Thomas L. Thurow

The influence of livestock trampling under intensive rotation grazing on soil hydrologic characteristics.

Infiltration rate decreased significantly and sediment production increased significantly on a site with a silty clay surface soil devoid of vegetation following periodic trampling typical of intensive rotation grazing systems. The deleterious impact of livestock trampling generally increased as stocking rate increased. Damage was augmented when the soil was moist at the time of trampling. Thirty days of rest were insufficient to allow hydrologic recovery. Soil bulk density, aggregate stability, aggregate size distribution and surface microrelief were related to the soil hydrologic response of the trampling treatments. Many of the world’s rangelands evolved in the presence and under the influence of grazing ungulates. However, the introduction and maintenance of domestic livestock on continuously or rotationally grazed pastures has the potential for altering botanical composition and cover (Ellison 1960) and soil physical properties (Klemmedson 1956, Reed and Peterson 1961). Modification of those parameters, either singly or in concert, may accelerate the natural erosion process and result in decreased on-site productivity, increased sediment production, and increased susceptibility of downstream flooding. As stocking rates of domestic livestock are increased under continuous year-long or season-long grazing, rainfall infiltration generally decreases while runoff and sediment loss increase (Alderfer and Robinson 1947, Rauzi and Hanson 1966, Rhoades et al. 1964). Heavy continuous grazing is generally detrimental to soil hydrologic characteristics, while the effects of moderate or light continuous grazing are significantly less deleterious and frequently not significantly different from each other (Blackburn 1984, Gifford and Hawkins 1978). Supporters of intensive rotation grazing (IRG) systems such as the short-duration grazing method propose that heavy stocking rates under some forms of rotational grazing may be advantageous Authors are presently environmental research scientist, USA-CERL, Environmental Divison, P.O. Box 400!, Champaign, Ill. 61820; range research scientist, P.O. Box 2954, Mogadishu, Somaha; professor of watershed management, Department of Range Science, Texas A&M University, College Station 77843; and research associate, Texas Agricultural Experiment Station, Sonora 76950. At the time of research the principal and second authors were graduate research assistants, Texas A&M University. This study was a cooperative project between U.S. Department of Agriculture and Texas Agricultural Experiment Station. Financial support of the Soil Conservation Service. RCA Special Study and a Science/ Education Grant is acknowledged. Pubhshed with approval of the Director, Texas Agricultural Experiment Stationas TA-20824. Manuscript accepted 7 July 1986. JOURNAL OF RANGE MANAGEMENT 39(0), November 1996 to the range ecosystem (Savory 1978, 1979). Under IRG, large numbers of livestock are concentrated on small areas for short periods of time, creating a “herd effect” or intensive trampling of the soil surface. Proponents of IRG contend that this “hoof action” will enhance infiltration of rainfall into the soil and reduce erosion, even when conventional stocking rates are doubled or tripled (Goodloe 1969, Savory 1983, Savory and Parsons 1980). Previous reviews of the impacts of grazing systems on watershed characteristics have concluded that there are no consistently significant advantages to be accrued by implementing specialized grazing systems (Blackburn 1984, Gifford and Hawkins 1976, Shiflet and Heady 1971, Van Poollen and Lacey 1979). Heavy stocking rates are almost universally detrimental to rainfall infiltration and sediment loss, regardless of the grazing system in use (Blackburn 1984; Gamougoun et al. 1984; M&alla et al. 1984a, 1984b; Pluhar 1984; Smith 1980; Thurow 1987, Weltz 1983). Unfortunately, it is often unclear if the effects are caused by livestock hooves on the soil surface or by the removal of vegetation which would otherwise protect the soil from raindrop impact, increase soil porosity through root activity, and provide an organic substrate for soil arthropod and microbe activity. Several studies have attempted to determine the impacts of livestock trampling in the absence of concomitant removal of vegetation, and have generally concluded that trampling compacts the soil, reduces rainfall infiltration rates, and increases soil erosion. Busby and Gifford (1981), Dadkhah and Gifford (1980), and Packer (1953) used mechanical trampling devices which imitated the compacting force of a livestock hoof but did not provide a rocking or churning effect caused by a hoof when walking. Bryant et al. (1972) used live animals, but neither they nor the former researchers related the degree of trampling disturbance to that which would occur in an intensive rotation grazing system. Edmond (1958,1963,1964) and Witschi and Michalk (1979) used sheep to reproduce the equivalent of a single day of grazing at several stocking rates. However, the studies were conducted on fertilized, irrigated pastures, a situation which is atypical of most of the world’s rangelands. Albeit ungrazed, the ameliorating presence of live plant cover was present in all of the aforementioned studies. The objective of this study was to evaluate the effect that livestock trampling has on infiltration rates and sediment production when a bare soil is subjected to trampling intensities incurred under

