Jared Whitaker

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.

Water deficit in field-grown Gossypium hirsutum primarily limits net photosynthesis by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis

Much effort has been expended to improve irrigation efficiency and drought tolerance of agronomic crops; however, a clear understanding of the physiological mechanisms that interact to decrease source strength and drive yield loss has not been attained. To elucidate the underlying mechanisms contributing to inhibition of net carbon assimilation under drought stress, three cultivars of Gossypium hirsutum were grown in the field under contrasting irrigation regimes during the 2012 and 2013 growing season near Camilla, Georgia, USA. Physiological measurements were conducted on three sample dates during each growing season (providing a broad range of plant water status) and included, predawn and midday leaf water potential (ΨPD and ΨMD), gross and net photosynthesis, dark respiration, photorespiration, and chlorophyll a fluorescence. End-of-season lint yield was also determined. ΨPD ranged from -0.31 to -0.95MPa, and ΨMD ranged from -1.02 to -2.67MPa, depending upon irrigation regime and sample date. G. hirsutum responded to water deficit by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis, thereby limiting PN and decreasing lint yield (lint yield declines observed during the 2012 growing season only). Conversely, even extreme water deficit, causing a 54% decline in PN, did not negatively affect actual quantum yield, maximum quantum yield, or photosynthetic electron transport. It is concluded that PN is primarily limited in drought-stressed G. hirsutum by decreased stomatal conductance, along with increases in respiratory and photorespiratory carbon losses, not inhibition or down-regulation of electron transport through photosystem II. It is further concluded that ΨPD is a reliable indicator of drought stress and the need for irrigation in field-grown cotton.

Inheritance of Evolved Glyphosate Resistance in a North Carolina Palmer Amaranth (Amaranthus palmeri) Biotype

Inheritance of glyphosate resistance in a Palmer amaranth biotype from North Carolina was studied. Glyphosate rates for 50% survival of glyphosate-resistant (GR) and glyphosate-susceptible (GS) biotypes were 1288 and 58 g ha−1, respectively. These values for F1 progenies obtained from reciprocal crosses (GR×GS and GS×GR were 794 and 501 g ha−1, respectively. Dose response of F1 progenies indicated that resistance was not fully dominant over susceptibility. Lack of significant differences between dose responses for reciprocal F1 families suggested that genetic control of glyphosate resistance was governed by nuclear genome. Analysis of F1 backcross (BC1F1) families showed that 10 and 8 BC1F1 families out of 15 fitted monogenic inheritance at 2000 and 3000 g ha−1 glyphosate, respectively. These results indicate that inheritance of glyphosate resistance in this biotype is incompletely dominant, nuclear inherited, and might not be consistent with a single gene mechanism of inheritance. Relative 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) copy number varied from 22 to 63 across 10 individuals from resistant biotype. This suggested that variable EPSPS copy number in the parents might be influential in determining if inheritance of glyphosate resistance is monogenic or polygenic in this biotype.

Distribution of Glyphosate- and Thifensulfuron-Resistant Palmer Amaranth (Amaranthus palmeri) in North Carolina

Glyphosate resistance in Palmer amaranth was first confirmed in North Carolina in 2005. A survey that year indicated 17 and 18% of 290 populations sampled were resistant to glyphosate and thifensulfuron, respectively. During the fall of 2010, 274 predetermined sites in North Carolina were surveyed to determine distribution of Palmer amaranth and to determine if and where resistance to fomesafen, glufosinate, glyphosate, and thifensulfuron occurred. Palmer amaranth was present at 134 sites. When mortality for each biotype was compared to a known susceptible biotype for each herbicide within a rate, 93 and 36% of biotypes were controlled less by glyphosate (840 g ae ha−1) and thifensulfuron (70 g ai ha−1), respectively. This approach may have underestimated resistance for segregating populations due to lack of homogeneity of the herbicide resistance trait and its contribution to error variance. When mortality and visible control were combined, 98% and 97% of the populations were resistant to glyphosate and the ALS inhibitor thifensulfuron, respectively, and 95% of the populations expressed multiple resistance to both herbicides. This study confirms that Palmer amaranth is commonly found across the major row crop production regions of North Carolina and that resistance to glyphosate and ALS-inhibiting herbicides is nearly universal. No resistance to fomesafen or glufosinate was observed.

