Residual Herbicides Research Articles

Cover crops and residual herbicides reduce selection pressure for Palmer amaranth resistance to dicamba‐applied postemergence in cotton

AbstractThe most widely planted cotton (Gossypium hirsutum L.) and soybean [Glycine max (L.) Merr.] varieties in the United States are resistant to the herbicide dicamba. Measures to reduce selection pressure for dicamba‐resistant weeds is paramount. Four studies evaluated strategies to reduce selection pressure for Palmer amaranth (Amaranthus palmeri S. Watson) resistance to dicamba applied postemergence (POST) in cotton. A split‐plot arrangement consisted of conventional tillage or cereal rye (Secale cereale L.) cover crop as the whole plot. The subplot included four herbicide systems: no herbicide, 3 POSTs, fomesafen plus diuron applied pre‐emergence (PRE) followed by (fb) 3 POSTs, and PRE fb 2 POSTs fb diuron plus monosodium methyl arsenate applied lay‐by post‐directed (LPD). The POST herbicide applications were glyphosate plus dicamba. The cover crop reduced Palmer amaranth density 75, 70, and 54% at the time of POST 1, POST 2, and POST 3 application, respectively. Pre‐emergence herbicides fb POST applications reduced Palmer amaranth densities 99, 99, and 96% at the aforementioned application timings, respectively. Cover crop plus PRE herbicides provided similar reductions. Cumulative for the season, Palmer amaranth exposure to dicamba was reduced 65% by the cover crop, 98% by both PRE herbicides and cover crop plus PRE herbicides compared to 3 POST applications. An LPD application reduced Palmer amaranth exposure to dicamba by 38,319 plants ha–1 compared to a third POST. Cotton stand was higher in conventional systems when soil temperatures were 30–37 °C during emergence; however, stand in the cover crop was greater when soil temperatures were 40–43 °C. Yields were reduced 14% in a total POST herbicide program, and were improved 14% in a cover crop. A cereal rye cover crop and residual PRE herbicides had the biggest impact on reducing selection pressure for Palmer amaranth resistance to dicamba while also improving yield.

Herbicide Options to Control Naturalised Infestations of Cereus uruguayanus in Rangeland Environments of Australia.

While there are many high profile Opuntioid cactus species invading rangeland environments in Australia, Cereus uruguayanus Ritt. ex Kiesl. has also naturalised and formed large and dense infestations at several locations. With no herbicides registered for control of C. uruguayanus in Australia, the primary aim of this study was to identify effective herbicides to control it using a range of techniques. This involved a large screening trial of twelve herbicides and four techniques, followed by a rate refinement trial for cut stump applications and another to test residual herbicides. Despite most treatments (except monosodium methylarsonate (MSMA)) taking a long time to kill plants, at least one effective herbicide was identified for basal bark (triclopyr/picloram), cut stump (aminopyralid/metsulfuron-methyl, glyphosate, metsulfuron-methyl, triclopyr/picloram, triclopyr/picloram/aminopyralid), stem injection (glyphosate, MSMA, triclopyr/picloram/aminopyralid) and foliar applications (aminopyralid/metsulfuron-methyl, MSMA, triclopyr, triclopyr/picloram/aminopyralid) due to their ability to kill both small and large plants. Ground application of residual herbicides was less conclusive with neither hexazinone nor tebuthiuron causing adequate mortality at the rates applied. This study has identified effective herbicides for the control of C. uruguayanus using several techniques, but further research is needed to refine herbicide rates and develop integrated management strategies for a range of situations and infestation sizes and densities.

Weed Management and Economics in Dicamba-Tolerant Cotton

Increasing populations of glyphosate-resistant Palmer amaranth [Amaranthus palmeri (S.) Wats.] have increased weed management costs for Texas High Plains cotton [Gossypium hirsutum (L.)] producers. The introduction of dicamba-tolerant cotton varieties and registration of dicamba formulations for postemergence use, combined with residual herbicides, can effectively control Palmer amaranth. Field studies were conducted in 2018 and 2019 near Lubbock, TX, USA to evaluate Palmer amaranth control and economics of weed management in dicamba-, glufosinate-, glyphosate-, and conventional cotton systems. The most consistent season-long Palmer amaranth control was achieved with the dicamba-tolerant system in both years. In 2018, greatest lint yields were achieved with dicamba-tolerant system when compared to the conventional and glufosinate-tolerant systems. In 2018, greatest gross margin above weed management costs were achieved with the dicamba-tolerant and glyphosate-tolerant systems. Greatest lint yield was achieved with the dicamba-tolerant and conventional systems in 2019 and greatest gross margins were achieved with the dicamba-tolerant system. Total variable costs were similar across all systems, with greater seed/technology and herbicide costs in dicamba-tolerant and glufosinate-tolerant systems, compared to higher tillage and hand hoeing costs in glyphosate-tolerant and conventional systems.

