What's Cropping Up? Blog

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Organic compared with conventional wheat once again has more rapid emergence, greater early-season plant densities, and fewer fall weeds when following soybean in no-till conditions

Bill Cox, Eric Sandsted, and Jeff Stayton

Organic wheat on the right (two 10-foot passes) was planted at 1.2 M seeds/acre, the same rate as the conventional wheat in the left. The variety is the same (P25R46) in both cropping systems but the conventional wheat on the left received a fungicide-insecticide seed treatment, whereas the organic wheat on the right received no seed treatment.

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 36-month transition period (2014-2017) from conventional to an organic cropping system. This article will focus on days to emergence, early plant densities, and fall weed densities of wheat in the fall of 2017.

Soybeans were harvested on September 26. We no-tilled wheat into the soybean stubble on the following day, September 27, because of the paucity of winter annual and winter perennial weeds. We used a John Deere 1590 No-Till Grain Drill to plant the treated (insecticide/fungicide seed treatment) soft red wheat variety, Pioneer 25R46, in the conventional cropping system; and the untreated 25R46 in the organic cropping system at two seeding rates, ~1.2 million seeds/acre (recommended input) and ~1.7 million seeds/acre (high input treatment). Soil conditions were dry so we planted ~2.0 inches deep to get into moisture. We applied about 200 lbs. /acre of 10-20-20 as a starter fertilizer to conventional wheat in both input treatments. In the organic cropping system, we applied the maximum amount of Kreher’s composted manure (5-4-3 analysis) that would flow through the drill, or about 100 lbs. of material/acre, as a starter fertilizer in both organic input treatments. We also broadcast Kreher’s composted manure to provide ~60 lbs. of actual N /acre (assuming 50% available N from the composted manure) in the high input treatment in the organic cropping system. In addition, we also added Sabrex, an organic seed treatment with Tricoderma strains, to the seed hopper of 25R46 in the high input treatment in the organic cropping system. Finally, we also applied Harmony Extra (~0.75 oz. /acre) to the high input conventional treatment at the early tillering stage (GS 2-October 27) for control of winter perennials (dandelion in particular).

We estimated plant emergence (>50% emergence) on October 6 and 7. We estimated plant densities on October 13 by counting all the plants in the four middle rows along a 1-m long meter stick in five different regions in the 100 foot long plots. We estimated weed densities on October 28 by counting all the visible winter weeds (there were a few summer annuals but numbers were low) along the entire 100 foot plot in the immediate 8 rows (on the way up) and the more distant 8 rows (on the way back). We also noted the dominant weeds in the plots (95% or more of the winter weeds were dandelions).

Organic compared to conventional wheat emerged 1.0 to 1.75 days earlier, had much better stands 2 weeks after planting, and fewer weeds 5 weeks after planting (Table 1), similar to our 2015 results (http://blogs.cornell.edu/whatscroppingup/2015/11/23/wheat-emergence-early-plant-populations-and-weed-densities-following-soybeans-in-conventional-and-organic-cropping-systems/). The experimental site received only 1.48 inches of precipitation in August and 2.55 inches in September so soil conditions were generally dry after soybean harvest. More specific to planting time, only 0.18 inches of precipitation were recorded in the 10-day period before planting to the 10-day period after planting. Dry soil conditions, the 2-inch planting depth, and the considerable soybean stubble (55-60 bushel/acre crop) undoubtedly contributed to the relatively long emergence time (8 to 10 days), despite warm conditions for the 8 to 10 days after planting (58.6 vs. 55.7 OF, average since 1980). As in 2015 wheat, we speculate that the seed treatment in the conventional wheat made the seed somewhat more impermeable to soil water imbibition under the relatively dry soil conditions, resulting in delayed emergence in the conventional cropping system by 1 to 1.75 days. We also noted more rapid emergence for organic compared with conventional corn in 2016 under very dry soil conditions. It would be interesting to test if seed treatment actually delays emergence of crops under dry soil conditions because of the consistency of observations across crops and dry years at planting in our study.

Organic compared to conventional wheat had greater plant densities (37%) 2 weeks after planting in part because of the delayed emergence of conventional wheat (Table 1). More conventional wheat emerged after our observations but we could not do another wheat density count because the earlier emerging wheat began to initiate tillers, which made counting too problematic. Another factor that may have influenced our results is that seed size differed between the untreated 25R46 (~11,000 seeds/lb.) and the treated 25R46 (~12,000 seeds/lb.) so drill settings were not consistent between the two plantings. Organic compared with conventional wheat also had 10 to 25% greater early plant densities 2 weeks after planting in the fall of 2016. Organic and conventional wheat, however, had a similar number of spikes or heads/m2 at harvest (~525 heads/m2) in 2016 so conventional wheat compensated for the lower plant densities with increased tillering, contributing in part to its 7.5% greater yield (http://blogs.cornell.edu/whatscroppingup/2016/09/26/organic-wheat-looked-great-but-yielded-7-5-less-than-conventional-wheat-in-20152016/).

Once again, organic compared with conventional wheat generally had fewer weed densities, especially in the field in which corn was the 2014 crop (Table 1). Weed densities, however, were very low so yields will probably not be compromised except in a couple of the plots in the conventional cropping system under recommended inputs (no herbicide) when corn was the 2014 crop. Dandelion was the dominant weed specie in all plots. Apparently, the last cultivation of soybean on July 20 removed existing or late-emerging dandelions, whereas the observed weeds in the conventional cropping system apparently emerged after the June 21 Roundup application.

In conclusion, organic compared with conventional wheat, no-tilled into soybean stubble, once again got off to a better start in the fall of 2017, as it did in the fall of 2015. Despite the better fall start in 2015, organic wheat yielded 7.5% lower. We could only apply ~100 lbs. /acre of Kreher’s compost to the organic wheat through the drill at planting in both years, due to flow problems of the composted manure, which may be a yield constraint (very little P or K applied). We will top-dress the recommended input treatment with ~75 lbs. N/acre of  Kreher’s material at green-up time in the early spring, and add an additional ~50 lbs. of N/acre with Kreher’s to the high input treatment at the end of tillering, as we did in the spring of 2016. The organic wheat, however, had much lower kernel N (1.66% N) compared with conventional wheat (2.03% N) at harvest in 2016, indicating that lack of available soil N in organic wheat probably contributed to the 7.5% lower yield in 2016. Wheat fertility and not stand establishment nor weed control appears to be the major challenge to successful organic wheat production under conditions in our study.


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