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

Progress report for ONC21-094

Project Type: Partnership
Funds awarded in 2021: $39,680.00
Projected End Date: 03/31/2023
Grant Recipient: Stute Farms
Region: North Central
State: Wisconsin
Project Coordinator:
Dr. James Stute
Stute Farms
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Project Information

Summary:

No-till (NT) and cover crops (CC) used in concert improve agricultural sustainability in our region of Southern Wisconsin by increasing crop yield, protecting soil while enhancing soil health, protecting water quality by reducing nutrient loss and their use is socially acceptable, giving farmers “license to farm”- overall public approval from keeping the landscape green. However, difficult to control weeds which are glyphosate tolerant or resistant (GTRW) are now prevalent, threatening the combination.  Current management recommendations include row cultivation or eliminating CC residue to increase herbicide efficacy, either of which will negate sustainability gains. We hypothesize that using winter rye as cover and managing its biomass differently than currently practiced will suppress problematic weeds, adding to CC functionality and overall sustainability.

Replicated strip-trials on 4 farms are evaluating cereal rye cover crop use/ management (planting rate and termination timing) for weed suppression compared to current Extension recommendations for management of GTRW. We are monitoring weed dynamics throughout the growing season and impact on crop yield.

Outreach activities (with evaluation) will include a hands-on field day, interactions with highly engaged, regional farmer-led watershed groups and direct farmer-consultant communication through one of our projects partner farmers who is also a practicing crop consultant.

Project Objectives:
  1. Determine if rye planting rate and delayed termination i.e. “planting green” are effective for controlling/suppressing problematic GTRW in no-till soybean. Soybean is the crop of interest because this rotational phase is where the problem is greatest
  2. Compare use of cover crops against current University of Wisconsin-Extension (UWEX) herbicide recommendations for GTRW suppression
  3. Determine if rye residue from traditional, preplant termination affects efficacy of preemergence residual herbicide
  4. Determine effect of termination timing on rye aboveground biomass and soybean yields
  5. Share results and experiences with farmers and technical advisors including UWEX Weed Science.

Cooperators

Click linked name(s) to expand/collapse or show everyone's info
  • Tom Novak (Researcher)
  • Tom Burlingham (Researcher)
  • Tyler Troiola (Researcher)
  • Nick Kau (Researcher)

Research

Materials and methods:

Replicated on-farm strip trials to evaluate the efficacy of rye management for suppressing GTRW compared to a cooperators routine practice and University of Wisconsin Extension (UWEX) recommendations were initiated in fall 2020. All four trial sites (S1-S4) have long no-till histories in corn-soybean rotation and have populations of one or more of the targeted, glyphosate tolerant weed species. By site, field specific concerns were: S1, water hemp (WH); S2, marestail (MT) and secondarily WH; S3, MT; and S4, both giant ragweed (GRW) and MT.

Treatments include:

  1. Routine farmer practice, late season rescue treatment acceptable, no rye (farmer control)
  2. UWEX recommendation: PRE herbicide (sulfentrazone + cloransulam); no rye (UWEX control)
  3. Rye cover 40 lb./a, early termination (“burndown”) including PRE herbicide
  4. Rye cover 80 lb./a, early termination including PRE herbicide
  5. Rye cover 40 lb./a, PRE herbicide, delayed termination (“planting green”)
  6. Rye cover 80 lb./a, PRE herbicide, delayed termination.

Treatments 2-6 included a planned post emergence application of Enlist Duo (2,4-D choline + glyphosate) to control escapes and newly emerged weeds. In practice, all cooperators follow basic UWEX recommendations relative to PRE herbicide use so plots designated for treatment 1 were managed identically to treatment 2, at least initially, allowing for future divergence if developing conditions warranted. Rye treatments were designed to evaluate the effect of rate and termination timing on biomass production and subsequent impact on weed suppression, but not intended as a substitute for PRE applied herbicide. Termination timings include preemergence (relative to soybean) designated PRE which is considered the traditional “burndown” treatment and postemergence, at rye anthesis designated POST, a practice referred to as “planting green” in the production agriculture community. Rye was terminated with glyphosate.