Population growth rates of weed species in response to herbicide programme intensity and their impact on weed community

AbstractThe adoption of dicamba‐tolerant crops has reduced the challenge of controlling glyphosate‐resistantAmaranthus palmeriin these crops. However, introducing herbicide programmes with new mechanisms of action and different intensities can favour major shifts in weed communities. This can affect not only the functionality of the agroecosystem, but also resource availability to the most problematic species. The objectives of this work were to evaluate how herbicide programme structure and intensity affect (a) weed diversity, (b) population growth rate (λ) of weed species and (c) weed community structure based on λ dynamics in a dicamba‐tolerant cotton monoculture. To achieve these objectives, a cotton field experiment was established to compare four herbicide programmes from 2011 to 2018. The herbicide programmes differed in the first 4 years (i.e. glyphosate every year, alternating glyphosate and glyphosate plus dicamba every other year, glyphosate plus dicamba every year, and residual herbicide and glyphosate plus dicamba every year). During the last 4 years, all programmes received glyphosate plus dicamba. The weed seedbank was evaluated every spring and λ calculated. Environmental variation from year to year was more important in determining weed richness than herbicide programmes, which exhibited similar richness across all treatments during the eight years of the study. Regardless of herbicide programme, most species maintained λ between zero and one. Only a few species were the exception with λ values above one, such asA.palmeri,Mollugo verticillataandEleusine indica. Although glyphosate plus dicamba reduced λ for several species, this herbicide mix was less effective in reducing λ for other weeds compared with glyphosate alone, suggesting a potential antagonism that could change weed community composition. The limited changes in λ for most species suggest the capacity of weeds to adjust reproduction and survival to overcome changes in mortality rates caused by increases in herbicide use intensity.

Cranberry Germination and Emergence Response to Environmental Factors and Seeding Depth

Cranberry (Vaccinium macrocarpon Ait.) cultivars are clonally propagated. Germination of cranberry seeds produces off-type varieties that are generally characterized by lower fruit productivity and higher vegetative vigor. Over time, the productivity of cranberry beds decreases as off-type frequency increases over time. Improved knowledge of cranberry germination biology would facilitate the use of targeted agronomic practices to reduce the emergence and growth of less productive off-types. The influences of light, temperature regime, pH, and water potential on cranberry seed germination were assessed in a growth chamber, whereas the effect of seeding depth on seedling emergence was evaluated in a greenhouse. Seeds stratified for 6 months at 3 °C were used for these experiments. Cranberry germination was influenced by light quality, with maximum germination reaching 95% after 15 minutes of exposure to red light but decreasing to 89% under far-red light. However, light was not required for inducing germination. Cranberry seeds germinated over a range of alternating diurnal/nocturnal temperatures between 5 and 30 °C, with an average maximum germination of 97% occurring for diurnal temperatures of 20 to 25 °C. The length of emerged seedlings was reduced by an average of 75% for pH 6 to 8 compared with pH 3 to 5. Seedlings that emerged at pH greater than 5 showed increasing chlorotic and necrotic injuries and were not considered viable at pH 7 or 8. Germination at 15 °C was reduced when seeds were subjected to water stress as low as −0.2 MPa, and no germination occurred below −0.4 MPa. Seeds incubated at 25 °C were more tolerant to water stress, with at least 70% maximum germination for osmotic potential (ψS) −0.6 MPa or greater. The average seedling emergence was 91% for seeds left on the soil surface or buried at a maximum depth of 1 cm; however, it was null at a burying depth of 4 cm. These results indicate that germination of cranberry seeds in cultivated beds in the northeastern United States likely occurs during the summer months, when temperatures are optimal and the moisture requirement is supported by irrigation. However, timely application of residual herbicide or sanding (a traditional cranberry agronomic practice) of open areas in cranberry beds could help prevent seed germination and reduce minimizing the onset of off-type varieties.

Effects of Atrazine on Chernozem Microbial Communities Evaluated by Traditional Detection and Modern Sequencing Technology.

