1Department of Plant Science, University of Manitoba, Winnipeg, MB
2Agriculture and Agri-Food Canada, Lacombe Research Centre, Lacombe, AB
3North Dakota State University, Dickinson Research and Extension Centre, Dickinson, ND
4North Dakota State University, Department of Plant Science, Fargo, ND
5Virginia Tech. Department of Crop, Soil and Environmental Sciences, Blacksburg, VA
6Agriculture and Agri-Food Canada, Brandon Research Centre, Brandon, MB
Cultivated forage crops are grown on almost 12 million hectares on the Northern Great Plains. This paper reviews the benefits of diversifying annual crop rotations with forage crops, and highlights innovations in forage systems. Agronomic benefits of rotating forage crops with annual grain crops include higher grain crop yields following forages (up to 13 years in one study); shifts in the weed population away from arable crop weeds, and improved soil quality. Perennial legumes in rotation also reduce energy requirements by adding significant amounts of N to the soil. Soil water availability may limit the extent to which forages benefit following crops. Under semi-arid conditions, forages can actually reduce yields of the following crops, and as such, soil water conserving tillage practices have been developed to partially address this problem. Forages in rotation provide environmental benefits, such as C sequestration, critical habitat for wildlife, and reduced nitrate leaching. A wider range of annual plant species are now used in forage systems in an effort to extend the grazing season, and to maximize use of water resources. Intensive pasture management using cultivated forages is on the increase, as is the use of alfalfa in grazing systems; in some cases using bloat-reduced alfalfa cultivars. Pasture-based systems appear to provide benefits for both animal and human health, and arguably the health of the environment. Pasture systems are less nutrient exhausting than hay systems. As a result, nutrient management strategies will differ in the following crop. Additional research is required to optimize the role of cultivated pastures in grain-based cropping systems.
Forage production in the Northern Great Plains (NGP) of the U.S. and Canada involves cultivated and native pasture and hay production. The area dedicated to cultivated forage crop production in the three Canadian prairie provinces (Manitoba, Saskatchewan and Alberta) and three U.S. states (North Dakota, South Dakota and Montana) totals 7.8 million hectares of cultivated hay and 3.8 million hectares of cultivated pasture (Provincial Government statistics, 1999; USDA, 1999). Many farmers and ranchers use cultivated forages to complement the approximate 44 million hectares (Provincial Government statistics, 1999; USDA, 1999) of native rangeland in this region.
Forage is produced and conserved during the short growing season, and fed during the remainder of the year. Hay is the predominant winter feed followed by straw, silage, stockpiled perennial pasture and swathed annual pastures (Small and McCaughey, 1999). The winter feeding period for beef cattle in western Canada is widely reported to exceed 200 days per year (Mathison, 1993). However this varies by region and year, mainly depending on period of snow cover. In Alberta during 1999, the mixed grassland region, most representative of the NGP area, had an average 155 winter feeding days compared to 201 in the boreal transition zone, which lies to the north of the prairie (Anonymous, 2000). Approximately 10% of forage production is used for dairy cows located in the NGP region. Some forage is also exported outside North America (e.g., dehydrated alfalfa [Medicago sativa L.] cubes and pellets and compacted timothy [Phleum pratense L.] hay). Very little forage is typically imported into this region, although redistribution of forage does occur when localized droughts reduce forage supply.
Alfalfa is the main forage legume and is grown on 61% of cultivated forage hayland in the U.S. NGP (USDA, 1998). Alfalfa's role in grazing systems is increasing (Smith and Singh, 2000). Other forage legumes are also grown where alfalfa is not adapted (e.g., red clover [Trifolium pratense L.] and alsike clover [Trifolium hybridium L.] in wetter and acid soil zones; sainfoin [Onobrychis viciafolia L.] in dryland pastures, or where a non-bloating legume is desired). There is significant potential to utilize these better adapted alternative perennial forage legumes in the region, though grower education is required. Many different grass species are used in cultivated forage systems, ranging from the drought and salt tolerant wheatgrasses (Agropyron spp.) to flooding tolerant reed canarygrasses (Phalaris spp.). Many annual C3 and C4 plant species are used to fill gaps in the feed supply (Kilcher and Heinrichs, 1961; Baron et al., 1992; Carr et al., 1998). Forage is defined as "Any plant whose vegetation is eaten by livestock" (Heath et al., 1973), and as such, many different plants are used including crop residues (e.g., corn stalks and chaff) and hay harvested for remnant areas and roadside ditches. These forage sources are especially important in drought years such as 2000 in Montana. Forage seed production is also an important industry in the region though it occupies a relatively small area compared with forage production.
