Climate and Traditional Cropping Practices in the Northern Great Plains

The US northern Great Plains comprise a vast, agriculturally important region consisting of North Dakota, most of South Dakota and Montana, and northeast portions of Wyoming and northwest Nebraska. The region is characterized by long, cold winters and short, warm summers, along with erratic precipitationReference Padbury, Waltman, Caprio, Coen, McGinn, Mortensen, Nielsen and Sinclair⁵. Precipitation typically is limited, ranging from roughly 300 to 500 mm annually, with declining amounts generally occurring from east to west. Prolonged dry periods accompanied with strong winds are common during summer monthsReference Padbury, Waltman, Caprio, Coen, McGinn, Mortensen, Nielsen and Sinclair⁵. Highly variable precipitation, along with seasonal and diurnal fluctuations in temperature, is a serious climatic challenge faced by farmers in the region. However, the unpredictability of the limited precipitation that is received poses the greatest impediment to sustained crop production.

Hard red spring wheat (Triticum aestivum L. emend Thell.) has been the dominant grain crop grown in much of the region, largely because of its drought-evasive or -resistant traitsReference Padbury, Waltman, Caprio, Coen, McGinn, Mortensen, Nielsen and Sinclair⁵. Spring wheat was grown in alternate years, with a long, 20- to 21-month fallow period between successive crops, throughout most of the 20th centuryReference Johnson, Larney and Lindwall⁶. Precipitation was stored in the soil during the fallow phase to mitigate the negative impacts of drought in the wheat–fallow systemReference Johnson, Larney and Lindwall⁶. The fallow period also allowed accumulation of plant-available nitrogen (N) from organic matter mineralization, and provided weed control opportunitiesReference Bauer and Conlon⁷. Tillage was the primary and, oftentimes, only tool used to kill weeds and stimulate organic matter breakdown by incorporating crop residue into soil.

The wheat–fallow system was a major contributor to severe soil erosion problems that plagued the northern Great Plains during much of the 20th century. Loss of soil organic matter was also an important problem with the wheat–fallow system. Bauer and BlackReference Bauer and Black⁸, as well as othersReference Haas, Evans and Miles⁹, reported declines in organic matter content of up to 60% within 40 years of adopting wheat–fallow compared with native prairie. Natural soil fertility, water-holding capacity and many other soil properties favoring sustained crop production were diminished considerably using the wheat–fallow system compared with uncultivated soil.

The Growth of Conservation Tillage and Intensification of Cropping Systems

The introduction of synthetic herbicides, beginning with 2,4-dichlorophenoxyacetic acid (2,4-D) in the late 1940s, was a boon to those concerned about the deleterious effects of tillage on the soil during the fallow period. Heavy reliance on tillage no longer was necessary for weed control, particularly as more effective herbicides were developed. By the mid-1990s, herbicides had effectively replaced tillage used for weed control in many farming operations across the US Great PlainsReference Lyon, Miller and Wicks¹⁰. Tillage also became unnecessary for seedbed preparation as planting equipment was developed which enabled farmers to establish crops in fields where crop residue was maintained on the soil surfaceReference Triplett and Dick².

An important purpose for the long fallow period was to recharge soil water for a subsequent wheat crop, but the transition to conservation tillage resulted in enhanced soil-water storage because of improvements in soil qualityReference Karlen, Wollenhaupt, Erbach, Berry, Swan, Eash and Jordahl^11, Reference Peterson, Schlegel, Tanaka and Jones¹². Water conservation was greatest under no-tillReference Peterson, Schlegel, Tanaka and Jones¹² and explains the interest in this form of conservation tillage in the US northern Great Plains. As a result, conservation tillage methods, particularly no-till, allowed cropping systems in this semi-arid region to intensify. That is, the long fallow period occurred less frequently between successive crops, or it was eliminated entirely between annual crop phases. This intensification in cropping systems also coincided with growing concerns about the economic efficiency of the wheat–fallow systemReference Lyon and Peterson^13, Reference Ali and Johnson¹⁴. By the late 20th century, the amount of cropland that was fallowed declined throughout the US northern Great PlainsReference Carr, Martin and Poland¹⁵.

