Jason A. Ferrell

Comparison of Glyphosate with Other Herbicides for Weed Control in Corn (Zea mays): Efficacy and Economics1

Abstract: Glyphosate was compared with other commonly used corn herbicides at three locations in Kentucky in 1998 and 1999. Sequential glyphosate treatments provided greater than 87% control of common cocklebur, ivyleaf morningglory, common lambsquarters, and giant ragweed. Control of these species with glyphosate treatments was similar to the control with atrazine plus other postemergence (POST) herbicides. Generally, treatments containing s-triazines and chloracetamides applied to the soil surface were not as effective as sequential glyphosate or atrazine plus POST herbicides. Corn yield was not significantly different at any location or for any year, whereas differences in net return occurred at one location in 1998. Nomenclature: Atrazine; chloracetamides; glyphosate; s-triazines; common cocklebur, Xanthium strumarium L. #3 XANST; common lambsquarters, Chenopodium album L. # CHEAL; giant ragweed, Ambrosia trifida L. # AMBTR; ivyleaf morningglory, Ipomoea hederacea L. Jacq. # IPOHE; corn, Zea mays L. ‘DeKalb 626RR’. Additional index words: Acetochlor, dicamba, Digitaria sanguinalis L. Scop. DIGSA, dimethenamid, giant foxtail, large crabgrass, 1-(4-methoxy-6-methyl-triazin-2-yl)-3-[2-(3,3,3-trifluoropropyl)-phenylsulfonyl]-urea, metolachlor, net return, primisulfuron, prosulfuron, Roundup Ready® corn, Setaria faberi Herrm. SETFA, simazine. Abbreviations: ASN, as needed; MP, mid-postemergence; POST, postemergence; PRE, preemergence; REG, regrowth.

Factors Affecting Seed Germination of Cadillo (Urena lobata)

Abstract Cadillo is an invasive species commonly found in pastures, rangelands, and disturbed areas. It is becoming a significant problem weed in Florida pastures and natural areas. The objectives of this research were to determine effective techniques to break seed dormancy and the effect of light, temperature, pH, water stress, and depth of seed burial on Cadillo germination. Cadillo seeds had significant levels of innate dormancy imposed by a hard seed coat; chemical scarification was the most effective technique for removing dormancy. Seeds germinated from 15 to 40 C, with an optimal temperature of 28 C. Germination was unaffected by pH levels. Water stress below −0.2 MPa reduced seed germination. Cadillo germination was not light-dependent and seeds emerged from depths up to 9 cm, with the greatest occurring emergence near the soil surface. Considering that Cadillo seed can germinate under a wide range of environmental conditions, it is not surprising that it has become a serious invasive weed in Florida. Nomenclature: Cadillo, Urena lobata L. URLO

Control of tropical soda apple (Solanum viarum) with aminopyralid

Experiments were conducted between 2002 and 2004 at multiple locations in Florida to determine the efficacy of aminopyralid and other herbicides on tropical soda apple (TSA) control. Aminopyralid applied at rates ≥0.08 kg ai/ha consistently provided >96% TSA control up to 335 d after treatment (DAT), while applications <0.06 kg/ha were less effective as well as inconsistent. Control of TSA with aminopyralid was often not statistically different from control with triclopyr, picloram, or dicamba. However, these responses were likely due to the variability in TSA control by triclopyr, picloram, or dicamba across several locations. For example, the standard error of the mean for TSA control with picloram at 335 DAT was 8, compared to 1 for aminopyralid. Herbicides were applied in April, January, and June, but time of year did not affect the efficacy of aminopyralid. Aminopyralid possesses soil residual activity and controlled 98% of germinating seedlings at 75 DAT, compared to 0% control for triclopyr or 2,4-D + dicamba. Therefore, aminopyralid controlled TSA from foliar applications and soil residual activity more consistently than any other herbicide evaluated in these experiments. Nomenclature: Aminopyralid, 4-amino-3,6-dichloropyridine-2-carboxylic acid; tropical soda apple, Solanum viarum Dunal. #3 SOLVI. Additional index words: Preemergence control. Abbreviations: DAT, days after treatment; TSA, tropical soda apple.

