Horseweed Control Research Articles

Enlist E3™ Soybean Sensitivity and Enlist™ Herbicide-Based Program Control of Sumatran Fleabane (Conyza sumatrensis)

AbstractEnlist E3™ soybean, resistant to 2,4-D, glufosinate, and glyphosate, provides options to control glyphosate-resistant Sumatran fleabane before planting and in crop. Twenty field trials were conducted in Argentina to determine Enlist E3 soybean sensitivity to POST applications of 2,4-D choline+glyphosate or glufosinate. Maximum injury from a single 2,4-D choline+glyphosate application at 1X (1140+1140 g ae ha−1) and 2X rate was 4% and 13%, respectively, at 3 days after treatment in the temperate Humid Pampa region. Slightly higher injury of 11 and 23% was observed in sub-tropical region of northern Argentina. Injury was transient with recovery occurring within 14 days. Injury caused by sequential applications was equivalent to that caused by single applications. Soybean yield was not affected by single nor sequential applications. In four trials, control programs containing 2,4-D choline+glyphosate applied PRE and POST provided greater GR Sumatran fleabane control and a 12 to 26% increase in yield compared to 2,4-D choline+glyphosate applied at PRE only. This research demonstrates the glyphosate-resistant control programs that include 2,4-D choline, glyphosate, and glufosinate provide excellent GR Sumatran fleabane control.

A Multy-Year Study Reveals the Importance of Residual Herbicides on Weed Control in Glyphosate-Resistant Soybean

ABSTRACT: A 3-year field study was conducted to assess the potential for using pre-emergent (PRE) herbicides tank mixed with glyphosate as a means of controlling weed species in soybean. In 2011/12, 2012/13 and 2013/14 growing sessions soybean cultivar Brasmax Apollo RR was planted under residues of rye. The herbicide treatments glyphosate (gly) (1,296 g a.i. ha-1), gly + S-metolachlor (1,296 + 1,920), gly + imazaquin (1,296 +161), gly + pendimethalin (1,296 + 1,000), gly + metribuzin (1,296 + 480), gly + 2.4-D amine (1,296 + 1,209) was applied in pre-emergence (PRE) over rye crop residues two days before soybean sowing. In addition, full season weed-free and weedy control plots were included. Gly + S-metolachlor and gly + pendimethalin reduced the horseweed density from 48 to 3 and 6 plants m-2, respectively. The mix containing gly + metribuzin and gly + 2.4-D amine and gly applied alone had no effect in the horseweed control. The mix containing gly + metribuzin, gly + 2.4-D amine, gly + imazaquin and gly applied alone had no effect in the crabgrass control. In contrast gly + S-metolachlor and gly + pendimethalin reduced the crabgrass density from 70 to 0 and 1 plant m-2, respectively. The soybean yield was higher with weed-free, S-metolachlor and metribuzin treatments. The use of an herbicide with residual effect had impact on weed management and soybean yield. In conclusion, a greater control of horseweed and crabgrass occurred when S-metolachlor or pendimethalin was applied PRE.

Associations of Herbicides on the Simultaneous Control of Horseweed and Italian Ryegrass

Weed competition negatively affects grain yield on wheat and soybean crops. Due to better practicality and timesaving it is usual the association of broadleaf herbicides with graminicides to control monocot and dicot weeds. However, little is known about the deleterious effects of broadleaf herbicides on graminicides. Thus, the objective of this experiment was to evaluate the effects of simple, double and triple association of broadleaf herbicides with graminicides to control of Italian ryegrass and horseweed. The experiment was conducted into greenhouse on completely randomized experimental design with four replications. The treatments consisted of the graminicides clethodim and clodinafop; the broadleaf herbicides 2,4-D, metsulfuron and saflufenacil and non-selective glyphosate, sprayed isolated and in double and triple association to be compared with a control. The variables evaluated were control (%) at 21 and 28 days after treatments (DAT), and dry weight (g) at 28 DAT. The isolated graminicides and the broadleaf herbicides controlled both Italian ryegrass and horseweed respectively. At the double association of broadleaf with graminicides herbicides, 2,4-D antagonized the effect of clodinafop, and glyphosate mixed with clodinafop decreased the control of the graminicide on Italian ryegrass. At the triple herbicide association, when mixed with glyphosate, metsulfuron decreased the control effect of Italian ryegrass by clethodim. The association of broadleaf herbicides with graminicides is effective on the control of monocot and dicot weeds, but it depends on which herbicide used for the combination. When it is evident the antagonism between broadleaf herbicides and graminicides, the detrimental effects go on grass control.

