| ||Native communities | Production constraints | Production Potential | Livestock Production | Conclusion | References
Key words: Biomass, above-ground, below-ground, water-use efficiency, reseeding, soil fertility, grazing efficiency
The mixed prairie represents the driest portion of Northern Great Plains in Canada. It is found in the southern prairies from the base of the foothills in the west to near the Saskatchewan-Manitoba border in the east, and extends north to about the 52° latitude. The soils are representative of the Chernozernic Order with Brown Chernozerns in the south-central region and Dark Brown Chernozems in a broad belt around their periphery and forming an interface with the Black Chernozemic soils in the mesic region. Major plant species found throughout the region include needle-and-thread (Stipa comata Trin. + Rupr.), porcupine grass (S. spartea (Trin.), blue grama [Bouteloua gracilis (H.B.K.) Lag. ex Steud.], western wheatgrass (Agropyron smithii Rydb.), northern wheatgrass [A. dasystachyum (Hook.) Scribn.], June grass [Koeleria cristata (L.) Pers], sand reed grass [Calamovilfa longifolia (Hook) Scribn.], thread-leaved sedge (Carex filifolia Nutt.), and pasture sage (Artemisia frigida Willd.). These and other species are found in various combinations, depending on site characteristics, and demonstrate considerable variability in production potential.
Native mixed prairie occupies about 6.5 M ha in Alberta and Saskatchewan and supports about 5.3 M animal-unit-month (AUM) or about 15% of all beef cattle (primarily beef cows) present on the Canadian prairies. In Alberta, livestock grazing on government owned mixed prairie range on 1. 1 M ha (Turnbull et al. 1993) generates about 100 M at the primary level and, with a multiplier of 2.7, this generates economic activity valued at about $270 M (Ruud and Wehrhahn 1990). The native prairie is often viewed by the livestock industry as a source of cheap forage since no cultivation is required to establish or maintain it. Yet, the sustainable character of the prairie, without requiring expensive inputs, is the key to maintaining the economic viability of the livestock industry in the region.
The grasslands developed with the northward retreat of the ice following the last glaciation about 10,000 years before the present (BP). Warm and dry climate about 6-7,000 BP caused the forest to retreat north and the grasslands to expand to about 54° latitude (Ritchie 1976). Forests again advanced south about 3,000 BP in response to a cooling trend, however, the advance to their natural limit appears to have been restricted by fires and buffalo.The mixed prairie developed under the influence of grazing and periodic fires and was sustained by limited inputs of water and nitrogen. The species, and communities they form, are adapted to recycle and conserve essential resources with a high root to shoot ratio and an ability to survive drought. The inherent character of the mixed prairie lies in those attributes that enable sustainability and it is these attributes that must be understood in order to manage the prairie for the multiple resources it supports. Therefore, the purpose of this paper is to identify those characteristics and review our current understanding of them with emphasis on their relationship to livestock impacts.Net primary production is one of the most important variables describing the character of the plant community. It defines the net assimilation of carbon by photosynthesis and is usually estimated by sampling plant biomass at sufficient intervals throughout the growing season to account for losses of senesced vegetation. Forage production is equivalent to net primary production above-ground but most estimates have been made with a single harvest of biomass at the end of the growing season. Therefore, estimates of forage
production are less than net primary production.
This paper consists of 4 major sections: (1) an introduction to native communities; (2) an examination of production constraints; (3) potential; and (4) a discussion of livestock effects on production. The paper will focus on primary production and major factors that have a demonstrated effect on it. Constraints, or factors that limit production, include soil moisture and nitrogen, temperature, and plant genetics. The potential is the latent opportunity for increasing production with human intervention. The effects and implications of increasing soil fertility and renovating prairie, as well as the opportunities for increasing production by breeding "superior" cultivars, are discussed. The livestock effects are examined relative to their impact, grazing efficiency, and the use of grazing systems.
Distribution. The vegetation of the mixed prairie is defined by species that are best adapted to the local climate, soils, and previous disturbances. Although many of the dominant species, including western wheatgrass, blue grama, needle-and-thread grass, and June grass, are found throughout the Great Plains region, their representation in any community is dependent on their competitive ability as modified by site conditions. Major plant communities recognized by Coupland (1961), defined by the dominant species, are: Stipa (spartea)-Agropyron (dasystachyum), Stipa (comata)-Bouteloua-Agropyron, Stipa (comata)-Bouteloua, Bouteloua-Agropyron (dasystachyum), and Agropyron (dasystachyum)-Koeleria. The agronomic potential of the sites diminishes, in order, from the Stipa-Agropyron to the Bouteloua-Agropyron communities due to increasing aridity, reduced permeability, or increased salinity of the soil. Most of the Agropyron-koeleria community has been converted to cultivated crops.
