Jerry W. Davis

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.

Glyphosate Hinders Purple Nutsedge (Cyperus rotundus) and Yellow Nutsedge (Cyperus esculentus) Tuber Production

Abstract The phase-out of methyl bromide requires alternative nutsedge management options in vegetable systems. Options that target tuber production, the primary means of reproduction, will be most beneficial. A study was conducted to evaluate the response of purple nutsedge and yellow nutsedge foliar growth and tuber production to a range of glyphosate rates. Glyphosate was applied at six rates between 0.41 and 2.57 kg ae ha−1 to 5-wk-old nutsedge plants with multiple shoots. The rate of glyphosate needed to reduce growth 50% (I50) was similar for purple nutsedge foliar growth (0.58 kg ha−1) and tuber biomass (0.55 kg ha−1). In contrast, I50 for yellow nutsedge foliar growth was 0.73 kg ha−1, which was greater than the I50 for tuber biomass (0.41 kg ha−1). First-order tubers, those directly attached to the initial tuber, had an I50 of 0.70 and 0.44 kg ha−1 of glyphosate for purple nutsedge and yellow nutsedge tuber biomass, respectively. For all higher-order tubers, I50 values ranged from 0.29 to 0.60 and 0.14 to 0.30 kg ha−1 of glyphosate for purple nutsedge and yellow nutsedge tuber biomass, respectively. Glyphosate at 0.74 kg ha−1 prevented fourth-order purple nutsedge and third-order yellow nutsedge tuber production (terminal tubers for yellow nutsedge). Fifth- and sixth-order purple nutsedge tuber production was eliminated by the lowest tested rate of glyphosate (0.41 kg ha−1). Effective nutsedge management options will require consistent control between spring and autumn crops. Glyphosate is economical, poses no herbicide carryover issues to vegetables, and minimizes nutsedge tuber production; therefore, it is a suitable candidate to manage nutsedges. Nomenclature: Glyphosate; purple nutsedge, Cyperus rotundus L. CYPRO; yellow nutsedge, Cyperus esculentus L. CYPES.

Influence of Sulfentrazone Rate and Application Method on Peanut Weed Control

Field studies were conducted in 2000 and 2001 in Plains, GA, to determine peanut and weed response to the residual herbicides sulfentrazone, imazapic, diclosulam, and flumioxazin. Herbicide treatments included sulfentrazone applied preemergence (PRE) or preplant incorporated (PPI) at 112, 168, 224, and 280 g ai/ha, imazapic postemergence (POST) at 71 g ai/ha, diclosulam PPI at 26 g ai/ha, and flumioxazin PRE at 88 g ai/ha. Peanut exhibited early-season injury from all herbicide treatments, ranging from 0 to 10% for sulfentrazone PPI or PRE, 10% for imazapic, 3 to 23% for flumioxazin, and 1 to 7% for diclosulam. Yields were similar for sulfentrazone PPI- or PRE-treated and flumioxazin-, imazapic-, and diclosulam-treated peanut. Yellow nutsedge control was 83% or greater with all rates of sulfentrazone PRE or PPI, 83 and 90% with diclosulam, and 96 and 99% with imazapic, respectively. Flumioxazin did not control yellow nutsedge or wild poinsettia. Tall morningglory control was 82% or greater with imazapic, diclosulam, flumioxazin, and sulfentrazone PPI or PRE at 168 g/ha or higher. Florida beggarweed control was 88% or greater with diclosulam, flumioxazin, and sulfentrazone PRE at 224 and 280 g/ha. Overall, peanut tolerance to sulfentrazone at 112 to 280 g/ha PPI and PRE was high and yield was equivalent to the currently registered peanut residual herbicides. Nomenclature: Diclosulam; flumioxazin; imazapic; sulfentrazone; Florida beggarweed, Desmodium tortuosum (Sec) L., #3 DEDTO; tall morningglory, Ipomoea purpurea (L.) Roth # PHBPU; wild poinsettia, Euphorbia heterophylla L. # EPHHL; Yellow nutsedge, Cyperus esculentus L. # CYPES; peanut, Arachis hypogaea L. Additional index words: Peanut injury, peanut yield, susceptibility to herbicides. Abbreviations: ALS, acetolactate synthase; POST, postemergence; PPI, preplant incorporated; PRE, preemergence; VE, vegetative emergence.

