Greenhouse experiments evaluated azafenidin and thiazopyr applied separately, as a tank mix, or in rapid succession, for preemergence control of yellow nutsedge (Cyperus esculentus) and hairy beggarticks (Bidens pilosa). Average of yellow nutsedge shoot dry weights at 21 and 42 d after treatment (DAT), indicated that azafenidin at 0.28, 0.56, and 1.12 kg ai/ha controlled 70, 86, and 90%, respectively, and thiazopyr at 1.12 kg/ha controlled 10% of yellow nutsedge. However, when thiazopyr at 1.12 kg/ha was tank-mixed with azafenidin at 0.28, 0.56, and 1.12 kg/ha, yellow nutsedge control averaged for 21 and 42 DAT was reduced to 41, 69, and 78%, respectively. Root plus tuber dry weight of yellow nutsedge increased threefold when 1.12 kg/ha thiazopyr was mixed with 0.28 kg/ha azafenidin, compared to azafenidin applied alone at the above rate. When the herbicides were applied sequentially, within minutes of each other, the antagonistic responses were reproduced, independent of the order of herbicide application. Thiazopyr applied at 0.56 and 1.12 kg/ha stimulated the emergence and establishment of hairy beggarticks and antagonized azafenidin for the control of yellow nutsedge based on a mathematical model. Although thiazopyr reduced the preemergence herbicidal efficacy of azafenidin on hairy beggarticks control, antagonism could not be determined because of the stimulatory effects of thiazopyr on hairy beggarticks emergence.

The objectives of this study were to determine the efficacy and risk of controlling weeds at reduced herbicide rates under various environmental and biotic conditions, through analysis of published data on the use of below-labeled rates of herbicides. A database was established by extracting information from previously published papers on weed control at below-labeled rates of herbicides in crop production systems over large geographical and temporal scales. The database was then analyzed to evaluate the efficacy and risk of using herbicides at various reduced rates under different management systems. Using below-labeled herbicide rates in conjunction with interrow cultivation is an effective way of reducing herbicide input in agricultural systems while maintaining satisfactory weed control. There are greater opportunities for herbicide reduction using preemergence (PRE) than preplant incorporated (PPI) or postemergence (POST) herbicides, in coarse-textured than in fine-textured soils, and in corn than in soybean or wheat. The success of reducing herbicide rates does not depend on whether the herbicides are applied in conventional or conservation tillage systems or whether they are used with or without adjuvants. The above conclusions are based on studies conducted in experimental fields where weed pressures may be subjectively chosen to be high. Greater potential for herbicide reduction may exist at locations or in cropping systems were weed pressure is low.

Experiments were conducted in subirrigated and upland Colorado pastures to compare herbicides applied alone in fall to the same herbicides preceded by one, two, or three mowings. Picloram controlled Canada thistle (Cirsium arvense) well at both sites, and in general, mowing did not improve its performance. Picloram at 560 g ai/ha and picloram plus 2,4-D at 280 plus 1,120 g ai/ha were the lowest rates that eliminated Canada thistle. Mowing did not improve chlorsulfuron performance at either site, but chlorsulfuron eliminated Canada thistle at the subirrigated site. Dicamba controlled 97% of Canada thistle at the subirrigated site, and mowing did not improve its performance; two or three mowings before spraying dicamba at the upland site improved performance, but 37% remained uncontrolled. Control from the reduced rate and the two lowest recommended rates of clopyralid plus 2,4-D was improved by two or three prior mowings at the subirrigated site, but only the highest rate benefited from two or three mowings at the upland site. Mowing alone three times per year for 2 yr controlled 85% of Canada thistle at the subirrigated site, but failed to control it at the upland site. Variability in results between experiments likely was due to a high water table at the subirrigated site, which may have restricted Canada thistle root growth and possibly made it easier to control. Inconsistent results prohibit concluding that mowing before spraying will consistently improve Canada thistle control, and such a treatment combination should not be commonly recommended.

An adjuvant is a material added to a tank mix to aid or modify the action of an agrichemical, or the physical properties of the mixture. For the most recent decade, responsible researchers, suppliers, and trade organizations have made significant progress to identify, understand, and standardize adjuvants. Adjuvant components, their purpose in application, terminology, and now, even tests that establish minimum performance expectations are under development. Certification of voluntary adjuvant composition and performance standards will soon be available. The adjuvant industry has grown into respectability. This paper will attempt to serve as a useful reference for current terminology and definitions accepted by weed science, and also provide a brief discussion of the chemistry favored in current adjuvant compositions.