The Stipa-Agropyron community is found throughout most of the dark-brown soil zone and a large portion of the more moist areas of the brown soil zone (Coupland 1961). It is a highly productive community and, although it represents about 40% of the mixed prairie, most has been cultivated for cereal production. The Stipa-Bouteloua-Agropyron community is the most representative of the drier region and, while wheatgrasses are a major component, they decrease during drought and ate reduced on sandy soils which result in the formation of the Stipa-Bouteloua community. However, wheatgrasses dominate on solonetzic soils and may form nearly pure stands where the topsoil (Ah horizon) has been removed by wind erosion.
Productivity. The dominant species represent the: production potential of each community and are key in their management for sustainability. Considerable differences exist among species in their production potential, resource allocation, and season of growth which, in turn, affects the production characteristics of the community. Relative aboveground production ratios between blue grama, needle- and-thread grass, and western wheatgrass are about 1:3:4, respectively (Clarke et al., 1947). However, carbon allocation between root and shoot differs so that aboveground production alone does not accurately reflect total carbon assimilation. For example, crested wheatgrass [Agropyron cristatum (L.) Gaertn.] produced about 400 kg ha-1 more biomass above-ground than short-grass native range but about 100 kg ha-1 less of total plant yield (Redente et al. 1989). Net primary production on ungrazed Agropyron-koeleria, Agropyron (smithii)Stipa (viridula), and Stipa-Bouteloua-Agropyron sites was about 7,000, 7,820, and 12,600 kg ha- 1, respectively, with about 80% below ground (Sims and Coupland 1979). Similarly, Lauenroth and Whitman (1977) estimated primary production on a Stipa-Bouteloua-Agropyron site at 2,400 to 3,040 kg ha-1 above-ground and from 9,310 to 12,210 kg ha-1 below-ground. Peak standing crop on various mixed prairie sites in Montana and North Dakota ranged from 950 to 1,680 kg ha-1 with variation attributed to year and specific community (Lauenroth and Whitman 1977).
Environmental. Biomass production on the mixed prairie is mostly limited by available water and soil nutrients while temperature has the greatest influence on the time and rate of growth, effectiveness of precipitation and water-use efficiency (WUE). Since the environmental effects and their interactions are species specific, their impact on the plant community will depend on its composition and the duration of the effect. While an altered environment may have a short-term effect reflected in annual variation, sustained change will affect the species composition and produce long lasting effects.
Available water during the growing season accounts for the greatest amount of annual variation in forage production on the Stipa-Bouteloua community in southeast Alberta (Smoliak 1986). Over a 50-year period, annual forage production estimated from standing crop after the growing season averaged 388 kg ha-1 and ranged from 96 to 925 kg ha-1. About 55% of the variation was explained by the April through July precipitation and expressed as a linear regression equation: Y=72.2+1.93X (r2 =0.55) where Y is production (kg ha-1) and X is precipitation (mm). Also correlated with production, but likely confounded with evapotranspiration, were the April through July temperature (r2 =0.38) and sunlight hours (r2=0.12), each having a negative effect. Lower yields than the previous minimum were measured in 1988 when 68 kg ha-1 were produced with 44 mm precipitation from April through July on a Stipa-Bouteloua community. In this case, the predicted value overestimated production by 90 kg ha-1 (Willms unpublished data). Although precipitation during the growing season has a significant effect on production, fall soil moisture is more important (Johnston et al. 1969) for species that begin growth early in spring, such as crested wheatgrass.
The timing of precipitation can be as important as the amount in relation to water demand for seedling establishment (Ries and Hofmann 1987) and forage production. Water must be available at the critical phases in seedling establishment which include imbibition, germination to radical emergence, seminal and adventitious root initiation and elongation. The degree of drought stress may be of lesser importance compared to the timing of stress relative to the phenological seedling growth stage. Timing of precipitation also significantly influences the advantage that species gain within the community and is discussed elsewhere in this article.
The limiting effect of soil fertility is demonstrated by a substantial increase in forage production with the addition of nitrogen from manure or chemical fertilizer. In a Stipa-Bouteloua community, forage production over an 8-year period was approximately doubled when manure was applied at about 65 t ha-1 or when fertilizer was applied at 340 kg N ha-1 (Smoliak 1965). In another experiment on a Bouteloua-Agropyron community, forage production increased linearly from 810 to 1938 and 2960 kg ha-1 with the addition of 0, 45 and 90 kg N ha-1 respectively (Lorenz and Rogler 1972). Increasing soil fertility affects production by increasing WUE (White and Brown 1972) and the proportion of more productive species (Smoliak 1965).
