Palmer Amaranth Control Research Articles

Comparison of Allyl Isothiocyanate and Metam Sodium with Methyl Bromide for Weed Control in Polyethylene-Mulched Bell Pepper

Methyl bromide (MeBr), classified as a Class I ozone-depleting substance, has been banned for ordinary agricultural uses. Weed control in commercial bell pepper production is complicated by the ban on MeBr and the lack of other available and effective soil fumigants. A field study was conducted to evaluate the effectiveness of allyl isothiocyanate (ITC) and metam sodium (methyl ITC generator) as MeBr alternatives for control of Palmer amaranth, large crabgrass, and yellow nutsedge; and for increasing marketable yields in low-density polyethylene (LDPE) –mulched bell pepper. Allyl ITC was applied at 450, 600, and 750 kg ha−1; metam sodium was applied at 180, 270, and 360 kg ha−1; and MeBr plus chloropicrin (67% and 33%, respectively) was applied at 390 kg ha−1. Allyl ITC and metam sodium did not injure bell pepper. Allyl ITC at 750 kg ha−1or metam sodium at 360 kg ha−1controlled Palmer amaranth (≥ 83%), large crabgrass (≥ 78%), and yellow nutsedge (≥ 80%) comparably to MeBr. Yellow nutsedge tuber density was ≤ 84 tubers m−2in plots treated with the highest rate of allyl ITC and metam sodium and was comparable to the tuber density in MeBr-treated plots. Although allyl ITC at 750 kg ha−1controlled weeds comparable to MeBr, total marketable bell pepper yield with allyl ITC was lower than the yield with MeBr. Conversely, total marketable bell pepper yield with the highest rate of metam sodium (53.5 ton ha−1) was equivalent to the yield (62.5 ton ha−1) in plots treated with MeBr. In conclusion, metam sodium at 360 kg ha−1is an effective MeBr alternative for weed control in LDPE–mulched bell pepper.

Efficacy and Economics of Herbicide Programs Compared to Methyl Bromide for Weed Control in Polyethylene-Mulched Tomato

Methyl bromide (MeBr), a widely used soil fumigant in tomato production, has been banned for ordinary agricultural uses. In the absence of MeBr, a viable alternative is imperative for weed control and prevention of economic loss in tomato production. A field study was conducted in the summers of 2010 and 2011 at Fayetteville, AR, to compare the efficacy and economics of herbicide programs consisting of pre-transplant followed by (fb) post-transplant herbicides in low-density polyethylene (LDPE) mulched tomato. Pre-transplant imazosulfuron at 0.112, 0.224, and 0.336 kg ai ha−1andS-metolachlor at 1.6 kg ai ha−1were fb a post-transplant mixture of trifloxysulfuron plus halosulfuron at 0.008 and 0.027 kg ai ha−1at 4 wk after transplant (WATP). The standard MeBr treatment (2:1 mixture of MeBr plus chloropicrin at 390 kg ai ha−1), weed-free (hand weeding) control, and nontreated weedy check were used for comparison. Pre-transplantS-metolachlor fb post-transplant herbicides controlled Palmer amaranth ≥ 89%, large crabgrass ≥ 88%, and yellow nutsedge ≥ 90%, which was comparable to the control with MeBr. Tomato recovered the injury (≤ 19% at 6 WATP) from post-transplant herbicides in the later weeks.S-metolachlor–containing herbicide programs yielded marketable tomato fruit comparable to the yield with MeBr. Economic evaluation of the herbicide programs demonstrated a net return of $3,758.50 ha−1from theS-metolachlor–containing herbicide program in LDPE-mulched tomato. Likewise, this herbicide program showed minimum loss of ≤ $671.61 ha−1in net return relative to MeBr. In conclusion, a herbicide program consisting of pre-transplantS-metolachlor fb post-transplant trifloxysulfuron plus halosulfuron is a viable alternative to MeBr for weed control and marketable yield in LDPE-mulched tomato production.