Hydrologic characteristics of vegetation types as affected by livestock grazing systems, Edwards Plateau, Texas.

Infiltration rate and sediment production were assessed in oak, bunchgrass and sodgrass vegetation types in moderate continuous (MCG), heavy continuous (HCG), and intensive rotation (shortduration, SDG) grazing systems and in a livestock exclosure (LEX). Infiltration rate was related to the total organic cover and bulk density characteristics of the site (RJ = 36). The amount of cover was more important than type, indicating that protection of soil structure from direct raindrop impact was the primary function of cover on infiltration. The SDG and HCG pastures had lower total organic cover with correspondingly lower infihration rates compared to the MCG and LEX pastures. Bulk density, an indicator of soil structure, was significantly lower in oak mottes than in the grass interspace, but there was no significant difference between pastures. Sediment production was related to the total aboveground biomass and the bunchgrass cover of the site (P q .79). Obstruction to overland sediment transport and protection from the disaggregating effect of direct raindrop impact were the primary functions of the total aboveground biomass and bunchgrass cover. Total aboveground biomass was greatest in the oak motte and least in the sodgrass interspace, consequently the sodgrass interspace had the greatest amount of sediment production and the oak mottes had the least sediment production. Midgrass cover and total aboveground biomass in the MCG and LEX pastures was significantly greater than in the SDG and HCG pastures; thus sediment production from the MCG and LEX pastures was significantly lower than from the SDG and HCG pastures. The hydrologic condition of a range site is the result of complex interactions of soil and vegetation factors. Infiltration rate and sediment production integrate these factors and are good indicators of hydrologic condition. The type of livestock grazing system and stocking rate differentially impact soil structure and vegetation growth in different plant communities. Successional trends in plant communities were directly proportional to grazing intensity, with the most severe changes occurring under heavy grazing (Ellison 1960). Palatable species decline as grazing pressure increases and are replaced by shrubs or other vegetation which are less preferred by livestock and more resistant to grazing (Dyksterhuis 1949). Perennial bunchgrasses are especially important indicators of rangeland condition in the Edwards Plateau of Texas. Many bunchgrass species are palatable and nutritious for livestock and provide good soil stabilization. However, many bunchgrasses have aboveground apical meristems and are not tolerant of repeated heavy grazing. Rich and Reynolds (1963) found that perennial bunchgrass basal cover was reduced by heavy grazing but was unaffected by moderate grazing. Rhoades et al. (1964) Sharp et al. (1964) and Dunford and Weitzman (1955) found that heavily grazed pastures were dominated by sod-forming grasses whereas bunchgrasses dominated moderate and non-grazed pastures. The authors are presently range research scientist, P.O. Box 2954, Mogadishu, Somalia; professor of watershed management, Department of Range Science, Texas A&M University, College Station 77843; and superintendent of the Texas Agricultural Experiment Station at Sonora 76950. Send reprint requests to second author. At the time of research, the senior author was graduate researchassistant, Department of Range Science, Texas A&M University. The authors would like to thank the personnel of the Texas Agricultural Research Station at Sonora and Ronnie Anson for their help during this study. This study was a cooperative project between U.S. Department of Agriculture and Texas Agricultural Expenment Station. Financial support of the Soil Conservation Service RCA Special Study and a Science/ Education Grant is acknowledged. Published with the approval of the Director, Texas Agricultural Experiment Station as TA 20746. Manuscript accepted 29 May 1986. JOURNAL OF RANGE MANAGEMENT 39(6), November 1966 Infiltration rates are generally observed to be highest under trees and shrubs, followed in decreasing order by bunchgrasses and sodgrasses (Smith and Leopold 1941, Woodward 1943, Reed and Peterson 1961, Box 1961, Blackburn 1975, Wood and Blackburn 1981, Knight et al. 1984). Each life form responds differently to grazing pressure and range improvement practices. The extent to which these rangeland uses alter the vegetation composition is a prime factor determining the effects on the soil structure and hydrologic condition of the site. The objective of this research was to study oak, bunchgrass, and sodgrass plant communities to determine how their infiltration rate and sediment production differ from each other. Also, the infiltration rate and sediment production of each of the 3 plant communities was studied to determine how the effects of livestock grazing systems differ for each plant community under moderate continuous, heavy continuous, and intensive rotation grazing systerns and livestock.

