Broadleaf Signalgrass Research Articles

Differential cysteine synthase activity and alachlor susceptibility in five crops and six weed species

Cysteine synthase (CS; EC 4.2.99.8) activities were compared among crops and grass weeds having different susceptibilities to alachlor. The order of alachlor tolerance of 11 grass species from most to least was: corn ≫ barley=rice=wheat > sorghum=johnsongrass=broadleaf signalgrass > barnyardgrass=large crabgrass > green foxtail=goosegrass, suggesting that corn was tolerant and all other species were susceptible to alachlor. In shoots, rice had the highest extractable CS activity ( 44.3 μmol/min/g fresh weight), and corn, barley, wheat, sorghum, johnsongrass, and broadleaf signalgrass had the second highest activity ranging between 20.6 and 26.4 μmol/min/g fresh weight. CS activity of large crabgrass, green foxtail, and goosegrass was relatively lower (15.5– 18.1 μmol/min/g fresh weight), and barnyardgrass had the lowest CS activity ( 10.9 μmol/min/g fresh weight). Generally, levels of CS activities in roots were similar to those in shoots in each grass species. However, barley had root CS activity lower than that in shoots and green foxtail had root CS activity higher than that in shoots. Alachlor treatment (GR 50 concentrations) had no effect on extractable CS activity in shoots of corn, barley, sorghum, and barnyardgrass. Content of non-protein cysteine in shoots ranged between 60.1 and 131.5 nmol/g fresh weight, and was not largely different among the grass species except for green foxtail where it was high, i.e., 334.6 nmol/g fresh weight. Content of other non-protein thiols was highest in corn (574.7 nmol/g fresh weight), which was tolerant to alachlor, and the contents were relatively low in other grass species which were susceptible. Alachlor susceptibility was more highly correlated to CS activity ( R 2=0.514) in shoots than to the contents of cysteine ( R 2=0.309) and to other thiols ( R 2=0.025) in grass species susceptible to alachlor. In addition, CS activity in roots ( R 2=0.159) was less correlated to the selectivity than shoots. These results indicate that CS activity in shoots could contribute to alachlor selectivity in grass species.

Weed Management with Diclosulam in Strip-Tillage Peanut (Arachis hypogaea)1

Experiments were conducted at three locations in North Carolina in 1999 and 2000 to evaluate weed management systems in strip-tillage peanut. Diclosulam was evaluated with standard preemergence (PRE), early postemergence, and postemergence (POST) herbicide systems in a factorial treatment arrangement. Preemergence treatments that contained diclosulam controlled common lambsquarters, common ragweed, and eclipta by 100%. Diclosulam PRE controlled entireleaf morningglory by 88%, ivyleaf morningglory by ≥ 90%, pitted morningglory by ≥ 81%, and prickly sida by ≥ 94%. Yellow nutsedge control with diclosulam ranged from 65 to 100% depending on location, whereas POST systems containing imazapic controlled yellow nutsedge by at least 89%, regardless of PRE herbicides. Peanut yields and net returns were reflective of levels of weed management. Systems that included diclosulam PRE plus POST herbicides consistently provided high yields and net returns. Clethodim late POST was required for full-season control of annual grasses, including broadleaf signalgrass, goosegrass, large crabgrass, and Texas panicum.Nomenclature: Clethodim; diclosulam; imazapic; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash #3 BRAPP; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; eclipta, Eclipta prostrata L. # ECLAL; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHG; goosegrass, Eleusine indica (L.) Gaertn. # ELEIN; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP; Texas panicum, Panicum texanum Buckl. # PANTE; yellow nutsedge, Cyperus esculentus L. # CYPES; peanut, Arachis hypogaea L. ‘NC 10C’ and ‘NC 12C’.Additional index words: Economic analysis.Abbreviations: EPOST, early postemergence; fb, followed by; POST, postemergence; PPI, preplant incorporated; PRE, preemergence.

