Walter D. Willms

Rotational Grazing on Rangelands: Reconciliation of Perception and Experimental Evidence

Abstract In spite of overwhelming experimental evidence to the contrary, rotational grazing continues to be promoted and implemented as the only viable grazing strategy. The goals of this synthesis are to 1) reevaluate the complexity, underlying assumptions, and ecological processes of grazed ecosystems, 2) summarize plant and animal production responses to rotational and continuous grazing, 3) characterize the prevailing perceptions influencing the assessment of rotational and continuous grazing, and 4) attempt to direct the profession toward a reconciliation of perceptions advocating support for rotational grazing systems with that of the experimental evidence. The ecological relationships of grazing systems have been reasonably well resolved, at the scales investigated, and a continuation of costly grazing experiments adhering to conventional research protocols will yield little additional information. Plant production was equal or greater in continuous compared to rotational grazing in 87% (20 of 23) of the experiments. Similarly, animal production per head and per area were equal or greater in continuous compared to rotational grazing in 92% (35 of 38) and 84% (27 of 32) of the experiments, respectively. These experimental data demonstrate that a set of potentially effective grazing strategies exist, none of which have unique properties that set one apart from the other in terms of ecological effectiveness. The performance of rangeland grazing strategies are similarly constrained by several ecological variables establishing that differences among them are dependent on the effectiveness of management models, rather than the occurrence of unique ecological phenomena. Continued advocacy for rotational grazing as a superior strategy of grazing on rangelands is founded on perception and anecdotal interpretations, rather than an objective assessment of the vast experimental evidence. We recommend that these evidence-based conclusions be explicitly incorporated into management and policy decisions addressing this predominant land use on rangelands.

EFFECT OF FORTY-FOUR YEARS OF GRAZING ON FESCUE GRASSLAND SOILS

A grazing study was initiated in the foothills of southwestern Alberta on the rough fescue grasslands (Festuca campestris (Rydb.) in 1949 comparing various grazing intensities. In 1992, soil samples were obtained from the Ah horizon of paddocks grazed at 1.2 (light), 2.4 (heavy), and 4.8 (very heavy) animal unit month ha-1 and from an ungrazed exclosure (control). The thickness of the Ah horizon of the control averaged 22 cm while that of the lightly, heavily and very heavily grazed paddocks averaged 18, 12, and 8 cm, respectively. Soil color changed from 10YR 2/1 (black) to 10YR 4/3 (dark brown to brown) in response to very heavy grazing. Grazing pressures decreased the mean-weight diameter of water-stable aggregates, total C and P, monosaccharide content and the galactose + mannose/xylose + arabinose ratio, while it increased bulk density, pH-CaCl2, and total N. The loss of P must be viewed with concern. Treatment effects on most soil parameters were most pronounced at the two heavier grazing pressures. Particularly, the heavy grazing pressure jeopardized the sustainability of the ecosystem by reducing fertility and water-holding capacity.