The percent arable cropland that is rotated with forage ranges from 5 to 15% in the region. Two recent surveys have characterized forage and beef production in western Canada (Entz et al., 1995; Small and McCaughey, 1999).
Objectives of this paper are: 1) to review agronomic, economic, and environmental benefits and risks of diversifying cropping systems with forage crops; 2) identify means to enhance the positive attributes of forages in NGP cropping systems, and to make forages a more important component of the cropping system; and 3) to highlight research challenges for the future.
Forage benefits have received less attention in the NGP than elsewhere, such as the humid U.S. midwest, where alfalfa has traditionally been rotated with grain crops, or areas of Australia, where unique self-regenerating forage species are grown in grain-based cropping systems (Grace et al., 1995). The short growing season and relatively dry conditions (i.e., low precipitation and high evaporative demand for water) in the NGP will modify rotational benefits of forages relative to wetter areas.
Some of the best information on forage rotational benefits in the NGP have come from long-term crop rotation studies, many of which were established soon after European settlement in the U.S. (Stoa and Zubriski, 1969) and Canada (Campbell et al., 1990).
Many NGP researchers have reported rotational yield benefits from perennial forages. In a long-term (1912 to 1956) study in Fargo, ND, Stoa and Zubriski (1969) reported that wheat yields were 50% higher from land previously cropped to alfalfa for three years than from non-legumes such as corn (Zea mays L.), wheat (Triticum aestivum L.) or flax (Linum usitatissimum L.).
Similar results continue to be reported from two ongoing classical long-term crop rotation studies in western Canada; The University of Alberta's Breton Plots (initiated in 1930) (Ellert, 1995) and the Agriculture and Agri-Food Canada's long-term study at Indian Head, SK (initiated in 1958) (Campbell et al., 1990), as well as studies at Melfort, SK (Campbell et al., 1990), Winnipeg, MB (Poyser et al., 1957), and Lethbridge, AB (Ellert, 1995).
In a survey of Manitoba and Saskatchewan forage producers, 71% of respondents indicated higher grain yields after forages than in annual crop rotations (Entz et al., 1995). Rotational yield benefits were greatest in eastern and northern zones and lowest in drier, western and southern zones. In one of the best studies ever published on the long-term residual yield benefits of including forage in a cropping system (McLennan, AB), Hoyt (1990) reported that for the first eight years after forage termination, wheat yields were 66 to 114% greater after forage relative to continuous wheat. Yield differences started to decline after eight years, although wheat yields in the alfalfa systems were still higher (P<0.05) than the control in the 10th and 13th year after sward breaking.
In areas of the NGP where water seriously limits crop productivity, inclusion of perennial forages can reduce crop yield in following crops due to forage-induced drought. Working in west central Saskatchewan, Brandt and Keys (1982) determined that available soil water in spring was lower after a 2-yr alfalfa crop than in a continuous grain rotation. A full year of fallow was insufficient to fully replenish the soil profile with water in the alfalfa relative to the grain system. In central Saskatchewan, Austenson et al. (1970) reported that alfalfa in rotation depressed wheat yield in the first crop after breaking even after a full year of summerfallow. Interestingly, they observed that alfalfa with bromegrass (Bromus inermis Leyss.) or bromegrass alone did not affect wheat yields significantly. Others (e.g., C.A. Campbell, 2000, personal communication) have suggested that low cereal yields after alfalfa could be due to allelopathic effects from alfalfa, and such effects are greatest under dry soil conditions. However, no studies have been conducted to substantiate this suggestion.
In wetter areas of the NGP, these water depleting characteristics of alfalfa and other perennial forages are often viewed as desirable. For example, de-watering characteristics of perennial forages play an important role in soil salinity management. Soil salinization is a threat to the long-term sustainability of crop production on approximately 25% of NGP cropland (Morrison and Kraft, 1994). Examples of successful salinity control with alfalfa (Eilers, p. 78 as cited in Morrison and Kraft, 1994) and perennial grasses (D. Wentz, Alberta Agriculture, Food and Rural Development, 1996, personal communication) have been documented. Hoyt and Leitch (1983) reported that the subsoil (60 to 135 cm) de-watering effect with perennial legumes lasted for at least two years after stand termination, and that alfalfa provided greater de-watering benefits than red clover. De-watering benefits with alfalfa on a clay soil in Manitoba resulted in higher wheat yields in alfalfa-based vs. annual grain-based rotations (Forster, 1998).