Atrazine is a long residual herbicide commonly used in maize fields. Although atrazine can effectively control weeds and improve crop yield, long-term application leads to continuous pollution in the agricultural ecological environment, especially in the soil ecosystem, and its impact on soil microorganisms is still not clear. Four methods were used in the experiment to clarify the effect of atrazine on the bacterial populations of cultivated soil layers of chernozem in a cold region in different periods: high-performance liquid chromatography (HPLC), colorimetry, microplate, and high-throughput sequencing. The level of residual atrazine in cold chernozem decreased from 4.645 to 0.077 mg/kg soil over time, and the residue gradually leached into deep soil and then decreased after accumulating to a maximum value. Atrazine significantly affected the activities of urease and polyphenol oxidase activity in the soil layers at different periods but had no significant effect on sucrase and phosphatase activity. Atrazine significantly reduced the diversity of microbial carbon source utilization and total activity in soil layers of 0–10 and 20–30 cm but only reduced the diversity of microbial carbon source utilization in the 10–20 cm layer. Atrazine had no significant effect on bacterial populations (10–12 phyla, 29–34 genera), but had a slight effect on the relative abundance of various groups. Atrazine significantly reduced the diversity of bacterial populations in cultivated soil layers of chernozem in a cold region, and the diversity of bacterial populations decreased with decreased residue. This lays a foundation for guiding the safe use of herbicides on farmland in Northeast China.

Integrated weed management with reduced herbicides in a no‐till dairy rotation

AbstractWith more than 65% of agronomic crops under no‐till in Pennsylvania, herbicides are relied on for weed management. To lessen the environmental impact and selection pressure for herbicide resistance, we conducted a 9‐yr experiment to test herbicide reduction practices in a dairy crop rotation at Rock Springs, PA. The rotation included soybean [Glycine max (L.) Merr.]–corn (Zea mays L.)–3‐yr alfalfa (Medicago sativa L.)–canola (Brassica napus L.). The following practices were used to reduce herbicide inputs: (a) banding residual herbicides over corn and soybean rows and using high‐residue inter‐row cultivation; (b) seeding a small grain companion crop with alfalfa; (c) plowing once in 6 yr to terminate the perennial forage. These practices were compared with standard herbicide‐based weed management (SH) in continuous no‐till. We hypothesized: (a) There would be more weed biomass in the reduced herbicide treatment (RH), (b) leading to more weeds in RH over time, but (c) the added weed pressure would not affect yield (d) or differences in net return. We sampled weed biomass in soybean, corn, and the first two forage years. In corn and soybean, weed biomass was often greater in RH than SH and increased over the years in the RH treatments. In the forage, weed biomass did not always differ between treatments. Yield and differences in net return were similar in most crops and years. Results suggest that weed management with reduced herbicide inputs supplemented with an integrated approach can be effective but may lead to more weeds over time.

Can the multi-walled carbon nanotubes be used to alleviate the phytotoxicity of herbicides in soils?

Herbicides are commonly used globally. However, residual herbicides in soils for ages often result in phytotoxicity and serious yield loss to subsequent crops. In this paper, the multi-walled carbon nanotubes (MWCNTs) were utilized to amend the herbicide polluted soil, and the adsorption performance of herbicides to MWCNTs amended soil was studied. Results indicate efficient alleviation of herbicide-induced phytotoxicity to rice and tobacco due to MWCNTs amendment. When 0.4% MWCNTs were applied, the concentration of sulfentrazone that inhibited the same rice height by 50% (IC50) increased to more than 3 times that of pure soil. When the MWCNTs were used to alleviate the phytotoxicity of quinclorac to tobacco, the MWCNTs not only alleviated the phytotoxicity of quinclorac but also promoted the growth of tobacco. The MWCNTs amended soil significantly increased the adsorption of herbicide to soil than biochar. The soil microbial analysis shows that MWCNTs had no significant effect on soil microbial community diversity, but the long-term exposure to MWCNTs could change the structure of the soil microbial community. Above all, our results highlighted the potential implication of the MWCNTs to ensure crop production by promoting crop growth and reducing the residual bioavailability of herbicides.

