With the release of dicamba-resistant crops, it is necessary to understand how technical and environmental conditions affect the application of dicamba. This study sought to evaluate drift from dicamba applications through flat-fan nozzles, under several wind speeds in a wind tunnel. Dicamba applications were performed through two standard (XR and TT) and two air induction (AIXR and TTI) 110015 nozzles at 0.9, 2.2, 3.6 and 4.9 ms−1 wind speeds. The applications were made at 276 kPa pressure and the dicamba rate was 561 g ae ha-1. The droplet spectrum was measured using a laser diffraction system. Artificial targets were used as drift collectors, positioned in a wind tunnel from 2 to 12 m downwind from the nozzles. Drift potential was determined using a fluorescent tracer added to solutions, quantified by fluorimetry. The air induction TTI nozzle produced the lowest percentage of dicamba drift at 2.2, 3.6 and 4.9 ms−1 wind speeds at all distances. Dicamba spray drift from XR, TT and AIXR nozzles increased exponentially as wind speed increased, whereas from TTI nozzle drift increased linearly as wind speed increased. Drift did not increase linearly as the volume percentage of droplets smaller than 100 µm and wind speed increased.

Field experiments were conducted to evaluate possible interactions of clethodim with imazapic applied as mixtures or sequentially for control of broadleaf signalgrass, fall panicum, goosegrass, and large crabgrass. Imazapic at 70 g ai/ha alone controlled grass weeds inconsistently, whereas clethodim at 140 g ai/ha alone controlled the same weeds at least 99%. Imazapic did not affect broadleaf signalgrass control by clethodim. Reduced control of fall panicum, goosegrass, and large crabgrass was observed when clethodim and imazapic were applied in mixture. Antagonism of clethodim occurred when clethodim was applied 1 d before or up to 3 d after application of imazapic (fall panicum and large crabgrass). Antagonism of goosegrass control was noted when imazapic was applied 3 d before or up to 7 d after application of clethodim. In other experiments, large crabgrass and Texas panicum control by clethodim (70 and 140 g/ha) applied alone or with imazapic (70 g/ ha) or bentazon (1.1 kg ai/ha) plus 2,4-DB (0.28 kg ai/ha) either with or without ammonium sulfate (2.8 kg/ha) was evaluated. Texas panicum control by clethodim was reduced by imazapic regardless of the ammonium sulfate rate. However, large crabgrass control by imazapic was not affected in these experiments. Control of both grasses by clethodim was reduced substantially by bentazon plus 2,4-DB, although in some instances ammonium sulfate improved control when in mixture. Ammonium sulfate improved control by clethodim in some instances irrespective of the broadleaf–sedge herbicide treatments.

In greenhouse experiments, unrefined corn oil was tested as an adjuvant with the mycoherbicidal fungus, Colletotrichum truncatum. A1:1 (V/V) fungus/corn oil tank mixture extended by at least 70 h the period of time in which the fungus remains infective between inoculation and the onset of dew. This formulation also reduced the dew period requirements for optimal weed pathogenesis and mortality from 12 h to 2 h, and reduced the required spray volume by 100-fold, from 500 L/ha to 5 L/ha. These results suggest that formulating C. truncatum in unrefined corn oil greatly improves the bioherbicidal potential of this pathogen.

Field experiments were conducted from 1997 to 1999 at the University of Minnesota Southern Research and Outreach Center in Waseca to evaluate the (1) effect of corn row spacing on grass and broadleaf weed species density and height, (2) optimal herbicide application timing in narrow- and wide-row systems, and (3) corn grain yield response to row spacing and herbicide application timing. Corn was planted in 51- and 76-cm row spacings. Within each row-spacing treatment, there were five herbicide application timings: a formulated mixture of acetochlor plus atrazine applied preemergence or a formulated mixture of imazethapyr and imazapyr tank-mixed with bromoxynil applied postemergence at 5-, 10-, 20-, or 30-cm giant foxtail plant height. Reducing the row spacing in corn from 76 to 51 cm did not influence early-season weed emergence or growth. Similarly, late-season weed density and growth were not influenced by row spacing except in 1997. But corn grain yield increased when corn was planted in narrow rows compared with wide rows in 2 out of 3 yr when averaged over herbicide application treatments. Herbicide application timing had a significant effect on late-season weed density and grain yield. But there was no interaction between herbicide application timing and row spacing on grain yield. Potential increases in crop competitiveness resulting from narrow-row corn did not appear to affect weed density or growth in this study.

