Broadleaf Signalgrass Control Research Articles

Effect of cereal rye cover crop termination timings on weed control and corn yield under a two-pass herbicide program

Cover crops, including cereal rye, are generally known to suppress weeds in various cropping systems; however, research on their effective use is lacking. This study investigated the effects of different cereal rye termination timings on weed control and corn performance in a two-pass herbicide program across 5 site-years (2021–2023) in Arkansas, USA. The herbicide program consisted of one application at cereal rye (Secale cereale) termination (cereal rye environment) or at corn (Zea mays) planting (conventional system) and another at the V4 corn stage. Palmer amaranth (Amaranthus palmeri) at V4 corn was better controlled in cereal rye environments regardless of its termination timing (>75%) than in the conventional corn system (<50%), and the control was generally excellent in both systems 4 weeks later, with subtly greater control in the cereal rye environments. In contrast, yellow nutsedge (Cyperus esculentus) control, in general, was slightly less in the cereal rye environments than in the conventional system. Barnyardgrass (Echinochloa crus–galli) and broadleaf signalgrass (Urochloa platyphylla) control was similar to that of Palmer amaranth, especially at the V4 corn stage, with clear benefits in its control with the later termination timings. Corn height was greater (by 8% to 10%) in cereal rye environments terminated at 1 or 2 weeks after corn planting compared to the conventional system or other cereal rye environments. Corn yield was similar between the conventional system and the cereal rye environments terminated before corn planting; however, the yield was 16% to 22% less with those terminated at or after corn planting. These findings indicate the constrained use of cereal rye in corn, with its termination time optimized for 2 weeks before corn planting. Further research could identify factors that mitigate the yield loss from delayed cereal rye termination and improve its use as a cover crop in corn.

Sensitivity of TamArk™ grain sorghum and monocot weed species to ACCase- and ALS-inhibiting herbicides

AbstractOnly a limited number of herbicides are available to provide postemergence (POST) control of selective monocot weeds in grain sorghum crops. The herbicides currently labeled for use with grain sorghum have strict use restrictions, low efficacy on johnsongrass, or weed resistance issues. To introduce a new effective herbicide mode of action for monocot control, multiple companies and universities have been developing herbicide-resistant grain sorghum that would allow producers to use herbicides that inhibit either acetolactate synthase (ALS) or acetyl coenzyme A carboxylase (ACCase) for POST monocot control. An experiment was conducted in Fayetteville, AR, in 2020 and 2021, to determine the effectiveness of two ALS-inhibiting herbicides and nine ACCase-inhibiting herbicides on TamArk™ grain sorghum, conventional grain sorghum, and problematic monocot weed species. Grain sorghum and monocot weeds (johnsongrass, broadleaf signalgrass, barnyardgrass, and Texas panicum) were sprayed when TamArk grain sorghum reached the 2- to 3-leaf stage. TamArk grain sorghum was tolerant of all ACCase-inhibiting herbicides tested, exhibiting ≤10% injury at all evaluation timings, except clethodim and sethoxydim, and had no resistance to the ALS-inhibiting herbicides that were evaluated. Additionally, all ACCase inhibitors except diclofop and pinoxaden controlled all monocots tested by >91% at 28 d after application (DAA). Conversely, the two ALS inhibitors, imazamox and nicosulfuron, provided ≤81% control of broadleaf signalgrass 28 DAA but still controlled all other monocots by >95%. TamArk grain sorghum has low sensitivity to multiple ACCase-inhibiting herbicides and thus provides an effective POST option for monocot weed control. In addition, unwanted volunteer TamArk plants can be controlled with cledthodim, sethoxydim, nicosulfuron, or imazamox. Although the ALS-inhibiting herbicides imazamox and nicosulfuron were not useful on TamArk grain sorghum, they are effective options for monocot control on Igrowth™ and Inzen™ grain sorghum crops, respectively.

