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
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 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/). This article will focus on corn emergence (days and %), plant densities at the V3 stage, and weed densities at the V14 stage in 2016.
A red clover green manure crop (~3.75 dry matter tons/acre) was mowed down on May 18. Because of the dry spring conditions (1.9 inches in March, 1.87 in April, and 0.75 inches from May 1-19) as well as the robust red clover crop, soil conditions were exceedingly dry and plow penetration was difficult in some regions of the fields on May 19. The fields were then cultimulched on the morning of May 20, the day of planting. We planted a treated (insecticide/fungicide seed treatment) GMO corn hybrid, P96AMXT, in the conventional system; and its isoline, the untreated non-GMO, P9675, in the organic cropping system at two seeding rates, ~29,600 kernels/acre (recommended input treatment) and 35,500 kernels/acre (high input). The high input organic treatment also received the organic seed treatment (in-hopper), Sabrex. We applied Roundup (Credit 41) at 32 oz. /acre for weed control in conventional corn at the V4-V5 stage (June 22) under both recommended and high input management. We used the rotary hoe to control weeds in the row in recommended and high input organic corn at the V1-2 stage (June 9). We then cultivated close to the corn row in both recommended and high input organic treatments at the V3 stage (June 15) with repeated cultivations between the rows at the V4-V5 stage (June 22) and again at the V7-V8 stage (July 1).
Weather conditions were warm and dry for the first 10 days after planting. Nevertheless, corn emergence required 8 to 9 days (Table 1), or 140 to 160 growing degree days, instead of the typical 110-120 growing degree days. Presumably, the dry soil conditions at the time of planting extended the emergence time beyond the typical thermal unit requirement. Surprisingly, the non-GMO P9675, with or without the organic seed treatment (only in high input), compared with its isoline, the GMO P9675AMXT with seed treatment (insecticide/fungicide), emerged at the same time or 0.25 to 0.50 days more rapidly. In 2015, the GMO hybrid emerged about 0.50 days more rapidly than the non-GMO hybrid (http://blogs.cornell.edu/whatscroppingup/2015/06/16/days-to-emergence-and-early-corn-and-soybean-plant-populations-under-conventional-and-organic-cropping-systems/). Perhaps, the treated seed coat of the GMO hybrid reduced permeability of the scarce soil water necessary to initiate the germination process. 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 corn emergence in 2016, similar to results in 2015.
We estimated corn plant densities in all treatments at the V3 stage (June 14), just prior to the cultivation close to the corn row on June 15, but unfortunately after the rotary hoeing that probably reduced corn densities in the organic cropping system. Corn emergence was relatively low in 2016 (Table 1) undoubtedly because limited rainfall resulted in dry soil conditions that was exacerbated by the red clover green manure crop. Conventional corn had only 71 to 85% plant establishment, whereas organic corn had 75 to 82% plant establishment (conventional corn had 91-100% and organic corn had 84 to 97% in 2015). Consequently, conventional corn in the recommended management treatment had plant densities of only ~22,500 to 24,000 plants/acre and organic corn in the recommended input treatment had plant densities of ~23,500 plants/acre (Table 1). In a typical year, plant densities of less than ~27,000 plants/acre would result in yield reductions. In a dry year, however, plant densities of 22,500 to 24,000 plants/acre may contribute to higher yields in recommended compared with the high input treatment (plant densities of ~25,000 to 30,000 plants/acre) because low plant densities tolerate drought stress better. If soil water conditions improve after silking, the high input treatment with plant densities of ~25,000 to 30,000 plants/acre may result in higher yields.
Weed densities were also quite low in 2016 (Table 1) because of the dry soil conditions and lack of significant rainfall events required to initiate weed emergence after cultivations in the organic cropping system or herbicide application in the conventional cropping system. Although weed densities were mostly higher in the organic cropping system, weed densities ranged from only 0.38 to 1.26 weeds/m2 (compared with 1.61 to 3.10 weeds/m2 in 2015), which probably will not reduce yields greatly. Weed densities in the conventional cropping system ranged from 0.08 to 0.38 weeds/m2, which indicates excellent efficacy of a Roundup application with drought-stressed weeds that emerged after the May 20 planting date and before the June 22 Roundup application.
In conclusion, organic corn compared with conventional corn emerged at the same time or earlier in dry soil conditions in 2016. Likewise, organic corn compared with conventional corn generally had similar plant densities at the V3 stage, despite a previous rotary hoeing operation to organic corn, which probably resulted in some corn damage. This contradicts findings in the wet 2015 spring when conventional corn compared with organic corn emerged more rapidly and had higher plant densities (http://blogs.cornell.edu/whatscroppingup/2015/06/16/days-to-emergence-and-early-corn-and-soybean-plant-populations-under-conventional-and-organic-cropping-systems/). Similar to the 2015 growing season, organic compared with conventional corn had mostly higher weed densities but weed control in organic corn in 2016 was generally satisfactory. Consequently, based on plant and weed densities, the yield potential for organic compared with conventional corn is similar. Organic compared with conventional corn, however, had lower pre-sidedress nitrate (PSNT) concentrations (11 and 19 ppm, respectively), although both averaged less than the threshold 25 ppm that signifies adequate soil N for maximum yields. We expected much higher PSNT values because red clover dry matter production averaged almost 4 tons/acre with average N concentrations of 3.35%. Apparently, the exceedingly dry soil conditions minimized N mineralization of the N in red clover. Organic corn also had PSNT concentrations of ~11 ppm in the wet 2015 growing season, which underscores the difficulty of providing adequate N for organic corn, as indicated by the 20 to 40% yield reductions in 2015 because of inadequate N (http://blogs.cornell.edu/whatscroppingup/2015/11/09/corn-yield-under-conventional-and-organic-cropping-systems-with-recommended-and-high-inputs-during-the-transition-year-to-organic/).