Available soil water limits forage yield potential but temperature controls the phenological development of native grasses (Frank and Hofmann 1989). Regressions of growth stage (Haun scale) on Growing Degree Days (GDD) were significant for the five species studied: western wheatgrass, blue grama, needle-and thread, green needle grass (Stipa viridula Trin.), and June grass [Koeleria pyramidata (Lam.) Beauv] with r2 values ranging from 0.62 for June grass to 0.97 for western wheatgrass. Because forage quality generally declines as phenological stage advances, high spring temperature may reduce livestock production from cool-season grasslands.
Communities dominated by cool-season (C3) plants produce most herbage in early summer while communities dominated by warm-season (C4) plants produce most herbage in mid-summer (Lauenroth and Whitman 1977). This corresponds to their chlorophyll content and their production efficiencies. The chlorophyll content in porcupine grass, a coolseason grass, declines from a high in spring while in prairie dropseed (Sporobolus heterolepsis A. Gray), a warm-season plant, it peaks in mid-summer before declining (Redmann 1975). Production efficiencies on a S. viridula and Sporobolus site followed a comparable pattern, varying from 0.36 to 0.09%, respectively, in spring and 0.18 and 0.62%, respectively, in summer. Similarly, seeds of western wheatgrass, a cool-season species, germinated more readily and produced about 2 times more biomass when grown in a cool environment while blue grama, a warm-season species, germinated more readily and produced almost 3 times more biomass in a warm environment (Kemp and Williams 1980). The offsetting growth between the species may be a strategy for reducing interspecific competition (Williams 1974) and increasing community production (Lorenz and Rogler 1972) but the most visible effect is on the community structure.
The high temperature requirements for optimal growth in warm-season grasses may limit their establishment in spring when precipitation events are most probable. Wilson (1981) reported the soil temperature optima for rate of root elongation, total weight of roots, and leaf growth of blue grama. seedlings were 31.8, 30.6, and 33.6°C, respectively, while adventitious root initiation occurred at 19.3ºC. Wilson (1981) suggests that seeding should be delayed until soil temperature is 15°C to encourage root elongation and reduce the initiation of adventitious roots. At this time the probability of precipitation on the semi-arid Canadian Prairie is low and adventitious root initiation is at risk.
The interaction between temperature, available water, and cool- or warm-season plants has a great influence on the species composition of native grasslands and has considerable implications in their management. Cool-season grasses produced more forage biomass than warm-season grasses in Montana (White 1986) where Russian wildrye (Psathyrostacftys juncea Fisch.) yielded 2.52 t ha-1 vs 0.84 t ha-1 for little bluestem [Schizachyrium scoparium (Michx.) Nees] and forage yield was positively correlated with evapotranspiration (r=0. 89 and 0.80 for Russian wildrye and little bluestem, respectively). Calculations based on White's (1986) data indicate that the WUE of the cool-season grasses was higher than the warm-season grass, probably due to lower atmospheric water
stress during the period of most active growth.
Long-term changes in climate will have a more lasting effect on production by allowing a shift in the species composition and thereby altering the community potential. During periods of drought, blue grama increased in basal area and became dominant at the expense of needle-and-thread grass and the wheatgrass species (Weaver and Albertson 1956). The effect on the community is reduced productivity and delayed production. The reverse trend was observed when the climate became cooler and wetter (Weaver and Albertson 1956) or the soil microclimate was modified with a straw mulch (Smoliak 1965).
Climate affects below-ground production and biomass accumulation through its effect on species composition and the rate of root decomposition (Sims and Coupland 1979). Although root production is greater in warm vs cool climates, root biomass is often greater in the latter due to reduced turnover which is a function of temperature (Redmann 1975, Sims and Coupland 1979). Root turnover on the mixed prairie was estimated to be 18% (Sims and Coupland 1979) while 62% of the roots were functional in a blue grama community (Singh and Coleman 1974). The effect of climate and species results in a root to shoot ratio that is about 6:1 on the mixed prairie and about 2:1 on the warmer and mesic tallgrass prairie.
Genetic. Genetic constraints to forage production have been discussed earlier with reference to cool- and warm-season species and their interaction with the environment. Within the context of a community, the mix of species tends to maximize resource extraction and conservation; attributes that are reduced in monocultures. However, considerable genotypic variation exists within a native species population presumably as a mechanism for ecological stability.