Utility of isoxaflutole-based herbicide programs in HPPD-tolerant cotton production systems

AbstractPalmer amaranth has developed resistance to at least seven herbicide sites of action in the Cotton Belt of the United States, leaving producers with fewer options to manage this weed. Previous research with corn and newly commercially released soybean systems have found the use of 4-hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides such as isoxaflutole (IFT) to be effective at managing Palmer amaranth. Consequently, a new transgenic cultivar of cotton is being developed with tolerance to IFT, allowing for in-crop applications of the herbicide. Two separate studies were conducted near Marianna, AR, in 2019 and replicated in 2020, to investigate the crop safety and utility of IFT when added to cotton herbicide programs. Herbicide programs featured IFT as a preemergence or early-postemergence option, residual herbicides in subsequent postemergence applications, and the presence or absence of a layby application. The use of IFT did not significantly impact cotton injury or yield, whereas the use of layered residual herbicides, including IFT, increased Palmer amaranth control compared to those without. Regardless of earlier use of IFT, layby applications were needed for season-long control of Palmer amaranth, entireleaf morningglory, broadleaf signalgrass, and johnsongrass, as evidenced by greater than a 20 percentage point improvement in control of all weeds when a layby application was made. Overall, findings from these studies indicate IFT to be a suitable tool for managing Palmer amaranth and will provide an additional site of action for cotton herbicide programs. Sequential herbicide applications and overlaying residuals were found to be paramount for managing Palmer amaranth throughout the season.

Using Quinclorac to Control Annual Grasses and Palmer Amaranth in Grain Sorghum (Sorghum bicolor L.)

Field studies were conducted during the 2015 and 2016 growing seasons in south-central Texas to determine control of Palmer amaranth and annual grasses along with grain sorghum tolerance to quinclorac alone and in various combinations when applied to weeds < 5 cm (EPOST) or 10 to 16 cm tall (LPOST). When evaluated late-season quinclorac alone at 0.43 kg ae ha-1 controlled broadleaf signalgrass 72% when applied EPOST and 91% when applied LPOST. Combinations of quinclorac with either atrazine, pyrasulfotole + bromoxynil, dicamba, or dimethenamid-P controlled Palmer amaranth 88 to 100% when applied EPOST or LPOST; however, broadleaf signalgrass control with these combination was better when applied LPOST (75 to 95%) compared with EPOST (37 to 72%) applications. Texas millet control with quinclorac was poor in both years and was never greater than 54%. Quinclorac plus either atrazine, pyrasulfotole + bromoxynil, dicamba, or atrazine + dimethenamid-P caused at least 20% sorghum injury at one of three locations. No yield reductions from the untreated check were noted in either year; however, in 2016 all treatments with the exception of quinclorac alone at 0.29 kg ha-1 applied EPOST, quinclorac + pyrasulfotole + bromoxynil applied LPOST, quinclorac + atrazine + pyrasulfotole + bromoxynil applied LPOST, and quinclorac + dicamba at either application timing produced yields that were greater than the untreated check.

Timing and Application Rate for Sequential Applications of Glufosinate are Critical for Maximizing Control of Annual Weeds in LibertyLink® Soybean

Preserving the utility of glufosinate in both LibertyLink soybean and other glufosinate-resistant crops is critical for managing herbicide-resistant weeds. An experiment with a two-factor factorial arrangement was conducted at the Arkansas Agricultural Research and Extension Center in Fayetteville, AR, in 2015, 2016, and 2017 to evaluate the efficacy of glufosinate in single and sequential applications at various rates on 8–32 cm tall Palmer amaranth, barnyardgrass, and broadleaf signalgrass. Herbicide treatments consisted of glufosinate applied at 454, 595, 738, and 882 g ai ha−1 (Factor 1) with either no sequential application or a sequential application occurring 7, 10, 14, or 21 days after the initial application (DAI) (Factor 2). For treatments that contained a sequential application, the same rate used in the initial application (e.g., 451 g ai ha−1) was also used in the sequential. Regardless of species and rate, sequential applications were always superior to single applications. Palmer amaranth control 3 weeks after the final treatment (WAF) was 8% greater when the sequential application occurred 10 DAI compared to 21 DAI, averaged over glufosinate rates. When at least 595 g ai ha−1 glufosinate was used in a treatment, no differences between the 7-, 10-, 14-, and 21-day sequential application intervals were observed for barnyardgrass or broadleaf signalgrass control, 3 WAF. Soybean yields were greater when the glufosinate applications occurred 7 or 10 d apart compared to 21 d, averaged over glufosinate rates. When large weeds are present in the field, these data suggest that glufosinate should be applied sequentially with a 7- to 14-day interval between applications. If sequential applications of glufosinate are used in combination with a comprehensive weed control management program, the value of the LibertyLink technology should be preserved by mitigating the risk of glufosinate resistance.

