*Corresponding author: krupinsj@mandan.ars.usda.gov, (701) 667-3011

The semi-arid environment of the Northern Great Plains has placed greater emphasis on the retention of crop residues on the soil surface, thereby increasing water storage and minimizing soil erosion (Peterson et al., 1996). Improved methods of soil water storage have led to the development of more intensive cropping systems than the alternate crop-fallow system (Greb, 1983; Tanaka and Anderson, 1997). With the adoption of conservation or reduced tillage systems, annual cropping, which includes alternative crops such as oilseeds, pulses, and forages, has become a viable option for producers. Proper sequencing of crops is recognized as an important component of cropping systems (Leighty, 1938; Pierce and Rice, 1988).

The influence of the previous crop and crop residues on crop production, plant diseases, weeds, residue persistence for erosion control, and soil quality need to be more fully understood in order to develop effective crop sequences for diverse cropping systems. A project was established in 1998 to determine the benefits and/or disadvantages of previous crop and crop residues in diverse cropping systems, the first short-term step for the development of long-term cropping systems. A multi-disciplinary team of scientists conducted a research project to evaluate the components of crop seed production, crop residue production, plant diseases, weeds, insects, economics, root growth, crop-water use, and soil quality, to develop guidelines for long-term diversified crop production systems and to provide producers with management flexibility for developing their own cropping systems. The objectives of this paper are to present an overview of the influence of crop sequencing of ten crops on crop seed production and plant diseases, and to introduce a computer program used to transfer research technology to producers.

A crop sequence research project was conducted at the Area IV Soil Conservation Districts/Agricultural Research Service Research Farm near the Northern Great Plains Research Laboratory, southwest of Mandan, North Dakota, USA, on a Wilton silt loam soil. A crop by crop residue matrix was formed so that ten crops could be seeded into the crop residue of the same ten crops (Figure 1). During the first year, ten crops (barley [Hordeum vulgare], bean [Phaseolus vulgaris], canola [Brassica napus], crambe [Crambe abyssinica], flax [Linum usitatissimum], pea [Pisum sativum], safflower [Carthamus tinctorius], soybean [Glycine max], sunflower [Helianthus annuus], and wheat [Triticum aestivum]) were no-till seeded in strips (30 ft, 9 m wide) into a uniform cereal residue. The ten cultivars were: ‘Montola 2000’ safflower, ‘Stander’ barley, ‘Dynamite’ canola, ‘Meyer’ crambe, ‘Shadow’ Black Turtle dry bean, ‘Profi’ dry pea, ‘Omega’ flax, ‘Jim’ soybean, ‘Cenex 803’ oilseed sunflower, and ‘Amidon’ spring wheat. During the second year, the same crops were no-till seeded perpendicular over the residue of the previous year’s crop. This established a 10 X 10 matrix with 100 treatment combinations, where each crop was grown on ten crop residues (Figure 1). The crop by crop residue matrix was present in the field for two consecutive years, 1999 and 2000. A uniform spring wheat crop was grown over the crop by crop residue matrix in 2000 and 2001. Growing season precipitation (May through August) was 197% in 1999, 112% in 2000, and 145% in 2001 of the long-term average (10 in, 25 cm). Seed yield was determined by harvesting (123 ft², 11.4 m²) with a plot combine. Spring wheat and barley were evaluated for leaf spot diseases. The total percentage of necrosis and chlorosis was visually assessed for individual leaves and used as an indicator of the severity of disease. Safflower was rated for Sclerotinia head blight (white mold; Sclerotinia sclerotiorum) incidence using the presence of sclerotia under the necrotic head to confirm the disease. Canola and crambe plants with white (bleached) stems, were rated positive for Sclerotinia.

Crop Sequence Calculator^z. The Crop Sequence Calculator (version 2) is a user-friendly program that runs directly from a CD-ROM eliminating the need for additional disk space or installation procedures. The program provides an introduction to the crop sequence research project and dynamic agricultural systems and contains information on crop production, economics, plant diseases, weeds, insects, water use, and surface soil properties to aid producers in their evaluation of management risks associated with different crop sequences. Once the previous crop (residue producing crop) and the expected crop are entered with a click of the mouse, summary statements appear regarding crop production, economics, plant diseases, weeds, insects, water use, and surface soil properties. The program can show the yield effect of ten crops (barley, canola, crambe, dry pea, dry bean, flax, safflower, soybean, sunflower, and spring wheat) grown in any two-year combination. Expected crop prices and expected loan deficiency payments and/or crop premiums can be entered to provide rapid calculations of potential returns. By selecting the "More Info" buttons adjacent to each summary statement, graphs, photos, internet resources and additional information is easily accessed. For example, additional information concerning plant diseases includes an introduction to plant diseases, research data, internet resources, and photographs of plant diseases to aid in their identification. The program also includes numerous photographs of weeds and insects to aid in identification.

