Lynn M. Sosnoskie

Loss of Glyphosate Efficacy: A Changing Weed Spectrum in Georgia Cotton

Abstract Introduction of glyphosate resistance into crops through genetic modification has revolutionized crop protection. Glyphosate is a broad-spectrum herbicide with favorable environmental characteristics and effective broad-spectrum weed control that has greatly improved crop protection efficiency. However, in less than a decade, the utility of this technology is threatened by the occurrence of glyphosate-tolerant and glyphosate-resistant weed species. Factors that have contributed to this shift in weed species composition in Georgia cotton production are reviewed, along with the implications of continued overreliance on this technology. Potential scenarios for managing glyphosate-resistant populations, as well as implications on the role of various sectors for dealing with this purported tragedy of the commons, are presented. Benghal dayflower, a glyphosate-tolerant species, continues to spread through Georgia and surrounding states, whereas glyphosate susceptibility in Palmer amaranth is endangered in Georgia and other cotton-producing states in the southern United States. Improved understanding of how glyphosate susceptibility in our weed species spectrum was compromised (either through occurrence of herbicide-tolerant or -resistant weed species) may allow us to avoid repeating these mistakes with the next herbicide-resistant technology. Nomenclature: Glyphosate; Benghal dayflower, Commelina benghalensis L.; Palmer amaranth, Amaranthus palmeri S. Wats; cotton, Gossypium hirsutum L.

Multiple Resistance in Palmer Amaranth to Glyphosate and Pyrithiobac Confirmed in Georgia

Abstract In 2006, Palmer amaranth with confirmed resistance to glyphosate (GLY-R) was not controlled effectively in cotton with pyrithiobac, an acetolactate synthase (ALS)-inhibiting herbicide. Glyphosate at 870 g ae ha−1 or pyrithiobac at 70 g ai ha−1 applied postemergence provided 5 to 28% control of a putative GLY/ALS-R Palmer amaranth biotype in the field. Glyphosate at 6,930 g ha−1 and pyrithiobac at 420 g ha−1 applied alone provided no more than 89 and 65% control 1 to 8 wk after treatment (WAT), respectively. When applied as a tank mixture, glyphosate plus pyrithiobac at 870 + 70 g ha−1 provided between 16 and 41% control; glyphosate plus pyrithiobac at 6,930 + 420 g ha−1 controlled the Palmer amaranth in the field 89 to 95%. Dose-response analyses developed from greenhouse data indicated that the estimated glyphosate rates required to produce 50% injury and reduce plant fresh weights by 50% relative to the nontreated control in a suspected GLY/ALS-R Palmer amaranth biotype were 12 and 14 times greater, respectively, than the estimated values for the susceptible (S) biotype. The predicted pyrithiobac rates required to produce the same responses in the putative resistant population were 151 (50% injury) and 563 times (50% fresh weight reduction) greater than the estimated rates for the S biotype. Field and greenhouse analyses confirm that the Palmer amaranth biotype evaluated in both studies is resistant to glyphosate and an ALS-inhibiting herbicide. Nomenclature: Glyphosate; pyrithiobac; Palmer amaranth, Amaranthus palmeri S. Wats; cotton, Gossypium hirsutum L.

Glyphosate-Resistant Palmer Amaranth (Amaranthus palmeri) Increases Herbicide Use, Tillage, and Hand-Weeding in Georgia Cotton

Abstract In 2005, the existence of glyphosate-resistance in Palmer amaranth was confirmed at a single 250 ha field site in Macon County, Georgia. Currently, all cotton producing counties in Georgia are infested, to some degree, with glyphosate-resistant Palmer amaranth. In 2010 and 2011, surveys were administered to Georgia growers and extension agents to determine how the development of glyphosate-resistance has affected weed management in cotton. According to respondents, the numbers of cotton acres that were treated with paraquat, glufosinate and residual herbicides effective against Palmer amaranth more than doubled between 2000 to 2005 and 2006 to 2010. Glyphosate use declined between 2000 to 2005 and 2006 to 2010 although, on average, the active ingredient was still applied to a majority of cotton acres. Although grower herbicide input costs have more than doubled following the evolution and spread of glyphosate resistance, chemically-based control of Palmer amaranth is still not adequate. As a consequence, Georgia cotton growers hand weeded 52% of the crop at an average cost of

