Wk After Treatment Research Articles

Aminocyclopyrachlor Enhances Fenoxaprop Efficacy for Smooth Crabgrass Control

Fenoxaprop effectively controls crabgrass in tall fescue turf, but antagonism with growth-regulating herbicides reduces potential to apply fenoxaprop in combination with many herbicides registered for broadleaf weed control. Aminocyclopyrachlor is a new broadleaf weed control herbicide that has not been evaluated in combination with fenoxaprop. Field experiments were conducted in Georgia, New Jersey, and Tennessee to investigate tank mixtures of fenoxaprop with aminocyclopyrachlor for smooth crabgrass and white clover control. Fenoxaprop alone exhibited substantial activity on smooth crabgrass but control was greater with fenoxaprop + aminocyclopyrachlor treatments. By 4 and 6 wk after treatment (WAT), approximately 22 and 44% less fenoxaprop was required to achieve 80% smooth crabgrass control when the herbicide was tank-mixed with aminocyclopyrachlor at 52.5 and 79 g ai ha−1, respectively. Fenoxaprop did not reduce white clover control with aminocyclopyrachlor because 97% control was achieved by 4 WAT for all aminocyclopyrachlor + fenoxaprop treatments. Tall fescue was not injured by any treatment. Results suggest aminocyclopyrachlor enhances fenoxaprop efficacy for smooth crabgrass control in tall fescue.

Multiple Resistance in Palmer Amaranth to Glyphosate and Pyrithiobac Confirmed in Georgia

In 2006, Palmer amaranth with confirmed resistance to glyphosate (GLY-R) was not controlled effectively in cotton with pyrithiobac, an acetolactate synthase (ALS)-inhibiting herbicide. Glyphosate at 870 g ae ha−1or pyrithiobac at 70 g ai ha−1applied postemergence provided 5 to 28% control of a putative GLY/ALS-R Palmer amaranth biotype in the field. Glyphosate at 6,930 g ha−1and pyrithiobac at 420 g ha−1applied alone provided no more than 89 and 65% control 1 to 8 wk after treatment (WAT), respectively. When applied as a tank mixture, glyphosate plus pyrithiobac at 870 + 70 g ha−1provided between 16 and 41% control; glyphosate plus pyrithiobac at 6,930 + 420 g ha−1controlled the Palmer amaranth in the field 89 to 95%. Dose-response analyses developed from greenhouse data indicated that the estimated glyphosate rates required to produce 50% injury and reduce plant fresh weights by 50% relative to the nontreated control in a suspected GLY/ALS-R Palmer amaranth biotype were 12 and 14 times greater, respectively, than the estimated values for the susceptible (S) biotype. The predicted pyrithiobac rates required to produce the same responses in the putative resistant population were 151 (50% injury) and 563 times (50% fresh weight reduction) greater than the estimated rates for the S biotype. Field and greenhouse analyses confirm that the Palmer amaranth biotype evaluated in both studies is resistant to glyphosate and an ALS-inhibiting herbicide.

Pyroxasulfone with and without Sulfentrazone in Sunflower (Helianthus annuus)

Pyroxasulfone (KIH-485) is a seedling growth-inhibiting herbicide developed by Kumiai America that has the potential to control weeds in sunflower. However, little is known about how this herbicide will interact with various soil types and environments when combined with sulfentrazone. The objective of this research was to evaluate sunflower injury and weed control with pyroxasulfone applied with and without sulfentrazone across the Great Plains sunflower production area. A multisite study was initiated in spring 2007 to evaluate sunflower response to pyroxasulfone applied PRE at 0, 167, 208, or 333 g ai ha−1. In 2008, pyroxasulfone was applied alone and in tank mixture with sulfentrazone. In 2007, no sunflower injury was observed with any rate of pyroxasulfone at any location except Highmore, SD, where sunflower injury was 17%, 4 wk after treatment (WAT) with 333 g ha−1. In 2008, sunflower injury ranged from 0 to 4% for all treatments. Adding sulfentrazone did not increase injury. Sunflower yield was only reduced in treatments in which weeds were not effectively controlled. These treatments included the untreated control and pyroxasulfone at 167 g ha−1. Sunflower yield did not differ among the other treatments of pyroxasulfone or sulfentrazone applied alone or in combination. The addition of sulfentrazone to pyroxasulfone improved control of foxtail barley, prostrate pigweed, wild buckwheat, Palmer amaranth, and marshelder, but not large crabgrass or green foxtail. The combination of pyroxasulfone and sulfentrazone did not reduce control of any of the weeds evaluated.