Weed Control from Herbicide Combinations with Glyphosate1

Greenhouse studies were initiated to evaluate glyphosate alone and tank-mixed with acifluorfen, CGA 277476, chlorimuron, cloransulam-methyl, fomesafen, imazaquin, or pyrithiobac on seedling johnsongrass, broadleaf signalgrass, pitted morningglory, and hemp sesbania. Johnsongrass and broadleaf signalgrass control by glyphosate was not affected by the selective herbicides applied in mixtures. Pitted morningglory control by glyphosate ranged from 0% with 280 g ai/ha to 67% with 840 g/ha. There was an additive effect when selective herbicides were added to 280 g/ha glyphosate 2 wk after treatment (WAT). When acifluorfen was added to 560 g/ha glyphosate, pitted morningglory control 2 WAT increased to 100% compared with 55% with glyphosate alone. Similarly, the addition of fomesafen or acifluorfen to 840 g/ha glyphosate controlled pitted morningglory 2 WAT by 90 and 98%, respectively, compared with 67% with glyphosate alone. Only tank mixtures of acifluorfen, CGA 277476, or fomesafen, and 840 g/ha glyphosate re...

Effect of Venturi-Type Nozzles and Application Volume on Postemergence Herbicide Efficacy1

Field studies were conducted to compare venturi-type nozzles to a fan nozzle with respect to the efficacy of postemergence herbicides applied to common cocklebur and broadleaf signalgrass. Spray solutions of glufosinate, glyphosate, and paraquat were applied through all combinations of three nozzles and two application volumes. Venturi nozzles were a Delavan Raindrop Ultra (RU) and a Spraying Systems AI Teejet (AI). A Spraying Systems XR Teejet (XR) fan nozzle was included as a standard. Previous work indicated droplet size spectra differed among these nozzles. There was a difference in common cocklebur control among nozzles (AI = XR > RU), although control was at least 90% for all nozzles. Herbicide choice had a greater effect on broadleaf signalgrass control than nozzle type. Broadleaf signalgrass control differed among herbicides (glufosinate = paraquat > glyphosate) and among nozzles (AI = XR > RU). Herbicide performance varied between nozzles (AI > RU), but the AI nozzle was as effective as the XR fan nozzle.Nomenclature: Glufosinate; glyphosate; paraquat; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash #3 BRAPP; common cocklebur, Xanthium strumarium (L.) # XANST.Additional index words: Drift.Abbreviations: AI, Spraying Systems AI Teejet; RU, Delavan Raindrop Ultra; VMD, volume median diameter; WAT, weeks after treatment; XR, Spraying Systems XR Teejet.

Weed Management in Transgenic Soybean Resistant to Glyphosate Under Conventional Tillage and No-Tillage Systems

A 3-year field research was conducted in 1997, 1998, and 1999 at Stoneville, MS on a Dundee silt loam soil to study efficacy and economics of glyphosate with or without preemergence (PRE) herbicides in glyphosate-resistant soybean under conventional tillage (CT) and no-tillage (NT) systems. Single or sequential glyphosate postemergence (POST) applications provided at least 91% control of Echinochloa crus-galli (L.) Beauv. (barnyardgrass), Brachiaria platyphylla (Griseb.) Nash (broadleaf signalgrass), Brachiaria ramosa (L.) Stapf (browntop millet), Sesbania exaltata (Raf.) Rydb. ex A.W. Hill (hemp sesbania), Sorghum halepense (L.) Pers. (johnsongrass), Senna obtusifolia (L.) Irwin and Barneby (sicklepod), Ipomoea lacunosa L. (pitted morningglory), Sida spinosa L. (prickly sida), and Cyperus esculentus L. (yellow nutsedge), and POST-only programs were as effective as PRE followed by POST programs. Two POST applications of glyphosate produced highest soybean yield (2,710 kg/ha) and net return ($280/ha). Single POST application of glyphosate resulted in greater net return ($226/ha) compared to the conventional standard (pendimethalin, imazaquin, acifluorfen, bentazon, and clethodim) herbicide program ($58/ha), although both had similar yields. Addition of PRE herbicides tend to increase herbicide costs and decrease net returns compared to two POST applications of glyphosate. Soybean yield and net return were lower in NT compared to CT systems, due to a lower soybean stand establishment in NT systems. These results suggest that when yields are comparable among various herbicide programs, net return is dictated by the cost of the herbicide program, including seeds and associated technology fee.