Cropping system intensification necessitated diversifying wheat-based cropping systems to avoid the build-up of certain pestsReference Peairs, Bean and Gossen¹⁶, and to optimize conversion of plant-available water to wheat grainReference Carr, Martin and Horsley¹⁷. Research in Montana and North Dakota documented the positive impact of growing legumes, such as field pea (Pisum sativum L.) and chickpea (Cicer arietinum L.), as well as other broadleaf crops, such as flax (Linum usitassimum L.), on subsequent wheat performance compared with annual wheat monocultureReference Carr, Martin and Horsley^17–Reference Miller and Holmes¹⁹. The positive rotation effect was attributed to disease suppression, soil-water recharge and biological N-fixation benefits in the case of legumes.

Cover Crops in Conventional Cropping Systems in the US Northern Great Plains

The benefits that cover crops can provide when incorporated into diversified cropping systems are well documented. Cover crops typically are grown to protect the soil from surface erosion, as well as to provide other benefits including soil improvement, disease suppression and to reduce N-leaching below the root zoneReference Snapp, Swinton, Labarta, Mutch, Black, Leep, Nyiraneza and O'Neil²⁰. TeasdaleReference Teasdale²¹ focused on the role that cover crops have in weed management in his review of the literature, while Cherr et al.Reference Cherr, Scholberg and McSorley²² discussed the use of cover crops as soil-amending green manures. The OZ systems developed in the eastern USA have been very reliant on winter annual cover cropsReference Mirsky, Ryan, Curran, Teasdale, Spargo, Maul, Moyer, Grantham, Weber and Way³.

Organic farmers have used legume cover crops as green manures in place of a cash crop during selective phases of crop rotations in the US northern Great Plains. In contrast, conventional farmers typically have not included cover crops in rotations, although research focused on the use of green manures to replace fallow (also known as green-fallow) was conducted during the 1990s. With the decline in fallow and the intensification of cropping systems in the northern Great Plains, there are few opportunities for growing green-fallow cover crops in conventional cropping systems at present.

Snapp et al.Reference Snapp, Swinton, Labarta, Mutch, Black, Leep, Nyiraneza and O'Neil²⁰ included in their review of the literature a discussion on the cover crop choices that are available to farmers, including a listing of the advantages and disadvantages of various crop species as well as groups (i.e., small-grain crops) when grown for this purpose. The authors noted researchReference Blackshaw, Moyer, Doran, Boswall and Smith²³ demonstrating the positive impact that a sweetclover (Melilotus officinalis L.) cover crop can have on subsequent wheat crop performance in the northern Great Plains. Other researchReference Carr, Poland and Tisor²⁴ demonstrated the positive rotation benefit of preceding wheat with sweetclover in this region. In contrast, wheat yield depression can result occasionally after a sweetclover cover cropReference Badaruddin and Meyer²⁵, presumably because of soil moisture shortages following the biennial legume, which can be a relatively heavy water user in some environmentsReference Badaruddin and Meyer^26–Reference Foster²⁸. The yield depression that can result following sweetclover is most evident when termination is delayed until later in the sweetclover growing seasonReference Foster²⁸. These studies support the observation by Snapp et al.Reference Snapp, Swinton, Labarta, Mutch, Black, Leep, Nyiraneza and O'Neil²⁰ that complex water relations govern successful incorporation of sweetclover and other cover crops in rotations with wheat and other cash crops in the northern Great Plains.

Townley-Smith et al.Reference Townley-Smith, Slinkard, Bailey, Biederbeck and Rice²⁹ echoed the concern of othersReference Tanaka, Bauer and Black³⁰ in the US northern Great Plains that legume cover crops, when green manured, were capable of depleting stored soil water to such an extent that grain yield of subsequent crops could be depressed. Of several legume species that were considered for cover crop use in the region, field pea was identified as the most promisingReference Townley-Smith, Slinkard, Bailey, Biederbeck and Rice²⁹. Field pea produced more cover crop dry matter than four other legume species [first-year alfalfa (Medicago sativa L.), fababean (Vicia faba L.), lentil (Lens culinaris Medik.) and Tangier flatpea (Lathyrus tingitanus L.)] and, along with fababean, fixed more N biologically than the other legume species in the seeding year. Likewise, other studiesReference Badaruddin and Meyer^26, Reference Walley, Clayton, Miller, Carr and Lafond³¹ identified field pea and fababean as being a superior choice for a green manure and cover crop compared with other annual legumes in the northern Great Plains.