Selection for genotypes among native species occurs naturally in response to the environment and grazing pressures but little information is available on their inherent potential. Of the species, the wheatgrasses are, perhaps, best understood. Western wheatgrass had greater variability for water status variables, including WUE, than crested or intermediate wheatgrasses [A. intermedium (Host) Beauv.] (Frank and Karn 1988). This suggests the possibility of selecting for more drought tolerant genotypes.
Increased partitioning of dry matter into stem rather than leaves, among western wheatgrass genotypes, resulted in a lower leaf area index (LAI) and digestibility but greater WUE and forage production (Frank and Karn 1988). In another test, crested wheatgrass was more WUE (4.1 g kg-1) than western wheatgrass (2.6 g kg-1) which was associated with reduced LAI.
Crested wheatgrass is particularly well suited of the species adapted to the semi-arid mixed prairie environment (Johnson 1986). Although the germination processes in crested wheatgrass seed are arrested by water stress, they will resume when soil moisture is replenished (Johnson 1986) and, in some cases, are enhanced by alternate periods of wetting and drying. The growth of the seminal root after release from water stress is also greater than in Russian wildrye (Johnson 1986). In established plants, leaf rolling and leaf xylem pressure potential are strongly related (Bittman and Simpson 1989). This adaptation strategy minimizes energy loading of leaves and adaxial conductance of water vapour.
Survival of the grasses on the mixed prairie depends on their response to water deficits and recovery. Leaf rolling and dormancy are common among the prairie grasses but photosynthetic activity and growth, in northern wheatgrass, is possible when turgor recovers in the early morning and after irrigation (Maxwell and Redmann 1974). On the other hand, water extraction is greater for blue grama than western wheatgrass (Majerus 1975). Root growth of blue grama ceased at a soil water potential of -8.0 MPa while root growth of western wheatgrass ceased at -3.0 MPa in the 0-5 cm soil layer. The soil water potential at the time of root growth cessation increased
with soil depth.
Considerable amount of effort has been invested in increasing forage production from the mixed prairie. The effort has been directed at increasing soil fertility, replacing the native range with seeded species, and breeding for more productive species. Forage production on the mixed prairie can be increased by overcoming the constraints of water, soil nutrition, or species but an agronomic solution is questionable because it may not be economical or its impact on the environment unacceptable.
Soil Fertility. Applied N is rapidly immobilized and forage production increases dramatically in the N-deficient environment of the mixed prairie. About 200 kg N ha-1 of 540 kg ha-1 applied in an Agropyron-Stipa-Bouteloua community was immobilized in roots, soil organic matter, microbial cells, or lost by gaseous means while about 30 kg N ha-1 was absorbed into grass tops (Power 1972). The remainder was present as inorganic N in the soil with about 10% still present after 6 years. The residual effect of applied N can persist for many years since leaching on the xeric grassland is virtually zero. Forage production after 8 years following a single application of 336 kg N ha-1 on a Stipa-Bouteloua community was about 60% greater than the control and, over the duration of the trial, produced about 6,500 kg ha-1 more forage (Smoliak 1965).
Forage production on a Bouteloua-Agropyron site increased linearly with an application of 45 and 90 kg N ha-1, from 810 kg ha-1 on an unfertilized range to 1,938 and 2,960 kg ha-1, respectively, while additional application improved yields marginally (Lorenz and Rogler 1972). Higher production due to the application of N may be attributed to greater WUE (White and Brown 1972) or an increase in the composition of more productive species (Smoliak 1965, Lorenz and Rogler 1972).
Water-use efficiency was increased up to 2.5 times when 140 kg N ha-1 were applied to a seeded stand of green needle grass (White and Brown 1972) or when 45 kg N ha-1 or more were applied to either a seeded stand of crested wheatgrass or an Agropyron-Stipa-Bouteloua community (Black 1968). Similar results were reported by Johnston et al. (1969) for native prairie sites on brown and dark brown soils, in southern Alberta, with the application of 475 to 705 kg N ha-1.
At least part of the measured increase in WUE from the application of N is related to increased water extraction from a larger root mass (Black 1968, White and Brown 1972). Goetz (1969) reported an increase in root mass and rooting depth on each of four sites when N was applied at rates as low as 37 kg ha-1 on sandy soils and 113 kg ha-1 on clay soils.
