Pest pressures, especially from soilborne pathogens and weeds, typically are highest when same or similar crop is grown continuously or near-continuously in the same field compared with a 3- and 4-year crop rotation. With conventional tillage, 2-year and 3-yr crop rotations may be long enough to manage pests, but with direct seeding, the rotation needs to be longer to provide the same level of control. This presents a major challenge, since the trend in much of the United States and especially in the Inland Northwest has been to use the same or shorter rather than longer crop rotations when shifting from conventional to direct seeding. ACereal-based rotations@ is used in this paper to mean rotation where cereals make up 50, 75 or even 100% of the cropping system.
There are many reasons for planting wheat and barley as the main cash crop in the same fields every year, every other year, or two years out of every three. These reasons include the economic advantages of specialization, the flexibility to recrop wheat when wheat prices are high, the difficulty of creating additional marketing infrastructures for new commodities, and, in the case of the Inland Northwest, accessibility to ports for export of commodities such as wheat or barley.
Furthermore, the dryland Inland Northwest is less suited that any other wheat- or barley-growing area in the United States to warm-season crops, thereby limiting the options for alternate crops mainly to brassicas (canola, rape, yellow mustard), pulses (peas, lentils and chickpeas), and grass seed (Kentucky bluegrass and ryegrass). Even areas suited to both warm- and cool-season crops, such as the North Central states, inevitably have some portion of the land replanted to small grains each year.
Potential for Direct-Seeded, Cereal-Based Cropping Systems in the Northwest
Growers such as Mort Swanson near Palouse, Wa, and ARS scientists such as Roland Schirman and Robert Papendick at Pullman began to experiment with direct-seeding in eastern Washington in the very early 1970s, yet 25 years later only about 6% of the cereal crops and 5% of all other crops are direct seeded in this region. Based on the trends world-wide, it seems clear that growers in the Inland Northwest must implement direct-seeding systems more rapidly just to remain competitive in the international markets. Wheat and barley yields are limited in this area by available water rather than the growing season or sunshine, and therefore any practice such as direct seeding that makes more water available for the crop will also increase the yield potential for that crop. Actual yields may not increase, initially, because of problems that still must be solved, but both theoretical and experimental measurements show that the yield potential for dryland wheat and barley is greater in this area with direct seeding than with conventional tillage.
Indeed, because of the exceptionally high yield potential for dryland wheat and barley in the Inland Northwest, owing to the favorable climate and deep soils, it is likely that Inland Northwest growers who perfect direct-seed, cereal-based cropping systems will be able to produce these crops as competitively or more competitively that any other area in the United States if not world-wide.
Inland Northwest growers attempting to direct-seed wheat or barley at a high frequency in the same fields must have methods to manage 1) diseases, especially those caused by soilborne pathogens, 2) weeds, especially grass weeds, 3) insect pests, especially those that survive in crop refuse, and 4) a dense layer of relatively fresh (undecomposed) straw and standing stubble, which makes planting difficult. With some of the highest yields for dryland wheat in the world, Inland Northwest growers must contend with more straw than other wheat-growing areas, except possibly northern Europe. Even one disease, weed, or insect pest problem, if severe, or the inability to handle the crop residue left in the field, can result in the decision by the grower to burn, invest more in herbicides, resort to tillage, or settle for a lower yield.
Some good news for the Inland Northwest is that leaf blights such as tan spot, helminthosporium net blotch and spot blotch, septoria leaf and glume blotches, and barley scald are virtually nonexistent problems in this area, because of the dry weather during the warmer months. Regions such as northern Europe, southern Brazil, and the wetter parts of the Great Plains must contend with severe pressures from these diseases as well as the root diseases when cereals are direct-seeded every year or every other year in the same fields.
Options for cereal-based Rotations in the Inland Northwest
The diversity of agronomic zones in the Inland Northwest makes it possible to produce club, common soft and hard white, and hard red wheats, triticale, and both feed and malting barley. Rotations among different kinds of small grain crops can provide options for pest and crop residue management as well as economic diversification not available with strict monoculture of the same kind of small grain crop. More importantly, this region is among the few wheat- and barley-growing areas of the world that is suited almost equally to both spring and winter cereals. Rotations between spring and winter cereals opens many options for control of grass weeds and soilborne pathogens of winter cereals. Likewise with brassica and pulse crops, the region is suited to several kinds of each of these crops and to both spring and winter types. This gives a wide range of options for cereal-based cropping systems.
