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 summary, 2021 began hot and dry, and these conditions persisted throughout the season, toggling between moderate and severe drought as classified by the National Drought Mitigation Center: US Drought Monitor https://droughtmonitor.unl.edu/ ). Conditions allowed timely fieldwork, but crops were under varying degrees of moisture stress throughout the season. In contrast, 2022 began wet and cool delaying field activities, a mid-season dry period led to crop moisture stress, then an abrupt change to abundant precipitation in early August caused a rebound in crop condition and growth. Overwinter precipitation, important for early rye growth and soil moisture recharge, was approximately 73% of normal averaged across sites in both years. A summary of monthly precipitation and GDD accumulation can be found in Table 2, monthly and seasonal precipitation departures from long-term mean are presented in Figure 1.

Timely rainfall in adequate amounts is required for activation of surface applied residual herbicides. Most Midwestern University Extensions suggest between 0.5 and 1 inch are required for activation, depending on the level of soil moisture, organic matter, and texture, and are required within 7 to 10 days post application (DAA, days after application). This is consistent with the label for the product used in this study, which was applied to dry soil in 2021, moist soil in 2022.

Daily precipitation from the time of application to 45 days post-application is presented in Figure 2. Timing and amount requirements appear to be met in 2021 in both trial regions with a single event and several smaller follow-up events at all locations within 10 DAA. This is not the case in 2022 when no precipitation fell within 10 DAA and singular events exceeding the suggested 0.5 inch minimum fell well beyond 10 DAA. This data is important to the weed suppression discussion below.

Rye biomass production and canopy

Rye AGB and percent canopy at termination results are presented in Figure 3. Delaying termination from planting to anthesis increased AGB approximately 13-fold. This relationship was similar in both years despite 60-66% greater AGB levels in 2022 which most likely resulted from greater early season precipitation to support rye growth (Table 2). The increased seeding rate increased rye AGB at early termination (p ≤ 0.007) and canopy (% green cover) at both termination timings (p≤ 0.006) but not AGB at the anthesis termination (p≥ 0.148).

Percent canopy at the PRE termination timing is an important consideration for both termination timings because it represents potential residual herbicide interception during application and thus, a reduction of herbicide efficacy. Mean canopy ranged from 7.9 to 17.2% and while significant (p≤ 0.006), increasing seeding rate had a minimal impact on canopy, +2.3% in 2001, +3.6 in 2022. Canopy increased with increasing biomass, 2022 was 118% greater than 2021. Coupled with the lack of an apparent “activation” rainfall event, increased cover probably compounded already reduced herbicide efficacy.                       

GTRW suppression

Rye in combination with residual PRE-applied herbicide was more effective in GTRW suppression compared to herbicide alone (Figure 4).  The degree of suppression was strongly affected by termination timing: late termination in the planting green system producing 94 to 100% suppression compared to control across GTRW species while early, PRE termination provided 40 to 64%. This relationship was consistent across sites and apart from WH, was largely unaffected by rye seeding rate.

Marestail was present at all sites in both years and in greater numbers in 2021. This relative abundance is somewhat surprising given the precipitation received in 2021 for residual herbicide activation. More importantly, the lack of rainfall for apparent activation in 2022 may give a better indication of rye’s suppressive effect alone. While the reduction in population was highly significant (p <0.001) for both termination timings in both years, the reduction was less in 2022 (PRE, 40.0%; POST, 93.6%) compared to 2021 (64.1 and 97.9% respectively), but still substantial and indicates the value of the cover crop as part of an integrated weed management system, especially when conditions do not favor herbicide activation.

Giant ragweed and WH only occurred at single site-years each, GRW at S4 in 2021, HW at S7 in 2022 and followed similar suppression trends in both cases. The GRW reduction: PRE, 49.8%; POST 99.0% (p<0.001) is interesting, not only in absolute terms but also in the reduction in population variability which is evident in the spread of the rye treatment boxplots (Figure 4). The large spread of the no rye treatment is assumed due to the patchy nature of the weed, and is not evident in the rye treatments, indicating more consistent performance. Waterhemp suppression: PRE, 54.2%; POST, 100% (p=0.180 for PRE, POST not analyzed statistically due to complete suppression, all zero values) in 2022 occurred in the year with questionable residual herbicide activation, therefore we believe this suppression is mostly due to rye/ rye residue. This is also the only case where seeding rate had an unquestioned (p<0.001) effect on suppression, 26.6 vs 81.8% (40 vs. 80 lb./ acre respectively), further supporting the rye/ rye residue suppression argument.