Leaf ontogeny strongly influences photosynthetic tolerance to drought and high temperature in Gossypium hirsutum

Temperature and drought are major abiotic limitations to crop productivity worldwide. While abiotic stress physiology research has focused primarily on fully expanded leaves, no studies have investigated photosynthetic tolerance to concurrent drought and high temperature during leaf ontogeny. To address this, Gossypium hirsutum plants were exposed to five irrigation treatments, and two different leaf stages were sampled on three dates during an abnormally dry summer. Early in the growing season, ontogenic PSII heat tolerance differences were observed. Photosystem II was more thermotolerant in young leaves than mature leaves. Later in the growing season, no decline in young leaf net photosynthesis (PN) was observed as leaf temperature increased from 31 to 37°C, as average midday leaf water potential (ΨMD) declined from -1.25 to -2.03MPa. In contrast, mature leaf PN declined 66% under the same conditions. Stomatal conductance (gs) accounted for 84-98% of variability in leaf temperature, and gs was strongly associated with ΨMD in mature leaves but not in young leaves. We conclude that young leaves are more photosynthetically tolerant to heat and drought than mature leaves. Elucidating the mechanisms causing these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future.

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.

Response of Herbicide-Resistant Palmer Amaranth (Amaranthus palmeri) Accessions to Drought Stress

Palmer amaranth is a very problematic weed in several crops in the southern USA due to its competitive ability and resistance to herbicides representing different mechanisms of action. Variation in growth and subsequent interference of North Carolina Palmer amaranth accessions has not been examined. A greenhouse experiment determined response of 15 North Carolina Palmer amaranth accessions to drought stress beginning 15 days after seedling emergence (DAE) for a duration of 3, 5, 7, and 9 days. Following exposure to drought, plants were grown under optimal moisture conditions until harvest at 30 DAE. Five accessions each of glyphosate-resistant (GR), acetolactate synthase inhibitor-resistant (ALSR), and acetolactate synthase inhibitor-susceptible and glyphosate-susceptible (ALSS/GS) were compared. Variation in response to drought stress, based on height and dry weight reduction relative to nonstressed controls, was noted among accessions. Stress for 3 or more days affected height and dry weight. Height and dry weight of GR and ALSR accession groups were reduced less by drought than the ALSS/GS accession group. Results suggest a possible relationship between herbicide resistance and ability of Palmer amaranth to withstand drought stress and thus a possible competitive advantage for resistant accessions under limited moisture availability.

Interference of Selected Palmer Amaranth (Amaranthuspalmeri) Biotypes in Soybean (Glycinemax)

Palmer amaranth (Amaranthus palmeri S. Wats.) has become difficult to control in row crops due to selection for biotypes that are no longer controlled by acetolactate synthase inhibiting herbicides and/or glyphosate. Early season interference in soybean [Glycine max (L.) Merr.] for 40 days after emergence by three glyphosate-resistant (GR) and three glyphosate-susceptible (GS) Palmer amaranth biotypes from Georgia and North Carolina was compared in the greenhouse. A field experiment over 2 years compared season-long interference of these biotypes in soybean. The six Palmer amaranth biotypes reduced soybean height similarly in the greenhouse but did not affect soybean height in the field. Reduction in soybean fresh weight and dry weight in the greenhouse; and soybean yield in the field varied by Palmer amaranth biotypes. Soybean yield was reduced 21% by Palmer amaranth at the established field density of 0.37 plant m−2. When Palmer amaranth biotypes were grouped by response to glyphosate, the GS group reduced fresh weight, dry weight, and yield of soybean more than the GR group. The results indicate a possible small competitive disadvantage associated with glyphosate resistance, but observed differences among biotypes might also be associated with characteristics within and among biotypes other than glyphosate resistance.