Emerging Issues in Rangeland Ecohydrology: Vegetation Change and the Water Cycle

Abstract Rangelands have undergone—and continue to undergo—rapid change in response to changing land use and climate. A research priority in the emerging science of ecohydrology is an improved understanding of the implications of vegetation change for the water cycle. This paper describes some of the interactions between vegetation and water on rangelands and poses 3 questions that represent high-priority, emerging issues: 1) How do changes in woody plants affect water yield? 2) What are the ecohydrological consequences of invasion by exotic plants? 3) What ecohydrological feedbacks play a role in rangeland degradation processes? To effectively address these questions, we must expand our knowledge of hydrological connectivity and how it changes with scale, accurately identify “hydrologically sensitive” areas on the landscape, carry out detailed studies to learn where plants are accessing water, and investigate feedback loops between vegetation and the water cycle.

Hydrologic characteristics of vegetation types as affected by prescribed burning.

The objective of this study was to determine how rangeland hydrology of oak, juniper, bunchgrass and shortgrass vegetation types is altered by fire. The research was conducted at the Sonora Agricultural Experiment Station on the Edwards Plateau, Texas. Infiltration rate and interrill erosion were measured using a drip-type rainfall simulator. Terminal infiltration rates of unburned areas were significantly greater on sites dominated by oak (Quercus virginiana Mill.) (200 mm hour-1) or juniper (Juniperus ashei Buchh.) (183 mm hour-1) than on sites dominated by bunch-grass (146 mm hour-1) or shortgrass (105 mm hour-1). Terminal infiltration rates on burned areas were significantly reduced on sites dominated by bunchgrass (110 mm hour-1), shortgrass (76 mm hour-1), and on oak sites that were cut and burned (129 mm hour-1). Soil organic matter content (r = .61), total organic cover (r = .59), and aggregate stability (r = .53) were the variables most strongly correlated with infiltration rate. Measured soil structure properties were not altered by fire, therefore, differences in infiltration rate between unburned and burned treatments were attributable to variations in the amount of cover. The terminal infiltration rate of cut and burned juniper sites (162 mm hour-1) was not changed significantly after the fire because the associated good soil structure properties allowed rapid infiltration even after cover was removed. Good soil structure properties were also present on the oak sites, but the infiltration rate significantly decreased as a result of the temporary hydrophobic nature of the soil on this site after burning. Prior to burning, interrill erosion was much lower under the tree sites (oak = 2 kg ha-1; juniper = 34 kg ha-1) than on bunchgrass (300 kg ha-1) or shortgrass (1,299 kg ha-1) sites. After burning, interrill erosion significantly increased for all vegetation types (shortgrass = 5,766 kg ha-1; bunchgrass = 4,463 kg ha-1; oak = 4,500 kg ha-1; juniper = 1,926 kg ha-1). Total organic cover (r = -.74) and bulk density at 0-30 mm (r = .46) were most strongly correlated with interrill erosion.

