Grazing management impacts on vegetation, soil biota and soil chemical, physical and hydrological properties in tall grass prairie

Abstract

To assess whether adaptive management using multi-paddock grazing is superior to continuous grazing regarding conservation and restoration of resources we evaluated the impact of multi-paddock (MP) grazing at a high stocking rate compared to light continuous (LC) and heavy continuous (HC) grazing on neighboring commercial ranches in each of three proximate counties in north Texas tall grass prairie. The same management had been conducted on all ranches for at least the previous 9 years. Impact on soils and vegetation was compared to ungrazed areas (EX) in two of the counties. MP grazing was managed using light to moderate defoliation during the growing season followed by adequate recovery before regrazing after approximately 40 days and 80 days during fast and slow growing conditions, respectively. The vegetation was dominated by high seral grasses with MP grazing and EX, and dominated by short grasses and forbs with HC grazing. LC grazing had a lower proportion of high seral grasses than MP grazing or EX. Bare ground was higher on HC than LC, MP and EX, while soil aggregate stability was higher with MP than HC grazing but not LC grazing and EX. Soil penetration resistance was lowest with MP grazing and EX and highest with HC grazing. Bulk density did not differ among grazing management categories. Infiltration rate did not differ among grazing management categories but sediment loss was higher with HC than the other grazing management categories. Soil organic matter and cation exchange capacity were higher with MP grazing and EX than both LC and HC grazing. The fungal/bacterial ratio was highest with MP grazing indicating superior water-holding capacity and nutrient availability and retention for MP grazing. This study documents the positive results for long-term maintenance of resources and economic viability by ranchers who use adaptive management and MP grazing relative to those who practice continuous season-long stocking.

Introduction

Prairie ecosystems prior to European settlement of the Great Plains of North America were characterized by free-ranging herds of large, migratory herbivores which moved constantly in response to changes in the vegetation due to topography, edaphic effects and variable and patchy precipitation to improve their diet quality and grazing efficiency (Frank et al., 1998). They also moved for a variety of other reasons including social factors, fire, predators, and movements by herders and hunters (Bailey and Provenza, 2008). Therefore, although grazing was intense at any particular site, such concentrated grazing seldom occurred at length and defoliated plants were usually afforded time and growing conditions to recover (Frank et al., 1998). This periodic vegetation defoliation and regrowth created by migratory herbivores contributed to ecosystem stability and the availability of high quality diet for these herbivores.

A further factor contributing to stability in these ecosystems is that grazers are important regulators of ecosystem processes in grazing ecosystems (Frank and Groffman, 1998). Ungulates in grazed ecosystems increase forage concentration, grazing efficiency, forage nutrient concentration and above-ground plant production (Frank et al., 1998). They also improve mineral availability by enhancing soil microbial nutrient enrichment and rhizospheric processes that ultimately feedback positively to plant nutrition and photosynthesis (Hamilton and Frank, 2001) in addition to increasing nutrient cycling within patches of their urine and excrement (Holland et al., 1992). Consequently, grazing is an optimization function with low levels of primary production at excessively low or high levels of herbivory and maximum productivity at intermediate levels of herbivory (McNaughton, 1979, Dyer et al., 1993, Turner et al., 1993). By increasing resource availability locally, herbivores can also diminish the adverse impacts of secondary compounds in plants by providing conditions more conducive to growth rather than to developing chemical defences (Bryant et al., 1983, Coley et al., 1985). Such grazer controls of carbon (C) and nitrogen (N) processes are as important as landscape effects of topography, catenal position and different soils (Frank and Groffman, 1998).

The relatively recent replacement of these free-ranging wild herbivores with livestock that have restricted movements has removed the key stabilizing element of periodic use. In addition, the maintenance of artificially high animal numbers with supplementary feed during less productive periods has led to widespread overgrazing (Oesterheld et al., 1992, Milchunas and Lauenroth, 1993). The result is degraded vegetation and soils, decreased palatable grasses, increased bare ground and erosion, leading to widespread declines in production, vigor and biodiversity of these grasslands (Archer and Smeins, 1991, West, 1993, Knopf, 1994, Frank et al., 1998) and ultimately a reduction in ecosystem resilience (Peterson et al., 1998). These developments have negative consequences for land-surface–atmospheric interactions (Archer et al., 2001).

