Introduction
Resource competition between weeds and soybean [Glycine max (L.) Merr.] has caused significant yield losses in the United States. Annually, potential soybean yield losses were estimated up to 52% in the absence of weed control measures (Soltani et al. Reference Soltani, Burke, Davis, Dille, Everman, Sikkema and VanGessel2017). Considering the U.S. soybean production from 2023 (112 million Mg; USDA-NASS 2023), these losses due to weed interference would be equivalent to approximately 58 million Mg of soybean or US$28 billion (US$473.99 Mg−1 of soybean; USDA-ERS 2023). Yield losses can be significantly reduced when chemical weed control measures are adopted. However, overreliance on herbicides to control weeds led to the development of more than 600 cases of herbicide resistance in the United States since 1957 (Heap Reference Heap2024). In this regard, integrated weed management (IWM) strategies that include cultural and mechanical methods to control weeds have become more popular to potentially delay the development of herbicide resistance.
Cover crops are one of the IWM tools often used to suppress weed growth (Swanton and Murphy Reference Swanton and Murphy1996). In addition, cover crops are known for improving the soil physical, chemical, and biological properties, as well as reducing soil erosion and nutrient leaching (Kladivko et al. Reference Kladivko, Kaspar, Jaynes, Malone, Singer, Morin and Searchinger2014; Rorick and Kladivko Reference Rorick and Kladivko2017; Ruffatti et al. Reference Ruffatti, Roth, Lacey and Armstrong2019; Villamil et al. Reference Villamil, Bollero, Darmody, Simmons and Bullock2006). Cereal rye (Secale cereale L.) is the most commonly used cover crop species and has the highest potential to suppress weeds through the competition for resources (light, water, and nutrients), allelopathy, and the physical barrier created by the residue after termination (Clark Reference Clark2007; Fernando and Shrestha Reference Fernando and Shrestha2023; Teasdale Reference Teasdale1996). Previous studies have indicated that biomass accumulation is the limiting factor for weed suppression and is directly proportional with weed suppression (Hodgskiss et al. Reference Hodgskiss, Young, Armstrong and Johnson2020; Teasdale and Mohler Reference Teasdale and Mohler1993; Wallace et al. Reference Wallace, Curran and Mortensen2019). In the presence of 3.9 Mg ha−1 of cereal rye biomass, Wallace et al. (Reference Wallace, Curran and Mortensen2019) observed horseweed [Erigeron canadensis L.; syn. Conyza canadensis (L.) Cronquist] densities reduced up to 95% at the time of spring cover crop termination compared with no cover crop.
The soybean planting green method consists of planting soybean into a live stand of cereal rye. Cereal rye is terminated at the anthesis growth stage, when the plants are close to their maximum biomass accumulation. The goals are to delay the termination of the cover crop so there is enough biomass to provide weed suppression, conserve soil moisture, reduce soil temperature fluctuations, and maximize carbon inputs to the soil (Balkcom et al. Reference Balkcom, Duzy, Kornecki and Price2015; Basche et al. Reference Basche, Kaspar, Archontoulis, Jaynes, Sauer, Parkin and Miguez2016; Reed and Karsten Reference Reed and Karsten2022). However, high-residue systems are challenging not only during planting but throughout the growing season. Reduced efficiency of closing the seed furrow while planting the cash crop or reduced soil-to-seed contact in the seed furrow are some of the problems faced during planting (Kornecki et al. Reference Kornecki, Raper, Arriaga, Schwab and Bergtold2009; Reed et al. Reference Reed, Karsten, Curran, Tooker and Duiker2019). Early in the season, the cover crop residue mat also retains soil moisture, thus providing near optimal environment for the growth of seedling pathogens (Acharya et al. Reference Acharya, Moorman, Kaspar, Lenssen and Gailans2022). The term “green bridge” is related to planting green systems, where insects migrate from the decaying cover crop and start feeding on newly emerged soybean plants, sometimes causing yield losses due to stand reductions early in the season (Dean et al. Reference Dean, Anderson and Hodgson2022; Dunbar et al. Reference Dunbar, O’Neal and Gassmann2016; Obermeyer Reference Obermeyer2020). Furthermore, the presence of high amounts of biomass can result in nutrient immobilization and therefore reduce nutrient availability to the soybean (Wells et al. Reference Wells, Reberg-Horton, Smith and Grossman2013). Nitrogen (N) and sulfur (S) are two examples of nutrients that can become unavailable if the cover crop residue has C:N and C:S ratios above 25:1 and 400:1, respectively (Tabatabai and Chae Reference Tabatabai and Chae1991; White et al. Reference White, Finney, Kemanian and Kaye2016).
The planting green impact on soybean yield is highly variable. For instance, several studies have reported soybean yield reductions varying from 14% to 45% when cover crop termination was delayed (Hodgskiss et al. Reference Hodgskiss, Young, Armstrong and Johnson2022; Liebl et al. Reference Liebl, Simmons, Wax and Stoller1992; Nunes et al. Reference Nunes, Arneson, DeWerff, Ruark, Conley, Smith and Werle2023b). Conversely, Reed et al. (Reference Reed, Karsten, Curran, Tooker and Duiker2019) did not observe soybean yield reductions due to the adoption of the planting green method.