Cotton (Gossypium hirsutum) Response to Simulated Imazapic Residues

Field trials were conducted in 2000, 2001, and 2002 at Tifton, GA, and Plains, GA, to evaluate the effects of simulated imazapic residues on cotton growth and yield. Preemergence applications of imazapic at 1, 2, 5, 9, 18, and 36 g ai/ha were made to four different cotton varieties (two at each location) and included a nontreated control. There were no differences in cotton variety response to imazapic. Each cotton variety responded to imazapic in a similar manner. Analysis of cotton yield as a percentage relative to the nontreated control indicated no difference in variety for location, so data for varieties were combined. At Tifton, cotton injury was exponentially related to imazapic rate with a maximum injury of 44% for 35 g/ha. Seed cotton yields at this location were reduced 0, 6, 6, 14, 16, 34, and 61% at 1, 2, 5, 9, 18, and 36 g/ha, respectively. For Plains, cotton exhibited extreme sensitivity with injury exceeding 70% for imazapic at 5 g/ha and greater than 95% for 18 g/ha. Seed cotton yields at this location were reduced 60% or more from imazapic rates of 5 g/ha and greater. These results indicated that soil type is a key factor in the response of cotton to imazapic. Nomenclature: Imazapic; cotton, Gossypium hirsutum L. Additional index words: Carryover injury, herbicide persistence, residual herbicide, simulated carryover. Abbreviations: CEC, cation-exchange capacity; DAT, days after treatment.

Combinations of Corn Gluten Meal, Clove Oil, and Sweep Cultivation are Ineffective for Weed Control in Organic Peanut Production

Abstract Weed control in organic peanut is difficult and lack of residual weed control complicates weed management efforts. Weed management systems using corn gluten meal in combination with clove oil and sweep cultivation were evaluated in a series of irrigated field trials. Corn gluten meal applied in a 30 cm band over the row at PRE, sequentially at PRE+2 wk after emergence, and PRE+2wk+4wk did not adequately control annual grasses and smallflower morningglory. Similarly, a banded application of clove oil applied POST did not adequately control weeds. The only treatment that improved overall weed control was sweep cultivation. Peanut yields were not measured in 2006 due to heavy baseline weed densities and overall poor weed control. Peanut yields were measured in 2007 and were not affected by any weed control treatment due to poor efficacy. While sweep cultivation improved weed control, weeds were controlled only in the row middles and surviving weeds in-row reduced peanut yield. Even when used in combination with sweep cultivation, corn gluten meal and clove oil were ineffective and offer little potential in a weed management system for organic peanut production. Nomenclature: Clove oil; corn gluten meal; crowfootgrass, Dactyloctenium aegyptium (L.) Willd.; goosegrass, Eleusine indica (L.) Gaertn.; smallflower morningglory, Jacquemontia tamnifolia (L.) Griseb.; southern crabgrass, Digitaria ciliaris (Retz.) Koel.; Texas millet, Urochloa texana (Buckl.) R. Webster; peanut, Arachis hypogaea L. Resumen El control de malezas en maní orgánico es difícil, y la carencia de control residual complica aún más los esfuerzos para el manejo de malezas. Se evaluaron sistemas de manejo de malezas incluyendo harina de gluten de maíz en combinación con aceite de trébol y con cultivadores de barrido en una serie de ensayos de campo bajo riego. El gluten de maíz aplicado en una banda de 30 cm sobre la línea de siembra en PRE, secuencialmente a PRE+2 semanas (wk) después de la siembra, y PRE+2wk+4wk no controló adecuadamente malezas gramíneas anuales ni Jacquemontia tamnifolia. Similarmente, una aplicación en banda de aceite de trébol realizada POST no controló adecuadamente las malezas. El único tratamiento que mejoró en forma general el control de malezas fue el cultivo de barrido. Los rendimientos del maní no fueron medidos en 2006 debido a las altas densidades de malezas iniciales y al pobre control de malezas. Los rendimientos del maní se determinaron en 2007 y no fueron afectados por ninguno de los tratamientos de malezas debido a su baja eficacia. Mientras que el cultivo de barrido mejoró el control, las malezas fueron controladas solamente en las zonas centrales entre las líneas de siembra y las malezas que sobrevivieron sobre las líneas de siembra redujeron el rendimiento del maní. Inclusive al usarse en combinación con cultivo de barrido, el gluten de maíz y el aceite de trébol no fueron efectivos y ofrecen muy poco potencial de control en sistemas de manejo de malezas para la producción orgánica de maní.

