Cover crops and soil residual herbicides are considered essential tools within integrated weed management practices. However, interception of soil-applied herbicides by cover crop residue can reduce weed control and crop yield. Field trials were conducted in 2022 and 2023, in Indiana, to investigate the effect of cereal rye (Secale cereale L.) termination strategies on the concentration of sulfentrazone, S-metolachlor, and cloransulam-methyl in soil, weed control, and soybean [Glycine max (L.) Merr.] yield. Soybean were planted at cereal rye anthesis. Termination strategies included roller crimped cereal rye, standing cereal rye, and a fallow control. The average cereal rye biomass in 2022 and 2023 was 4,061 and 14,211 kg ha-1, respectively. Soybean stands were unaffected in 2022 but were reduced by 24 and 69% in the presence of roller crimped and standing cereal rye, respectively, in 2023. On average, 75 and 84% of the soil residual herbicides applied were intercepted by the roller crimped cereal rye residue in 2022 and 2023, respectively. The use of cereal rye did not improve overall weed control relative to fallow at 18 WAP, in 2022 and 2023. In 2022, roller crimped cereal rye reduced soybean yields by up to 13% in comparison to the presence of to the fallow. In 2023, regardless of management strategy, the use of cereal rye as cover crop reduced soybean yields by an average 44% in comparison to the fallow. Results from this research suggest that the adoption of the planting green system can significantly reduce soybean yield primarily due to stand losses if proper planting equipment is not used. Furthermore, the high levels of cereal rye biomass achieved in both years of the study did not provide additional season-long weed suppression relative to the non-cover crop control.

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. 2017). Considering the U.S. soybean production from 2023 (112 million tons; NASS 2023), these losses due to weed interference would be equivalent to approximately 58 million tons of soybean or $28 billion ($473.99 t-1 or $12.9 bu-1 of soybean; USDA-ERS 2023). Yield losses can be significantly reduced when chemical weed control measures are adopted. However, the overreliance on herbicides to control weeds led to the development of more than 600 cases of herbicide resistance in the U.S. since 1957 (Heap 2024). 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 1996). 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. 2014; Rorick and Kladivko 2017; Ruffatti et al. 2019; Villamil et al. 2006). 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 2007; Fernando and Shrestha 2023; Teasdale 1996). Previous studies have indicated that biomass accumulation is the limiting factor for weed suppression, and was directly proportional with weed suppression (Hodgskiss et al. 2020; Teasdale and Mohler 1993; Wallace et al. 2019). In the presence of 3,910 kg ha-1 of cereal rye biomass, Wallace et al. (2019) observed horseweed [Conyza canadensis (L.) Cronquist; syn. Erigeron canadensis L.] densities reduced up to 95% at the time of spring cover crop termination, compared to 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. 2015; Basche et al. 2016; Reed and Karsten 2022). However, high-residue systems are challenging not only https://doi.org/10.1017/wsc.2025.10059 Published online by Cambridge University Press 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. 2009; Reed et al. 2019). 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. 2022). 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. 2022; Dunbar et al. 2016; Obermeyer 2020).

Furthermore, the presence of high amounts of biomass can result in nutrient immobilization and therefore, reduce nutrient availability to the soybean (Wells et al. 2013). Nitrogen (N) and sulfur 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 1991; White et al. 2016).

The impact of planting green 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 (Nunes et al. 2023; Hodgskiss et al. 2022; Liebl et al. 1992). Conversely, Reed et al. (2019) 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 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. 2011). 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. 2014; Wells et al. 2014). The use of a roller crimper, however, does not eliminate the need for herbicides even when high amounts of biomass are present (Davis 2010; Dorn et al. 2013). 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 1993). Previous studies have reported inadequate weed control when cover crops were used as the sole weed management strategy (Teasdale et al. 2005). Conversely, adequate weed control was achieved when cover crops were used in combination with comprehensive weed management programs https://doi.org/10.1017/wsc.2025.10059 Published online by Cambridge University Press including pre- and post-emergence herbicides (Cornelius and Bradley 2017; Whalen et al. 2020; Wiggins et al. 2016).

The use of soil residual herbicides at cover crop termination has been suggested to extend the weed control through the critical weed-free period (Nunes et al. 2023a; Whalen et al. 2020). 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 1982, 1984; Ghadiri et al. 1984; Nunes et al. 2023a; Whalen et al. 2020). The extent of herbicide interception has been correlated with biomass accumulation, with highresidue systems intercepting more herbicide than cover cropping systems with early terminations (less biomass) (Nunes et al. 2023a; Whalen et al. 2020). Research conducted by Whalen et al. (2020) suggested that when cover crop termination was delayed from 21 to 7 days prior to 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. 2019). 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 1982). Similarly, Ghadiri et al. (1984) demonstrated that after 50 mm of rain, atrazine concentration in the soil increased more than 2-fold, while the amount retained in the wheat 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 1991), and lower herbicide solubility (Khalil et al. 2019) tend to limit the amount of residual herbicides reaching the soil at application and after rainfall or irrigation. Khalil et al. (2019) 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. 2013; Neve and Powles 2005). For instance, a multiple-resistant rigid ryegrass (Lolium rigidum Gaudin) population that was subjected for three generations to low doses of https://doi.org/10.1017/wsc.2025.10059 Published online by Cambridge University Press 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. 2012).

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 (1) to determine if 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) determine the concentration of sulfentrazone, s-metolachlor, and cloransulam-methyl in soil at seven sample timings, and (3) determine if roller crimping cereal rye increases soybean yield relative to standing cereal rye.

To read the full manuscript, "Influence of Cover Crop Termination Strategies on Weed Suppression, Concentration of Residual Herbicides in the Soil, and Soybean Yield", click the link.