Several species have been evaluated as potential cover crops in the US northern Great Plains. PowerReference Power²⁷ concluded that fababean and field pea were the best cover crop replacements for fallow in a wheat–fallow system of the 10 legume species that were considered, including sweetclover. This researcher noted that the two pulse crops produced more above-ground biomass than other cover crop species when seeded in early May and harvested in late June, averaging from 1550 kg (fababean) to 3940 kg (field pea) of dry matter per hectare. Total N accumulation in above-ground biomass was also greater for fababean and field pea than other legume cover crops when terminated in June. These two legume species, along with lentil, are superior biological N-fixers compared with most other annual pulse crops grown in the northern Great PlainsReference Walley, Clayton, Miller, Carr and Lafond³¹.

Tangier flatpea and field pea were considered as cover crop replacements for fallow in rotation with wheat in a 4-year study in central North DakotaReference Tanaka, Bauer and Black³⁰. The researchers concluded that both species produced sufficient dry matter to protect the soil surface from erosion when terminated at or before flowering, and that wheat grain yield following legume cover crops was not depressed compared with the fallow control. Delaying cover crop termination until pod formation or later sometimes reduced subsequent wheat grain yield because of insufficient recharge of soil water prior to planting the wheat crop. Similarly, wheat yield following a lentil cover crop was depressed by 25% compared with the fallow control when cover crop termination was delayed until full bloom or more advanced growth stagesReference Pikul, Aase and Cochran³². However, Pikul et al.Reference Pikul, Aase and Cochran³² concluded that a lack of available N following the legume cover crop explained the negative rotation effect on wheat grain yield rather than a depletion of stored soil water. Studies conducted by other researchersReference Miller, Gan, McConkey and McDonald³³ in the northern Great Plains support the contention that N deficiencies, rather than stored soil-water deficits, can explain grain depression when wheat follows legume crop species in some low-N environments when no supplemental N is provided. These studiesReference Pikul, Aase and Cochran^32, Reference Miller, Gan, McConkey and McDonald³³ demonstrate that biological N-fixation by legume cover crops may be insufficient to supply N needs of subsequent cash crops under some conditions in the US northern Great Plains.

Organic Farming Research in the US Northern Great Plains

Research on organic farming methods in the US northern Great Plains was initiated in the mid- to late 1980s after the widespread adoption of synthetic fertilizers and pesticides. Small-grain cultivar studies were completed under organic managementReference Kandel, Porter and Carr^34–Reference Carr, Kandel, Porter, Horsley and Zwinger³⁶. More recently, StofferahnReference Stofferahn³⁷ summarized the environmental–ethical and other factors that explained the motivation of North Dakota farmers to grow crops organically. However, results of these research efforts fail to provide insight on OZ methods being considered for adoption in the region.

Recognition of the soil quality benefits that result when conservation tillage practices are adopted by conventional farmers who rely on synthetic herbicides for weed control has resulted in criticism directed at the frequent use of tillage in organic systemsReference Trewevas³⁸. However, potentially mineralizable N was higher in a pea–wheat–lentil–barley (Hordeum vulgare L.) cropping system transitioning to certified organic management compared with various cropping systems transitioning to conventional zero-till management in MontanaReference Miller, Buschena, Jones and Holmes³⁹. The 4-year organic rotation included a winter pea cover crop that was green manured prior to seeding winter wheat, whereas cover crops were not included in the conventional zero-till systems. Miller et al.Reference Miller, Buschena, Jones and Holmes³⁹ established the cover crop phase by sowing pea seed in the fall directly into barley stubble without any pre-plant tillage, and avoided tillage after wheat, lentil or barley grain was harvested, unless absolutely necessary, to control weeds in the organic system.