Cereal rye can be terminated chemically with herbicides or mechanically with a roller-crimper, mower, or tillage. Roller-crimper use has gained popularity among growers that are adopting the soybean planting green method as an alternative to lay the residue flat above the soil surface and potentially increase the ground cover (Mirsky et al. Reference Mirsky, Curran, Mortenseny, Ryany and Shumway2011). Effective termination of cereal rye with a roller-crimper is only possible if the plants have reached the reproductive stage and is more effective as the plants mature (Parr et al. Reference Parr, Grossman, Reberg-Horton, Brinton and Crozier2014; Wells et al. Reference Wells, Reberg-Horton and Mirsky2014). The use of a roller-crimper, however, does not eliminate the need for herbicides, even when high amounts of biomass are present (Davis Reference Davis2010; Dorn et al. Reference Dorn, Stadler, van der Heijden and Streit2013). The season-long effects of high-residue accumulation such as moisture conservation and reduced temperature fluctuations can favor the germination and emergence of weeds later in the season (Teasdale and Mohler Reference Teasdale and Mohler1993). Previous studies have reported inadequate weed control when cover crops were used as the sole weed management strategy (Teasdale et al. Reference Teasdale, Pillai and Collins2005). Conversely, adequate weed control was achieved when cover crops were used in combination with comprehensive weed management programs including pre- and postemergence herbicides (Cornelius and Bradley Reference Cornelius and Bradley2017; Whalen et al. Reference Whalen, Shergill, Kinne, Bish and Bradley2020; Wiggins et al. Reference Wiggins, Hayes and Steckel2016).
The use of soil-residual herbicides at cover crop termination has been suggested to extend weed control through the critical weed-free period (Nunes et al. Reference Nunes, Arneson, Wallace, Gage, Miller, Lancaster, Mueller and Werle2023a; Whalen et al. Reference Whalen, Shergill, Kinne, Bish and Bradley2020). However, when used at cover crop termination, only some of the herbicide applied reaches the soil, while the rest is intercepted by the cover crop biomass, hence reducing initial herbicide concentrations in the soil that would be biologically available to germinating weed seeds (Banks and Robinson Reference Banks and Robinson1982, Reference Banks and Robinson1984; Ghadiri et al. Reference Ghadiri, Shea and Wicks1984; Nunes et al. Reference Nunes, Arneson, Wallace, Gage, Miller, Lancaster, Mueller and Werle2023a; Whalen et al. Reference Whalen, Shergill, Kinne, Bish and Bradley2020). The extent of herbicide interception has been correlated with biomass accumulation, with high-residue systems intercepting more herbicide than cover-cropping systems with early terminations (less biomass) (Nunes et al. Reference Nunes, Arneson, Wallace, Gage, Miller, Lancaster, Mueller and Werle2023a; Whalen et al. Reference Whalen, Shergill, Kinne, Bish and Bradley2020). Research conducted by Whalen et al. (Reference Whalen, Shergill, Kinne, Bish and Bradley2020) suggested that when cover crop termination was delayed from 21 to 7 d before planting, sulfentrazone concentration in the soil at the time of application was reduced by approximately 57% due to cover crop biomass.
Once intercepted, the herbicides can only move to the soil with rainfall or irrigation, with greater water volumes washing off more herbicide from the biomass to the soil (Khalil et al. Reference Khalil, Flower, Siddique and Ward2019). Previous studies have reported differences in metribuzin concentration in the soil varying from 1% to 15% relative to what was applied after 20 mm of rainfall (Banks and Robinson Reference Banks and Robinson1982). Similarly, Ghadiri et al. (Reference Ghadiri, Shea and Wicks1984) demonstrated that after 50 mm of rain, atrazine concentration in the soil increased more than 2-fold, while the amount retained in the wheat (Triticum aestivum L.) straw was reduced by 90%. Furthermore, ground cover, age of cover crop residue, and herbicide solubility are other factors that will influence how much and how fast the herbicide will move from the cover crop biomass to the soil. Generally, greater ground cover, older residue (Dao Reference Dao1991), and lower herbicide solubility (Khalil et al. Reference Khalil, Flower, Siddique and Ward2019) tend to limit the amount of residual herbicides reaching the soil at application and after rainfall or irrigation. Khalil et al. (Reference Khalil, Flower, Siddique and Ward2019) reported that with 5 mm of rainfall, more pyroxasulfone (3.49 mg L−1 water solubility) leached from the residue to the soil than trifluralin (0.3 mg L−1 water solubility), largely due to differences in water solubility between these two compounds. Reduced herbicide concentrations in the soil due to interference from cover crop residue may contribute to the selection of non–target site herbicide resistance (Busi et al. Reference Busi, Neve and Powles2013; Neve and Powles Reference Neve and Powles2005). For instance, a multiple-resistant rigid ryegrass (Lolium rigidum Gaudin) population that was subjected for three generations to low doses of pyroxasulfone had more than 30% survival rate after the application of 240 g ai ha−1 (2.4-fold the label rate) (Busi et al. Reference Busi, Gaines, Walsh and Powles2012).
