return of cropland to pasture in central and eastern United States (Burdine, 2014). Figure 1. View largeDownload slide Grassland area in different regions of the continental United States in 2002 and 2012 (USDA-NASS, 2004, 2014). Regions include the West (Arizona, California, Idaho, Nevada, Oregon, Utah, and Washington), northern Great Plains (Montana, Nebraska, North Dakota, South Dakota, and Wyoming), southern Great Plains (Colorado, Kansas, New Mexico, Oklahoma, and Texas), the Midwest (Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, and Wisconsin), the Southeast (Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, and Virginia), and the Northeast (Connecticut, Delaware, Massachusetts, Maryland, Maine, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and West Virginia). Figure 1. View largeDownload slide Grassland area in different regions of the continental United States in 2002 and 2012 (USDA-NASS, 2004, 2014). Regions include the West (Arizona, California, Idaho, Nevada, Oregon, Utah, and Washington), northern Great Plains (Montana, Nebraska, North Dakota, South Dakota, and Wyoming), southern Great Plains (Colorado, Kansas, New Mexico, Oklahoma, and Texas), the Midwest (Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, and Wisconsin), the Southeast (Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, and Virginia), and the Northeast (Connecticut, Delaware, Massachusetts, Maryland, Maine, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, and West Virginia). Beyond grazing, grasslands provide ecological services including carbon sequestration (Follett and Reed, 2010), water infiltration (Franzluebbers, 2002), soil and nutrient retention (Haan et al., 2006), and wildlife habitat (Anderson and McCuistion, 2008). However, because unmanaged grasslands become dominated by a few grass species with invasion of weeds and woody plants (Tracy and Sanderson, 2000; Leffler et al., 2014), grasslands must be exposed to disturbances such as grazing to maintain the diverse plant community required to maximize these ecological services (Rook and Tallowin, 2003). Therefore, grazing of grasslands managed to provide ecological services ++can provide forage for livestock while enhancing environmental quality. However, the effects of grazing on environmental quality are controlled by the location, timing, duration, and intensity of grazing (CAST, 2002). Therefore, management practices to enhance environmental quality may include management of stocking rate (Briske et al., 2011; Sollenberger et al., 2012) and system (Sollenberger et al., 2012). But as stocking rate will vary with stocking density and the number of grazing units and the duration of stocking period and the length of rest periods in rotational or deferred stocking systems may range from weeks to years, these factors need to be defined when evaluating the effects of grazing on environmental quality. BIODIVERSITY OF PLANT COMMUNITIES Because of its relations with quality of soil, water, and wildlife habitat, biodiversity within grassland plant communities is an important consideration in the optimization of the ecological services provided by a landscape. A diverse plant community will enhance the amount and stability of total biomass production within a grassland as a result of niche complementarity in species-rich mixtures that optimize capture of light, water, and nutrient resources (Tilman, 1996; Tilman et al., 2001). Studies have shown that plots planted with some monocultures or binary mixtures are as or more productive than more diverse plant communities over a short time period, but the most productive species depended on the site and season of measurement (Sanderson et al., 2004). Therefore, the advantage of a diverse plant community may be the greater probability of presence of the most productive species under the variety of environmental conditions to which a pasture is exposed over time. As a result, greater diversity in grassland plant communities is more likely to maintain ecosystem functions during periods of climatic stress (Tilman, 1996; Yachi and Loreau, 1999). Greater biodiversity of plant communities has resulted in increased aboveground productivity and carbon sequestration as soil OM, because nitrogen fixed by legume species enhanced belowground biomass production by warm-season grass species (Fornara and Tilman, 2008). In addition, because the variety of species in a diverse plant community will fulfill the variety of niches within a landscape, there is less opportunity for invasive species to establish (Wilsey and Polley, 2002). However, for success in controlling invasive species, the plant species selected need to be functionally unique and correspond to the growth form of the invasive species (Leffler et al., 2014). Furthermore, diverse plant communities provide the variety in feed resources and plant structure needed to sustain a diverse population of vertebrate and invertebrate animal species (Rook and Tallowin, 2003). As a result of these benefits, plant diversity is considered a measure of rangeland health when existing vegetation can be compared with native vegetation in a reference site (Pellant et al., 2005). However, because of the large amount of natural variability, the lack of research demonstrating how it is affected by management, and the ranges of acceptable and unacceptable conditions, quantifying the relationship between species diversity and rangeland health is difficult (Symstad and Jonas, 2011). As grasslands require some disturbance to prevent succession toward woody invasive species, properly managed grazing will maintain a vigorous plant community (Wilsey and Polley, 2002). Grazing affects the composition of plant communities by the effects of herbivory and trampling associated with grazing along with transfer of seeds either attached to coats or excreted in feces (Olff and Ritchie, 1998). Stocking rate relative to the potential productivity of a location will control the effects of grazing on plant diversity in tallgrass prairie (Hickman et al., 2004) and Nebraska Sandhills meadows (Volesky et al., 2004). Whereas grazing at low stocking rates will increase plant diversity on productive grasslands, high stocking rates can decrease diversity to a few grazing-tolerant species (Briske, 1996; Olff and Ritchie, 1998). Under conditions of high productivity associated with nutrient inputs from fertilization or atmospheric