Conventional Midwestern U.S. row crop agriculture has created significant environmental problems and made the farm economy reliant on government subsidies. Environmentally friendly and economically profitable alternatives are badly needed. This study addresses production characteristics of strip intercropping, a system that may meet both requirements. Two experiments were conducted in 1989 and 1990: one on a cooperating farmer's field with ridge tillage and the second at a university research farm with conventional tillage. The objective was to evaluate grain yields of different rows in adjoining strips (3.8 or 4.6 m wide) of three crops. Corn, soybean, and oat strips were either inter seeded with nondormant alfalfa or seeded with hairy vetch as a cover crop after oat grain harvest. Outside corn rows had significantly higher yields than center rows in 1990, when plant water stress was low, but under dry conditions in 1989, early season competition for water caused corn to yield less in the row bordering oat than in the row bordering soybean. Comparative soybean yields in border and center rows also depended on rainfall; with adequate water, soybean yield next to the oat strip was greater than or equal to yield in the center of the strip. Oat border rows yielded higher than those in the oat strip center. Timing differences in crop life cycles and water availability seem to influence how these crop species interact, particularly at the border positions. Overall, the strip intercropping system seems a suitable alternative to current practices.

Sustainability of Iowa agriculture may require change from predominantly a corn (Zea muys L.)‐soybean [Glycine max (L.) Merr.] rotation to more diverse cropping systems. Alternative crops are vital for providing temporal diversity. Reincorporating small grains into a three‐crop rotation with corn and soybean can provide greater temporal diversity, especially if a forage legume is included as a companion crop. A field study was established in 1991 on a Kenyon (fine‐loamy, mixed, mesic Typic Mapludoll) soil to evaluate the economic and biological benefits of an oat (Avena sativa L.) crop under‐seeded with berseem clover (Trifolium alexandrinum L.) in a three croprotation. Two rotation treatments were compared: (i) corn‐soybean‐oat and (ii) corn‐soybean‐oat intercropped with berseem clover. In 1992, 1993, 1994, and 1995, oat grain yield was not significantly changed when berseem clover was underseeded with the oat crop. However, in 5 yr, oat under‐seeded with berseem clover produced up to 70% more biomass (harvested material without the grain) than sole‐crop oat straw. The biomass (40% oat straw and 60% berseem clover forage) also had adequate digestible material (51%) to be considered as low quality forage. Berseem clover regrowth after oat grain harvest produced an average 1.2 tons/acre of forage, which could have been harvested for hay or left in the field as green manure. During this trial, berseem clover regrowth was left as groundcover and green manure, which contributed an average of 39 lb N/acre to the succeeding corn crop. Corn grain yields following berseem clover were 10% higher over the trial period. Soybean grain yields were the same for both treatments. Intercropping berseem clover with oat returned an average of %39/acre more than sole‐crop oat. This study demonstrated both economic and biological advantages for more diverse cropping practices. Research Question Alternative crops are vital for providing diversity and enhancing sustainability. In Iowa, approximately 78% of the 27 million harvested acres are managed with an annual corn and soybean production. Inclusion of oat as third crop in rotations has been unpopular, largely because of low grain market value and the high year‐to‐year variability in grain yield. However, oat allows inclusion of a forage legume as a companion crop, which may improve profitability. The objectives of this study were to evaluate the economic and biological benefits of an oat crop underseeded with berseem clover in two crop‐rotation systems. Literature Summary Increased corn and soybean production throughout the Midwest has decreased crop diversity, contributed to significant environmental problems, and limited opportunities to integrate livestock into the cropping system. Inclusion of an alternative small grain as a third crop can significantly improve soil productivity. Research in Minnesota indicated that total production increased when a small graidegume is added to a corn‐soybean rotation. Legume cover crops may also contribute N to the subsequent crop, reducing the...

Time, fertilizer, tillage, and cropping systems may alter soil organic carbon (SOC) levels. Our objective was to determine the effect of long-term cropping systems and fertility treatments on SOC. Five rotations and two N fertility levels at three Iowa sites (Kanawha, Nashua, and Sutherland) maintained for 12 to 36 yr were evaluated. A 75-yr continuous corn (Zea mays L.) site (Ames) with a 40-yr N-P-K rate study also was evaluated. Soils were Typic and Aquic Hapludolls and Typic Haplaquolls. Four-year rotations consisting of corn, oat (Avena saliva L.), and meadow (alfalfa [Medicago sativa L.], or alfalfa and red clover [Trifolium pratense L.]) had the highest SOC (Kanawha, 32.1 g/kg; Nashua, 21.9 g/kg; Sutherland, 27.9 g/kg). Corn silage treatments (Nashua, < 18.9 g/kg; Sutherland, <23.2 g/kg) and no-fertilizer treatments (Kanawha, 25.3 g/kg; Nashua, <20.9 g/kg; Sutherland, <23.5 g/kg) had the lowest SOC. A corn-oat-meadowmeadow rotation maintained initial SOC (27.9 g/kg) after 34 yr at Sutherland. Continuous corn resulted in loss of 30% of SOC during 35 yr of manure and lime treatments. SOC increased 22% when N-P-K treatments were imposed. Fertilizer N, initial SOC levels, and previous management affected current SOC levels. Residue additions were linearly related to SOC (Ames, r 2 = 0.40; Nashua, r 2 = 0.82; Sutherland, r 2 = 0.89). All systems had 22 to 49% less SOC than adjacent fence rows. Changing cropping systems to those that conserve SOC could sequester as much as 30% of C released since cropping began, thereby increasing SOC. T HE ATMOSPHERIC CC>2 CONCENTRATION has gained much attention for its potential contribution to global warming. Agriculture affects atmospheric COz concentrations through consumption of fossil fuels, clearing of forested lands for food production (U.S. Congress, 1991; Wallace et al., 1990), and alteration of SOC levels by agricultural management practices. Agricultural fuel consumption and N fertilizer production release 35.4 Tg C yr~' into the atmosphere. Based on current production practices, the Council for Agricultural Science and Technology (1992) estimates another 2.7 Tg C yr~' are released into the atmosphere from cultivated soils in the USA alone. Changes in SOC can be attributed to crop species grown, cropping systems (including rotations), residue management practices, fertilizer applications, tillage practices, and other management factors (Havlin et al., 1990; Unger, 1968). Anderson et al. (1990), Bauer and Black (1981), and Havlin et al. (1990) independently showed that SOC losses were directly related to tillage intensity. Manure applications modified the tillage-SOC relation, increasing SOC even with high-intensity conventional tillage (Anderson et al., 1990). Crop rotations may retard SOC losses relative to those observed in