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.

Efficacy of trifloxysulfuron with and without surfactant was evaluated against balsamapple, cat's claw vine, Florida beggarweed, hairy beggarticks, ivyleaf morningglory, johnsongrass, prickly sida, redroot pigweed, sicklepod, strangler vine, tall morningglory, and yellow nutsedge at 21, 42, and 63 g ai/ha applied at the four- or six-leaf stages and compared with glyphosate at 280, 560, and 840 g ae/ha. Delayed application from the four- to six-leaf stage significantly reduced trifloxysulfuron efficacy; reduction was less with glyphosate. Trifloxysulfuron plus 0.25% X-77 was more effective on the four-leaf stage than on the six-leaf stage plants of redroot pigweed, johnsongrass, hairy beggarticks, strangler vine, and prickly sida; effect was similar on yellow nutsedge, sicklepod, Florida beggarweed, balsamapple, ivyleaf morningglory, and tall morningglory. Trifloxysulfuron at 63 g/ha plus surfactant reduced the fresh weight of all test plants more than 80% compared with control, except prickly sida, strangler vine, and cat's claw vine. Glyphosate was less effective than trifloxysulfuron plus surfactant against tall morningglory, sicklepod, ivyleaf morningglory, and yellow nutsedge but was significantly better against balsamapple, prickly sida, and cat's claw vine. None of the herbicides provided satisfactory control of cat's claw vine, strangler vine, and prickly sida.

Abrasive weed control is a novel weed management tactic that has great potential to increase the profitability and sustainability of organic vegetable cropping systems. The objective of this study was to determine the effect of air-propelled organic abrasive grits (e.g., organic fertilizers) on weed seedling emergence and growth and vegetable crop growth. A series of thirteen greenhouse trials were conducted to determine the susceptibility of weeds to abrasive weed control with one of six organic materials including: corn cob grits, corn gluten meal, greensand fertilizer, walnut shell grits, soybean meal, and bone meal fertilizer. In addition, crop injury was quantified to determine the potential utility of each organic material as abrasive grits in tomato and pepper cropping systems. Of the six organic materials, corn gluten meal, greensand fertilizer, walnut shell grits, and soybean meal provided the broadest range of POST weed control. For example, one blast of corn gluten meal and greensand fertilizer reduced Palmer amaranth (one-leaf stage) seedling biomass by 95 and 100% and green foxtail (one-leaf stage) biomass by 94 and 87%, respectively. None of the organic materials suppressed weed seedling emergence when applied to the soil surface, suggesting that residual weed control with abrasive grits is unlikely. Tomato and pepper stems were relatively tolerant of abrasive grit applications, though blasting with select materials did increase stem curvature in tomato and reduced biomass (corn cob grit) and relative growth rate (corn gluten meal and greensand) in pepper. Results suggest that organic fertilizers can be effectively used as abrasive grits in vegetable crops, simultaneously providing weed suppression and supplemental crop nutrition. Field studies are needed to identify cultural practices that will increase the profitability of multifunctional abrasive weed control in organic specialty crops.

Field studies were conducted in 1996 and 1997 at three locations throughout southern Ontario with the objective of developing dose-response curves of RPA 201772 for weed control and crop tolerance in corn. The biologically effective doses required to control redroot pigweed, velvetleaf, and wild mustard were 100, 90, and 80 g/ha, respectively. Yellow foxtail was controlled with 100 to 120 g/ha, while rates for common lambsquarters varied from 60 to 130 g/ha, depending on the year and location. Wild buckwheat control was poor (> 30%) at all of the doses tested. RPA 201772 did not reduce corn grain yield; however, temporary crop injury was evident on coarse sandy soils.