deposition, grazing by domestic or wild herbivores increased grassland plant diversity primarily by controlling ground-level light (Borer et al., 2014). In contrast to the effects of stocking rate, Briske et al. (2011) found little effect of stocking system on plant species diversity on rangelands. However, Sollenberger et al. (2012) observed that, although not as significant as stocking rate, stocking method will influence the botanical composition and persistence in pasturelands in the central and eastern United States. For example, both continuous and rotational stocking reduced the proportion of cool-season grasses while increasing the proportion of weed and legume species, respectively, in comparison to a nongrazed exclosure in southern Iowa pastures (Guretzky et al., 2005). Within legume species, continuous stocking increased the proportion of the prostrate-growing species, white clover, whereas rotational stocking increased proportion of the upright-growing species, red clover (Guretzky et al., 2005). The response of plant communities to grazing is subject to variations caused by landscape position, soil properties, and climate (Stohlgren et al., 1999; Symstad and Jonas, 2011). Whereas grazing with large herbivores may increase diversity of plant communities in productive regions such as temperate grasslands, grazing may reduce diversity in less productive regions such as arid ecosystems (Bakker et al., 2006). However, at a local scale, the effects of grazing with different stocking systems on the proportions of cool-season perennial grass, legume, and weed species within plant communities were greater on hill slopes than either the summits or toe slopes of hills (Harmoney et al., 2001; Guretzky et al., 2005). This variation in response may be related to the soil fertility and/or water availability of the soils with the greatest plant diversity occurring where these factors are limiting (Huston, 1994). Tracy and Sanderson (2000) found soil phosphorus concentration to be negatively related to species richness of pastures in the northeast United States. As livestock prefer to graze at the base or on top of hills, topographic distribution of grazing livestock may have a role in the differences in botanical composition across landscape in pasturelands (Sollenberger et al., 2012). However, current literature is inadequate to make conclusions on this relationship. Temporal variations in the response of plant communities to grazing are likely related to variations in weather (Symstad and Jonas, 2011). In southern Iowa, a single grazing event at a stocking density of 529,000 kg/ha with movement of cattle 4 times daily (high-density, short-duration stocking) or 148,000 kg/ha with movement of cattle once daily (moderate-density, moderate-duration stocking) during a period with 32.8 cm precipitation initiated a succession of a grassland plant community toward annual grasses followed by legume species in the spring of the subsequent year (Bisinger, 2014; Fig. 2). Furthermore, within legume species, there was succession from birdsfoot trefoil (Lotus corniculatus L.) in nongrazed grasslands to red clover (Trifolium pratense L.) in pastures grazed by a single spring grazing event at either stocking density. However, because of dry conditions from 14 to 30 mo after the single spring grazing event, the proportions of legume species in pastures grazed by high-density, short-duration or moderate-density, moderate-duration stocking that were either nongrazed or grazed by rotational stocking, thereafter, did not differ from nongrazed pastures not exposed to an initial single spring grazing event. Furthermore, when the same stocking treatments were used at a different location during a period with 2.9 cm precipitation in the subsequent year, there was no effect of grazing on the composition of the plant communities as there was little soil disturbance. Figure 2. View largeDownload slide The effects of a single spring grazing event at stocking densities of 520,000 kg/ha with movement 4 times once daily (HDSD) or 148,000 kg/ha with movement once daily (MDMD) with or with subsequent rotational stocking (HDSDR or MDMDR) on the portions of cool-season grass (A), annual grass (B), or legume species (C). NG = nongrazed. a,bMeans within month not bearing a common superscript differ (P < 0.05; Bisinger, 2014). Figure 2. View largeDownload slide The effects of a single spring grazing event at stocking densities of 520,000 kg/ha with movement 4 times once daily (HDSD) or 148,000 kg/ha with movement once daily (MDMD) with or with subsequent rotational stocking (HDSDR or MDMDR) on the portions of cool-season grass (A), annual grass (B), or legume species (C). NG = nongrazed. a,bMeans within month not bearing a common superscript differ (P < 0.05; Bisinger, 2014). These variations in the response of botanical composition to grazing imply that any effects of grazing management may be superseded by landscape, soil, and climate factors (Stohlgren et al., 1999; Symstad and Jonas, 2011). Therefore, both the spatial and temporal scale of measurement of plant diversity must be defined, as selective grazing and disturbance may locally increase plant diversity whereas long-term selection of the most palatable plants is likely to promote dominance of grazing tolerant plants over a large area over time (Olff and Ritchie, 1998). SOIL ORGANIC MATTER Soil OM, derived from above- and belowground plant residues, plant root exudates, and microbial communities, is a critical component of highly functioning soil in grassland ecosystems (Doran and Parkin, 1994; Franzluebbers, 2002; Wander, 2004). Greater levels of soil OM are associated with increased levels of carbon sequestration, cation exchange capacity, and soil aggregation (Conant et al., 2001; Follett and Reed, 2010). As a result, soils with greater OM have been associated with reduced nutrient leaching and increased water infiltration and holding capacity, thus increasing the resilience of landscapes to droughts and reducing potential sediment and nutrient transport to water bodies in precipitation runoff (Tiessen et al., 1994; Follett and Reed, 2010). Soil OM, measured as organic carbon (SOC), accumulates over time in the soil profile. However, the rate and extent of SOC accumulation is, in part, dependent on factors such as the carbon to nitrogen ratio of substrate additions, topography, soil type, and climate (Kalbitz et al., 