Nitrogen fertilizer also affects the species composition over several years which can influence the root mass. Nitrogen applied at rates of up to 180 kg ha-1 increased western wheatgrass but almost eliminated blue grama (Lorenz and Rogler 1972). On a Stipa-Bouteloua cornmunity, 300 kg N ha-1 alone had no effect on blue grama but when combined with a straw mulch it was severely reduced, together with other shallow rooted species, while needle-and-thread and the wheatgrasses were increased (Smoliak 1965). The effect may not necessarily be related to the original rooting depth but to the time of growth, whether cool- or warm-season, which determines the time of resource demand and their competitive relations.
Renovation. Native range has been reseeded to increase forage production and extend the grazing season. The primary species used on the mixed prairie are mostly introduced and include crested wheatgrass, intermediate wheatgrass, Russian wildrye, Altai wildrye: [Leymus angustus (Trin.), Pilger] and meadow brome (Bromus biebersteinii Roem. + Schult.). Although some studies report no difference in production potential between monocultures of introduced and improved native species (Hanson et al. 1976), others, have clearly demonstrated greater productivity of the introduced species (Kilcher and Looman 1983, Knowles 1987, Lawrence and Ratzlaff 1989) possibly because of deeper rooting and water extraction capability (Black and Reitz 1969). Since the role of each species in grazing management has been well defined, the crucial question on most sites is not which species to seed but whether reseeding should be done at all.
Relative above-ground production of native mixed prairie vs seeded monocultures is not consistent among production years or studies. For example, Black (1968) reported greater yields from at) gropyron-Bouteloua-Stipa community than from a stand of crested wheatgrass in 3 of 4 yr. Lower production from the native site in one year was attributed to drought conditions and the early growth of crested wheatgrass which enabled it to utilize available spring moisture (Black 1968). Similarly, Smoliak et al. (1967) estimated greater forage production from a stand of crested wheatgrass than from a Stipa-Bouteloua community in 5 of 7 yr when annual precipitation was below average but greater production from the latter in one year when precipitation was about 2 times the average. However, below-ground production is consistently greater on native prairie so that total production, if not greater, is at least equivalent.
Comparisons on the relative merits of seeded vs native grasslands are based on land use requirements and often without a comprehensive recognition of the important variables, such as below-ground production and plant-soil relationships, and without an adequate measurement of above-ground production. The most serious error made in comparing production is with the use of a single estimate of standing crop after the growing season. Such an estimate will seriously underestimate net annual production because weathering losses are not included. Consequently, net production on an Agropyron-Koeleria site is about 3 times greater than the standing crop after the growing season (Coupland and Abouguendia 1974). Lauenroth and Whitman (1977) estimated decomposition
of net above-ground production on a mixed-prairie site of about 50% over a 7-mo-period from May to December.
Smoliak also observed a discrepancy between estimates obtained from multiple harvests vs a single harvest made at the end of the growing season on a Stipa-Bouteloua grassland. Production estimate were made at biweekly intervals, on a range continuously grazed by sheep (Smoliak 1968), with the use of portable cages which were moved after each harvest. Forage production was calculated as the sum of increments made over the season from April or May to November. From 1957 to 1960 yields based on these production estimates were 2,380, 2,747, 2,530, and 2,571 kg ha-1 respectively, while production estimates from a single harvest were 706, 664, 529, and 446 kg ha-1 for the same years, respectively (Smoliak and Donnaar 1985). Estimates from multiple harvests were similar to those reported by Lauenroth and Whitman (1977) and were from 5.6 to 9.5 times greater than estimates from a single harvest. Presumably, the larger estimates are at least partly due to capturing dry matter before weathering losses occurred. Seeded grasslands will also be subject to weathering losses but likely with a smaller error because they are a monoculture without a large complement of forbs.
The environmental effect of seeding native prairie is most obvious by the replacement of native communities with a simplified community of introduced species. Other wildlife species of both plants and animals are also affected but, less obvious, is the effect of seeding on the soil resource. The impact of agriculture on the soil is poorly understood and yet the soil is vital for food production and the support of other resources.
There is no doubt that seeded species do not support the same soil environment as the native prairie and that quality tends to deteriorate in a seeded stand. Crested wheatgrass releases far less carbon into the rhizosphere than does western wheatgrass or blue grama (Biondini et al. 1988). The significance of microflora to a healthy grassland is unclear. However, Coleman et al. (1983) suggest that they provide nutrients to the plants and act as a buffer mechanism to allow plants to cope with environmental stress. Twenty-three years after seeding to crested wheatgrass or Russian wildrye, soils had less organic matter (1.42 vs 1.26 and 1.21%), a lower pH (6.8 vs 6.6 and 6.2), and more Na (0.4 vs 0.5 and 0.6 meq L-1) while the root mass was about 300 kg ha-1 less (Smoliak and Dormaar 1985). Effects were also detected 40 and 49 yr after seeding to crested wheatgrass and abandonment that resulted in an increase of the native species (Dormaar et al. 1978). In that study, soil bulk density was greater, water stable aggregates less, and energy input less on the seeded vs adjacent native range.