Grain yield of winter wheat following winter pea cover crop in an organic system was equal or superior to wheat yield levels in a conventional zero-till system that was diversified, did not include cover crops and received up to 157 kg ha⁻¹ of fertilizer NReference Miller, Buschena, Jones and Holmes³⁹, depending on the year. The relatively high grain yields in the organic system were attributed to slower wheat growth and, conversely, more efficient uptake and conversion of soil-stored water by plants in organic plots as the growing season progressed. However, soil nitrate-N, phosphorus (P) and potassium (K) content were lower under organic management. These results demonstrated that legume cover crops may not be sufficient to supply the N needs of many cash crops in the US northern Great Plains, leading Miller et al.Reference Miller, Buschena, Jones and Holmes³⁹ to suggest that manure from livestock might be needed on organic farms to maintain soil fertility at a level that supports sustained crop production over the long term. Entz et al.Reference Entz, Guilford and Gulden⁴⁰ reported that soil nitrate-N, along with K and sulfur (S) levels, generally were sufficient on 14 organic farms, including one in North Dakota. However, the Canadian researchers noted that soil P levels were deficient for sustained crop yields on a majority of the farms. Russelle et al.Reference Russelle, Entz and Franzluebbers⁴¹, as well as Krall and SchumanReference Krall and Schuman⁴², argued that livestock offer many ecosystem services when integrated into cropping systems on farms, including soil fertility improvements.

Liebig and DoranReference Liebig and Doran⁴³ found that organic carbon (C) and total N levels tended to be elevated in organic fields compared with conventional fields where some tillage is used in the US northern Great Plains. These two scientists speculated that the higher organic C and total N levels in fields farmed organically may have been ‘… due to less frequent tillage and inclusion of cover crops in the crop sequence [on the organic farm].’ The surprising observation that tillage was more common under conventional management was explained by the inclusion of forages (e.g., alfalfa) in crop rotations on the organic farm, and the absence of forages in crop rotations on the conventional farms. These results suggest that increasing crop rotation diversity, using forage and cover crops, and reducing tillage in organic systems is an important goal for improving soil quality in organic cropping systems in the northern Great Plains.

Thirty-two farmers, roughly half of whom were farming organically, were surveyed about their farming practices and contrasted with conventional farming practices that were used in the late 1980s in South DakotaReference Taylor, Dobbs and Smolik⁴⁴. Results of the survey and follow-up interviews revealed that the 32 farmers used more diverse crop rotations, which oftentimes included cover crops used as green manures, compared with conventional farmers. Sweetclover and, to a lesser extent, ‘forage sudan’ [i.e., presumably sudangrass, Sorghum sudanense (Piper) Stapf.], were the most common cover crop species, grown at the time as an alternative to traditional fallow. Although conventional farmers on panels questioned the use of such ‘low value’ crops in cropping systems, organic and other non-conventional farmers felt that soil fertility and pest control benefits provided by cover crops and crop rotation diversity offset any immediate economic concerns when they were grown.

Menalled et al.Reference Menalled, Jones, Buschena and Miller⁴⁵ considered the transition from conventional to organic farming management in the northern Great Plains. The researchers acknowledged the importance of legume cover crops grown as green manures as a source of N when followed by cash crops in rotations. Use of cover crops and vegetative mulches for weed suppression also was noted as a way to prevent sunlight from reaching weeds and weed seeds on the soil surface and, in some instances, as sources of allelopathic substances. No mention was made of OZ in the publication, although Menalled et al.Reference Menalled, Jones, Buschena and Miller⁴⁵ did caution readers to be careful when relying on tillage for weed control as it can lead to increased risk of soil erosion.

Organic Zero-till Research in the Northern Great Plains

Much of the recent published literature on conservation tillage in the context of organic farming was generated from studies in the eastern USA or in Canada within North America, and is referenced in other papers in this special issueReference Mirsky, Ryan, Curran, Teasdale, Spargo, Maul, Moyer, Grantham, Weber and Way^3, Reference Shirtliffe and Johnson^4, Reference Reberg-Horton, Grossman, Kornecki, Meijer, Price, Place and Webster⁴⁶. Results of recent studies on the use of cover crops in organic management systems within the US northern Great Plains have been publishedReference Miller, Buschena, Jones and Holmes^39, Reference Miller, Lighthiser, Jones, Holmes, Rick and Wraith^47, Reference Rick, Jones, Engel and Miller⁴⁸, but none of this research considered the use of cover crops in an OZ system. Several studies on the potential of OZ in the US northern Great Plains have been completed recently, though results have not been published. A brief summary of these studies follows, as well as discussion of some other strategies for reducing or eliminating tillage in organic farming systems in the region.