Texas Panicum (Panicum texanum) Control in Irrigated Field Corn (Zea mays) with Foramsulfuron, Glyphosate, Nicosulfuron, and Pendimethalin'

Texas panicum is considered to be the most troublesome weed of field corn in the Southeast. Field trials were conducted in Georgia in 2003 and 2004 to compare pendimethalin, nicosulfuron, foramsulfuron, and glyphosate for Texas panicum control in irrigated field corn and to determine which herbicide provided the greatest economic returns. Pendimethalin applied early POST (EPOST), 10 to 12 d after planting (DAP), controlled Texas panicum less than 35% late in the season and resulted in reduced corn yield and net returns in 2004. Glyphosate applied sequentially POST at 21 to 24 DAP and again late POST (LPOST) at 35 to 38 DAP controlled Texas panicum 82 to 94% late in the season compared with 43 to 80% control by nicosulfuron and foramsulfuron applied POST. Texas panicum control, corn yield, and net returns were similar with glyphosate applied POST and LPOST at 0.53 or 1.1 kg ai/ha. Glyphosate applied POST and LPOST was more effective than glyphosate POST, but net returns were greater only in 2004. Nomenclature: Atrazine, foramsulfuron, nicosulfuron, pendimethalin, potassium salt of glyphosate, Texas panicum, Panicum texanum L. #3 PANTE, corn, Zea mays L. ‘DeKalb 67-60RR’. Additional index words: Economics, herbicide-resistant crops, postemergence. Abbreviations: AMS, ammonium sulfate; COC, crop oil concentrate; GR, glyphosate resistant; MSO, methylated seed oil.

Implements and Cultivation Frequency to Improve In-Row Weed Control in Organic Peanut Production

Abstract Weed control in organic peanut production is difficult and costly. Sweep cultivation in the row middles is effective, but weeds remain in the crop row, causing yield loss. Research trials were conducted in Ty Ty, GA to evaluate implements and frequencies of cultivation to improve in-row weed control in organic peanut. Implements were a tine weeder and power takeoff-powered brush hoe that targeted weeds present in the row. Frequencies of cultivation were at vegetative emergence of peanut (VE), 1 wk after VE (1wk), 2 wk after VE (2wk), sequential combinations of VE/1wk, VE/2wk, and VE/1wk/2wk. All plots were cultivated with a sweep cultivator to control weeds in row middles. The tine weeder tended to be easier to operate and performed more consistently than the brush hoe. Both implements performed best when initial cultivation was at VE. Delaying the initial cultivation reduced overall effectiveness. Plots with the best in-row weed control were hand-weeded once to control escapes and harvested for peanut yield. The best overall combination of weed control, minimal use of salvage hand-weeding, and maximum peanut yield resulted from sequential cultivation at VE/1wk using either the tine weeder or brush hoe, row middle sweep cultivation, and preharvest mowing. Nomenclature: Smallflower morningglory, Jacquemontia tamnifolia (L.) Griseb.; southern crabgrass, Digitaria ciliaris (Retz.) Koel.; Texas millet, Urochloa texana (Buckl.) R. Webster; peanut, Arachis hypogaea L.

Cultural Practices to Improve In-Row Weed Control with Cultivation in Organic Peanut Production