2000). Although Skinner and Dell (2015) found that a cool-season grass-dominated pasture fertilized with less than 60 kg N/yr had no change in SOC measured from soil cores, a similar pasture fertilized with 90 to greater than 200 kg N/yr lost 504 g SOC m–2 yr–1 with the amounts lost increasing at depths from 30 to 100 cm. Nitrogen fertilizers accelerate the decomposition of light-fraction SOC, which has a turnover on decadal timescales, while stabilizing the mineral-associated heavy-fraction SOC with longer turnover timescales (Neff et al., 2002). In comparison, additions of substrates with a greater carbon to nitrogen ratio, such as grass hay, increased both light- and heavy-fraction SOC, indicating less mobilization and greater accumulation of soil carbon (Cookson et al., 2005). Within grassland landscapes, Burke et al. (1995) found greater levels of SOC in toe slopes in comparison to summit locations. Geographic region was also a significant factor in the amount of SOC by Burke et al. (1995), likely due to differences in soil type and climatic conditions. Regions with higher mean annual temperatures and coarser soil textures have faster SOC decomposition rates and less plant productivity resulting in lower levels of SOC accumulation (Parton et al., 1987; Burke et al., 1989). In comparison to other climatic variables, Jenny (1980) found mean annual precipitation to have the greatest effect on SOC pools. Areas with greater mean annual precipitation have greater accumulation of SOC as a result of greater plant productivity. Nonetheless, within the pool of SOC, increased precipitation during the year will increase loss of dissolved organic carbon (Neff and Asner, 2001; Jacinthe et al., 2004). Beyond the influence of climatic conditions, human-induced disturbances in grassland landscapes, such as conversion to row crop agriculture, have had significant influence on the levels of SOC (West and Post, 2002; Fornara and Tilman, 2008). Tillage, associated with row-crop agriculture, reduces aggregate stability and exposes OM in the soil profile to microbial degradation. Although no-till agriculture increases the formation of soil aggregates associated with greater levels of SOC, SOC content in perennial plant communities is greater than in fields managed by no-till crop production (Franzluebbers, 2005). Perennial plant communities produce greater root mass and support larger communities of soil organisms than annual plant monocultures (Chantigny et al., 1997; Milne and Haynes, 2004). Within perennial plant communities, Fornara and Tilman (2008) found a positive relationship between functional diversity and the rate of SOC accumulation, likely a result of plant complementarity between legume and grass species and more diverse soil microbial communities (Jastrow et al., 2007; De Deyn et al., 2011). Grazing in perennial grasslands has a significant effect on SOC distribution and accumulation (Schuman et al., 1999; Piñeiro et al., 2010). The spatial distribution of SOC is influenced by grazing as fecal carbon is concentrated in loafing areas (Franzluebbers et al., 2000a). However, grazing livestock also accelerates the return of carbon and nitrogen to the soil profile through fecal and urine additions, potentially increasing net primary productivity and, therefore, accumulation of SOC in pasture areas where excessive treading does not inhibit plant growth (Holdo et al., 2007; Semmartin et al., 2008). Although grazing has been associated with both greater and lesser net primary productivity depending on climate (Piñeiro et al., 2010), grazing has consistently increased the amount of root mass, which is the most significant contributor to SOC (Johnson and Matchett, 2001; Pucheta et al., 2004; Derner et al., 2006,). Therefore, in a 12-yr study, Franzluebbers and Stuedemann (2009) found that grazing increased SOC by approximately 0.82 t C∙ha–1∙yr–1 at a depth between 0 and15 cm and reduced SOC by 0.16 t C∙ha–1 yr–1 below 30 cm compared with nongrazed exclosures in grasslands in the southeastern United States (Table 1). However, high biomass removal rates by grazing and hay harvest prevent grasslands from acting as carbon sinks (Skinner, 2008). In addition to the loss of biomass, soil temperature is increased and soil moisture is reduced by litter removal, increasing the decomposition of soil OM (Bremer et al., 1998; Burke et al., 1998; Savadogo et al., 2007). Similarly, soil compaction associated with short duration grazing at greater than 4 animal units∙d–1∙ ha–1 increased decomposition of soil OM because of lower water infiltration rates (Savadogo et al., 2007). Table 1. Effects of stocking management on soil organic carbon in pastures in the humid region of the United States Comparison  Duration of treatments, yr  Measurement depth, cm  Response in soil organic C content,1 %  Reference  Grazed vs. grazing exclosure  12  0 to 15  25.5  Franzluebbers and Stuedemann (2009)  Grazed vs. hay harvest  12  0 to 15  40.2  Franzluebbers and Stuedemann (2009)  Low (5.8 steers/ha) vs. high (8.7 steers/ha) grazing pressure  12  0 to 15  8.5  Franzluebbers and Stuedemann (2009)  Management intensive vs. extensive grazing  21  0 to 50  15.3  Conant et al. (2003)  Management intensive vs. hay harvest  25  0 to 50  NS2  Conant et al. (2003)  Management intensive vs. extensive grazing  5  0 to 50  31.9  Conant et al. (2003)  Management intensive vs. extensive grazing  3  0 to 50  18.9  Conant et al. (2003)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities  3  0 to 7.5  NS  Bisinger (2014)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities with subsequent rotational stocking  3  0 to 7.5  NS  Bisinger (2014)  Mob grazing vs. rotational grazing  2  0 to 7.5  NS  Dunn (2013)  Comparison  Duration of treatments, yr  Measurement depth, cm  Response in soil organic C content,1 %  Reference  Grazed vs. grazing exclosure  12  0 to 15  25.5  Franzluebbers and Stuedemann (2009)  Grazed vs. hay harvest  12  0 to 15  40.2  Franzluebbers and Stuedemann (2009)  Low (5.8 steers/ha) vs. high (8.7 steers/ha) grazing pressure  12  0 to 15  8.5  Franzluebbers and Stuedemann (2009)  Management intensive vs. extensive grazing  21  0 to 50  15.3  Conant et al. (2003)  Management intensive vs. hay harvest  25  0 to 50  NS2  Conant et al. (2003)  Management intensive vs. extensive grazing  5  0 to 50  31.9  Conant et al. (2003)  Management intensive vs. extensive grazing  3  0 to 50  18.9  Conant et al. (2003)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities  3  0 to 7.5  NS  Bisinger (2014)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities with subsequent rotational stocking  3  0 to 7.5  NS  Bisinger (2014)  Mob grazing vs. rotational grazing  2  0 to 7.5  NS  Dunn (2013)  1Response of first variable relative to response of second variable of comparison. 