Organic zero-till in North Dakota

Organic zero-till research in North Dakota has focused on identifying cover crop species and cultivars which provide adequate vegetative mulch when killed to suppress subsequent weed growth. A 3-year study at Dickinson (46.8792 lat., −102.7892 long., 735 m elev.) on loam soils compared dry matter production by weeds following five cover crop treatments terminated (i) mechanically without tillage using a blade roller, (ii) with non-inversion tillage using a Versatile 5000 blade plow⁵² and (iii) with inversion tillage using a light tandem disk. The cover crop treatments were fall-seeded (late-September to mid-October, depending on the year) and included hairy vetch, winter rye (Secale cereale L.), winter wheat and combinations of each small-grain crop with hairy vetch, with cereal and legume components seeded at sole-crop rates in the mixtures. Above-ground biomass production of cover crops and weeds was determined from a 0.5-m² area in each plot just prior to imposing termination methods, which ranged from June 22 to July 2, depending on cover crop, method and growing conditions that year. Results demonstrated that winter rye produced equal or greater amounts of above-ground biomass compared with other cover crop treatments (Table 1). Weed production roughly 75 days after cover crops were killed ranged from 75 to 640 kg ha⁻¹ in rye plots, whereas weed dry matter ranged from 584 to 2436 kg ha⁻¹ in hairy vetch plots, depending on the year.

Table 1. Impact of cover crop termination method and species or species mixture on dry matter production of cover crops (CC) and weeds in 3 years at Dickinson, North Dakota, USA.

¹ Cover crop treatments were established in the fall prior to the year in which data were collected.

² Termination methods were applied at the same vegetative growth stage in 2007–2008 , and so aboveground dry matter production was determined for subplot treatments (cover crop species) but not whole plot treatments (termination method); disking was done prior to rolling in both 2008–2009 and 2009–2010 growing seasons, and so impact of whole plot and subplot treatments were determined.

³ Different letters after numbers indicate that differences in treatment means were detected at P<0.05.

⁴ Cereal and legume components each were seeded at their sole crop rate in polycultures.

Termination of cover crops should be delayed until cover crops reach advanced growth stages when the blade roller is used. Otherwise, small-grain and legume plants may not be killed effectivelyReference Mirsky, Curran, Mortensen, Ryan and Shumway^53, Reference Mischler, Dulker, Curran and Wilson⁵⁴. In the study at Dickinson, small-grain cover crops were not killed effectively unless blade rolling was delayed until the early milk growth stage of kernel development (Zadoks growth stage 73)Reference Zadoks, Chang and Konzak⁵⁵, and hairy vetch at the flat pod growth stage (BBCH growth stage 70)Reference Lancashire, Bleiholder, Van Den Boom, Langeluddeke, Stauss, Weber and Witzenberger⁵¹. Delaying blade rolling compared with disking resulted in greater amounts of weed-suppressive vegetative mulch being produced in the study at Dickinson (Table 1). However, delaying killing of cover crops also allowed further growth of weeds that were established before termination methods were imposed, sometimes resulting in greater amounts of dry matter production by weeds following blade rolling compared with disking, as was the case in 2010. Delaying cover crop termination also prevented subsequent cash crops [buckwheat (Fagopyrum esculentum Moench), corn (Zea mays L.) and pinto bean (Phaseolus vulgaris L.)] from producing grain because too few heat units remained before fall freeze-up (data not presented). In contrast, disking needed to be done earlier than blade rolling or vegetative growth was excessive and interfered with effective kill of cover crop plants by inversion tillage methods (data not presented). Killing of cover crops by disking, if done early in the growing season, would allow subsequent crops to produce grain within one growing season.