Breeding. Plant breeding may be a tool for improving the forage production potential of the mixed prairie. The selection criteria used to produce improved cultivars of crested wheatgrass and Russian wildrye have been seed weight and emergence from deep planting (Johnson and Asay 1987). While selection was conducted in the laboratory and greenhouse, they were correlated to seedling emergence, forage yield, and final plant density in field plantings. Carbon isotope discrimination (?) was negatively correlated with WUE in crested wheatgrass and Altai wildrye in: a greenhouse pot experiment (Johnson et al. 1990). In a field experiment where crested wheatgrass clones were grown under a water gradient, ? was positively correlated to forage yield. Broad-sense heritability estimates were sufficiently large to indicate that A is a promising selection tool for development of more WUE crested wheatgrass cultivars.
Blue grama grass exhibits variation in seed weight (Carren et al. 1987b), water uptake of the seminal root system, and seedling shoot biomass (Nason et al. 1987). Heavier seed classes had greater emergence from planting depths greater than 3.0 cm. (Carren et al. 1987b) and were correlated to greater shoot biomass and adventitious root weight. Under soil moisture stress, high seed weight was required for emergence from a depth of 2.0 cm (Carren et al. 1987a). Therefore breeding for large seed weight will allow deeper planting depth and improve the establishment success.
Ecotypes of blue grama that were thought to be 'fast' and 'slow' spreading based on their spread onto abandoned farmland, showed variation in basal spread, forage yield, height, and anthesis date (Samuel 1985). The two types did not differ in forage yield and since genetic factors contributed to the observed variation selection for higher yielding blue grama was deemed possible.
Impacts. Grazing affects the productivity of the mixed prairie in the short term, by its effect on the environment and energy relations within the plant, and over the long term, by its effect on the species composition and selection of ecotypes. The species in the mixed prairie are mostly resistant to grazing and their response will depend on the seral stage, affected by previous disturbances and defined relative to the climax community, and on the current grazing pressure. Since the species composition tends toward equilibrium for any steady state of conditions, any changes will alter the competitive relationship among species and produce a trend, whether succession or retrogression, relative to the climax plant community.
The species composition of the mixed prairie is resistant to change but heavy grazing pressure will favour an increase of blue grama, and other shallow-rooted species, at the expense of the dominants. The selection pressures that favour blue grama are spring grazing, creation of a warmer and drier micro-environment, and preferential grazing of the taller cool-season grasses. Spring grazing is more detrimental to cool-season grasses since they are available for grazing earliest in spring while blue grama avoids grazing pressure with delayed growth and a small stature (Heitschmidt et al. 1990). Blue grama is also favoured by a warmer and drier soil environment which is created with grazing when litter is removed (Weaver and Roland 1952) and infiltration reduced (Naeth et al. 1990).
Grazing affects the plant directly by disrupting physiological activity and indirectly through its effect on the soil environment. On the mixed prairie, the direct effect is not clear and certainly not as significant to the community as is the indirect effect. Severe grazing may retard growth of tall grasses but the effect is less likely With short and mid grasses of the mixed prairie which have few floral tillers and apical meristems that are accessible to cattle (Knight 1973). While Heitschmidt et al. (1990) found that grazing stimulated growth among the mid grasses, and concluded that self-shading was a factor, Knight (1973) reported a leafarea index (proportion of leaf area to ground surface) of less than 0.6, which suggests that seIf-shading is insignificant. Furthermore, compensatory photosynthesis may occur but has no ecological significance because of the small proportion of leaves affected (Nowak and Caldwell
Grazing may also affect the WUE by reducing evapotranspiration and soil moisture depletion (Wraith et al. 1987). The ungrazed plants of crested wheatgrass had better predawn leaf Water status than grazed plants after 1 July, suggesting that the deferral of soil moisture depletion by grazing resulted in improved water status during the summer period. However, the delay in growth that is caused will likely result in greater water demands at a time when less is available (Nowak and Caldwell 1984). The competition for soil moisture in Stipa-Bouteloua grasslands of western Canada is an important factor in species success.
