Palmer amaranth is a troublesome weed species displaying the ability to adapt and evolve resistance to multiple herbicide modes of action, and there is a need for additional weed suppression tactics. Growing interest in the use of cover crops (CC) has led to questions regarding the most appropriate forms of CC management prior to cash crop planting in order to maximize weed suppression benefits. Experiments were conducted between 2021 to 2023 to test: 1) cover crop termination timing (i.e., green or brown), 2) CC biomass amount and 3) CC termination method (i.e., rolled or left standing) on Palmer amaranth suppression. Treatments included “planting brown” (cereal rye terminated two weeks before soybean planting), “planting green” (cereal rye terminated at soybean planting), and a no CC (winter fallow) check. Palmer amaranth emergence was evaluated at 4 and 6 weeks after soybean planting, and yield was calculated at harvest. CC reduced Palmer amaranth emergence when compared to the no CC check, and more suppression was observed as CC biomass increased. This decrease in emergence is potentially due to a decrease in light penetration reaching the soil surface and physical suppression as CC biomass increased. Yield, however, was unaffected by any CC management practice, indicating that growers can tailor CC termination practices for weed suppression. This information will provide better recommendations for farmers interested in using cover crops for weed suppression. Overall, the importance of CC biomass accumulation to achieve weed suppression is highlighted in our findings. Additionally, we add to the growing body of documentation that soybean yield may be variable from year to year as a result of CC presence.

Delaying cover crop termination until cash crop planting (i.e., planting green) is an emerging no-till practice. Improved management recommendations are needed for optimizing weed suppression benefits while minimizing other pest, fertility, and crop management risks when planting green in corn production systems. In a 2-yr field experiment, we evaluated the interaction between cereal rye residue management tactics (standing residue, roll-crimping, roll-crimping with row cleaners) and herbicide programs (1-pass preemergence [PRE], 2-pass postemergence [POST]) when planting green on weed recruitment spatial patterns and corn performance compared to standard termination (14 d preplant [DPP]) and ryelage harvest (14 DPP) practices. In a 2-yr on-farm experiment, we evaluated corn performance in response to the same residue management tactics. Cereal rye biomass production varied significantly across years in on-station experiments, with average (4.9 Mg ha−1) and anomalous (9.9 Mg ha−1) levels observed in 2020 and 2021, respectively. In 2020, planting green with an integrated roll-crimper/row cleaner system resulted in greater intrarow weed density compared with planting green into standing cereal rye. Interrow weed density was lower when roll-crimping was employed compared to early termination (14 DPP). Planting green into standing cereal rye resulted in greater mean corn height (V5 stage) compared to other treatments, but corn population and yield did not differ. In 2021, few differences in weed recruitment patterns were observed, but corn population and yield were significantly lower in planting green treatments compared to early termination. In both years, late-season weed biomass was lower in two-pass POST programs compared to one-pass PRE programs. On-farm trials showed that planting green into standing residue increases corn height and can reduce corn populations, which may lead to reduce yields. Our results suggest that management recommendations for optimizing herbicide application timing should consider intrarow and interrow weed recruitment dynamics associated with residue management tactics needed to optimize corn performance.

Region- and system-specific research is needed to understand the viability of delayed cover-crop termination (i.e., planting green) as an integrated weed management (IWM) tactic in no-till soybean. In a 3-yr field experiment, we evaluated the potential for planting green to facilitate elimination of soil-applied, preemergence residual herbicides within a soybean phase of a 6-yr grain–forage cropping systems experiment in Pennsylvania. This IWM tactic was contrasted with a Standard treatment, which included 14 to 21 d pre-plant termination of cereal rye and a two-pass herbicide program with preemergence herbicides. A 63% increase in cereal rye biomass production was observed within the IWM treatment in 2019, but only a 22% and 33% increase in 2020 and 2021, respectively. In 2020, significantly lower volumetric water content (%VWC) was observed within the IWM treatment in dates closest to planting and greater %VWC at multiple dates in June and July compared to the Standard treatment. No differences occurred in soybean populations, but soybean biomass at the V4 growth stage was reduced in the Standard treatment compared to the IWM treatment, which we attribute to injury from preemergence applications. The Standard treatment resulted in greater soybean yield (2,590 kg ha–1) than the IWM treatment (1,870 kg ha–1) in 2020, but yields were similar in other years. The IWM treatment resulted in 58% fewer herbicide inputs, as measured by the number of active ingredients applied, compared to the Standard over the 3-yr study. Yet, peak weed biomass did not differ between treatments. However, the IWM treatment resulted in greater total horseweed density and the number of horseweed plants that exceeded recommended size-based height thresholds (10 cm) compared to the Standard treatment just prior to postemergence applications (35–42 d after planting) in 2020 and 2021, underscoring the importance of integrating surface mulch residues with effective herbicide sites of action.

