Bill Cox and Eric Sandsted
Soil and Crop Sciences Section, School of Integrative Plant Science, Cornell University
We initiated a 4-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 corn article (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/). Unfortunately, we were unable to plant wheat after soybean in the fall of 2016 because green stem in soybean compounded with very wet conditions in October and early November delayed soybean harvest until November 9, too late for wheat planting. Consequently, corn followed soybean as well as wheat/red cover in 2017 (Table 1). This article will focus on corn and soybean plant densities after the rotary hoeing operation in the organic cropping system.
We reported in a previous article (http://blogs.cornell.edu/whatscroppingup/2017/06/05/organic-and-conventional-corn-have-similar-emergence-and-early-plant-densities-in-2017/) that organic corn and conventional corn had similar plant densities in all treatments at the V1-2 stage (June 2), a couple of hours before the rotary hoeing operation (Table 2). Results were surprising because we presumed that the seed treatment of conventional corn, P96AMXT, would result in higher plant establishment rates compared to the non-treated organic isoline, P9675. In contrast, organic soybean (P92Y21) with no seed treatment had greater plant establishment rates compared with conventional soybean (P22T41R2) with seed treatment (Table 3). We attributed difference due to variety or genetic factors and not to organic management factors (http://blogs.cornell.edu/whatscroppingup/2017/06/06/soybean-emergence-and-early-plant-densities-v1-v2-stage-in-conventional-and-organic-cropping-systems-in-2017/).
Conventional corn had similar plant densities at the V3 stage compared to the V1-2 stage (Table 2). In contrast, organic corn had 5.5% lower plant densities on June 12 compared to June 2. Although we observed limited visual plant damage when inspecting the organic plots during the operation, the rotary hoeing must have reduced plant stands, especially because conventional corn plant densities remained similar.
Conventional soybean had similar plant densities at the V3 stage compared to the V1-2 stage (2.4% higher at V3 compared to the V1-2 stage in the field with a small grain in 2014 probably because of uneven and delayed emergence in soybean, Table 3). Likewise, organic soybean generally had similar plant densities at the V3 stage compared to the V1-2 stage. Apparently, the rotary hoeing operation results in limited damage to soybean, unlike corn, at the V1-2 stage.
We have recommended seeding rates of ~30,000 kernels/acre for conventional corn in New York, despite criticism from some industry personnel, farmers, and academic colleagues in other states. We have maintained these recommended seeding rates because on most occasions final stands of 26,000 to 28,000 plants/acre result in maximum economic yields (https://scs.cals.cornell.edu/sites/scs.cals.cornell.edu/files/shared/documents/wcu/WCUvol23no1.pdf). After the rotary hoeing operation, however, most of the plant densities of organic corn at the V3 stage ranged from ~24,000 to 25,500 plants/acre. This is before the close cultivation, performed on June 12, and 2 in-row cultivations that occurred on June 27 and again on July 5, which will further reduce plant densities. We attributed the 7% lower yield in organic compared with conventional corn in 2016 to lower plant densities (http://blogs.cornell.edu/whatscroppingup/2016/11/28/organic-corn-only-yields-7-lower-than-conventional-corn-during-the-second-transition-year/). Organic corn apparently should be planted at seeding rates of at least 33,000 plants/acre to maintain final plant densities above 26,000 plants/acre after the myriad of weed control operations, including rotary hoeing, close cultivation, and in-row cultivation.
We have recommended seeding rates of ~150,000 seeds/acre for soybeans in New York based on numerous studies (https://scs.cals.cornell.edu/sites/scs.cals.cornell.edu/files/shared/documents/wcu/WCU21-2.pdf). Some organic soybean producers and researchers believe that seeding rates should be higher, ~200,000 seeds/acre, because of the delayed planting date and more importantly for improved weed control. Data from this study in 2015 and 2016, however, indicated that organic soybean with recommended management practices (~150,000 seeds/acre) compared with high input management (~200,000 seeds/acre) yielded similarly (http://blogs.cornell.edu/whatscroppingup/2016/11/28/organic-soybean-once-again-yields-similarly-to-conventional-soybean-during-the-second-transition-year/). Weed densities were indeed higher in organic soybean at the lower seeding rate in both years but seed yield did not correlate with weed densities in 2016. We are only in the 3rd year of growing soybeans organically and perhaps weed densities will increase to such an extent that higher seeding rates will be justified. At this time, however, we see no justification for increasing the recommended organic soybean in 30-inch rows from ~150,000 seeds/acre to ~200,000 seeds/acre, especially because the rotary hoeing operation did not lower early plant densities in soybean as it did in corn.