Antagonism in mixtures of glufosinate + glyphosate and glufosinate + clethodim on grasses

AbstractProper management of glufosinate in glufosinate-resistant crop technologies is needed to mitigate the likelihood of resistance evolution. Antagonism may result from mixtures of glufosinate and other commonly used POST herbicides in soybean and cotton. Two experiments were conducted at the Arkansas Agricultural Research and Extension Center in Fayetteville, AR, in 2015 and 2016 to evaluate mixtures of glufosinate + clethodim and glufosinate + glyphosate on barnyardgrass, broadleaf signalgrass, johnsongrass, and large crabgrass. Furthermore, droplet spectra analyses were conducted to determine if droplet size was associated with identification of herbicide interactions. Antagonism was dependent on the herbicide rates and the weed species. For barnyardgrass and large crabgrass control 4 wk after treatment, glufosinate + glyphosate was antagonistic at all rates evaluated. When large crabgrass was evaluated, some mixtures (e.g., 595 g ha–1 glufosinate + 76 g ha–1 clethodim) had a significant reduction in control relative to one of the herbicides applied alone. Glufosinate (451 and 595 g ai ha–1) + glyphosate (867 and 1,735 g ae ha–1) was antagonistic at all four possible rate combinations for broadleaf signalgrass control. Fewer instances of antagonism were observed for seedling johnsongrass control than for other species, but certain treatments were identified as antagonistic (e.g., glufosinate at 451 g ai ha–1 + clethodim at 76 g ai ha–1). Overall, antagonism was less likely and greater control was observed when the highest rates of both herbicides in a given mixture were used. The addition of glyphosate or clethodim to glufosinate can increase the volume median diameter and decrease the percentage volume of fines, compared to glufosinate alone. The droplet spectra analyses indicate that the glufosinate performance may be negatively affected by the addition of glyphosate or clethodim.

Evaluation of thiencarbazone‐methyl for use in acetolactate synthase‐resistant soybean

AbstractTo combat herbicide‐resistant weeds, new options are needed to effectively rotate herbicide modes of action and slow the development of additional herbicide resistance. Thiencarbazone‐methyl (TCM), an acetolactate synthase (ALS)‐inhibiting herbicide, is currently being evaluated for preemergence (PRE) and postemergence (POST) activity on many troublesome summer annual and perennial weeds. Multiple experiments were conducted to assess the utility of this product in soybean (Glycine max) production systems. Field experiments were conducted across Arkansas in 2016 and 2017 to determine the tolerance of ALS‐resistant soybean varieties to PRE and POST applications of TCM at either 0.03 or 0.06 lb acre–1. For all locations, no significant interactions or main effects were observed for any parameters evaluated. Overall, soybean injury remained consistently low, with <3% injury at 21 days after treatment (DAT) on all ALS‐resistant varieties. However, the ALS‐susceptible variety showed high levels of injury for both PRE (75%) and POST (98%) applications at 21 DAT. Two additional experiments were conducted in 2016, 2017, and 2018 to evaluate the efficacy of PRE and POST applications of TCM in ALS/glufosinate‐resistant soybean. For PRE applications, residual activity of TCM, imazethapyr, chlorimuron, nicosulfuron, halosulfuron, and flumetsulam at labeled rates for soybean or corn were evaluated. TCM at 0.06 lb acre–1was effective in controlling ivyleaf morningglory (91 %) and broadleaf signalgrass (97%) at 21 DAT, but was not effective in controlling ALS‐resistant Palmer amaranth (58%). In the POST experiment, TCM at 0.03 or 0.06 lb acre–1was compared to imazethapyr, chlorimuron, nicosulfuron, and halosulfuron with and without glufosinate (0.5 lb acre–1). Alone, TCM produced higher control of ivyleaf morningglory (99 to 100%) and broadleaf signalgrass (94 to 99%) at 21 DAT compared to all other ALS‐inhibiting herbicides. However, TCM alone was not effective in controlling ALS‐resistant Palmer amaranth (55 to 61%) at 21 DAT, but when combined with glufosinate control increased to 95 to 99%. The results from this research suggest that TCM could have utility in soybean weed control systems due to the high level of resistance exhibited by ALS‐resistant soybean varieties and the efficacy of TCM on common soybean weed species at both the PRE and POST application timings.