Grazing management and plant species impact soil water availability and potential evapotranspiration. Perennials begin to de-water soil as soon as growth begins in the spring (April), whereas annuals only begin to reduce soil available water when ground cover has been achieved (mid June) (Twerdorff et al., 1999a). In research in central Alberta, greater evapotranspiration by perennials reduced surface (0 to 7.5 cm) soil water more than annuals until mid July, after which annuals and perennials had similar surface soil water contents. Generally, surface soil water was higher under heavy compared with light grazing intensities for perennial grasses (Twerdoff et al., 1999a). Seasonal evapotranspiration was generally greater for perennials than annuals. Water use efficiency for perennials (16.6 kg DM ha-1 mm-1 ) was 1.4 times greater than annuals (11.6 kg DM ha-1 mm-1 ). However, heavy grazing intensities reduced water use efficiency from 14.9 kg DM ha-1 mm-1 (five cycles of grazing) to 13.0 kg DM ha-1 mm-1 (three cycles of grazing) (Twerdoff et al., 1999a).
The N benefits of forage legumes grown in the NGP have been documented by many workers over the past 75 years (e.g., Badaruddin and Meyer, 1989; Hoyt and Leitch, 1983). Working in dry sub-humid, southern Manitoba, Kelner et al. (1997) determined that net N additions of an alfalfa hay crop were 84, 148, and 137 kg ha-1 in the first, second, and third years of the stand, respectively. This suggests that relatively short-term alfalfa stands could maximize N input. Ferguson and Gorby (1971), on the other hand, recorded a slightly higher long-term N benefit between an 8-yr and a 4-yr alfalfa stand. No similar study has been conducted in drier areas of the NGP.
Ferguson and Gorby (1971) reported that most N benefits from alfalfa or alfalfa/bromegrass stands to following grain crops occurred in the first two years after forage termination. Hoyt and Leitch (1983) reported that N benefits from a number of different forage legumes occurred in the second and third year after forage termination. Both reported significant N-benefits up to seven years after forage termination. Hoyt and Leitch (1983) determined that rotational N benefits to following grain crops were greatest for alfalfa>alsike clover>bird's foot trefoil (Lotus corniculatus L.).
Forster (1998) attempted to separate N and non-N yield benefits from an alfalfa hay crop in Manitoba. He reported increased wheat yields (over the control) of 1100, 500, 200, 250, and 400 kg ha-1, due to N for the first five wheat crops after alfalfa, respectively, vs. yield increases of 200, 450, 400, 200, and 200 kg ha-1, due to non-N factors, for the first five wheat crops after alfalfa, respectively. Therefore, after the second grain crop, rotational yield benefits from alfalfa were similar for N and non-N factors.
Several NGP researchers evaluated N benefits of single year "dual purpose" - hay and late-season forage re-growth plowdown systems. Badaruddin and Meyer (1989) reported a fertilizer replacement value of legume (cut for hay and re-growth fall incorporated) equivalent to the addition of up to 150-kg N ha-1 on continuous wheat. Kelner and Vessey (1995) reported a net soil N contribution of 121 kg N ha-1 for 'Nitro' alfalfa in Manitoba. Working in Montana, Westcott et al. (1995) compared N contributions of single year Nitro alfalfa and 'Bigbee' Berseem clover (Trifolium alexandrinun L.). They concluded that "if the goal in managing annual forage legume in a fall plowdown system is primarily for forage yield, then berseem clover in a two-harvest system may be preferable. If plowdown N-benefits are of greater priority, then Nitro alfalfa in a zero- or one-harvest system should be considered."
By adding N to the soil system, forages in rotation also decrease energy requirements for crop production. Effects of including alfalfa on energy use (Rice and Biederbeck, 1983, as cited in Campbell et al., 1990) and energy use efficiency (Hoeppner et al., 1999) have been documented for NGP cropping systems.