Biologically effective dose of pyraflufen-ethyl/2,4-D, applied preplant alone or mixed with metribuzin on glyphosate-resistant horseweed in soybean

AbstractGlyphosate resistance in weed species has presented immense challenges for farmers in Ontario. The co-application of burndown plus residual herbicides provides control of glyphosate-resistant (GR) horseweed in soybean. Pyraflufen-ethyl/2,4-D is a premixed herbicide formulation sold under the tradename Blackhawk®. Five field experiments were conducted over a 2-yr period (2019, 2020) in fields in southwestern Ontario to ascertain the biologically effective dose of pyraflufen-ethyl/2,4-D, applied alone, or mixed with metribuzin, for GR horseweed control when applied preplant to soybean. Soybean visible injury for all treatments was <15%. At 8 wk after application (WAA), the calculated doses of pyraflufen-ethyl/2,4-D for 50%, 80%, and 95% control of GR horseweed were 390, 1,148, and >2,108 g ha−1, respectively. The addition of metribuzin to pyraflufen-ethyl/2,4-D reduced the doses of pyraflufen-ethyl/2,4-D for 50%, 80%, and 95% control of GR horseweed to 19, 46, and 201 g ha−1, respectively. Pyraflufen-ethyl/2,4-D + metribuzin controlled GR horseweed by 97%, which is comparable to the current industry standards. Based on these results, pyraflufen-ethyl/2,4-D + metribuzin (527 + 400 g ha−1) applied preplant can be used for GR horseweed control in soybean.

Rice response to sublethal rates of paraquat, metribuzin, fomesafen, and cloransulam-methyl at different application timings

AbstractThe application of paraquat mixtures with residual herbicides before planting rice is a common treatment in Mississippi, and rice in proximity is susceptible to off-target movement of these applications. Four concurrent studies were conducted in Stoneville, MS, to characterize rice performance following exposure to a sublethal rate of paraquat, metribuzin, fomesafen, and cloransulam-methyl at different application timings. Herbicides were applied to rice at the growth stages of spiking to one-leaf (VEPOST), two- to three-leaf (EPOST), three- to four-leaf (MPOST), 7 d postflood (PFLD), and panicle differentiation (PD). Regardless of application timing, rice injury following exposure to paraquat was ≥45%. Delays in maturity were increased by 0.3 d d−1 following paraquat from emergence through PD. Dry weight, rough rice yield, panicle density, and germination were reduced by 18.7 g, 131.5 kg ha−1, 5.6 m−2, and 0.3%, respectively, per day from application of paraquat at emergence through PD. By 28 d after treatment (DAT), metribuzin injured rice 3% to 6%, and that injury did not translate into a yield reduction. Regardless of application timing, rice injury following fomesafen application ranged from 2% to 5% 28 DAT. Rice exposed to cloransulam-methyl EPOST exhibited the greatest root and foliar injury 21 DAT and 28 DAT, respectively. Additionally, when rice was exposed to cloransulam-methyl EPOST, yield was reduced to 6,540 kg ha−1 compared with a yield of 7,850 kg ha−1 from nontreated rice. Rice yield was negatively affected after paraquat was applied any time after rice emergence. However, applications of paraquat to rice at early reproductive growth stages reduced rough rice yield and seed germination the greatest. Application timing is crucial in determining severity of rice injury. Early-season injury to rice following paraquat application had less effect on yield compared with injury at later stages. Additionally, fields devoted to seed rice production are at risk for reduced seed germination if they are exposed to paraquat during early reproductive growth stages.

Residual herbicides affect critical time of Palmer amaranth removal in soybean

AbstractGlyphosate‐resistant (GR) Palmer amaranth (Amaranthus palmeri S. Watson) is one of the most difficult to control weeds in soybean [Glycine max (L.) Merr.] production fields. Residual pre‐emergence (PRE) herbicide applied at planting is one of the recommendations for management of herbicide‐resistant Palmer amaranth; however, information is not available about the effect of residual herbicides on critical time of Palmer amaranth removal (CTPAR) to prevent an unacceptable yield loss in soybean. The objective of this study was to determine the CTPAR in soybean affected by residual PRE herbicides compared with the no PRE herbicide in southcentral Nebraska. Field experiments were conducted in 2018 and 2019 in a grower's field infested with GR Palmer amaranth near Carleton, NE. The treatments were arranged in a split‐plot design with PRE herbicides (no PRE herbicide, flumioxazin, and a premix of flumioxazin/metribuzin/pyroxasulfone) as the main plot and Palmer amaranth removal timings as subplot treatments (a weed‐free control; a nontreated control; and Palmer amaranth removal timing at the V1, V3, V6, R2, and R5 soybean growth stages). In the absence of a PRE herbicide, the CTPAR at 5% soybean yield loss occurred at V1 and V6 soybean growth stages, equivalent to 194 and 480 Celsius growing degrees days (GDDc) in 2018 and 2019, respectively. When flumioxazin was applied alone, the CTPAR was delayed until the V3 and V6 soybean growth stages, or 341 and 501 GDDc. When flumioxazin/metribuzin/pyroxasulfone premix was applied, the CTPAR was delayed until the V2 and R1 soybean growth stages, corresponding to 255 and 546 GDDc, in 2018 and 2019, respectively.

