Alan C. York

Glyphosate-resistant Palmer amaranth ( Amaranthus palmeri ) confirmed in Georgia

Abstract A glyphosate-resistant Palmer amaranth biotype was confirmed in central Georgia. In the field, glyphosate applied to 5- to 13-cm-tall Palmer amaranth at three times the normal use rate of 0.84 kg ae ha−1 controlled this biotype only 17%. The biotype was controlled 82% by glyphosate at 12 times the normal use rate. In the greenhouse, I50 values (rate necessary for 50% inhibition) for visual control and shoot fresh weight, expressed as percentage of the nontreated, were 8 and 6.2 times greater, respectively, with the resistant biotype compared with a known glyphosate-susceptible biotype. Glyphosate absorption and translocation and the number of chromosomes did not differ between biotypes. Shikimate was detected in leaf tissue of the susceptible biotype treated with glyphosate but not in the resistant biotype. Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats; AMAPA.

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

Abstract: 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.

Tropical Spiderwort (Commelina benghalensis) Control in Glyphosate-Resistant Cotton

Tropical spiderwort has recently become the most troublesome weed in Georgia cotton. Most of Georgias cotton is glyphosate resistant (GR), and glyphosate is only marginally effective on tropical spiderwort. An experiment was conducted at four locations to determine tropical spiderwort control in GR cotton by 27 herbicide systems. Treatments consisted of three early-postemergence over-the-top (POT) herbicide options and nine late–postemergence-directed (LPD) options arranged factorially. Glyphosate POT controlled tropical spiderwort only 53% 21 d after treatment (DAT). Glyphosate plus pyrithiobac or S-metolachlor controlled tropical spiderwort 60 and 80%, respectively. Pyrithiobac improved control of emerged spiderwort, whereas S-metolachlor provided residual control. Pooled over POT treatments, glyphosate LPD controlled tropical spiderwort 70% 21 DAT. Dimethipin mixed with glyphosate did not improve control. Carfentrazone, diuron, or flumioxazin mixed with glyphosate LPD improved control 9 to 15%. MSMA and MSMA plus flumioxazin were 8 and 19% more effective than glyphosate LPD. At time of cotton harvest, systems without residual herbicides at LPD controlled tropical spiderwort 42 to 45% compared with 64 to 76% with LPD treatments that included diuron or flumioxazin. Nomenclature: Carfentrazone; dimethipin; diuron; flumioxazin; glyphosate; MSMA; pyrithiobac; S-metolachlor; tropical spiderwort, Commelina benghalensis L.; cotton, Gossypium hirsutum L. ‘DP 458 B/RR’, ‘FM 989 B/RR’, ‘ST 4793 BR’. Additional index words: Invasive weed, noxious weed, weed shift. Abbreviations: DAP, days after planting; DAT, days after treatment; GR, glyphosate resistant; LPD, late postemergence directed; POT, postemergence over-the-top.

Tropical Spiderwort (Commelina benghalensis): A Tropical Invader Threatens Agroecosystems of the Southern United States1

Tropical spiderwort (more appropriately called Benghal dayflower) poses a serious threat to crop production in the southern United States. Although tropical spiderwort has been present in the United States for more than seven decades, only recently has it become a pest in agricultural fields. Identified as an isolated weed problem in 1999, tropical spiderwort became the most troublesome weed in Georgia cotton by 2003. Contributing to the significance of tropical spiderwort as a troublesome weed is the lack of control afforded by most commonly used herbicides, especially glyphosate. Vegetative growth and flower production of tropical spiderwort were optimized between 30 and 35 C, but growth was sustained over a range of 20 to 40 C. These temperatures are common throughout much of the United States during summer months. At the very least, it appears that tropical spiderwort may be able to co-occur with cotton throughout the southeastern United States. The environmental limits of tropical spiderwort have not yet been determined. However, the rapid spread through Georgia and naturalization in North Carolina, coupled with its tolerance to current management strategies and aggressive growth habit, make tropical spiderwort a significant threat to agroecosystems in the southern United States. Additional index words: Exotic invasive weed, federal noxious weed, Benghal dayflower.