Evaluation of Quizalofop-Resistant Rice for Arkansas Rice Production Systems

Due to the ongoing evolution of herbicide-resistant weeds, new technologies are needed to maintain effective levels of control. A new rice variety that will be resistant to quizalofop, an acetyl coenzyme A carboxylase- (ACCase-) inhibiting herbicide, is currently under development. With the anticipated launch of this technology in 2018, multiple experiments were conducted to determine effectiveness of the quizalofop-resistant rice system for common grass weed species found in Arkansas rice production. One hundred and twenty-six barnyardgrass populations were collected across Arkansas and treated with quizalofop at 80 g ai ha−1 to determine a baseline of response. All populations evaluated were effectively controlled (≥92%) by quizalofop, with only 13 populations resulting in lower than 98% control. A greenhouse and field trial were conducted to compare efficacy of quizalofop to currently labeled rice graminicides for control of common rice grass weeds. Results from the greenhouse experiment showed that quizalofop treatments resulted in greater efficacy of common grass weeds compared to cyhalofop or fenoxaprop. This was especially apparent at the larger grass growth stages. A field experiment conducted compared season-long weed control programs of quizalofop to fenoxaprop and cyhalofop. The quizalofop-containing treatments were no better than fenoxaprop and cyhalofop for barnyardgrass and broadleaf signalgrass control. Barnyardgrass and broadleaf signalgrass control were greater than 96% for all herbicide treatments. An additional field experiment was conducted to determine the best rate structure for sequential applications of quizalofop in rice. Sequential applications of quizalofop at 120 g ha−1 followed by 120 g ha−1 two weeks later resulted in the highest barnyardgrass and broadleaf signalgrass control. Likewise, applying the full seasonal use rate of 240 g ha−1 of quizalofop resulted in greater control compared to 200 and 160 g ha−1. Results from this research indicate a strong benefit from quizalofop use in rice.

Weed Control and Peanut (Arachis hypogaea L.) Response to Acetochlor Alone and in Combination with Various Herbicides

ABSTRACT Acetochlor, a chloroacetamide herbicide, is now registered for preplant (PPI), preemergence (PRE), and postemergence (POST) application in peanut. Field research was conducted during 2011 and 2012 in Georgia and North Carolina to determine peanut response and weed control by acetochlor compared with S-metolachlor alone and in programs with other herbicides. In weed-free experiments, peanut tolerance to acetochlor (1.26 and 2.52 kg ai/ha) and S-metolachlor (1.42 kg ai/ha) were evaluated when applied PPI, PRE, early postemergence (EPOST), or POST. Peanut tolerance to acetochlor was similar to S-metolachlor with no negative impact of either herbicide on peanut yield compared with non-treated peanut in absence of weed interference. When applied PRE, acetochlor controlled Palmer amaranth, pitted morningglory, sicklepod, and Texas millet similarly to S-metolachlor while control of broadleaf signalgrass was greater with S-metolachlor. Weed control programs containing EPOST and/or POST applications of herbicides following PRE herbicides provided the best overall weed control but did not affect yellow nutsedge control regardless of whether acetochlor or S-metolachlor were applied. Herbicide programs including PRE, EPOST, and POST herbicides most often resulted in the greatest yields. There was no difference in peanut yield regardless of the presence of acetochlor or S-metolachlor in a comprehensive herbicide program.

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.