Picloram and Aminopyralid Sorption to Soil and Clay Minerals

Abstract Research was conducted to determine picloram and aminopyralid sorption in five soils and three clay minerals and to determine if the potential for off-target movement of aminopyralid in soil is less than that of picloram. Nearly all sorption of picloram and aminopyralid occurred between 0 and 8 h, and the maximum theoretical sorption of picloram and aminopyralid were 10.3 and 15.2%, respectively. Freundlich distribution coefficients (Kf) for picloram ranged from 0.12 in a Cecil sandy loam to 0.81 in an Arredondo fine sand, while Kf values for aminopyralid ranged from 0.35 in a Cecil sandy loam to 0.96 in an Arredondo fine sand. Furthermore, Kf values of aminopyralid were higher than those of picloram in all soils tested. Kf values of picloram in clay minerals were 0.25 (kaolinite), 1.17 (bentonite), and 1,016.4 (montmorillonite), and those of aminopyralid were 5.63 (kaolinite), 2.29 (bentonite), and 608.90 (montmorillonite). It was concluded that soil sorption of aminopyralid was greater than that of picloram and that the potential for off-target movement of aminopyralid is less than that of picloram. Nomenclature: Aminopyralid; picloram.

Sulfentrazone absorption by plant roots increases as soil or solution pH decreases

Abstract Sulfentrazone is a herbicide that has been observed to injure crops in an unpredictable manner. Therefore, experiments were conducted to determine whether root absorption of sulfentrazone was dependent on the pH of the rooting medium. Studies were initiated to examine sulfentrazone uptake of whole plants from soil and hydroponic solution, as well of excised roots in solution. These experiments demonstrated that transpiration decreased as soil pH decreased and herbicide rate increased; it was our intention to use this measure as a description of herbicide injury. Likewise, plants grown for 24 h in 14C-sulfentrazone hydroponic solution accumulated a greater herbicide concentration in roots as solution pH decreased below 6.5. This trend of increased absorption with reduced solution pH was again demonstrated when excised cotton roots were placed for durations of 10 to 120 min in hydroponic solution containing 14C-sulfentrazone. However, when excised roots were placed in solution containing the weak acid herbicide glyphosate, no trend of increased absorption was observed with changes in solution pH. Therefore, it was concluded that the accompanying change in solubility, as sulfentrazone was converted from the ionic to the neutral form, was responsible for the increased absorption by plant roots. Localized differences in soil pH could be responsible for greater sulfentrazone uptake and explain the unpredictable patterns of injury that have been observed. Nomenclature: Glyphosate; sulfentrazone; cotton, Gossypium hirsutum L. ‘Fibermaxx 989 RR/BG’; tobacco, Nicotiana tobacum ‘TN 90’, ‘KY 14’.

Dogfennel (Eupatorium capillifolium) Size at Application Affects Herbicide Efficacy

Abstract Dogfennel is one of the most problematic weeds in Florida pasturelands and its control can become inconsistent as the plant matures. A premix of triclopyr + fluroxypyr has been recently introduced for weed control in pastures and rangeland; however, little published information exists concerning the control of dogfennel in pastures with this herbicide combination. Therefore, experiments were initiated to determine the efficacy of triclopyr + fluroxypyr compared with commonly used pasture herbicides on dogfennel at three heights. All herbicides utilized in this study are commonly used for dogfennel control. Dogfennel control was affected by both herbicide treatment and dogfennel height. In general, 0.80 + 0.28 kg ai/ha of 2,4-D amine + dicamba resulted in inconsistent control, especially as dogfennel plants increased in size. Increasing the rate of 2,4-D amine + dicamba to 1.21 + 0.42 kg/ha increased the consistency. Triclopyr + fluroxypyr provided similar levels of control as that of 1.21 + 0.42 kg/ha 2,4-D amine + dicamba. In all locations, control of 154-cm dogfennel was signficanatly lower than that of 38-cm dogfennel. These data indicate that triclopyr + fluroxypyr is an effective option for dogfennel control, but dogfennel height at the time of application is an important factor for optimizing control. Nomenclature: 2,4-D Amine; dicamba; fluroxypyr; triclopyr; Dogfennel, Eupatorium capillifolium L.