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 improve control. Bromoxynil plus MSMA early POST was more effective than bromoxynil alone only on sicklepod. Pendimethalin plus fluometuron at planting followed by bromoxynil plus MSMA early POST controlled sicklepod 62%, Palmer amaranth 81%, goosegrass 83%, and all other species at least 95%. Glyphosate early POST did not adequately control many species due to sustained weed emergence. Glyphosate early POST followed by glyphosate late POST (after last effective bloom date) controlled all species except pitted morningglory and tall morningglory at least 93%. Pendimethalin plus fluometuron followed by glyphosate early POST was the most effective glyphosate system overall, and it controlled sicklepod 88%, pitted morningglory 90%, and other species at least 93%. Glyphosate late POST did not increase control in systems with pendimethalin plus fluometuron at planting followed by glyphosate early POST. Yields and net returns were similar with all herbicide/cultivar systems at two of five locations. At other locations, yields and net returns were similar with systems of pendimethalin plus fluometuron at planting followed by pyrithiobac plus MSMA early POST, pendimethalin plus fluometuron followed by bromoxynil plus MSMA early POST, and glyphosate early POST. Greatest yields and net returns were obtained with pendimethalin plus fluometuron at planting followed by glyphosate early POST. Herbicide systems did not affect fiber quality. Nomenclature: Bromoxynil, 3,5-dibromo-4-hydroxybenzonitrile; fluometuron, N,N-dimethyl-N′-[3-(trifluoromethyl)phenyl]urea; glyphosate, N-(phosphonomethyl)glycine; MSMA, monosodium methanearsonate; pendimethalin, N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine; pyrithiobac, 2-chloro-6-[(4,6-dimethoxy-2-pyrimidinyl)thio]benzoic acid; broadleaf signalgrass, Brachiaria platyphylla (Griseb.) Nash #3 BRAPP; carpetweed, Mollugo verticillata L. # MOLVE; common cocklebur, Xanthium strumarium L. # XANST; common lambsquarters, Chenopodium album L. # CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; goosegrass, Eleusine indica (L.) Gaertn. # ELEIN; jimsonweed, Datura stramonium L. # DATST; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; Palmer amaranth, Amaranthus palmeri S. Wats. # AMAPA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; 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; cotton, Gossypium hirsutum L. ‘Deltapine 51,’ ‘Paymaster 1220RR,’ ‘Stoneville BXN 47.’ Additional index words: Bromoxynil-resistant cotton, cotton yield, fiber quality, glyphosate-resistant cotton, transgenic herbicide-resistant cotton. Abbreviations: POST, postemergence; PPI, preplant incorporated; PRE, preemergence; UNR, ultra narrow row; WAP, weeks after planting.

Weed Control and Crop Response to Glufosinate Applied to ‘PHY 485 WRF’ Cotton

Abstract Field experiments were conducted in Georgia to evaluate weed control and crop tolerance with glufosinate applied to ‘PHY 485 WRF®’ cotton. This glyphosate-resistant cotton also contains a gene, used as a selectable marker, for glufosinate resistance. Three experiments were maintained weed-free and focused on crop tolerance; a fourth experiment focused on control of pitted morningglory and glyphosate-resistant Palmer amaranth. In two experiments, PHY 485 WRF cotton was visibly injured 15 and 20% or less by glufosinate ammonium salt at 430 and 860 g ae/ha, respectively, applied POST two or three times. In a third experiment, glufosinate at 550 g/ha injured cotton up to 36%. Pyrithiobac or glyphosate mixed with glufosinate did not increase injury compared to glufosinate applied alone; S-metolachlor mixed with glufosinate increased injury by six to seven percentage points. Cotton injury was not detectable 14 to 21 d after glufosinate application, and cotton yields were not reduced by glufosinate or glufosinate mixtures. A program of pendimethalin PRE, glyphosate applied POST twice, and diuron plus MSMA POST-directed controlled glyphosate-resistant Palmer amaranth only 17% late in the season. S-metolachlor included with the initial glyphosate application did not increase control, and pyrithiobac increased late-season control by only 13 percentage points. Palmer amaranth was controlled 90% or more when glufosinate replaced glyphosate in the aforementioned system. Pitted morningglory was controlled 99% by all glufosinate programs and mixtures of glyphosate plus pyrithiobac. Seed cotton yields with glufosinate-based systems were at least 3.3 times greater than yields with glyphosate-based systems because of differences in control of glyphosate-resistant Palmer amaranth. Nomenclature: Diuron; glufosinate; glyphosate; MSMA; pendimethalin; pyrithiobac sodium; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Wats AMAPA; pitted morningglory, Ipomoea lacunosa L. IPOLA; cotton, Gossypium hirsutum L.

