Quintin R. Johnson

Herbicides for Potential Use in Lima Bean (Phaseolus lunatus) Production

Abstract: Herbicides registered for lima bean (Phaseolus lunatus L.) do not consistently control many troublesome weeds. Some herbicides registered for soybean (Glycine max) will control these weeds, but tolerance to lima bean is not known. Two field and two greenhouse studies were conducted to evaluate recently registered soybean herbicides for lima bean tolerance. Field studies were conducted in Delaware from 1996 to 1998, and in North Carolina during 1997 and 1998. The first field study evaluated the preemergence (PRE) herbicides cloransulam at 0.01, 0.02, 0.03, and 0.04 kg ai/ha; flumetsulam at 0.04, 0.05, 0.06, and 0.07 plus metolachlor at 1.3, 1.6, 1.8, and 2.1 kg ai/ha; sulfentrazone at 0.1, 0.15, 0.2, and 0.25 kg ai/ha; lactofen at 0.2 and 0.25 kg ai/ha; and the commercial standard treatment of imazethapyr plus metolachlor at 0.05 and 1.7 kg ai/ha, respectively. Lima bean injury 5 to 8 wk after emergence was lowest for imazethapyr plus metolachlor (standard treatment) and all four rates of cloransulam. Crop injury with flumetsulam plus metolachlor ranged from 0 to 18% and sulfentrazone ranged from 3 to 75% depending on location and rate. Lactofen treatments caused unacceptable lima bean injury. Yield in plots treated with cloransulam were consistently greater than in the plots treated with other herbicides. The second field study examined the postemergence (POST) herbicides cloransulam (0.013 or 0.02 kg ai/ha), bentazon (1.1 kg ai/ha), imazethapyr (0.035 or 0.053 kg ai/ha), and imazamox (0.018 or 0.036 kg ai/ha), applied when the crop was at the first trifoliolate stage. Cloransulam caused 0 to 13% crop injury and imazamox caused 3 to 25% injury depending on rate and location. In greenhouse studies, no differences were observed among eight common processing lima bean cultivars in tolerance to sulfentrazone applied PRE or to cloransulam, imazamox, imazethapyr, or bentazon applied POST. Nomenclature: Bentazon, 3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide; cloransulam, 3-chloro-2-[[(5-ethoxy-7-fluoro[1,2,4]triazolo[1,5-c]pyrimidine-2yl)sulfonyl]amino]benzoic acid; flumetsulam, N-(2,6-difluorophenyl)-5-methyl[1,2,4]triazolo[1,5-α]pyrimidine-2-sulfonamide; imazamox, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid; imazethapyr, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid; lactofen, (±)-2-ethoxy-1-methyl-2-oxoethyl-5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoate; metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide; sulfentrazone, N-[2,4-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]phenyl]methanesulfonamide; lima bean, Phaseolus lunatus L., ‘M-15’, ‘F1072’, ‘M-408’, ‘Packers’, ‘Concentrated Fordhook’, ‘8-78’, ‘Eastland’; soybean, Glycine max (L.) Merr. Additional index words: Crop tolerance; varietal sensitivity. Abbreviations: COC, crop oil concentrate; NIS, nonionic surfactant; POST, postemergence; PRE, preemergence; WAT, weeks after treatment.

Influence of Glyphosate-Resistant Horseweed (Conyza Canadensis) Growth Stage on Response to Glyphosate Applications

Abstract Infestations of glyphosate-resistant (GR) horseweed have become widespread in the eastern United States. This biotype is problematic in no-tillage production that relies extensively on glyphosate for weed control. Because horseweed is treated at various stages of growth, a greenhouse study explored rate response of glyphosate-resistant and -susceptible horseweed at three growth stages. GR horseweed was more responsive to glyphosate at the seedling stage than at the large rosette or bolting stages. A field study evaluated GR horseweed response when treated with glyphosate at soybean planting time, POST in-crop (about 45 d after planting), or both at planting and POST in-crop. There was a cumulative effect of the at-planting followed by POST in-crop glyphosate applications. When evaluating single glyphosate applications, the at-planting application was more effective at suppressing GR horseweed than a POST in-crop application. Because glyphosate cannot control GR horseweed, this biotype should be controlled with an herbicide with an alternate mode of action and applied at the most effective timing. Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq.; soybean, Glycine max (L.) Merr

Evaluating Drift Control Agents to Reduce Short Distance Movement and Effect on Herbicide Performance1

Spray drift to unintended areas is more of a concern as applications of nonselective herbicides associated with herbicide-resistant crops and the proximity of residential land to agricultural land increase. This research evaluated the benefit of three commercial drift control agents for effectiveness in reducing drift in 19 to 24 km/h wind and their potential effect on weed control. The drift control agents were added to a spray mixture of glyphosate at 0.8 kg ae/ha applied in 140 L/ ha and applied with either flat-fan or flood nozzles. None of the drift control agents reduced drift compared with the spray mixture without drift control agent over a distance of 6 m, as measured by water-sensitive cards or grain sorghum bioassay, regardless of nozzle type used. In a separate study, drift control agents did not reduce the weed control of glyphosate or acifluorfen. Nomenclature: Acifluorfen; glyphosate; grain sorghum, Sorghum bicolor (L.) Moench. Additional index words: Adjuvants, bioassay, crop injury, flat-fan nozzles, flood nozzles, Glycine max, off-site movement, soybean, spray drift. Abbreviation: WS, water sensitive.