Herbicidal Control of Largeleaf Lantana (Lantana camara)

Abstract Largeleaf lantana is a perennial shrub that commonly infests pastures, roadsides, and natural areas. Many experiments have been conducted to manage this weed, but few successful herbicides have been found. Little information is available for the effectiveness of fluroxypyr, aminopyralid, or aminocyclopyrachlor on largeleaf lantana. Experiments were conducted in central Florida on dense, natural infestations of largeleaf lantana. Aminopyralid (0.12 kg ha−1), fluroxypyr (0.56 kg ha−1), and aminocyclopyrachlor (0.2 kg ha−1) were either applied in the fall (approximately 2 mo before frost) or in the fall followed by a spring application. Aminopyralid was ineffective on largeleaf lantana, and neither one nor two applications resulted in > 20% control 1 yr after treatment (YAT). Fluroxypyr applied once in the fall resulted in 12% control at 1 YAT, but two applications resulted in 80% control after 1 yr. The combination of fluroxypyr + aminopyralid, applied twice, resulted in approximately 90% control 1 YAT. A single application of fluroxypyr + aminopyralid failed to provide greater than 20% control. Conversely, aminocyclopyrachlor applied once in the fall provided 98% control of largeleaf at 1 YAT. Where aminocyclopyrachlor was applied twice, largeleaf lantana control was 100%. From these data, largeleaf lantana can be effectively controlled by two applications of fluroxypyr, two applications of fluroxypyr + aminopyralid, or a single application of aminocyclopyrachlor. Individual plant treatments were also investigated using herbicides applied as basal or cut surface applications. At 1 YAT, only triclopyr + aminopyralid provided > 90% control as a basal application. The other herbicide combinations appeared to be effective earlier, but significant regrowth had occurred by 1 YAT. Cut surface applications were similar with triclopyr + aminopyralid and triclopyr + fluroxypyr providing effective control. Neither triclopyr alone nor imazapyr provided effective control for 1 YAT with basal or cut surface applications. Nomenclature: Aminocyclopyrachlor; aminopyralid; fluroxypyr; imazapyr; triclopyr; largeleaf lantana, Lantana camara L. LANCA.

Influence of Herbicide and Application Timing on Blackberry Control

Abstract Blackberry is a troublesome species across much of the southeastern United States. Control of blackberry with the pyridine herbicides is often variable among different locations. Experiments were conducted to determine whether application timing, either spring or fall, affected efficacy of the pyridine herbicides triclopyr, fluroxypyr and picloram, and metsulfuron. The pyridine herbicides provided greater control when applied in the fall. At 12 mo after treatment, fluroxypyr plus picloram and fluroxypyr plus triclopyr provided 83% control when applied in the fall and 65% when applied in the spring. Conversely, metsulfuron provided 85% control, and application timing was not significant. Although metsulfuron effectively controls blackberry, it is also highly injurious to bahiagrass. Therefore, chlorosulfuron was tested to determine whether it would provide blackberry control while not injuring bahiagrass. Blackberry control with chlorosulfuron was similar to metsulfuron. These data indicated blackberry control in bahiagrass pastures can be effectively accomplished with chlorosulfuron. Nomenclature: Chlorosulfuron; fluroxypyr; metsulfuron; picloram; triclopyr; blackberry, Rubus spp.; bahiagrass, Paspalum notatum Flüggé, ‘Pensacola’.

The effect of selected herbicides on CO2 assimilation, chlorophyll fluorescence, and stomatal conductance in johnsongrass (Sorghum halepense L)

Abstract Greenhouse studies were initiated to determine the duration of time after herbicide treatment required to render johnsongrass physiologically noncompetitive. Nicosulfuron, imazapic, clethodim, and glyphosate were applied to rhizomatous johnsongrass at 35, 70, 140, and 840 g ai ha−1, respectively. Net carbon assimilation, stomatal conductance, chlorophyll meter readings, and maximum (dark adapted) efficiency of photosystem II were measured. Net carbon assimilation (AN) was assumed to be the best indicator of johnsongrass competitiveness. Johnsongrass was considered to be physiologically noncompetitive when AN declined below 50% of that of nontreated check. From these data, it was concluded that glyphosate rendered johnsongrass noncompetitive most readily, 4.3 d after treatment, whereas no differences were detected between nicosulfuron, imazapic, or clethodim throughout the experiment. Stomatal conductance (gs) was highly correlated to AN and was determined to be an adequate substitute for AN when determining johnsongrass competitiveness. It was concluded that chlorophyll meter readings and photosystem II efficiency were poor indicators of johnsongrass competitiveness. Nomenclature: Clethodim; glyphosate; imazapic; nicosulfuron; johnsongrass, Sorghum halepense L., SORHA.