Pollen-Mediated Dispersal of Glyphosate-Resistance in Palmer Amaranth under Field Conditions

57 per hand-weeded ha; this represents a cost increase of at least 475% as compared to the years prior to resistance. In addition to increased herbicide use and hand weeding, growers in Georgia are also using mechanical, in-crop cultivation (44% of acres), tillage for the incorporation of preplant herbicides (20% of the acres), and post-harvest deep-turning (19% of the acres every three years) for weed control. Current weed management systems are more diverse, complex and expensive than those employed only a decade ago, but are effective at controlling glyphosate-resistant Palmer amaranth in glyphosate-resistant cotton. The success of these programs may be related to producers improved knowledge about herbicide resistance, and the biological attributes that make Palmer amaranth so challenging, as well as their ability to implement their management programs in a timely manner. Nomenclature: 2,4-D; diuron; fomesafen; flumioxazin; fluometuron; glyphosate; glufosinate; MSMA; paraquat; pendimethalin; pyrithiobac; S-metolachlor; trifluralin; Palmer amaranth, Amaranthus palmeri (S. Wats); cotton, Gossypium hirsutum L.

Glyphosate Resistance Does Not Affect Palmer Amaranth (Amaranthus palmeri) Seedbank Longevity

Abstract In addition to being a strong competitor with cotton and other row crops, Palmer amaranth has developed resistance to numerous important agricultural herbicides, including glyphosate. The objective of this study was to determine if the glyphosate-resistance trait can be transferred via pollen movement from a glyphosate-resistant Palmer amaranth source to a glyphosate-susceptible sink. In 2006 and 2007 glyphosate-resistant Palmer amaranth plants were transplanted in the center of a 30-ha cotton field. Susceptible Palmer amaranth plants were transplanted into plots located at distances up to 300 m from the edge of the resistant pollen source in each of the four cardinal and ordinal directions. Except for the study plots, the interior of the field and surrounding acreage were kept free of Palmer amaranth by chemical and physical means. Seed was harvested from 249 and 292 mature females in October 2006 and 2007, respectively. Offspring, 14,037 in 2006 and 13,685 in 2007, from glyphosate-susceptible mother plants were treated with glyphosate when the plants were 5 to 7 cm tall. The proportion of glyphosate-resistant progeny decreased with increased distance from the pollen source; approximately 50 to 60% of the offspring at the 1- and 5-m distances were resistant to glyphosate, whereas 20 to 40% of the offspring were resistant at the furthest distances. The development of resistance was not affected by direction; winds were variable with respect to both speed and direction during the peak pollination hours throughout the growing season. Results from this study indicate that the glyphosate-resistance trait can be transferred via pollen movement in Palmer amaranth. Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; cotton, Gossypium hirsutum L.

Seedbank and Emerged Weed Communities Following Adoption of Glyphosate-Resistant Crops in a Long-Term Tillage and Rotation Study