There are four kinds of cereal-based rotations available to growers in the Inland Northwest. Keep in mind that Adirect seeding@ must include all the crops grown in the rotation and not just the wheat and barley. Also keep in mind that these rotations are based entirely on cool-season crops; other rotations are possible in those parts of the region suited to certain warm season crops, and research is in progress to find or develop other suitable crops. The cereal-based rotations with cool-season crops are (Fig. 1):
$ Flexible 4-year rotations of back-to-back cereals (e.g., winter wheat followed by either spring wheat or spring barley) and back-to-back broadleaf crops, (e.g., winter or spring pulse crop followed by a winter or spring brassica crop or vice versa), or a 4-year cycle of spring or winter cereal, broadleaf crop, spring cereal, broadleaf crop;
$ Optimal 3-year rotations of winter wheat, spring wheat or barley, and a pulse crop, brassica, or fallow, depending on the rainfall of the area;
$ Traditional 2-year rotations of winter alternated with a pulse or brassica crop in the intermediate and high precipitation areas and with fallow, including chem fallow, in the intermediate and low-precipitation areas; and
$ Continuous cereals, including one year of winter wheat followed by two or more years of either spring wheat or spring barley or continuous spring cereals but not continuous winter wheat.
Any one of these four cereal-based rotations, in turn, can be implemented after Kentucky bluegrass, alfalfa hay, pasture, CRP, or other perennial crop. Perennial grass crops should be considered like any other potential green bridge--a source of infection by pathogens with ability to live on the roots of grasses. In general, it works best to follow a perennial grass such as Kentucky bluegrass with a broadleaf crop, such as lentils, and a perennial broadleaf crop, such as alfalfa, with wheat or barley, thereafter continuing with the normal rotation cycle.
A key factor with all but the 2-year rotations is that winter wheat is grown no more than once in three years. This is critical for control of cephalosporium stripe, snow molds, and psuedocercosporella foot rot, and it helps for control of fusarium root and crown rot, cheat grass and jointed goat grass. The risks of snow molds and pseudocercosporells foot rot can be managed with the resistant varieties now available for these diseases, and resistance many soon be available for cephalosporium stripe, bit the grass weeds are still a problem in 2-year rotations, especially when direct seeding is used. Fusarium root and crown rot caused by F. graminearum has been most damaging on winter wheat seeded early into conventional fallow in the low- and intermediate-precipitation areas and may actually subside in importance with the shift to direct seeding, because of later seeding (or later emergence) and less chance for predisposing drought stress on the host plant in direct-seeded soils.
The Mediterranean climate in this region, where most precipitation comes in the fall, winter, and spring and summers are dry, makes it necessary in the wheat-fallow areas to maintain a dust mulch during the summer months to hold soil moisture. Experience and research have shown that without the dust mulch, by the time of planting, the soil will have dried out below the depth of seed placement. Planting earlier, while seed-zone moisture is still adequate, exposes the crop to greater pressure from cheat grass and foot rots. Planting later, after the rains have resumed, forfeits a significant part of the growing season for the winter wheat. Major research and extension efforts are under way, some of which will be reported at this conference, on replacement of the winter wheat/fallow system with continuous direct-seeded spring crops. The flexible 4-year, optimal 3-year and continuous spring cereals rotations each are under investigation to reduce or eliminate dependence of the dryland Inland Northwest of fallow.
Root diseases, namely take-all, rhizoctonia root rot, and pythium root rot, become major limiting factors to the growth and yield of cereals planted directly into standing stubble of cereals. In the four kinds of rotations listed above, root diseases will be potentially important each year in the continuous cereals and in the years that spring barley or spring wheat are planted directly into winter wheat stubble in the 3-year and 4-year rotations. On the other hand, the 6-month period between harvest and planting, as when spring cereals are planted, means less risk from these diseases than a 2-month period between harvest and planting, as when a winter cereal is planted. The risk for spring cereals results mainly from the volunteer and grass weeds (green bridge); left unmanaged in the stubble, these host plants of the root pathogens can negate the benefit of a 6-month break between harvest planting. The many options available for reducing the risks of root diseases of cereals seeded directly into cereal stubble are discussed in more detail in the next section.