Activator adjuvants for herbicides increase herbicide activity and encompass a wide variety of surfactant chemistry. Activator adjuvants may be included in the product formulation by the manufacturer or tank-mixed by the herbicide applicator. Activator adjuvant efficacy is a function of not only the adjuvant but also the herbicide, the particular weed species, and environmental conditions. Proposed modes of activator action include reduction of spray solution surface tension to enhance contact area, solibilization of the leaf cuticle, emulsifier action, increased spray retention, protection of the herbicide in the spray solution, promotion of rainfastness, acting as a cosolvent or copenetrant, modification of spray deposition on plant foliage, and enhanced movement on the foliage surface to areas of greater absorption.

Pitted morningglory control with norflurazon or fluometuron preemergence (PRE), each at 1.12 kg/ha, was 72% or less 14 d after a postemergence (POST) application of pyrithiobac in 1993 and 1994. Pyrithiobac POST at 70 g/ha following fluometuron or norflurazon PRE cultivated at any timing controlled pitted morningglory at least 76%. Pyrithiobac POST controlled common cocklebur equal to the weed-free in 1993 at 14 d after the POST application. In 1994, when rated 28 d after pyrithiobac POST, cultivation 3 d before a single application of pyrithiobac controlled less common cocklebur than any other herbicide treatment containing pyrithiobac. At 28 d after pyrithiobac POST, no treatment controlled common cocklebur as well as the weed-free. In 1993 and 1994, seed cotton yield was equal to the weed-free when pyrithiobac followed fluometuron PRE cultivated at any timing or a single application of pyrithiobac cultivated prior to 7 d after POST application.

Field studies were conducted in 1996 and 1997 to determine postemergence control of purple (Cyperus rotundus) and yellow nutsedge (Cyperus esculentus) by single and sequential applications of bentazon, halosulfuron, and sulfentrazone; sequential applications of MSMA; and single applications of imazaquin and imazaquin plus MSMA. Sequential applications of halosulfuron at 70 g ai/ha controlled purple nutsedge more effectively at 10 wk after initial application (WAI) than all other herbicides. Single applications of imazaquin (0.56 kg ai/ha) or imazaquin plus monosodium salt of methylarsonic acid (MSMA, 2.24 kg ai/ha) controlled purple nutsedge adequately 6 WAI (78 and 90%, respectively) but not 18 WAI. Single applications of either halosulfuron (70 g/ha) or imazaquin plus MSMA or sequential applications of either bentazon (1.12 kg ai/ha), MSMA (2.8 kg/ha), or halosulfuron controlled yellow nutsedge greater than 80% by the end of the season in 1996 when nutsedge was in bermudagrass (Cynodon dactylon L.). However, in absence of bermudagrass in 1997, only sulfentrazone or the halosulfuron sequential treatments controlled yellow nutsedge 13 WAI. In 1996, single and sequential applications of halosulfuron reduced viability of purple nutsedge tubers to 33 and 30%, respectively, compared to 84% in the nontreated control. However, herbicide treatments did not reduce tuber viability in 1997. Sulfentrazone (0.28 kg ai/ha) plus MSMA (2.24 kg/ha) and sequential MSMA applications reduced viability of yellow nutsedge tubers to 6 and 31%, respectively, in 1997.

A greenhouse study was conducted to determine the effects of sublethal dicamba concentrations in the nutrient media on hydroponically grown tomato plants. Tomato leaf area was the most sensitive vegetative growth parameter measured in response to dicamba concentrations, ranging from 0 to 22 µg/L. Leaf area was reduced 31 and 76%, and specific leaf weights, a relative measure of leaf thickness (g/cm2), increased 26 and 121% after 30-d exposure to dicamba concentrations of 2.2 and 22 µg/L, respectively. In long-term experiments conducted until plants produced first ripe fruit, regression analysis indicated leaf area reductions of 8 and 66% from initial dicamba concentrations of 1 and 10 µg/L, respectively. Reductions in total fruit fresh weight were highly correlated (r = 0.93) with leaf area reductions caused by dicamba. A hyperbolic regression model gave predicted losses in fruit fresh weight per plant of 6% at 1 µg/L dicamba and 73% at 10 µg/L dicamba (r 2 = 0.87). Results generally indicated that the level of dicamba in the nutrient media of hydroponically grown tomatoes that produced no observable effect was ≤ 1 µg/L.