Forage legumes, especially in hay systems, remove large amounts of minerals from the soil (Woodhouse and Griffith, as cited in Heath et al., 1973). For example, in the long-term study (1958 to present) at Indian Head, SK, inorganic soil P levels were 37 kg ha-1 in continuous fertilized wheat, 27 kg ha-1 in continuous unfertilized wheat, and 21 kg ha-1 in the unfertilized forage-containing rotation (Campbell et al., 1993). In a southern Alberta study, including alfalfa-crested wheatgrass (Agropyron cristatum [L.] Gaertn.) in a 6-yr forage-wheat rotation reduced the rate of soil N depletion, but increased slightly, the decline in exchangeable K levels (Pittman, 1977, as cited in Campbell et al., 1990). Forage legumes also affect soil chemical properties. For example, at the Breton plots, long-term use of forage legumes (1930 to present) has decreased the soil pH to the point where liming is critical to maintain crop production (Robertson, 1992).
Forage-based rotations that include pasture systems, where nutrients are recycled to the soil, are less nutrient exhausting than hay systems. This may be particularly so in the moister, northern area of the NGP. In a short-term rotational pasture study over four years in the Alberta parkland, where annual and perennial species were compared at three grazing intensities, soil-C in the surface 0-5 cm (Typic Haplustall silt loam) increased for perennial grasses, but decreased for annuals and was unaffected by grazing intensity; total-N and C:N ratio were unaffected by species or grazing intensity (Mapfumo et al., 2000). However, the mineral-N fraction was much more dynamic and responsive to grazing intensity. Very intensive grazing (five cycles of grazing) resulted in soil mineral-N levels exceeding 200 kg N ha-1 compared to 95 kg N ha-1 (0 to 60 cm depth) for less intensive grazing (three cycles of grazing) averaged over annual and perennial species (Baron et al., 2001). Nuttall et al. (1980) reported that economic returns from fertilizing mixed alfalfa-grass pastures was maximized at 90 kg N ha-1 and 20 kg P ha-1 when the stocking rate was 3.7 head ha-1. In the same study, herbage yields increased with N applications up to 185 kg N ha-1, but nitrate-N accumulated in the 30 to 60 cm depth of the soil profile. In the drier areas, and perhaps on long-term grassland, little mineral-N remains in the soil solution at any time. Soil micro-organisms are much less active when soil available water content is low (Biederbeck et al., 1984), and even in moist periods, immobilization of mineralized-N by soil microflora predominates mineralization of organic matter (Woodmansee et al., 1981). In the drier areas, little fertilizer-N is applied to pastures and grazing is mostly of an extensive nature. Because differences for nutrient dynamics exist between short-term and long-term sequences, moist and dry, and intensive vs. extensive grazing regimes, more research is needed on impacts of nutrient cycling as well as fertilizer requirements for pastures in the NGP.
Many non-N benefits of forage in a crop rotation are attributed to improved soil quality. This is especially important given that NGP soils have undergone serious degradation since the early part of the 20th century (McGill et al., 1981). Many soil physical condition improvements by forage have been attributed to greater soil-C in forage-based compared with annual crop systems (e.g., Spratt, 1966). Pavlychenko (1942) reported a much higher proportion of large soil aggregates for various introduced and native perennial grass species than for a wheat-oat (Avena sativa L.) rotation, but "at depths exceeding 10 cm, none of the cultivated grasses had an appreciable effect upon the soil structure". Native grass species such as Porcupine grass (Stipa spartea Trin.) and blue grama (Bouteloua gracilis [H.B.K.] lag.exStead) improved soil structure between 10 and 60 cm soil depth more than the very popular crested wheatgrass (Pavlychenko, 1942).
From a management perspective, perennial pastures provide a large litter base and root system that promotes greater storage of C in the soil compared to annuals. In short-term pasture sequences in the moister NGP, Baron et al. (2001) estimated total C contribution (roots and litter) for perennials was 2.7 times more than annuals; contribution of roots and litter was 1.5 times greater with light compared to heavy grazing.