Abstract A greater understanding of the factors that regulate weed seed return to and persistence in the soil seedbank is needed for the management of difficult-to-control herbicide-resistant weeds. Studies were conducted in Tifton, GA to (1) evaluate whether glyphosate resistance, burial depth, and burial duration affect the longevity of Palmer amaranth seeds and (2) estimate the potential postdispersal herbivory of seeds. Palmer amaranth seeds from glyphosate-resistant and glyphosate-susceptible populations were buried in nylon bags at four depths ranging from 1 to 40 cm for intervals ranging between 0 and 36 mo, after which the bags were exhumed and seeds evaluated for viability. There were no detectable differences in seed viability between glyphosate-resistant and glyphosate-susceptible Palmer amaranth seeds, but there was a significant burial time by burial depth interaction. Palmer amaranth seed viability for each of the burial depths declined over time and was described by exponential decay regression models. Seed viability at the initiation of the study was ≥ 96%; after 6 mo of burial, viability declined to 65 to 78%. As burial depth increased, so did Palmer amaranth seed viability. By 36 mo, seed viability ranged from 9% (1-cm depth) to 22% (40-cm depth). To evaluate potential herbivory, seed traps with three levels of exclusion were constructed: (1) no exclusion, (2) rodent exclusion, and (3) rodent and large arthropod exclusion. Each seed trap contained 100 Palmer amaranth seeds and were deployed for 7 d at irregular intervals throughout the year, totaling 27 sample times. There were seasonal differences in seed recovery and differences among type of seed trap exclusion, but no interactions. Seed recovery was lower in the summer and early autumn and higher in the late winter and early spring, which may reflect the seasonal fluctuations in herbivore populations or the availability of other food sources. Seed recovery was greatest (44%) from the most restrictive traps, which only allowed access by small arthropods, such as fire ants. Traps that excluded rodents, but allowed access by small and large arthropods, had 34% seed recovery. In the nonexclusion traps, only 25% of seed were recovered, with evidence of rodent activity around these traps. Despite the physically small seed size, Palmer amaranth is targeted for removal from seed traps by seed herbivores, which could signify a reduction in the overall seed density. To be successful, Palmer amaranth management programs will need to reduce soil seedbank population densities. Future studies need to address factors that enhance the depletion of the soil seedbank and evaluate how these interact with other weed control practices. Nomenclature: Palmer amaranth, Amaranthus palmeri (S.) Wats. AMAPA.

Italian Ryegrass (Lolium perenne) Control and Winter Wheat Response to POST Herbicides

Abstract The compositions of the germinable weed seedbank and aboveground weed communities in a long-term tillage and rotation study were characterized 4, 5, and 6 yr (2002 to 2004) after the adoption of glyphosate-tolerant corn and soybean. Averaged across rotation, mean germinable weed seed density and diversity were greatest in the no-tillage treatment as compared to the minimum- and conventional-tillage treatments. Averaged over tillage, density and diversity were greater in the corn–oat–hay (ryegrass + alfalfa) system as compared to the continuous corn and corn–soybean rotations. Similar trends in density and diversity were observed for the aboveground weed communities. Differences in community composition among treatments were quantified with the use of a multiresponse permutation procedure. Results indicated that the weed seedbank community in a corn–oat–hay rotational system differed from the communities associated with the continuous corn and corn–soybean rotational systems. Weed seedbank communities developing under a no-tillage operation differed from those in minimum- and conventional-tillage scenarios. Compositional differences among the aboveground weed communities were less pronounced in response to tillage and rotation. Indicator species analyses indicated that the number of significant indicator weed species was generally higher for no tillage than minimum or conventional tillage for both the seedbank and the aboveground weed communities. The number of significant indicator species for the seedbank and weed communities was generally greater in the three-crop rotation as compared to the continuous corn and corn–soybean rotations. The trends observed in density, diversity, and community composition after the adoption of glyphosate-tolerant corn and soybeans, and a glyphosate-dominated weed management program, were also observed when soil-applied herbicides were included in the study. We suggest that the switch to a POST-glyphosate protocol did not significantly alter weed communities in the short term in this study. Nomenclature: Glyphosate; alfalfa, Medicago sativa L.; corn, Zea mays L.; oat, Avena sativa L.; ryegrass, Lolium perenne L.; soybean, Glycine max (L.) Merr.

Evaluating the Volatility of Three Formulations of 2,4-D When Applied in the Field