Growth and Seed Production of Horseweed (Conyza canadensis) Populations after Exposure to Postemergence 2,4-D

Horseweed can be a problematic weed in no-till soybean fields and populations can vary in their response to 2,4-D. The objective of this study was to evaluate the growth and seed production of four horseweed populations after exposure to 2,4-D. 2,4-D amine was applied at 0, 140, 280, and 560 g ae ha−1 to 5- to 10-cm-tall horseweed plants. An additional treatment of 280 g ha−1 of 2,4-D + 840 g ae ha−1 of glyphosate was included in the study. At 2 wk after treatment (WAT), injury ranged from 47 to 98%, but by 6 WAT the injury ranged from 89 to 100% for all four populations. Between 6 and 12 WAT some individual horseweed plants started to recover. No differences in dry weights were observed between the four populations in the untreated checks at 0, 2, 6, and 12 WAT. At 280 g ha−1 of 2,4-D amine, seed production was reduced by greater than 95%. However, three of the four horseweed populations had plants that survived and produced seed after exposure to 840 g ha−1 of glyphosate + 280 g ha−1 of 2,4-D. One plant produced seed after exposure to 560 g ha−1 of 2,4-D. These results suggest that horseweed can evolve resistance to 2,4-D and no fitness penalities were observed in populations that had higher levels of tolerance to 2,4-D.

Postemergence Weed Control in Acetolactate Synthase–Resistant Grain Sorghum

Postemergence herbicides to control grass weeds in grain sorghum are limited. Acetolactate synthase (ALS) –inhibiting herbicides are very effective at controlling many grass species in many crops; unfortunately, use of ALS-inhibiting herbicides is not an option in conventional grain sorghum because of its susceptibility to these herbicides. With the development of ALS-resistant grain sorghum, several POST ALS-inhibiting herbicides can be used to control weeds in grain sorghum. Field experiments were conducted in 2007 and 2008 to evaluate the efficacy of tank mixtures of nicosulfuron + rimsulfuron applied alone or in combination with bromoxynil, carfentrazone–ethyl, halosulfuron + dicamba, prosulfuron, 2,4-D, or metsulfuron methyl + 2,4-D. In addition, these treatments were applied with and without atrazine. Nicosulfuron + rimsulfuron controlled barnyardgrass, green foxtail, and giant foxtail 99, 86, and 91% 6 wk after treatment (WAT), respectively. A decrease in annual grass control was observed when nicosulfuron + rimsulfuron was tank mixed with some broadleaf herbicides, although the differences were not always significant. In addition, nicosulfuron + rimsulfuron controlled velvetleaf and ivyleaf moringglory 64 and 78% 6 WAT, respectively. Control of velvetleaf was improved when nicosulfuron + rimsulfuron was tank mixed with all broadleaf herbicides included in this study with the exception of atrazine, bromoxynil, and prosulfuron + atrazine. Control of ivyleaf morningglory was improved when nicosulfuron + rimsulfuron was tank mixed with all of the herbicides included in this study with the exception of metsulfuron methyl + 2,4-D. Weed populations and biomass were lower when nicosulfuron + rimsulfuron were applied with various broadleaf herbicides than when it was applied alone. Grain sorghum yield was greater in all herbicide treatments than in the weedy check, with the highest grain yield from nicosulfuron + rimsulfuron + prosulfuron. This research showed that postemergence application of nicosulfuron + rimsulfuron effectively controls grass weeds, including barnyardgrass, green foxtail, and giant foxtail. The research also showed that velvetleaf and ivyleaf morningglory control was more effective when nicosulfuron + rimsulfuron were applied with other broadleaf herbicides.