Efficacy and Dissipation of Pyroxasulfone and Three Chloroacetamides in a Tennessee Field Soil

Field studies were conducted in Knoxville, TN, over a 2-yr period (2007 and 2008) to determine the field dissipation rate and efficacy of pyroxasulfone, acetochlor, dimethenamid, ands-metolachlor to broadleaf signalgrass. Depending on rainfall patterns, pyroxasulfone at 209 g ai ha−1provided broadleaf signalgrass control of > 75%, which was equal to or superior to acetochlor at 1,740 g ai ha−1, dimethenamid at 1,500 g ai ha−1ands-metolachlor at 1,420 g ai ha−1. Pyroxasulfone provided residual control into the growing season and provides a tool for resistance management of later-emerging weeds. Herbicide dissipation was rapid in all soils (half-life usually < 20 d), although it was slower in a dry year. The order of herbicide dissipation and half-life in days in the 2 yr was acetochlor (3.5, 5 d) > dimethenamid (5, 9 d) >s-metolachlor (8.8, 27 d) > pyroxasulfone (8.2, > 71 d). There was poor correlation between observed weed control at 45 d after treatment and chemically determined herbicide concentrations at that same time, with ∼ 40% difference in 2007 and ∼ 50% difference in 2008.

Urea Ammonium Nitrate Effects on Bispyribac and Penoxsulam Efficacy

Experiments were conducted at the Lonoke Extension and Applied Research Center greenhouse at Lonoke, AR, to evaluate the effects of urea ammonium nitrate (UAN) on bispyribac and penoxsulam efficacy on barnyardgrass, hemp sesbania, and broadleaf signalgrass. Herbicide treatments included bispyribac at 17.9 or 35.8 g ai/ha or penoxsulam at 24.4 or 48.9 g ai/ha tank mixed with (1) no adjuvant, (2) a nonionic organosilicone (OSL) adjuvant at 0.125% v/v, (3) a methylated seed oil/organosilicone (MSO/OSL) adjuvant at 0.37 L/ha, (4) a proprietary blend of MSO/OSL/UAN at 2% v/v, (5) UAN at 2% v/v, (6) OSL at 0.125% plus UAN at 2% v/v, or (7) MSO/OSL at 0.37 L/ha plus UAN at 2% v/v. In addition to these adjuvants, penoxsulam was also applied with crop oil concentrate (COC) at 2.34 L/ha and with COC at 2.34 L/ha plus UAN at 2% v/v. The addition of UAN to either herbicide plus an adjuvant increased herbicide efficacy on barnyardgrass in the greenhouse, with 95 to 99% biomass reduction of three- to four-leaf barnyardgrass and 88 to 92% biomass reduction of one- to three-tiller barnyardgrass. UAN did not generally increase efficacy on hemp sesbania, as control was 90% or higher with treatments containing either herbicide and a recommended adjuvant. Adding UAN did not increase efficacy on broadleaf signalgrass. Broadleaf signalgrass control was highly variable and no treatment provided more than 65% biomass reduction.

Imazethapyr Use with and without Clomazone for Weed Control in Furrow-Irrigated, Imidazolinone-Tolerant Rice

Research on weed management in furrow-irrigated rice is needed as water availability becomes more limited in rice production regions of Arkansas. Research was conducted at Keiser and Pine Tree, AR, with the objectives being to determine (1) whether the addition of clomazone to imazethapyr would improve PRE weed control in furrow-irrigated, imidazolinone-tolerant rice, and (2) whether increasing the imazethapyr rates would improve weed control without injuring rice. Imazethapyr was applied at 70, 87.5, and 105 g ai/ha PRE with and without clomazone followed by imazethapyr POST at the same rate as used PRE. No rice injury was observed during the growing season at either site. Clomazone plus imazethapyr applied PRE did not improve early season control of Palmer amaranth, pitted morningglory, prickly sida, barnyardgrass, or broadleaf signalgrass over imazethapyr alone. Increasing the PRE imazethapyr rate to 105 g/ha did not improve Palmer amaranth or pitted morningglory control. Imazethapyr applied PRE on a clay soil generally provided lower weed control than on the silt loam soil. Increasing the imazethapyr rate did not improve rice yields.Nomenclature: Clomazone, imazethapyr, barnyardgrass, Echinochloa crus-galli (L.) Beauv. ECHCG, broadleaf signalgrass, Urochloa platyphylla (Nash) R.D. Webster BRAPP. Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA, pitted morningglory, Ipomoea lacunosa L. IPOLA, prickly sida, Sida spinosa L. SIDSP, rice, Oryza sativa L. ‘CL 730’ ORYSA