The most common form of grazing management on rangeland with livestock is continuous year-round stocking. Livestock grazing large paddocks exhibit spatial patterns of repetitive use; heavily using preferred plants, patches and areas while avoiding or lightly using others (Willms et al., 1988, O’Connor, 1992, Ash and Stafford-Smith, 1996, Bailey et al., 1996, Gerrish, 2004, Witten et al., 2005). Therefore, the effective stocking rate on heavily used patches is much higher than that intended for the area as a whole and if threshold amounts of biomass and litter are not maintained on preferred areas, a degradation spiral is initiated and this is accelerated during periods of below average precipitation (O’Connor, 1992, Thurow, 1991, Fuls, 1992, Ash and Stafford-Smith, 1996, Snyman, 1998, Teague et al., 2004). Even under light stocking rates, grasslands can deteriorate with continuous grazing because of constant, high grazing pressure on preferred areas and plants (Merrill, 1954, Thurow et al., 1988, Norton, 1998, Tainton et al., 1999) and this becomes more prevalent as the size of the paddock increases (Teague et al., 2004, Müller et al., 2007, Bailey and Provenza, 2008, Provenza, 2008).

The principal objective of conservation-oriented livestock grazing management is to maintain or improve forage production and forage harvesting efficiency. Maintenance or improvement of forage production is directly related to water infiltration rates and water-holding capacity (Thurow, 1991, Belsky et al., 1993, Snyman, 2003). Therefore, the long-term success of grazing management depends on how well increased livestock harvest efficiency, which reduces herbaceous cover and biomass, is balanced with the need to maintain soil chemical, physical and hydrological properties. Pioneer conservation ranchers and scientists achieved range improvement using growing season rest (Smith, 1895, Sampson, 1913, Rogler, 1951, Scott, 1953, Matthews, 1954, Merrill, 1954, Hormay, 1956, Hormay and Evanko, 1958, Hormay and Talbot, 1961) and subsequent research confirmed the successful improvement of rangeland using growing season rest often in conjunction with multi-paddock grazing (Reardon and Merrill, 1976, Booysen and de Tainton, 1978, Smith and Ownsby, 1978, Daines, 1980, Danckwerts et al., 1993, Taylor et al., 1993, Kirkman and Moore, 1995, Müller et al., 2007).

In the early 1970s a more intensive form of management was developed based on the writings of Voisin (1959) and Acocks (1966) which involved multiple paddocks per herd, high animal densities, very short periods of grazing, long recovery periods and higher stocking rates than were traditionally considered sustainable (Savory and Parsons, 1980, Savory and Butterfield, 1999, Gerrish, 2004). Subsequently, ranchers worldwide have used adaptive management with multi-paddock grazing to refine management protocols and achieve excellent animal productivity and vegetation improvement objectives. Many ranchers who have practiced multi-paddock grazing for decades have reported a high degree of satisfaction with the economic and ecological results and changes in management lifestyle and social environment of their ranch businesses (Goodloe, 1969, Tainton et al., 1977, Cumming, 1989, McCosker, 1994, Dagget, 1995, Earl and Jones, 1996, Stinner et al., 1997, Norton, 1998, Norton, 2003, Berton, 2001, Sayre, 2001, Gordon, 2002, Howell, 2008). Many of these ranchers have received conservation awards from scientific societies and from the ranching and wildlife industries.

In contrast, based on the results of small-scale field research, the efficacy of multi-paddock grazing management for maintaining or improving rangeland condition has been questioned by Briske et al. (2008). However, most of the grazing management research cited by Briske et al. (2008) did not take into account plant and animal processes at appropriate spatial and temporal scales and was not adaptively managed to achieve desirable soil, vegetation and livestock goals, thus resulting in incorrect interpretations for rangeland management on commercial ranches (Norton, 1998, Teague et al., 2009).

Long-term ecosystem health and profitability are the goals of conservation-oriented ranchers. They plan their grazing management within an adaptive, goal-oriented management framework using basic knowledge of plant and animal physiology and ecology. Also, ranchers must manage in environments with all the inherent variability of unique landscapes and the vagaries of the weather and market place. So to achieve desired goals they view grazing schedules and stocking rates as variables to be applied in an adaptive management context to meet a variety of management objectives under constantly changing circumstances (Teague et al., 2009). This contrasts with research protocols such as those cited by Briske et al. (2008) that have almost invariably applied grazing variables as fixed “treatments” to avoid confounding “grazing management” with other variables, and reduce variability by using small plots, thus ignoring landscape effects (Teague et al., 2009). Accordingly, there is a need to determine the influence of different grazing management categories at the ranch-scale on soil C, ecosystem goods and services and profitability in the southern Great Plains rangelands when they are managed adaptively to achieve desirable soil, vegetation, livestock and ecosystem service goals.

This project aimed to assess the effect of multi-paddock grazing when managed adaptively at the ranch scale to achieve desirable vegetation and animal production goals relative to the two most common grazing practices of light and moderate to heavy continuous grazing on the vegetation and soil chemical, physical, microbial and hydrological properties. We hypothesized that, at the ranch management scale, if multi-paddock grazing were managed adaptively to achieve dominance by high seral vegetation and moderate growing season defoliation levels, it would result in superior vegetation composition and standing crop, and superior soil physical, chemical, microbial and hydrological properties compared to season-long continuous grazing.