Soybean yield loss and economic thresholds due to glyphosate resistant hairy fleabane interference

ABSTRACT: The interference of glyphosate resistant hairy fleabane (Conyza bonariensis) has caused yield losses in soybean crop. Knowledge of the economic threshold is an important aspect for the economic management of resistant weeds. Because of it, the interference of the glyphosate resistant hairy fleabane density on the soybean yield loss was evaluated, as well as to estimate the economic threshold. Two experiments were conducted to verify the effect of different weed densities on the yield of two soybean cultivars (BRS Estância and BMX Turbo). Weed density ranged from 0 to 124 plants m-2. The yield of the BRS Estância decreased by 1.4% to the increase of one single plant, while for the BMX Turbo the loss was 25.9%. Soybean yield and economic threshold were lower for the BMX Turbo cultivar compared to the cultivar BRS Estância, which can be attributed to the genotype, competitive ability, weed development stage and/or environment. Increases in crop yield expectation, soybean price and herbicide efficiency reduce the economic threshold, indicating that the adoption of control measures should be taken when the weed density is low. Due to the high cost of hairy fleabane control, it is only feasible at high densities.

Effect of Time of Day of Application of 2,4-D, Dicamba, Glufosinate, Paraquat, and Saflufenacil on Horseweed (Conyza canadensis) Control

A study to evaluate the effect of application time of day (TOD) on the efficacy of five burndown herbicides was conducted in Alabama and Tennessee. Treatments of 2,4-D, dicamba, glufosinate, paraquat, and saflufenacil were applied at sunrise, midday, or sunset to a native population of horseweed and analyzed separately. Control of glyphosate-resistant (GR) horseweed with 2,4-D, dicamba, glufosinate, and saflufenacil was greatest from the midday application. Percentage of living horseweed counts for all of these herbicides followed a similar pattern. Control from paraquat was lowest at the midday timing and greatest from the sunset application with surviving horseweed plant populations reflecting those control ratings. Application TOD significantly affected all of the herbicides in this research. Applications of 2,4-D, dicamba, glufosinate, and saflufenacil are more efficacious when applied during the middle portion of the day, while paraquat is more efficacious when applied at sunset for maximum horseweed control.

English

Fleabane (Conyza spp.) has spread in no-tillage areas of Paraná State (PR), Brazil, and currently represents one of the main challenges related to weed control, particularly during off-season period. For this purpose, two experiments were carried out in Campina da Lagoa (PR) and Floresta (PR) with the aim of assessing the efficacy of herbicide treatments applied to fleabane areas during the off-season period between maize harvest (June to July) and soybean sowing (November). Treatments consisted of herbicide treatments (glyphosate + 2, 4-D; glufosinate, MSMA) targeting the control of fleabane plants in POST, tank-mixed with residual herbicides (metsulfuron, chlorimuron, diclosulam, imazethapyr, imazaquin, flumioxazin, metribuzin, amicarbazone and isoxaflutole) to control the emergence and growth of new flushes in PRE. Applications were performed when fleabane plants reached a height of 2 cm. Evaluation on both efficacy and residual weed control was accomplished from the first day of application day to 75 days after application (soybean crop sowing). Glyphosate + 2, 4-D mixture was efficient for burndown of Conyza spp. in all situations. During 75 day off season period, diclosulam and chlorimuron were the best options for controlling fleabane emergence when mixed with any other options of herbicide treatments, for the control in POST. Mixtures of glyphosate + 2, 4-D with metribuzin and glufosinate with flumioxazin, metribuzin and isoxaflutole maintained good fleabane control throughout the whole 75 days off season period. Fall management was an effective option for fleabane control.   Key words: Conyza spp., herbicide resistance, management, residual effect, weeds.

Biologically effective rate of metribuzin for glyphosate-resistant Canada fleabane control in soybean

Eight field experiments were conducted during 2013–2015 to determine the dose response of glyphosate-resistant Canada fleabane to preplant metribuzin in a tank mixture with glyphosate in soybean. T...

