Thomas C. Mueller

Proactive Versus Reactive Management of Glyphosate-Resistant or -Tolerant Weeds1

The value of glyphosate has been compromised in some fields where weed populations have developed resistance or tolerant species increased. Three case studies related to reduced control from glyphosate are: (1) a weed population that has become resistant to glyphosate, with horseweed in Tennessee as an example; (2) a weed population increases due to lack of control in “glyphosate only” systems, with tropical spiderwort in Georgia cotton used as an example; and (3) the hypothetical resistance of common waterhemp to glyphosate in Illinois. For each of these case studies, an economic analysis was performed using a partial budget approach. This economic analysis provides the cost of control to the farmer when glyphosate fails to control these weeds and gives a critical time in years to compare different glyphosate resistance management philosophies (applicable only before resistance has evolved). The cost of glyphosate-resistant horseweed in cotton-soybean-corn rotation in Western Tennessee was calculated to be

Palmer Amaranth (Amaranthus palmeri) in Tennessee Has Low Level Glyphosate Resistance

30.46/ha per year. The cost of tropical spiderwort in cotton in southern Georgia was calculated to be

Biotic and abiotic factors influence horseweed emergence

35.07/ha per year. The projected cost if common waterhemp were to develop glyphosate resistance in a corn-soybean rotation in southern Illinois was projected to be

A proposal to standardize soil/solution herbicide distribution coefficients

44.25/ha per year, and the critical time was determined to be greater than 20 yr, indicating that a resistance management strategy would extend the value of glyphosate-resistant crops. Nomenclature: glyphosate; common waterhemp, Amaranthus rudis Sauer. #3 AMATA; horseweed, Conyza canadensis L. Cronq # ERICA; tropical spiderwort, Commelina benghalensis L. # COMBE. Additional index words: economic analysis, herbicide cost, herbicide resistance. Abbreviations: ALS, acetolactate synthase; Cmanaging, economic cost of managing resistance; Cresistance, economic cost of resistance; GR, glyphosate resistant; NPV, net present value; NPVproactive, net present value of proactive resistance management; NPVreactive, net present value of reactive management; PPO, protoporphyrinogenoxidase; Rresistance, net economic return once resistance has occurred; Rwithout, net economic return without resistance; Tcritical, time at which net present value for reactive and proactive resistance management are equal; Tresistance, time at which resistance occurs.

Sulfentrazone dissipation in a Tennessee soil.

Many agricultural producers apply glyphosate to glyphosate-resistant crops to control weeds, including Palmer amaranth. Populations of this weed in Tennessee not completely controlled by glyphosate were examined. Field and greenhouse research confirmed that two separate populations had reduced biomass sensitivity (1.5× to 5.0×) to glyphosate compared to susceptible populations, although the level of resistance was higher based on plant mortality response (about 10×). Shikimate accumulated in both resistant and susceptible plants, indicating that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) was inhibited in both biotypes. These results suggest that an altered target site is not responsible for glyphosate resistance in these Palmer amaranth biotypes. Nomenclature: Glyphosate, Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA

Comparison of Glyphosate Salts (Isopropylamine, Diammonium, and Potassium) and Calcium and Magnesium Concentrations on the Control of Various Weeds'

Abstract Factors affecting horseweed emergence are important for management of this weed species, particularly because of the presence of herbicide-resistant biotypes. Horseweed emergence was highly variable and not strongly correlated to soil temperature (r2 = 0.21), air temperature (r2 = 0.45) or rainfall (r2 = 0.32). Horseweed emerged mainly during April and September in Tennessee when average daytime temperatures fluctuate between 10 and 15.5 C. However, some horseweed plants emerged during almost any month when temperatures ranged from 10 to 25 C and adequate moisture was available at the soil surface. Horseweed densities ranged from a low of 30 to 50 plants m−2 to a high of > 1,500 plants−2 at one location. These extremely high densities illustrate the ability of horseweed to be an effective ruderal plant that can produce stands that approach monoculture densities if not controlled. The amount of crop residue remaining after harvest from the previous field season was in the order of corn > cotton > soybean > fallow. Residue from a previous corn crop reduced horseweed emergence compared with soybean and cotton residues in a no-tillage situation. Decreased horseweed density due to crop residue presence indicates that a systems approach may help reduce horseweed populations. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; corn, Zea mays L.; cotton, Gossypium hirsutum L.; soybean, Glycine max (L.) Merr.

Control of Glyphosate-Resistant Horseweed (Conyza canadensis) with Saflufenacil Tank Mixtures in No-Till Cotton

Abstract Herbicide soil/solution distribution coefficients (Kd) are used in mathematical models to predict the movement of herbicides in soil and groundwater. Herbicides bind to various soil constituents to differing degrees. The universal soil colloid that binds most herbicides is organic matter (OM), however clay minerals (CM) and metallic hydrous oxides are more retentive for cationic, phosphoric, and arsenic acid compounds. Weakly basic herbicides bind to both organic and inorganic soil colloids. The soil organic carbon (OC) affinity coefficient (Koc) has become a common parameter for comparing herbicide binding in soil; however, because OM and OC determinations vary greatly between methods and laboratories, Koc values may vary greatly. This proposal discusses this issue and offers suggestions for obtaining the most accurate Kd, Freundlich constant (Kf), and Koc values for herbicides listed in the WSSA Herbicide Handbook and Supplement. Nomenclature: Readers are referred to the WSSA Herbicide Handbook and Supplement for the chemical names of the herbicides.