Blueberry and blackberry are tolerant to repeated indaziflam applications

AbstractNumerous perennial horticultural crops are grown across the southeastern United States. Blueberry and blackberry (also known as caneberry) are commonly found in roadside stands, promote agritourism via pick-your-own markets, are important for fresh market commercial production in the region, and when processed, provide desirable value added products. Season-long weed control using residual herbicides is crucial for these perennial fruit crops to maximize berry quality and yield. Studies performed from 2012 to 2014 in Lanier and Clinch counties in Georgia evaluated the effects of repeated applications of indaziflam at 35, 75, or 145 g ai ha−1 applied biannually in March and September (five total applications) on growth of ‘Alapaha’ rabbiteye and ‘Palmetto’ highbush blueberry, and ‘Apache’ thornless blackberry. All indaziflam treatments were mixed with glufosinate, and a glufosinate-only treatment was included as a check. Minor leaf chlorosis (<10%) was noted within 30 d after application for all blueberries for all treatments, but this was always transient. Blueberry stem diameter was not different for any treatment, even when indaziflam was applied up to 725 g ai ha−1 over 3 yr as compared to glufosinate alone. There was no chlorosis or stem diameter differences for blackberry noted for any treatment. Indaziflam applied in blueberry and blackberry production provides season-long control of numerous troublesome weed species, without causing injury or negatively impacting crop growth.

Field evaluation of fluorochloridone for broad-spectrum weed control in carrot (Daucus carota L.) in north-west India

Weed management has been one of the major components of carrot production where the critical period of weed competition is longer as the development of a carrot canopy is very slow. Thus, hand weedings must be applied to control weeds. In this context, use of residual herbicides assumes greater significance in this important root vegetable. The field efficacy of fluorochloridone, a residual pre-emergence herbicide, was evaluated in carrot over two years at Ludhiana, India. Fluorochloridone at 625 and 750 g ha−1 provided >90% control of annual grasses [Digitaria sanguinalis (L.) Scop, Echinochloa colona (L.) Link, and Eleusine indica (L.) Gaertn.], and broad leaf weeds [Digera arvensis Forssk., Chenopodium album (L.), and Amaranthus viridis (L.)] until 60 days after application. Fluorochloridone at 625 and 750 g ha−1 gave the highest carrot root yield (28.4–31.7 t ha−1) which was similar to the hand weeded check (29.5–32.6 t ha−1) and 25.4–26.3% higher than the weedy check (23.1–25.1 t ha−1) in both years. Findings imply that application of fluorochloridone at 625 g ha−1 could be a suitable option for pre-emergence residual control of annual grass and broadleaved weeds in carrot.

Russian thistle (Salsola tragus L.) control with soil-active herbicides in no-till fallow

AbstractThe benefits of no-till fallow, which include reduced soil erosion, improved soil health, and increased stored soil water, are in jeopardy because of the widespread development of glyphosate resistance in Russian thistle. The objective of this research was to evaluate the efficacy of soil-active, residual herbicides for Russian thistle control in no-till fallow. The combinations of sulfentrazone + carfentrazone and flumioxazin + pyroxasulfone, and metribuzin alone were each applied in late fall, late winter, and split-applied in late fall and late winter at three sites: Adams, OR, in 2017–2018; Lind, WA, in 2018–2019; and Ralston, WA, in 2019–2020. All treatments provided good to excellent control of the initial flush of Russian thistle when assessed in mid-May, except the late-fall application of metribuzin at all three sites, and the late-fall application of sulfentrazone + carfentrazone at Adams. Cumulative Russian thistle densities, evaluated monthly throughout the fallow season, were lowest for the sulfentrazone + carfentrazone treatments, except for the late-fall application at Adams. However, flumioxazin + pyroxasulfone and metribuzin provided greater control of tumble mustard and prickly lettuce than did sulfentrazone + carfentazone. Sulfentrazone + carfentrazone, flumioxazin + pyroxasulfone, and metribuzin can all be used for Russian thistle control in fallow. To reduce the risk for crop injury to subsequently planted winter wheat, a late-fall application of sulfentrazone + carfentrazone may be the preferred treatment in low-rainfall regions where winter wheat–fallow is commonly practiced. A late-winter application may be preferred in higher rainfall regions where a 3-year rotation (e.g., winter wheat–spring wheat–fallow) is common. Flumioxazin + pyroxasulfone should be considered if other broadleaf weeds, such as tumble mustard or prickly lettuce, are of concern. The use of these soil-applied herbicides will reduce the need for the frequent application of glyphosate for Russian thistle control in no-till fallow.