Herbicide Combinations for Control of Volunteer Potato

Field studies were conducted to determine if POST applications of the p-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitors mesotrione (105 g ai/ha), topramezone (18 g ai/ha), and tembotrione (92 g ai/ha) applied alone and in mixtures with the photosystem-II (PSII) inhibitors atrazine (560 g ai/ha), bentazon (560 g ai/ha), or bromoxynil (280 g ai/ha) would control volunteer potato. Mesotrione alone controlled potato 62%, but topramezone and tembotrione only provided 10 to 22% control by 6 wk after treatment (WAT). All PSII inhibitors applied alone provided less than 36% control of potato. Overall, mixtures of PSII inhibitors plus mesotrione improved initial potato control by 2 WAT; however, by 6 WAT, few differences were observed between mesotrione applied alone or in mixtures with PSII inhibitors. PSII inhibitors did not always improve activity of topramezone or tembotrione on potato, and in some instances appeared to antagonize control. HPPD inhibitors applied alone or in combinations with PSII inhibitors reduced potato yields in comparison to the untreated check.

Effect of Postemergence Mesotrione Application Timing on Grain Sorghum

Field experiments were conducted at Belleville, Colby, Hays, Hesston, Garden City, and Manhattan, KS, to determine grain sorghum response to POST application of mesotrione at three application timings. Mesotrione was applied at 52, 105, 157, and 210 g ai/ha in combination with 280 g ai/ha atrazine to grain sorghum at heights of 5 to 8, 15 to 20, and 30 cm, which correspond to early POST (EPOST), mid-POST (MPOST), and late POST (LPOST), respectively. All mesotrione rates caused injury at all application timings. Overall, grain sorghum injury from mesotrione was greatest at 1 wk after treatment (WAT); plants partially recovered from injury by 4 WAT. Mesotrione applied EPOST injured grain sorghum more than when applied at MPOST and LPOST timings. The EPOST application injured grain sorghum 19 to 88%, whereas injury from MPOST and LPOST application was 1 to 66% and 0 to 69%, respectively, depending on rate. Mesotrione injury was least at Belleville and most at the Hesston and Garden City (irrigated) sites regardless of growth stage. Correlation coefficient analyses indicated that observed mesotrione injury symptoms were not well correlated with grain sorghum yield; thus, mesotrione injury to grain sorghum did not influence grain yield. However, initial grain sorghum injury was severe, and this will likely be a major concern to producers.

Pitted and Hybrid Morningglory Accessions Have Variable Tolerance to Glyphosate

Two greenhouse studies were conducted to investigate the variability in tolerance to a sublethal dose of glyphosate among accessions of pitted morningglory, hybrid morningglory (a fertile hybrid between pitted and sharppod morningglory), and sharppod morningglory, collected from several states in the southern United States. The first study was conducted to evaluate the variability in tolerance to glyphosate among accessions. Glyphosate at 420 g ae/ha was applied to plants at the four- to five-leaf stage, and control (percent shoot fresh weight reduction) was determined 2 wk after treatment (WAT). Pitted morningglory response ranged from −9% (indicating no response to glyphosate) to 39% control. A similar trend was observed in hybrid morningglory. Control of two related species, cypressvine morningglory and red morningglory, averaged 40 and 29%, respectively, and was similar to control of the most susceptible pitted morningglory and hybrid morningglory accessions. Ivyleaf morningglory control was 9%. Sharppod morningglory control was highest (48%) among the morningglories studied. A second study was conducted to determine levels of tolerance to glyphosate based onGR50(dose required to cause a 50% reduction in plant growth) in 10 accessions that were least to most sensitive to glyphosate (7 pitted, 2 hybrid, and 1 sharppod morningglory). GlyphosateGR50doses ranged from 0.65 to 1.23 kg/ha, a two-fold variability in tolerance to glyphosate among the 7 pitted morningglory accessions. Increasing levels of tolerance were associated with the absence of a leaf notch. These results indicate the existence of variable tolerance to a sublethal dose of glyphosate among accessions of pitted morningglory.