Compatibility of Diclosulam with Postemergence Herbicides and Fungicides

Diclosulam is registered for residual and postemergence control of several broadleaf weeds and suppression of annual sedges in peanut in the southeastern United States. Many producers apply herbicides and other pesticides simultaneously to increase the spectrum of pest control or to increase efficiency of operations. However, compatibility of coapplication of pesticides is a concern. Field trials were conducted to evaluate the compatibility of diclosulam with other herbicides and fungicides. Horseweed control by diclosulam in combination with glyphosate, sulfosate, or paraquat was compared to combinations of these herbicides with flumioxazin, tribenuron plus thifensulfuron, or 2,4-D. All treatments that contained diclosulam controlled horseweed at least 86%. Broadleaf signalgrass control by clethodim and sethoxydim was not affected by diclosulam; however, large crabgrass control was reduced when graminicides were coapplied with diclosulam. Common ragweed control was reduced when diclosulam was applied with chlorothalonil and pyraclostrobin but not by azoxystrobin or tebuconazole.

Rice Response to Clomazone as Influenced by Application Rate, Soil Type, and Planting Date

Clomazone is an effective herbicide widely used for PRE grass control in rice. However, use of clomazone on sandy textured soils of the western Texas rice belt can cause serious rice injury. Two field experiments at three locations were conducted in 2002 and 2003 to determine the optimum rate range that maximizes barnyardgrass and broadleaf signalgrass control and minimizes rice injury across a wide variety of soil textures and planting dates. At Beaumont (silty clay loam), Eagle Lake (fine sandy loam), and Ganado (fine sandy loam), TX, PRE application of 0.34 kg ai/ha clomazone applied to rice planted in March, April, or May optimized barnyardgrass and broadleaf signalgrass control and rice yield while minimizing rice injury. Data suggest that, although injury might occur, clomazone is safe to use in rice on sandy textured soils.

Rice (Oryza sativa) Response and Annual Grass Control with Graminicides

Field studies were conducted to evaluate rice injury and control of propanil-resistant and -susceptible (natural infestation) barnyardgrass, broadleaf signalgrass, and Amazon sprangletop with BAS 625, cyhalofop, and fenoxaprop plus the safener isoxadifen in rice. BAS 625 at 100 g ai/ha applied to two- to three-leaf rice resulted in 19 to 72% injury in three of four experiments. Fenoxaprop plus isoxadifen at 90 + 98 g ai/ha injured rice 11 to 31%, and cyhalofop at 280 g ai/ha consistently resulted in minimal rice injury. The most effective control (84 to 99%) of propanil-resistant and propanil-susceptible barnyardgrass across all experiments was achieved with sequential applications of the BAS 625 at 75 and 100 g ai/ha, cyhalofop at 210 and 280 g ai/ha, and fenoxaprop plus isoxadifen at 68 + 74 and 90 + 98 g ai/ha. When the graminicides were applied to four- to six-leaf rice (one tiller), propanil-resistant and propanil-susceptible barnyardgrass control was generally very poor. Fenoxaprop plus isoxadifen controlled broadleaf signalgrass 91 to 100%, even when applied once to four- to six-leaf rice. BAS 625 at 75 and 100 g ai/ha and cyhalofop at 210 and 280 g ai/ha applied sequentially provided consistent broadleaf signalgrass control (≥98%). Amazon sprangletop control was good (85 to 99%) with fenoxaprop plus isoxadifen at 45 + 49, 68 + 74, and 90 + 98 g ai/ha (applied in a single application or sequentially), BAS 625 at 100 g ai/ha applied to two- to three-leaf and four- to six-leaf rice or 50, 75, and 100 g ai/ha applied sequentially, and cyhalofop at 140, 210, and 280 g ai/ha applied to two- to three-leaf rice or sequentially.