Weed Management in Glufosinate- and Glyphosate-Resistant Soybean (Glycine max)1

An experiment was conducted at six locations in North Carolina to compare weed-management treatments using glufosinate postemergence (POST) in glufosinate-resistant soybean, glyphosate POST in glyphosate-resistant soybean, and imazaquin plus SAN 582 preemergence (PRE) followed by chlorimuron POST in nontransgenic soybean. Prickly sida and sicklepod were controlled similarly and 84 to 100% by glufosinate and glyphosate. Glyphosate controlled broadleaf signalgrass, fall panicum, goosegrass, rhizomatous johnsongrass, common lambsquarters, and smooth pigweed at least 90%. Control of these weeds by glyphosate often was greater than control by glufosinate. Mixing fomesafen with glufosinate increased control of these species except johnsongrass. Glufosinate often was more effective than glyphosate on entireleaf and tall morningglories. Fomesafen mixed with glyphosate increased morningglory control but reduced smooth pigweed control. Glufosinate or glyphosate applied sequentially or early postemergence (EPOST) following imazaquin plus SAN 582 PRE often were more effective than glufosinate or glyphosate applied only EPOST. Only rhizomatous johnsongrass was controlled more effectively by glufosinate or glyphosate treatments than by imazaquin plus SAN 582 PRE followed by chlorimuron POST. Yields and net returns with soil-applied herbicides only were often lower than total POST herbicide treatments. Sequential POST herbicide applications or soil-applied herbicides followed by POST herbicides were usually more effective economically than single POST herbicide applications.Nomenclature: Chlorimuron, ethyl 2-[[[[(4-chloro-6-methoxy-2-pyrimidinyl)amino]carbonyl] amino]sulfonyl]benzoate; SAN 582 (proposed name, dimethenamid), 2-chloro-N-[(1-methyl-2-methoxy)ethyl]-N-(2,4-dimethyl-thien-3-yl)-acetamide; fomesafen, 5-[2-chloro-4-(trifluoromethyl)phenoxy]-N-(methylsulfonyl)-2-nitrobenzamide; glufosinate, 2-amino-4-(hydroxymethylphosphinyl) butanoic acid; glyphosate, N-(phosphonomethyl)glycine; imazaquin, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash #2 BRAPP; carpetweed, Mollugo verticillata L. # MOLVE; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; cutleaf groundcherry, Physalis angulata L. # PHYAN; eclipta, Eclipta prostrata L. # ECLAL; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray # IPOHG; fall panicum, Panicum dichotomiflorum Michx. # PANDI; goosegrass, Eleusine indica (L.) Gaertn. # ELEIN; johnsongrass, Sorghum halepense (L.) Pers. # SORHA; prickly sida, Sida spinosa L. # SIDSP; sicklepod, Senna obtusifolia L. Irwin and Barneby # CASOB; smooth pigweed, Amaranthus hybridus L. # AMACH; tall morningglory, Ipomoea purpurea (L.) Roth # PHBPU; soybean, Glycine max (L.) Merr. ‘Asgrow 5403 LL’, ‘Asgrow 5547 LL’, ‘Asgrow 5602 RR’, ‘Hartz 5566 RR’, ‘Southern States FFR 595’.Additional index words: Herbicide-resistant crops, Liberty Link soybean, nontransgenic soybean, Roundup Ready soybean.Abbreviations: DAT, days after treatment; EPOST, early postemergence; EPSPS, 5-enolpyruvylshikimate-3-phosphate synthase; LPOST, late postemergence; POST, postemergence; PRE, preemergence; THR, transgenic, herbicide-resistant; WAA, weeks after late postemergence application; WAP, weeks after planting.

