Charles E. Snipes

Weed Control from Herbicide Combinations with Three Formulations of Glyphosate1

Abstract: Greenhouse studies were conducted to evaluate weed control from various formulations of glyphosate alone and in combination with postemergence herbicides. Tank mixtures did not increase barnyardgrass control 2 wk after treatment (WAT) when compared with glyphosate alone; however, tank mixtures did reduce barnyardgrass fresh weight 4 WAT when compared with glyphosate alone in several instances. Antagonism was observed when chlorimuron was combined with all formulations of glyphosate 4 WAT, but control was not reduced when compared with glyphosate alone. Selective herbicides added to glyphosate had an additive or antagonistic effect on prickly sida fresh-weight reductions. Antagonism of pitted morningglory fresh-weight reductions occurred when glyphosate was combined with all herbicides except acifluorfen, which had an additive effect. Fomesafen or lactofen effectively controlled hemp sesbania 2 WAT without the addition of glyphosate. Acifluorfen and chlorimuron combined with glyphosate Cheminova, Monsanto, or Zeneca reduced hemp sesbania fresh weight nearly twofold more than glyphosate alone. Nomenclature: Acifluorfen; chlorimuron; fomesafen; glyphosate; lactofen; barnyardgrass, Echinochloa crus-galli (L.) Beauv. #3 ECHCG; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex. A. W. Hill # SEBEX; pitted morningglory, Ipomea lacunosa L. # IPOLA; prickly sida, Sida spinosa L. # SIDSP. Additional index words: Additive effects, antagonism, reduced rates, synergism. Abbreviations: ALS, acetolactase synthase; C, Cheminova; LSD, least significance difference; M, Monsanto; POST, postemergence; WAT, weeks after treatment; Z, Zeneca.

Comparison of Glyphosate-Resistant and Nontransgenic Soybean (Glycine max) Herbicide Systems1

Studies were conducted in 1997 and 1998 to evaluate the efficacy and economics of glyphosate-resistant and nontransgenic soybean systems. The three highest yielding glyphosate-resistant and nontransgenic soybean cultivars were chosen each year for three Mississippi locations based on Mississippi Soybean Variety Trials. Treatments within each cultivar/herbicide system included nontreated, low input (one-half of the labeled rate), medium input (labeled rate), and high input level (labeled rate plus an additional postemergence application). In 1997, all systems controlled hemp sesbania by more than 80% but nontransgenic systems controlled hemp sesbania more than the glyphosate-resistant systems in most instances in 1998. High input levels usually controlled pitted morningglory more than low or medium inputs in 1997. In 1998, both systems controlled pitted morningglory by 90% or more at Shelby; however, at other locations control was less than 85%. Soybean yield in 1997 at Shelby was more with the glyphosate-resistant system than with the nontransgenic systems at medium and high input levels, primarily because of early-season injury to a metribuzin-sensitive cultivar in the nontransgenic system. In 1998, soybean yield at Shelby was more with the nontransgenic system than the glyphosate-resistant system, regardless of input level, due to poor late-season hemp sesbania control with glyphosate. Net returns were often more with the glyphosate-resistant system at Shelby in 1997. Within the glyphosate-resistant system, there were no differences in net return between input levels. Within the nontransgenic system, low input level net returns were higher compared to medium and high input levels due to higher soybean yield and less herbicide cost. At Brooksville, using high input levels, the glyphosate-resistant systems net returns were

Influence of Row Spacing and Residual Herbicides on Weed Control in Glufosinate-Resistant Soybean (Glycine max)1

55.00/ha more than the nontransgenic system. Net returns were higher with the nontransgenic system compared to the glyphosate-resistant system at Shelby in 1998, regardless of input level. Nomenclature: Glyphosate; metribuzin; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex. A. W. Hill #3 SEBEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; soybean, Glycine max (L.) Merr. Additional index words: Economics, reduced rates, glyphosate-resistant soybean systems. Abbreviations: MSBG, Mississippi state budget generator; POST, postemergence.