Infiltration and interrill erosion responses to selected livestock grazing strategies, Edwards Plateau, Texas.

Understanding the temporal response of infiltration rate and interrill erosion to selected livestock grazing strategies is necessary for the continued soil and water conservation of rangeland. Infiltration rate and interrill erosion were sampled bimonthly from 1978-1984 on pastures grazed continuously (MCG) and moderately stocked (8.1 ha AU-1); continuously (HCG) and heavily stocked (4.6 ha AU-1); high-intensity, low-frequency (HILF) and moderately stocked (8-1; 17:119 day, stocked at 8.1 ha AU-1); short duration (SDG) and heavily stocked (14-1; 4:50 day, stocked at 4.6 ha AU-1). The MCG and HILF pastures were able to recover from droughts and maintain initial infiltration rates and interrill erosion. In contrast, infiltration rates decreased and interrill erosion increased on HCG and heavily stocked SDG pastures. The trend of infiltration rate and interrill erosion deterioration in the heavily stocked SDG and HCG pastures was not gradual; rather, it followed a stair-step pattern typified by decreasing condition during drought and an inability to recover to pre-drought level during periods of above-normal precipitation. The heavy stocking rate and climate rather than grazing strategy were the primary factors influencing the hydrologic responses. Infiltration rates were seasonally cyclic in the SDG and HCG pastures, but no significant seasonal trend could be identified in the MCG pasture. This was attributed to greater midgrass cover and litter accumulation in the MCG pasture which provided cover stability compared to less litter accumulation and a greater dominance of seasonal shortgrasses and forbs in the SDG and HCG pastures. Total organic cover was the most important factor determining infiltration rate. The midgrass bunch growth form and litter accumulation were the most important factors influencing interrill erosion. Both factors increased microrelief, and obstructed sediment transport and interrill erosion.

Vegetation and water yield dynamics in an Edwards Plateau watershed

Woody cover, when expressed at the scale of the 207 km Cusenbary Draw basin, remained unchanged (~23%) from 1955 to 1990. When expressed at the scale of range sites, woody cover declined on sites with relatively high production potential and increased on sites with relatively low production potential. Change in woody cover distribution at sub-range site scales, increased low and high woody covers and decreased intermediate woody cover, would be expected to lead to increased water yield at the basin scale because there was an apparent threshold woody cover (~20%) above which simulated evapotranspiration (ET) changed little with increasing woody cover. This potential increase, however, was more than offset by the decreased water yield due to increased ET loss associated with compositional changes of woody vegetation from oak to juniper. A set of woody cover-ET regression curves was developed for different range sites based on simulation studies using the SPUR-91 hydrologic model. Based on these woody cover-ET regression curves and GIS analysis, no brush management would result in a 35% decrease in water yield, while a hypothetical brush management cost-share program would increase water yield by 43% over the 1990 level. Benefits in water yield and forage production from brush management differ in different range sites. A brush management cost-share program that preferentially allocated brush management to sites with deep soil and the highest forage production potential increased water yield by 50%, compared to a 100% increase if brush management were preferentially allocated on sites with shallow soil and highest water yield potential. These model results illustrate that the spatial scale of assessment and spatial distribution of brush management among range sites should be important concerns associated with developing and evaluating brush management policies.

Some Vegetation Responses to Selected Livestock Grazing Strategies, Edwards Plateau, Texas