Weed Management in Ultra Narrow Row Cotton (Gossypium hirsutum)1

Abstract: New weed management tools and growth regulators make production of ultra narrow row (UNR) cotton possible. Weed control, cotton yield, fiber quality, and net returns were compared in UNR bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton. Weeds included broadleaf signalgrass, carpetweed, common cocklebur, common lambsquarters, common ragweed, goosegrass, jimsonweed, large crabgrass, Palmer amaranth, pitted morningglory, prickly sida, sicklepod, smooth pigweed, and tall morningglory. Pendimethalin preplant incorporated (PPI) in conventional-tillage or preemergence (PRE) in no-till systems plus fluometuron PRE did not adequately control many of these weeds. Pyrithiobac plus MSMA early postemergence (POST) often was more effective than pyrithiobac alone. Pendimethalin plus fluometuron at planting followed by pyrithiobac plus MSMA early POST controlled sicklepod 82%, goosegrass 89%, Palmer amaranth 92%, and the other species at least 95% late season. Pyrithiobac at mid-POST did not...

Influence of Adjuvants on Interactions of Sethoxydim with Selected Broadleaf Herbicides Used in Corn (Zea mays)

Field experiments were conducted during 1996 and 1998 to determine the effect of atrazine, bentazon, atrazine + bentazon, the amine salt of 2,4-D, and bromoxynil on broadleaf signalgrass and barnyardgrass control when applied in mixture with sethoxydim. Herbicide combinations were applied with crop oil concentrate, crop oil concentrate + ammonium sulfate, or BCH 815. Bentazon and atrazine + bentazon reduced broadleaf signalgrass and barnyardgrass control by sethoxydim. Bromoxynil reduced barnyardgrass control but had no affect on broadleaf signalgrass control. Including ammonium sulfate with crop oil concentrate or substituting BCH 815 for crop oil concentrate increased barnyardgrass and broadleaf signalgrass control by sethoxydim when applied with bentazon. Ammonium sulfate and BCH 815 increased barnyardgrass control when sethoxydim was applied with bromoxynil but did not affect control when sethoxydim was applied with atrazine + bentazon. Atrazine and the ammonium salt of 2,4-D did not reduce barnyardgrass and broadleaf signalgrass control by sethoxydim regardless of adjuvant.

Weed Management in Glufosinate-Resistant Corn (Zea mays)

An experiment was conducted at five locations in North Carolina to compare management systems utilizing glufosinate applied postemergence (POST) in glufosinate-resistant corn with standard systems of metolachlor plus atrazine preemergence (PRE) or nicosulfuron plus atrazine POST Glufosinate alone and both standard systems controlled common ragweed and prickly sida at least 98%, whereas sicklepod control was < 20% late in the season. Late-season control of common lambsquarters, smooth pigweed, pitted morningglory, and tall morningglory was generally less with glufosinate alone than with the standard systems. However, late-season control of common lambsquarters, smooth pigweed, pitted morningglory, tall morningglory, and sicklepod by mixtures of glufosinate plus atrazine was at least 99, 100, 89, 93, and 81%, respectively, and was equal to or greater than control by either standard. Broadleaf signalgrass, large crabgrass, and fall panicum were controlled similarly by glufosinate and the standards. Goosegrass control by glufosinate was similar to control by nicosulfuron plus atrazine, but it was less than control by metolachlor plus atrazine. Metolachlor applied PRE or atrazine mixed with glufosinate increased goosegrass control to that achieved with metolachlor plus atrazine. Mixing atrazine with glufosinate did not affect fall panicum control, but metolachlor PRE followed by glufosinate controlled fall panicum as well as the standards. Cultivation or ametryn applied at layby increased control when PRE or POST herbicides alone controlled weeds less than about 90%. Ametryn was generally more effective than cultivation. Glufosinate POST followed by ametryn at layby controlled sicklepod > 90% and other species > 95% late in the season. Corn yield and net returns were similar in the glufosinate and standard systems.