Efficacy of Saflufenacil for Control of Glyphosate-Resistant Horseweed (Conyza canadensis) as Affected by Height, Density, and Time of Day

Control of glyphosate-resistant (GR) horseweed in soybean with glyphosate (900 g ai ha−1) plus saflufenacil (25 gaiha−1) has been variable. The objective of this research was to determine the effect of GR horseweed height, density, and time of day (TOD) at application on saflufenacil plus glyphosate efficacy in soybean. All experiments were completed six times during a 2 yr period (2014, 2015) in fields previously confirmed with GR horseweed. Applications from 0900 to 2100 hours provided optimal control of GR horseweed 8 WAA. Soybean yield paralleled GR horseweed control with the highest yield of 3000kgha−1at 1500 hours, and the lowest yield of 2400kgha−1at 0600 hours. The height and density of GR horseweed at application had minimal effect on saflufenacil efficacy. Saflufenacil provided>99% control of GR horseweed when applied to small plants and low densities; however, control decreased to 95% when>25 cm tall, and to 96% in densities>800 plants m−2at 6 WAA due to some plant regrowth. TOD of application had a greater influence on GR horseweed control with saflufenacil than height or density. To optimize control of GR horseweed, saflufenacil should be applied during daytime hours to small plants at low densities. Optimizing GR horseweed control minimizes weed seed return and weed interference.

Glyphosate-Resistant Horseweed (Conyza canadensis) Dose Response to Saflufenacil, Saflufenacil plus Glyphosate, and Metribuzin plus Saflufenacil plus Glyphosate in Soybean

The control of glyphosate-resistant (GR) horseweed (Conyza canadensis) in soybean has been variable with glyphosate plus saflufenacil. The objective of this research was to determine the biologically effective rate (BER) of saflufenacil, saflufenacil mixed with glyphosate, and metribuzin mixed with saflufenacil and glyphosate applied preplant (PP) for the control of GR horseweed in no-till soybean; a study was conducted to determine each of the three treatments. For each study, seven field sites infested with GR horseweed were used over a 2-yr period (2014, 2015). Saflufenacil alone at 25 and 36 g ai ha–1 provided 90 and 95% control of GR Horseweed 8 wk after application, while the BER to achieve 98% control was outside of the treatment range tested. The saflufenacil plus glyphosate (900 g ai ha–1) BER experiment found less saflufenacil was required as 25, 34, and 47 g ha–1 provided 90, 95, and 98% control of GR horseweed respectively. The metribuzin BER experiment found 61, 261, and 572 g ha–1 was required to provide 90, 95 and 98% control of GR horseweed, respectively, mixed with saflufenacil (25 g ha–1) and glyphosate (900 g ha–1). The addition of metribuzin with the recommended rate of saflufenacil (25 g ha–1) plus glyphosate improved control and a second effective herbicide mode of action for the control of GR horseweed. The use of a threeway herbicide mixture can be an effective weed management strategy to control GR horseweed in soybean.

Postemergence herbicides for control of glyphosate-resistant Canada fleabane in corn

Post-emergence herbicides were evaluated for glyphosate-resistant Canada fleabane control in corn (Zea mays L.) from 2013 to 2015. By 8 wk after treatment, dicamba/atrazine (96% control), dicamba (...

Influence of Carrier Water pH, Hardness, Foliar Fertilizer, and Ammonium Sulfate on Mesotrione Efficacy

Carrier water pH, hardness, coapplied foliar fertilizer, water conditioning agents, and plant height are critical considerations for optimum herbicide performance. Field studies were conducted to evaluate the effect of carrier water pH (4, 6.5, and 9) and zinc (Zn) or manganese (Mn) foliar fertilizer on mesotrione for horseweed and Palmer amaranth control. Additionally, effect of carrier water pH and foliar fertilizer was evaluated on 7.5-, 12.5-, and 17.5-cm tall horseweed. Greenhouse treatments consisted of carrier water pH and foliar fertilizer (Zn, Mn, or without fertilizer); or water hardness (0 to 1,000 mg L−1) in the presence or absence of ammonium sulfate (AMS) for mesotrione control of giant ragweed, horseweed, and Palmer amaranth. Mesotrione activity was greater on horseweed with carrier water pH 6.5 compared to pH 4 or 9. Coapplied Zn fertilizer reduced mesotrione activity on Palmer amaranth in the field study in 2014 and horseweed in the greenhouse study. Mesotrione efficacy was greatly influenced by horseweed height. Percent control ranged from 96 to 99%, 75 to 89%, or 61 to 64% with mesotrione applied on 7.5-, 12.5-, or 17.5-cm tall horseweed, respectively, and results were similar for plant height and dry weight reduction. Increasing carrier water hardness from 0 to 1,000 mg L−1reduced mesotrione efficacy 28, 18, and 18% (or greater) on giant ragweed, horseweed, and Palmer amaranth, respectively. The addition of AMS enhanced mesotrione efficacy 9, 6, or 9% (or greater) for giant ragweed, horseweed, and Palmer amaranth control, respectively. Mesotrione should be applied at near neutral carrier water pH, hardness < 200 mg L−1, and with AMS for achieving optimum weed control.