Utility adjuvants are adjuvants that are tank-mixed in the spray solution to improve the spray application process, but do not directly influence herbicide efficacy. However, by improving the spray application process, utility adjuvants can indirectly improve herbicide efficacy. There are five primary utility adjuvant types: compatibility agents, deposition agents, drift control agents (sometimes referred to as antidrift agents or drift retardants), defoaming agents, and water conditioning agents, and three secondary utility adjuvant types: acidifying agents, buffering agents, and colorants (dye markers). Herbicides can react either physically or chemically with other spray mixture components to form an unsprayable mixture. Compatibility agents prevent these reactions from occurring. Drift control agents and deposition agents increase the amount of herbicide deposited on target surfaces. The primary function of drift control agents is to reduce the amount of spray solution that moves off-target. Indirectly, the amount of herbicide reaching target surfaces can be increased. A defoaming agent will reduce or prevent foam produced in the spray mixture. Ions in the spray solution can interact with various herbicides, decreasing efficacy. Water conditioning agents will counteract the effect of the ions on herbicides. Water conditioning agents must be added before the herbicide to prevent herbicide–ion interaction. Acidifying and buffering agents function in a similar fashion, reducing or increasing spray solution pH. A buffering agent will maintain a pH range, whereas an acidifying agent will not. Colorants are dyes that are added to the spray solution to produce a visible color on the sprayed area to assist the applicator in applying the herbicide.

The impact of Palmer amaranth on mechanical harvesting, ginning, and fiber quality in dryland cotton was documented. Only the highest Palmer amaranth density (3,260 weeds/ha) reduced lint and seed yields. However, all weed densities increased harvesting time 2- to 3.5-fold. Two factors increased the time required for stripper harvesting: slower ground speeds due to large weeds and work stoppages that required hand removal of weed stems lodged in the harvester. Ninety-eight percent of the weedy plant material was discarded in the field by the harvester, and the remaining 2% was successfully removed in ginning and lint-cleaning processes. Weed infestations did not result in any differences in moisture content of seed cotton, ginning time, fiber quality, or the percentage of cleaned lint.

Field experiments were conducted in 1996 and 1997 near Arlington, WI, to compare the efficacy of glyphosate applied below registered rates in sequential and tank-mix combinations with residual herbicides to no-till, narrow-row, glyphosate-resistant soybean. In the sequential combinations of preplant (PP) residual herbicides and postemergence (POST) glyphosate study, glyphosate followed one of eight burndown treatments. Clomazone applied PP controlled 86% of common lambsquarters in 1996 when followed by 420 g ae/ha glyphosate early POST (EPOST). All other herbicide treatments controlled 94% or greater regardless of weed species, PP treatment, glyphosate timing, or glyphosate rate. The greatest soybean yields occurred in EPOST glyphosate applications in 1996 and late POST (LPOST) glyphosate applications in 1997. The only time PP residual herbicides were beneficial was prior to the LPOST glyphosate application in 1996. In the tank-mix POST combinations of glyphosate and residual herbicides study, glyphosate was applied alone or in combination with four residual herbicides. Soybean injury did not exceed 5% except in the glyphosate and imazethapyr combination in 1997. Control of common lambsquarters, velvetleaf, and giant foxtail was 90% or greater when averaged across all residual combinations and glyphosate timings and rates. Imazethapyr alone controlled velvetleaf 99% and giant foxtail 92% in 1997. When glyphosate was applied alone, soybean yields were similar at all glyphosate rates and application timings, except the 630 g/ha glyphosate LPOST resulted in a lower yield than 420 g/ha glyphosate LPOST. Only one residual herbicide, SAN 582, combined with glyphosate produced yields equivalent to the highest yielding treatments when averaged over both glyphosate rates and timings. Cloransulam added to 420 g/ha glyphosate EPOST and chlorimuron plus thifensulfuron and imazethapyr added to 420 g/ha glyphosate LPOST resulted in lower soybean yields compared to the same rate of glyphosate applied alone at the respective timings. Thus, no herbicide combination preformed better than glyphosate applied in a timely manner alone. However, in situations where early-season weed competition is severe and a timely glyphosate application is not possible, a PP residual herbicide may be beneficial.