Weed Management and Net Returns With Transgenic, Herbicide-Resistant, and Nontransgenic Cotton (Gossypium hirsutum)

Weed management systems were compared in bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton. A standard system of pendimethalin preplant incorporated (PPI), fluometuron preemergence (PRE), fluometuron plus MSMA early postemergence-directed (POST-DIR), and cyanazine plus MSMA late POST-DIR in combination with cultivation controlled broadleaf signalgrass, large crabgrass, common lambsquarters, jimsonweed, pitted morningglory, prickly sida, sicklepod, and smooth pigweed 98 to 100% late season. Weed control, cotton yield, and net returns were similar when pyrithiobac or bromoxynil plus MSMA postemergence (POST) replaced fluometuron plus MSMA POST-DIR. Fluometuron PRE had little to no effect in bromoxynil systems. Glyphosate POST to three- to four-leaf-stage cotton followed by cyanazine plus MSMA late POST-DIR and cultivation controlled weeds 96 to 100%. Glyphosate POST followed by glyphosate POST-DIR and cultivation controlled pitted morningglory and large crabgrass 89 to 90% and other species at least 94%. Yields and net returns at one location were similar for glyphosate applied twice or glyphosate POST followed by cyanazine plus MSMA POST-DIR and the standard system. Pendimethalin plus fluometuron in glyphosate systems did not increase yield or net returns. At a location severely infested with large crabgrass, pendimethalin plus fluometuron in glyphosate systems increased yield 37 to 44% and net returns 85 to 108%, respectively, when glyphosate was applied to cotton at the three-to four-leaf stage, but not if glyphosate was applied to cotton at the one-leaf stage. Yield and net returns were similar when bromoxynil-resistant, glyphosate-resistant, and nontransgenic cotton were treated using the standard system.

Influence of CGA-277476 on Efficacy of Postemergence Graminicides

Field experiments were conducted at three Mississippi locations to evaluate potential antagonism when postemergence graminicides were tank-mixed with CGA-277476 and to determine if the antagonism could be overcome with an increase in graminicide rate. At all locations when the 1X rate of clethodim, fluazifop-P, or quizalofop was tank-mixed with 79 g ai/ ha CGA-277476, johnsongrass, broadleaf signalgrass, and barnyardgrass control was reduced 5 to 30%. When the graminicide rates were increased to 1.5X, antagonism was still present for fluazifop-P and quizalofop. However, 156 g ai/ha clethodim overcame the antagonism when tank-mixed with 79 g/ha CGA-277476 compared to 105 g/ha clethodim. CGA-277476 alone controlled barnyardgrass and broadleaf signalgrass 38 to 63% and johnsongrass 60 to 85%. There were no differences in soybean yields with graminicides applied alone or tank-mixed with CGA-277476.

Absorption, translocation, and metabolism of primisulfuron and nicosulfuron in broadleaf signalgrass (Brachiaria platyphylla) and corn