2NS = no significant differences (P > 0.10). View Large Table 1. Effects of stocking management on soil organic carbon in pastures in the humid region of the United States Comparison  Duration of treatments, yr  Measurement depth, cm  Response in soil organic C content,1 %  Reference  Grazed vs. grazing exclosure  12  0 to 15  25.5  Franzluebbers and Stuedemann (2009)  Grazed vs. hay harvest  12  0 to 15  40.2  Franzluebbers and Stuedemann (2009)  Low (5.8 steers/ha) vs. high (8.7 steers/ha) grazing pressure  12  0 to 15  8.5  Franzluebbers and Stuedemann (2009)  Management intensive vs. extensive grazing  21  0 to 50  15.3  Conant et al. (2003)  Management intensive vs. hay harvest  25  0 to 50  NS2  Conant et al. (2003)  Management intensive vs. extensive grazing  5  0 to 50  31.9  Conant et al. (2003)  Management intensive vs. extensive grazing  3  0 to 50  18.9  Conant et al. (2003)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities  3  0 to 7.5  NS  Bisinger (2014)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities with subsequent rotational stocking  3  0 to 7.5  NS  Bisinger (2014)  Mob grazing vs. rotational grazing  2  0 to 7.5  NS  Dunn (2013)  Comparison  Duration of treatments, yr  Measurement depth, cm  Response in soil organic C content,1 %  Reference  Grazed vs. grazing exclosure  12  0 to 15  25.5  Franzluebbers and Stuedemann (2009)  Grazed vs. hay harvest  12  0 to 15  40.2  Franzluebbers and Stuedemann (2009)  Low (5.8 steers/ha) vs. high (8.7 steers/ha) grazing pressure  12  0 to 15  8.5  Franzluebbers and Stuedemann (2009)  Management intensive vs. extensive grazing  21  0 to 50  15.3  Conant et al. (2003)  Management intensive vs. hay harvest  25  0 to 50  NS2  Conant et al. (2003)  Management intensive vs. extensive grazing  5  0 to 50  31.9  Conant et al. (2003)  Management intensive vs. extensive grazing  3  0 to 50  18.9  Conant et al. (2003)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities  3  0 to 7.5  NS  Bisinger (2014)  Grazing exclosure vs. single grazing event at 529,000 or 148,000 kg/ha stocking densities with subsequent rotational stocking  3  0 to 7.5  NS  Bisinger (2014)  Mob grazing vs. rotational grazing  2  0 to 7.5  NS  Dunn (2013)  1Response of first variable relative to response of second variable of comparison. 2NS = no significant differences (P > 0.10). View Large Improved grazing management practices, primarily through control of stocking rate, increases grassland carbon sequestration (Conant et al., 2001; Schuman et al., 2002). Most studies on carbon sequestration in the United States have focused on western rangelands (Conant et al., 2001; Derner and Schuman, 2007). Schuman et al. (2002) found that carbon sequestration in western rangelands could be increased from 0.1 to 0.3 t C ha–1 yr–1 by improving grazing management through control of stocking rate. The improved practices would allow greater root elongation and biomass (Davidson, 1978; Holland and Detling, 1990) and increase the proportion of legume species in the plant community (Mortenson et al., 2004). However, Briske et al. (2011) found the response of SOC to stocking rate to be equivocal because of limited number of studies and decreased response of SOC with the longevity of the management practice. Beyond western rangelands, temperate grasslands in the eastern half of the United States may provide additional areas for carbon sequestration because the greater annual precipitation likely increases the accumulation of SOC (Franzluebbers and Follett, 2005). The potential for increased carbon sequestration is greatest in marginal lands converted from cropland to perennial vegetation as SOC in established well-managed pastures is likely near saturation (Johnson et al., 2005). Unfortunately, the number of studies evaluating the effects of grazing management on soil SOC in the eastern United States is limited (Franzluebbers et al., 2002; Johnson and Matchett, 2001; Johnson et al., 2005). After 12 yr, soils in pastures grazed at a low stocking rate of 5.8 steers/ha contained 8.5% greater SOC than pastures grazed at a high stocking rate of 8.7 steers/ha (Franzluebbers and Stuedemann, 2009). In Virginia, Conant et al. (2003) reported that pastures on loam and silt loam soils and managed by management-intensive grazing for 3 to 25 yr contained 22% greater SOC to a depth of 50 cm than pastures managed by extensive grazing or hay harvest. However, beyond stating that management-intensive grazing was defined as short-rotation grazing compared with pastures that were either extensively grazed or harvested for hay, the stocking rate and management of either system were not defined in that paper. Furthermore, although there were no samples taken before the initiation of these grazing practices, future resampling was planned for determination of SOC sequestration rates. In contrast to those studies, SOC concentration to a depth of 7.5 cm did not differ among pastures that were grazed by season-long rotation, strip, or mob stocking for 2 yr (Dunn, 2013) or after 3 yr in pastures that had been either nongrazed or subjected to a single grazing event by high-density, short-duration or moderate-density, moderate-duration stocking with or without subsequent rotational stocking in Iowa (Bisinger, 2014). A major challenge in studies evaluating total SOC is the length of time required to observe differences resulting from management, reported to be up to 10 yr (Smith, 2004; Conant et al., 2011). The extended length of studies required to observe management-induced changes in total SOC is caused by the variation in SOC fractions susceptible to oxidation by microbes (Knorr et al., 2005). Microbial activity is dependent on soil temperature and moisture, making seasonal weather patterns a significant factor in the temporal and spatial variability of total SOC (Davidson and Janssens, 2006). To overcome the long period required to quantify changes in SOC, soil particulate OM, representing the slowly decomposing OM fraction, has been suggested to be