Crop Seed Production. Previous crop and crop residue significantly influenced crop seed yields (Crop Sequence Calculator, v.2). The previous crop significantly influenced flax and safflower in 1999 and 2000 (Figure 2). The previous crop residue also influenced seed yield for canola, sunflower, wheat, and barley in 2000. Thus, crop seed yields most influenced by previous crop and crop residues were flax and safflower, followed by canola, sunflower, wheat, and barley. Crop seed production least influenced by previous crop and crop residues were crambe, dry bean, and dry pea. In general seed yield was usually the lowest when the crop was grown on its own residue. In contrast, one anomaly was soybean grown on soybean residue, which produced a higher seed yield than the other treatments.

The spring wheat crop grown on the matrix in 2000 and 2001 was influenced by the various crop sequences (Crop Sequence Calculator, v.2). For example, spring wheat grown on spring wheat residue (ten treatments, four replicates per yr) produced a two-yr average of 45 bu/a. Spring wheat grown after other crops (ten treatments, 1 yr out of wheat) averaged 50 bu/a, for example, wheat/flax/wheat (Figure 3). Spring wheat production grown after another crop for two years (83 treatments, 2 yr out of wheat) averaged 53 bu/a, for example, sunflower/dry pea/wheat (Figure 3).

Plant Diseases. A good example of high disease severity when a crop was seeded for two consecutive years was flax seeded after flax (Figure 2). Flax wilt (Fusarium oxysporum f. sp. lini) and pasmo (Septoria linicola) were the most common diseases present.

Sclerotinia head blight on safflower ranged from 0% to 3% in 1999 and from 0% to 2% in 2000, with the highest level following crambe. With canola, sclerotinia stem rot ranged from 0% to 6% in 1999 and from 1% to 5% in 2000. With crambe, sclerotinia stem rot ranged from 2% to 15% in 1999 and from 10% to 60% in 2000. Thus, the higher levels of sclerotinia on canola and crambe followed safflower. With the variation in disease incidence among plots, it was difficult to demonstrate significant differences in Sclerotinia among previous residue treatments at a probability level of P < 0.05 (Crop Sequence Calculator, v.2).

Leaf spot diseases on Amidon spring wheat included: stagonospora nodorum blotch (Stagonospora nodorum) and tan spot (Pyrenophora tritici-repentis), the most common diseases, followed by septoria tritici blotch (Septoria tritici), septoria avenae blotch (Stagonospora avenae f. sp. triticea), and spot blotch (Bipolaris sorokinianum). Leaf spot diseases on Stander barley included: net blotch (Helminthosporium teres), the most common disease, followed by septoria speckled leaf blotch (Septoria passerinii), stagonospora avenae leaf blotch (Stagonospora avenae f. sp. triticea), and spot blotch (Bipolaris sorokinianum). With early evaluations of leaf spot diseases on wheat and barley, diseases were generally more severe following a wheat and barley crop, respectively, compared with the other nine crops but not later in the season. For example, the disease severity on seven of the nine treatments without wheat residue was significantly less than the wheat residue treatment with the July 6^th rating (Figure 4).

Producers should not rely exclusively on a single management practice to minimize disease risk but rather integrate a combination of practices to develop a consistent long-term strategy for disease management that is suited to their production system and location. For example, plant disease risks can be lowered through crop and cultivar selection, crop sequence/crop rotation, fungicide application, seeding rate and seeding date, balanced fertility, control of weeds and volunteer crop plants, and modification of the micro-environment within the crop canopy using tillage practices and stand density (Krupinsky et al. 2002).

We thank D. Wetch, J. Hartel, C. Flakker, M. Hatzenbuhler, D. Schlenker, M.K. Tokach C. Klein, J. Bullinger, and L. Renner for technical assistance.

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