Understanding the temporal response of vegetation to selected livestock grazing strategies is necessary for the continued maintenance or increased productivity of rangelands. Vegetation cover and above-ground biomass were sampled bimonthly from 1978-1984 on pastures grazed continuously (MCG) and moderately stocked (8.1 ha AU-1); continuously (HCG) and heavily stocked (4.6 ha AU-l); high-intensity, low-frequency (HILF) and moderately stocked (8-1; 17:119 day stocked at 8.1 ha AU-); short-duration grazing (SDG) and heavily stocked (14-1; 4:50 day, stocked at 4.6 ha AU-1); and livestock exclusion (LEX). Prior grazing history, vegetation cover, soils, and slope were similar among pastures. Midgrass cover was eliminated in the HCG pasture, and declined in the heavily stocked SDG pasture. Midgrass cover was maintained under the moderately stocked HILF grazing strategy and increased under MCG or LEX. During 1984, sideoats grama (Bouteloua curtipendula (Michx.) Torr.) basal diameter in the MCG and LEX pastures was significantly greater than in the SDG pasture. By the end of the study, total organic cover and total aboveground biomass in the MCG or LEX pastures were significantly greater than in the SDG and HCG pastures. The heavy grazing intensity used in this study, regardless of the grazing strategy, does not appear suited for long-term maintenance of midgrass species.

Long-Term Water Balance in a Semiarid Shrubland

Abstract Baseline information on the water balance is essential for adequately understanding ecohydrological relationships on rangelands. Unfortunately, such information is not always available, because insufficient data have been collected and/or the data do not represent relevant temporal or spatial scales. In particular, for many rangelands long-term records of runoff at the small catchment or larger scales are relatively rare. In this study, we used catchment-scale data, collected over nearly a decade, to estimate the long-term water balance for mesquite–juniper rangelands in the Rolling Plains of Texas. The data include precipitation, surface runoff, soil water, and vegetation cover; these were measured on 9 microcatchments, each about 1.4 ha in size. Soil water was determined by neutron-probe measurements to a depth of 120 cm. Surface runoff was, surprisingly, a very small component (< 1%) of the water budget and occurred only during extraordinary precipitation events. Soil-water recharge resulted mainly from winter precipitation. Evapotranspiration, which took place during the entire year, ranged from less than 1 mm·day−1 during the winter to almost 8 mm·day−1 during the summer. This study is important because it clearly documents how water is allocated on these rangelands at spatial and temporal scales that are relevant to management. The results from this study, in conjunction with other work in the Rolling Plains, suggests that there is little potential for increasing water yield via brush control in these landscapes.

Differences in soil water use by annual broomweed and grasses

The use of water in the upper 1 m of the soil profile by 3 common herbaceous species of the southern Great Plains was examined by labeling soil water with 2H2O and H2(18)O. Uptake of labeled water from the 15 cm depth was approximately equal for all species. However, water uptake from the 75 cm depth was significantly greater by annual broomweed [Amphiachyris dracunculoides (DC.) Nutt] than either sideoats grama [Bouteloua curtipendula (Michx.) Torr] or curlymesquite [Hilaria belangeri (Steud.) Nash]. Although both grasses had greater root length density than annual broomweed at the 75 cm depth, annual broomweeds rate of water extraction from the 75 cm depth was nearly twice that of sideoats grama or curlymesquite. Greater access to and more rapid utilization of deeper soil water by annual broomweed relative to the grass species may partially explain annual broomweeds success at invading grasslands and reducing grass production in semi-arid rangelands.

Observations on vegetation responses to improved grazing systems in Somalia.

Vegetation community response is an important factor determining the potential for improvement of rangeland dormant season forage availability through implementation of grazing systems. Heavy continuous grazing (HCG) (5 ha AU-1) of communal rangelands in coastal southern Somalia has resulted in a herbaceous vegetation community dominated by short-lived annual forbs of low palatability that provide little forage during the dormant season. Changes in the plant community resulting from implementation of 2 grazing systems were compared: complete livestock deferral (LEX) and moderately stocked short-duration grazing (MSDG) (10-1; 3:30 day, stocked at 10 ha AU-1). After 2 years, the LEX pasture was dominated by palatable forbs (primarily Commelina forskalaei and Ipomoea garekeana), which formed a vine mat that overtopped other herbaceous species. These vines died and decomposed soon after the rainy season ended and thus were not a useful source of dry season forage. The periodic grazing in the MSDG opened the vine mat and enabled grasses to establish, thus grass cover became significantly greater on the MSDG pasture compared to either the LEX or HCG pastures and provided forage for livestock in the dry season.