more sensitive to soil management than total soil OM (Cambardella and Elliott, 1992). However, because of the large variation in particulate OM measurements, Conant et al. (2003) found particulate OM to be an inadequate measure of the effects of grazing management on total SOC. More recently, novel methods to investigate microbial-mediated soil processes, such as metabolic activity in soil aggregates, have been developed to detect rapid ecological changes that result in long-term soil organic matter accrual (Bach and Hofmockel, 2014). SOIL PHYSICAL PROPERTIES Although SOC and its associated soil aggregates have important effects on physical properties, such as soil bulk density and water infiltration rate (Warren et al., 1986; Angers, 1990; Andrews et al., 2004), the effects of grazing on soil physical properties are much more rapid than changes in SOC (Greenwood and McKenzie, 2001). Soil compaction is a reduction in the fractional air volume of a soil resulting from an applied load that forces soil particles and aggregates together as air or water are forced from soil pores (Bilotta et al., 2007). Measured as bulk density or penetration resistance, soil compaction will inhibit plant production by obstructing root growth and thereby reducing water and nutrient uptake (Unger and Kaspar, 1994; Evans et al., 2012). However, the high concentration of roots near the soil surface in mature pasture plants may compensate for poor soil conditions in that region (Greenwood and McKenzie, 2001). Soil compaction may also inhibit water infiltration that will increase precipitation runoff, soil erosion, and nutrient loading of surface water sources while reducing soil water storage (Unger and Kaspar, 1994). The static compression force by a standing beef cow has been estimated to be 123 kPa (Betteridge et al., 1999), causing soil compaction. However, because of added force resulting from the kinetic energy associated with walking that may be carried on 2 or 3 legs, the compression forces of walking cows on soils is more than double the static force (Scholefield et al., 1985), enhancing the effects of cattle treading on soil compaction. Although this force would be greater than the 74 to 84 kPa exerted by an unloaded tractor (Blunden et al., 1994), the smaller area of contact by hooves reduces the depth of influence of the force below the soil surface, resulting in the largest effects of grazing livestock near the soil surface (Greenwood and McKenzie, 2001). But assuming a hoof contact area of 100 cm2 (Scholefield et al., 1985), a distance walked per cow of 4.05 km/d in a 12.1-ha cool-season pasture (Davis et al., 2011), and a number of steps walked per kilometer of 995 by grazing cows calculated from Aharoni et al. (2009), each unit of land in a 12.1-ha cool-season pasture stocked with 15 cows would have been trodden 2.8 times over a 140-d grazing season if cattle were evenly distributed. As 10.1% of observations of cows grazing in 12.1-ha pastures were within 33 m of a stream bisecting the pastures, which represented 7.2% of pasture area (Haan et al., 2010; Bisinger et al., 2014), each unit of land within this ecologically sensitive area would have been trodden 3.9 times over a 140-d grazing season if cattle had unrestricted access to the stream. Furthermore, as the proportion of observations within 33 m of a stream increased to 28.2% when the pasture size was reduced from 12.1 to 4.04 ha (Bisinger et al., 2014), each unit of land within 33 m of the stream of a 4.04-ha pasture would have been trodden as many as 10.8 times over a 140-d grazing season, if the stocking rate was proportional in the smaller pasture. In addition to compaction resulting from vertical compression, smearing of the soil surface and tearing of vegetation may be caused by the tangential force exerted horizontally by the hooves of a walking animal (Bilotta et al., 2007). Because of these forces, cattle grazing has increased soil compaction compared with nongrazed exclosures by 6.8 to 9.0% at a depth of 3 to 8 cm when measured as bulk density and by 32 to 84% at a depth of 5 to 10 cm when measured as penetration resistance as a result of increased soil strength and fewer, smaller, and less continuous macropores of soils in pastures (Greenwood et al., 1997; Greenwood and McKenzie, 2001) and grazed cover crops (Franzluebbers and Stuedemann, 2008). The depth of the effects of grazing livestock on soil compaction have ranged from 2.5 to 14.0 cm from the surface (Van Haveren, 1983; Haan et al., 2006), with damage occurring at the greater depths when treading occurs at high soil moistures (Greenwood et al., 1997). Because of the repeated exposure to treading, soil bulk densities in the upper 2.5 to 7.5 cm soil may be 18 to 32% greater near cattle congregation sites such as trails or supplementation sites or near shade or water sources than sites at distances as close as 25 m from these congregation points (Franzluebbers et al., 2000b; Tate et al., 2004). Although grazing increased soil compaction, different stocking rates have had little or no effect on soil bulk density or penetration resistance in some studies, particularly on soils subjected to treatments for extended periods of time (Warren et al., 1986; Chanasyk and Naeth, 1995; Greenwood et al., 1997; Daniel et al., 2002). However, shortgrass prairie pastures in Colorado grazed at 0.73 ha∙heifer–1∙mo–1 for 30 yr had a greater soil bulk density to a depth of 2.5 cm than pastures grazed at 1.26 or 1.66 ha∙heifer–1∙mo–1, primarily from grazing effects on fine textured loam soils (Van Haveren, 1983). Similarly, soil bulk density in a North Dakota pasture grazed at 2.3 steers/ha in late spring to early summer and 1.2 steers/ha thereafter each year for 72 yr was greater than pastures grazed at 0.39 or 1.1 steers/ha for 88 yr (Liebig et al., 2014). Soil penetration resistance measurements to a depth of 10 cm in pastures with an irrigated short-duration grazing system (3 d grazing and 33 d rest over 10 cycles) in Brazil were greater in pastures stocked at 5.68 than at 4.42 or 3.50 animal units/ha (da Silva et al., 2003). Over an entire grazing season, a range of stocking systems including moderate continuous, heavy continuous, and short-duration stocking (Thurow et al., 1986); continuous, rotationally deferred, and short-duration stocking at 3 stocking rates ( Abdel-Magid et al., 1987); no stocking or continuous and rotational stocking to