Christopher L. Main

Palmer Amaranth (Amaranthus palmeri) in Tennessee Has Low Level Glyphosate Resistance

Many agricultural producers apply glyphosate to glyphosate-resistant crops to control weeds, including Palmer amaranth. Populations of this weed in Tennessee not completely controlled by glyphosate were examined. Field and greenhouse research confirmed that two separate populations had reduced biomass sensitivity (1.5× to 5.0×) to glyphosate compared to susceptible populations, although the level of resistance was higher based on plant mortality response (about 10×). Shikimate accumulated in both resistant and susceptible plants, indicating that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was inhibited in both biotypes. These results suggest that an altered target site is not responsible for glyphosate resistance in these Palmer amaranth biotypes. Nomenclature: Glyphosate, Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA

Biotic and abiotic factors influence horseweed emergence

Abstract Factors affecting horseweed emergence are important for management of this weed species, particularly because of the presence of herbicide-resistant biotypes. Horseweed emergence was highly variable and not strongly correlated to soil temperature (r2 = 0.21), air temperature (r2 = 0.45) or rainfall (r2 = 0.32). Horseweed emerged mainly during April and September in Tennessee when average daytime temperatures fluctuate between 10 and 15.5 C. However, some horseweed plants emerged during almost any month when temperatures ranged from 10 to 25 C and adequate moisture was available at the soil surface. Horseweed densities ranged from a low of 30 to 50 plants m−2 to a high of > 1,500 plants−2 at one location. These extremely high densities illustrate the ability of horseweed to be an effective ruderal plant that can produce stands that approach monoculture densities if not controlled. The amount of crop residue remaining after harvest from the previous field season was in the order of corn > cotton > soybean > fallow. Residue from a previous corn crop reduced horseweed emergence compared with soybean and cotton residues in a no-tillage situation. Decreased horseweed density due to crop residue presence indicates that a systems approach may help reduce horseweed populations. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; corn, Zea mays L.; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.

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. Nomenclature: Glyphosate; diammonium sulfate; broadleaf signalgrass, Brachiaria platyphylla L. #3 BRAPP; Palmer amaranth, Amaranthus palmeri L. # AMAPA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; yellow nutsedge, Cyperus esculentus L. # CYPES; soybean, [Glycine Max (L). Merr. var ‘Asgrow 5602’]. Additional index words: Antagonism, hard water. Abbreviations: DAT, days after treatment; ICAP, inductively coupled argon plasma.

Evaluating Rates and Application Timings of Saflufenacil for Control of Glyphosate-Resistant Horseweed (Conyza canadenis) Prior to Planting No-Till Cotton

Abstract Managing glyphosate-resistant (GR) horseweed in no-till cotton continues to be a serious challenge for midsouthern producers. Field studies were conducted in 2008 and 2009 to evaluate spring burndown applications of saflufenacil on GR horseweed prior to planting cotton. Saflufenacil controlled GR horseweed at least 94% up to 7 d before planting (DBP) without causing significant cotton injury. Saflufenacil applied at 7 or 14 DBP controlled GR horseweed while still providing residual control until planting. Moreover, saflufenacil, on silt loam soil evaluated in this study, showed no more injury than dicamba applied 7 or more DBP. Results indicated that saflufenacil is an option in cotton for controlling GR horseweed much closer to cotton planting than 42 DBP (current saflufenacil label). At 25 g ha−1, which is the standard labeled rate in cotton, saflufenacil provided > 90% control of GR horseweed. Saflufenacil as a GR horseweed burndown, could replace the current dicamba standard every other year to reduce the probability of horseweed developing resistance to dicamba or salflufenacil. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; cotton, Gossypium hirsutum L. Phytogen 485WRF

A Survey for Diclofop-Methyl Resistance in Italian Ryegrass from Tennessee and How To Manage Resistance in Wheat