Control of glyphosate resistant Canada fleabane with saflufenacil plus tankmix partners in soybean

Six field trials were conducted over a two-year period (2014, 2015) to determine the level and consistency of glyphosate-resistant (GR) Canada fleabane control with glyphosate plus saflufenacil plu...

Control of Glyphosate-Resistant Horseweed in Winter Wheat with Pyrasulfotole Premixed with Bromoxynil

Five experiments were conducted over a 2-yr period (2013 and 2014) to evaluate POST herbicides in winter wheat fields with a history of glyphosate-resistant (GR) horseweed. Control 4 wk after treatment (WAT) with pyrasulfotole + bromoxynil was 95%. Control 8 WAT with 2,4-D, dicamba + MCPA + mecoprop, clopyralid, and pyrasulfotole + bromoxynil ranged from 89 to 97%; these herbicides also reduced GR horseweed density and biomass by 97 to 99%. Single mode of action herbicides like 2,4-D controlled GR horseweed; however, multiple modes of action should be used to prevent populations from becoming incrementally more resistant under repeated selection pressure.

Eficiência do cloransulam-metílico no controle em pós-emergência de Conyza bonariensis na cultura da soja RR®

Com a introdução da soja transgênica resistente ao glyphosate, a qual foi amplamente difundida na região sul do Brasil, intensificou-se o número de aplicações e a seleção de biótipos de buva (Conyza bonarienses) resistente ao herbicida. Desta maneira, são fundamentais novas alternativas químicas para o controle eficiente de buva. Diante disso, o objetivo deste trabalho foi avaliar a eficácia do cloransulam-metílico (Pacto®), aplicado em pós-emergência, no controle de C. bonariensis e produtividade da soja SYN 1059 RR Vtop®. O experimento foi conduzido no município de Assis Chateaubriand-PR, em delineamento experimental de blocos ao acaso, com quatro repetições. Os tratamentos constituíram das doses de cloransulam-metílico (25, 30, 35 e 40 g i.a. ha-1), clorimuron-etílico (20 g i.a. ha-1) e testemunhas capinada e sem capina. As plantas de buva encontravam-se entre 5 a 10 cm de altura e densidade média de 35 plantas por m2. Cloransulam-metílico, nas doses de 25 e 30 g ha-1, não controlou de forma eficiente a buva (<80%) a partir dos 28 DAA (dias após aplicação), sendo inferior ao clorimuron-etílico. Entretanto, cloransulam-metílico a 35 e 40 g ha-1 apresentou controle de buva (87,0% e 90,7%) não diferindo significativamente do clorimuron-etílico aos 42 DAA (85,7%), assim como não ocasionando sintomas de injúrias e redução de produtividade à cultura da soja. Cloransulam-metílico apesar de não ter sido eficiente no controle de C. bonariensis a 30 g ha-1, apresentou produtividade da soja semelhante à testemunha capinada, e superior ao clorimuron-etílico. A matointerferência ocasionada pela infestação de buva reduziu a produtividade da soja em 47,5%.

Influence of Application Variables on the Foliar Efficacy of Saflufenacil on Horseweed (Conyza canadensis)

Greenhouse studies were conducted to determine the influence of spray-solution pH, adjuvant, light intensity, temperature, and glyphosate on the efficacy of saflufenacil on horseweed. Control of glyphosate-resistant horseweed from saflufenacil alone was greatest with a spray-solution pH of 5, compared with pH 7 or 9. However, when glyphosate was added to saflufenacil, similar GR50values were measured with spray solutions adjusted to pH 5 and 9, and horseweed control at pH 9 was 38% greater than at pH 7. The efficacy of saflufenacil on horseweed was 36% greater when crop oil concentrate was used as an adjuvant compared with nonionic surfactant, regardless of the addition of glyphosate or the sensitivity of the horseweed population to glyphosate (resistant vs. susceptible). The addition of glyphosate to low rates of saflufenacil increased control over saflufenacil applied alone on glyphosate-susceptible and -resistant horseweed. Saflufenacil activity was greater under low light intensity (300 μmol m−2s−1) than high light intensity (1,000 μmol m−2s−1). Although initial horseweed control was greater under high temperature (27 C) compared with low temperature (10 C), by 21 d after treatment horseweed dry weight was similar from saflufenacil applied under high and low temperatures.