McGill et al. (1986) studied the dynamics of soil microbial C and N in two systems in the Breton plots: wheat-fallow, and wheat-oat-barley (Hordeum vulgare L.) -forage-forage. They found that the 5-yr rotation contained 38% more N and 117% more microbial N than did the wheat-fallow system. In addition to increasing long-term soil biological fertility, N additions to NGP soils are also known to increase soil aggregation (Biederbeck et al., 1984). Therefore, both the C and N additions from forages reduce soil erosion potential of NGP soils. Working in the semi-arid zone of the NGP, Naeth et al. (1991) reported that high soil microbial populations associated with pasture grass rhizospheres produce polysaccharide mucigels that promote aggregation in the short term, while in the long term, the build-up of humic materials will stabilize aggregates. On a sandy soil in southern Manitoba, Banjeree et al. (2000) observed a reduction in soil microbial biomass-C when predominantly alfalfa pastures were grazed at heavy compared to light stocking rates (i.e., 2.2 vs. 1.1 steers ha-1). Cultivation of long-term prairie reduces soil biomass-C, as well as concentrations of soluble sugars and amino-N (Deluca and Keeney, 1994).
Cavers (1996) reported that a 4-yr alfalfa hay crop resulted in saturated soil hydraulic conductivity 10 times higher than in a small grain rotation on a clay soil in Manitoba; this difference was measured at 25-, 50- and 75-cm soil depths. Wheat root activity on these same plots was found to be significantly deeper after alfalfa compared with an annual crop rotation (Forster, 1998).
Some suggest that intensive rotational grazing may increase water infiltration (e.g., Savory, 1978). Mapfumo et al. (1999) questioned this assertion since soil pressures from animal hooves can be as much as 200 kPa, which is considerably greater than the pressure exerted on the soil by a tractor (30 to 50 kPa) (Profitt et al., 1993). However, in an Alberta study comparing intensive vs. extensive grazing of annual and perennial forages, Mapfuno et al. (1999) found only a few negative effects of intensive grazing on soil physical properties, and suggested that "natural processes such as freeze-thaw action, wet-dry cycles, and earthworm activity likely reduced the effects of animal trampling". In fact, their evidence suggests that extensive grazing causes soil compaction and restricts soil water movement. On this excellent loamy soil, bulk density increased in a curvilinear fashion with increasing exposure to animal traffic, but increased faster for annual than perennial pastures (Twerdoff et al., 1999b). However, it was concluded that intensive rotational grazing systems may not cause serious compaction problems for soils of this type.
Weed suppression with forages, especially perennial hay crops, has been documented in various NGP studies over the past 50 years. Siemens (1963) described results of a long-term crop rotation study at Brandon, MB where wild oat (Avena fatua L.) "dockage" (i.e., percent yield consisting of wild oat seeds) in grain crops averaged less than 1% in forage-containing rotations and 15% in continuous grain or fallow-grain systems. In a survey of Canadian prairie farmers, 83% of respondents reported fewer weeds after alfalfa vs. grain rotations, with good suppression of wild oat, green foxtail (Setaria viridis L. [Beauv.].) and Canada thistle (Cirsium arvense [L.] Scop.) (Entz et al., 1995). Ominski et al. (1999) reported that wheat after perennial alfalfa or alfalfa/grass hay crops had significantly fewer problem weeds than wheat in annual grain rotations, and that forage in the rotation shifted the weed community composition away from wild oat, green foxtail and Canada thistle. Alfalfa did, however, select for dandelion (Traxacum officianale Weber in Wiggers).
Even single year forage crops have been found to provide significant weed control benefits (Schoofs and Entz, 2000). These workers concluded that "the ideal annual forage system for weed management should combine the early season vigour of a biennial crop, the continuous competition of a long-season crop, and the intense mid-summer competition of a C4 crop. Therefore, a combination of two, or possibly more crops grown together, may be required." Harker et al. (2000) studied the impact of grazing intensity on weed populations in perennial and annual pastures. They found that dandelion increased with increasing grazing intensity and years of grazing in perennial grass pastures at a rate of 4 plants m-2 for every unit increase in intensity (three to five cycles of grazing); annual weeds, mostly shepherd's purse (Capsella bursa-pastoris [L.] Medikus), increased at a rate of 51 plants m-2 for each unit increase in grazing intensity. However in annual pastures, where tillage was a factor, shepherd's purse was higher at low vs. high grazing intensities; high grazing intensity served to reduce annual weed populations.
Fewer studies have considered forage effects of plant diseases or insects. It is important to recognize that perennial forages are sometimes continuous monoculture, and therefore pathogen or insect pest populations can build up in the crop (e.g., Sclerotinia sclerotiorum - both alfalfa and canola [Brassica spp.] are susceptible). By the same token, a perennial forage stand provides a long period for pathogens to decline, thereby reducing damage to a following susceptible crop. Both Penning and Orr (1988) and Tinline (as cited in Campbell et al., 1990) reported that the only rotation to effectively reduce inoculum of common root rot (Cochiablus sativus) was a 3-yr forage hay stand.
