Can "planting green" suppress troublesome glyphosate tolerant/ resistant weeds in no-till soybean?

Growing season conditions had a major influence on crop growth and development and influenced weed suppression and soybean yield. In general, both the overwinter period (Oct.-Mar.) which influences rye growth and development, and the growing season (Apr.-Sept.) were much drier and warmer than normal and followed a regionally dry 2020 (Table 2). Overwinter and growing season precipitation were both approximately 62% of the 20-year normal (2000-19) and temperature (as GDD) 120% of normal as measured at the nearby NOAA National Weather Service Office, Sullivan. The Sullivan data is used to present a regional characterization using its long-term record which is not available for the individual sites.

Limited overwinter precipitation resulted in little profile recharge which was evident during soil sampling and later in the season when soybean failed to close canopy, independent of treatment. Dry conditions also allowed development of widespread frost May 28-29, forcing abandonment of S1 due to differential soybean mortality between treatments and general frost injury independent of treatment at S2-3.

Rye biomass production and canopy

The increased seeding rate affected rye AGB at early termination and canopy (% green cover) at both termination timings but had no effect on system response variables, so data are averaged over seeding rate. Rye AGB production is reported in Table 4. As expected, delayed termination resulted in increased AGB (Table 4) and canopy (Figure 1). Sites 1, 4 had greater AGB and canopy than sites 2 and 3 which appear unrelated to factors which could influence yield such as soil type, planting date, residue from the previous corn, soil fertility or precipitation (Table 2, Table 3).

MT suppression

Rye with PRE herbicide was more effective in MT suppression compared to herbicide alone, measured mid-season (Table 3). This suppression was not affected by seeding rate but was strongly affected by termination timing, with late termination producing the greatest effect (Figure 2). This was consistent across sites, however the difference between and early and late termination was not significant at S3 with both providing equivalent suppression (Figure 3). Enhanced suppression of the early termination is unlikely due to AGB quantity because this site was intermediate to the others for this variable (Table 4). As with rye AGB data, this difference cannot be explained by site specific conditions.

The concern over rye biomass interception of soil-applied preemergence herbicide application leading to reduced efficacy appears unfounded, at least in 2021. This observation is based on the consistent difference between the herbicide only treatment and the early terminated rye treatment. Had the AGB interfered with effective herbicide deposition, we would expect reduced suppression, but the opposite was observed. Further evidence is provided by the lack of influence of seeding rate on suppression. In the preemergent termination timing, rye seeding rate affected both AGB and percent canopy, measures of two factors which could influence herbicide interception, yet no difference in weed population was observed.

Lack of weeds in the late terminated rye treatments led to the decision not to apply post emergence herbicide, a systems level decision based on the principals of integrated pest management (IPM). This resulted in significantly greater marestail populations from later emerging plants at harvest at all sites and is partially responsible for soybean yield reduction (Table 3). Because marestail populations did not rebound after treatment in the no rye and early termination treatments relative to late termination, indicating no post treatment “flush,” we conclude the suppression provided by late rye termination delayed the in-season flush. In a year with adequate moisture, we would anticipate a second flush of marestail following treatment, necessitating a second herbicide application. The practical implication of this observation is that suppression by late rye termination may result in the need for only one post emergence herbicide application, reducing chemical control costs and the potential for resistance development by subjecting fewer weeds to less active ingredient.

We are evaluating slight trial modification including untreated subplots for 2022 to help evaluate weed management options and effects.

GRW suppression

Rye suppression of GRW at Site 4 was consistent with that of MT (Figure 4).

Soybean yield

Soybean yield is reported in Table 3, shown in Figure 5. The comparison between no rye and early terminated rye is considered a pure treatment effect due of identical herbicide applications while the comparison between late terminated rye and the other treatments is partially confounded by differential herbicide application. Early terminated rye had no impact on yield at S2 and S3 but reduced yield at S4 (Table 3). Because weed suppression at S4 was similar to the other sites, yield difference is attributed to greater soil moisture use by rye at this site as indicated by greater rye AGB (Table 4). Lack of post emergence weed control in the late terminated rye treatment is partially responsible for the significant yield reduction. Early season moisture use by rye and direct competition with soybean is probably also partially responsible. Both reduced soybean plant size and greater moisture stress were observed in this treatment and are evident in Figure 6, Figure 7.

Profile nitrate

Soil profile nitrate-N was measured at both rye termination timings to validate additional ecosystem services from the cover crop. Early terminated rye had no effect on PN compared to no rye, while late terminated rye significantly reduced it (Figure 8). Interestingly, PN increased significantly without rye in the ~40-day period between sampling/ termination while remaining unchanged with rye which captured PN as it became available. The increase in PN is probably the result of nitrification of residual applied N from 2020. While not measured, the potential for carryover of ammoniacal-N from 2020 corn fertilization exists because lack of moisture limited corn yield. Because these soils are leaching prone and moisture limited soybean yield (and presumably nutrient uptake), this nitrogen scavenging may have reduced leaching potential.

These results, coupled with evidence of weed suppression further support the multi-functional role of rye cover crops in creating a more sustainable agriculture.