Abstract Italian ryegrass resistance to diclofop has been documented in several countries, including the United States. The purpose of this research was to screen selected putative resistant populations of Italian ryegrass for resistance to the acetyl-CoA carboxylase (ACCase)–inhibiting herbicides diclofop and pinoxaden and the acetolactate synthase (ALS)–inhibiting herbicides imazamox, pyroxsulam, and mesosulfuron in the greenhouse and to use field experiments to develop herbicide programs for Italian ryegrass control. Resistance to diclofop was confirmed in eight populations from Tennessee. These eight populations did not show cross-resistance to pinoxaden. One additional population (R1) from Union County, North Carolina, was found to be resistant to both diclofop and pinoxaden. The level of resistance to pinoxaden of the R1 population was 15 times that of the susceptible population. No resistance was confirmed to any of the ALS-inhibiting herbicides examined in this research. Field experiments demonstrated PRE Italian ryegrass control with chlorsulfuron (71 to 94%) and flufenacet + metribuzin (84 to 96%). Italian ryegrass control with pendimethalin applied PRE or delayed preemergence (DPRE) was variable (0 to 85%). POST control of Italian ryegrass was acceptable with pinoxaden, mesosulfuron, flufenacet + metribuzin, and chlorsulfuron + flucarbazone (> 80%). Application timing and herbicide treatment had no effect on wheat yield, except for diclofop and pendimethalin treatments, in which uncontrolled Italian ryegrass reduced wheat yield. Nomenclature: Chlorsulfuron; diclofop; flucarbazone; flufenacet; imazamox; mesosulfuron; metribuzin; pendimethalin; pinoxaden; pyroxsulam; Italian ryegrass, Lolium perenne L. ssp. multiflorum Lam. Husnot LOLMU; wheat, Triticum aestivum L.

Evaluation of Spring and Fall Burndown Application Timings on Control of Glyphosate-Resistant Horseweed (Conyza canadensis) in No-Till Cotton

Abstract Field studies were conducted in 2007 and 2008 to evaluate fall applications of herbicides to control glyphosate-resistant (GR) horseweed before planting cotton. Fall treatments were compared with spring treatments for control of GR horseweed and effect on seed cotton yield. Fall and spring treatments with and without residual herbicides were also compared. No differences were observed for control of GR horseweed or seed cotton yield between fall and spring application timings. However, a difference was observed between fall applications with and without a residual herbicide. Fall applications that contained residual herbicides provided 86% control of GR horseweed and yielded 2,360 kg/ha of seed cotton. Fall applications that did not contain a residual herbicide only provided 70% control of GR horseweed and yielded 2,010 kg/ha of seed cotton. No benefit was observed from spring applications that contained a residual herbicide. This research indicates that glyphosate-resistant horseweed can be controlled with fall- or spring-applied burndown herbicides, and fall applications should include a residual herbicide for best results. Nomenclature: Dicamba; diuron; flumioxazin; fomesafen; trifloxysulfuron; glufosinate; glyphosate; paraquat; prometryn; horseweed, Conyza canadensis (L.) Cronq ERICA; Cotton, Gossypium hirsutum L. ‘PhytoGen 485 WRF’.

Sulfentrazone Persistence in Southern Soils: Bioavailable Concentration and Effect on a Rotational Cotton Crop'

Field studies were conducted from 1998 to 2000 in Tennessee, North Carolina, Arkansas, and Oklahoma to determine the effects of sulfentrazone carryover to a cotton rotational crop from sulfentrazone applied the previous year. Sulfentrazone applied the previous year at 400 g/ha caused no yield loss in Tennessee, >30% yield reduction in Oklahoma, and 20% yield loss in Arkansas and North Carolina. In most experiments in this study, visual evaluations of injury closely correlated with final cotton lint yield (r2 =0.84). Nomenclature: Sulfentrazone; cotton, Gossypium hirsutum L. Additional index words: Bioassay, herbicide carryover, PROTOX inhibitor, rotational crops.

