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Emergence, Plant Densities (V2 Stage) and Weed Densities (R3 Stage) of Soybean in Conventional and Organic Cropping Systems in 2016

 Bill Cox1, Eric Sandsted1,  Phil Atkins2 and Brian Caldwell1
1Soil and Crop Sciences Section – School of Integrated Plant Science, Cornell University; 2New York Seed Improvement Project – Cornell University

Organic soybean (July 27) during the dry 2016 growing season has not canopied or filled in completely, especially with the recommended seeding rate (6 rows on the right), which may allow for some late-season weed escapes to affect soybean yield.

Organic soybean (July 27) during the dry 2016 growing season has not canopied or filled in completely, especially with the recommended seeding rate (6 rows on the right), which may allow for some late-season weed escapes to affect soybean yield.

We initiated a 3-year study at the Aurora Research Farm in 2015 to compare different sequences of the corn, soybean, and wheat/red clover rotation in conventional and organic cropping systems under recommended and high input management during the 3-year transition period (2015-2017) from conventional to an organic cropping system. We provided a detailed discussion of the various treatments and objectives of the study in a previous soybean article (http://blogs.cornell.edu/whatscroppingup/2015/09/16/emergence-early-v2-stage-plant-populations-and-weed-densities-r4-in-soybeans-under-conventional-and-organic-cropping-systems/). This article will focus on soybean emergence (days and %), plant densities at the 2nd leaf (V2 stage), and weed densities at the early pod (R3 stage) in 2016.

Corn preceded soybean in the rotation in this study. Despite the dry spring conditions (1.9 inches in March, 1.87 in April, and 0.75 inches from May 1-19), ideal soil conditions prevailed for plowing on May 19 (full soil water profile in contrast to corn that followed red clover that had very little stored soil water in the top 8 inches). The fields were then cultimulched on the morning of May 20, the day of planting. We used the White Air Seeder to plant the treated (insecticide/fungicide) GMO soybean variety, P22T41R2, and the non-treated non-GMO, 92Y21, at two seeding rates, ~150,000 (recommended input) and ~200,000 seeds/acre (high input). 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, the soybean comparison is not as robust as the corn comparison for emergence and early plant establishment because of the different row spacing and genetics between the two cropping systems.

We applied Roundup (Credit 41) at 32 oz. /acre for weed control to conventional soybean at the V4 stage (June 22) under both recommended and high input treatments. We used the tine weeder to control weeds in the row in both recommended and high input organic treatments at the V1-V2 stage (June 9). We then cultivated close to the soybean row in both recommended and high input organic treatments at the V3 stage (June 15) with repeated cultivations between the entire row at the V4 stage (June 22), beginning flowering (R1) stage (July 1), and full flowering (R2) stage (July 14). The high input soybean treatment in the conventional cropping system also received a fungicide application on July 27, the R3 stage, for potential disease problems and overall plant health.

Weather conditions were warm and dry for the first 10 days after planting. Soybean emergence required 7 to 8 days, which was more rapid than corn emergence, probably because the lack of a green manure crop (red clover preceded corn) resulted in more soil water in the seed depth zone. As in 2015, organic compared to conventional soybean emerged 0.50 to 0.75 days earlier (Table 1). As mentioned in a 2015 article (http://blogs.cornell.edu/whatscroppingup/2015/06/16/days-to-emergence-and-early-corn-and-soybean-plant-populations-under-conventional-and-organic-cropping-systems/), variety differences between the cropping systems probably influenced days to emergence. 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 probably contributed to the more rapid emergence in the organic 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 recommended and high input treatments in the organic cropping system, indicating that Sabrex, the organic seed treatment, did not hasten soybean emergence in 2016, similar to results in 2015.

Cox Soybean Table 1

We estimated soybean plant densities at the V2-V3 stage (June 13), prior to the close cultivation to the soybean row on June 15, but unfortunately after tine weeding the organic soybeans. Conventional soybean consistently had higher plant establishment rates (83 to 92%) compared with organic soybean (71 to 82%, Table 1). This was inconsistent with the 2015 data in which soybean in the organic cropping system under recommended input management had greater stand establishment than its counterpart treatment in the conventional cropping system (http://blogs.cornell.edu/whatscroppingup/2015/07/23/emergence-early-v4-stage-and-final-plant-populations-v10-psnt-values-v4-and-weed-densities-v12-in-corn-under-conventional-and-organic-cropping-systems/).  In 2015, we were able to estimate plant populations before tine weeding. In 2016, we estimated plant densities after tine weeding, which probably reduced plant establishment rates because of crop damage. Nevertheless, plant densities exceeded 114,000 plants/acre in all organic treatments, the threshold density below which soybean yields decrease under typical growing conditions (http://scs.cals.cornell.edu/sites/scs.cals.cornell.edu/files/shared/documents/wcu/WCU21-2.pdf). We will estimate soybean densities again at harvest to determine the extent of crop damage during the subsequent four cultivations in the organic cropping system to determine if final stands fall below the 114,000 plant/acre threshold.

As with corn, weed densities were exceedingly low in soybean in 2016 (Table 1), but even more so in soybean. As with corn, the dry soil conditions and lack of significant rainfall events required to initiate weed emergence after cultivations in the organic cropping system and herbicide application in the conventional cropping system contributed to very low weed densities. Weed densities were essentially the same between the organic (0.21 to 0.53 weeds/m2) and conventional cropping systems (0.07 to 0.38 weeds/m2) under both management systems. Such low weed densities should not contribute to yield losses in soybean in either cropping system in 2016.

In conclusion, organic soybean compared with conventional soybean consistently emerged 0.5 to 0.75 days earlier in 2016 probably because of variety and row spacing differences between cropping systems. Surprisingly, organic soybean compared with conventional soybean consistently had lower plant establishment rates at the V2-V3 stage, resulting in lower plant densities. This contradicts findings in the wet 2015 spring when organic compared with conventional soybean emerged more rapidly and had higher plant densities (http://blogs.cornell.edu/whatscroppingup/2015/09/16/emergence-early-v2-stage-plant-populations-and-weed-densities-r4-in-soybeans-under-conventional-and-organic-cropping-systems/). Plant densities of ~115,000 plants/acre at the V2-V3 stage in organic soybean with recommended inputs are close to the ~ 114,000 plant/acre threshold below which soybean yields decrease under typical growing conditions. Consequently, organic soybean yield with recommended inputs could be compromised a bit, especially if close cultivation or cultivations between the rows further reduced plant densities. On the other hand, the lower plant densities may provide a yield benefit, if weather conditions remain dry during August. The dry conditions, however, have resulted in limited soybean growth in 2016. Consequently, organic soybean in 30-inch row spacing has not canopied or filled in as of July 28, which could result in some weed escapes putting on enough growth to perhaps reduce yields in a dry growing season. Nevertheless, despite uncertainty on final plant and weed densities in organic soybean, the yield potential between the organic and conventional cropping system appears to be similar, which would reinforce 2015 results when organic and conventional soybean yields were mostly similar   (http://blogs.cornell.edu/whatscroppingup/2015/11/09/soybean-yield-under-conventional-and-organic-cropping-systems-with-recommended-and-high-inputs-during-the-transition-year-to-organic/).

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