Introduction

Winter-annual cover crops have become more readily used as a soil conservation practice across the United States. This conservation technique has been proven to improve soil quality, increase soil organic matter, conserve soil moisture, reduce soil erosion, and provide early-season weed suppression when implemented in an agronomic cropping system (Reddy 2001b; White and Worsham 1990). Winter-annual grasses and legumes have been planted as cover crops in soybean, cotton (Gossypium hirsutum L.), and corn (Zea mays L.) (Reddy 2001b; White and Worsham 1990). Cover crops have been documented to provide early-season weed suppression by both physical and chemical interference (Barnes and Putnam 1986; Reddy 2001b; Teasdale and Mohler 2000). According to a recent United States Department of Agriculture survey, two-thirds of growers who planted a cereal rye cover crop have noticed improved control of multiple herbicide–resistant weeds across the United States (SARE 2017). Research has shown that cover-crop residues are allelopathic, that is, they release phytotoxins that inhibit germination and early growth of some weed species (Blackshaw et al. 2001; Davis and Liebman 2003; Yenish et al. 1996). In light of the uncertainty about the commercialization of new herbicide sites of action, the need for biological, cultural, and mechanical weed control measures is paramount (Heap 2018; Norsworthy et al. 2012). The integration of these cover crops with new herbicide-resistant crops can be effective alternatives for managing multiple herbicide–resistant Amaranthus spp. and other problematic weeds (Cahoon et al. 2015; Culpepper et al. 2000; DeVore et al. 2013; Montgomery et al. 2017; Ryan et al. 2011; Wiggins et al. 2015, 2016).

Palmer amaranth is the most troublesome and economically damaging summer-annual weed across the mid-South (Beckie 2011; Van Wychen 2016). Palhano et al. (2017) reported that cereal rye cover-crop plots had 83% less Palmer amaranth emergence compared with plots with no cover crop. Wiggins et al. (2016) reported 20% greater control of Palmer amaranth when integrating multiple cover-crop species with glufosinate and glyphosate in cotton. Palmer amaranth biotypes have evolved resistance to six different herbicide sites of action in agronomic crops in the United States (Heap 2018). Therefore, successful herbicide programs must focus on use of multiple, effective herbicide sites of action and sequential applications of residual herbicides for season-long control of Amaranthus spp. (Cahoon et al. 2015; Riar et al. 2013).

Table 1.

Details of field experiments conducted in multiple states to evaluate efficacy of residual herbicides influenced by cover-crop residue for control of Amaranthus spp. in soybean.

Waterhemp is another annual Amaranthus species that demonstrates a very rapid growth rate and can be very competitive with crops––specifically, soybean (Horak and Loughin 2000). When Palmer amaranth and waterhemp emerged at a density of 8 weeds m^–2, soybean yield was reduced 78% and 56%, respectively (Bensch et al. 2003). Waterhemp at a density of 42 plants m^–2 reduced soybean yield by 10% when they emerged as late as the V4 soybean growth stage (Steckel and Sprague 2004a).

Cover crops have been adopted for use as a conservation practice, because cover crops have been documented to increase soil quality, increase soil organic matter, increase soil moisture retention, reduce erosion, and provide supplemental weed control (Hartwig and Hoffman 1975). Cereal rye and winter wheat (Triticum aestivum L.) are commonly used winter-annual grass cover crops that reduce pressure of several weed species (Moore et al. 1994). Two legume cover crops, hairy vetch and crimson clover (Trifolium incarnatum L.), have been investigated for weed suppression as well as their ability to biologically fix atmospheric nitrogen that becomes available for the subsequent crop (Duck and Tyler 1996; Fisk et al. 2001; Norsworthy et al. 2014). Winter-annual grasses and legumes have been implemented in several crops, such as corn, cotton, and soybean (Reddy 2001b; White and Worsham 1990). Although cover crops suppress many winter-annual weed during the early spring, cover-crop residues typically do not provide adequate in-season weed control for agronomic crops (Teasdale and Mohler 2000. Thus, herbicides are commonly needed alongside cover-crop residues to achieve adequate weed control.