Weed Control and Response to Herbicides during Tifton 85 Bermudagrass Establishment from Rhizomes

Field studies were conducted in 2001 to 2003 to determine the effects of herbicides and application timings during establishment of ‘Tifton 85’ hybrid bermudagrass (Cynodon dactylon Pers. × C. nlemfuensis Vanderyst). The experimental design was a split plot with three replications. Main plot consisted of three herbicide application timings of 1, 14, and 28 d after planting (DAP), and subplots were 16 herbicide treatments. Hormone‐like herbicides (picloram + 2,4‐D amine and 2,4‐D amine + dicamba) applied 1 DAP controlled large crabgrass [Digitaria sanguinalis (L.) Scop.] 70 to 82%, did not injure Tifton 85, and consistently aided in Tifton 85 establishment, resulting in 23 to 30% ground cover in 2001 and 2002 compared with 3 to 4% in the untreated control. Imazapic (0.02, 0.035, and 0.05 kg a.i. ha−1) applied 1 and 14 DAP injured Tifton 85, ranging from 4 to 48% injury across years, yet these plots consistently had greater ground cover compared with the control, ranging from 9 to 56%. Glyphosate (0.28 kg a.i. ha−1) applied 14 DAP injured Tifton 85 less than 9%, controlled large crabgrass 76%, broadleaf signalgrass (Urochloa platyphylla Munroe ex C. Wright) 91%, and had 38, 45, and 17% ground cover in 2001, 2002, and 2003, respectively. Trifloxysulfuron (0.02 kg ha−1) applied 1 and 14 DAP controlled broadleaf signalgrass (>90%) and yellow nutsedge (Cyperus esculentus L.) (>90%), did not injure Tifton 85, and resulted in 41 to 67% ground cover in 2002 and 2003. These data illustrate that Tifton 85 establishment was improved when weeds were controlled.

Comparison of Glyphosate Salts (Isopropylamine, Diammonium, and Potassium) and Calcium and Magnesium Concentrations on the Control of Various Weeds

Greenhouse and field experiments were conducted near Knoxville, TN, during 2002 and 2003 to investigate the effects of calcium and magnesium ions on the performance of three glyphosate formulations with and without diammonium sulfate (AMS). Weed species investigated in the greenhouse were broadleaf signalgrass, pitted morningglory, Palmer amaranth, and yellow nutsedge. Three glyphosate formulations (isopropylamine salt, diammonium salt, and potassium salt) and two glyphosate application rates (0.42 and 0.84 kg ae/ha) were applied to weeds in water fortified with either calcium or magnesium at concentrations of 0, 250, 500, 750, and 1,000 ppm. In all comparisons, there were no differences in the three glyphosate formulations. Glyphosate activity was reduced only when cation concentration was >250 ppm, and this antagonism was not observed when 2% w/ w AMS was added to the spray solution. A chemical analysis of the calcium and magnesium concentrations in water collected from farmers indicated that water samples from eight different producers contained relatively low amounts of cations, with calcium at <40 ppm and magnesium at <8 ppm. In the field results using these and other waters as the herbicide carrier, broadleaf signalgrass control was greater with the 0.84 kg ae/ha than 0.42 kg ae/ha glyphosate rate regardless of water source or addition of AMS. Pitted morningglory responded similarly to glyphosate with water from all farms and with AMS added, and the addition of AMS gave similar results for both glyphosate rates. In 2003, common cocklebur was evaluated and control was >93% regardless of glyphosate rate, water source, or AMS addition. Based on these results, the addition of AMS-based adjuvants to many glyphosate applications may not be warranted.

Weed Management withS-Metolachlor and Glyphosate Mixtures in Glyphosate-Resistant Strip- and Conventional-Tillage Cotton (Gossypium hirsutumL.)