A three‐crop strip intercropping system including corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and oat (Avena sativa L.) interseeded with nondormant alfalfa (Medicago sativa L.), was established in south‐central Iowa on a poorly drained Haig soil (fine, smectitic, mesic Vertic Argiaquoll). In 1989 (a dry year) and 1990 (a wet year), we studied the effect of tillage treatment (conventional, CT; reduced, RT; and no‐till, NT) and row position on soil water content, canopy and air temperatures, corn grain yield, and yield components. No‐till resulted in the most favorable soil water status, plant water status, and grain yield in 1989. No‐till had the poorest performance in 1990, mainly because of excessive soil water. The opposite was true for conventional till in both years. Reduced till yield equaled that for the most productive tillage treatment in both years. Conventional and reduced tillage resulted in lower corn grain yield at the border with oat‐alfalfa than with soybean because oat depleted soil water more than soybean in both years. In contrast, for no‐till the corn grain yield at the border with oat‐alfalfa was 6% greater than corn yield bordering soybean in 1989 and 13% greater in 1990. When water was not limiting, in 1990, both corn borders outyielded the center rows by an average of 14% in NT, 27% in RT, and 28% in CT. Soil water content rankings throughout the 1989 season were NT > RT > CT and row position rankings were soybean border > center > oats‐alfalfa border. In 1990, there were no soil water content differences between tillage treatments and row positions. Reduced tillage is the most suitable soil management system for corn production with three‐crop strip intercropping on this soil, considering consistently high relative yields for both wet and dry years.

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Strip intercropping seeks to capture the biological efficiency of intercropping in traditional agricultural systems and is compatible with agricultural equipment used in the U.S. This efficiency stems from complementary use of resources by constituent crops and is a function of crop selection, strip width and orientation, weed control, and other factors. Strip intercropping requires a high level of management; further, some reports suggest the gains and losses more-or-less balance in actual production situations. These questions are best addressed by the performance of strip intercropping as implemented by farmers in production situations. Practical Farmers of Iowa (PFI) members have worked with Iowa State University agronomists to evaluate strip intercropping. For three years six farmers compared strip intercropping to field blocks of individual crops. The strip intercrop systems employed three crops: corn, soybeans, and small grains with a forage legume underseeding. The comparison systems, crops grown in sole-crop blocks, consisted of the same three crops on four farms (planting pattern comparison) or, on two farms, just corn and soybeans in rotation (systems comparison). Yields and field operations were recorded and entered in the Iowa State University Crop EnterpriseRecord System (CER) to derive gross profit, total production cost, and net profit for each crop component and for each cropping system on every farm. Strip intercropping net profit was generally greater than that infield blocks, and intercropping compared favorably with CER results obtained from corn-soybean rotations on other farms around Iowa. Land equivalent ratios (LER) were usually greater than 1.0, indicating satisfactory biological efficiency. Despite occasional problems, in this set of 18 site-years strip intercropping was associated with greater stability of net profit.

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To improve soil erosion prediction technology, the mechanics of each erosion process must be understood sufficiently to predict soil loss on an event basis. The mechanics of the initial erosion process, soil detachment caused by falling raindrops, requires greater understanding to improve mechanics‐based prediction. This laboratory study addressed the effect of soil shear strength and raindrop impact angle on soil detachment. Loess (fine‐silty, mixed, superactive, mesic Typic Hapludoll) and glacial till (fine‐loamy, mixed, superactive, mesic Aquic Hapludoll) A and C horizon soil materials were used. To vary soil shear strength, soybean protein material was added to each soil material at concentrations of 0.0, 0.5, and 1.0% by weight. Soil shear strength and soil detachment were measured on preformed soil cores. Soil detachment tests were performed at water drop impact angles of 90, 80, 70, and 60°. Soil strength increased and detachment decreased with increasing soybean protein concentrations. Shear strength of the loess C horizon increased 0.61 to 1.85 Mg m−2, while that of the till C horizon material increased 0.57 to 0.98 Mg m−2 with addition of 1% soybean protein. A 1%–soybean protein addition reduced soil detachment 26% compared with unamended soil. Significant soil detachment interactions existed between waterdrop impact angle and the other variables. These interactions were due to different mechanical behavior of the soils and changing strength caused by soybean protein additions. Interactions observed are explained based on differences in the lateral jet for varying impact angles and for elastic vs. inelastic impacts.