Broadleaf signalgrass is sensitive to nicosulfuron and resistant to primisulfuron, but corn is resistant to both. Research was conducted to determine the effect of varying light level and air temperature on absorption, translocation, and metabolism of nicosulfuron and primisulfuron in broadleaf signalgrass and corn. Corn absorbed between 60 and 85% of the applied nicosulfuron and primisulfuron within 72 h after treatment (HAT), depending on environmental treatment. Absorption, translocation, and metabolism all tended to be more rapid at higher temperature and light intensity. Nicosulfuron and primisulfuron translocation out of the treated leaf was < 4.5% of herbicide absorbed through 72 HAT. Corn rapidly metabolized both herbicides in both environments. However, primisulfuron was metabolized more rapidly (high = 99%, low = 92%) than nicosulfuron (high = 95%, low = 78%). Broadleaf signalgrass absorbed 20% more nicosulfuron than primisulfuron through 72 HAT. Nicosulfuron translocation out of the treated leaf in broadleaf signalgrass was ≤ 15% absorbed through 72 HAT, while primisulfuron translocation was ≤ 4% during the same time period. Primisulfuron metabolism was more rapid than nicosulfuron in broadleaf signalgrass. During the first 4 HAT, broadleaf signalgrass metabolized > 20 times more primisulfuron than nicosulfuron. By 72 HAT, broadleaf signalgrass under conditions of high light and temperature had metabolized nearly 90% of the primisulfuron absorbed but ≤ 7% of the nicosulfuron absorbed was metabolized during the same time. These results suggest that differential activity of nicosulfuron and primisulfuron on broadleaf signalgrass may be based on differential rates of metabolism to nonphytotoxic compounds; uptake and translocation differences agree with the differential broadleaf signalgrass activity. Additionally, environment has the potential to affect rates of sulfonylurea absorption, translocation, and metabolism.

Quinclorac belongs to a new class of highly selective auxin herbicides

Substituted quinolinecarboxylic acids, including quinclorac (BAS 514H), are a new class of highly selective auxin herbicides, which are chemically similar to naturally occurring compounds isolated from plants and soils. Quinclorac is used in rice to control important dicot and monocot weeds, particularly barnyardgrass. The herbicide has also been developed for application in turfgrass areas, spring wheat, and chemical fallow. Quinclorac is readily absorbed by germinating seeds, roots, and leaves and is translocated in the plant both acropetally and basipetally. By mimicking an auxin overdose, quinclorac affects the phytohormonal system in sensitive plants. The compound stimulates the induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and thus promotes ethylene biosynthesis. In susceptible dicots, increased levels of ethylene trigger an accumulation of abscisic acid (ABA), which, as part of the intrinsic auxin activity of quinclorac, plays a major role in growth inhibition and the induction of epinasty and senescence. In sensitive grasses, such as barnyardgrass species, large crabgrass, broadleaf signalgrass, and green foxtail, quinclorac leads particularly to an accumulation of tissue cyanide, formed as a co-product during increased ACC and ethylene synthesis. This causes phytotoxicity characterized by the inhibition of root and particularly shoot growth with tissue chlorosis and subsequent necrosis. These effects were not observed in tolerant rice and a resistant biotype of barnyardgrass. No significant differences in uptake, translocation, or metabolism of quinclorac between resistant and sensitive grasses were found. Hence, a target-site-based mechanism of selectivity is suggested. The induction process of the ACC synthase activity plays the primary role in the selective herbicide action of quinclorac. This is a common effect of auxin herbicides and auxins, which lead to the accumulation of cyanide and/or ABA depending on the plant species and tissues, the compound concentration in the tissue, and their biological activity.

Spray Adjuvant, Formulation, and Environmental Effects on Synergism from Post-Applied Tank Mixtures of SAN 582H with Fluazifop-P, Imazethapyr, and Sethoxydim

The effect of adding a spray adjuvant to SAN 582H tank mixtures with fluazifop-P, imazethapyr, and sethoxydim was evaluated. SAN 582H synergistically increased broadleaf signalgrass control with reduced rates of all three postemergence (POST) herbicides when no spray adjuvant was used and when crop oil concentrate was added. For example, broadleaf signalgrass control increased from 50% to 83% when SAN 582H was tank-mixed with 52 g ai/ha sethoxydim and crop oil concentrate. In another experiment, several formulations of SAN 582H, including blank solvent-only formulations (no SAN 582H), were evaluated in combination with a reduced rate of sethoxydim to determine the source of synergism from tank mixtures. The SAN 582H molecule, not the carrier solvents in formulated product, was determined to be the source of synergism. The synergistic properties of SAN 582H were compared to other chloroacetamides. Synergism from acetochlor was similar to SAN 582H when applied POST with a reduced rate of either fluazifop-P, imazethapyr, or sethoxydim for grass control. Metolachlor also synergistically increased the control of grasses with the POST herbicides; however, metolachlor caused considerable phytotoxicity when applied alone and synergistic interactions were detected less frequently. The efficacy of sethoxydim mixed with SAN 582H was evaluated under different soil moisture conditions. Broadleaf signalgrass control increased from 81% to 93% under dry, moisture-stressed conditions when 210 g/ha sethoxydim was tank- mixed with SAN 582H.