Strategies for Control of Horseweed (Conyza canadensis) and Other Winter Annual Weeds in No-Till Corn

Field studies were conducted to determine if mesotrione alone or in combinations with other corn herbicides would control horseweed and other winter annual weeds associated with no-till corn. Mesotrione alone controlled horseweed 52 to 80% by 3 wk after treatment (WAT); however, by 7 WAT control diminished to between 37 to 68%, depending on mesotrione rate. Mesotrione at 0.16 kg ai/ha plus atrazine at 0.28 kg ai/ha controlled 99% of horseweed and annual bluegrass and 88% of yellow woodsorrel. Combinations of mesotrione at 0.16 kg/ha plus acetochlor at 1.79 kg ai/ha plus 1.12 kg ai/ha glyphosate (trimethylsulfonium salt of glyphosate) or 0.7 kg ai/ha paraquat provided 93% or greater control of all three weed species. Glyphosate alone also controlled all weed species 97 to 99%, while paraquat alone provided 99% control of annual bluegrass, 72% control of horseweed, and 36% control of yellow woodsorrel. Mixtures of paraquat plus acetochlor improved control of horseweed (93%) and yellow woodsorrel (73%) over control with either herbicide applied alone.

Zinc Antagonizes Glyphosate Efficacy on Yellow Nutsedge (Cyperus esculentus)

Growers of glyphosate-resistant crops apply micronutrients tank-mixed with glyphosate to save time and production costs. Therefore, effect of zinc (Zn), as Zn sulfate, on absorption, translocation, and efficacy of glyphosate on yellow nutsedge was investigated. Glyphosate at 850 g ae ha−1provided 90% yellow nutsedge control at 5 wk after treatment (WAT). Presence of Zn at 1,000 ppmw in the glyphosate spray solution reduced yellow nutsedge control to 24 and 8%, 3 and 5 WAT, respectively. Yellow nutsedge control decreased with increasing Zn level (500 to 2,000 ppmw) in the spray solution. Yellow nutsedge treated with higher rates of Zn tank-mixed with glyphosate produced more tubers and tillers per plant than untreated plants. An abrupt decrease in absorption and translocation of14C–glyphosate occurred between 500 and 1,000 ppmw Zn. The antagonistic effect of Zn on glyphosate depended mainly on reduced absorption and translocation of14C–glyphosate within treated tissues. Less than 10% of applied14C–glyphosate was absorbed when glyphosate was mixed with 1,000, 2,000, or 4,000 ppmw Zn as compared with 85% absorption for glyphosate alone. These treatments inhibited > 90% of14C–glyphosate translocation out of the treated leaf and > 50% of tuber translocation relative to glyphosate alone. Results indicate that micronutrients containing Zn are not suitable for tank-mixing with glyphosate.

Influence of Hexazinone on ‘Tifton 85’ Bermudagrass Growth and Forage Quality

Smutgrass is a clump-type grassy weed that is commonly found in grazed pastures throughout the Southeast. Currently, the only herbicide registered for control of smutgrass is hexazinone and it is unknown if hexazinone negatively impacts bermudagrass forage growth and quality. Experiments were conducted to determine the impact of hexazinone rate on bermudagrass yield and crude protein. All rates of hexazinone reduced bermudagrass yield at 4 wk after treatment (WAT). From 6 to 12 WAT, no bermudagrass yield reduction was observed except for the 2 kg/ha treatment. The yield reducing effects of hexazinone were transitory at the 0.5 and 1.0 kg/ha rates; cumulative yield for the season was reduced by 17 and 23%, respectively. Although yield was negatively influenced by hexazinone, crude protein generally increased in a linear fashion with increased hexazinone rate.