Influence of Malathion Timing on Cotton (Gossypium hirsutum) Response to Pyrithiobac

Field experiments were conducted in 1998 and 1999 at two locations in Mississippi to determine weed control efficacy of postemergence soybean herbicides alone or following pendimethalin + imazaquin preemergence in glufosinate-tolerant soybean planted in 38- or 76-cm rows. Glufosinate applications controlled pitted morningglory better than conventional herbicide treatments, regardless of row spacing. Pendimethalin + imazaquin did not increase the efficacy of glufosinate on pitted morningglory. Pitted morningglory control was increased in narrow rows when compared to wide rows with all treatments. Sicklepod control ranged from 90 to 100% in narrow rows with glufosinate, regardless of rate. Residual herbicides alone controlled sicklepod 54%, regardless of row spacing. With grass species, two applications of 420 g ai/ha glufosinate controlled weeds 82 to 100%. Residual herbicides followed by 420 g/ha glufosinate controlled grass species 80% or more, regardless of row spacing. Hemp sesbania control ranged from 80 to 92% in 76- and 38-cm rows with one application of 560 g/ha glufosinate. Glufosinate at 420 g/ha used as sequential applications controlled hemp sesbania better than the conventional treatment in 76-cm rows. Residual herbicides in combination with glufosinate did not increase hemp sesbania control. There were no differences in yield due to row spacing at Stoneville in either year due to extremely dry growing conditions during pod set. At Starkville, two applications of 420 g/ha glufosinate resulted in higher yields than pendimethalin + imazaquin followed by 420 g/ha glufosinate in both years. Pendimethalin + imazaquin followed by 420 g/ha glufosinate increased yield in narrow rows compared to wide rows at Starkville in 1998. Nomenclature: Glufosinate; imazaquin; pendimethalin; hemp sesbania, Sesbania exaltata (Raf.) Rydb. ex. A. W. Hill #3 SEBEX; pitted morningglory, Ipomoea lacunosa L. # IPOLA; sicklepod, Senna obtusifolia (L.) Irwin and Barnaby # CASOB; soybean, Glycine max (L.) Merr. Additional index words: Glufosinate-tolerant soybean. Abbreviations: POST, postemergence; POST-3, POST application 3 wk after planting; POST-5, POST application 5 wk after planting; PRE, preemergence; WAP, weeks after planting.

Imazaquin mobility and persistence in a Sharkey clay soil as influenced by tillage systems

Experiments were conducted from 1994 through 1996 to determine the effect timing of malathion applications have on cotton response relative to postemergence applications of pyrithiobac. Treatments were initiated approximately 1 mo after planting (four to six leaves) and included pyrithiobac at 70 g ae/ha applied alone or in combination or with malathion applied at 1 or 3 d before and at 1 or 3 d after pyrithiobac. At 7 d after treatment (DAT), pyrithiobac plus malathion applied in combination caused 34, 28, and 21% cotton injury in 1994, 1995, and 1996, respectively. Malathion applied 1 d before pyrithiobac resulted in 18% injury in 1994, 11% in 1995, and 15% injury in 1996, whereas malathion applied 1 d after pyrithiobac injured cotton 15 and 11% in 1994 and 1996, respectively. Injury of cotton subjected to malathion applied 3 d before pyrithiobac was not different from pyrithiobac applied alone. When malathion was applied 3 d after pyrithiobac, injury occurred only in 1995. At 14 and 21 DAT, the combination of pyrithiobac plus malathion caused the most cotton injury. Seed cotton yield was not adversely affected by any treatment when compared with pyrithiobac applied alone. Nomenclature: Malathion, O,O-dimethyl phosphorodithioate of diethyl mercaptosuccinate; pyrithiobac; cotton, Gossypium hirsutum L. ‘DES 119’, ‘DPL 50’. Additional index words: Crop injury, insecticide, interaction, pesticide, timing. Abbreviations: DAT, days after treatment; POST, postemergence; PRE, preemergence.