Five studies were conducted at Clayton, Rocky Mount, and Lewiston-Woodville, NC, in 2001 and 2002, to evaluate weed management, crop tolerance, and yield in strip- and conventional-tillage glyphosate-resistant cotton. Cotton was treated with two glyphosate formulations; glyphosate-IP (isopropylamine salt) or glyphosate-TM (trimethylsulfonium salt), early postemergence (EPOST) alone or in a mixture withS-metolachlor. Early season cotton injury was minimal (3%) with either glyphosate formulation alone or in mixture withS-metolachlor. Weed control and cotton yields were similar for both glyphosate formulations. The addition ofS-metolachlor to either glyphosate formulation increased control of broadleaf signalgrass, goosegrass, large crabgrass, and yellow foxtail 14 to 43 percentage points compared with control by glyphosate alone.S-metolachlor was not beneficial for late-season control of entireleaf morningglory, jimsonweed, pitted morningglory, or yellow nutsedge. The addition ofS-metolachlor to either glyphosate formulation increased control of common lambsquarters, common ragweed, Palmer amaranth, smooth pigweed, and velvetleaf 6 to 46 percentage points. The addition of a late postemergence-directed (LAYBY) treatment of prometryn plus MSMA increased control to greater than 95% for all weed species regardless of EPOST treatment, and control was similar with or withoutS-metolachlor EPOST. Cotton lint yield was increased 220 kg/ha with the addition ofS-metolachlor to either glyphosate formulation compared with yield from glyphosate alone. The addition of the LAYBY treatment increased yields 250 and 380 kg/ha for glyphosate plusS-metolachlor and glyphosate systems, respectively.S-metolachlor residual activity allowed for an extended window for more effective LAYBY application to smaller weed seedlings instead of weeds that were possibly larger and harder to control.

Weed Control in Corn (Zea mays) with an Imazethapyr Plus Imazapyr Prepackaged Mixture1

Pendimethalin plus atrazine (1,120 2,240 g ai/ha) or metolachlor plus atrazine (2,170 1,790 g ai/ha) applied PRE over 2 yr controlled broadleaf signalgrass and itchgrass 78 to 88% 30 d after treatment (DAT). The imazethapyr plus imazapyr prepackaged mixture (PREMIX) applied PRE at 47 16 g ai/ha controlled these weeds no more than 35%. With pendimethalin, metolachlor plus atrazine, or atrazine PRE followed by (fb) imazethapyr plus imazapyr POST, broadleaf signalgrass was controlled 89 to 98% and itchgrass 76 to 83% 28 d after POST application. When only imazethapyr plus imazapyr was applied POST, broadleaf signalgrass control was 88% the first year, but only 64% the second year. Itchgrass was controlled 71% when imazethapyr plus imazapyr was applied alone POST, but control was nearly 86% when pendimethalin or nicosulfuron was applied POST with imazethapyr plus imazapyr. Consistently high corn yields were obtained when pendimethalin was applied POST with imazethapyr plus imazapyr, and these corn yields were greater than those following POST-only applications of imazethapyr plus imazapyr in 1 of 2 yr. When imazethapyr plus imazapyr was applied alone POST, corn yield was at least 1.3 times that when imazethapyr plus imazapyr was applied alone PRE. Weed control with imazethapyr plus imazapyr POST at 47 16 g/ha was not improved when ammonium sulfate was added with nonionionic surfactant or when methylated seed oil with or without ammonium sulfate or organosilicone adjuvants were used in the spray solution compared with using only nonionic surfactant in the spray solution. When imazethapyr plus imazapyr plus the various adjuvants was applied to weeds 5 to 8 cm tall, broadleaf signalgrass was controlled 93%, itchgrass 74 and 88%, and pitted morningglory 87 and 93% 28 DAT over 2 yr.Nomenclature: Atrazine; imazapyr; imazethapyr; metolachlor; nicosulfuron; pendimethalin; broadleaf signalgrass, Brachiaria platyphylla (Griesb.) Nash #3 BRAPP; itchgrass, Rottboellia cochinchinensis (Lour.) W. D. Clayton # ROOEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; corn, Zea mays L. ‘Pioneer 3395 IR.’Additional index words: Adjuvants, application timing, herbicide combinations, imidazolinone-resistant corn, tank mixture.Abbreviations: DAT, days after treatment; EPOST, early postemergence; fb, followed by; IR, imidazolinone-resistant; LPOST, late postemergence; PREMIX, prepackaged mixture.