Management of Italian ryegrass in cereal-based cropping systems continues to be a major production constraint in areas of the United States, including the soft white winter wheat producing regions of the Pacific Northwest. Pyroxasulfone is a soil-applied herbicide with the potential to control broadleaf and grass weed species, including grass weed biotypes resistant to group 1, 2, and 7 herbicides, in several crops for which registration has been completed or is pending, including wheat, corn, sunflower, dry bean, and soybean. Field experiments were conducted from 2006 through 2009 near Corvallis, OR, to evaluate the potential for Italian ryegrass control in winter wheat with applications of pyroxasulfone. Application rates of PRE treatments ranged from 0.05 to 0.15 kg ai ha−1. All treatments were compared to standard Italian ryegrass soil-applied herbicides used in winter wheat, including diuron, flufenacet, and flufenacet + metribuzin. Visual evaluations of Italian ryegrass and ivyleaf speedwell control and winter wheat injury were made at regular intervals following applications. Winter wheat yields were quantified at grain maturity. Ivyleaf speedwell control was variable, and Italian ryegrass control following pyroxasulfone applications ranged from 65 to 100% and was equal to control achieved with flufenacet and flufenacet + metribuzin treatments and greater than that achieved with diuron applications. Winter wheat injury from pyroxasulfone ranged from 0 to 8% and was most associated with the 0.15–kg ha−1 application rate. However, this early-season injury did not negatively impact winter wheat yield. Pyroxasulfone applied at the application rates and timings in these studies resulted in high levels of activity on Italian ryegrass and excellent winter wheat safety. Based on the results, pyroxasulfone has the potential to be used as a soil-applied herbicide in winter wheat for Italian ryegrass management and its utility for management of other important grass and broadleaf weeds of cereal-based cropping systems should be evaluated.

Glyphosate-resistant Palmer amaranth has become a major threat to soybean production in the southern United States. Arkansas soybean producers rely heavily on glyphosate-resistant soybean; hence, an alternative solution for controlling resistant Palmer amaranth is needed. A field experiment was conducted at Marianna, AR, during 2009 and 2010 in which soybean production systems were tested in combination with deep tillage and no tillage to determine the impact on Palmer amaranth emergence. To establish a baseline population, 250,000 glyphosate-resistant Palmer amaranth seeds were placed in a 1-m2 area in the middle of each plot and incorporated in the soil, and emergence was evaluated five times during the season. Production systems of full-season soybean with a rye cover crop or soybean double-cropped with wheat, which had high amounts of plant residue on the soil surface reduced Palmer amaranth emergence more than systems without deep tillage and a cover crop or wheat. When used in combination with deep tillage, these systems reduced Palmer amaranth emergence by 98 and 97%, respectively, in 2009 and by 73 and 82%, respectively, in 2010. Deep tillage alone caused an 81% reduction in emergence averaged over both years. Soybean double-cropped with wheat used in combination with deep tillage provided a 95% reduction in Palmer amaranth emergence over the 2-yr period. This research shows that deep tillage in combination with soybean production systems that have high amounts of residue on the soil surface are alternative means for providing a high level of control of glyphosate-resistant Palmer amaranth and could lessen the dependence on chemical weed control.