Pre‐plant and in‐crop herbicide programs for prickly sida control in soybean

AbstractPrickly sida (Sida spinosa L.) is emerging as a prevalent and troublesome weed in the Mid‐South glyphosate‐resistant (GR) soybean [Glycine max (L.) Merr.] cropping systems as reliance on residual herbicides has decreased. It emerges throughout the growing season and postemergence (POST) control has been reported to be variable. Combinations of residual herbicides applied pre‐plant and at‐planting followed by POST herbicides in‐crop may be needed to maximize control. This research utilized separate studies to address the value of currently available residual herbicides applied pre‐plant, at‐planting, and POST in glyphosate‐resistant soybean prickly sida management systems. At 35 days after application (DAA), prickly sida control from glyphosate plus 2,4‐D pre‐plant with flumioxazin plus chlorimuron‐ethyl, flumioxazin plus pyroxasulfone, flumioxazin plus chlorimuron‐ethyl plus thifensulfuron‐methyl, metribuzin plus chlorimuron‐ethyl or metribuzin plus sulfentrazone was 70–89% and similar among treatments. In a separate study, based on prickly sida control and density 21 days after late postemergence (LPOST) application, improvement in control was most affected by inclusion of glyphosate or glyphosate plus residual herbicides flumioxazin, chlorimuron‐ethyl, and thifensulfuron‐methyl at‐planting. Weed control was not improved when residual herbicides chlorimuron‐ethyl plus tribenuron‐methyl were included pre‐plant or when residual herbicides chlorimuron‐ethyl, s‐metolachlor, and fomesafen were applied with glyphosate POST. Late‐season prickly sida control was as high as 93% following at‐planting and POST applications that did not include residual herbicides. Soybean yield was maximized with glyphosate applied both at‐planting and POST, whether or not residual herbicides flumioxazin, chlorimuron‐ethyl, and thifensulfuron‐methyl were included at‐planting or residual herbicides chlorimuron‐ethyl, s‐metolachlor, or fomesafen were included POST. When averaged across at‐planting and POST treatments, a 10% yield increase was observed when residual herbicides chlorimuron‐ethyl and tribenuron‐methyl were applied with glyphosate plus 2,4‐D pre‐plant compared with glyphosate plus 2,4‐D alone.

Ecological Mowing with Residual Herbicides: A Viable Weed Management Tool for Citrus Orchards

Ecological Mowing with Residual Herbicides: A Viable Weed Management Tool for Citrus Orchards

Influence of Cereal Rye Management on Weed Control in Soybean

Cereal rye as a cover crop is often used to improve soil health and as part of integrated weed management programs. Despite this, cereal rye biomass is often not managed for optimal weed suppression. This study evaluated the effects of managing cereal rye as part of an integrated weed management strategy in soybean. Factors consisted of levels of cereal rye management (no cereal rye, no nitrogen, or 20 kg/ha of nitrogen); cereal rye termination timing (20 or 10 d before soybean planting); and residual herbicide treatment applied at cereal rye termination (with or without). Winter annual weed control with cereal rye was generally greater compared to no cereal rye. Winter annual weed control was consistently better when cereal rye was terminated at 20 d before soybean planting compared to 10 d; while summer annual weed control was improved if termination was delayed. Effect of cereal rye management on summer annual weed control varied by weed species. In the absence of residual herbicides, Palmer amaranth control responded to the different levels of cereal rye management. However, morningglory spp. only responded to rye with supplemental N applications. Large crabgrass control was similar for treatments containing cereal rye, regardless of nitrogen input. Our results demonstrate the importance of cover crop management when incorporating cereal rye into an integrated weed management program for soybean.