Herbicide-Based Weed Management Programs for Palmer Amaranth (Amaranthus palmeri) in Sweetpotato

Studies were conducted in 2010 and 2011 to determine the effect of herbicide-based Palmer amaranth management systems in ‘Covington' sweetpotato. Treatments consisted of three herbicide application times. Pretransplant applications were flumioxazin at 107 g ai ha−1, fomesafen at 280 g ai ha−1, flumioxazin at 70 g ha−1plus pyroxasulfone at 89 g ai ha−1, or no herbicide. A second herbicide application was applied within 1 d after transplanting (DAP) and consisted ofS-metolachlor at 800 g ai ha−1, clomazone at 630 g ai ha−1, or no herbicide. Two weeks after planting (WAP) plots receivedS-metolachlor at 800 g ha−1, metribuzin at 140 g ai ha−1, a tank mix ofS-metolachlor at 800 g ha−1plus metribuzin at 140 g ha−1, hand-weeding followed by (fb)S-metolachlor at 800 g ha−1, or no herbicide. Crop tolerance, Palmer amaranth control, and sweetpotato yield in systems containing fomesafen pretransplant were similar to flumioxazin-containing systems. Systems containing flumioxazin plus pyroxasulfone pretransplant resulted in increased crop stunting and decreased sweetpotato yield in 2010, compared with systems containing flumioxazin or fomesafen, but were similar to systems with flumioxazin or fomesafen in 2011. In 2010, systems containingS-metolachlor applied within 1 DAP resulted in increased sweetpotato injury, similar Palmer amaranth control, and reduced no. 1, jumbo, and total sweetpotato yield, compared with systems with clomazone. In 2011, systems containing clomazone were more injurious to sweetpotato than systems receivingS-metolachlor, but Palmer amaranth control and sweetpotato yield were similar. Systems containing metribuzin 2 WAP resulted in increased sweetpotato injury and Palmer amaranth control (in 2010) but similar no. 1 and total sweetpotato yields, compared with systems containingS-metolachlor at 2 WAP. Hand-weeding fbS-metolachlor provided greater Palmer amaranth control and no. 1 sweetpotato yield than did systems ofS-metolachlor without a preceding hand-weeding event in 2010.

Palmer Amaranth (Amaranthus palmeri) Control by Glufosinate plus Fluometuron Applied Postemergence to WideStrike®Cotton

Glyphosate-resistant (GR) weeds, especially GR Palmer amaranth, are very problematic for cotton growers in the Southeast and Midsouth regions of the United States. Glufosinate can control GR Palmer amaranth, and growers are transitioning to glufosinate-based systems. Palmer amaranth must be small for consistently effective control by glufosinate. Because this weed grows rapidly, growers are not always timely with applications. With widespread resistance to acetolactate synthase-inhibiting herbicides, growers have few herbicide options to mix with glufosinate to improve control of larger weeds. In a field study using a WideStrike®cotton cultivar, we evaluated fluometuron at 140 to 1,120 g ai ha−1mixed with the ammonium salt of glufosinate at 485 g ae ha−1for control of GR Palmer amaranth 13 and 26 cm tall. Standard PRE- and POST-directed herbicides were included in the systems. Glufosinate alone injured the WideStrike® cotton less than 10%. Fluometuron increased injury up to 25% but did not adversely affect yield. Glufosinate controlled 13-cm Palmer amaranth at least 90%, and there was no improvement in weed control nor a cotton yield response to fluometuron mixed with glufosinate. Palmer amaranth 26 cm tall was controlled only 59% by glufosinate. Fluometuron mixed with glufosinate increased control of the larger weeds up to 28% and there was a trend for greater yields. However, delaying applications until weeds were 26 cm reduced yield 22% relative to timely application. Our results suggest fluometuron mixed with glufosinate may be of some benefit when attempting to control large Palmer amaranth. However, mixing fluometuron with glufosinate is not a substitute for a timely glufosinate application.

Influence of Deep Tillage, a Rye Cover Crop, and Various Soybean Production Systems on Palmer Amaranth Emergence in Soybean

Glyphosate-resistant Palmer amaranth has become a major threat to soybean production in the southern United States. Arkansas soybean producers rely heavily on glyphosate-resistant soybean; hence, an alternative solution for controlling resistant Palmer amaranth is needed. A field experiment was conducted at Marianna, AR, during 2009 and 2010 in which soybean production systems were tested in combination with deep tillage and no tillage to determine the impact on Palmer amaranth emergence. To establish a baseline population, 250,000 glyphosate-resistant Palmer amaranth seeds were placed in a 1-m2area in the middle of each plot and incorporated in the soil, and emergence was evaluated five times during the season. Production systems of full-season soybean with a rye cover crop or soybean double-cropped with wheat, which had high amounts of plant residue on the soil surface reduced Palmer amaranth emergence more than systems without deep tillage and a cover crop or wheat. When used in combination with deep tillage, these systems reduced Palmer amaranth emergence by 98 and 97%, respectively, in 2009 and by 73 and 82%, respectively, in 2010. Deep tillage alone caused an 81% reduction in emergence averaged over both years. Soybean double-cropped with wheat used in combination with deep tillage provided a 95% reduction in Palmer amaranth emergence over the 2-yr period. This research shows that deep tillage in combination with soybean production systems that have high amounts of residue on the soil surface are alternative means for providing a high level of control of glyphosate-resistant Palmer amaranth and could lessen the dependence on chemical weed control.