Annual Bluegrass (Poa Annua) Control in Creeping Bentgrass (Agrostis Stolonifera) Putting Greens with Bispyribac-sodium

Annual bluegrass is one of the most difficult-to-control weeds in creeping bentgrass putting greens. Field trials were conducted in 2003 and 2005 to evaluate bispyribac-sodium for annual bluegrass management in creeping bentgrass greens maintained at a 3 mm mowing height. Bispyribac-sodium applied weekly at 12 or 24 g ai/ha controlled annual bluegrass 86% 12 wk after initial treatment (WAIT). In 2003, bispyribac-sodium applied at 12 and 24 g/ha/wk injured creeping bentgrass approximately 15 and 50% by 4 WAIT, respectively. However, injury was transient and was not evident by 12 WAIT. In 2005, the 12 and 24 g/ha/wk injured creeping bentgrass 15 and 85% by 8 WAIT, respectively, and was still evident throughout the trial. Putting green quality was reduced when compared to nontreated creeping bentgrass by the same treatments. The removal of annual bluegrass caused soil exposure until creeping bentgrass grew over the bare areas, contributing to decreased quality evaluations. Management of annual bluegrass in creeping bentgrass putting greens is possible with bispyribac-sodium. However, these results indicate bispyribac-sodium can cause excessive injury when applied to creeping bentgrass mowed at 3 mm. Nomenclature: Bispyribac-sodium; annual bluegrass, Poa annua L. POANN; creeping bentgrass, Agrostis stolonifera L. AGSST, ‘Penncross’.

Horsenettle (Solanum carolinense) Control in Tall Fescue (Festuca arundinacea) and Clover (Trifolium sp.) Pastures with Mixtures of 2,4-D and Picloram1

Herbicide treatments (4:1 ratio of 2,4-D amine:picloram) at 0.7 and 1.4 kg ae/ha at early postemergence (10- to 15-cm horsenettle height), midpostemergence (early flower), and late postemergence (fruit initiation) applied both early and late in the growing season provided >80% horsenettle control. Horsenettle density at seasons end in all treated plots was less than 0.25 stems/m2, whereas untreated plots contained about 5 stems/m2. Horsenettle control the next spring was between 47 and 66% for all rates and application timings, and horsenettle density in treated plots was less than 3 stems/m2 as opposed to about 6 stems/m2 in the untreated plots. Clover drilled into the treated area the year after herbicide application was injured, indicating clover establishment the season after application of this package mixture would be difficult. Nomenclature: 2,4-D; picloram; clover sp., Trifolium sp.; horsenettle, Solanum carolinense L. #3 SOLCA. Additional index words: Crop tolerance. Abbreviations: EPOST, early postemergence; LPOST, late postemergence; MPOST, midpostemergence.

Star-of-Bethlehem (Ornithogalum umbellatum) Control with Postemergence Herbicides in Dormant Bermudagrass (Cynodon dactylon) Turf1

Field studies were conducted near Knoxville, TN, during late March and early April 2002 and 2003, respectively, for star-of-Bethlehem control in dormant bermudagrass turf that was established over 25 yr ago. Halosulfuron, imazaquin, metsulfuron, 2,4-D plus dicamba plus mecoprop, and triclopyr plus clopyralid controlled star-of-Bethlehem 35% at most 35 d after treatment (DAT). Bromoxynil alone or mixed with halosulfuron, imazaquin, or metsulfuron controlled star-of-Bethlehem at least 80% at 35 DAT. Imazaquin and imazaquin plus bromoxynil injured bermudagrass 51% 35 DAT. This injury was characterized by decreased bermudagrass postdormancy transition and was transient. Nomenclature: Bromoxynil; clopyralid; 2,4-D; dicamba; halosulfuron; imazaquin; mecoprop; metsulfuron; triclopyr; common bermudagrass, Cynodon dactylon (L.) Pers. #3 CYNDA; star-of-Bethlehem, Ornithogalum umbellatum L. # OTGUM. Additional index words: Glyphosate, golf course, postdormancy transition. Abbreviation: DAT, days after treatment.