The effect of removing plant litter and altering the microenvironment is immediate and can be long lasting. Plant litter moderates the soil environment and removing it results in increased soil temperature, increased evaporation, and reduced water available for plant growth (Facelli and Pickett 1991). Mechanically removing litter during the dormant season reduced forage production on the mixed prairie by up to 60% (Willms et al. 1986). The effectiveness of litter was dependent on available water for growth; with little response when water was either not available or in sufficient quantities that conservation was not required (Willms, unpublished data). A coefficient of 1 (i.e., 1 unit of litter resulting in 1 unit of production) was obtained in a year when precipitation was below normal but occurred in substantial showers at well spaced intervals. On the other hand, when water was not limiting, a coefficient of 0.0 was obtained which increased to 1.0 and 2.1 as 150 and 300 kg N ha-1 were applied. This suggests that as fertilizer was applied, water demands increased, the need for conservation increased, and litter became more effective.
The immediate effect of overgrazing can be overcome by reducing grazing pressure and allowing litter to accumulate. However, persistent overgrazing leads to an increase in species that are more competitive in a warmer environment and under heavy grazing pressure. High stocking rates, defined as 1. 13 ha per animal unit month (AUM) for cows (Clarke et al. 1947) and 1. 7 ha per AUM for ewes (Smoliak 1974), over a 6- to 17-year period favoured blue grama and Sandberg's blue grass (Poa secunda Presl.) at the expense of western wheatgrass and needle-and-thread. The result of replacing the taller grasses with grasses having a shorter stature is reduced production potential that will persist until grazing pressure is relieved and the range allowed to recover. However, recovery may take many years because blue grama resists being replaced by other species unless the soil is mechanically disturbed.
The ratio of root to shoot production tends to increase with grazing and aridity (Sims and Coupland 1979); presumably in response to a shift in species having a high root to shoot ratio. However, on range in a low seral state or having a high proportion of grazing tolerant species, heavy grazing pressure may not alter the composition substantially and small reductions in root mass will occur with heavy grazing pressure (Milchunas and Lauenroth 1989, Willms et al. 1990).
Resistance to further change may be partly the result of selection of ecotypes that are more resistant to grazing. For example, blue grama plants that had developed under prolonged heavy grazing pressure allocated a greater proportion of resources to the root system than plants that had not been subjected to that pressure (Jaramillo and Detling 1988) while western wheatgrass plants, that had developed under heavy grazing, accumulated shoot biomass and nitrogen more rapidly than plants that had no history of grazing (Polley and Detling 1988).
Livestock may also modify the species composition with trampling and manuring but both these effects are only achieved with heavy grazing pressure and high animal densities with the use of specialized grazing systems. Mechanical impact may be responsible for a reduction in moss phlox (Phlox hoodii Richardson) and little club-moss (Selaginella densa Rydb.) where heavy grazing pressure by livestock was applied (Clarke et al. 1947, Smoliak 1974, Willms et al. 1990) while manuring favours western wheatgrass and is detrimental to blue grama (Smoliak 1965). Western wheatgrass ecotypes that had developed under heavy grazing pressure gained an advantage in their ability to accumulate N (Polley and Detling 1988) while blue grama did not (Jaramillo and Detling 1988), possibly reflecting a different rhizosphere ecology. Blue grama supports more rapid mycorrhizal infection due to greater root exudates (Biondini et al. 1988) and, apparently, does not gain an advantage from additional N.
Grazing Efficiency. Grazing efficiency, defined as the proportion of forage intake/forage disappearance, is a function of other losses due to weathering, decomposition and other herbivory. Dry matter losses can be extremely high but may be reduced by controlling the degree and distribution of grazing pressure. Grazing efficiency on a Stipa-Bouteloua community [S. leuchotticha Trin. and Rupr. and B. curlipenduld (Michx.) Toff.] was estimated at 1.06, 0.75, 0.71, and 0.55 as grazing pressure, measured by allowable forage, decreased from 10, 20, 40, and 50 kg AU-1 day-1 (AUD), respectively (Allison et al. 1982). On continuously grazed fescue prairie, the grazing efficiency was estimated at 0.57 and 0.40 when the allowable forage was increased from 35 to 62 kg AUD-1 and the average intake was assumed to be 12 kg AUD-1 (Willms, unpublished data). However, on small field grazing trials, having similar grazing pressures achieved over a 7-d period, grazing efficiency was about 1 regardless of pressure (Willms, unpublished data). On the other hand, within the context of a specialized grazing system where livestock distribution was controlled within 17 pastures and with grazing pressure sufficient to remove about 80% of available dry matter, grazing efficiency was estimated from 2 to 3 with the same assumptions of forage intake (Willms, unpublished data). While the latter estimate of grazing efficiency is impossible and perhaps due to errors in estimating dry matter disappearance and overestimating intake, it demonstrates the potential for increasing grazing efficiency by controlling grazing pressure and the distribution of use.