A survey conducted across agricultural ecoregions of Saskatchewan in 1996 revealed that wild oat (Avena fatua) populations resistant to acetyl-CoA carboxylase (ACCase) inhibitors were present in approximately 10% of Saskatchewan fields (2.4 million ha). In the Aspen Parkland and Boreal Transition ecoregions, this increased to 17%. The objective of this study was to determine if agronomic practices promoted or delayed resistance and to assess producer awareness of herbicide resistance. Weed resistance and management questionnaire data from the 1996 resistance survey and management questionnaire data from the 1995 Saskatchewan weed survey were submitted to multiway frequency analysis. The frequency of occurrence of herbicide-resistant wild oat was related directly to ACCase inhibitor use. Resistance to cyclohexanedione (CHD) herbicides was not related to CHD use but to frequency of ACCase inhibitor use (i.e., CHD + aryloxyphenoxypropanoate [AOPP]), suggesting that the pressure imposed by AOPPs contributed to the selection of CHD resistance in wild oat. ACCase inhibitor use was more extensive in the Aspen Parkland and Boreal Transition ecoregions than in the Mixed and Moist Mixed Grassland ecoregions. Crop rotations were not conducive to rotation of herbicides with different sites of action. Frequency of ACCase inhibitor use increased with frequency of annual crops, in spite of the inclusion of cereal and dicot crops in the rotation. Producers utilizing conservation tillage practices in the Grassland ecoregions used proportionally more ACCase inhibitors than those using conventional tillage practices. This increase in ACCase use in conservation tillage systems did not result in an increased incidence of wild oat populations resistant to ACCase inhibitors. Producers reporting troublesome wild oat populations tended to have proportionally more ACCase-resistant wild oat. Producers who reported practicing weed sanitation were less likely to have resistant wild oat than those who were less careful. Increased awareness and implementation of management practices that will reduce the dependency on ACCase herbicides are required to better enable producers to prevent, delay, or manage herbicide-resistant wild oat populations.

The physicochemical properties of adjuvants determine their function and impact upon biological activity. Various physicochemical parameters are key to modifying both the preretention events and postretention consequences of adjuvant usage, irrespective of whether the adjuvants are tank-mix additives or built into a formulation. This paper discusses several key adjuvant parameters for a range of adjuvant chemistries alone and in mixtures. In addition, the misleading use of terms such as nonionic surfactant and hydrophile–lipophile balance is addressed. From a more coherent understanding of the parameters involved, it can be shown that there are ways of predicting the required properties of an adjuvant to solve specific delivery problems. The recognition that different problems often require quite different approaches illustrates that good adjuvants do not exist per se, only materials that should be rationally selected for specific reasons. The chemistry of the herbicide and the nature of its targets will dictate adjuvant selection criteria.

Three commercial formulations of glyphosate were spiked with 14C-glyphosate and applied via a spray nozzle to velvetleaf plants. The use of 14C-glyphosate as a marker caused minimal alteration to formulation properties, and the use of spray application simulated field practices. Formulation retention, calculated based on maximum plant-leaf area, showed that only 27 to 33% of the available area retained the spray. The small differences in retention among the formulations suggest that they contribute little to differences in efficacy. Following spray application, plants were harvested at various times to measure the levels of glyphosate uptake into the plant and translocation into roots. Significant differences were observed among the formulations in the rate of glyphosate uptake. The most efficient formulation absorbed about one third of the dose by 24 hr after treatment. Root translocation of glyphosate was approximately proportional to uptake and accounted for less than one third of the absorbed dose. The relationship between uptake and rainfastness was examined in greenhouse studies with simulated rainfall at various times after glyphosate application. A direct correlation was observed between rainfastness with the speed and quantity of glyphosate uptake.

The effect of nicosulfuron application timing on wild-proso millet (Panicum miliaceum) control and sweet corn (Zea mays) yield was evaluated in the field during 1991, 1992, and 1993. Sweet corn yields were equal to hand weeded controls when nicosulfuron was applied to wild-proso millet shorter than 9 cm, but the best (> 95%) wild-proso millet control occurred when nicosulfuron was applied to wild-proso millet plants between 11 and 19 cm tall. However, nicosulfuron controlled wild-proso millet 90 to 95%, 13 wk after planting, when nicosulfuron was applied to plants 8 to 10 cm tall. Nicosulfuron applications made to wild-proso millet plants less than 8 cm were made early in the season and failed to control subsequent wild-proso millet flushes. Additionally, nicosulfuron failed to control completely wild-proso millet plants larger than 20 cm. Consequently, nicosulfuron applied to wild-proso millet plants shorter than 8 cm or taller than 20 cm resulted in a rapid decline in wild-proso millet control 13 wk after planting. Nicosulfuron applied following 6.7 kg ai/ha EPTC plus 2.2 kg ai/ha cyanazine PPI in 1991, could be applied to wild-proso millet 25 cm tall without subsequent losses in wild-proso millet control or sweet corn yield. Wild-proso millet control was 93% and sweet corn yields were equal to hand-weeded controls when nicosulfuron was applied to wild-proso millet 4 cm tall and followed by one cultivation 14 d after application.