Response of LibertyLink and WideStrike Cotton to Varying Rates of Glufosinate

Field studies were conducted in Alabama, Arkansas, Georgia, Louisiana, Mississippi, North Carolina, and Tennessee during 2010 and 2011 to determine the effect of glufosinate application rate on LibertyLink and WideStrike cotton. Glufosinate was applied in a single application (three-leaf cotton) or sequential application (three-leaf followed by eight-leaf cotton) at 0.6, 1.2, 1.8, and 2.4 kg ai ha−1. Glufosinate application rate did not affect visual injury or growth parameters measured in LibertyLink cotton. No differences in LibertyLink cotton yield were observed because of glufosinate application rate; however, LibertyLink cotton treated with glufosinate yielded slightly more cotton than the nontreated check. Visual estimates of injury to WideStrike cotton increased with each increase in glufosinate application rate. However, the injury was transient, and by 28 d after the eight-leaf application, no differences in injury were observed. WideStrike cotton growth was adversely affected during the growing season following glufosinate application at rates of 1.2 kg ha−1 and greater; however, cotton height and total nodes were unaffected by glufosinate application rate at the end of the season. WideStrike cotton maturity was delayed, and yields were reduced following glufosinate application at rates of 1.2 kg ha−1 and above. Fiber quality of LibertyLink and WideStrike cotton was unaffected by glufosinate application rate. These data indicate that glufosinate may be applied to WideStrike cotton at rates of 0.6 kg ha−1 without inhibiting cotton growth, development, or yield. Given the lack of injury or yield reduction following glufosinate application to LibertyLink cotton, these cultivars possess robust resistance to glufosinate. Growers are urged to be cautious when increasing glufosinate application rates to increase control of glyphosate-resistant Palmer amaranth in WideStrike cotton. However, glufosinate application rates may be increased to maximum labeled rates when making applications to LibertyLink cotton without fear of reducing cotton growth, development, or yield. Nomenclature: Glufosinate; Palmer amaranth, Amaranthus palmeri S. Wats; cotton, Gossypium hirsutum L. Estudios de campo fueron realizados en Alabama, Arkansas, Georgia, Louisiana, Mississippi, North Carolina, y Tennessee durante 2010 y 2011 para determinar el efecto de la dosis de aplicación de glufosinate sobre algodón LibertyLink y WideStrike. Se aplicó glufosinate en una aplicación sencilla (algodón con tres hojas) o en aplicaciones secuenciales (algodón con tres hojas seguido de ocho hojas) a 0.6, 1.2, 1.8, y 2.4 kg ai ha−1. La dosis de aplicación de glufosinate no afectó el daño estimado visualmente o los parámetros de crecimiento medidos en algodón LibertyLink. No se observaron diferencias en el rendimiento del algodón LibertyLink debido a la dosis de glufosinate. Sin embargo, el algodón LibertyLink tratado con glufosinate produjo un rendimiento ligeramente mayor que el testigo sin tratamiento. Estimaciones visuales de daño en el algodón WideStrike aumentaron con el incremento en la dosis de aplicación de glufosinate. Sin embargo, el daño fue transitorio, y a 28 d después de la aplicación a ocho hojas, no se observó ninguna diferencia entre dosis. El crecimiento del algodón WideStrike fue afectado adversamente durante la temporada de crecimiento después de las aplicaciones de glufosinate a dosis de 1.2 kg ha−1 o mayores. Sin embargo, la altura del algodón y el total de nudos no fueron afectados por la dosis de aplicación de glufosinate al final de la temporada. La madurez del algodón WideStrike fue retrasada, y los rendimientos reducidos después de la aplicación de glufosinate a 1.2 kg ha−1 o más. La calidad de la fibra de los algodones LibertyLink y WideStrike no fue afectada por la dosis de aplicación de glufosinate. Estos datos indican que se puede aplicar glufosinate a algodón WideStrike a dosis de 0.6 kg ha−1 sin inhibir el crecimiento, el desarrollo, o el rendimiento del algodón. Debido a la ausencia de daño o reducción en el rendimiento después de la aplicación de glufosinate al algodón LibertyLink, estos cultivares poseen una resistencia robusta a glufosinate. Se urge a los productores a ser precavidos cuando incrementen las dosis de aplicación de glufosinate para aumentar el control de Amaranthus palmeri resistente a glyphosate en algodón WideStrike. Sin embargo, las dosis de aplicación de glufosinate podrían ser aumentadas a la dosis máxima de la etiqueta cuando se hacen aplicaciones a algodón LibertyLink sin tener temor de reducir el crecimiento, el desarrollo, o el rendimiento del algodón.