Field research in 2002 and 2003 evaluated spring-interseeded winter rye (Secale cereale L.) at 67, 134, or 200？kg？ha？1 at two soybean (Glycine max (L.) Merr.) row spacings (19- and 76-cm) on weed control, yield, and gross margins. Based on regression analysis, wide-row (76-cm) soybean grain yield and gross margins were greatest when winter rye was interseeded at 114 and 106？kg？ha？1, respectively. Yields and gross margins for wide-row soybean were 8 to 55% greater than narrow-row (19-cm) soybean seeded at 494,000 or 742,000 seeds ha？1 which was probably due to flexibility for implementing cultivation. As interseeded rye rates increased from 67 to 200？kg？ha？1, yields and gross margins for narrow-rows decreased. Soybean row spacing had minimal impacts on specific weed species and total weed biomass or density. The use of wide-row soybean and spring-interseeded rye at 67？kg？ha？1 was more cost-effective compared to narrow rows. 1. Introduction Demand for organic crop production in the USA has increased, resulting in over 4.2 million certified organic crop production hectares in 2005 [1]. In 2005, more than 55% of the organic field crop production hectares in Missouri were soybean, while Missouri ranked 23rd in the USA for the number of certified organic producers in 2007 [1]. Missouri farmers have noted increased profit margins with organic versus conventional crop production systems in mid-west states [2, 3] and are more likely to consider using land previously set aside in the conservation reserve program for organic crop production [4]. Claypan soils in Missouri enrolled in the conservation reserve program generally are highly erodible. Using preplant primary tillage followed by two or three rotary hoeings and at least two in-crop cultivations has been an acceptable method of controlling weeds in organic soybeans [4–6]. Long-term research indicated that tillage-intensive, organic systems had greater soil benefits than conventional no-till on drought-prone, erodible soils [7]. However, the intensity of labor and tillage required for organic crop production has limited the widespread adoption of organic soybean production, especially on highly erodible soils. Weed interference has affected soybean yields more in organic production systems than in conventional systems [8, 9], especially in narrow rows [10]. Weed control using companion crops such as winter rye seeded and narrow-row soybean was described in the 1950s [11]. Winter rye was preferred as a companion crop over wheat (Triticum aestivum L.). Cover crops such as winter rye have been used to suppress
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