Integrated Palmer Amaranth Management in Glufosinate-Resistant Cotton: II. Primary, Secondary and Conservation Tillage

A three year field experiment was conducted to evaluate the role of soil inversion, cover crops and spring tillage methods for Palmer amaranth between-row (BR) and within-row (WR) management in glufosinate-resistant cotton. Main plots were two soil inversion treatments: fall inversion tillage (IT) and non-inversion tillage (NIT). Subplots were three cover treatments: crimson clover, cereal rye or none (i.e., winter fallow); and the sub subplots were four secondary spring tillage methods: disking followed by (fb) cultivator (DCU), disking fb chisel plow (DCH), disking fb disking (DD) and no tillage (NT). Averaged over years and soil inversion, the crimson clover produced maximum cover biomass (4390 kg ha−1) fb cereal rye (3698 kg ha−1) and winter fallow (777 kg ha−1). Two weeks after planting (WAP) and before the postemergence (POST) application, Palmer amaranth WR and BR density were two- and four-times less, respectively, in IT than NIT. Further, Palmer amaranth WR and BR density were reduced two-fold following crimson clover and cereal rye than following winter fallow at 2 WAP. Without IT, early season Palmer amaranth densities were 40% less following DCU, DCH and DD, when compared with IT. Following IT, no spring tillage method improved Palmer amaranth control. The timely application of glufosinate + S-metolachlor POST tank mixture greatly improved Palmer amaranth control in both IT and NIT systems. The highest cotton yields were obtained with DD following cereal rye (2251 kg ha−1), DD following crimson clover (2213 kg ha−1) and DD following winter fallow (2153 kg ha−1). On average, IT cotton yields (2133 kg ha−1) were 21% higher than NIT (1766 kg ha−1). Therefore, from an integrated weed management standpoint, an occasional fall IT could greatly reduce Palmer amaranth emergence on farms highly infested with glyphosate-resistant Palmer amaranth. In addition, a cereal rye or crimson clover cover crop can effectively reduce early season Palmer amaranth emergence in both IT and NIT systems. For effective and season-long control of Palmer amaranth, one or more POST applications of glufosinate + residual herbicide as tank mixture may be needed in a glufosinate-based cotton production system.

Comparison of Weed Control, Yield, and Net Income in Conventional, Glyphosate‐Resistant, and Glufosinate‐Resistant Soybean

Separate field experiments were conducted in central and southeast Missouri during 2009 and 2010 to evaluate the effect of preemergence (PRE) and postemergence (POST) herbicide programs on Palmer amaranth (Amaranthus palmeri S. Wats.) and waterhemp (Amaranthus rudis Sauer) control, soybean [Glycine max (L.) Merr.] yield, and net income in conventional, glyphosateresistant, and glufosinate-resistant soybean production systems. Visual control evaluations 10 wk after emergence at the waterhemp site revealed that all preemergence only applications (PRE-only) and preemergence followed by a postemergence applications (PRE fb POST) provided greater than 92% waterhemp control in either soybean system and at the Palmer amaranth site and all PREonly provided greater than 83% Palmer amaranth control across soybean systems. Averaged across all herbicide programs at both locations, glufosinate-resistant soybean provided the highest grain yield and net return followed by glyphosate-resistant and conventional soybean systems. Furthermore, with the exception of the conventional PRE-only program at the waterhemp site, all glyphosate-resistant soybean herbicide programs provided greater net return than all conventional herbicide programs. Collectively, the results from both trials indicate that programs containing PRE herbicide treatments provide the best opportunity for season-long control of waterhemp and Palmer amaranth and highest grain yields and net returns in conventional, glyphosate-resistant, or glufosinateresistant soybean systems. The results from these experiments also suggest that Palmer amaranth may be particularly difficult to control in conventional soybean systems. Problematic Weeds and Weed Management Systems Palmer amaranth and waterhemp, both members of the Amaranthus family, are two of the most troublesome agronomic weeds in the United States due to their extended period of emergence, rapid growth at high light intensities and temperatures, and prolific seed production (10,14,18,27). In Missouri, the most common pigweed (Amaranthus spp.) species Published in Crop Management DOI 10.1094/CM-2013-0028-RS © Plant Management Network All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Kristin K. Rosenbaum, Graduate Research Assistant, Division of Plant Sciences, 205 Waters Hall, University of Missouri, Columbia, MO 65211; Raymond E. Massey, Extension Professor, Department of Agricultural and Applied Economics, 223D Mumford Hall, University of Missouri, Columbia, MO 65211; and Kevin W. Bradley, Associate Professor, Division of Plant Sciences, 201 Waters Hall, University of Missouri, Columbia, MO 65211. Received 1 Aug. 2013. *Corresponding author (bradleyke@missouri.edu). Abbreviations: 1-pass POST, one-pass postemergence application; 1-pass POST W/Res, one-pass postemergence application with a residual herbicide; 2-pass POST, two-pass postemergence application; ALS, acetolactate synthase; POST, ; PRE, preemergence; PRE fb POST, preemergence followed by a postemergence application; PREonly, preemergence only application; WAE, weeks after emergence.