Chemical Management of Cut-Leaved Teasel (Dipsacus Laciniatus) in Missouri

Cut-leaved teasel is an invasive weed along highway corridors and is classified noxious in four states, including Missouri. Few herbicides have been examined for cut-leaved teasel control. Herbicides were evaluated for efficacy on established plants and residual activity for suppressing seedling emergence. Various growth regulator herbicides, amino acid inhibitors, and paraquat were applied on established teasel at two locations in central Missouri in fall 2003 and spring 2004, and two additional locations in fall 2004 and spring 2005. At 2 wk after treatment (WAT), paraquat resulted in the highest injury of teasel (85%), but injury declined over time. At 4 WAT, teasel control was most consistent with dicamba + diflufenzopyr applied in spring, ranging from 75 to 94% control. At 8 wk, glyphosate, dicamba + diflufenzopyr, metsulfuron-methyl, imazapyr, and combinations of growth regulator herbicides with 2,4-D were most effective, with teasel control from 86 to 100%. Control with 2,4-D alone was inconsistent; sulfosulfuron, sulfometuron-methyl, and paraquat were ineffective. Residual herbicides did not reduce teasel seedling emergence the following year. A number of herbicides were effective in managing emerged plants but reinfestations of treated sites is likely, even with the residual herbicides used in this research.

Genetic Basis of Sensitivity in Sweet Corn to Tembotrione

Tembotrione inhibits 4-hydroxyphenyl-pyruvate-dioxygenase (HPPD) and was recently registered for use in all types of corn. Some sweet corn hybrids are killed by tembotrione, yet a mechanistic understanding of sensitivity has not been reported. Sensitivity of mesotrione, another HPPD-inhibitor, is conditioned by a single allele. Two hypotheses were tested: (1) response to tembotrione and mesotrione are conditioned by alleles at the same or closely linked loci and (2) the extent of early-season injury from tembotrione and mesotrione is similar on hybrids. The first hypothesis was tested by comparing responses to tembotrione and mesotrione in 136 F3:5families derived from a cross of mesotrione-sensitive and mesotrione-tolerant sweet corn inbreds. F3families cosegregated for responses to tembotrione and mesotrione: 94% of the families had the same response to both herbicides. Thus, the same gene or very closely linked genes condition response to both herbicides. On the basis of chi-square goodness of fit tests, responses of families to tembotrione fit a 3 : 2 : 3 sensitive : segregating : tolerant ratio (P = 0.24), which would be expected if sensitivity to tembotrione was conditioned by a single recessive allele. The second hypothesis was tested in six field experiments by quantifying the extent of early-season injury to 249 sweet corn hybrids 1 wk after treatment (WAT) of tembotrione (184 g ha−1) or mesotrione (210 g ha−1). One hundred ninety-three hybrids were tolerant to both herbicides. Seven sensitive hybrids that were severely injured by both herbicides 1 WAT differed in their response 3 to 4 WAT; sensitive hybrids treated with mesotrione had apparently resumed normal growth, whereas those treated with tembotrione died. Conversely, hybrids with intermediate levels of injury (> 10 to 50%) 1 WAT with mesotrione had no visual symptoms of injury from applications of tembotrione. Despite the common genetic basis for response to mesotrione and tembotrione, hybrids with sensitive or intermediate responses to mesotrione did not have identical responses to tembotrione.

