John D. Nalewaja

Spray Volume, Formulation, Ammonium Sulfate, and Nozzle Effects on Glyphosate Efficacy1

Field experiments were conducted to examine the influence of spray volume on glyphosate efficacy in relation to glyphosate rate, formulation, ammonium sulfate addition, and type of sprayer nozzle. Using several grass species it was shown that glyphosate efficacy increased as spray volume decreased from 190 to 23 L/ha. To obtain equal efficacy, glyphosate rates can be reduced by at least one-third when glyphosate is applied in 23 or 47 L/ha spray volume compared with 94 or 190 L/ha. The amount of surfactant in formulated glyphosate at 35 to 140 g ae/ha rates was insufficient when glyphosate was applied in 94 or 190 L/ha spray volumes. Additional surfactant enhanced glyphosate efficacy at these rates when applied in 94 or 190 L/ha spray volume, but efficacy was still less than when applied in 23 or 47 L/ha without additional surfactant. Thus, low spray volumes maximized glyphosate efficacy primarily through high herbicide concentration in the spray deposit and reduced salts from the carrier to antagonize efficacy. Glyphosate applied in 23 L/ha spray volume with drift-reducing nozzles provided control equal to that provided by glyphosate applied with standard flat-fan nozzles. Grass control also was equal from several glyphosate formulations that contained surfactants, regardless of spray volume. Nomenclature: Glyphosate. Additional index words: Adjuvant, application methods, spray deposit, spray droplet. Abbreviations: AMS, ammonium sulfate; DAT, days after treatment; NIS, nonionic surfactant.

Glyphosate Efficacy on Velvetleaf (Abutilon theophrasti) is Affected by Stress

Abstract Greenhouse and growth chamber studies were conducted to determine the effect of drought, flooding, and cold stress on the efficacy of glyphosate for velvetleaf control, and the interaction between these stresses and adjuvant and posttreatment temperature. Glyphosate activity on velvetleaf decreased when plants were stressed with drought ≥ flooding > cold. Leaf blades of environmentally stressed velvetleaf angled downward, which increased tolerance to glyphosate but was not as great a cause of tolerance as the stress effects. Glyphosate applied to 6- and 12-leaf velvetleaf was two and eight times more phytotoxic on nonstressed compared with drought-stressed plants, respectively. Glyphosate was most effective on nonstressed plants, followed by plants recovering from stress, and least effective on plants still under stress. None of the adjuvants completely overcame the adverse affects of stress on glyphosate efficacy; use of a nonionic surfactant and ammonium sulfate resulted in a 9–13 percentage point improvement in control of stressed plants compared with glyphosate applied without an adjuvant. Low temperatures (5 or 12 C) maintained for 48 h after herbicide treatment enhanced glyphosate phytotoxicity to stressed and nonstressed velvetleaf. Glyphosate at a low rate stressed velvetleaf, which made them more tolerant to subsequent glyphosate application compared with velvetleaf not pretreated with glyphosate. Nomenclature: Glyphosate, velvetleaf, Abutilon theophrasti Medik. ABUTH

Quinclorac Efficacy as Affected by Adjuvants and Spray Carrier Water

Laboratory and greenhouse experiments were conducted to determine quinclorac efficacy as influenced by surfactants, methylated seed oil (MSO), basic pH compounds, and salts in the spray carrier water. Quinclorac efficacy for green foxtail control generally increased with an increase in linear alcohol ethoxylate (LAE) surfactant carbon-chain length and percentage of ethoxylation. With LAE surfactants, quinclorac phytotoxicity to green foxtail was nearly doubled (average from 44 to 81%) when triethanolamine (TEA) was included in the spray mixture. Combination of LAE surfactants with TEA also enhanced quinclorac absorption. Enhancement of quinclorac absorption and phytotoxicity by LAE surfactants and TEA was related to spray deposits that had close contact with the cuticle and without apparent quinclorac crystals. Sodium and calcium ions strongly antagonized quinclorac efficacy when applied with a block copolymer surfactant or MSO. Ammonium sulfate or ammonium nitrate adjuvants were more effective than urea–ammonium nitrate liquid fertilizer in overcoming antagonism from salts in spray carrier waters. These results demonstrate the potential for maximizing quinclorac efficacy by careful selection of surfactants, nitrogen fertilizer, and basic pH additives. Nomenclature: Quinclorac; green foxtail, Setaria viridis (L.) P. Beauv. #3 SETVI. Additional index words: Herbicide absorption, nitrogen fertilizer, nonionic surfactant, pH, spray deposit. Abbreviations: AMS, ammonium sulfate; AN, ammonium nitrate; DEA, diethanolamine; EO, ethylene oxide chain; LAE, linear alcohol ethoxylate; MEA, monoethanolamine; MSO, methylated seed oil; TEA, triethanolamine; UAN, urea–ammonium nitrate (28% N) liquid fertilizer.