The most comprehensive economic analysis of forage-based cropping systems has been conducted by Zentner and coworkers (Agriculture and Agri-Food Canada, Semi-arid Prairie Agriculture Research Centre). Using information from long-term crop rotation studies at Indian Head, Scott, and Melfort, SK, they determined input costs, net returns, and income variability associated with forage-based and annual grain crop-based rotations (Zentner et al., 1986). Cost of production for forage-based systems was lower than continuous grain production, but higher than a wheat-fallow system. Net returns tended to be more stable across a range of crop prices for the forage-based systems than annual systems. Including 2- or 3-yr forage crops in a 6-yr rotation was found to significantly reduce income variability or risk. At both locations, adding a 2-yr or 3-yr forage phase into the 6-yr crop rotation decreased income variability significantly more than crop insurance. Therefore, in order to reduce risk, a biological solution appeared to be superior to a government program.
Agronomists and farmers are interested in knowing the minimum economic optimal length of a forage hay crop. This question was partially addressed by Zentner et al. (1986), who reported that 2- or 3-yr forage stands in a 6-yr rotation are economical. Other NGP research suggests that alfalfa and other forage legume monocultures should be terminated after four or five years for maximum economic efficiency of the rotation (Jeffrey et al., 1993). Most forage stands in dryland regions are currently maintained for at least seven years (Entz et al., 1995).
Perennial forages can scavenge nutrients from greater soil depths than annual crops because of their deep root systems (Pavlychenko, 1942). In the long-term study at Indian Head, SK, Campbell et al. (1994) found that a 3-yr alfalfa/bromegrass crop in a 6-yr crop rotation reduced buildup of subsoil (to 240 cm) nitrates. Entz et al. (2001b) observed nitrate extraction to depths of 90, 180, 210 and 270 cm in the first four years of an alfalfa stand, respectively. Working on the same trial, Kelner et al. (1997) determined that these high subsoil nitrates did not reduce N fixation of alfalfa in years one, two, or three of the stand.
Campbell et al. (1994) reported significant nitrate leaching from alfalfa in the Indian Head rotation. They attributed this observation to the fact that, in the Indian Head study, alfalfa is followed by a year of fallow. Under these conditions, legumes increased soil N supply, but the net downward movement of water during the fallow year resulted in nitrate leaching. Using no-till vs. tillage methods to terminate alfalfa crops improves the synchrony of N release from alfalfa and uptake by following cereal grain crops, thereby reducing the risk of nitrate leaching from perennial alfalfa (Mohr et al., 1999). The role of perennial forages to extract deep-leached nitrates is becoming more important as large-scale livestock production increases in the NGP region.
Forage crops play an increasingly important role in providing critical habitat for many species, including migrating waterfowl. These programs have increased in sophistication over time; they have evolved from simply establishing perennial forage crops to use of locally adapted native grass species, often in a sculpted seeding system (Jacobson et al., 1994). Development of native plant materials and ecotypes for multiple land use systems has been in place in the U.S. for some time (e.g., USDA at Mandan, ND, John Berdahl, 2000, personal communication). In Canada, Ducks Unlimited Canada in cooperation with Agriculture and Agri-Food Canada and University scientists have recently initiated a program to develop ecological varieties of approximately 20 native grass species, 7 forbs and 4 shrubs (Wark, 1998, Ducks Unlimited Canada, Brandon, MB, personal communication).
The potential to sequester atmospheric carbon dioxide as soil organic C in forage-based cropping systems is well recognized (Spratt, 1966). Carbon sequestration in cropland seeded to perennial grasses averaged 1.1 Mg C ha-1 yr-1 over a 5-yr period in a survey of land under the Conservation Reserve Program in the U.S. (Gebhardt et al., 1994) . Because of their deeper root systems, perennial forage plants can place C deeper into the soil system than annual plants, resulting in better long-term C storage. Some previous studies in the NGP region have focused on long-term effects of fertilization on long-term carbon storage (e.g., Nyborg et al., 1999; Cihacek and Meyer, 2000). As Baron et al. (2001) pointed out, perennial forage systems result in greater soil C accumulation than annual forage systems.