Manejo de plantas daninhas eudicotiledôneas na cultura da soja Roundup Ready

As espécies eudicotiledôneas sempre foram um desafio no manejo de plantas daninhas na cultura da soja. Essa dificuldade foi aumentada pelo surgimento de plantas de difícil controle com os herbicidas atualmente existentes no mercado. Diante disso, o objetivo deste trabalho foi o de buscar alternativas de manejo que diminuam a pressão de seleção devido ao uso contínuo de glifosato na soja Roundup Ready®. O trabalho foi implantado no delineamento em blocos casualizados, em esquema de parcelas sub-subdivididas, com quatro repetições. Foram avaliadas cinco coberturas de inverno (parcelas), seis manejos químicos em pré-semeadura da soja (subparcelas) e três manejos químicos em pós-emergência da soja (sub-subparcelas). Nenhum dos tratamentos avaliados mostrou fitotoxicidade visual às plantas de soja. No entanto, as aplicações sequenciais em pós-emergência diminuíram a altura da soja, mas sem impactar negativamente a sua produtividade. Nos tratamentos com cobertura de aveia, trigo ou azevém, qualquer combinação de aplicações em pré e pós-semeadura propiciou controle de buva acima de 90%. De maneira geral, os tratamentos que propiciaram os maiores controles para leiteiro foram as combinações de glifosato + diclosulam e glifosato + sulfentrazone em pré-semeadura. Quando a soja foi semeada em área de pousio de inverno, a produtividade foi em média 8% inferior do que quando semeada em áreas com cobertura de aveia e trigo. De modo geral, os tratamentos que resultaram em maiores produtividades foram aqueles em que se utilizou glifosato + diclosulam ou glifosato + sulfentrazone na pré-semeadura da soja.

Canada fleabane (Conyza canadensis) control with preplant applied residual herbicides followed by 2,4-D choline/glyphosate DMA applied postemergence in corn

Ford, L., Soltani, N., Robinson, D. E., Nurse, R. E., McFadden, A. and Sikkema, P. H. 2014. Canada fleabane (Conyza canadensis) control with preplant applied residual herbicides followed by 2,4-D choline/glyphosate DMA applied postemergence in corn. Can. J. Plant Sci. 94: 1231–1237. Glyphosate resistant (GR) Canada fleabane (Conyza canadensis) is an extremely problematic weed in no-tillage farming operations. A total of five field trials were conducted over a 2-yr (2012 and 2013) period in Ontario to determine the level of GR Canada fleabane control with a two-pass weed control program of a pre plant (PP) residual herbicide followed by 2,4-D choline/glyphosate dimethylamine (DMA) applied POST. Among residual herbicide treatments evaluated, s-metolachlor (1600 g a.i. ha−1)+flumetsulam (50 g a.i. ha−1)+clopyralid (135 g a.e. ha−1) provided the most consistent (95–99%) control across all sites 8 wk after application (WAA). S-metolachlor/atrazine (1800 g a.i. ha–1) did not provide effective GR Canada fleabane control (21–86%) 8 WAA. The preplant residual herbicides followed by 2,4-D choline/glyphosate DMA (1720 g a.e. ha−1) POST provided 97–100% control. Glyphosate (900 g a.e. ha−1) applied PP followed by 2,4-D choline/glyphosate DMA POST provided 80–93% control 8 WAA. The application of 2,4-D choline/glyphosate DMA POST following any PP residual herbicide resulted in 97% or greater control of GR Canada fleabane. Results from this research demonstrate that residual herbicides applied PP followed by 2,4-D choline/glyphosate DMA POST provides excellent control of GR Canada fleabane, and also incorporates different modes of action thereby limiting the selection of resistant weeds.