residual sward heights of 5 or 10 cm (Haan et al., 2006); and rotational, strip, or mob stocking at equal stocking rates (Dunn, 2013) had little effect on soil compaction. However, although Bisinger (2014) found greater soil bulk densities to 7.5 cm and penetration resistance measurements to 10.0 cm for 3 yr subsequent to pastures being exposed to a single spring grazing by moderate-density, moderate-duration stocking (moved once daily) with or without subsequent grazing than in nongrazed exclosures during a period with 32.8 cm precipitation, there were no differences in soil bulk densities or penetration resistance measurements between grazing exclosures and pastures exposed to a single grazing event by high-density, short-duration stocking (moved 4 times daily) if not subsequently grazed, even though both grazing treatments had equal stocking rates. This result implies that even at a high stocking density, the effects of grazing on soil compaction may be minimized by reducing the length of time a given area is exposed to treading. However, although the length of the rest period has been reported to have greater effects on the hydrologic condition of pasture soils than stocking density or length of the grazing period (Warren et al., 1986), there has been little research directly evaluating the effects of rest period length on soil bulk density or penetration resistance. While increasing soil compaction, grazing may also reduce water infiltration and thereby increase risks of soil erosion and non-point-source pollution of surface water sources (Pietola et al., 2005; Haan et al., 2006). Water infiltration rate is more sensitive to stocking rate than soil bulk density, because water infiltration is consistently less at increased stocking rates (Warren et al., 1986; Abdel-Magid et al., 1987; Daniel et al., 2002). The sensitivity of water infiltration is shown not only by the extent of the decrease in water infiltration but also in rate of change, as water infiltration was reduced after treading at different levels for 40 min (Russell et al., 2001). This greater sensitivity of water infiltration than soil bulk density is likely related to factors such as vegetative (Russell et al., 2001; Haan et al., 2006; Schwarte et al., 2011a) and organic cover (Thurow et al., 1988), plant community composition (Thurow et al., 1986, 1988), ambient soil moisture (Bisinger, 2014), soil surface roughness (Russell et al., 2001), and slope (Haan et al., 2006) that affect water infiltration beyond the loss of macropores. In addition to stocking rate, water infiltration is also more responsive to stocking management than soil bulk density or penetration resistance. Although water infiltration rates of Texas pastures grazed by continuous stocking at a moderate rate or by high-intensity, low-frequency stocking maintained initial water infiltration rates, grazing by continuous stocking at a high rate or by short-duration stocking reduced water infiltration rates over 2 to 6 yr (Thurow et al., 1988). Haan et al. (2006) found that whereas grazing of smooth bromegrass (Bromus inermis L) pastures by continuous or rotational stocking to a residue height of 5 cm reduced water infiltration rate and increased P transport in simulated precipitation runoff, pastures grazed by rotational stocking to a residual height of at least 10 cm or harvested for hay during summer and grazed in the fall had no greater water runoff or P transport in simulated precipitation runoff than nongrazed exclosures (Fig. 3). Similarly, although there were no differences in the infiltration rates of bare areas along stream banks in pastures grazed by continuous stocking or rotational stocking in which grazing in the riparian paddock was managed to maintain a sward height of 10 cm, the lower amount of bare ground along stream banks in pastures grazed by rotational stocking resulted in lower amounts of sediment and P transport in rotationally stocked pastures (Schwarte et al., 2011a,b). Therefore, maintaining a sward height of 10 cm is valuable in minimizing precipitation runoff and sediment transport in Midwest pastures (Haan et al., 2006; Schwarte et al., 2011b). But the most desirable sward height will be dependent on forage species, as a canopy height of 2 cm was adequate to limit the effects of treading on precipitation runoff and sediment transport in dense sodgrass pastures dominated by brown top (Agrostis tenuis Sibth.) in New Zealand hill country (Russell et al., 2001). The differences in the sward height necessary to minimize precipitation runoff are likely associated with the proportion of bare soil, which has been identified as the major factor affecting water infiltration (Greenwood and McKenzie, 2001; Haan et al., 2006; Schwarte et al., 2011a). Once bare, the homogenization of muddy soils by hoof traffic in animal congregation sites further reduces water infiltration (Pietola et al., 2005). Figure 3. View largeDownload slide Effects of stocking systems on water infiltration rate and sediment and P loading of runoff from rainfall simulations with precipitation applied at 7.1 cm/h for 90 min. Treatments included nongrazed, hay/stockpile (forage harvested as 2 harvests followed by stockpiling and November grazing at 7.5 cows/ha to a residual sward height of 5 cm measured with a falling plate meter [4.8 kg/m2; 63 cow-days{cow-d} ha–1 yr–1], Rotational 10 cm (paddocks stocked at 7.5 cow/ha to a residual sward height of 10 cm followed by 35 d rest from May through October [272 cow-d ha–1 yr–1], Rotational 5 cm (paddocks stocked at 7.5 cow/ha to a residual sward height of 5 cm followed by 35 d rest from May through October [337 cow-d ha–1 yr–1], and Continuous 5 cm (paddocks stocked at 7.5 cow/ha to a residual sward height of 5 cm followed by 7 to 10 d rest from May through October [429 cow-d ha–1 yr–1]). a,bWithin a variable, means with different superscripts differ (P < 0.05;Haan et al., 2006). Figure 3. View largeDownload slide Effects of stocking systems on water infiltration rate and sediment and P loading of runoff from rainfall simulations with precipitation applied at 7.1 cm/h for 90 min. Treatments included nongrazed, hay/stockpile (forage harvested as 2 harvests followed by stockpiling and November grazing at 7.5 cows/ha to a residual sward height of 5 cm measured with a falling plate meter [4.8 kg/m2; 63 cow-days{cow-d} ha–1 yr–1], Rotational 10 cm (paddocks stocked at 7.5 cow/ha to a