Research regarding the fate of residual herbicides in high-residue cover-cropping systems and its impact on weed control is still limited. The objectives of this research were to determine (1) whether the practice of roller-crimping cereal rye increases ground cover and reduces the density of giant ragweed (Ambrosia trifida L.) and grasses relative to standing cereal rye; (2) the concentration of sulfentrazone, S-metolachlor, and cloransulam-methyl in soil at seven sample timings; and (3) whether roller-crimping cereal rye increases soybean yield relative to standing cereal rye.
Materials and Methods
Field experiments were established in the fall of 2021 and 2022 at the Throckmorton Purdue Agricultural Center (TPAC; 40.29°N, 86.90°W). Trial locations varied from one year to another respecting the corn (Zea mays L.)–soybean rotation at TPAC (adjacent fields; same soil type). Soil was Toronto-Millbrook silt loam with 20% sand, 53% silt, 26% clay, with organic matter and pH ranging from 2.9% to 4.2% and 6.0 to 6.5, respectively, between 2022 and 2023. The fields were previously managed under a corn–soybean rotation for more than 10 yr and were in corn during the 2021 and 2022 growing seasons. Before cereal rye planting in fall of 2021, the corn crop was mowed, and residue was incorporated into the top 10 cm of soil using a rotary tiller. Conversely, in the fall of 2022, the previous corn crop was also mowed, but the field was disked and cultivated to eliminate crop residue and weeds, which resulted in a deeper incorporation of the corn residue compared with the management practice adopted in 2021. The target seeding rate for this experiment was 50 kg ha−1 of cereal rye (‘Elbon’, Cisco Company, Indianapolis, IN, USA). However, due to excessive soil moisture in October of 2021, the use of a box drill seeder was not possible. Thus, 20 kg ha−1 of extra seed were added to account for losses postplanting (e.g., lack of seed incorporation, animal feeding, rotting), and on October 23, 2021, cereal rye was spread at 70 kg ha−1 on the soil surface using a chest-mounted spreader (421-S, Solo, Newport News, VA, USA). Conversely, soil conditions were adequate in 2022, and cereal rye was planted at 50 kg ha−1 on September 16, 2022, using a John Deere 1590 box drill (Deere & Company, Moline, IL). Soil samples were taken in March of 2022 and 2023, at 0- to 10-cm depth to determine the physicochemical properties of the soil (Table 1).
Table 1. Chemical and physical properties of the soil at 0- to 10-cm depth from experiments conducted in 2022 and 2023, Lafayette, IN.
a Trials were conducted in adjacent fields between 2022 and 2023.
b OM, organic matter.
c Difference in OM content is due to management practices adopted before cereal rye planting in the fall of 2021 and 2022.
The experiment was laid out under a randomized complete block design with a 3 by 2 factorial arrangement having four replications for a total of 24 experimental units that measured 9.1 by 4.6 m in size. Treatments included two cereal rye termination management strategies (roller-crimped and standing cereal rye) as well as a fallow control, and two residual herbicide programs for a total of six treatments. The herbicide programs consisted of (1) with residual: glyphosate at 1,750 g ae ha−1, glufosinate at 737 1,750 g ai ha−1, sulfentrazone at 280 g ai ha−1, S-metolachlor at 1,790 g ai ha−1, and cloransulam-methyl at 44 g ai ha−1; and (2) no residual: glyphosate at 1,750 g ae ha−1 plus glufosinate at 737 g ai ha−1. All herbicides within each program were applied in tank mix and at cover crop termination, immediately after the use of the roller-crimper. Herbicides were applied using a CO2-pressurized spray boom equipped with eight AIXR 11002 nozzles (TeeJet® Spraying Systems, Wheaton, IL, USA). Nozzles were spaced 38 cm apart and calibrated to deliver 140 L ha−1 while traveling at 4.8 km h–1 and operating at 165 kPa. Nonionic surfactant (Class Act® Ridion®, Winfield Solutions, St Paul, MN, USA) at 0.5 % v/v and ammonium sulfate (34%, Amsol®, Winfield Solutions) at 5% v/v were added to all herbicide applications.
Glyphosate (Roundup PowerMax®, Bayer Crop Science, St Louis, MO, USA) was applied in March of 2022 and 2023 at 1,540 g ae ha−1 to eliminate cereal rye plants from the fallow control plots. At that point, plants were up to 10 cm in height with little biomass accumulation. The residue had, therefore, approximately 2 mo to decay and did not provide interception of residual herbicides at the time of treatment application.
Soybeans (‘AG30XF2’, Asgrow®, Bayer Crop Science; glyphosate, glufosinate, and dicamba tolerant) were planted at 350,000 seeds ha−1, in 76-cm row spacing when cereal rye plants reached anthesis (i.e., Zadoks 60; Zadoks et al. Reference Zadoks, Chang and Konzak1974). Immediately after planting, plots assigned to the roller-crimper treatment were rolled using a tractor-mounted 2.4-m-wide roller-crimper filled with water to increase weight. Following the use of the roller-crimper, all plots were sprayed with their specific herbicide treatments on May 20, 2022, and May 18, 2023. One postemergence application of glyphosate plus glufosinate at 1,740 g ae ha−1 and 737 g ai ha−1, respectively, was made 4 wk after glyphosate-resistant soybean planting (WAP).