Cultivated forages are sown on 5 to 15% of the arable landbase in the NGP region. Hence, only a small percentage of the landbase can receive the benefits of forages at any one time. Since the total forage acreage (especially perennial forages) is not likely to increase dramatically in the future, the best approach for increasing exposure of arable lands to forage benefits is to cycle forages through the crop rotation more quickly.
While the minimum alfalfa stand lengths to achieve weed control (Ominski et al., 1999), N (Kelner et al., 1997), subsoil nitrate extraction (Entz et al., 2001b), and economic (Jeffrey et al., 1993) benefits are five years or less, forage stand length is currently over seven years in the region (Entz et al., 1995). Therefore, the potential exists to use the existing forage hectarage more efficiently by shortening forage stand length and moving forages from field to field more rapidly. However, farmers are reluctant to terminate forage stands for two main reasons: difficulties encountered when establishing and terminating forage stands (Entz et al., 1995).
Forage seedlings are especially vulnerable to soil moisture deficits because the small seeds are sown near the soil surface. Conventional seedbed preparation techniques result in dry seedbeds and increase the risk of soil erosion. No-till forage establishment increases soil water available to germinating forage seeds, and increases establishment success, especially when post-seeding precipitation is absent (Allen and Entz, 1994). The long-term crop rotation study at Indian Head, SK is now conducted under no-till, and since this change, alfalfa/bromegrass establishment has improved greatly (Lafond, 1999, personal communication).
Most forages in the NGP are seeded with a companion crop (Entz et al., 1995). Companion crops tend to reduce forage establishment and reduce first and sometimes even second year forage yields (Smith et al., 1997). However, despite the loss in forage yield potential from companion crops, most workers agree that use of companion crops is economical. For example, working in Alberta, Smith et al. (1997) concluded that companion crops for alfalfa establishment significantly enhanced economic performance over three years, compared to where no companion crop was used; especially when the companion crop was removed early (as silage). Jefferson and Zentner (1994) concluded that forage yield would have to be negatively affected by companion crops for two years after forage establishment to be less profitable than establishment without a companion crop.
It is useful to note that many of the establishment year benefits of companion crops can be achieved with no-till forage seeding (i.e., reduced blowing soil damage; shading and lower soil temperatures) without the competition for resources, especially water (Allen and Entz, 1994). Smith et al. (1997) concluded that herbicides are not economically feasible during the forage establishment year.
Most producers currently use some tillage to terminate forage stands (Entz et al., 1995). This represents a significant investment of time and machinery. Use of herbicides instead of tillage to terminate alfalfa is feasible (Bullied et al., 1999) and has been shown to increase soil water conservation and grain yields in following crops (Bullied and Entz, 1999). No-till seeding winter cereals into herbicide-killed forages (Entz and Bamford, 2000, unpublished data) has the advantage that winter cereals use the limited water supply more efficiently than spring cereals (Gan et al., 2000). Other benefits of no-till alfalfa termination include fewer weeds in subsequent crops due to less soil disturbance (Ominski and Entz, 2001).
The choice of grain crop after forage depends on the forage species and the moisture environment. Pulse crops are best suited to production after grass forage crops, including annual ryegrass (Chescu and Entz, unpublished). When following alfalfa (or stands that have less than 50% grasses), large-seeded cereals such as oat, barley and wheat provide the best results. Three years of trials in Manitoba showed that winter wheat performed very well after alfalfa. Following forages with small-seeded crops such as canola or flax increases the risk of stand failure, especially under dry conditions.
Annual forages play an important role in the feed supply. In addition to supplying winter feed (e.g., silage), annual forages are being promoted as a means to extending the length of the grazing season; a very important goal for livestock producers.
Traditional annual forage species in the NGP region include barley, oat, fall rye (Secale cereale L.) and wheat (Kilcher and Heinrichs, 1961). For example, barley for silage is the choice of the feedlot industry in southern Alberta (MacAlpine et al., 1997). Triticale (x Triticosecale Wittmack) is a newer cereal that has outperformed traditional cereals in the semiarid regions of western Canada (McLeod et al., 1998) and Montana (Stallknecht and Wichman, 1998). Other novel annual species such as sunflower (Helianthus annuus L.), canola, corn and pulses have been tested for forage potential previously (Berkenkamp and Meeres, 1987). Annual forage mixtures, while typically not enhancing yield (Baron et al., 1992) can enhance quality (Carr et al., 1998), and can greatly improve seasonal dry matter distribution (Baron et al., 1992; Carr et al., 1998; Manske and Nelson, 1995).