residual sward height of 10 cm followed by 35 d rest from May through October [272 cow-d ha–1 yr–1], Rotational 5 cm (paddocks stocked at 7.5 cow/ha to a residual sward height of 5 cm followed by 35 d rest from May through October [337 cow-d ha–1 yr–1], and Continuous 5 cm (paddocks stocked at 7.5 cow/ha to a residual sward height of 5 cm followed by 7 to 10 d rest from May through October [429 cow-d ha–1 yr–1]). a,bWithin a variable, means with different superscripts differ (P < 0.05;Haan et al., 2006). The response of soil to treading is related to the structural stability of the soil and the ability of soils to recover its structural form through the actions of roots, soil animals, and weather cycles (Greenwood and McKenzie, 2001). Therefore, similar to other pasture characteristics, soil, landscape, and climatic factors result in large temporal-spatial variation in soil physical properties. Susceptibility of soils to compaction is increased with increasing clay content of the soil and decreased with increasing concentration of sand in the soil and SOC in the clay (Van Haveren, 1983; Angers, 1990). Also, prior stresses caused by seasonal variations in weather, dynamic kneading, trampling, or tillage affect the susceptibility of a soil to compaction by disturbing the mechanical behavior between soil particles (Scholefield et al., 1985; Warren et al., 1986; Pietola et al., 2005). Therefore, when evaluating the effects of grazing management on soil physical properties, it is valuable to report not only the classification but also the previous management of the soils at the specific locations on which the treatments are imposed. Soil moisture at the time of grazing will have a large impact on the effects of grazing on soil physical properties (Scholefield et al., 1985; da Silva et al., 2003). Soil compaction will increase as soil moisture at the time of treading increases until the soil moisture content is high enough for the soil to undergo plastic deformation resulting in particle movement with or without a reduction in total pore volume referred to as pugging (Bilotta et al., 2007). As soil water contents increase above the soil's plastic limit, soils are more likely to be subjected to particle displacement than compaction (Scholefield et al., 1985), resulting in poaching of the soil, defined as hoof-print depressions greater than 40 mm deep caused by remolding of the soil (Greenwood and McKenzie, 2001). As a result of differences in soil moisture, grazing grasslands in southern British Columbia in the spring for 20 yr resulted in greater soil bulk density to 7.5 cm and penetration resistance to 15.0 cm than fall grazing (Evans et al., 2012). Soil moisture at the time of physical measurements also may affect results by reducing soil strength (da Silva et al., 2003; Dexter et al., 2007) or altering the degree of saturation of the soils (Franzluebbers and Stuedemann, 2008) In addition to soil moisture, soil temperatures will also influence the extent and recovery of soils from soil compaction. Grazing on frozen soils prevented increases in soil penetration resistance in the upper 10 cm of soil observed in corn fields grazed when soils were not frozen (Clark et al., 2004). Furthermore, freeze–thaw activity in the soil has mitigated increases in soil compaction (Jabro et al., 2014) or decreases in water infiltration (Abdel-Magid et al., 1987; Wheeler et al., 2002) associated with grazing. WILDLIFE HABITAT Midwest grasslands provide habitat for many wildlife species that generate income and enhance the ecological stability of grassland ecosystems. Economic contributions from upland game bird hunting in Iowa were estimated at US$39 million in 2011; however, all hunting enthusiasts generated more than $673 million in economic activity in Iowa (National Shooting Sports Foundation, 2012). In addition to upland game birds, grasslands also provide habitat for beneficial arthropods that contribute up to $8 billion annually as biological controls and native pollinators in the U.S. agriculture sector alone (Losey and Vaughan, 2006). Unfortunately, loss of quality habitat and adverse climatic conditions are negatively impacting the populations of grassland bird and arthropod species throughout the midwestern states (Brennan and Kuvlesky, 2005; Isaacs et al., 2009). However, grazing systems that enhance wildlife habitat and conservation programs designed to increase the availability of perennial grasslands to beef producers have the potential to enhance the ecological stability and economic sustainability of perennial grasslands (Boyles et al., 2001; Brennan and Kuvlesky, 2005; Anderson and McCuistion, 2008). Wildlife habitat relates the presence of a species with the physical and biological environment (Block and Brennan, 1993). Therefore, high-quality habitat for wildlife describes an environment with biological and physical characteristics capable of supporting a stable wildlife population (Van Horne, 1983). In addition to a water source, quality wildlife habitat requires appropriate feed resources. Grassland birds rely on insects and seeds for feed (Martin et al., 2000; Hurst, 1972). As a result, habitat for grassland birds requires areas of diverse plant communities with abundant forb species to attract insects and provide high-energy seeds (Haddad et al., 2001; Knops et al., 1999; O'Leske et al., 1996). Within a grassland landscape, quality wildlife habitat for many species requires microhabitats with characteristics distinct to areas for raising young and protection from predators, weather, and other threats (White et al., 2005; Fisher and Davis, 2010). For example, brood-rearing areas for ground-nesting birds, such as bobwhite quail (Colinus virgianus), have a greater proportion of bare ground and forb species with taller vegetation (Taylor et al., 1999). In comparison, although nest sites also had a greater proportion of forb species, there was greater residual plant material for nest structure and protection from predators (White et al., 2005). However, the habitat requirements for grassland wildlife, particularly grassland birds, vary depending on species (Derner et al., 2009; McCoy et al., 1999). Habitat quality for many wildlife species is, in part, dependent on diversity of plant species and successional stages of plant communities within a habitat (Siemann et al., 1998; Fuhlendorf and Engle, 2001; Harper 2007). As plant communities