Evaluating Rates and Application Timings of Saflufenacil for Control of Glyphosate-Resistant Horseweed (Conyza canadenis) Prior to Planting No-Till Cotton

Abstract: Sulfentrazone dissipation in soil was examined in field experiments in 1995, 1996, and 1997 at Knoxville, TN, on a Sequatchie loam soil. Sulfentrazone 50% disappearance time (DT50) varied from 24 to 113 d. Cotton injury was observed the year following sulfentrazone application when half-lives were ≥85 d. Sulfentrazone degradation under controlled laboratory conditions was slower in autoclaved soil than in nonautoclaved surface soil and subsurface soil, with DT50 of 198, 93, and 102 d, respectively. The difference due to autoclaving the soil implied that sulfentrazone degradation was influenced by both microbial and chemical mechanisms. Nomenclature: Glyphosate, isopropylamine salt of N-(phosphonomethyl)glycine; pyrithiobac, sodium 2-chloro-6-[(4,6 dimethoxy-2-pyrimidinyl)thio]benzoate; sulfentrazone, N-[2,4-dichloro-5-[4-(difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]phenyl]methanesulfonamide; soybean, Glycine max (L.) Merr. ‘Asgrow 5601’; cotton, Gossypium hirsutum L. ‘Paymaster 1220RR’. Additional index words: Degradation, half-life, HPLC. Abbreviations: DAT, days after treatment; DT50, 50% disappearance time; HPLC, high performance liquid chromatography; WAT, weeks after treatment.

Soybean Tolerance to Early Preplant Applications of 2,4-D Ester, 2,4-D Amine, and Dicamba

Greenhouse and field experiments were conducted near Knoxville, TN, during 2002 and 2003 to investigate the effects of calcium and magnesium ions on the performance of three glyphosate formulations with and without diammonium sulfate (AMS). Weed species investigated in the greenhouse were broadleaf signalgrass, pitted morningglory, Palmer amaranth, and yellow nutsedge. Three glyphosate formulations (isopropylamine salt, diammonium salt, and potassium salt) and two glyphosate application rates (0.42 and 0.84 kg ae/ha) were applied to weeds in water fortified with either calcium or magnesium at concentrations of 0, 250, 500, 750, and 1,000 ppm. In all comparisons, there were no differences in the three glyphosate formulations. Glyphosate activity was reduced only when cation concentration was >250 ppm, and this antagonism was not observed when 2% w/ w AMS was added to the spray solution. A chemical analysis of the calcium and magnesium concentrations in water collected from farmers indicated that water samples from eight different producers contained relatively low amounts of cations, with calcium at <40 ppm and magnesium at <8 ppm. In the field results using these and other waters as the herbicide carrier, broadleaf signalgrass control was greater with the 0.84 kg ae/ha than 0.42 kg ae/ha glyphosate rate regardless of water source or addition of AMS. Pitted morningglory responded similarly to glyphosate with water from all farms and with AMS added, and the addition of AMS gave similar results for both glyphosate rates. In 2003, common cocklebur was evaluated and control was >93% regardless of glyphosate rate, water source, or AMS addition. Based on these results, the addition of AMS-based adjuvants to many glyphosate applications may not be warranted. Nomenclature: Glyphosate; diammonium sulfate; broadleaf signalgrass, Brachiaria platyphylla L. #3 BRAPP; Palmer amaranth, Amaranthus palmeri L. # AMAPA; pitted morningglory, Ipomoea lacunosa L. # IPOLA; yellow nutsedge, Cyperus esculentus L. # CYPES; soybean, [Glycine Max (L). Merr. var ‘Asgrow 5602’]. Additional index words: Antagonism, hard water. Abbreviations: DAT, days after treatment; ICAP, inductively coupled argon plasma.

Glyphosate-Resistant Goosegrass (Eleusine indica) Confirmed in Tennessee

Abstract Glyphosate-resistant (GR) horseweed management continues to be a challenge in no-till cotton systems in Tennessee and Mississippi. Field studies were conducted in 2009 and 2010 to evaluate saflufenacil in tank mixtures with glyphosate, glufosinate, or paraquat on GR horseweed prior to planting cotton. Saflufenacil and saflufenacil tank mixtures were applied 7 d before planting (DBP). Three broad spectrum herbicides were tank-mixed with saflufenacil at rates of 0, 6.3, 12.5, 25, and 50 g ai ha−1. Saflufenacil at 25 and 50 g ai ha−1 in tank mixture with all three broad-spectrum herbicides provided similar GR horseweed control when compared to the current standard of glyphosate + dicamba. Across all saflufenacil rates, lint cotton yield among the glyphosate, glufosinate, and paraquat tank mixture treatments did not differ from each other. Control of horseweed with 25 or 50 g ha−1 of saflufenacil across all tank mixtures also was not different from the standard of glyphosate + dicamba. Moreover, saflufenacil, on silt loam soil evaluated in this study, showed no more cotton injury than glyphosate applied 7 d or more before planting. Saflufenacil applied alone at 25 g ha−1 provided lower control of GR horseweed than the standard, which translated to lower lint yield compared to the glyphosate + dicamba treatment or saflufenacil with each tank mixture partner. The 12.5 g ha−1 rate of saflufenacil tank mixed with either paraquat or glufosinate provided less horseweed control (< 85%) than if higher rates of saflufenacil were used (> 95%). However, lint cotton yield was not different between these treatments. This research suggests that saflufenacil at 25 g ha−1 is the most optimal rate for tank mixtures with glyphosate, glufosinate, or paraquat. It also reaffirms earlier research that the 25 g ha−1 saflufenacil rate safely can be applied inside the currently labeled 42-d waiting period between a saflufenacil application and cotton planting. Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA; cotton, Gossypium hirsutum L. ‘Phytogen 375 WRF’.