Morningglory (Ipomoea spp.) and Large Crabgrass (Digitaria sanguinalis) Control with Glyphosate and 2,4-DB Mixtures in Glyphosate-Resistant Soybean (Glycine max)1

Abstract: Glyphosate effectively controls most weeds in glyphosate-resistant soybean. However, it is sometimes only marginally effective on Ipomoea spp. A field experiment was conducted at five locations in North Carolina to determine the effects of mixing 2,4-DB with glyphosate on Ipomoea spp. control and on soybean injury and yield. The isopropylamine salt of glyphosate at 560, 840, and 1,120 g ai/ha controlled mixtures of tall morningglory, entireleaf morningglory, and red morningglory at least 96% at two locations. Mixing the dimethylamine salt of 2,4-DB at 35 g ae/ha with glyphosate did not increase control but reduced soybean yield 6%. At two other locations, 2,4-DB increased control of tall morningglory and a mixture of entireleaf morningglory and ivyleaf morningglory 13 to 22% when mixed with glyphosate at 560 g/ha, but not when mixed with glyphosate at 840 or 1,120 g/ha. Soybean yield was reduced 31% at one location and was unaffected at the other. At the fifth location, 2,4-DB increased control of tall morningglory 25, 11, and 7% when mixed with glyphosate at 560, 840, and 1,120 g/ha, respectively. Soybean yield was increased 15%. In separate field experiments, glyphosate at 560, 840, and 1,120 g/ha controlled large crabgrass at least 99%. Mixing 2,4-DB at 35 g/ha with glyphosate did not affect control. In the greenhouse, mixing 2,4-DB at 35, 70, 140, or 280 g/ha with glyphosate at 70 to 560 g/ha did not affect large crabgrass control by glyphosate. Nomenclature: Glyphosate; 2,4-DB; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray #3 IPOHG; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. # IPOHE; large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; red morningglory, Ipomoea coccinea L. # IPOCC; tall morningglory, Ipomoea purpurea (L.) Roth # PHBPU; soybean, Glycine max (L.) Merr. ‘Hartz 5566 RR’. Additional index words: Herbicide combinations, herbicide interactions, herbicide-resistant crops. Abbreviations: WAT, weeks after treatment.

Weed Management with Glyphosate- and Glufosinate-Based Systems in PHY 485 WRF Cotton

Abstract Glyphosate-resistant (GR) Palmer amaranth has become a serious pest in parts of the Cotton Belt. Some GR cotton cultivars also contain the WideStrike™ insect resistance trait, which confers tolerance to glufosinate. Use of glufosinate-based management systems in such cultivars could be an option for managing GR Palmer amaranth. The objective of this study was to evaluate crop tolerance and weed control with glyphosate-based and glufosinate-based systems in PHY 485 WRF cotton. The North Carolina field experiment compared glyphosate and glufosinate alone and in mixtures applied twice before four- to six-leaf cotton. Additional treatments included glyphosate and glufosinate mixed with S-metolachlor or pyrithiobac applied to one- to two-leaf cotton followed by glyphosate or glufosinate alone on four- to six-leaf cotton. All treatments received a residual lay-by application. Excellent weed control was observed from all treatments on most weed species. Glyphosate was more effective than glufosinate on glyphosate-susceptible (GS) Palmer amaranth and annual grasses, while glufosinate was more effective on GR Palmer amaranth. Annual grass and GS Palmer amaranth control by glyphosate plus glufosinate was often less than control by glyphosate alone but similar to or greater than control by glufosinate alone, while mixtures were more effective than either herbicide alone on GR Palmer amaranth. Glufosinate caused minor and transient injury to the crop, but no differences in cotton yield or fiber quality were noted. This research demonstrates glufosinate can be applied early in the season to PHY 485 WRF cotton without concern for significant adverse effects on the crop. Although glufosinate is often less effective than glyphosate on GS Palmer amaranth, GR Palmer amaranth can be controlled with well-timed applications of glufosinate. Use of glufosinate in cultivars with the WideStrike trait could fill a significant void in current weed management programs for GR Palmer amaranth in cotton. Nomenclature: Diuron; glufosinate; glyphosate; MSMA; pyrithiobac; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; cotton, Gossypium hirsutum L