Hybrid Kentucky Bluegrass Tolerance to Preemergence and Postemergence Herbicides

Field studies were conducted near Knoxville, TN, from 2003 to 2005 to evaluate the response of ‘Thermal Blue’, a new interspecific hybrid Kentucky bluegrass to commonly applied PRE and POST herbicides for weed management. Dithiopyr, oryzalin, oxadiazon, pendimethalin, prodiamine, quinclorac, and trifluralin applied at seeding injured hybrid bluegrass greater than 81% and reduced hybrid bluegrass cover greater than 57%. In a second study, established hybrid bluegrass was treated POST with acetolactate synthase–inhibiting herbicides including bispyribac-sodium, chlorosulfuron, foramsulfuron, halosulfuron, imazapic, imazaquin, metsulfuron, rimsulfuron, sulfosulfuron, and trifloxysulfuron at low and high rates (one and two times the suggested use rates in Kentucky bluegrass or other turfgrasses). By 5 wk after treatment (WAT), foramsulfuron at 88 g ai/ha and trifloxysulfuron at 35 g ai/ha injured hybrid bluegrass greater than 26% and reduced visually estimated quality and chlorophyll meter indices. However, hybrid bluegrass injury was no longer evident at 10 WAT. In a third study, established hybrid bluegrass was treated with clethodim, diclofop-methyl, fluazifop-p-butyl, and sethoxydim applied at low, medium, and high rates (0.5, 1, and 2 times the registered Kentucky bluegrass or other turfgrass use rates). Clethodim applied at 280 and 560 g ai/ha, fluazifop at 420 g ai/ha, and sethoxydim at 630 g ai/ha injured hybrid bluegrass 5 WAT. These treatments also reduced quality (to less than 5 on a scale of 1 to 9) and chlorophyll meter indices (24 to 37%) when compared to the untreated control. By 10 WAT, only clethodim at 560 g ai/ha caused injury (14%). By 10 WAT, hybrid bluegrass had recovered and injury was only observed in plots treated with clethodim at 560 g ai/ha. No differences in chlorophyll indices or quality were observed at 10 WAT for any POST graminicides.Nomenclature: Bispyribac-sodium, chlorosulfuron, clethodim, diclofop-methyl, dithiopyr, fluazifop-p-butyl, foramsulfuron, halosulfuron, imazapic, imazaquin, metsulfuron, oryzalin, oxadiazon, pendimethalin, prodiamine, quinclorac, rimsulfuron, sethoxydim, sulfosulfuron, trifloxysulfuron, trifluralin, Kentucky bluegrass, Poa arachnifera Torr. × P. pratensis L. ‘Thermal Blue’

Glyphosate-resistant Horseweed (Conyza canadensis) Control Using Glyphosate-, Paraquat-, and Glufosinate-Based Herbicide Programs

Field studies were conducted in 2005 and 2006 to determine the most effective chemical options within three individual herbicide-based burndown programs, glyphosate, paraquat and glufosinate, for controlling glyphosate-resistant horseweed in Mississippi. Burndown treatments were applied April 5, 2005 and March 15, 2006 to horseweed plants 15 to 30 cm in height. Glyphosate at 0.86 kg ae/ha alone provided 60 to 65% horseweed control 4 wk after treatment (WAT). Control 4 WAT ranged from 73 to 74% when the glyphosate rate was increased to 1.25 kg/ha. Glyphosate at 0.86 kg/ha applied in combination with 2,4-D at 0.84 kg ae/ha or dicamba at 0.28 ae/ha maximized control of horseweed (≥ 90%) 4 WAT and soybean yield. Horseweed control 4 WAT with paraquat alone at 0.84 kg ai/ha ranged from 55 to 63% and control did not improve by increasing the rate to 0.98 kg/ha. Addition of 2,4-D or dicamba to paraquat maximized horseweed control both years (78 to 89%), whereas soybean yield was maximized with addition of dicamba or metribuzin at 0.42 kg ai/ha. Glufosinate applied alone at 0.47 kg ai/ha resulted in at least 88% control of horseweed and maximized soybean yield. Results indicate that effective management of glyphosate-resistant horseweed can be obtained in glyphosate-resistant soybean in glyphosate-, paraquat-, and glufosinate-based preplant weed control programs.Nomenclature: Carfentrazone, chlorimuron, 2,4-D, dicamba, flumioxazin, glufosinate, glyphosate, linuron, metribuzin, oxyfluorfen, paraquat, thifensulfuron, tribenuron, horseweed, Conyza canadensis (L.) Cronq. ERICA, soybean, Glycine max. (L.) Merr. DK 4763RR, AG 4801RR