HERBAGE PRODUCTION FOLLOWING LITTER REMOVAL ON ALBERTA NATIVE GRASSLANDS

Studies were conducted to determine the effects on herbage yield of removing mulch and standing dead plant litter during dormancy for up to 3 or more consecutive years. This inlormation is required to obtain a better understanding of the hnpiications of dormant season grazing on forage production. In 2 studies, mulch and standing litter were harvested at 3 or more annual frequencies from 2 X 2 m plots. One study was repeated in both the Fescue Prairie and Mixed Prairie communities and plant response was measured annually as the yield of herbage produced from treated and control plots. The second study was conducted in the Fescue Prairie on 3 sites and designed as a 3 X 3 Latin sqmre. The treatments consisted of removing mulch and standing litter, removing and replacing this material, and a control. Estimates were made of the yield, species composition, and morphological characteristics of the grasss. A third study was made, in the Fescue Prairie, by defoliating individual rough fescue (Festuca scabrella Torr. var. m&r Vasey) plants a single time, at 5 and 15 cm above ground, and comparing them with a control. Herbrge yields decreased as the annual frequency of mulch and litter harvests increased in the Mixed Prairie but not in the Fescue Prairie. In the Mixed Prahie, yields deetied to 43% of the control after 3 years of treatment. Removing mulch and standing litter from rough fescue plants resulted in shorter but a greater number of tillers than in the control. The results were similar after 1 or 3 years of treatment. Native grasslands in southern Alberta provide relatively good quality forage for fall and winter grazing (Johnston and Bezeau 1962). Although photosynthetic tissue is not removed when plants are harvested during dormancy, their subsequent growth is affected. In the first year after removing mulch and standing dead plant litter of rough fescue (Fesrucu h&ii (Vasey) Piper (Sinton 1980) or bluebunch wheatgrass (Agropyron spicutum (Pursh) Scribn. + Smith) (Sauer 1978, Willms et al. 1980), forage yields decreased while tiller densities increased. However, after 2 consecutive years of removing standing dead plant litter from rough fescue, yields were marginally greater and tiller densities were substantially increased (Sinton 1980). Plant litter helps conserve soil moisture by reducing soil temperature and evaporation (Weaver and Rowland 1952, Hopkins 1954). However, reduced soil temperatures in spring will delay plant growth (Weaver and Rowland 1952) and may result in reduced herbage yield (Dyksterhuis and Schmutz 1947, Penfound 1964) and in a reduced diversity in the plant community (Weaver and Rowland 1952). Native grasslands in southern Alberta have evolved with buffalo grazing on the Mixed Prairie in summer and on the Fescue Prairie in winter (Johnston and MacDonald 1967). This would suggest that grasses of the Fescue Prairie should be tolerant of winter grazing. However, the effects of removing mulch and harvesting standing dead plant litter on herbage production have not been examined very extensively. Clarke et al. (1947) reported yields from plots harvested after plant senescence, over a 9-year period, but assumed no effect of removing standing dead plant material Authors are range ecologists, Agriculture Canada, Research Station, Lethbridge, Alberta TIJ 481; and professor of range management, Dept. of Plant Science, the University of Alberta, Edmonton T6G 2P5. The authors are grateful for the statistical advice given by B. Schaalje. Technicians B. W. Kesler and R.G. Gschaid assisted with the field work. The studv at Kinselia was partly supported by a grant from the Agricultural Research Colincil of Alberta, Farming for the Future, to Dr. A.W. Bailey. Manuscript accepted 17 April 1986. 636 and had no control treatment. These studies were initiated to determine the effects of removing mulch and standing litter during the dormant season on forage production in the Fescue Prairie and Mixed Prairie grasslands. Materials and Methods Site Description Two study areas were located on the Fescue Prairie and one on the Mixed Prairie of southern Alberta. One Fescue Prairie site was at the University of Alberta Ranch, Kinsella (150 km SE of Edmonton), and the other at the Agriculture Canada Range Research Substation, Stavely (90 km NW of Lethbridge). The soils, climate, and vegetation of the Kinsella area have been described by Bailey and Anderson (1978) while the Stavely area was described by Willms et al. (1985). Major differences in the plant communities at 2 Fescue Prairie sites were the associated species and the presence or absence of rhizomes on rough fescue. At Kinsella, the rough fescue is rhizomatous and is a species (F. hallii) distinct from the tufted rough fescue (F. cumpestris Rydb.; or F. scabrellu Torr. var. major Vasey) found at Stavely (Looman and Best 1979, Pavlick and Looman 1984). The rhizomatous form is associated with western porcupine grass (Stipu spurtea Trin. var. curtiseta Hitchc.) while the tufted form is associated with Parry oat grass (Danthonia parryi Scribn.). The soils at both areas were orthic black chernozemic (Argic Cryoboroll). Precipitation averaged 432 mm at Kinsella and 614 mm at Stavely (Table 1). The Table 1. Precipitation (mm) from April to August during the period of studiea at 3 locations in Alberta.