It is generally accepted that perennial pastures are the least expensive feed sources for the beef cow herd. However, novel annual forage systems can fill a void at specific points in the livestock enterprise, resulting in significant savings for the entire enterprise. Motivation for novel pasture systems are: 1) conventional pasture system, while low cost, cannot keep up to with the demands of cows, calves or stocker cattle, all of which are gaining in size and weight; 2) it is less expensive to overwinter beef cows conventionally if they enter the winter feeding period in good body condition (Willms et al., 1993); and 3) it is cheapest to feed some classes of livestock (e.g., beef cattle) on pasture than in dry lots (Adams et al., 1994). Manske and Nelson (1995), working in western ND, reported that oat-pea (Pisum sativum L.) intercrops, millet (Panicum spp.) and fall rye worked well in annual grazing systems. Novel systems aimed at improving seasonal forage dry matter distribution have also been developed. Mixtures of spring and winter cereals, for example, provide earlier grazing than winter cereals alone, but continue producing dry matter later in the season than spring cereals planted alone (Baron et al., 1993a, 1993b). Mixtures of winter cereals and Italian ryegrass (Lolium multiflorum L.) respond similarly to winter-spring cereal mixtures (Thompson et al., 1992). A novel system for late-season and winter grazing is swath grazing. Cereals are swathed from heading until dough stage and animals graze the swaths. A wide range of C3 and C 4 cereal crops are being tested for use in swath grazing (McCaughey, 1999, unpublished data). Cereals for swath grazing are sown later in spring - a proven tactic to reduce wild oat populations (Schoofs and Entz, 2000).
Low cost cereal straw and chaff represent a vast potential feed source for gestating beef cows across the NGP (Coxworth et al., 1981). In Alberta, MacAlpine et al. (1997) estimated 1.2 t of straw or 2.2 t of chaff are required to winter (200 days) a 450 kg cow. Oat and barley straw are generally considered to have a higher feeding value than triticale and wheat straw (Coxworth et al., 1981; Wedin and Klopfenstein, 1995).
Coxworth et al. (1981) reviewed straw and chaff management for the dry part of the Canadian NGP. They observed that stage of maturity and N fertilizer application could have significant effects on nutritive value of wheat straw, but location could have a greater effect. They noted differences among wheat cultivars for feeding value, that chaff had greater feeding value than straw, and that ammonia application could have substantial positive impacts on straw and chaff nutritive value. Whether ammoniation of straw and chaff is economical depends on relative costs of anhydrous ammonia and alternative feedstuffs, such as cereal grains.
Agronomic benefits of chaff collection include residue management and weed and volunteer crop control. High chaff yields in moist areas pose a limitation to direct seeding. Weed control benefits of chaff collection were identified by Shirtliffe (1999) who determined that wild oat and green foxtail patch dispersal was virtually eliminated when chaff was collected. This is another example of how integrating livestock and grain production provides important synergy within the farming system.
A novel forage-based cropping system has been used successfully for decades in Australia (Grace et al., 1995). Here, self-regenerating subterranean clover (Trifolium subterraneum L.) and annual medic (Medicago spp.) are grown in pasture-grain systems. There has been considerable interest in adapted these systems to the NGP region. Sims and Slinkard (1991) concluded that black medic (Medicago lupulina L.) had potential for replacing summerfallow in a wheat-fallow cropping system in Montana. Long-term field trials demonstrated that 'George' black medic (Sims et al., 1985) successfully re-seeded itself and boosted wheat yields by 1300-kg ha-1 compare with wheat on summerfallow. In this system, black medic can be grazed during the fallow year.
Self-regenerating annual medics can also be integrated into continuous grain production systems. Three annual medic species were established on 40 ha. on a North Dakota farm in 1991. The medics have been regenerating successfully in a continuous cropping system for the past 8 years (K. Aldridge, NDSU extension agent, Sheridan county, ND, 1998, personal communication, 1998). The medics provide significant forage for late-season grazing, as well as weed suppression, plus they produce enough seed to successfully re-establish themselves each year. Thiessen-Martens and Entz (2001) determined that a large area of the NGP has sufficient heat and water resources for late-season growth, including seed production of several medic and subclover species. Selection of suitable medic species, and management practices is currently underway (Entz and Carr, 2000, unpublished data).