INTRODUCTION Organic acreage, to meet the feed and food industry demand is increasing. Between 1997 and 2003, certified organic acres grew by 10% per year (Dimitri and Oberholtzer, 2006). Common questions are however, “How productive are organic grain systems?” How do they compare with conventional grain systems? In this poster we report on yield comparisons between organically and conventionally grown corn and soybeans. MATERIALS AND METHODS In 1990 a large cropping systems trial was established at two sites [Arlington ( , Elkhorn ( )] on prairie-derived soils about 70 miles apart in SC Wisconsin (Photo 1). Two of the six systems under study were a conventional no-till corn (Zea mays) and soybean (Glycine max) rotation (CONV), common throughout the upper mid-west, and a three phase organic rotation that included corn- soybeans/wheat-wheat/red clover (Triticum aestivum/Trifolium pratense) (ORG). Best management practices within each production philosophy were followed: Due to high initial soil test values, P & K fertilizer additions were limited to starter fertilizer in the CONV system and N rates were dictated by the Pre-sidedress Nitrate Tests (PSNT). Herbicides were used with the former, and generally 1-2 rotary hoeings and 1-2 cultivations were conducted on the ORG system. Corn hybrids and soybean varieties were not the same in the CONV and ORG systems due to delayed planting and organic regulations in the ORG system. In a RCB design with four replicates, 12 rows by 510 ft. were harvested from the middle of each plot to estimate production and grab samples were used to estimate moisture. Due to the unequal number of observations (years) the yields were analyzed within sites SAS PROC MIXED (SAS Institute Inc., 2003) with years and blocks as random and system as a fixed variable. RESULTS Average results from the two locations indicate that organic corn produced 87% and organic soybeans 88% of the production in the conventional system (Table 1). At each location however, there was a significant year × system interaction. Inconsistent weed control in the organic system explains a large portion of this interaction. For UNDERSTANDING ORGANIC CORN AND SOYBEAN YIELDS: THE IMPORTANCE OF WEED PRESSURE Jon Baldock 1 Joshua Posner 2, Janet Hedtcke 2 1 = AGSTAT, Verona, WI 2=Univ. of Wisc. Madison, WI example, in the years that mechanical weed control was difficult at Arlington for corn, the organic systems produced only 72% as much grain as the conventional systems (Fig 1). But in the years with better mechanical weed control, the organic systems yielded 96% as well as the conventional systems. At Elkhorn, in the two years the field crew reported problems controlling weeds, the organic corn produced only 76% as much grain as the conventional system. In the other years, when mechanical weed control was more successful, the organic system yielded 94% as much as the conventional systems. The results for soybean were nearly the same as those for corn (Fig 2). Overall, the organically managed system at Arlington produced 90% as much soybean grain as the conventionally managed system (Table 1). However, during five of the study years weed control was poor and the organic soybean yields averaged only 79% of the conventional soybean yields. On the other hand, in the 8 years with better mechanical weed control, organic soybean production averaged 96% of that in the conventional system. At Elkhorn, organically managed soybean yielded 85% as much grain as the conventionally managed soybean (Table 1). However, during four years it was difficult to control weeds mechanically and organic soybean grain production averaged only 78% of that in the CONV system (Fig 2). In the five years with better mechanical weed control, organic soybean grain yields averaged 91% of conventional yields. Wet spring weather was the major reason mechanical weed control was difficult in some years. Although monthly total rainfall does not exactly capture soil wetness after planting, of the six site-years in which mechanical weed control in corn was a problem all had more than 140% of the normal May plus June rainfall. And of the nine site-years with difficulty controlling weeds in the organically managed soybean crop, six had above normal May plus June rainfall. In Minnesota, Porter et al (2003) also found that wet springs diminished the efficacy of mechanical weed control and adversely affected organic corn production which use primarily untreated seed. CONCLUSION We conclude that on prairie derived soils of the upper Midwest: 1- As an overall average, organically managed corn and soybean can produce 85 to 90% as well as conventionally managed grain crops. 2- When differences in the effectiveness of mechanical weed control are considered, organic corn and soybean yields are 60 to 80% of conventional yields when mechanical weed control is poor; when it is successful, which it is in the majority of years, the organic corn and soybean yields are 90 to 98% of conventional yields. An important focus for future organic grain production must be weed control strategies. REFERENCES Dmitri, C and L. Oberholtzer A brief retrospective on the U.S. organic sector: 1997 and Plant Management Network. Porter, P.M., D. R. Huggins, C.A. Perillo, S. R. Quiring, and R.K. Crookston Organic and other management strategies with two and four year crop rotations in Minnesota. Agron J. 95: SAS V.9. Copyright © 2003 by SAS Institute Inc., Cary, NC, USA. Photo 1. Aerial view of WICST at Arlington (top) and Elkhorn (bottom), WI Table 1. Mean yields (bu/a) for organic and conventional corn and soybeans, WICST plots (1990’s). ArlingtonElkhornMean Bu/acre CornCONV ORG p<0.0001p< SoybeanCONV ORG p<0.0005p< N=4N=2N=6N=4 N=5N=4N=8N=5 ORG CONV % %