Precipitation data (mm) were recorded at 1-h intervals by an automatic weather station (WatchDog Weather Station 2700, Spectrum Technologies, Aurora, IL, USA) placed within 10 m of the edge of the trial and averaged daily (Figure 1).
Figure 1. Daily precipitation during the 2022 and 2023 growing seasons, Lafayette, IN. Herbicide treatments were applied on May 23, 2022, and May 19, 2023. Data were collected from a weather station placed in the trial area.
In 2022 and 2023, the cereal rye stand was uniform across the trial area. Thus, average cereal rye biomass was measured for the whole trial and not per plot. Ten 0.25-m2 quadrats were randomly placed within the trial area, and all aboveground plant material was harvested by cutting the plants at the base (1 cm above soil surface). Samples were placed in a forced-air oven at 80 C for 48 h. The average cereal rye biomass was 4 and 14.2 Mg ha−1 in 2022 and 2023, respectively.
Density of A. trifida and grasses was determined before the postemergence application (4 WAP) and at 18 WAP (Table 2). Two 0.25-m2 quadrats were randomly placed in each plot (one in the first half of the plot area and one in the second half). The number of plants was recorded, averaged for each plot, and converted to plants per square meter.
Table 2. Ambrosia trifida and grass density in each cereal rye management strategy at 4 and 18 wk after soybean planting (WAP) a , Lafayette, IN.
a Numbers followed by the same letter or no letters within year, WAP, and weed species are not significantly different according to Fisher’s protected LSD (P ≤ 0.05).
b Ambrosia trifida and grass density data at 4 WAP were log transformed. However, original mean values are presented.
c Ambrosia trifida and grass density data at 18 WAP were square-root transformed. However, original mean values are presented.
d Data are presented as pooled over herbicide treatments. However, there was a significant interaction between cover crop and herbicide treatments for grass density at 4 WAP in 2022. The interactions are discussed in the text.
Soybean stands were determined at 18 WAP by counting the number of soybean plants per meter of row in the two center rows of each plot (Table 3). The first count was done in the front half of the plot and second count was done in the back half of the plot. The number of plants per meter of row was then converted to plants per hectare. Soybean yield (in Mg ha−1) was determined by harvesting all six soybean rows from each plot with a plot combine.
Table 3. Soybean final stand and yield from each cereal rye management strategy from experiments conducted in 2022 and 2023 a , Lafayette, IN.
a Numbers followed by the same letter within year are not significantly different according to Fisher’s protected LSD (P ≤ 0.05).
The concentration of residual herbicides in the soil throughout the growing season was determined by collecting soil samples immediately after herbicide application and at 10, 14, 28, 56, 84, and 112 d after application (Figure 2). Fifteen soil cores (2 cm in diameter by 5 cm in depth) were collected using a Gator Probe (AMS, American Falls, ID, USA), placed in a plastic bag (composite sample), and stored in a cooler at 4 C. The soil probe was cleaned with a 50% acetone solution to prevent sample contamination from one plot to another. No later than 1 d after sampling, soil cores from each plot were sieved (2 mm) to remove debris and homogenize the sample and then stored in a −20 C freezer until further processing.
Figure 2. Dissipation of sulfentrazone, cloransulam, and S-metolachlor from 0 to 112 d after herbicide application and under three cereal rye management strategies in 2022 and 2023, Lafayette, IN. Data points represent mean ± SE of four replications. Lines represent the first-order regression equations for each cereal rye management strategy. Parameter estimates for each regression line are detailed in Table 4.
The concentration of sulfentrazone, S-metolachlor, and cloransulam-methyl in soil samples was determined using the QuEChERS (Quick-Easy-Cheap-Effective-Rugged-Safe) method as previously described by Olaya-Arenas and Kaplan (Reference Olaya-Arenas and Kaplan2019) with modifications. All samples were analyzed within 4 mo of collection in an Agilent 1290 Infinity II ultra-high performance liquid chromatography (UHPLC) with a 6470 triple-quadrupole mass spectrometer and an EclipsePlus C18 RRHD 1.8-μm, 2.1 × 50 mm column (Agilent Technologies, Santa Clara, CA, USA) at the Bindley Bioscience Center at Purdue University. Recoveries from fortified untreated soil samples indicated that recovery was 112%, 80%, and 74% for sulfentrazone, S-metolachlor, and cloransulam-methyl, respectively.