Weed management strategies are needed to control weeds in corn that emerge after postemergence (POST) application of glyphosate or glufosinate. Field trials were conducted from 1996 to 1999 to determine if residual herbicides could be used with glyphosate or glufosinate to provide season-long weed control in glyphosate-resistant or glufosinate-resistant corn. Preemergence (PRE) applications of several residual herbicides followed by POST applications of glyphosate or glufosinate were compared with POST tank mixtures of glyphosate or glufosinate with residual herbicides. All residual herbicides used in combination with glyphosate, when compared with glyphosate alone, increased control of redroot pigweed and common lambsquarters by an average of 20% and resulted in a 4 to 19% increase in control of giant foxtail. Each residual herbicide tank mixture with glufosinate, when compared with glufosinate alone, increased control of common ragweed and giant foxtail. In most instances, weed control was similar for total POST and for PRE followed by POST systems. Velvetleaf control was reduced by 12% when glyphosate was tank mixed with a half rate of atrazine compared with a full rate of atrazine. Common lambsquarters control was reduced by 13% when glyphosate was tank mixed with a half vs. a full rate of acetochlor. Glufosinate tank mixed with half rates of acetochlor + atrazine, flumetsulam, or pendimethalin reduced control of velvetleaf compared with the full rate of these products. Corn yields were not affected by residual herbicide application timings or rates of residual herbicides.

Field experiments were conducted in 2002 and 2003 to evaluate weed control and dry bean response to fomesafen and fomesafen tank mixtures. In the first experiment, six market classes of dry bean were treated at the first trifoliate growth stage with fomesafen + nonionic surfactant (NIS) + urea ammonium nitrate solution (UAN) at rates ranging from 210 to 840 g/ha. In the second experiment, fomesafen + NIS + UAN at 280 g/ha was applied alone or combined with imazamox at 36 g/ha, bentazon at 560 g/ha, or clethodim at 140 g/ha, and each treatment was applied postemergence at either the unifoliate, first trifoliate, or third trifoliate growth stage to six dry bean cultivars. Crop injury from fomesafen in the form of stunting and leaf crinkling was apparent 7 d after treatment (DAT), but crop injury was temporary and plants recovered. Common lambsquarters and hairy nightshade control increased from 61 to 71% and 74 to 92%, respectively, as fomesafen rate increased from 210 to 280 g/ha. Redroot pigweed, kochia, and common purslane were controlled at the ≥90% level by fomesafen at 210 g/ha. Applying fomesafen and fomesafen tank mixtures at the unifoliate growth stage caused less dry bean injury and improved redroot pigweed, common lambsquarters, and hairy nightshade control compared with treatments made at the first or third trifoliate growth stage. Decreased weed control caused by delaying herbicide application to the third trifoliate growth stage resulted in a 17% decrease in crop yield, compared with treatments where herbicides were applied at the unifoliate growth stage. Combining fomesafen with other herbicides increased crop injury 4%, 14 DAT. A tank mixture of fomesafen plus imazamox caused more crop injury than fomesafen plus bentazon. Combining fomesafen with imazamox or bentazon improved hairy nightshade control to 92 and 87%, respectively; however, common lambsquarters control was improved only with tank mixtures of fomesafen with bentazon.

Field studies were conducted in 1999 to 2000 on a clay soil and a sandy-loam soil in Londrina and Palmeira, PR, Brazil, respectively, to determine the persistence and carryover effect of a mixture of imazapic and imazapyr, applied to imidazolinone-tolerant corn, on rotational crops of soybean, edible bean, wheat, and corn in two different planting systems (no till and tillage). Main plots were herbicide treatments (0, 52.5 + 17.5, and 105 + 35 g ai/ha for imazapic and imazapyr, respectively) and subplots were five intervals (0, 30, 60, 90, and 120 d) between the herbicide application and rotational crop planting. Soil samples were collected for a cucumber bioassay and chemical residues analysis at each time interval. The dissipation time (DT50) of the herbicides in the soil was greater in Londrina than Palmeira, for both imazapic (54 d vs. 27 d, respectively) and imazapyr (40 d vs. 33 d, respectively), probably due to the lower pH and greater clay content of the soil in Londrina compared with Palmeira. The DT50 for both herbicides tended to increase slightly in no-till compared with conventional tillage but the differences were not great. Soybean was the least sensitive rotational crop, with a period for no yield drag (PINYD) of 87 d in Londrina and 88 d in Palmeira. Wheat and edible bean showed intermediate sensitivity. The PINYD for wheat and edible bean was 99 and 98 d for Londrina and 91 and 97 d for Palmeira, respectively. Corn was the most sensitive, with a PINYD of 117 d in Londrina and 97 d in Palmeira. Cucumber was more sensitive to imazapic and imazapyr residues than the rotational crops and should be an effective bioassay to indicate when rotational crops can be safely planted.