Efficacy of Herbicides When Spray Solution Application Is Delayed

Information is limited concerning the impact of delaying applications of pesticides after solution preparation on efficacy. Experiments were conducted to determine weed control when diclosulam, dimethenamid-P, flumioxazin, fomesafen, imazethapyr, pendimethalin, andS-metolachlor were applied preemergence the day of solution preparation or 3, 6, and 9 days after solution preparation. Herbicide solutions were applied on the same day regardless of when prepared. Control of broadleaf signalgrass, common lambsquarters, entireleaf morningglory, and Palmer amaranth by these herbicides was not reduced regardless of when herbicide solutions were prepared. Surprisingly entireleaf morningglory control by all herbicides increased when herbicide application was delayed by 9 days. In separate experiments, control of broadleaf signalgrass by clethodim, common ragweed by glyphosate and lactofen, entireleaf morningglory by lactofen, Italian rye grass by glyphosate and paraquat, and Palmer amaranth by atrazine, dicamba, glufosinate, glyphosate, imazethapyr, lactofen, and 2,4-D was affected more by increase in weed size due to delayed application than the time between solution preparation and application.

Comparison of Pyroxasulfone to Soil Residual Herbicides for Glyphosate Resistant Palmer Amaranth Control in Glyphosate Resistant Soybean

Comparison of Pyroxasulfone to Soil Residual Herbicides for Glyphosate Resistant Palmer Amaranth Control in Glyphosate Resistant Soybean

Grain Sorghum Tolerance and Weed Control with Pyrasulfotole plus Bromoxynil Combinations

Research was conducted in Texas from 2010 through 2012 to evaluate combinations of pyrasulfotole plus bromoxynil for weed control and grain sorghum [Sorghum bicolor (L.) Moench] response. In most instances, combinations of pyrasulfotole plus bromoxynil applied early postemergence controlled Palmer amaranth (Amaranthus palmeri S. Wats), devil's-claw [Proboscidea louisianica (Mill.) Thellung], pitted morningglory (Ipomoea lacunosa L.), and smellmelon [Cucumis melo L. subsp. agrestis (Naudin) Pangalo var. dudaim (L.) Naudin (syn. Cucumis melo L. var. dudaim Naud.)] at least 81%. However, these combinations controlled Texas millet [(Urochloa texana (Buckl.) L.] no more than 72%. Horse purslane (Trianthema portulacastrum L.) was controlled completely when pyrasulfotole plus bromoxynil was applied to plants ≤10 cm in height but no more than 65% when applied to taller plants. Pyrasulfotole plus bromoxynil combinations injured grain sorghum transiently and injury was not apparent later in the season. Early season injury caused by combinations of pyrasulfotole plus bromoxynil did not adversely affect yield except in the case of pyrasulfotole at 0.03 kg a.i./ha plus bromoxynil at 0.20 kg a.i./ha plus atrazine at 0.58 kg a.i./ha plus dicamba at 0.13 kg a.i./ha. Results from this research indicate that combinations of pyrasulfotole and bromoxynil can be effective in managing problematic weeds in Texas.