Abstract Field studies were conducted to evaluate Italian ryegrass control and winter wheat tolerance to applications of diclofop, mesosulfuron plus methylated seed oil (MSO) alone or with 30% urea ammonium nitrate (UAN), mesosulfuron plus thifensulfuron plus tribenuron plus MSO, mesosulfuron plus MCPA plus MSO, or flufenacet plus metribuzin. Treatments were applied to wheat PRE, two- to three-leaf wheat (2–3 LF) at Feekes stage 1.0 or to one- to two-tiller wheat (TILL) at Feekes stage 3.0, depending on label recommendations. Studies were conducted in Williamson, GA, and Plains, GA, from autumn 2003 to spring 2005. Italian ryegrass control was variable, depending on location and year. Maximum and most-consistent Italian ryegrass control (> 90%) occurred with mesosulfuron plus MSO and UAN. Without UAN, control of Italian ryegrass with mesosulfuron varied from 44 to 97%. That variability was partially attributed to unfavorable environmental conditions associated with cold night time temperatures at or below 0 C, following applications. Wheat injury observed in response to herbicide treatments was minimal (< 15%) and transient; wheat recovered with no differences in yield. Nomenclature: Diclofop; flufenacet; mesosulfuron; metribuzin; thifensulfuron; tribenuron; Italian ryegrass, Lolium perenne L. ssp. multiflorum (Lam.) Husnot; wheat, Triticum aestivum L. Resumen Se realizaron estudios de campo para evaluar el control de Lolium perenne ssp. multiflorum y la tolerancia del trigo de invierno a aplicaciones de diclofop, mesosulfuron más aceite de semilla metilado (MSO) solo o con 30% urea ammonium nitrate (UAN), mesosulfuron más thifensuluron más tribenuron más MSO, mesosulfuron más MCPA más MSO o flufenacet más metribuzin. Los tratamientos se aplicaron al trigo en PRE, a dos a tres hojas del trigo (2–3 LF) en el estado Feekes 1.0 o a uno a dos hijuelos (TILL) en el estado Feekes 3.0, dependiendo de las recomendaciones de las etiquetas. Los estudios se realizaron en Williamson, GA y en Plains, GA, desde el otoño de 2003 a la primavera de 2005. El control de L. perenne fue variable dependiendo del sitio y el año. El máximo control (>90%) se dio con mesosulfuron más MSO y UAN. Sin UAN, el control de L. perenne con mesosulfuron varió entre 47% y 97%. La variabilidad fue parcialmente atribuida a condiciones ambientales desfavorables asociadas a temperaturas nocturnas de 0 C o inferiores, después de las aplicaciones. El daño observado en el trigo en respuesta a los tratamientos con herbicidas fue mínimo (<15%) y transitorio. El trigo se recuperó y no hubo diferencias en rendimiento.