A 3-year study at Carrington (46.4497 lat., −99.1261 long., 484 m elev.) on loams soils compared ten winter rye cultivars for dry matter production just prior to rolling with a blade roller in fields with soil nitrate levels at seeding ranging from 48 to 73 kg ha⁻¹. The rye treatments were planted at 247 live kernels m⁻² on September 18–28, depending on the year. Rye was rolled at early anthesis (Zadoks growth stages 60–62)Reference Zadoks, Chang and Konzak⁵⁵ on June 14–23, depending on the year and cultivar treatment. The cultivars Boreal and Dacold both produced roughly 6000 kg ha⁻¹ in 2010, compared with less than 5000 kg ha⁻¹ for the other cultivars that were grown (Table 2). Dry matter production differences were not detected among a subset of the ten cultivar treatments in 2008 and 2009, which included Dacold. Results also demonstrated the inability of blade rolling to kill a winter rye cover crop consistently when termination was attempted at early anthesis. These results demonstrated that cultivar selection can affect dry matter production by cover crop species in some environments, but other factors confound cultivar recommendations when growing winter rye for production of weed-suppressive vegetative mulch across environments.

Table 2. Effectiveness of blade rolling on termination of winter rye cultivars and dry matter (DM) production when rolling–crimping over a 3-year period at Carrington, North Dakota, USA.

¹ Rating scale was from 1 to 10 where 1=no plants killed and 10=all plants killed with ratings assigned approximately 30 days after blade rolling.

² Different letters after numbers indicate that differences in treatment means were detected at P<0.05.

³ Experimental cultivars being considered but not available for commercial use.

Recommendations for OZ Research in the US Northern Great Plains

Preliminary results of OZ studies in the northern Great Plains indicate that cover crops can be killed mechanically without tillage if termination methods are delayed until crops reach advanced growth stages in late spring or early summer. However, this delays the planting of subsequent cash crops and prevents maturation of annual crops before the onset of the first killing frost in the fall. Alternative strategies must be developed so that cash crops can be grown successfully following termination of cover crop growth. This could involve selecting cover crop species that can be terminated earlier, or adjusting crop rotations to include winter annual cash crops that are planted in the fall after cover crop termination and harvested during the following growing season.

Researchers in Canada have had success in killing spring-planted annual cover crops in late summer or fall and then seeding a cash crop the following springReference Vaisman, Entz, Flaten and Gulden⁴⁹. This strategy should be explored within the US northern Great Plains and should include the screening of cover crop species adapted to OZ systems that encompass multiple years and growing seasons. Tillage implements that kill cover crops effectively but with minimal burial of vegetative mulch, such as the blade plow⁵², should be investigated as tools to control weeds between crop plantings in OZ farming systems.

Weed control will continue to be the greatest obstacle for successful adoption of OZ methods among organic farmers. Perennials pose a particular challenge since it is unlikely that any amount of crop residue mulch produced by killed cover crops under actual field conditions could be thick enough to suppress weed species like Canada thistle [Circium arvense (L.) Scop.] and field bindweed (Convolvulus arvensis L.). Use of forages in rotations with cash grain crops for the purpose of weed control is a widely documented practice in the Great PlainsReference Entz, Bullied and Katepa-Mupondwa⁵⁸, and establishment of alfalfa pasture has been used effectively to control Canada thistle in the regionReference Entz, Baron, Carr, Meyer, Smith and McCaughey⁵⁹. However, perennial forages pose the same challenges as perennial weeds to organic farmers wanting to convert perennial stands back to crop land without tillage. Livestock grazing in forage pasture, on crop aftermath and weeds after cash grain crop harvest, or at other selective times can also be used to control perennial weeds and, conceptually, could be used to weaken perennial forages so they are prone to winterkill in the northern Great Plains. The ability of livestock to kill perennial forage pasture consistently through targeted grazing requires greater attention in the region and, if successful, could be an important part of an integrated strategy for controlling weeds in OZ farming systems.

Synthetic herbicides are an important weed control tool in conventional systems, but are not allowed in organic farming systems. Although many organic farmers oppose the use of herbicides for controlling weeds philosophically, some bio-herbicides are comprised of substances that are allowed for use in certified organic systems. These bio-herbicides, including an experimental compound reportedly with systemic activity but not yet commercialized, are being field-tested in North Dakota (data not presented) and may offer organic farmers who are not opposed to the infrequent use of bio-herbicides an additional weed control tool in OZ systems. Future research should focus on integrating cover crops with forages, grazing, the judicious use of high-residue tillage equipment and approved bio-herbicides in developing a multi-faceted weed control program for use in OZ farming systems in the US northern Great Plains.