Synergism of Grass Weed Control with Postemergence Combinations of SAN 582 and Fluazifop-P, Imazethapyr, or Sethoxydim

Greenhouse and field experiments were conducted to evaluate early postemergence (POST) tank mixtures of SAN 582 with fluazifop-P, imazethapyr, or sethoxydim. In the greenhouse, SAN 582 synergistically improved barnyardgrass, broadleaf signalgrass, and johnsongrass control from imazethapyr and sethoxydim. Half-rates of imazethapyr and sethoxydim tank-mixed with SAN 582 controlled grass weeds as well as full rates of either herbicide applied alone. Grass weed control with imazethapyr increased up to 40% with the addition of SAN 582. In field experiments, SAN 582 increased grass control with imazethapyr to a lesser degree than observed in the greenhouse. In a multispecies study, grass weed control increased up to 15% when SAN 582 was tank-mixed with a reduced rate of imazethapyr, although the full rate of imazethapyr applied POST with or without SAN 582 controlled grass weeds 80% or less. The combination of SAN 582 with sethoxydim was synergistic for barnyardgrass and johnsongrass control in this experiment. When applied POST in soybean, SAN 582 plus fluazifop-P or sethoxydim controlled barnyardgrass throughout the season better than a single POST application of a graminicide.

Annual Grass Control by Glyphosate plus Bentazon, Chlorimuron, Fomesafen, or Imazethapyr Mixtures

The isopropylamine salt of glyphosate at 420, 560, and 840 g ae/ha applied alone or mixed with the sodium salt of bentazon at 840 g ai/ha, chlorimuron at 9 g ai/ha, the sodium salt of fomesafen at 350 g ai/ha, or the ammonium salt of imazethapyr at 70 g ae/ha was evaluated for control of large crabgrass and broadleaf signalgrass. Neither grass was controlled by bentazon, fomesafen, or chlorimuron. Imazethapyr controlled large crabgrass and broadleaf signalgrass 30 and 72%, respectively, 3 weeks after treatment (WAT). Glyphosate at all rates controlled both grasses 100%. Control 3 WAT was unaffected by mixing bentazon, chlorimuron, fomesafen, or imazethapyr with glyphosate. Broadleaf signalgrass control 1 WAT was reduced 4 to 15% by mixing bentazon with glyphosate.

Glyphosate Tank Mixtures with SAN 582 for Burndown or Postemergence Applications in Glyphosate-Tolerant Soybean (Glycine max)

Field experiments were conducted to evaluate postemergence (POST)-applied tank mixtures of 560, 1,120, and 1,680 g ai/ha glyphosate with or without 1,120 g ai/ha SAN 582 (proposed name, dimethenamid) as burndown treatments or POST in glyphosate-tolerant soybean. SAN 582 was not antagonistic with glyphosate at the glyphosate rates evaluated. In the burndown study, glyphosate controlled horseweed 98% or more and curly dock 82% or more with or without SAN 582. However, broadleaf signalgrass emerged after the burndown treatments were applied. All tank mixtures that included SAN 582 controlled broadleaf signalgrass 84 to 96%, 6 wk after treatment. In the glyphosate-tolerant soybean study, glyphosate controlled barnyardgrass and johnsongrass present at the time of application 89% or more, regardless of rate. Tank mixtures of SAN 582 with glyphosate controlled late-season flushes of barnyardgrass through residual activity of the SAN 582. Applying SAN 582 with glyphosate improved soybean yield 500 kg/ha over glyphosate applied alone.