Increasing the grazing efficiency is not independent of the grassland ecosystem but the nature of the effect is speculative. Forage that disappears to sinks other than the 'cow' is not necessarily wasted but contributes to a variety of consumers and pathways not yet defined. Therefore, while livestock production may increase with increased grazing efficiency, other components may be negatively impacted in an immediate or delayed process.
Grazing Systems. Grazing systems provide the opportunity for increasing the efficiency of forage use and production but also contain financial risk due to the greater investment in fencing and intensified management. Low production of the mixed prairie increases the risk and, therefore, the temptation to increase livestock numbers to offset the cost. Livestock production may be safely increased with a properly applied system but considerable care is required to avoid overgrazing. Carryover is essential to stabilize forage production and to 'stockpile' forage for drought years and no grazing system can overcome the negative effects of excessive grazing pressure.
Grazing systems are developed for specific objectives usually directed at improving grassland condition and production by controlling the time and distribution of livestock. Season long grazing on a single pasture is the least intensive with few controls on animal distribution and management input only on setting the stocking rate. Grazing efficiencies are low as a result of greater trampling and the presence of overgrazed and undergrazed patches but the heterogeneity introduced may be environmentally desirable in supporting bio-diversity. Rotational grazing systems are applied by moving animals as a group from one pasture to the next on a fixed or flexible schedule that may take several months or several years to complete.
Rotational grazing systems function by resting certain areas in spring and allowing plants to recover after grazing. Their effectiveness on improving the plant community is dependent on the initial condition of the vegetation and on individual application. In a review of published experiments, Gammon (1978) concluded that fewer than half of the experiments improved range condition while individual animal performance usually declined. However, total production will likely increase if the carrying capacity increases.
Although a considerable amount of effort has been spent in testing results, very little has been directed at examining the process that would expand the scope and develop improved guidelines.
Water and soil, nutrients impose the greatest constraints on primary production on the mixed prairie. Agronomic practices that increase above-ground production are generally not feasible or have undesirable environmental consequences related to the establishment of monocultures. However, WUE can be increased by promoting species that are deep rooted and complete their growth before the summer drought.
Increasing grazing efficiency may offer the greatest opportunity for increasing secondary production. This might be accomplished with the use of grazing systems but the environmental consequences of that practice are uncertain.
The interaction of the several environmental constraints with man-made factors such as grazing management require further study at the ecosystem level. Competition for scarce resources such as water and nutrients, the impact of grazing on plant competition, and the potential for seeding native species into native communities are unclear. While environmental constraints will limit productivity, economic decisions, often based on short-term decision-making models that maximize short-term profits, will probably continue to influence range management actually practised in the farming community. A better understanding of the long-term impacts of those decisions is essential to the sustainability of the mixed grass prairie of western Canada.
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W. D. Willms1 and P. G. Jefferson2
'Research Station, Agriculture Canada, Lethbridge, Alberta, Canada T1J 4B 1.
2ReSearch Station, Agriculture Canada, Swift Current, Saskatchewan, Canada S9H 3X2.
With permission from.
Willms, W. D. and Jefferson, P. G. 1993. Production characteristics of the mixed prairie: constraints and potential. Can. J. Anim Sci. 73: 765-778. The mixed prairie represents the most and region of the Northern Great Plains in Canada. Approximately 6.5 M ha of the original total of 24 M ha have retained their native character. The native prairie supports about 5.3 M animal-unit-months or about 15% of all beef cattle present on the Canadian prairies. A large portion of the area is dominated by either needle-and-thread (Stipa comata Trin. + Rupr.) of western wheatgrass (Agropyron smithii Rydb.), both cool season grasses, and associated with blue grama [Bouteloua gracifis (H.B.K.) Lag. ex Steud.] a warm season grass. These species define the major plant communities of the mixed prairie and determine their production potential. However, their production is limited by available water during the growing season and by soil nutrients; factors which also influence their species composition. Grazing imposes a significant impact on the grasslands by altering the water and nutrient cycles, through defoliation and reduced plant litter, and eventually by affecting e species composition. Removing litter may reduce forage production by up to 60% and repeated defoliation will favour the more drought tolerant but less productive species. Forage production may be increased by seeding introduced species, which have a greater shoot to root ratio than native grasses, or with fertilizer application. Livestock production may be increased with the use of grazing systems. However, the benefits of each practice on the mixed prairie must be assessed in terms of their cost, their impact on the environment, and the reduced or lost value for other users.