Abstract Cultivation is a proven means of weed control in organic peanut. However, weeds present in-row often escape control. Research trials were conducted in Ty Ty, GA to modify cultural practices to help suppress weed emergence in-row. Modified cultural practices were three row pattern/seeding rate combinations; twin rows (four rows on a seedbed) seeded at the recommended (1X) seeding rate that produced 13 seed m−1 in each row, twin rows seeded at the 2X seeding rate that produced 23 seed m−1 in each row, and wide rows (two rows on seedbed) seeded at the recommended seeding rate that produced 23 seed m−1. Four cultivation regimes were evaluated; cultivation with a tine weeder at weekly intervals for 6 wk, cultivation with a tine weeder at weekly intervals for 8 wk, scouring with a brush hoe at vegetative emergence followed by the tine weeder for 6 wk, and a noncultivated control. There were no interactions between row pattern/seeding rates and cultivation regimes for any parameter measured. There was inconsistent response of weed control and peanut yield to row pattern/seeding rates. Weed control and peanut yields were similar with tine weeding for 6 wk, 8 wk, or with the brush hoe followed by the tine weeder. Weed management in organic peanut was not improved by altering peanut cultural practices that facilitate quicker canopy closure, and the use of narrow row patterns should not be based on attempts to improve weed control in organic peanut. Narrow row patterns provide other benefits to organic peanut production and those attributes should influence decisions on the choice of row pattern, not weed control. Nomenclature: Crowfootgrass, Dactyloctenium aegyptium (L.) Willd.; Florida beggarweed, Desmodium tortuosum (Sw.) DC.; pitted morningglory, Ipomoea lacunosa L.; sicklepod, Senna obtusifolia (L.) H.S. Irwin & Barneby; smallflower morningglory, Jacquemontia tamnifolia (L.) Griseb.; southern crabgrass, Digitaria ciliaris (Retz.) Koel.; peanut, Arachis hypogaea L. Resumen Cultivar es un método comprobado para el control de malezas en maní orgánico. Sin embargo, las malezas presentes en la línea de siembra a menudo escapan al control. Se realizaron estudios en Ty Ty, GA para modificar las prácticas culturales para ayudar a suprimir la emergencia de malezas en la línea de siembra. Las prácticas culturales modificadas fueron tres combinaciones de patrones y de densidades de siembra; líneas gemelas (cuatro líneas en cada cama de siembra) sembradas a la densidad recomendada (1X) que produjo 13 semillas m−1 en cada línea, líneas gemelas sembradas a una densidad 2X que produjo 23 semillas m−1 en cada línea, y líneas amplias (2 líneas por cama de siembra) sembradas a la densidad recomendada lo que produjo 23 semillas m−1. Se evaluaron cuatro regímenes de cultivo; cultivo semanal con un rastrillo de púas durante 6 semanas, cultivo semanal con rastrillo de púas durante 8 semanas, barrido con azadón de cepillo al momento de emergencia vegetativa seguido por un cultivo con el rastrillo de púas durante 6 semanas, y un tratamiento testigo sin cultivo. No hubo interacciones entre el patrón/densidad de siembra y el régimen de cultivo para ninguno de los parámetros medidos. La respuesta al patrón/densidad de siembra en control de malezas y rendimiento del maní fue inconsistente. El control de malezas y los rendimientos del maní fueron similares con el cultivo con el rastrillo de púas durante 6 y 8 semanas, o con el azadón de cepillo seguido por el rastrillo de púas. El manejo de malezas en maní orgánico no mejoró al alterar las prácticas culturales que facilitaron un cierre del dosel más rápido. El uso de patrones angostos de líneas de siembra no debería enfocarse en intentos para mejorar el control de malezas en maní orgánico. Patrones angostos de líneas de siembra brindan otros beneficios en la producción de maní orgánico y deberían ser estos otros atributos, y no el control de malezas, los que influencien las decisiones sobre la escogencia de patrones de siembra.

Imidazolinone-Resistant Sunflower Tolerance to Imazapic

Abstract Field trials were conducted in Georgia in 2007 to 2008 to evaluate the tolerance of three imidazolinone-resistant sunflower cultivars to POST applications of imazapic. There was no interaction between sunflower cultivar and herbicide treatment. When averaged over sunflower cultivars, imazapic, at 70 and 140 g ai/ha and applied at 30 d after planting, had no effect on sunflower above-ground biomass, plant height, seed-heads per meter row, and seed-head weights. Sunflower response to imazapic was similar to that of imazamox. Imazapic could be used in imidazolinone-resistant sunflower production systems without risk of unacceptable crop injury. Nomenclature: Imazamox; imazapic; sunflower, Helianthus annuus L. ‘Dekalb 3880CL’, ‘Mycogen 8H419CL’, and ‘Mycogen 8N386CL’