Previous research has shown that the use of residual herbicides in cover crops prolong in-season weed control (Cornelius and Bradley 2017; Wiggins et al. 2016). Herbicides that are applied PRE can reduce early-season weed interference and often improve season-long control of Amaranthus spp. (Culpepper and York 1998; Keeling et al. 2006; Reddy 2001a; Toler et al. 2002; Whitaker et al. 2008). Residual herbicides applied PRE are actively promoted to aid management of glyphosate-resistant Amaranthus spp. and to delay further evolution of resistance (Steckel 2020; Stephenson et al. 2008; York and Culpepper 2009). The research reported here was conducted to determine the potential efficacy of different soil-residual herbicides on Palmer amaranth and waterhemp in the presence of cover-crop residue and develop recommendations for the best residual herbicides for use in cover-crop systems where soybean is the crop.

Table 2.

Herbicide active ingredient and application rates based on soil texture and organic matter content applied in field experiments conducted in multiple states to evaluate efficacy of residual herbicides influenced by cover-crop residue for control of Amaranthus spp. in soybean.

Materials and Methods

This experiment was conducted in 10 environments total across the mid-South. The environments were located in Fayetteville, AR, Jackson, TN, Farmland, IN, and Carbondale, IL in 2018 and 2019; in Champaign, IL in 2018; and in Columbia, MO in 2019. Palmer amaranth was the recorded species in Tennessee and Arkansas. Waterhemp was the recorded species in Missouri, Illinois, and Indiana. The coordinates for each location can be referenced in Table 1.

The experimental design was a randomized complete block with nine treatments replicated four times in plots 3 m wide and 9.1 m long. Cover crops were planted in the previous fall in 18-cm row spacings and consisted of cereal rye at a rate of 67 kg ha^–1 plus hairy vetch at a rate of 8 kg ha^–1. Cover-crop species and planting rates were selected as suggested by Wiggins et al. (2016, 2017). Cover-crop planting dates, soybean planting dates, and soil characteristics for each site can be found in Table 1. Cover crops were terminated 3 wk pre-plant with an application of glyphosate at 1,260 g ae ha^–1 + dicamba at 560 g ae ha^–1. Soybeans were planted in 75-cm-wide rows to varieties that were Liberty Link (glufosinate resistant). Treatments consisted of a nontreated or no-residual plot, acetochlor, dimethenamid-P, flumioxazin, metribuzin, pendimethalin, pyroxasulfone, pyroxasulfone + flumioxazin, and S-metolachlor. Application rates were based on label specification for those herbicides. Active ingredient, trade names, and rates can be found in Table 2. Treatments were applied at planting with a CO₂-pressurized backpack sprayer calibrated to deliver 140 L ha^–1 at 220 kPa using AIXR 11003 or a XR 11003 nozzles spaced 50 cm apart (AIXR; TeeJet Technologies, Wheaton, IL).

Table 3.

The effect of cover crop (cereal rye + hairy vetch) with and without soil-residual herbicide on number of days required for Amaranthus spp. to reach 10-cm height, density, and soybean yield during field experiments conducted in multiple states to evaluate efficacy of residual herbicides influenced by cover-crop residue for control of Amaranthus spp. in soybean.

The number of days until Amaranthus spp. reached 10 cm height was recorded to estimate the residual capability from each herbicide treatment and corresponds with the plant height limit for POST control with most herbicides available for soybean (Anonymous 2020a, 2020b, 2020c). Density of Amaranthus spp. was estimated by counting the number of Palmer amaranth and waterhemp plants from two 1-m² quadrants recorded at each of the following evaluation timings: 14, 21, 28, 35 d. Amaranthus spp. plant heights were recorded at the 35-d evaluation timing. Soybean yield was taken by harvesting with a plot combine, and grain moisture was adjusted to 13% moisture.

Data were subjected to ANOVA using the PROC GLIMMIX procedure in SAS (version 9.4; SAS Institute, Cary, NC). Palmer amaranth and waterhemp data were analyzed separately for this analysis. Each year–location combination was considered an environment sampled at random from a population as described by Blouin et al. (2011) and Carmer et al. (1989). Designating environments random broadens the possible inference space to which the experimental results are applicable. Environments, replications (nested within environments), and interactions containing these effects were declared random effects in the model; herbicide treatments were designated fixed effects. Type III statistics were used to test the fixed effects, and least square means were separated using Fisher's protected LSD at α = 0.05.

Results and Discussion