Management of Palmer Amaranth (Amaranthus palmeri) in Glufosinate-Resistant Soybean (Glycine max) with Sequential Applications of Herbicides

Palmer amaranth (Amaranthus palmeri S. Wats.) is one of the most difficult weeds to control in soybean (Glycine max (L.) Merr.) in North Carolina. Research was conducted during 2010 and 2011 to determine if Palmer amaranth control and soybean yield were affected by soybean plant population and combinations of preemergence (PRE) herbicides followed by a single application of glufosinate postemergence (POST) versus multiple applications of glufosinate POST. Palmer amaranth was controlled more and soybean yield was greater when soybean was established at 483,000 plants ha−1 in 3 of 4 experiments compared with soybean at 178,000 plants ha−1 irrespective of herbicide treatments. In separate experiments, application of PRE herbicides followed by POST application of glufosinate or multiple POST applications of glufosinate provided variable Palmer amaranth control, although combinations of PRE and POST herbicides controlled Palmer amaranth the most and provided the greatest soybean yield. In 1 of 3 experiments, sequential applications of glufosinate were more effective than a single application. Yield was higher in 2 of 3 experiments when glufosinate was applied irrespective of timing of application when compared with the nontreated control. In the experiment where glufosinate was applied at various POST timings, multiple applications of the herbicide provided the best control and the greatest yield compared with single applications.

Influence of Late-Season Herbicide Applications on Control, Fecundity, and Progeny Fitness of Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Biotypes from Arkansas

Experiments were conducted in 2008 and 2009 in Fayetteville, AR, to determine the influence of late-season herbicide applications on control, seed reduction, seed viability, and seedling fitness of two glyphosate-resistant (GR) Palmer amaranth biotypes, one from Mississippi County (MC) and the other from Lincoln County (LC) in Arkansas. Glyphosate (870 g ae ha−1), glufosinate (820 g ai ha−1), 2,4-D amine (1,060 g ae ha−1), dicamba (280 g ae ha−1), and pyrithiobac (70 g ai ha−1) were each applied at the first visible sign of inflorescence of GR Palmer amaranth plants. Glufosinate, 2,4-D, and dicamba provided 52 to 74% control of MC GR Palmer amaranth plants 28 d after treatment (DAT). The LC biotype was larger (94 cm tall) than the MC biotype was (64 cm tall) at application and was more difficult to control. Although control of GR Palmer amaranth was inadequate, late-season applications of glufosinate, 2,4-D, and dicamba reduced seed production of the LC biotype by 75 to 87% and production of the MC biotype by 94 to 95% compared with nontreated plants. Irrespective of biotypes, glufosinate, 2,4-D, and glyphosate reduced 100-seed weight by 22% compared with the nontreated control, and viability of seeds produced by treated plants was only 45 to 61% compared with 97% seed viability in nontreated plants. Glyphosate, glufosinate, 2,4-D, or dicamba reduced cumulative seedling emergence by an average of 84% compared with the nontreated check. Seedling biomass was four times greater for the LC than for the MC biotype, suggesting greater vigor and fitness for the LC progeny. This research demonstrates that a single, late-season (early inflorescence stage) application of glufosinate, 2,4-D, or dicamba could potentially reduce seedbank replenishment of GR Palmer amaranth. Additionally, reduction in seed weight, viability, and seedling recruitment would impair the success of GR Palmer amaranth progeny in the following season.

Integrated Palmer Amaranth Management in Glufosinate-Resistant Cotton: I. Soil-Inversion, High-Residue Cover Crops and Herbicide Regimes