Spring barley is especially well-suited to direct seeding into cereal stubble, either its own stubble or wheat stubble. Rhizoctonia root rot is the only serious disease problem encountered for this crop species so far, and the tools are now becoming available to manage this disease (see below).
The 4-year rotations are Aflexible@ both in the sequence of the four crops grown and the use of winter versus spring types of the crops. A complete break from cereals for 2 years, as achieved with back-to-back broadleaf crops (e.g., a pulse followed by a brassica crop, or vice versa), is necessary to free the soil of the majority of inoculum of the important soilborne pathogens of cereals. Again, the high-risk crop in this system is the spring cereal grown back-to-back with the winter wheat. Likewise, the second broadleaf crop could be vulnerable to white mold caused by Sclerotinia sclerotiorum, particularly in the higher rainfall areas.
The flexibility of growing either winter or spring types of these crops allows additional opportunities to control weeds, distributes the work load between fall and spring, and provides the option of either fall or spring planting, depending on when soil conditions are most suitable for direct seeding.
The Potential for Continuous Cereals in the Inland Northwest
Direct-seeded cropping systems that use 3- and 4-year rotations should be used to the maximum extent possible, because of the pest control benefits. However, there are also opportunities in the Inland Northwest for success with continuous cereals, including continuous wheat, either continuous spring wheat, or winter wheat/spring wheat rotations. Growers such as John Rea at Touchet, Wa, and Frank Mader near Echo, OR have shown that annual spring wheat is not only technically possible, but economically competitive with a 2-year winter wheat/fallow rotation in areas with only 8-10 inches of precipitation per year. It is important to remember that wheat and barley evolved over millennia under conditions that required them to reseed themselves in the same soil year after year without tillage.
At Pullman, a 1-acre research plot was direct seeded for the 17th consecutive year in the fall of 1997, during which time the plot has been planted to wheat 14 times, chem-fallowed once (1987), planted to spring barley once (1993), and planted to spring peas once (1994). The yields were progressively more suppressed and weeds became unmanageable by the 5th consecutive direct-seeded wheat crop (1986), hence the site was chem-fallowed in 1987. The 1988 winter wheat crop after this 1 year of chem fallow averaged 130 bu/A. The site was planted to spring wheat (Penawawa) in each of the next four years, with the volunteer and grass weeds sprayed at least two weeks before planting all fertilizer shanked below the seed depth at the time of planting. The yields of spring wheat averaged 63 bu/A in 1989, 74 bu/A in 1990, 64 bu/A in 1991, and 65 bu/A in 1992. The 1993 spring barley (Steptoe) averaged slightly greater than 5,000 lbs/A. The peas were planted in the spring of 1994 but yields were not measured. Thereafter, the 1995 spring wheat (Penawawa) averaged 105 bu/A, the 1996 winter wheat (Madsen) averaged 101 bu/A, and the 1997 spring wheat (Penawawa) averaged 72 bu/A.
Soils cropped continuously to wheat or barley eventually convert from conducive to suppressive take-all, including when the wheat or barley is direct seeded. The Pullman plot described above, for example, has become highly suppressive to take-all, but is still conducive to rhizoctonia and pythium root rots. The suppressiveness to take-all is due to the build-up of naturally-occurring root-associated bacteria (rhizobacteria) with ability to produce the antibiotic 2,4-diacetylphloroglucinol inhibitory to the wheat take-all fungus in the rhizoshere of wheat. In the Inland Northwest, this process takes 12-15 years of cereals, although there is now evidence that it can be augmented by the introduction of certain strains of these antibiotic-producing bacteria on the seeds of wheat at planting. Initially, a soil suppressive to take-all may then favor greater damage from rhizoctonia root rot, but research carried out in Australia and by R.W. Smiley at the OSU station at Pedleton suggest that soils cropped intensively to cereals with direct seeding also eventually convert from conducive to suppressive to this disease.