The concern over rye biomass interception of soil-applied residual herbicide leading to reduced efficacy also appears unwarranted, at least under the conditions of this study. This observation is based on the consistent reduction from the herbicide only treatment to the early terminated rye treatment: better system performance regardless of interception. Further evidence is provided by the lack of seeding rate influence on suppression. In the PRE termination timing, the greater rye seeding rate increased both AGB and percent canopy, both indicators of potential herbicide interception, yet little difference in weed population was observed (WH at S7 is the exception for seeding rate influence but occurred in the year of questionable activation).

Lack of weeds in the late terminated rye treatments led to the decision not to apply post emergence herbicide in 2021, a systems level decision based on the principals of integrated pest management (IPM). This resulted in significantly greater marestail populations (p=0.008) from later emerging plants at harvest in this treatment and is partially responsible for soybean yield reduction (Figure 5). Because marestail populations did not rebound after POST herbicide application in the no rye and early termination treatments relative to the late termination treatment, we conclude a post application “flush” did not occur, thus the suppression provided by the late termination timing delayed the in-season flush. In a year with adequate moisture, we would anticipate a second flush of marestail following application, 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 in years with adequate moisture, reducing both chemical control costs and the potential for resistance development by subjecting fewer weeds to less active ingredient. In review, we should have delayed the POST herbicide application in the late-terminated treatments for a more objective comparison.

Soybean yield

Soybean yield is presented in Figure 5. Yield response to rye management followed the same pattern in both years: equivalent to slightly increased yield for PRE termination relative to no rye and a significant (p<0.001) yield reduction for late termination. The yield reduction is mostly attributed to direct competition for moisture and additional moisture stress during a season-long drought (2021, 28.8% reduction) or midseason drought (2022, 15.1% reduction). Secondary contributors could include shading and mechanical interference from standing rye (Figure 6), especially in 2022 with greater AGB levels (Figure 2). Figure 7, Figure 8 show developmental differences at different times of the growing season. The 2021 late-season marestail flush in the untreated POST treatment presumably also contributed although the population was relatively low at harvest (810 plants/a) and conversely, these soybean plants were not subject to the additional stress of herbicide exposure for during the mid-season application.

In general, plants grown in the late termination system were smaller than either other treatment and exhibited moisture stress sooner as soils dried following rain. This treatment demonstrated compensatory growth as precipitation began to exceed normal in mid-2022 and plants were similar in size and appearance to the other treatments at maturity. In general, the timely rains in mid-2022 resulted in yields which are considered good in this region.

What the data doesn’t show   

The following observations deserve mention but don’t necessarily fit neatly in the above categories:

The delayed termination treatment delayed maturity by approximately 7 days across all sites (based on maturity ratings) and resulted in greater (~2%) grain moisture at harvest. Delayed maturity is evident in Figure 9.

Rye residue in the delayed termination treatment remained standing by harvest at some locations (Figure 10). While not possible in an experimental situation, combine adjustment may be necessary to manage the additional material flowing through the combine and to obtain a clean sample.

All rye treatments reduced pressure of late emerging grass weeds (crab and witchgrass, Figure 11) which are prevalent on the lighter soils of our study region. Cooperating farmers made this observation and note this may eliminate the need for a late-season application of a residual grass herbicide, reducing production costs.

Figure 12 depicts a giant ragweed plant which survived the post emergence application of 2,4-D choline and glyphosate at Site 4 in 2021. This was the first ever post emergence application of this herbicide combination to traited soybean at this site and follow-up investigation could find no fault with the application. The plant was mistakenly destroyed: it should have been submitted to the University of Wisconsin for further resistance investigation as the population has already been confirmed resistant to three other herbicide modes of action.

This finding reinforces the potential of cereal rye use as a component of a resistance management plan. Both termination timings provided substantial population reductions. In addition, the delay in weed emergence caused by late termination may make complete control possible with one postemergence application. We only needed one application during this study but in years with more favorable precipitation causing more, prolonged weed emergence, multiple control applications are often required. Fewer weeds with less herbicide active ingredient exposure is a cornerstone of resistance management. 

Profile nitrate

Soil profile nitrate-N was measured at both rye termination timings in 2021 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 13). 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.