Soil samples were thawed, and a 3-g (± 0.01) subsample of wet soil was transferred from each composite sample into 50-ml tubes (Falcon 50-ml centrifuge tubes, Thermo Fisher Scientific, Waltham, MA, USA). The exact weight of each sample was recorded and later used to calculate the dry weight based on the moisture content from each composite sample. The moisture content from each sample was determined from a 5-g subsample of wet soil that was placed in a forced-air oven at 105 C for 24 h. Next, 15 ml of double deionized water, 15 ml of acetonitrile enriched with 1% formic acid (all reagents MS grade; Thermo Fisher Scientific), and 10 μl of an isotopically labeled internal standard containing sulfentrazone, S-metolachlor, and cloransulam-methyl (Pestanal® standard 98.8%, Sigma-Aldrich, MO, USA) were added to the 50-ml tube containing the 3-g soil sample. The tube was agitated for 30 s with a Mini Vortex Mixer (VWR, Radnor, PA, USA). Once agitation was complete, anhydrous salts of magnesium sulfate (6 g) and sodium acetate (1.5 g) (Thermo Fisher Scientific) were added to the tubes, followed by another agitation of 30 s. Tubes were then transferred to the Geno/Grinder 2010 (SPEX sample prep, Metuchen, NJ, USA) and shaken for 3 min at 1,100 rpm and then centrifuged at 2,500 rpm for 10 min. Twelve milliliters of the supernatant was transferred into 15-ml dispersive solid-phase extraction tubes (part no: 5982-5158; Agilent Technologies, Santa Clara, CA, USA) that were shaken for 3 min at 1,100 rpm in the Geno/Grinder 2010 and then centrifuged at 4,000 rpm for 5 min. The supernatant was transferred into Falcon 15-ml centrifuge tubes (Thermo Fisher Scientific) and dried overnight in a speed vacuum (SC250EXP; Thermo Fisher). The dried pellet was re-suspended with 150 μl of a 50% acetonitrile solution, and the tube was agitated with a Mini Vortex Mixer until the pellet was dissolved. The 15-ml tubes were then centrifuged at 4,000 rpm for 5 min, and 130 μl of the supernatant was transferred to 96-well microplates (Nunc™ low-binding polypropylene, Thermo Fisher Scientific) before analysis in the UHPLC.
All data were subjected to an ANOVA using the PROC GLIMMIX procedure in SAS 9.4 (Supplementary Tables S1–S3). There was a significant treatment by year interaction for the weed density, soybean stand and yield, and herbicide concentration in the soil. Therefore, results are presented separately by year. The interaction between cereal rye management and herbicide treatments for weed density and soybean stand and yield were nonsignificant; therefore, data were combined over herbicide treatments within each year. One exception was in 2022, for the grass density at 4 WAP, when a significant interaction between cereal rye management and herbicide treatments was observed. In this case, the results are discussed separately by herbicide treatment. Assumptions of normality and homogeneity of variance were evaluated by visual assessment of residual plots. Data were log or square-root transformed when needed. However, original mean values are presented. Treatment means were separated using Fisher’s protected LSD (P ≤ 0.05). Nonlinear regression analysis of herbicide concentration over time (0 to 112 d after application) was performed to determine the first-order-dissipation rate constants for each herbicide within each year and each cereal rye management strategy (Table 4).
Table 4. Parameter estimates for sulfentrazone, cloransulam, and S-metolachlor from each cereal rye management strategy in 2022 and 2023, Lafayette, IN.
a First-order regression equation: nonlinear regression of herbicide concentration over time (0 to 112 d after application).
Results and Discussion
Cereal Rye Biomass
The amount of cereal rye biomass produced in the spring of 2022 (4 Mg ha−1) was 18% above the average biomass produced in eastern half of the United States (3.4 Mg ha−1; n = 5,695) (Huddell et al. Reference Huddell, Thapa, Marcillo, Abendroth, Ackroyd, Armstrong, Asmita, Bagavathiannan, Balkcom, Basche, Beam, Bradley, Canisares, Darby and Davis2024). In 2023, however, the biomass accumulated (14.2 Mg ha−1) was 4-fold greater than the same average, which can be considered excessive and not common for most growers utilizing cover crops in the United States. We attribute this difference in biomass accumulation mainly to the difference in planting dates. Because conditions for planting were not ideal, cereal rye was broadcast on October 23, 2021. However, in the fall of 2022, cereal rye was drilled in on September 16. Therefore, the 37 extra days allowed the plants to grow more in the fall of 2022 and resulted in bigger plants in the spring of the following year when growth resumed. Similar results were reported in previous studies that showed a 21% increase in biomass accumulation when triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum]) was planted in mid-September in comparison to a late-October planting date (Lyons et al. Reference Lyons, Ketterings, Godwin, Cherney, Czymmek and Kilcer2017). In addition to planting date, planting method was another factor contributing to the difference in biomass between 2022 and 2023, with drilling of cereal rye resulting in greater stands due to increased soil-to-seed contact in comparison to broadcasting (Fisher et al. Reference Fisher, Momen and Kratochvil2011).