Influence of Cultivation Timing on Pyrithiobac Performance in Cotton (Gossypium hirsutum) 1

Abstract Field studies were conducted at Delta Research and Extension Center, Stoneville, MS, in 1996, 1997, and 1998 to assess the effect of tillage systems (conventional tillage and subsoiling) on the environmental fate of imazaquin in a Sharkey clay soil. Imazaquin was applied preemergence at 140 g ai ha−1. Subsoiling in the fall did not affect imazaquin dissipation, total volume of runoff, imazaquin concentration in runoff, or imazaquin concentration in soil, as determined by chemical extraction. A corn root bioassay revealed no differences due to tillage systems in plant-available imazaquin in soil. Imazaquin concentration measured by chemical extraction or bioassay diminished over time, with a half-life ranging from 8 to 25 d. A field bioassay utilizing cotton and corn was conducted in 1997 and 1998 using plots that had received imazaquin the previous year. In 1997, 2 wk after planting, cotton and corn injury ranged from 3 to 15%, whereas no injury was observed in 1998. Injury symptoms declined over time, with no injury 5 wk after planting in either year. Although early-season cotton stunting and slight discoloration of corn was apparent in 1997, imazaquin residues did not affect subsequent vegetative and reproductive growing patterns of cotton or corn. In 1998, corn and cotton height were significantly greater in subsoiled plots compared to conventional tillage. Nomenclature: Corn, Zea mays L. ‘HyPerformer HS 9773’, ‘Pioneer 3167’; cotton, Gossypium hirsutum L. ‘DPL 50’; soybean, Glycine max (L.) Merr. ‘DPL 3589’.

Competition of common cocklebur (Xanthium pensylvanicum) with cotton (Gossypium hirsutum).

Abstract: Pitted morningglory control with norflurazon or fluometuron preemergence (PRE), each at 1.12 kg/ha, was 72% or less 14 d after a postemergence (POST) application of pyrithiobac in 1993 and 1994. Pyrithiobac POST at 70 g/ha following fluometuron or norflurazon PRE cultivated at any timing controlled pitted morningglory at least 76%. Pyrithiobac POST controlled common cocklebur equal to the weed-free in 1993 at 14 d after the POST application. In 1994, when rated 28 d after pyrithiobac POST, cultivation 3 d before a single application of pyrithiobac controlled less common cocklebur than any other herbicide treatment containing pyrithiobac. At 28 d after pyrithiobac POST, no treatment controlled common cocklebur as well as the weed-free. In 1993 and 1994, seed cotton yield was equal to the weed-free when pyrithiobac followed fluometuron PRE cultivated at any timing or a single application of pyrithiobac cultivated prior to 7 d after POST application. Nomenclature: Fluometuron, N,N′-dimethyl-N′-[3-(trifluoromethyl)phenyl]urea; norflurazon, 4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-pyridazinone; pyrithiobac, 2-chloro-6-[(4,6-dimethoxy-2-pyrimidinyl)thio] benzoic acid, sodium salt; common cocklebur, Xanthium strumarium L. #3 XANST; pitted morningglory, Ipomoea lacunosa L. # IPOLA; cotton, Gossypium hirsutum L. ‘DES-119’. Additional index words: Fluometuron, norflurazon, Ipomoea lacunosa, Xanthium strumarium, IPOLA, XANST. Abbreviations: ALS, acetolactate synthase (EC 4.1.3.18); DAPA, days after POST application; DBPA, days before POST application; DREC, Delta Research and Extension Center; fb, followed by; IAPA, immediately after POST application; POST, postemergence; PRE, preemergence; PSRC, Plant Science Research Center; SWSRU, Southern Weed Science Research Unit Farm.