A three year field experiment was conducted to evaluate the role of soil-inversion, cover crops and herbicide regimes for Palmer amaranth between-row (BR) and within-row (WR) management in glufosinate-resistant cotton. The main plots were two soil-inversion treatments: fall inversion tillage (IT) and non-inversion tillage (NIT). The subplots were three cover crop treatments: crimson clover, cereal rye and winter fallow; and sub subplots were four herbicide regimes: preemergence (PRE) alone, postemergence (POST) alone, PRE + POST and a no herbicide check (None). The PRE herbicide regime consisted of a single application of pendimethalin at 0.84 kg ae ha−1 plus fomesafen at 0.28 kg ai ha−1. The POST herbicide regime consisted of a single application of glufosinate at 0.60 kg ai ha−1 plus S-metolachlor at 0.54 kg ai ha−1 and the PRE + POST regime combined the prior two components. At 2 weeks after planting (WAP) cotton, Palmer amaranth densities, both BR and WR, were reduced ≥90% following all cover crop treatments in the IT. In the NIT, crimson clover reduced Palmer amaranth densities >65% and 50% compared to winter fallow and cereal rye covers, respectively. At 6 WAP, the PRE and PRE + POST herbicide regimes in both IT and NIT reduced BR and WR Palmer amaranth densities >96% over the three years. Additionally, the BR density was reduced ≥59% in no-herbicide (None) following either cereal rye or crimson clover when compared to no-herbicide in the winter fallow. In IT, PRE, POST and PRE + POST herbicide regimes controlled Palmer amaranth >95% 6 WAP. In NIT, Palmer amaranth was controlled ≥79% in PRE and ≥95% in PRE + POST herbicide regimes over three years. POST herbicide regime following NIT was not very consistent. Averaged across three years, Palmer amaranth controlled ≥94% in PRE and PRE + POST herbicide regimes regardless of cover crop. Herbicide regime effect on cotton yield was highly significant; the maximum cotton yield was produced by the PRE + POST herbicide regime. Averaged over three years, the PRE, POST and PRE + POST cotton yields were about three times higher than no herbicide regime. In a conservation tillage production system, a PRE + glufosinate POST herbicide based regime coupled with a cereal rye cover crop may effectively control Palmer amaranth and maximize cotton yields.

Influence of Soybean (Glycine max) Population and Herbicide Program on Palmer Amaranth (Amaranthus palmeri) Control, Soybean Yield, and Economic Return

Palmer amaranth (Amaranthus palmeri S. Wats) has become one of the most prominent and difficult weeds to control in soybean (Glycine max (L.) Merr.) in North Carolina. A survey was conducted in North Carolina during fall 2010 to estimate the magnitude of this problem. Palmer amaranth was present in 39% of 2,512 fields representing 0.24% of soybean ha in North Carolina. In recent years, growers have reduced soybean seeding rates in an effort to decrease production costs associated with technology fees. However, given the increase in prevalence of Palmer amaranth and the difficultly in controlling this weed due to herbicide resistance, growers may need to reconsider reductions in seeding rates. Therefore, research was conducted during 2010 and 2011 to determine if Palmer amaranth control, soybean yield, and economic return were affected by soybean plant population, preemergence (PRE) and postemergence (POST) herbicides, and herbicide resistant traits (glufosinate-resistant and glyphosate-resistant cultivars). Applying PRE or POST herbicides and increasing soybean population increased Palmer amaranth control, soybean yield, and economic return when compared with POST herbicides only or when lower soybean populations were present. Efficacy of glufosinate and glyphosate did not vary in most instances, most likely because these herbicides were applied timely, and the frequency of glyphosate resistance did not exceed 10% in these fields.

Weed Management and Peanut Response from Applications of Saflufenacil

Saflufenacil is a new protoporphyrinogen oxidase–inhibiting herbicide registered for use before establishment of field corn and soybean. Generally, peanut plants are tolerant to other herbicides in this class, and no reports document the utility of saflufenacil for in-season weed control. Experiments were conducted to determine whether saflufenacil applied at 12, 25, and 50 g ha−1could effectively control Benghal dayflower and Palmer amaranth. It was observed that saflufenacil, applied either PRE or POST, was ineffective for Benghal dayflower. The maximum control at 28 d after treatment (DAT) was 79% when 50 g ha−1was applied to 5- to 10-cm plants. Control of Palmer amaranth from PRE applications was less effective than flumioxazin at 28 DAT. However, POST applications provided > 87% control at 28 DAT when applied to plants 5 to 10 cm in height. For plants 10 to 15 cm in height, > 90% Palmer amaranth control was only achieved by the 50 g ha−1application rate. For plants 15 to 20 cm in height, no POST application provided > 70% control. Peanut response, in a weed-free environment, to saflufenacil rate and application timing were also evaluated. Peanut stunting ranged from 0 to 36%, relative to application timing. Applications made at 0 d after emergence (DAE) were least injurious, whereas those made at 15 DAE were most injurious. Application of 50 g ha−1provided the greatest amount of stunting and foliar injury. However, stunting and saflufenacil application rate did not correspond to yield reduction. Saflufenacil application timing did influence peanut yield. Applications made between 0 and 30 DAE did not result in yield loss, whereas applications made at 45 and 60 DAE resulted in a 5 and 19% reduction, respectively. Though saflufenacil has many positive characteristics, higher application rates are required for optimum weed control. However, these higher use rates also resulted in unacceptable levels of injury.