Effects of Stocking Rate on a Rough Fescue Grassland Vegetation

their construction, to a stable range condition, took from 14 years in the lightly grazed field to more than the length of the study in the A study was conducted to examine the effects of 4 stocking rates on the vegetation in a Rough Fescue Grassland vegetation in southwestern Alberta. Stocking at a light rate (1.2 AUM/ha) for 32 very heavily grazed field. The duration required for recovery was years did not affect range condition. However, a modest increase in stocking rate (1.6 AUM/ha) led to a marked decline in range related to the original range condition of the exclosures. condition. This was associated with a change in the composition of rough fescue from 38 to 21% of basal area. Rough fescue (FestzCQ ScQbreflQ) was nearly eliminated with a stocking rate of 2.4 AUM/ha. Rough fescue was repiaced by Parry oat grass (DQnthoni~ parryi) which increased from 24% at 1.2 AUM/ha to 48% at 2.4 AUM/ha. However, stocking at 4.8 AUM/ha resulted in severe deterioration of the grassland. This required annual adjustment of the stocking rate to avoid animal losses. The recommended stocking rate for good condition range in the area is 1.6 AUM/ha. Recovery of the vegetation within the exclosures, from the time of setting stocking rates. The stocking rate, expressed in this paper as animal unit months per hectare (AUM/ ha), defines the relative quantity of forage that will be harvested during the grazing period in a given year. The effect of increasing the stocking rate is to increase the proportion of forage utilized, which may lead to changes in species composition of grasslands (Ellison 1960). The species that increase because of an increased stocking rate are often sub-dominants in the plant community and are potentially less productive than the dominant species (Looman 1969). The purpose of this study was to examine the changes that occur in species composition of a Rough Fescue Grassland as a result of stocking with cattle at different rates. The work described is part of a final assessment of the study which was reported earlier (Johnston 1961, Peake and Johnston 1965).

Vegetation and soil responses to short-duration grazing on fescue grasslands.

The effects of animal impact on soil chemical and physical properties as well as range condition were measured over a 5-year period to test tbc bypotbesls that animal impact can improve the nutrient and water status of the soil and promote grassland succeadon. A seventeen-pasture short-duration graxing system was establlsbed in 1981 on 972 ba. The pastures were stocked on average wltb 278 cows wltb calves from 1982 to 1986, wbicb was about twice to triple tbe recommended rate of 0.8 AUM/ba. Increased grazing pressure reduced range condition as reflected by a loss of de&able species such as rougb fescue (Festuccr scubrelkh Torr.). Soil molsture was always bigber in soils of ungrazed exclosures. Soli bulk density increased wblle hydraulic conductivity decreased with grazing. Litter was not signlflcantiy incorporated into the soll with hoof action. Cbltln-N, as a measure of fungrl biomass, decreased significantly under tbe increased grazing pressure. Tbe bypotbesis that animal impact would improve range condition was rejected since impact, in tbe manner applled during tbe study, resulted ln retrogression of the grasslands.

Influence of litter on herbage production in the mixed prairie.

Lifter (dead plant material) increases production in xeric environments but the nature of this effect is uncertain. The purpose of this study was to determine the relationship between liner quantity and herbage production over a 4-year period as well as to determine the effect of repeated removal of lifter on production. The study was made in a Stipa-Bouteloua-Agropyron faciation of the Mixed Prairie association, near Lethbridge, Alberta, Canada. Litter quantity was altered by mechanical removal before spring growth and the residue separated into coarse and fine components. In Experiment 1, the effect of lifter on herbage production was tested by removing lifter at 0, medium, and high levels that resulted in an average residue of coarse liner of 1,171, 787, and 377 kg ha-1. Coarse litter was related to an increase in herbage production (P < 0.05) in 3 of the 4 years studied. The effects of lifter were related to the growing conditions of each year. The linear regression coefficients describing the response (herbage production related to litter) ranged from 0.114 to 0.802 with the smallest effect under either very dry or very wet conditions. In Experiment 2, lifter was removed at high levels in either 0, 1, 2, or 3 successive years. These treatments resulted in an average residue of coarse litter of 1,300, 164, 149, and 188 kg ha-1. Herbage production was not affected by removing litter for more than 1 year but plant height, tiller weight, and herbage yield of some plant species were.

Effect of native prairie, crested wheatgrass (Agropyron cristatum (l.) Gaertn.) and Russian wildrye (Elymus junceus Fisch.) on soil chemical properties.

Crested wheatgrass and Russian wildrye are used extensively as seeded pastures in the prairie region of western Canada. Their long-term impact on soil quality was studied at 4 sites, each including plant communities of native mixed prairie rangeland and 17to 27-year-old monocultures of crested wheatgrass and Russian wildrye, in southern Alberta, Canada. Root mass and soil chemical properties were determined on the soil samples collected. Native rangeland had about 7.6 times more root mass than the seeded species from the 0to 7.5-cm depth and about equivalent mass from the 7.5to 40-cm depth. For the seeded species, root mass was significantly less between rows than within rows. Soils in the native rangeland community had significantly greater soil organic matter and lower NO3-N, chemical index, urease activity, and available phosphorus than those in the seeded pastures. Altering the plant community from native mixed prairie to either a sequence of cropping followed by an introduced grass monoculture, or directly to an introduced grass monoculture, resulted in decreased root mass and organic matter, and monosaccharide content of dry aggregates. The seeded grasses could neither return nor maintain the chemical quality of the soils in relation to that of the native rangeland.

