Traits of Interest for Improving Weed Suppressive Ability in Soybeen During the Critical Period for Weed Competition

Organic soybean producers must rely on a variety of tactics for weed management. The use of soybean varieties with enhanced ability to compete with weeds may be one such tactic to increase weed control. Experimental design utilized for this project was a split plot design with soybean trait of interest as the main plot, genotype as the subplot and weedy and weed free strips across the block for side by side comparison of treatments in weedy and weed free conditions. Differences in weed biomass were detected between genotypes. Larger seeded genotypes resulted in increased percent ground cover estimated at three weeks after emergence but seed size had little influence on percent ground cover estimates at five weeks after emergence. Optimum models from multiple regression showed seed size to be the most significant trait measured in overall genotype competitive ability.

Introduction

Currently over 90% of soybeans grown in the United States utilize glyphosate resistance technology (Cerdeira and Duke 2006). An even greater percentage of soybeans are cultivated with the use of herbicides. But the acreage of organic soybeans is increasing as the organic milk, beef and egg markets grow each year (Dimitri 2008). The profit margin for organic soybeans can be substantial (Archer et al., 2007) but farmers making the transition to organic soybean production cite weed management as their top challenge (Archer and Kludze 2006; Cavigelli et al. 2008; Hamilton et al. 2007; Walz 1999). Without the use of herbicides as a management option, organic soybean producers must rely on a variety of tactics to reduce weed pressures (Liebman et al., 1997). The utilization of more competitive soybean cultivars may be another supplemental weed management tactic. A highly competitive cultivar would not only be useful for organic producers but conventional producers could also benefit if less herbicide applications are needed in season (Norsworthy & Shipe, 2006). Genotypic differences in competitiveness for weeds have been identified for several agricultural species including: wheat (Triticum aestivum L.) (Ramsel & Wicks 1988), rice (Oryza sativa L.) (Haefele et al., 2004), cowpea (Vigna unguiculata L.) (Remison 1978), corn (Zea mays L) (Wooley & Smith 1986) and many others (Callaway 1992). Similar studies have suggested differences in competitiveness of soybean genotypes (Jannink et al., 2000; Rose et al., 1984) but high variation often overshadows possible differences (Norsworthy & Shipe, 2006; Bussan et al., 1997). Soybean breeding programs have typically focused on improving characteristics such as yield and disease resistance with little or no attention to weed competitive improvement since the majority of soybean breeding trials are conducted in weed free conditions (Baenziger et al. 2006; Egli 2008; Gepts and Hancock 2006; Heisey et al. 2001). However, genetic variation in soybean competitive ability has been described by other research groups. Reports of traits that may be related to competitive ability have included height (Jannink et al. 2000), leaf area (Jordan 1993), and early vigor (Guneyli et al., 1969; Rose et al., 1984). However, identifying characteristics imparting competitive advantage has been difficult (Norsworthy & Shipe, 2006). Root characteristics may also influence soybean competitiveness (Dunbabin 2007; Place et al. 2008), but screening for and selection on canopy characteristics that improve competitive ability will be most feasible for soybean breeders. Because traits of interest for increased competitiveness such as canopy cover or height may be variable depending on the growth stage, such traits should be investigated during the most critical period for weed competition. This period is defined as the interval in the life cycle of the crop when it must be kept weed free to prevent yield loss (Zimdahl 1980; Van Acker et al., 1993). This period is variable depending on environmental conditions (Van Acker et al., 1993) but has been estimated between the soybean stages V2 and V8 (Eyherabide et al., 2002) which occur at approximately 2 and 7 weeks after emergence (WAE), respectively.

We investigated the weed competitiveness of 27 soybean genotypes that were selected based on differing characteristics of seed size, petiole length, petiolule length, leaflet width and length, and main stem height. Our main objectives were to (1) determine if differences in weed competitive ability exist between cultivars of varying canopy traits and seed sizes and (2) determine the relation of these traits on the competitive ability of a genotype.