Interception of Residual Herbicides by Cereal Rye Biomass
Although herbicide concentration in the cereal rye biomass was not measured on this study, we postulate that nearly all the herbicide applied should reach the soil in the absence of biomass, with only a negligible amount being lost through drift, volatility, and/or photodecomposition. Similarly, in the presence of biomass, part of the herbicide applied should be intercepted by the biomass, while the remaining part reaches the soil. This concept aligns with previous research showing that the reduction in the concentration of residual herbicide in the soil at the time of application is positively correlated with cereal rye biomass accumulation (Nunes et al. Reference Nunes, Arneson, DeWerff, Ruark, Conley, Smith and Werle2023b). Therefore, herbicide interception was calculated as the percent reduction from the concentration of the residual herbicide in the soil of plots with cereal rye relative to plots without cereal rye, at 0 DAT. In 2022, the presence of 4 Mg ha−1 of roller-crimped cereal rye biomass resulted in about 75% interception of the residual herbicides (Figure 2; Supplementary Table S4). As cereal rye biomass increased from 2022 to 2023 (3.5-fold), herbicide interception also increased. Up to 94% and 95% of the applied sulfentrazone and cloransulam-methyl, respectively, were intercepted by roller-crimped cereal rye biomass in 2023. These results corroborate those reported by previous studies (Banks and Robinson Reference Banks and Robinson1984, Reference Banks and Robinson1986; Crutchfield et al. Reference Crutchfield, Wicks and Burnside1986; Khalil et al. Reference Khalil, Flower, Siddique and Ward2018; Nunes et al. Reference Nunes, Arneson, DeWerff, Ruark, Conley, Smith and Werle2023b). Investigating the effect of increasing amounts of wheat straw, going from 2,240 and up to 6.7 Mg ha−1, Banks and Robinson (Reference Banks and Robinson1986) observed between 67% and 97% interception of acetochlor. Previous research has also reported a 12-fold reduction in spray coverage underneath 12.2 Mg ha−1 of cereal rye biomass in comparison to a no cover crop control (Nunes et al. Reference Nunes, Arneson, DeWerff, Ruark, Conley, Smith and Werle2023b).
Concentration of Residual Herbicide in Soil during the Growing Season
The application of residual herbicides in 2022 and 2023 was followed by 37 and 15 mm of rainfall, respectively, within 7 d (Figure 2). In 2022, the concentration of all residual herbicides tested was similar for all cereal rye and fallow treatments in nearly all sample timings, except at cereal rye termination and at 84 DAT (sulfentrazone and S-metolachlor only) (Figure 2; Supplementary Table S4). In 2023, with 14.2 Mg ha−1 of cereal rye biomass, the concentrations of sulfentrazone and S-metolachlor in the soil with roller-crimped cereal rye were lower than the concentrations measured in the soil of fallow plots during most of the growing season (at least four out of seven sample timings). With roller-crimped cereal rye biomass ranging from 5.2 to 12.2 Mg ha−1, Nunes et al. (Reference Nunes, Arneson, DeWerff, Ruark, Conley, Smith and Werle2023b) observed, on average, 49% and 77% reductions in the concentrations of sulfentrazone and metolachlor in the soil, respectively, when compared with the concentrations measured in the tilled soil, at 25 d after herbicide application.
Ambrosia trifida and Grass Density at 4 and 18 Weeks after Soybean Planting
No cereal rye management strategy by year interactions were observed for weed density. Therefore, data were pooled over years (Table 2). The primary weed species present in the field trial areas in 2022 and 2023 were: A. trifida, giant foxtail (Setaria faberi Herrm.), yellow foxtail [Setaria pumila (Poir.) Roem. & Schult.; syn.: Setaria glauca (L.) P. Beauv.], barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.), and fall panicum (Panicum dichotomiflorum Michx.).
Ambrosia trifida density in 2022 was similar for all treatments at the two evaluation timings (Table 2). However, in 2023, there were an average of 15 A. trifida plants m−2 in fallow plots, at 4 WAP, whereas in plots with cereal rye there were no A. trifida plants. Other researchers have reported A. trifida density being reduced by 56% with the use of cereal rye relative to the fallow treatment at the time of termination (DeSimini et al. Reference DeSimini, Gibson, Armstrong, Zimmer, Maia and Johnson2020). There was a significant interaction between cereal rye management strategy and herbicide treatments in 2022 at 4 WAP for grass density. Therefore, results are discussed separately by herbicide treatment. In that year, the inclusion of residual herbicides at cover crop termination and to the tank mixture applied to the fallow plots provided 95% and 50% control of grasses at 4 WAP, respectively, in comparison to the application of glyphosate plus glufosinate. These results were corroborated by those from Essman et al. (Reference Essman, Loux, Lindsey and Dobbels2023), who observed 85% to 90% lower S. faberi density in plots with cereal rye and sprayed with a residual herbicide, relative to the densities from plots without cereal rye or residual herbicide.
The 3.5-fold increase in cereal rye biomass from 2022 to 2023 contributed to the complete control of A. trifida at 4 WAP (Table 2). At that same evaluation timing, the average A. trifida density in fallow plots was 15 plants m−2. Investigating the effect of cereal rye termination timing in weed control, Essman et al. (Reference Essman, Loux, Lindsey and Dobbels2023) reported reductions in A. trifida density going from 60% to 90% when cereal rye termination was delayed from 7 to 21 d after soybean planting in comparison to the densities observed at preplant termination timing (7 d before planting). Other recent studies also suggest that increased amounts of cereal rye biomass (achieved with delayed termination) can result in greater suppression of E. canadensis (Schramski et al. Reference Schramski, Sprague and Renner2021) and waterhemp [Amaranthus tuberculatus (Moq.) Sauer] (Hodgskiss et al. Reference Hodgskiss, Young, Armstrong and Johnson2022). The use of a roller-crimper resulted in a grass control that was similar to the fallow treatment in both years and evaluation timings. In 2023, the standing cereal rye treatment resulted in an average of 8 grass plants m−2, while there were no grasses in plots with roller-crimped cereal rye or fallow, at 18 WAP.