Dark Tobacco (Nicotiana tabacum) Tolerance to Trifloxysulfuron and Halosulfuron

Field experiments were conducted at two locations in 2003 and 2004 to determine dark tobacco tolerance and weed control from postemergence over-the-top (POT) and postemergence-directed (PD) applications of trifloxysulfuron and halosulfuron. Trifloxysulfuron was applied at 3.6 or 5.3 g ai/ha and halosulfuron was applied at 36 or 53 g ai/ha. Trifloxysulfuron POT injured ‘Narrowleaf Madole’ dark tobacco 16 to 33% and halosulfuron POT caused 32 to 33% injury at 1 wk after treatment (WAT). Tobacco plant height at 1 WAT with POT applications was also reduced by up to 31% at one of two locations. Tobacco injury from POT applications had dissipated to 6 to 12% by 4 WAT. Despite extensive early-season injury, most POT herbicide applications did not significantly reduce tobacco yield, mean grade index, and gross revenue compared to tobacco that only received pretransplant applications of sulfentrazone plus clomazone. Dark tobacco was much more tolerant to PD applications of either herbicide, with no more than 4% injury observed at 1 WAT. Neither herbicide controlled horsenettle more than 57% at either application rate or method. Although trifloxysulfuron and halosulfuron could contribute to tobacco weed control programs by providing postemergence control of several common weed species, PD applications would be the only acceptable method of application due to excessive injury observed from POT applications of these herbicides.

Ragweed Parthenium (Parthenium hysterophorus) Control with Preemergence and Postemergence Herbicides

Field and greenhouse experiments were conducted during 2005 and 2006 at Stoneville, MS, to determine control of ragweed parthenium with several preemergence (PRE) and postemergence (POST) herbicides registered for use in corn, cotton, peanut, rice, and soybean. Norflurazon, pendimethalin, clomazone, diuron, fluometuron, pyrithiobac, dimethenamid, flumetsulam, imazaquin, s-metolachlor, metribuzin, chlorimuron, atrazine, simazine, flumioxazin, and quinclorac were applied PRE. Ragweed parthenium control was highest with norflurazon (100%) and clomazone (100%) followed by fluometuron (96%), metribuzin (90%), diuron (87%), flumioxazin (84%), chlorimuron (77%), and quinclorac (67%) at 6 wk after treatment (WAT) under greenhouse conditions. Control of ragweed parthenium was less than 58% with all other herbicides. Ragweed parthenium appears to be highly sensitive to pigment and photosynthetic inhibitors compared to herbicides with other modes of action. Glyphosate, glufosinate, paraquat, bentazon, acifluorfen, chlorimuron, halosulfuron, MSMA, bromoxynil, atrazine, 2,4-D, flumioxazin, trifloxysulfuron, and clomazone were applied POST to field-grown rosette and bolted plants. Glyphosate, glufosinate, chlorimuron, and trifloxysulfuron applied at rosette stage provided greater than 93% control of ragweed parthenium at 3 WAT. Halosulfuron, MSMA, bromoxynil, 2,4-D, and flumioxazin controlled 58 to 90% rosette ragweed parthenium at 3 WAT. Ragweed parthenium control with all other POST herbicides was less than 38%. At bolted stage, glyphosate, glufosinate, and trifloxysulfuron controlled 86 to 95% ragweed parthenium and control was 61 to 70% with chlorimuron, halosulfuron, and 2,4-D 3 WAT. Overall, efficacy of POST herbicides was better on rosette plants than on bolted plants. Amino acid synthesis and glutamine synthase inhibitors were more active than herbicides with other modes of action. These results indicate that norflurazon, clomazone, fluometuron, flumioxazin, halosulfuron, chlorimuron, and trifloxysulfuron could provide effective control of ragweed parthenium.

