Organic systems in the southeastern USA offer unique challenges and solutions to crop production due to regional soil and climate characterized by highly weathered soil types, high precipitation and the capacity to grow cover crops in the winter. Recently, the interest of producers and researchers in high-residue cover crops and conservation tillage systems has increased. Various designs of the roller-crimper to manage cover crops have been invented and demonstrated to growers in the southeastern region of the USA over the past 17 years. The impacts of high-residue cover crop mulches on the agronomic systems in the region are diverse. Legume cover crops assist with meeting N demand from cash crops though they decompose rapidly and are seldom sufficient for N demanding crops such as corn. Cereal cover crop mulches can have the opposite effect by immobilizing N and have a longer impact on soil moisture and weed dynamics. While undesirable for many crops, N immobilization is one possible mechanism for weed suppression in legume cash crops planted into cereal residues. Other cover crop weed suppression mechanisms include physical impedance, light availability, allelopathy and microclimate effects. Regardless of the cause, successful weed control by mulches is highly dependent on having substantial biomass. The southeastern region is capable of producing cover crop biomass in excess of 9000 kg ha − 1 , which is sufficient for weed control in many cash crops, although supplementary weed control is sometimes necessary. Long-term data are needed to predict when farmers should add supplementary weed control. More work is also needed on how much additional N is required for the cash crops and how best to deliver that N in a high-residue environment using organic sources.

Rising demand for organic soybeans and high price premiums for organic products have stimulated producer interest in organic soybean production. However, organic soybean producers and those making the transition to organic production cite weed management as their main limitation. Current weed management practices heavily rely on cultivation. Repeated cultivation is expensive and has negative consequences on soil health. Research is needed to improve organic reduced tillage production. Rye cover crop mulches were evaluated for weed suppression abilities and effects on soybean yield. Experiments were planted in 2008 and 2009 at three sites. Rye was planted in the fall of each year and killed at soybean planting with a roller/crimper or flail mower, creating a thick weed-suppressing mulch with potential allelopathic properties. The mulch was augmented with one of three additional weed control tactics: preemergence (PRE) corn gluten meal (CGM), postemergence (POST) clove oil, or postemergence high-residue cultivation. Roll-crimped and flail-mowed treatments had similar weed suppression abilities at most sites. There were no differences between CGM, clove oil, or cultivation at most sites. Sites with rye biomass above 9,000 kg ha−1of dry matter provided weed control that precluded soybean yield loss from competition. In Goldsboro 2008, where rye biomass was 10,854 kg ha−1of dry matter, the soybean yield in the rolled rye treatment was not significantly different from the weed-free treatment, yielding at 2,190 and 2,143 kg ha−1, respectively. Likewise, no difference in soybean yield was found in Plymouth 2008 with a rye biomass of 9,256 kg ha−1and yields of 2,694 kg ha−1and 2,809 kg ha−1in the rolled rye and weed-free treatments, respectively. At low rye biomass levels (4,450 to 6,606 kg ha−1), the rolled rye treatment soybean yield was 628 to 822 kg ha−1less than the weed-free treatment. High rye biomass levels are critical to the success of this production system. However, high rye biomass was, in some cases, also correlated with soybean lodging severe enough to cause concern with this system.

The organic grain sector is one of the fastest growing sectors of the organic market, but farmers in the mid-Atlantic cannot meet the organic grain demand, including the demand for organic soybean. Weed management is cited by farmers as the largest challenge to organic soybean production. Recent soybean population studies show that lower seeding rates for genetically modified organism soybean farmers provide maximum economic return due to high seed technology fees and inexpensive herbicides. Such economic analysis may not be appropriate for organic soybean producers due to the absence of seed technology fees, stronger weed pressures, and price premiums for organic soybean. Soybean seeding rates in North Carolina have traditionally been suggested at approximately 247,000 live seeds/ha, depending on planting conditions. Higher seeding rates may result in a more competitive soybean population and better economic returns for organic soybean producers. Experiments were conducted in 2006 and 2007 to investigate seeding rates of 185,000, 309,000, 432,000, and 556,000 live seeds/ha. All rates were planted on 76-cm row spacing in organic and conventional weed management systems. Increased soybean seeding rates reduced weed ratings at three of the five sites. Increased soybean seeding rates also resulted in higher yield at three of the four sites. Maximum economic returns for organic treatments were achieved with the highest seeding rate in all sites. Results suggest that seeding rates as high as 556,000 live seeds/ha may provide organic soybean producers with better weed control, higher yield, and increased profits.

Organic soybean [Glycine Max (L.) Merr.] producers rely on a variety of tactics for weed management. The use of soybean cultivars with enhanced ability to compete with weeds may increase weed control. Our objective was to identify genetic traits that may enhance soybean's competitive ability to suppress weeds. Experimental design was a split-split plot with sets of contrasting soybean genotypes assigned to the main plots, individual genotypes assigned to subplots, and weedy and weed-free sub-subplots stripped across blocks for side by side comparison of treatments in weedy and weed-free conditions. Differences in weed biomass were detected among genotypes in both years at 7 wk after emergence. Narrow-leaflet small-seeded natto types were generally poor competitors with weeds. Cultivars released as forage types did not have a consistent advantage over the control. Larger-seeded tofu genotypes were also variable in weed suppressive ability. However, N04-8906, the genotype with the greatest 100-seed weight in this study (24 g), stood out as being among the best for weed suppression, soybean biomass accumulation, ground cover at 3 wk, and early season plant height. Optimum models from multiple regression showed seed size to be the most significant trait measured in overall genotype competitive ability in both years. However, seed size effects on ground cover largely dissipated by 5 wk after emergence.

Weed control remains one of the greatest problems in agro-ecological systems. An important factor controlling crop and weed competition for below ground resources is the presence of compacted soil layers or 'hard pans'. In a series of experiments, we investigated the ability of roots of soybean (Glycine max L.) and the weeds sicklepod (Senna obtusifolia L.) and Palmer amaranth (Amaranthus palmeri S. Wats) to penetrate through a compacted soil layer and acquire N from lower in the soil profile. Soil columns were constructed to simulate a compacted soil layer with different bulk densities~8 cm beneath the soil surface. Results indicated that roots of the two weed species penetrated high bulk density soil layers more effectively than those of four soybean lines. Root penetration was not related with growth rates among the species or soybean genotypes. Overall root and shoot growth of the weeds was sustained when downward root growth was inhibited, while both declined with soybean, even under high fertility conditions. The weeds also acquired relatively high amounts of 15 N À NO À 3 from buried patches beneath the high bulk density layers compared to soybean. The results indicate that the weed species would have a competitive advantage when plow pans are present, and an attempt is made to relate this advantage with competitive dynamics observed in the field.