Soybean Stand and Yield
No cereal rye management strategy by herbicide interactions were observed for soybean stand and yield, while the main effect of cereal rye management strategy was significant. Therefore, data were pooled over herbicide treatments within each year (Table 3). No differences were observed in soybean stand in 2022 (Table 3). In 2023, when the biomass increased to 14.2 Mg ha−1 (3.5-fold increase relative to 2022), standing cereal rye reduced soybean stands by an average of 69% in comparison to the fallow. The use of roller-crimper that year increased soybean stands relative to the standing cereal rye by 58% but was still 29% lower than the soybean stands from fallow plots. Our findings are consistent with the range of 20% to 60% reduction in soybean stand reported for the use of cereal rye as cover crop (Westgate et al. Reference Westgate, Singer and Kohler2005; Reddy Reference Reddy2001; Williams et al. Reference Williams, Mortensen and Doran2000). We attribute the soybean stand reductions observed in this research mostly to poor seed slot closure that led to reduced soil-to-seed contact. Furthermore, the use of roller-crimper likely increased the soil-to-seed contact, resulting in greater soybean stands than standing cereal rye. During planting, the presence of large amounts of cereal rye biomass in combination with higher soil moisture (in comparison to the soil moisture from fallow plots; data not shown) resulted in hairpinning, when the front coulters were pushing the residue into the furrow and required more down force than what the planter was capable of providing to the closing wheels. Similar issues during soybean planting were also reported in previous studies (Eckert Reference Eckert1988; Hovermale et al. Reference Hovermale, Camper and Alexander1979; Liebl et al. Reference Liebl, Simmons, Wax and Stoller1992; Wagner-Riddle et al. Reference Wagner-Riddle, Gillespie and Swanton1994; Williams et al. Reference Williams, Mortensen and Doran2000). Currently, there is a wide range of planter adaptations that can be made and that would minimize or prevent the issues observed in our field trial. Successful soybean establishment in high levels of green cereal rye biomass is possible with the use of planters capable of providing a much greater down force to the row cleaners, coulters, closing wheels, and gauge wheels than planters that are used to plant into conventional tilled ground (Lal et al. Reference Lal, Reicosky and Hanson2007; Triplett and Dick Reference Triplett and Dick2008; Triplett et al. Reference Triplett, Johnson and Van Doren1963). In addition to the down force, these planters also have seed firmers to enhance soil-to-seed contact and capacity to inject insecticide in-furrow. Insect feeding was another cause of soybean stand reduction. The cover crop biomass accumulated in 2023 created the optimal environment for true armyworms [Mythimna (Psuedaletia) unipuncta] to start feeding on newly emerged soybean seedlings. In the last few years, Iowa State University extensionists have reported several occurrences of heavy infestations of this insect species in cover crop systems (Dean et al. Reference Dean, Anderson and Hodgson2022; Dean and Hodgson Reference Dean and Hodgson2023).
In 2022, soybean yields from plots with standing cereal rye were similar to the fallow control (Table 3). The use of roller-crimper in that year resulted in 13% lower soybean yield in comparison to the fallow treatment. Also in 2022, no significant effects of cereal management strategy or fallow treatments were observed for the A. trifida densities at 4 and 18 WAP. In that year, the densities of grasses were similar between treatments at 18 WAP. This result suggests that the yield reduction was not a result of competition for resources between the soybean and weeds. It does suggest that there may have been a substantial nutrient immobilization during the degradation of the roller-crimped cereal rye residue, consequently reducing the availability of essential nutrients for soybean growth. Many previous studies have demonstrated that cereal rye residue decomposition can immobilize N, thus reducing the pool of N available in the soil for plant uptake (Krueger et al. Reference Krueger, Ochsner, Porter and Baker2011; Nevins et al. Reference Nevins, Lacey and Armstrong2020; Preza-Fontes et al. Reference Preza-Fontes, Miller, Camberato, Roth and Armstrong2022; Tollenaar et al. Reference Tollenaar, Mihajlovic and Vyn1993; Wells et al. Reference Wells, Reberg-Horton, Smith and Grossman2013). Although soybean plants are able to fix N from the atmosphere, 40% to 50% of its total N demand comes from mineral sources from the soil (Salvagiotti et al. Reference Salvagiotti, Cassman, Specht, Walters, Weiss and Dobermann2008). In addition to nutrient immobilization, some studies have also suggested that cereal rye residue decomposition may release allelochemicals into the soil (Burgos et al. Reference Burgos, Talbert and Mattice1999; Raimbault et al. Reference Raimbault, Vyn and Tollenaar1990; Rice et al. Reference Rice, Park, Adam, Abdul-Baki and Teasdale2005). To date, there are no reports of yield reductions in soybean following cereal rye cover crop due to allelopathy. However, considering that most research in this area is relatively recent and that there are few researchers studying allelopathy from cereal rye (Brooks et al. Reference Brooks, Danehower, Murphy, Reberg-Horton and Burton2012; Burgos et al. Reference Burgos, Talbert and Mattice1999; Reberg-Horton et al. Reference Reberg-Horton, Burton, Danehower, Guoying, Monks, Murphy, Ranells, Williamson and Creamer2005), there could be an allelopathic compound yet to be studied that could be affecting soybean growth (Koehler-Cole et al. Reference Koehler-Cole, Everhart, Gu, Proctor, Marroquin-Guzman, Redfearn and Elmore2020).