mature, diversity of grassland plant communities decreases, decreasing the habitat quality for many wildlife species (Millenbah et al., 1996; Harper, 2007). To provide the opportunity for more diverse or early successional plant communities to become established, disturbance of the plant community must occur (Harper, 2007). There are many methods of plant community disturbance, but one potential method is strategic grazing of livestock, which can provide the necessary disturbance while making use of the available forage (Patterson and Best, 1996; Boyles et al., 2001; Coppedge et al., 2008). The timing and level of disturbance from strategically managed grazing has the potential to influence plant community composition and available habitat for wildlife species (Fuhlendorf and Engle, 2001; Tews et al., 2004; Bisinger et al., 2014). For example, whereas high-density, short-duration stocking decreased the proportion of cool-season grass species, it increased the proportion of annual grass and legume species (Bisinger, 2014). Frisina (1992) found that spring grazing in Montana rangelands enhanced the quality of forage available for elk during summer months. Additionally, Derner et al. (2009) determined that specific grazing management could be implemented to influence plant communities and structure and height of forage to create habitat for specific grassland bird species. Besides large mammalian wildlife and grassland birds, research has shown that modified grazing systems that increase landscape heterogeneity also increase the diversity of small arthropods (Dennis et al., 1998; Engle et al., 2008). Grazing of grasslands to promote wildlife habitat has several benefits, but there are also many challenges. A major challenge to the use of grazing to enhance wildlife habitat is developing a grazing system that meets the needs of different wildlife species and grazing cattle. Grasslands managed for ground-nesting bird and some song bird habitats require long periods to allow birds to nest. During this time, forage may senesce, reducing forage nutritional quality (Brennan and Kuvlesky, 2005; Briske et al., 2008). As a result, when grazing grasslands managed for grassland birds, cattle producers should attempt to match the nutritional requirements of the livestock to the composition of the forage. Another challenge is balancing conservation goals. Many ground-nesting bird species prefer grassland areas with as much as 25 to 50% bare ground to allow chick movement and collecting arthropods (White et al., 2005). This level of bare ground is difficult to achieve in an established cool-season grassland even with a single spring grazing event at high-density, short-duration stocking (Bisinger, 2014). Furthermore, managing areas for more than 30% bare ground has an inherent risk of increasing the erosion of soil and nutrients into water bodies (Hofmann and Ries, 1991; Haan et al., 2006). As a result, the timing, location, and extent of grazing need to be strategically implemented to optimize the goals of animal production and the conservation goals of the land owner. SUMMARY AND CONCLUSIONS In comparison with land used for row crop production, the perennial vegetation provided by grasslands will increase SOC content and water infiltration while reducing soil bulk density (Tiessen et al., 1994; Franzluebbers, 2002; Follett and Reed, 2010). With appropriate management of stocking rate and system for the plant species and soil conditions within a grassland ecosystem, plant community diversity (Hickman et al., 2004; Volesky et al., 2004; Guretzky et al., 2005), SOC content (Schuman et al., 2002; Conant et al., 2003; Franzluebbers and Stuedemann, 2009), and wildlife habitat (Fuhlendorf and Engle, 2001; Tews et al., 2004; Bisinger et al., 2014) of grasslands may be improved by grazing. Furthermore, whereas grazing may increase soil bulk density, water infiltration rate can be sustained by grazing management practices that maintain adequate ground cover. However, the minimum amount of cover to optimize water infiltration will depend on the measurement of cover and the structural characteristics of the plants, topography of the landscape, and physical characteristics of the soils in the ecosystem. Therefore, water infiltration increased to 86.5% organic ground cover on Texas rangeland (Thurow et al., 1988), 94.7% vegetative cover (10 cm sward height) in Iowa smooth bromegrass pastures (Haan et al., 2006), and 90.1% vegetative cover (2 cm sward height) in New Zealand Hill Country pastures (Russell et al., 2001). Grazing management practices that optimize ecological services will control the density and temporal-spatial distribution of stocking with the most effective systems having a low or moderate stocking rate (Briske et al., 2011; Sollenberger et al., 2012; Liebig et al., 2014) and some form of rotational stocking management (Sollenberger et al., 2012). Within rotational stocking management, short grazing periods (Bisinger, 2014) and long rest periods (Warren et al., 1986) may enhance the effects of stocking management on environmental quality, but the effects of these management practices need further study. As the intrinsic properties of plant community, soil, and landscape and extrinsic properties of prior management and climate govern grazing management effects on each component of an ecosystem (Stohlgren et al., 1999; Guretzky et al., 2005; Symstad and Jonas, 2011; Bisinger, 2014), research studies conducted in small pastures or plots have limited value in assessing the effects of grazing on ecosystem characteristics. Therefore, long-term large-scale integrated research projects are necessary to appropriately evaluate such relationships. Furthermore, as the plant community and soil characteristics required to optimize individual ecological services differ, grazing management systems must be identified and implemented that optimize the primary goals of land managers while not harming other characteristics of the ecosystem. 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TI - FORAGES AND PASTURES SYMPOSIUM: Improving soil health and productivity on grasslands using managed grazing of livestock
JO - Journal of Animal Science
DO - 10.2527/jas.2014-8787
DA - 2015-06-01
UR - https://www.deepdyve.com/lp/oxford-university-press/forages-and-pastures-symposium-improving-soil-health-and-productivity-oMR5u0cUW2
SP - 2626
EP - 2640
VL - 93
IS - 6
DP - DeepDyve
ER -