Effect of Tillage and Soil-Applied Herbicides on Broadleaf Signalgrass (Brachiaria platyphylla) Control in Corn (Zea mays)

Broadleaf signalgrass control from preemergence (PRE) herbicides was usually lower in no-till than in tilled plots. Broadleaf signalgrass control was most nearly complete in tilled plots treated with metolachlor in 1995, a year that favored an herbicide with more soil persistence. Broadleaf signalgrass control was most nearly complete in tilled plots treated with acetochlor in 1996, a year in which rainfall to activate the herbicides did not occur until 9 d after planting. The 1996 data indicated that acetochlor was more stable on the soil surface under the drier conditions. There was no difference in broadleaf signalgrass control between the two acetochlor formulations. Alachlor, metolachlor, and dimethenamid controlled broadleaf signalgrass > 80% for about 4 wk, acetochlor provided control for about 4 wk under no-till conditions and about 8 wk in tilled plots, and pendimethalin provided about 2 wk broadleaf signalgrass control. Acetochlor provided consistent control regardless of the rainfall pattern after application.

Weed Management in No-Till Cotton (Gossypium hirsutum) with Thiazopyr

Thiazopyr at 0.14, 0.28, and 0.42 kg ai/ha and pendimethalin at 1.1 kg ai/ha applied preemergence (PRE) were compared as components in weed management systems for no-till cotton. Mid- and late-season control of mixtures of large crabgrass, goosegrass, and fall panicum by thiazopyr at 0.28 kg/ha was 89 to 97% and 11 to 50%, respectively, compared with 11 to 38% midseason and 0 to 5% late-season control by pendimethalin. Thiazopyr at 0.42 kg/ha and pendimethalin controlled broadleaf signalgrass 44 and 0%, respectively, late in the season. Adding fluometuron PRE at 1.7 kg ai/ha had little to no effect on large crabgrass, goosegrass, and fall panicum control but increased broadleaf signalgrass control 47 to 79 percentage points compared with thiazopyr or pendimethalin alone. Late-season control of annual grasses by thiazopyr or pendimethalin plus fluometuron PRE followed by methazole plus MSMA early postemergence (POST)-directed and cyanazine plus MSMA late POST-directed was at least 95% at two locations and 80% at the third location. Common lambsquarters was controlled 54 and 95% in systems without and with fluometuron PRE, respectively. Acceptable control of ivyleaf, pitted, and tall morningglories at all locations and smooth pigweed at two of three locations was achieved only in systems with POST-directed herbicides. Adding POST-directed herbicides to systems with thiazopyr or pendimethalin plus fluometuron PRE increased cotton yield at two of three locations. Treatments had no effect on fiber quality.

Weed Hosts of the Sugarcane Root Rot Pathogen, Pythium arrhenomanes.

Weeds in the Poaceae and Cyperaceae families prevalent in sugarcane fields were evaluated as potential hosts for the root rot pathogen, Pythium arrhenomanes. In greenhouse studies, bermudagrass, broadleaf signalgrass, browntop panicum, barnyardgrass, large crabgrass, goosegrass, itchgrass, johnsongrass, Italian ryegrass, and purple nutsedge became infected when grown in steam-treated soil infested with P. arrhenomanes. However, the extent of root colonization, symptom severity, and growth reductions varied among species. Symptom severity and root colonization by P. arrhenomanes were less when weeds were grown in sugarcane field soil in the greenhouse than when weeds were grown in Pythium-infested, steam-treated field soil. Levels of root colonization by P. arrhenomanes in both experiments were greatest for johnsongrass and itchgrass and lowest for browntop panicum, goosegrass, and Italian ryegrass. For weeds collected from sugarcane fields, frequencies for colonized plants were moderate to high, but the extent of root colonization by P. arrhenomanes was low for all except johnsongrass. The results indicate that weeds can serve as hosts for P. arrhenomanes and may play roles in the epidemiology of Pythium root rot on sugarcane.