Greenhouse and field research evaluated yellow nutsedge growth, vegetative control, and tuber production after application of glyphosate, various acetolactase synthase (ALS)–inhibiting herbicides, and tank mixtures thereof. Yellow nutsedge was controlled by the herbicides halosulfuron at 35 g ai/ha, chlorimuron at 12 g ai/ha, and imazethapyr–imazapyr at 62 g ai/ha (> 70% control); imazethapyr at 70 g ai/ha, glyphosate at 840 g ae/ha, cloransulam at 17.5 g ai/ha, and rimsulfuron at 17.5 g ai/ha (40 to 70% control); and imazamox at 45 g ai/ha (< 40% control). Compared with the untreated control, tuber fresh weight in the field was reduced 45 to 91%, and tuber density was reduced 33 to 90% by all herbicide treatments 42 wk after treatment (WAT) except imazamox and rimsulfuron. Tuber sprouting was reduced to 19% in plots treated with halosulfuron and pyrithiobac compared with untreated yellow nutsedge 42 WAT. Chlorimuron and imazethapyr–imazapyr controlled yellow nutsedge at least 90%, prevented panicle formation, and reduced tuber density and fresh weight by 90% or more 14 WAT in the greenhouse. The addition of glyphosate to cloransulam or imazethapyr increased yellow nutsedge control and reduced tuber density and fresh weight when compared with either ALS-inhibiting herbicide or glyphosate applied alone. Tuber density data indicated that there were 8 tubers for every gram of tubers harvested. Yellow nutsedge height was 15 to 20 cm 4 to 5 wk after tillage, using growth analysis data. Long-term yellow nutsedge management may be aided with treatments that reduce tuber production.

Eleven field experiments were conducted over a 3-yr period (2010, 2011, and 2012) in conventional- and no-till soybean with a flumioxazin and pyroxasulfone premix. PRE and preplant applications were evaluated for soybean injury, weed control, and yield compared to standard herbicides. Early-season soybean injury from flumioxazin/pyroxasulfone ranged from 1 to 19%; however, by harvest, soybean yields were similar across labeled rates (160 and 200 g ai ha−1), standard treatments, and the nontreated control. Flumioxazin/pyroxasulfone provided excellent control (99 to 100%) of velvetleaf, pigweed species (redroot pigweed and smooth pigweed), and common lambsquarters across almost all rates tested (80 to 480 g ai ha−1). Common ragweed, green foxtail, and giant foxtail control increased with flumioxazin/pyroxasulfone rate. The biologically effective rates varied between tillage systems. The flumioxazin/pyroxasulfone rate required to provide 80% control (R80) of pigweed was 3 and 273 g ai ha−1 under conventional- and no-till, respectively. For common ragweed, the R80 was 158 g ai ha−1 under conventional tillage; yet, under no-till, the rate was nonestimable. The results indicate that flumioxazin/pyroxasulfone can provide effective weed control as a setup for subsequent herbicide applications.

Plant responses to various doses of herbicides usually follow a sigmoid model where the potency is given by the 50% inhibition (I 50) value. To assess the potency of a herbicide under a range of environmental conditions, a series of independent bioassays are necessary to account for assay-to-assay variation. Analysis has conventionally been done by separate analysis of the individual bioassays or by simply pooling data. Analyzing the individual bioassays separately throws up relevant information on interassay variation. Such a model becomes too complex because a full set of model parameters is needed for each data set. Pooling data instead, and analyzing the bioassay jointly, inflates parameter uncertainty because of oversimplification. Such a simple model would have too few variables, and the fixed-effect estimates would be more uncertain because they would be explaining the interassay random effects. This means that the underlying statistical model is not realistic. Therefore, we propose a new technique of intermediate complexity that outperforms either technique and provides biologically realistic estimates that allow us to compare herbicide potencies. With this technique, we simultaneously analyze independent experiments by using a combination of nonlinear regression and mixed models. The case study uses a group of independently run bioassays with two photosystem II–inhibiting herbicides, diuron and bentazon, by measuring the oxygen evolution of thylakoid membranes. The introduction of random elements in the nonlinear regression parameters reduces the uncertainty in the parameters of interest. We demonstrate that it is possible to pool data from independent experiments to assess which parameters can be assigned a random element, to conduct hypothesis testing, and to calculate stable confidence limits and thus obtain a more precise interpretation of the biologically relevant parameters, such as I 50, compared with the conventional nonlinear regression models of the individual bioassays.