Multiple Resistance to Glyphosate and Pyrithiobac in Palmer Amaranth (Amaranthus palmeri) from Mississippi and Response to Flumiclorac

Greenhouse and laboratory studies were conducted to confirm and quantify glyphosate resistance, quantify pyrithiobac resistance, and investigate interaction between flumiclorac and glyphosate mixtures on control of Palmer amaranth from Mississippi. The GR50(herbicide dose required to cause a 50% reduction in plant growth) values for two glyphosate-resistant biotypes, C1B1 and T4B1, and a glyphosate-susceptible (GS) biotype were 1.52, 1.3, and 0.09 kg ae ha−1glyphosate, respectively. This indicated that the C1B1 and T4B1 biotypes were 17- and 14-fold resistant to glyphosate, respectively, compared with the GS biotype. The C1B1 and T4B1 biotypes were also resistant to pyrithiobac, an acetolactate synthase (ALS) inhibitor, with GR50values of 0.06 and 0.07 kg ai ha−1, respectively. This indicated that the C1B1 and T4B1 biotypes were 7- and 8-fold, respectively, more resistant to pyrithiobac compared with the GS biotype, which had a GR50value of 0.009 kg ha−1. Flumiclorac was antagonistic to glyphosate by reducing glyphosate translocation. The C1B1 and T4B1 absorbed less glyphosate 48 h after treatment (HAT) compared with the GS biotype. The majority of the translocated glyphosate accumulated in the shoot above the treated leaf (that contains the apical meristem) in the GS biotype and in the shoot below the treated leaf in the resistant biotypes, C1B1 and T4B1, by 48 HAT. The C1B1 biotype accumulated negligible shikimate levels, whereas the T4B1 and GS biotypes recorded elevated levels of shikimate. Metabolism of glyphosate to aminomethylphosphonic acid was not detected in either of the resistant biotypes or the susceptible GS biotype. The above results confirm multiple resistance to glyphosate and pyrithiobac in Palmer amaranth biotypes from Mississippi and indicate that resistance to glyphosate is partly due to reduced absorption and translocation of glyphosate.

Influence of Selected Fungicides on Efficacy of Clethodim and 2,4-DB

Abstract A range of fungicides and herbicides can be applied to control pests and optimize peanut yield. Experiments were conducted in North Carolina to define biological and physicochemical interactions when clethodim and 2,4-DB were applied alone or with selected fungicides. Pyraclostrobin consistently reduced large crabgrass [Digitaria sanguinalis (L.) Scop.] control by clethodim. Chlorothalonil and tebuconazole plus trifloxystrobin reduced large crabgrass control by clethodim in two of four experiments while prothioconazole plus tebuconazole and flutriafol did not affect control. Palmer amaranth [Amaranthus palmeri S. Wats] control by 2,4-DB was not affected by these fungicides. Although differences in spray solution pH were noted among mixtures of clethodim plus crop oil concentrate or 2,4-DB and fungicides, the range of pH was 4.40 to 4.92 and 6.72 to 7.20, respectively, across sampling times of 0, 6, 24, and 72 h after solution preparation. Permanent precipitates were formed when clethodim, crop oil concentrate, and chlorothalonil were co-applied at each sampling interval. Permanent precipitates were not observed when clethodim and crop oil concentrate were included with other fungicides or when 2,4-DB was mixed with fungicides. Significant positive correlations were noted for Palmer amaranth control by 2,4-DB and solution pH but not for clethodim and solution pH.

Weed Control and Crop Safety with Premixed <i>S</i>-Metolachlor and Sulfentrazone in Sunflower