Impact of Exposure to 2,4-D and Dicamba on Peanut Injury and Yield

Abstract The potential widespread adoption of cotton and soybean varieties with 2,4-D and dicamba resistance traits in the southeastern US will increase the risk of accidental exposure of peanut to these herbicides because of drift or application errors. When such accidents occur, growers must decide between continuing the crop and terminating it. In order to make this decision, growers need to estimate the potential yield reduction caused by 2,4-D or dicamba. Dose-response studies were conducted under field conditions in Citra and Jay, FL in 2012 and 2013 to determine peanut injury and yield reduction after exposure to 70, 140, 280, 560, and 1120 g ae ha−1 of 2,4-D or to 35, 70, 140, 280, and 560 g ae ha−1 of dicamba at 21 and 42 d after planting (DAP). Only herbicide by rate interactions were significant (P < 0.04). Dicamba caused 2 to 5 times higher peanut injury and 0.5 to 2 times higher yield reductions than 2,4-D. Injury ranged from 0 to 35% when peanut plants were treated with 2,4-D and from 20 to 78% with dicamba. The maximum yield reduction was 41% with 1,120 g ha−1 of 2,4-D and 65% with 560 g ha−1 of dicamba. Linear regression indicated that the intercept for yield reduction was 12% for 2,4-D and 23% for dicamba, and there was a 2.5% and 7.7% increase in yield reduction per additional 100 g ha−1, respectively. Although high variability was observed for the different variables, there was a positive correlation between injury and peanut yield reduction (P < 0.0001) with Pearsons Rho values ranging from 0.45 to 0.59 for 2,4-D and from 0.27 to 0.55 for dicamba, suggesting that growers can use injury data to make rough projections of yield reduction and decide if they continue their crop, especially when injury is evident. Nomenclature: 2,4-D; dicamba; peanut; Arachis hypogaea L. Resumen La amplia adopción potencial de variedades de algodón y soya con resistencia a 2,4-D y dicamba en el sureste de los Estados Unidos aumentará el riesgo en maní de exposición accidental a estos herbicidas debido a deriva o errores de aplicación. Cuando estos accidentes ocurran, los productores deberán decidir entre continuar con el cultivo o terminarlo. Para tomar esta decisión, los productores necesitan estimar el potencial de reducción del rendimiento a causa de 2,4-D o dicamba. Se realizaron estudios de respuesta a dosis bajo condiciones de campo en Citra y Jay, FL en 2012 y 2013 para determinar el daño y reducción de rendimiento en el maní después de la exposición a 70, 140, 280, 560 y 1120 g ae ha−1 de 2,4-D o a 35, 70, 140, 280, y 560 g ae ha−1 de dicamba a 21 y 42 d después de la siembra (DAP). Solamente interacciones entre el herbicida y la dosis fueron significativas (P<0.04). Dicamba causó de 2 a 5 veces mayor daño al maní y de 0.5 a 2 veces mayor reducción en el rendimiento que 2,4-D. La mayor reducción del rendimiento fue 41% con 1,120 g ha−1 de 2,4-D y 65% con 560 g ha−1 de dicamba. Regresiones lineales indicaron que el intercepto para la reducción del rendimiento fue 12% para 2,4-D y 23% para dicamba, y hubo un incremento de 2.5% y 7.7% en la pérdida de rendimiento por cada 100 g ha−1 adicionales de estos herbicidas, respectivamente. Aunque se observó una alta variabilidad para las diferentes variables, hubo una correlación positiva entre el daño y la reducción en el rendimiento del maní (P<0.0001) con valores de Rho de Pearson de 0.45 a 0.59 para 2,4-D y 0.27 a 0.55 para dicamba, lo que sugiere que los productores pueden usar datos de daño para hacer proyecciones aproximadas de pérdida de rendimiento y así decidir si continúan el cultivo, especialmente cuando el daño es evidente.