Flumioxazin plus metolachlor mixtures preemergence (PRE) were evaluated with or without postemergence (POST) herbicides for weed control and peanut (Arachis hypogaea) response in three North Carolina studies. Metolachlor PRE at 2.24 kg ai/ha controlled goosegrass (Eleusine indica) and yellow nutsedge (Cyperus esculentus) 93 and 80%, respectively, and control was not improved with flumioxazin or norflurazon. Metolachlor plus flumioxazin PRE at 0.07 or 0.11 kg ai/ha controlled common lambsquarters (Chenopodium album); entireleaf (Ipomoea hederacea var. integriuscula), ivyleaf (I. hederacea), and pitted morningglory (I. lacunosa); and prickly sida (Sida spinosa) better than metolachlor plus norflurazon PRE at 1.34 kg ai/ha. Morningglories (Ipomoea spp.) were controlled 77 and 86% with flumioxazin PRE at 0.07 and 0.11 kg/ha, respectively, and control was increased to nearly 100% with acifluorfen plus 2,4-DB or lactofen plus 2,4-DB POST. Peanut injury by flumioxazin and norflurazon was observed at one location in 1997; however, yields were not reduced. Peanut treated with metolachlor plus flumioxazin PRE at either rate yielded at least 3,750 kg/ha compared to 3,120 kg/ha with metolachlor plus norflurazon PRE or 1,320 kg/ha with metolachlor PRE.

Two previously identified triazine-resistant annual bluegrass (Poa annua) biotypes in Mississippi were evaluated to determine the level as well as the mode of resistance compared to a triazine-susceptible biotype. Annual bluegrass seeds were collected at two sites (Meridian and Tupelo, MS) from field plots previously treated with 11.2 kg ai/ha simazine. Seeds were planted on agar containing simazine (0, 0.001, 0.01, 0.1, 1, 10, or 100 mM) or diuron (0, 0.01, 0.1, 1, 10, 100, or 1,000 μM), placed in the growth chamber with an 8 h daylength and alternating temperatures of 15 and 20 C, and allowed to grow for 3 wk, after which fresh weight was determined. Data were subjected to regression analyses and the amount of simazine or diuron required to reduce fresh weight to 50% of the untreated plants was determined. These data indicated a greater than 1,000-fold level of resistance to simazine with either biotype. Diuron concentrations of 5.0, 3.0, and 3.2 μM were required to reduce fresh weight to 50% of the untreated for the triazine-susceptible, Meridian, and Tupelo biotypes, respectively. Sequencing a portion of the chloroplast psbA gene revealed a serine 264 to glycine mutation, which reduced the affinity of the triazine herbicides for the QB-binding niche on the D1 protein in photosystem II.

Field experiments were conducted in Oklahoma to determine the effects of row spacing, cultivar, seeding rate, and water or ammonium polyphosphate fertilizer injection in the row at seeding, on the competitiveness of hard red winter wheat with cheat. Decreasing row spacing from 23 to 8 cm increased yield of weed-free wheat at two of three locations and cheat-infested wheat in six of ten experiments. Increasing seeding rate from 265 to 530 seeds m-2 increased wheat yield. Injecting water at 20 ml m-1 of row at seeding did not increase wheat emergence or yield. Cheat seed production was not consistently suppressed by any one cultivar. Juvenile growth habit was unrelated to wheat competitiveness.

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.