Bundleflower (Desmanthus bicornutus) Response to Postemergence Herbicides

Field studies were conducted from 2003 through 2006 to evaluate bundleflower tolerance to several postemergence (POST) herbicides commonly used in pastures. All herbicides except imazapic, metsulfuron, and 2,4-DB injured bundleflower at least 88% when rated 4 wk after treatment (WAT). When rated 8 WAT, metsulfuron, picloram, and picloram plus 2,4-D injured bundleflower greater than 90%. Imazapic stunted bundleflower (10 to 35% injury) throughout the growing season. When rated 52 WAT, dicamba plus 2,4-D, picloram alone, and picloram plus 2,4-D injured bundleflower at least 97% in one of two years. Emergence of bundleflower with picloram-containing treatments ranged from 16 to 67% of the nontreated control, whereas emergence after dicamba-containing treatments ranged from 67 to 97%.

Oat and Rye Tolerance to Mesosulfuron and Tribenuron

The affect of mesosulfuron and tribenuron applied POST to oat and rye was determined at three locations in Georgia during 2004 and 2005. Five herbicide treatments were applied to oat and rye in Feekes scale (FS) 1.2 or 2 developmental stage. Herbicide treatments consisted of tribenuron at 13 or 26 g ai/ha, tribenuron at 13 g/ha plus MCPA at 561 g ai/ha, MCPA at 561 g/ha, and mesosulfuron at 15 g ai/ha. Tribenuron, MCPA, and tribenuron plus MCPA injured both crops less than 4% and did not affect grain yield. Mesosulfuron applied to rye in FS 1.2 injured the crop 36% at 2 wk after treatment (WAT), 12% at 5 WAT, and 0% at harvest. Mesosulfuron applied at FS 2 injured rye less than 9%. Mesosulfuron did not influence rye-grain yield with either application timing. Mesosulfuron injured oat 65 to 86% at 5 WAT and reduced yield at least 70% regardless of application timing.

Rimsulfuron for Postemergence Weed Control in Corn in Humid Tropical Environments of Nigeria

Cogongrass and guineagrass are serious perennial weeds in small-scale farms in lowland subhumid zones of West Africa. Field studies were conducted in 2002 and 2003 at two sites in Ibadan, Nigeria [Ijaye and the International Institute of Tropical Agriculture (IITA)], to evaluate the effect of rimsulfuron on weed communities dominated by cogongrass and guineagrass in corn. At both sites, treatments were rimsulfuron dosages of 0 (nontreated control), 10, 20, 30, 40, 50, 60, 70, and 80 g ai/ha. Rimsulfuron did not cause any visible phytotoxicity on the corn at any dosage at either site. There was a rapid increase in weed control as the dosage of rimsulfuron increased from 0 to 20 g/ha. Weed control was not improved at rates higher than 20 g/ha. Rimsulfuron was very effective against sedges,Ipomoea involucrata, Bengal dayflower, gulf leafflower, old-world diamond-flower, and wild jute providing more than 80% control at dosages between 10 and 20 g/ha at Ijaye. Rimsulfuron was less effective for cogongrass, with a maximum of only 38% control observed. At IITA, the herbicide was very effective against guineagrass, Bengal dayflower, nodeweed, coat buttons, redfruit passionflower, and waterleaf; all of which were controlled more than 70% with any rate of rimsulfuron. Regression analysis showed that the dosage of rimsulfuron required to reduce shoot dry biomass by 70% was 5 g/ha for guineagrass and 35 g/ha for cogongrass at 3 wk after treatment (WAT). At crop maturity, the dosage of rimsulfuron required to reduce shoot dry biomass by 70% was 43 g/ha for guineagrass and 200 g/ha for cogongrass. The dry biomass of cogongrass and guineagrass was higher at crop harvest than at 2 WAT regardless of herbicide dosage. Corn grain yield was 1.8 times higher at IITA than at Ijaye. At both sites, corn grain yield increased with increased herbicide dosage. Maximum corn grain yields were obtained at a rimsulfuron dosage of 20 g/ha.