Sequential Applications for Mesosulfuron and Nitrogen Needed in Wheat

Abstract Cotton genetically engineered to be resistant to topical applications of 2,4-D could provide growers with an additional tool for managing difficult-to-control broadleaf species. However, the successful adoption of this technology will be dependent on the ability of growers to manage off-target herbicide movement. Field experiments were conducted in Moultrie, GA, to evaluate cotton injury resulting from the volatilization of 2,4-D when formulated as an ester, an amine, or a choline salt. Each formulation of 2,4-D (2.24 kg ha−1) was applied in mixture with glyphosate (2.24 kg ha−1) directly to the soil surface (10 to 20% crop residue) in individual square blocks (750 m2). Following herbicide applications, replicate sets of four potted cotton plants (five- to seven-leaf stage) were placed at distances ranging from 1.5 to 48 m from the edge of each treatment. Plants were allowed to remain in-field for up to 48 h before being removed. Cotton exposed to 2,4-D ester for 48 h exhibited maximum injury ratings of 63, 57, 48, 29, 13, and 2% at distances of 1.5, 3, 6, 12, 24, and 48 m, respectively. Less than 5% injury was noted for the amine and choline formulations at any distance. Plant height was also affected by formulation and distance; plants that were located closest to the ester-treated block were smaller than their more distantly-positioned counterparts. Exposure to the amine and choline formulations did not affect plant heights. Additionally, two plastic tunnels were placed inside of each treated block to concentrate volatiles and maximize the potential for crop injury. Injury ratings of 76, 13, and 5% were noted for cotton exposed to the ester, amine, and choline formulations, respectively when under tunnels for 48 h. Results indicate that the choline formulation of 2,4-D was less volatile and injurious to cotton than the ester under the field conditions in this study. Nomenclature: 2,4-dichlorophenoxyacetic acid, glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; cotton, Gossypium hirsutum L. Resumen El algodón genéticamente diseñado para ser resistente a las aplicaciones tópicas de 2,4-D podría brindar a los productores una herramienta adicional para el manejo de especies de hoja ancha difíciles de controlar. Sin embargo, la adopción exitosa de esta tecnología dependerá de la habilidad de los productores de manejar el movimiento del herbicida a lugares no deseados. Se realizaron experimentos de campo en Moultrie, Georgia, para evaluar el daño en algodón resultante de la volatilización de 2,4-D cuando se formuló como ester, amine, o sal choline. Cada formulación de 2,4-D (2.24 kg ha−1) fue aplicada en mezcla con glyphosate (2.24 kg ha−1) directamente a la superficie del suelo (10 a 20% de residuos de cultivos) en parcelas cuadradas individuales (750 m2). Seguido de las aplicaciones del herbicida, grupos replicados de cuatro plantas de algodón en contenedores (en el estado de cinco a siete hojas) fueron colocados a distancias que variarían de 1.5 a 48 m del borde de cada tratamiento. Las plantas fueron mantenidas en el campo por períodos de hasta 48 h antes de ser removidas. El algodón expuesto a 2,4-D ester por 48 h mostró evaluaciones de daño máximas de 63, 57, 48, 29, 13, y 2% a distancias de 1.5, 3, 6, 12, 24, y 48 m, respectivamente. Para las formulaciones amine y choline, el daño notado fue menor a 5% en cualquiera de las distancias evaluadas. La altura de planta también fue afectada por la formulación y la distancia; las plantas que estaban más cerca de la parcela tratada con ester fueron más pequeñas que aquellas que estaban a mayor distancia. La exposición a las formulaciones amine y choline no afectó la altura de las plantas. Adicionalmente, se colocaron dos túneles de plástico dentro de cada parcela tratada para concentrar los compuestos volátiles y maximizar el potencial de daño del cultivo. Las evaluaciones de daño de 76, 13, y 5% fueron notadas para el algodón expuesto a las formulaciones ester, amine, y choline, respectivamente, bajo los túneles por 48 h. Los resultados indican que la formulación choline de 2,4-D fue menos volátil y menos dañina al algodón que la formulación ester bajo condiciones de campo en este estudio.

Response of Seeded and Transplanted Summer Squash to S-Metolachlor Applied at Planting and Postemergence

Abstract Mesosulfuron is often applied to wheat at a time of year when top-dress nitrogen is also applied. Current labeling for mesosulfuron cautions against applying nitrogen within 14 d of herbicide application. Soft red winter wheat response to mesosulfuron and urea ammonium nitrate (UAN) applied sequentially and in mixtures was determined at three locations in North Carolina and Georgia during 2005 and 2006. Mesosulfuron at 0, 15, and 30 g ai/ha was applied in water to wheat at Feekes growth stage (GS) 3 followed by UAN at 280 L/ha 2 h, 7 d, 14 d, and 21 d after mesosulfuron. Mesosulfuron applied in UAN was also evaluated in 2006. Mesosulfuron injured wheat 6 to 9% in 2005 and 12 to 23% in 2006 when UAN was applied 2 h or 7 d after the herbicide. Wheat injury did not exceed 8% when UAN was applied 14 or 21 d after the herbicide. Greatest injury, 35 to 40%, was noted when mesosulfuron and UAN were combined. Wheat yield was unaffected by mesosulfuron or time of UAN application in 2005. In 2006, yield was affected by the timing of UAN application relative to mesosulfuron; wheat yield increased as the interval, in days, between UAN and herbicide applications increased. To avoid crop injury and possible yield reduction, mesosulfuron and UAN applications should be separated by at least 7 to 14 d. These findings are consistent with precautions on the mesosulfuron label. Nomenclature: Mesosulfuron (proposed common name), 2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-4-[[(methylsulfonyl)amino]methyl]benzoic acid; soft red winter wheat, Triticum aestivum L. ‘26R61’, ‘Coker 9184’, ‘SS 8308’.