Soils that have become suppressive to take-all with continuous cropping to cereals may become conducive again when planted to crops such as canola, oats, alfalfa, or toher nonhost crops. Field observations suggest that the suppressiveness will return relatively more quickly during a second or subsequent sequence of continuous wheat following one or more years of a break crop, but the second wheat crop after the break can again be damaged by this disease. For this reason, and because of the potential for success with continuous cereals, including in the higher-precipitation areas, consideration shoul be given to didicating some fields to continuous direct-seeded cereals, specifically winter wheat/spring cereal rotations, and the rest of the farm to 3-year or 4-year rotations, rather than switching bacj and forth between these very different strategies for management of root diseases. Dedicating some fieldsto continuous cereals and the rest to a 3-year optimal or 4-year flexible rotation would allow for the advantage o both the crop rotation effect and the long-term cereal monoculture effect, without risk of the setbacks that can occur by switching between these two cropping systems.
Reducing the Risks of Root Diseases with Continuous Direct-Seeded Cereals
The following practices have been shown to reduce either the incidence or the yield-depressing effectsof root diseases that can be expected with continuous direct-seeded cereals.
Some of these practices, such as timely management of the green bridge, treatment of seeds, and precision-placement of fertilizerat planting, are good agronomic practices regardless of the cropping system. However, most of these practices become redundant when root diseases are adequately managed by crop rotation.
Future Prospects for Management of Soilborne Plant Pathogens with Direct Seeding in Cereal-based Rotations
Major progress has been made during the past 10-15 years in the identification and implementation of cultural practices and sepecially methods of planting for management or soilborne plant pathogens of wheat and barley with direct seeding. These developments followed the equally important advances in diagnosis of the important root diseases favoredby direct seeding, namely take-all, pythium root rot, and, most recently, rhizoctonia root rot. The demonstration that soils convert from conducive to suppressive to take-all with long-term cereal monoculture, including with direct seeding, and the discovery of the microbiological basis for this suppressive effect represent still another scientific advance on the road to successful direct-seeded cereal-based cropping systems. Still another breakthrough has beenthe discovery of a source of resistance n a wild relative of wheat to both take-all and rhizoctonia root rot, and in barley to rhizoctonia root rot.
Atleast two new biologically-based technologies are on the horizon for management of soilborne pathogens with direct seeding in cereal based rotations. One is the availability of resistance genes, which must now be transferred into agronomically acceptable varieties. The availabilty of wheat and barley varieties with resistance to take-all and rhizoctonia root rot would greatly reduce the risks of direct-seeding wheat or barley intocereal stubble in the same way that Madsen and Hyak with resistance to pseudocercosporella foot rot greatly reduced the risks of early seeding winter wheat n the traditional 2-year rotations. The other new biologically-based technology is the availability of unique strains of rhizobacteria with ability, when introduced with the seeds of wheat, to suppress take-all and possibly also rhizoctonia and pythium root rots. Similar work by Ann Kennedy is in progress on use of the plant-deleterious rhizobacteria to suppress the growth and competitive ability of cheat grass and goat grass. These projects collectively present major opportunities, including though use of the new tools of biotechnology, for helping wheat barley acheive the high yields possible inthis region with direct seeding.
The research on development of warm-season crops or additional cool-season cropsfor use in cereal-based rotations is critical. These crops,in turn, will be vulnerable to their own diseases that must be diagnosed and methods developed for their control. In this respect, every precaution should be taken when introducing new crops into this region to assure, as a minimum, that pathogens of these crops are not introduced with the seed.
Equally or most importantly, we need a greater effort in fundamental studies to provide the knowledge base so critical to the sustainability of direct-seeded cropping systems. Fundamental research, in turn, opens new direction for applied research. Understanding plant ecology, soil biology, and plant-soil-microbe interactions in direct-seeded cropping systems represents one of the major frontiers in science today and deserve much greater study as a sourse of clues to sustainable soil, pest, and crop management in direct-seeded cropping systems.