Palmer Amaranth (Amaranthus palmeri) Control in Soybean with Glyphosate and Conventional Herbicide Systems

Abstract Glyphosate typically controls Palmer amaranth very well. However, glyphosate-resistant (GR) biotypes of this weed are present in several southern states, requiring the development of effective alternatives to glyphosate-only management strategies. Field experiments were conducted in seven North Carolina environments to evaluate control of glyphosate-susceptible (GS) and GR Palmer amaranth in narrow-row soybean by glyphosate and conventional herbicide systems. Conventional systems included either pendimethalin or S-metolachlor applied PRE alone or mixed with flumioxazin, fomesafen, or metribuzin plus chlorimuron followed by fomesafen or no herbicide POST. S-metolachlor was more effective at controlling GR and GS Palmer amaranth than pendimethalin; flumioxazin and fomesafen were generally more effective than metribuzin plus chlorimuron. Fomesafen applied POST following PRE herbicides increased Palmer amaranth control and soybean yield compared with PRE-only herbicide systems. Glyphosate alone applied once POST controlled GS Palmer amaranth 97% late in the season. Glyphosate was more effective than fomesafen plus clethodim applied POST. Control of GS Palmer amaranth when treated with pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST was equivalent to control achieved by glyphosate applied once POST. In fields with GR Palmer amaranth, greater than 80% late-season control was obtained only with systems of pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE followed by fomesafen POST. Systems of pendimethalin or S-metolachlor plus flumioxazin, fomesafen, or metribuzin plus chlorimuron applied PRE without fomesafen POST controlled GR Palmer amaranth less than 30% late in the season. Systems of pendimethalin or S-metolachlor PRE followed by fomesafen POST controlled GR Palmer amaranth less than 60% late in the season. Nomenclature: Chlorimuron; clethodim; flumioxazin; fomesafen; glyphosate; metribuzin; pendimethalin; S-metolachlor; Palmer amaranth, Amaranthus palmeri S. Wats.; soybean, Glycine max (L.) Merr.

Weed control and yield with flumioxazin, fomesafen, and S-metolachlor systems for glufosinate-resistant cotton residual weed management.

Abstract Field studies were conducted near Clayton, Lewiston, and Rocky Mount, NC in 2005 to evaluate weed control and cotton response to preemergence treatments of pendimethalin alone or in a tank mixture with fomesafen, postemergence treatments of glufosinate applied alone or in a tank mixture with S-metolachlor, and POST-directed treatments of glufosinate in a tank mixture with flumioxazin or prometryn. Excellent weed control (> 91%) was observed where at least two applications were made in addition to glufosinate early postemergence (EPOST). A reduction in control of common lambsquarters (8%), goosegrass (20%), large crabgrass (18%), Palmer amaranth (13%), and pitted morningglory (9%) was observed when residual herbicides were not included in PRE or mid-POST programs. No differences in weed control or cotton lint yield were observed between POST-directed applications of glufosinate with flumioxazin compared to prometryn. Weed control programs containing three or more herbicide applications resulted in similar cotton lint yields at Clayton and Lewiston, and Rocky Mount showed the greatest variability with up to 590 kg/ha greater lint yield where fomesafen was included PRE compared to pendimethalin applied alone. Similarly, an increase in cotton lint yields of up to 200 kg/ha was observed where S-metolachlor was included mid-POST when compared to glufosinate applied alone, showing the importance of residual herbicides to help maintain optimal yields. Including additional modes of action with residual activity preemergence and postemergence provides a longer period of weed control, which helps maintain cotton lint yields. Nomenclature: Flumioxazin; fomesafen; glufosinate; pendimethalin; prometryn; S-metolachlor; common lambsquarters, Chenopodium album L. CHEAL; goosegrass, Eleusine indica ELEIN; large crabgrass, Digitaria sanguinalis L. DIGSA; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; pitted morningglory, Ipomoea lacunosa L. IPOLA; cotton, Gossypium hirsutum L.