Quantifying drylands' drought resistance and recovery: the importance of drought intensity, dominant life history and grazing regime

Projected global change will increase the level of land-use and environmental stressors such as drought and grazing, particularly in drylands. Still, combined effects of drought and grazing on plant production are poorly understood, thus hampering adequate projections and development of mitigation strategies. We used a large, cross-continental database consisting of 174 long-term datasets from >30 dryland regions to quantify ecosystem responses to drought and grazing with the ultimate goal to increase functional understanding in these responses. Two key aspects of ecosystem stability, resistance to and recovery after a drought, were evaluated based on standardized and normalized aboveground net primary production (ANPP) data. Drought intensity was quantified using the standardized precipitation index. We tested effects of drought intensity, grazing regime (grazed, ungrazed), biome (grassland, shrubland, savanna) or dominant life history (annual, perennial) of the herbaceous layer to assess the relative importance of these factors for ecosystem stability, and to identify predictable relationships between drought intensity and ecosystem resistance and recovery. We found that both components of ecosystem stability were better explained by dominant herbaceous life history than by biome. Increasing drought intensity (quasi-) linearly reduced ecosystem resistance. Even though annual and perennial systems showed the same response rate to increasing drought intensity, they differed in their general magnitude of resistance, with annual systems being ca. 27% less resistant. In contrast, systems with an herbaceous layer dominated by annuals had substantially higher postdrought recovery, particularly when grazed. Combined effects of drought and grazing were not merely additive but modulated by dominant life history of the herbaceous layer. To the best of our knowledge, our study established the first predictive, cross-continental model between drought intensity and drought-related relative losses in ANPP, and suggests that systems with an herbaceous layer dominated by annuals are more prone to ecosystem degradation under future global change regimes.

Effect of grazing and abandoned cultivation on a Stipa-Bouteloua community.

A Stipa-bouteloua community, cultivated in the autumn of 1928 and abandoned in the spring of 1932, reverted to a community dominated by needle-and-thread (Stipa comata Trin. and Rupr.). An exclosure to prevent grazing was constructed in 1978 to include equal portions of previously cultivated and adjacent native range, while the remainder of the area continued to be subjected to moderate to heavy grazing pressure. This permitted a study to determine the effects of the brief period of cultivation on forage production, species recovery, and soil physical and chemical characteristics compared to those of native prairie. After 14 years of protection from grazing, needle-and-thread accounted for 79% of foliar cover of the abandoned cultivation and 18% of the untreated range while blue grama [Bouteloua gracilis (HBK.) Lag. ex Steud] occupied 1 and 51% on the same treatments, respectively. After 60 years, the soil on the abandoned cultivated area showed reduced carbon, total nitrogen, available phosphorus, and hydraulic conductivity but increased N03-N. Grazing reduced hydraulic conductivity, NH4-N, available mineralizable nitrogen (chemical index), available phosphorus, and total carbohydrates but increased carbon, total nitrogen, and N03-N. Cultivation and grazing resulted in reduced root mass. To facilitate a rapid transition from blue gramb to needle-and-thread stable communities, input of energy, such as cultivation, may well be required.

Effect of grazing and cultivation on some chemical properties of soils in the mixed prairie.

Components of the organic matter were studied in soil under 3 Mixed Prairietypes: grasslnd dominated by needle-and-thread/blue grama (Stipa comata Trin. and Rupr./Bouteloua gracilis (HBK.) Lag. ex Steud.) in good range condition; grassland significantly modified by grazing, dominated by blue grama and in poor range condition; and grassland, dominated by needle-and-thread/blue grama in good range condition, but converted to cropland and under continuous wheat for 4 years. The soils were sampled on 13 April 1988. Concentrations of total organic carbon in the upper 2 cm were 1.39, 2.70, and 1.87%, respectively. The higher organic carbon under blue grama was caused by an active, ramified, fine rootmass which gave rise to most of the monosaccharides being of microbial origin. The monosaccharides in the lower Ap horizon in the cropland were generally of plant origin from incorporated