Emergence pattern of Palmer amaranth (Amaranthus palmeri) influenced by tillage timings and residual herbicides

AbstractThe evolution of multiple herbicide-resistant weeds, including Palmer amaranth, has necessitated the implementation of an integrated weed management (IWM) program. Understanding weed emergence patterns is critical for developing effective IWM strategies. The objective of this study was to evaluate the effect of tillage timings and residual herbicides on cumulative emergence and emergence pattern of Palmer amaranth. Field experiments were conducted in 2015 and 2016 in a field naturally infested with photosystem (PS) II and 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor-resistant Palmer amaranth near Shickley, Nebraska, in a bare ground study, with no crop planted in the plots, although residues from the preceding corn crop were present on the soil surface. Treatments consisted of shallow tillage timings (early, mid, and late), three premix corn or soybean residual herbicides, and a nontreated control. The Weibull function was fitted to cumulative Palmer amaranth emergence with day of year (DOY) and thermal time (TT) as independent variables. Year by treatment interaction was significant for time to 10%, 25%, 50%, 75%, and 90% Palmer amaranth emergence and cumulative emergence. The majority of Palmer amaranth seedlings emerged early, following early tillage with 90% cumulative emergence occurring on DOY 172 compared with DOY 210 to 212 for mid- and late-tillage, and DOY 194 for the nontreated control in 2015. In 2016, 90% of cumulative emergence following early-, mid-, and late-tillage (DOYs 201 to 211) were similar, and that of the nontreated control (DOY 188) was similar to that of early tillage. Nontreated control and PRE herbicide treatments had similar DOY values for 90% emergence in both years. The number of emerged Palmer amaranth seedlings over the season was higher with shallow tillage than no tillage or with the use of PRE herbicides.

A holistic approach in herbicide resistance research and management: from resistance detection to sustainable weed control

Agricultural weeds can adapt rapidly to human activities as exemplified by the evolution of resistance to herbicides. Despite its multi-faceted nature, herbicide resistance has rarely been researched in a holistic manner. A novel approach combining timely resistance confirmation, investigation of resistance mechanisms, alternative control solutions and population modelling was adopted for the sustainable management of the Amaranthus palmeri weed in soybean production systems in Argentina. Here, we show that resistance to glyphosate in the studied population from Cordoba province was mainly due to a P106S target-site mutation in the 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) gene, with minor contributions from EPSPS gene duplication/overexpression. Alternative herbicides, such as fomesafen, effectively controlled the glyphosate-resistant plants. Model simulations revealed the tendency of a solo herbicidal input to primarily select for a single resistance mechanism and suggested that residual herbicides, alongside chemical diversity, were important for the sustainable use of these herbicides. We also discuss the value of an interdisciplinary approach for improved understanding of evolving weeds.

Evaluating cereal rye and crimson clover for weed suppression within buffer areas in dicamba-resistant soybean

AbstractAs herbicide-resistant weeds become more problematic, producers will consider the use of cover crops to suppress weeds. Weed suppression from cover crops may occur especially in the label-mandated buffer areas of dicamba-resistant soybean where dicamba use is not allowed. Three cover crops terminated at three timings with three herbicide strategies were evaluated for their effect on weed suppression in dicamba-resistant soybean. Delaying termination until soybean planting or after and using cereal rye or cereal rye + crimson clover increased cover-crop biomass by at least 40% compared to terminating early or using a crimson clover–only cover crop. Densities of problematic weed species were evaluated in early summer before a blanket POST application. Plots with cereal rye had 75% less horseweed compared to crimson clover at two of four site-years. Cereal rye or the mixed cover crop terminated at or after soybean planting reduced waterhemp densities by 87% compared to early termination timings of crimson clover and the earliest termination timing of the mix at one of two site-years. Cover crops were not as effective in reducing waterhemp densities as they were in reducing horseweed densities. This difference was due to a divergence in emergence patterns; waterhemp emergence generally peaks after termination of the cover crop, whereas horseweed emergence coincides with establishment and rapid vegetative growth of cereal rye. Cover crops alone were generally not as effective as was using a high-biomass cover crop combined with an herbicide strategy that contained dicamba and residual herbicides. However, within label-mandated buffer areas where dicamba cannot be used, a cover crop containing cereal rye with delayed termination until soybean planting combined with residual herbicides could be used to improve suppression of horseweed and waterhemp.