In the Mid-Atlantic United States, there is increasing interest in delaying cereal rye termination until after soybean planting (i.e., planting green). Improved understanding of cereal rye seeding rate effects is needed to balance weed and agronomic management goals. We investigated the effects of cereal rye seeding rates on weed control and crop performance when planting green in complementary experiments in two Mid-Atlantic regions. The Pennsylvania experiment was replicated at three site-years and the Delaware experiment at two site-years. In both experiments, population-level weed responses were evaluated across four cereal rye seeding rates: 0, 51, 101, and 135 kg ha−1. The Delaware experiment also implemented a nitrogen treatment factor (0 and 34 kg N ha−1; spring applied). Both experiments showed that integrating cereal rye in the fall significantly improved winter- and summer-annual weed suppression compared with the fallow control, but no differences in total cereal rye biomass production or weed suppression were found among alternative cereal rye seeding rates (51 to 135 kg ha−1). Soybean yield did not differ among treatments in any of the studies. These results show there is no reason to increase cereal rye seeding rates for weed suppression services or to decrease seeding rates for agronomic reasons (i.e., soybean population and yield) when employing planting-green tactics in no-till soybean production within the Mid-Atlantic region of the United States.

Alternative strategies are needed for management of glyphosate-resistant (GR) horseweed in soybean. Integrating a cereal rye cover crop with soybean planted in narrow rows may improve control and reduce herbicide selection pressure for herbicide-resistant horseweed biotypes. Four site-years of experiments were conducted in Michigan to determine whether fall-planted cereal rye terminated with glyphosate 1 wk prior to (early termination) or 1 wk after (planting green) planting in combination with narrow-row soybean improved GR horseweed management. At postemergence (POST) herbicide application, horseweed biomass was reduced by 71% to 90% when soybean was planted into cereal rye, regardless of termination time, compared with no cover across all row widths. Planting green or narrow-row soybean suppressed horseweed through soybean harvest. When glyphosate was applied POST (noneffective), horseweed biomass was 36% to 46% lower when planting green compared with early terminated cereal rye and no cover. Similarly, planting soybean in 19- and 38-cm rows reduced horseweed biomass by 48% and 28%, respectively, compared with 76-cm rows. Cereal rye did not affect soybean yield pooled over 3 of 4 site-years; however, narrow row soybean yielded 11% to 18% higher than 76-cm rows. Soybean yield was 11% higher when an effective POST herbicide was applied. In conclusion, fall-seeded cereal rye or narrow-row soybean suppressed horseweed compared with no cover and 76-cm rows; however, the effects of early termination did not last throughout the growing season in most cases. Delaying cover crop termination by planting green reduced horseweed biomass and density through soybean harvest, but reduced yield in 1 site-year due to an increased incidence of white mold. These cultural practices have a positive influence on suppressing horseweed that should be part of an overall horseweed management strategy; however, the use of an effective POST herbicide is still needed for complete season-long horseweed management.

The evolution of herbicide-resistant weeds has resulted in the necessity to integrate nonchemical control methods with chemicals for effective management in crop production systems. In soybean, control of the pigweed species, particularly herbicide-resistant waterhemp and Palmer amaranth, have become predominant concerns. Cereal rye planted as a winter cover crop can effectively suppress early-season weed emergence in soybean, including waterhemp, when planted at a rate of 123 kg ha−1. The objectives of this study were to determine the effects of different cereal rye seeding rates (0, 34, 56, 79, 110, and 123 kg ha−1) on early-season waterhemp suppression and soybean growth and yield. Soybean was planted into fall-seeded cereal rye, which was terminated within 4 d of soybean planting. The experiment was conducted over the 2018, 2019, and 2020 growing seasons in Columbia, Missouri. Effects of cereal rye on early-season waterhemp suppression varied by year and were most consistent at 56 kg ha−1 or higher seeding rates. Linear regression analysis of cereal rye biomass, height, or stand at soybean planting showed inverse relationships with waterhemp emergence. No adverse effects on soybean growth or yield were observed at any of the cereal rye seeding rates relative to plots that lacked cereal rye cover. Result differences among the years suggest that the successfulness of cereal rye on suppression of early-season waterhemp emergence is likely influenced by the amount of waterhemp seed present in the soil seed bank.

Integrated strategies for management of glyphosate-resistant (GR) horseweed are needed to reduce reliance on herbicides. Planting a cover crop after corn or soybean harvest in the Upper Midwest may reduce horseweed establishment and growth. Experiments were conducted in Michigan to determine if cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, and terminated with glyphosate at 1.27 kg ae ha−1 1 wk before planting (early termination) or 1 wk after soybean planting (planting green) would suppress establishment and growth of GR horseweed. Cover-crop biomass was 212% to 272% higher when termination was delayed by planting green compared with early termination. At the time of termination, cover crops reduced GR horseweed biomass 41% to 89% compared with no cover. Planting green increased the C:N ratio of cover-crop residue, which improved residue persistence and GR horseweed suppression at the time of POST herbicide application, approximately 5 wk after planting. Planting green reduced GR horseweed biomass 46% to 93% compared with no cover at the time of POST herbicide application; early termination provided less consistent suppression. Cover crops alone did not suppress GR horseweed through soybean harvest. Soybean yield was 30% to 108% greater when planting green compared with early termination at 2 site-years. Cereal rye and winter wheat, seeded at 67 or 135 kg ha−1, provided early-season GR horseweed suppression. Results from this research indicate that the practice of planting green may improve GR horseweed suppression through the time of POST herbicide application.