Emergence, Early (V2 Stage) Plant Populations, and Weed Densities (R4) in Soybeans Under Conventional and Organic Cropping Systems

Bill Cox1, Eric Sandsted2, Phil Atkins3, and Brian Caldwell1
1Soil and Crop Sciences Section, 2Horticulture Section – School of Integrated Plant Science; 3New York Seed Improvement Project – Cornell University

We initiated a 3-year study at the Aurora Research Farm in 2015 that will compare the corn, soybean, and wheat/red clover rotation under conventional and organic cropping systems during the 3-year transition period (2015-2017) from a conventional to an organic cropping system. We used three entry points or previous crops in 2014 to initiate the study: 1) small grain, 2) grain corn, and 3) soybean. Two of the many objectives of the study are to determine the best previous crop and the best crop to plant in the first year of the transition from conventional to organic cropping systems. This article will discuss soybean emergence, soybean populations at the 2nd node stage (V2) and weed populations in soybean at the full pod stage (R4 stage).

We used the White Air Seeder to plant the treated GMO soybean variety, P22T41R2, and the non-treated non-GMO, 92Y21, at two seeding rates, ~150,000 and 200,000 seeds/acre. Unlike the corn comparison, P96Y21 is a not an isoline of P22T41R2 so only the maturity of the two varieties and not the genetics are similar between the two cropping systems. As with corn, we treated the non-GMO, 92Y21, in the seed hopper with the organic seed treatment, Sabrex, in the high input treatment (high seeding rate). Unlike corn, however, we used different row spacing in the two cropping systems with the typical 15” row spacing in the conventional cropping system and the typical 30” row spacing (for cultivation of weeds) in the organic cropping system. Consequently, in comparing days to emergence and the early plant populations of the two crops under two cropping systems, the soybean comparison is not as robust as the corn comparison because of the different row spacing and genetics between soybean varieties in the two cropping systems.

We applied Roundup (PowerMax) at 32 oz/acre for weed control to conventional soybean at the V3-4 stage (June 27) under both recommended and high input treatments. We used the tine weeder to control weeds in the row in organic soybean at the V1 stage (June 5), if corn was the previous crop but not in the other two experiments. We then cultivated close to the soybean row in both recommended and high input organic treatments at the V2 stage (June 20) with repeated cultivations between the entire row at the V3 stage (June 25), beginning flowering (R1) stage (July 6), and full flowering (R2) stage (July 16). The high input soybean treatment in the conventional cropping system also received a fungicide application at the beginning pod (R3) stage (July 31) for potential disease problems and overall plant health.

Weather conditions were warm and dry for the first 7 days after planting. Soybean emergence was consistently 1 day earlier in the organic vs. the conventional cropping system for all three entry points (Table 1). Pioneer rated P92Y21, the variety used in the organic system, with a higher field emergence score (8 out of 10 rating) compared with P22T41R2 (7 out of 10), which is probably the main reason for the earlier emergence of soybean in the organic cropping system. The organic cropping system also was planted in 30 inch rows so there were 8.5 or 11.5 seeds emerging through the soil in 1 foot of row in the organic system compared with 4.25 or 5.75 seeds emerging in 1 foot of row in the conventional system, which may have hastened emergence in the organic cropping system. Days to emergence did not differ between the conventional and high organic cropping system, indicating that Sabrex, the organic seed treatment, did not hasten soybean emergence in 2015.

We estimated soybean plant populations in at the V2 stage (June 18), just prior to the cultivation close to the soybean row on June 20. Soybean in the organic cropping system under conventional inputs had greater plant populations or stand establishment than its counterpart treatments in the conventional cropping system in all three studies (Table 1). Soybean in the organic cropping system under high inputs, however, had similar plant populations compared with soybean in the organic cropping system under high inputs. It is not clear why there was an interaction between cropping system and management inputs for plant populations. Nevertheless, plant populations exceeded 119,000 plants/acre in all treatments, which should provide adequate plant populations in all treatments for soybean to realize its yield potential. We will estimate soybean populations again at harvest to determine the extent of crop damage during the four to five cultivations in the organic cropping system.

Weed densities in all conventional soybean treatments were less than 0.3 weeds m-2, indicating excellent control of weeds with a timely Roundup application (Table 1). Also, weather conditions were dry (~2.0 inches) after the July 16 cultivation through the R6 stage (August 20). Weed densities in all organic soybean treatments were less than 0.9 weeds m-2, indicating good control of weeds with timely cultivations (Table 1). We believe that weed densities of less than 0.9 weeds m-2 will result in minimum yield reduction. Weed densities were lower in the high input vs. the recommended input treatment in the organic system only when corn was the previous crop, presumably because the higher soybean plant populations resulted in a more competitive crop in a field with inherent higher weed densities. Still, any yield differences associated with improved weed control could be offset by greater crop stress at the higher plant populations during very dry and warm conditions during the first 10 days of September (R5.5 to R6.5 stage). In addition, the dry conditions resulted in zero visual symptoms of disease in any of the soybeans from the R3 through R6.5 stage, which probably negated the need for a fungicide application at the R3 stage in the high input conventional management treatment.

Cox - Soybeans Table 1

In conclusion, organic soybeans, which do not require fertilizer N so did not lose any N during the wet spring conditions, looked to have the same yield potential as the conventional soybeans at the physiological maturity (R7.0) stage (~September 15). Plant populations were similar or higher in the organic management systems. Although more weeds were observed in the organic compared with the conventional cropping system, weed densities averaged less than 0.9 weeds m-2 in all organic treatments, which probably had limited impact on yields. Based on visual observations, soybean compared with corn looks like a better crop during the first transition year from conventional to organic management because of much better N status of soybean, the similar plant populations, and the more competitive nature of soybeans vs. corn with weeds.

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