Soil disinfestation options in Aydın province, Turkey, strawberry cultivation

Two field experiments were conducted in the two successive seasons of 2005/2006 and 2006/2007 in strawberry-growing areas of Aydin province of Turkey to (a) compare soil solarization and metam sodium (MS) combinations with low-density polyethylene (LDPE) and virtually impermeable film (VIF); and (b) evaluate strawberry fruit yield, soilborne disease and weed control efficacy. Disinfestation treatments were as follows: (i) raised bed solarization for about 6 weeks, (ii) raised bed solarization for 3 weeks, (iii) raised bed solarization for 2 weeks + MS (50 ml m-2), (iv) MS (50 ml m-2) with LPDE (v) MS (100 ml m-2) with LDPE, (vi) MS (50 ml m-2) with VIF, (vii) MS (100 ml m-2) with VIF. Results indicated that Macrophomina phaseolina and Fusarium spp. were the major fungal pathogens isolated from the crown of dead or dying plants in both cropping seasons. MS treatment (100 ml m-2) through the drip irrigation system under LDPE sheet or VIF reduced soilborne diseases significantly while reduced dosage of MS (50 ml m-2) under LDPE or VIF and in combination with 2-week soil solarization controlled the soilborne diseases to a greater extent than soil solarization treatments. All treatments controlled the following weed species: Portulaca oleracea, Amaranthus spp., Poa annua, Alopecurus myosuroides, Matricaria chamomilla, Digitaria sanguinalis, Raphanus raphanistrum, Seteria verticillata, Chenopodium album, Stelleria media, Capsella bursa-pastoris, Echinochloa cruss-galli, Veronica hederifolia, Lamium amplexicaule and Silybum marianum. None of the treatments provided acceptable control of horseweed (Conyza canadensis). Metam sodium application (100 ml m-2) significantly increased yields by 18.5% and 14.5% in the 2005/2006 and 21.6% and 18.5% in the 2006/2007 season under LDPE and VIF, respectively, while marketable fruit yield was not significantly increased by soil solarization treatments in either trial. There was no significant difference between MS applications under LDPE and VIF for the weed and disease control, and marketable fruit yield.

Saflufenacil efficacy on horseweed and its interaction with glyphosate

Saflufenacil is a new herbicide on the market and its effectiveness on horseweed, several populations of which have evolved resistance to glyphosate, is not clear. In this research, the effect of adjuvants on the control of horseweed with saflufenacil in the field, the effect of the interaction between glyphosate and saflufenacil on glyphosate‐resistant and glyphosate‐susceptible horseweed and the patterns of uptake and translocation of glyphosate applied alone and in combination with saflufenacil in horseweed were evaluated. The addition of methylated seed oil to saflufenacil provided the best control of horseweed, with crop oil concentrate being intermediate in effect and non‐ionic surfactant ranking as the least‐effective adjuvant. The interaction between glyphosate and saflufenacil was additive with regards to the control of glyposate‐resistant horseweed. The glyphosate‐susceptible horseweed population absorbed 6–13% more 14C‐glyphosate than the glyphosate‐resistant population. The addition of saflufenacil reduced 14C‐glyphosate translocation in both the glyphosate‐resistant and the glyphosate‐susceptible horseweed populations by at least 6%; however, due to the exceptional efficacy of saflufenacil, these reductions did not reduce the level of control. Saflufenacil holds great potential as an alternative control option for glyphosate‐resistant horseweed and is a valuable tool in the management of resistant weeds.

Influence of Plant Height and Glyphosate on Saflufenacil Efficacy on Glyphosate-Resistant Horseweed (Conyza canadensis)

A field study was conducted in 2007 and 2008 near Murphysboro, IL to determine the effect of plant height and addition of glyphosate on control of glyphosate-resistant horseweed with saflufenacil. Saflufenacil was applied at rates ranging from 25 to 125 g ai ha−1alone and in combination with glyphosate at 840 g ae ha−1, and the efficacy compared to paraquat at 840 g ai ha−1. Control of horseweed with glyphosate applied alone was less than 30%, confirming the presence of glyphosate-resistant plants. At 14 d after application, all treatments with saflufenacil or paraquat provided at least 90% control. Saflufenacil applied alone at the lowest rate of 25 g ha−1provided less control (92%) than all other treatments that included saflufenacil, and efficacy was reduced as horseweed height at application increased. Horseweed control from saflufenacil at 50 g ha−1was reduced as plant height increased in 2007 but not in 2008. However, saflufenacil applied at 50 g ha−1or greater resulted in at least 98% control, regardless of horseweed height at application or tank mixture with glyphosate. Combining glyphosate with saflufenacil at 25 g ha−1increased horseweed control compared with saflufenacil applied alone and resulted in control similar to saflufenacil applied at 50 g ha−1. Control of horseweed from paraquat declined over time as the growth continued from the apical meristem. The extent of horseweed regrowth from applications of saflufenacil alone was less than that observed from paraquat. The addition of glyphosate to saflufenacil further reduced the frequency of horseweed regrowth compared with saflufenacil applied alone.