A preliminary study conducted in the central USA near Colby and Hays, Kansas (KS) in 2010 indicated a premix of S-metolachlor & sulfentrazone codenamed F7583 (Broadaxe?) had good potential for use in sunflower (Helianthus annuus L.). Additional studies were conducted in 2011 at Colby, Hays, Manhattan, KS to refine rate and application timing of F7583 for weed control and crop safety. Four rates of F7583 (860, 1100, 1350 and 1840 g·ha-1) were compared to single rates of S-metolachlor and pendimethalin, and applied 21 days preplant versus preemergence (PRE). F7583 at ≥1100 g·ha-1 applied preplant or PRE controlled Palmer amaranth (Amaranthus palmeri S. Wats.) and kochia [Kochia scoparia (L.) Schrad.] ≥95% and 100%, respectively in neutral pH soils. In slightly acidic soils, PRE application of F7583 was more effective against Palmer amaranth and grass weeds compared to preplant application. No benefit was gained by increasing the rate of F7583 from 1100 to 1350 g·ha-1 at either application timing. Puncturevine (Tribulus terrestris L.) control was not commercially satisfactory with F7583 at any rate or time of application. Both S-metolachlor at 1070 g·ha-1 and pendimethalin at 1600 g·ha-1 applied either preplant or PRE were considerably less effective on all three broadleaf weeds compared to F7583 treatments. Individually, S-metolachlor and pendimethalin were more effective when applied PRE compared to preplant application. F7583 did not reduce sunflower plant population or visibly injure sunflower anytime during the season.

Joint Action Analysis Utilizing Concentration Addition and Independent Action Models

In weed science literature, models such as concentration addition, independent action, effect summation, and the parallel line assay technique have been used to predict and analyze whole-plant response to herbicide mixtures. Although a joint action reference model is necessary for determining whether the herbicide mixture provides less than (antagonistic), equal to (zero-interaction or additive), or greater than (synergistic) expected control, model selection often occurs with little regard to the model's underlying biological assumptions. The joint action models of concentration addition (CA) and independent action (IA) are the appropriate models to consider for analysis of herbicide mixtures of two active components. CA assumes additivity of dose, with herbicides differing only in potency, whereas IA assumes multiplicativity of effects, in which herbicides behave independently and sequentially within the plant. CA and IA predicted mixture responses were computed for a sample mixture data set of mesotrione plus atrazine. IA predicted lower mixture responses than CA; for example, mesotrione at 17.5 g ha−1+ atrazine at 140 g ha−1was predicted to provide 45% (IA) compared with 53% (CA) control of Palmer amaranth. Joint action claims of synergism and antagonism were shown to be dependent on the reference model selected. Although mesotrione plus atrazine combinations were synergistic under IA assumptions, analysis under CA assumptions indicated mesotrione plus atrazine to be synergistic, additive, and antagonistic according to the selected effective concentration (ECx) level and fixed-ratio mixture. Because it is not possible to determine the appropriate joint action model on the basis of fit of predicted to observed mixture data, the appropriateness of underlying biological assumptions was considered for the sample mixture data set. Additionally, we provide decision criteria to aid researchers in their selection of an appropriate joint action reference model.

Weed Management with Glyphosate- and Glufosinate-Based Systems in PHY 485 WRF Cotton

Glyphosate-resistant (GR) Palmer amaranth has become a serious pest in parts of the Cotton Belt. Some GR cotton cultivars also contain the WideStrike™ insect resistance trait, which confers tolerance to glufosinate. Use of glufosinate-based management systems in such cultivars could be an option for managing GR Palmer amaranth. The objective of this study was to evaluate crop tolerance and weed control with glyphosate-based and glufosinate-based systems in PHY 485 WRF cotton. The North Carolina field experiment compared glyphosate and glufosinate alone and in mixtures applied twice before four- to six-leaf cotton. Additional treatments included glyphosate and glufosinate mixed withS-metolachlor or pyrithiobac applied to one- to two-leaf cotton followed by glyphosate or glufosinate alone on four- to six-leaf cotton. All treatments received a residual lay-by application. Excellent weed control was observed from all treatments on most weed species. Glyphosate was more effective than glufosinate on glyphosate-susceptible (GS) Palmer amaranth and annual grasses, while glufosinate was more effective on GR Palmer amaranth. Annual grass and GS Palmer amaranth control by glyphosate plus glufosinate was often less than control by glyphosate alone but similar to or greater than control by glufosinate alone, while mixtures were more effective than either herbicide alone on GR Palmer amaranth. Glufosinate caused minor and transient injury to the crop, but no differences in cotton yield or fiber quality were noted. This research demonstrates glufosinate can be applied early in the season to PHY 485 WRF cotton without concern for significant adverse effects on the crop. Although glufosinate is often less effective than glyphosate on GS Palmer amaranth, GR Palmer amaranth can be controlled with well-timed applications of glufosinate. Use of glufosinate in cultivars with the WideStrike trait could fill a significant void in current weed management programs for GR Palmer amaranth in cotton.