Cotton Stage of Growth Determines Sensitivity to 2,4-D

The anticipated release of EnlistTM cotton, corn, and soybean cultivars likely will increase the use of 2,4-D, raising concerns over potential injury to susceptible cotton. An experiment was conducted at 12 locations over 2013 and 2014 to determine the impact of 2,4-D at rates simulating drift (2 g ae ha−1) and tank contamination (40 g ae ha−1) on cotton during six different growth stages. Growth stages at application included four leaf (4-lf), nine leaf (9-lf), first bloom (FB), FB + 2 wk, FB + 4 wk, and FB + 6 wk. Locations were grouped according to percent yield loss compared to the nontreated check (NTC), with group I having the least yield loss and group III having the most. Epinasty from 2,4-D was more pronounced with applications during vegetative growth stages. Importantly, yield loss did not correlate with visual symptomology, but more closely followed effects on boll number. The contamination rate at 9-lf, FB, or FB + 2 wk had the greatest effect across locations, reducing the number of bolls per plant when compared to the NTC, with no effect when applied at FB + 4 wk or later. A reduction of boll number was not detectable with the drift rate except in group III when applied at the FB stage. Yield was influenced by 2,4-D rate and stage of cotton growth. Over all locations, loss in yield of greater than 20% occurred at 5 of 12 locations when the drift rate was applied between 4-lf and FB + 2 wk (highest impact at FB). For the contamination rate, yield loss was observed at all 12 locations; averaged over these locations yield loss ranged from 7 to 66% across all growth stages. Results suggest the greatest yield impact from 2,4-D occurs between 9-lf and FB + 2 wk, and the level of impact is influenced by 2,4-D rate, crop growth stage, and environmental conditions. Nomenclature: 2,4-D; cotton, Gossypium hirsutum L. La anticipada liberación de cultivares Enlist™ de algodón, maíz, y soja probablemente incrementará el uso de 2,4-D, aumentando así la preocupación del daño potencial en algodón susceptible. Se realizó un experimento en 12 localidades durante 2013 y 2014 para determinar el impacto de 2,4-D a dosis de deriva simulada (2 g ae ha−1) y de contaminación en tanque (40 g ae ha−1) sobre algodón durante seis estadios de crecimiento diferente. Los estadios de crecimiento al momento de aplicación incluyeron cuatro hojas (4-lf), nueve hojas (9-lf), primer brote florar (FB), FB + 2 semanas (wk), FB + 4 wk, y FB + 6 wk. Las localidades fueron agrupadas según el porcentaje de pérdida de rendimiento al compararse con el testigo sin tratamiento (NTC), teniendo el grupo I la menor pérdida de rendimiento y el grupo III la mayor. La epinastia producto de 2,4-D fue más pronunciada con aplicaciones durante los estadios de crecimiento vegetativo. Importantemente, la pérdida en el rendimiento no correlacionó con la sintomatología visual, pero siguió de cerca los efectos en el número de frutos. La dosis de contaminación a 9-lf, FB, o FB + 2 wk tuvo el mayor efecto en todas las localidades, reduciendo el número de frutos por planta cuando se comparó con el NTC, pero sin tener efecto cuando se aplicó en FB + 4 wk o después. La reducción en el número de frutos no fue detectable con la dosis de deriva excepto en el grupo III cuando se aplicó en el estadio FB. El rendimiento fue influenciado por la dosis de 2,4-D y el estadio de crecimiento del algodón. Considerando todas las localidades, las pérdidas de rendimiento mayor a 20% ocurrieron en 5 de 12 localidades cuando se aplicó la dosis de deriva entre 4-lf y FB + 2 wk (mayor impacto a FB). Para la dosis de contaminación, la pérdida en rendimiento fue observada en todas las 12 localidades. Al promediar todas las localidades, la pérdida de rendimiento varió entre 7 y 66% entre todos los estadios de crecimiento. Los resultados sugieren que el mayor impacto en el rendimiento causado por 2,4-D ocurre entre 9-lf y FB + 2 wk, y el nivel de impacto es influenciado por la dosis de 2,4-D, el estadio de crecimiento, y las condiciones ambientales.