Evaluating Postemergence Herbicides, Safener, and Tolerant Hybrids for Corn Response

Crop safety is one of the many considerations when deciding which POST herbicide to use. This research examined relative corn injury as a result of POST herbicides and the effect of including the safener isoxadifen, the choice of a sensitive or tolerant hybrid, or both. The herbicides included commercial combinations of dicamba, diflufenzopyr, nicosulfuron, rimsulfuron, and thifensulfuron, all at twice the labeled rate. Isoxadifen reduced twisting from dicamba plus diflufenzopyr but not with dicamba plus rimsulfuron. Isoxadifen had negligible effect on chlorosis. In general, rimsulfuron plus thifensulfuron caused the most corn stunting, whereas including isoxadifen or using a tolerant hybrid often reduced corn injury. In two of the four years, treatments with rimsulfuron plus thifensulfuron resulted in yield reductions. Although using products with isoxadifen or selecting tolerant hybrids may influence injury, herbicide selection will have the greatest effect on corn injury. Nomenclature: Dicamba; diflufenzopyr; nicosulfuron; rimsulfuron; thifensulfuron; isoxadifen; corn, Zea mays L.

Relative Safety of Preemergence Corn Herbicides Applied to Coarse-Textured Soil

Crop safety is an important consideration in determining PRE herbicide application, especially when multiple herbicide sites-of-action are used. This research examined relative corn injury as the result of PRE applications containing ALS- and/or HPPD-inhibiting herbicides to a sandy loam soil. Herbicide premixes containing clopyralid, flumetsulam, isoxaflutole, mesotrione, rimsulfuron, tembotrione, thifensulfuron, and thiencarbazone were applied at twice the labeled rate. In general, isoxaflutole alone was the safest herbicide evaluated, while PRE applications of rimsulfuroncontaining herbicides caused the most corn stunting, had a lower recovery rate, and lower yields. However, POST applications of mesotrione plus rimsulfuron stunted corn less than 2%. Although there was little correlation between corn injury and yield, growers should be aware of the other factors, such as soil texture and environment that may impact crop production. Nomenclature: Clopyralid; flumetsulam; isoxaflutole; mesotrione; rimsulfuron; thiencarbazone; thifensulfuron; tembotrione; corn, Zea mays L.

Effect of Application Timing on Winter Wheat Response to Metribuzin

Metribuzin will control many problematic weed species in winter wheat in the mid-Atlantic states, including herbicide-resistant biotypes, but it has not been recommended due to crop safety concerns. In a three-year trial, metribuzin was applied at 105 or 210 g ai ha−1 to wheat at the PRE, 2-leaf (Feekes stage 1 to 2), early spring (Feekes stage 3 to 4), and late spring (Feekes stage 4 to 6) growth stages using wheat cultivars sensitive to metribuzin. Early spring applications had the least amount of injury, and injury at this timing was transient and yield was not reduced. Yield loss was observed with the other application timings in at least one out of three years. Rainfall shortly after application appears to increase the risk of wheat injury. Nomenclature: metribuzin; wheat, Triticum aestivum L. Metribuzin controlará muchas especies de malezas problemáticas en trigo de invierno en los estados del Atlántico medio, incluyendo biotipos resistentes a herbicidas, pero no ha sido recomendado debido a preocupación sobre su seguridad en el cultivo. En un ensayo durante tres años, se aplicó metribuzin a 105 ó 210 g ai ha−1 al trigo en los estadios de crecimiento PRE, 2-hojas (estadios Feekes 1 a 2), temprano en la primavera (estadios Feekes 3 a 4), y tarde en la primavera (estadios Feekes 4 a 6) usando cultivares de trigo sensibles a metribuzin. Las aplicaciones temprano en la primavera tuvieron la menor magnitud de daño, y el daño en este momento de aplicación fue temporal y el rendimiento no se redujo. Se observó pérdida en el rendimiento con otros momentos de aplicación en uno de los tres años. Eventos de lluvia poco después de las aplicaciones parecen aumentar el riesgo de daño en el trigo.

Impact of Soybean Leaf Interference and Row Spacing on Preharvest Glyphosate Application1

Preharvest applications of glyphosate can be useful in controlling perennial weeds. Experiments were conducted from 1996 to 1999 to determine whether preharvest glyphosate applications are affected by differences in the amount of soybean canopy present at the time of application by measuring spray deposition and subsequently horsenettle or Canada thistle control. Soybean leaf interference levels were achieved by use of three soybean cultivars with different maturity groups to achieve no leaf interference, moderate leaf interference, and maximum leaf interference, and soybean was planted in three row spacings ranging from 19 to 76 cm. As soybean leaf interference increased, spray coverage of spray deposition cards decreased. There was a similar trend for relative spray volume, determined by intensity of the color change with water-sensitive cards. Row spacing did not influence spray coverage or relative spray volume. Percent change in horsenettle or Canada thistle stems from fall to spring counts was inconsistent. Differences detected in spray coverage did not influence weed control or weed stem density the following spring. Nomenclature: Glyphosate; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR; horsenettle, Solanum carolinense L. # SOLCA; soybean, Glycine max (L.) Merr. Additional index words: CIRAR, Cirsium arvense, cultural practices, integrated weed management, perennial weed control, Solanum carolinense, SOLCA, spray deposition.