The biggest obstacle to organic soybean production in the Southeast is weed management. Current organic soybean systems rely on mechanical weed control. The rotary hoe is one implement that allows for within row weed control through early broadcast cultivation, also called “blind cultivation”, because the rotary hoe goes over the whole field without regard for the soybean row position. Pre plant rotary hoeing, also called stale seed bedding, is also a practice that has been utilized for weed control. By pulling the rotary hoe over the field following field disking and conditioning but prior to planting, small weed seeds are induced to germinate and subsequently killed. In two years of experimentation at two different locations, results indicate that multiple rotary hoeing, while effectively managing weeds, may reduce soybean yields because of increased damage to soybean stands. Stale seed bedding was effective at times in reducing weed pressures but the effect of stale seed bedding dissipated after one effective post plant rotary hoeing.
Soybean seeding rates have traditionally been suggested at 100,000 to 150,000 live seeds per acre. For farmers accustomed to using round up ready seed technology, less seed is better to reduce the technology fee. Such a technology fee is not a factor for organic soybean producers. It is well known that drilling soybeans on 7 inch rows provides a more competitive soybean stand, closes the canopy faster and often results in higher yields than soybeans planted on 30 inch or 38 inch rows. But drilling soybeans on narrow rows eliminates the ability of a farmer to cultivate, often a critical part of the weed management program. Experiments were conducted to investigate differing seeding rates, with all rates planted on 30 inch row spacing. Two years of experiments at two locations have shown that seeding rates as high as 280,000 live seeds per acre may provide producers with better weed control and higher yields.
Results from some of this research have been compiled in an Extension Bulletin: Planting Rate Recommendations for Organic Soybean Producers (AG 723w).
The development of a more competitive soybean genotype would be useful for organic producers. Typical soybean breeding trials are maintained weed free and, consequently, new soybean cultivars are not selected based on their ability to compete with weeds. Genotypic differences in competitiveness for weeds have been identified for several agricultural species including: wheat, rice, cowpea, corn and many others. Similar studies have suggested differences in competitiveness of soybean genotypes. Competitiveness of 29 genotypes maintained in the North Carolina USDA soybean breeding program with varying characteristics of seed size, petiole length, leaf shape and height were measured in the 2007 season. These experiments will be repeated in 2008 with the goal of identifying soybean traits most related to improving weed competitiveness.