Rye was established in fall 2020 following corn (grain) for 2021. Fields were selected based on presence/uniformity of GTRW. Plots were scaled at individual sites to match equipment width with length determined by populations of GTRW to ensure uniformity across all plots. Partners used their routine practices for crop management not related to treatments. Site characteristics are presented in Table 1

Rye was sampled before termination (with glyphosate) to determine aboveground biomass and percent canopy as estimated by fractional green canopy cover using Canopeo (Patrignani and Ochsner, 2015). Biomass is reported on a dry matter basis and canopy as percent cover. Soils were concurrently sampled to a 2-foot depth in 1-foot increments to measure profile nitrate-N (PN). Profile measurements are outside the scope of the current work but were included with outside support (USDA ARS Cooperative Agreement 58-5090-7-072) to increase the utility of project results for Producer-led Watershed Protection Groups in Wisconsin.

Soybean was planted immediately before the first rye termination date (S1) or shortly after (S2-4) and preemergent herbicide applied. The second rye sampling and termination occurred at anthesis, 25-41 days after soybean planting (DAP). Postemergence herbicide (2,4-D choline plus glyphosate) was applied 44-60 DAP.

Weed emergence was monitored periodically throughout the season beginning at planting. Populations of target species were measured just before postemergence herbicide application and at harvest. Individual plot counts by species were converted to plants/acre. Areas measured by site were: S2, 0.1223 acre; S3, 0.1291 acre; S4, 0.0689 acre. Plot size at S4 was smaller to accommodate siting of the entire trial within a “patch” of giant ragweed which is under investigation for glyphosate resistance by UW Weed Science. Lack of weeds in the late terminated rye treatments led to the decision to not apply herbicide. This decision led to soybean yield impacts making discussion of yield effect more difficult but led to observations on weed control system management discussed below. Soybean yield was determined by mechanical harvest of the center two rows for a minimum of 100’.

The experimental design is a randomized complete block with 4 replicates. Data were subject to analysis of variance procedures using RStudio (ver. 2021.09.1) and means separated with a protected LSD (Fishers, p=0.05) where significant treatment effects were detected. Rye seeding rates were combined within termination timing following analysis of the subset of rye containing treatments which indicated the effect of seeding rate was nonsignificant for all response variables except preemergence AGB and % canopy (Tables 3, 4). Treatments 1 and 2 were combined to create one control due to identical experimental treatment (described above) and to equalize the number of observations per treatment for the analysis. Data were analyzed over sites as well as by site to provide site specific information for cooperators and to explain interactions which may be caused by unique site conditions including preplant weed management. Combined data is presented to the extent possible. Waterhemp populations were inconsequential in 2021 so not discussed.

Rye AGB and canopy data were submitted to the UW-Madison Dept. of Soil Science for inclusion in the SnapPlus Nutrient Management Planning software database. SnapPlus is Wisconsin’s official NM planning software program and uses this data to estimate soil and nutrient loss based on agronomic practices. Inclusion of more cover crop conservation performance data should improve estimates of sediment and nutrient loss reductions associated with cover use.

Research results and discussion:

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.

 

Participation Summary
4 Farmers participating in research

Educational & Outreach Activities

1 Other educational activities: Shop talk, please see below.

Participation Summary:

4 Farmers
1 Ag professionals participated
Education/outreach description:

A formal cooperator update meeting was held February 4, 2022. Activities included discussion of 2021 research results and 2022 outreach planning.

A formal field day will be held August 12th at Langmanor Farm, Palmyra. This event will be cosponsored by the Jefferson County Soil Builders, a newly formed Producer-led Watershed Production group whose membership includes all of our cooperating farmers. 

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.