In this study, we suggested that weed interference was not a factor contributing to soybean yield losses. In 2022, when the grass density reached an average of 31 plants m−2 in plots with standing cereal rye at 4 WAP and 2 plants m−2 at 18 WAP, soybean yields were similar to the fallow and reached an average of 5.2 Mg ha−1. However, in 2023, when the grass density varied from 0 plants m−2 at 4 WAP to an average of 8 plants m−2 at 18 WAP in plots with standing cereal rye, soybean yield was reduced by 55%. In 2023, soybean yields from fallow plots were, on average, 61% higher than soybean yields from plots with cereal rye. These results are partially corroborated by those reported by Nord et al. (Reference Nord, Curran, Mortensen, Mirsky and Jones2011), who observed reduced weed biomass and soybean yield with increasing amounts of cereal rye biomass.
In 2023, soybean yields were reduced by up to 55% with the use of cereal rye (Table 3). When the roller-crimper was used at cereal rye termination, yield losses were reduced to an average of 33% in comparison to the fallow. Different than 2022, when soybean stands were similar for all treatments, and we suggest that the yield losses may have been due to nutrient immobilization, in 2023, lower soybean stands in plots with cereal rye were the main cause for yield reductions. Soybean yield reductions due to stand losses in cover-cropping systems were also reported in previous studies (Eckert Reference Eckert1988; Moore et al. Reference Moore, Gillespie and Swanton1994; Reddy Reference Reddy2001). In 2023, the use of a roller-crimper resulted in a 12- to 15-cm-thick residue layer imposing a significant barrier for the early development of the soybean plants that ended up growing taller than plants from the fallow plots (Figure 3). However, once the plants had grown enough to surpass the residue layer, they had full access to sunlight. Conversely, soybean plants growing under standing cereal rye did not have full access to sunlight for several weeks after planting, which may have contributed to the slow growth and delayed maturity in 2023. Overall, in 2022, we observed that soybean plants growing in plots with cereal rye were always one growing stage behind plants growing in the fallow plots. In 2023, this difference increased to an average of three growing stages throughout the growing season (Figure 4).
Figure 3. Physical barrier created by roller-crimped cereal rye residue in 2023, Lafayette, IN. Soybean plants showing spindly growth.
Figure 4. Difference in soybean growth between fallow (left) and roller-crimped cereal rye treatments (right). Pictures taken 8 wk after soybean planting in 2023, Lafayette, IN.
In conclusion, the results of this research showed that the planting green method imposes significant challenges to the successful establishment of a soybean crop. We have demonstrated that the excessive biomass accumulation inherent from this method resulted primarily in soybean stand losses that led to substantial yield losses. In addition, roller-crimped cereal rye residue consistently reduced the concentration of residual herbicides in the soil immediately after application. On average, 75% and 83% of the soil-residual herbicides applied were intercepted by the roller-crimped cereal rye residue in 2022 and 2023, respectively. Despite the significant interception, the application of soil-residual herbicides at cereal rye termination reduced the density of grassy weed species in 2022 at 4 WAP relative to termination without residual herbicides. The rainfall events following the herbicide applications likely washed some of the herbicide off the residue and onto the soil. For all other evaluation timings in 2022 and 2023, the effect of herbicide treatments was nonsignificant for grass densities. Moreover, the use of residual herbicides did not reduce the density of A. trifida at any of the evaluation timings during the 2 yr of the study. The utilization of proper planting equipment with the correct adjustments is critical for the successful establishment of a soybean crop in high-residue cover-cropping systems and is one alternative to prevent the issues encountered in our field trials. In addition, our observations (consistent within the 2 yr of field trials) suggest that planting green systems may require a more comprehensive nutrient management program in order to overcome the potential nutrient immobilization. More work is needed to understand the correct cereal rye seeding rate and application timing for residual herbicides in planting green systems. Perhaps a substantial reduction (e.g., 50%) in the seeding rate normally used for early-terminated cereal rye would be adequate to prevent the accumulation of excessive amounts of biomass like we observed in 2023. Although, the options of residual herbicide for after planting are limited in soybean, the delay in the application of residual herbicide to an early postemergence timing may reduce the interception relative to the application onto green biomass. Finally, recent studies have shown promising results with the adoption of the precision planting of cover crops (Kurtz et al. Reference Kurtz, Acharya, Kaspar and Robertson2021; Sadeghpour et al. Reference Sadeghpour, Adeyemi, Hunter, Luo and Armstrong2021), which consists of planting the cover crop only in the space between the cash crop rows, thus reducing the interference of the cover crop residue with the cash crop.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/wsc.2025.10059
Acknowledgments
The authors would like to thank Pete Illingworth for technical assistance with fieldwork.
Funding statement
This research was partially funded by the Indiana Corn Marketing Council.
Competing interests
The authors declare no conflicts of interest.
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