Spray Deposits from Nicosulfuron with Salts that Affect Efficacy1

Abstract: Nicosulfuron efficacy varies with surfactant, natural salts in the spray water carrier, and added nitrogen fertilizer salts. Scanning electron micrographs (SEM) were taken of nicosulfuron spray droplet residue on large crabgrass in the greenhouse. Spray residue characteristics differ for nicosulfuron applied with surfactants alone and with specific salts. Uniform deposits with close contact to the leaf epicuticular surface generally related positively to nicosulfuron efficacy. Ammonium salt enhancement of nicosulfuron phytotoxicity when with surfactant X-77® related to a change from a ring to a uniform deposit. Spray mixtures containing Tween 20 or Atplus 300F surfactants that gave distinct dark amorphous deposits over anticlinal cell walls generally related to effective nicosulfuron treatments. Salts that were antagonistic to nicosulfuron phytotoxicity left a large amorphous deposit, including ammonium nitrate antagonism of nicosulfuron applied with Pluronic® P85 surfactant and general antagonism from sodium bicarbonate. The SEM information indicates that the effect of surfactants and salts on spray deposit characteristics influence nicosulfuron efficacy. Nomenclature: Nicosulfuron, 2-[[[[(4,6-dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-N,N-dimethyl-3-pyridinecarboxamide; large crabgrass, Digitaria sanguinalis (L.) Scop. Additional index words: Adjuvant, antagonism, ammonium nitrate, urea, sodium bicarbonate, spray technology. Abbreviations: SEM, scanning electron micrographs; 28% N, liquid fertilizer containing ammonium nitrate and urea.

Efficacy of Glyphosate Plus Bentazon or Quizalofop on Glyphosate-Resistant Canola or Corn

Greenhouse experiments were conducted to determine the effect of glyphosate on efficacy of bentazon for glyphosate-resistant (GR) canola control and of quizalofop for GR corn control. Control also was evaluated for glyphosate plus bentazon on wild buckwheat and wheat and glyphosate plus quizalofop on velvetleaf. Glyphosate plus bentazon synergistically controlled GR canola and wild buckwheat but were antagonistic for wheat control. Glyphosate plus quizalofop were additive for control of GR corn and velvetleaf. Inert ingredients in glyphosate formulations, i.e., cationic surfactant, NH4, or K, contributed to glyphosate synergism of bentazon, but the major contribution came from glyphosate itself. Efficacy of glyphosate plus bentazon on GR canola was enhanced by ammonium nitrate (AMN), ammonium sulfate (AMS), nonionic surfactant (NIS), or silicone surfactant (SiS) but was slightly decreased by methylated seed oil (MSO) or petroleum oil concentrate. AMN, AMS, NIS, and SiS partially overcame the antagonism of bentazon to glyphosate for wheat control. NIS enhanced phytotoxicity of glyphosate plus quizalofop to GR corn and velvetleaf, but the enhancement was less than by SiS or MSO to GR corn and SiS or AMS to velvetleaf. Nomenclature: Bentazon, glyphosate, quizalofop, velvetleaf, Abutilon theophrasti (L.) Medic. ABUTH, wild buckwheat, Polygonum convolvulus L. POLCO, canola, Brassica napus L, corn, Zea mays L, wheat, Triticum aestivum L

Effect of Hard Water and Ammonium Sulfate on Weak Acid Herbicide Activity

Glyphosate is a weak acid herbicide and can bind with calcium in the spray carrier. Diammonium sulfate is commonly used as an adjuvant with glyphosate to enhance phytotoxicity and overcome antagonistic effect of these cations. Most postemergence herbicides are also weak acid herbicides. Data is limited for other weak acid herbicides and the effect of diammonium sulfate in enhancing herbicide activity and overcoming antagonism. Field studies were conducted with aminopyralid, tembotrione, dicamba plus diflufenzopyr, and glufosinate to determine if (1) these weak acid herbicides are enhanced by ammonium, (2) if they are antagonized by calcium and magnesium in the spray solution, (3) if diammonium sulfate overcomes salt antagonism, and (4) if a previously published equation for the amount of ammonium sulfate required to overcome salt antagonism of glyphosate based on cation concentration in spray water correctly predicts to other weak acid herbicides. The activity of the four weak acid herbicides increased with the addition of ammonium to the spray solution, all were antagonized by calcium and magnesium, and diammonium sulfate overcame the antagonism. The previously published equation to calculate the amount the diammonium sulfate needed to overcome 500 and 1000 ppm hardness was accurate and can be used for these herbicides and possibly other weak acid herbicides.

Interference between Spring Cereals and Related to Environment and Photosynthetic Pathways

Kochia [Kochia scoparia (L.) Schrad.; syn. Bassia scoparia (L.) A.J. ScottJ is a weed that infests cereal crops in the Great Plains of the USA, often severely reducing yields. Herbicides have controlled kochia, but recently kochia has developed resistance to many herbicides. Nonherbicide alternatives are therefore needed for the integrated management of kochia. Greenhouse and growth chamber competition studies were conducted between kochia, a C 4 weed, and barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) to determine the environmental conditions that would render kochia most vulnerable to competition by a small-grain crop. Replacement-series experiments between kochia and wheat or barley were conducted under various temperature, soil moisture, and light conditions. Unlike wheat, kochia growth and photosynthesis were suppressed under cool temperatures. Barley suppressed kochia more than wheat did because of its larger canopy, despite its lower photosynthetic rates. Under high radiation conditions and warm temperatures, growth and photosynthesis were greater for kochia than wheat. Warm temperatures also increased dark respiration and reduced water use efficiency under low radiation conditions, however, thus limiting kochias competitiveness under a closed canopy. Water stress did not affect competition, although net photosynthetic rates of kochia were greater at photosynthetically active radiation (PAR) values > 400 μmol m s 1 . Growth and CO 2 exchange rates varied among four different kochia accessions, but growth of all accessions was reduced by shade. Results suggest that a leafy, cold-tolerant crop or cultivar, grown early in the season to produce necessary ground cover, should provide opportunity to suppress kochia.