The development of glufosinate-resistant corn represents a new weed management system for corn growers. Field experiments were conducted from 1995 to 1997 at four locations in southwestern Ontario. The objective of this study was to determine the effect of timing of weed control relative to the growth stage of corn with glufosinate applied alone or in combination with residual herbicides. Control of all species tested improved with the addition of atrazine plus dicamba to glufosinate, applied from the two- to eight-leaf stage of corn growth. Based on a 90% weed dry matter reduction, glufosinate with atrazine plus dicamba controlled common ragweed, common lambsquarters, and pigweed species at the three-leaf stage of corn and yellow foxtail, barnyardgrass, and large crabgrass at the two-, four-, and eight-leaf stage of corn, respectively. Weed control with glufosinate alone was improved when applied at the later growth stages of corn. Glufosinate applied alone at the four-leaf stage of corn controlled common ragweed and common lambsquarters, whereas pigweed species were controlled effectively at the eight-leaf stage of corn growth. Corn grain yield was consistently higher when glufosinate was applied in combination with residual herbicides, compared to glufosinate alone. Glufosinate in combination with residual herbicides applied to corn at the three- to five-leaf stage may represent the best timing for weed control. Our data suggested that a tank mixture of glufosinate with other postemergence residual herbicides or a split application of glufosinate in combination with cultivation may be required for weed control in glufosinate-resistant corn.

The objective of this study was to compare the consistency and accuracy of visually estimated weed biomass and weed control data to data obtained through image analysis. Weed biomass and weed control were evaluated in soybean herbicide efficacy trials conducted at the University of Nebraska–Lincoln during 1992 and 1993. Measurements were based on visual estimates and on aerial photographs taken at a height of 3.5 m above the soil surface. Photographs were digitized and classified, producing pixel values for broadleaf weeds, grass weeds, soybean, and soil. Percent weed cover was calculated in relation to the crop canopy, based on the respective number of pixels per image. Visual and photographic ratings of weed biomass and of weed control were not closely correlated. In the first year the visual method discriminated between more treatments than the photographic method, but the opposite occurred in the second year. The photographic method predicted yield more closely than the visual estimates. We concluded that visual estimates were less consistent and more subject to observer bias than measurements obtained with the photographic method.

Field experiments were conducted in 1997 and 1998 near Rossville in northeast Kansas to evaluate control of imazethapyr-resistant common sunflower with selected herbicides in corn and soybean. In soybean, common sunflower control was unacceptable with imazethapyr, chlorimuron, CGA-277476, and cloransulam applied alone. Lactofen or acifluorfen applied with these herbicides slightly improved control, whereas glyphosate applied with them controlled more than 97% of common sunflower. All herbicides reduced common sunflower populations in the corn. The greatest reductions occurred when atrazine was applied with dicamba, 2,4-D, RPA 201772, or CGA-152005 plus primisulfuron. These herbicide combinations also stunted common sunflower that survived treatment. Several herbicide alternatives are available to control imazethapyr-resistant common sunflower in corn. However, glyphosate was the only herbicide that gave adequate control in soybean.

Field experiments were conducted in 1996 and 1997 to evaluate barnyardgrass (Echinochloa crus-galli) control, rice (Oryza sativa) yield, and grain quality with cyhalofop-butyl applied early postemergence (EPOST) or late postemergence (LPOST) in drained or flooded plots at rates of 0.15 to 0.4 kg ai/ha and standard treatments of molinate or propanil. Control with cyhalofop-butyl at 0.15 and 0.2 kg/ha applied in drained plots was greater compared to propanil and generally increased when treatments were applied EPOST to plots reflooded 2 d after treatment. Barnyardgrass control with cyhalofop-butyl at 0.15 and 0.2 kg/ha applied in flooded plots was similar to or better than the control obtained with molinate. Rice yields generally reflected differences in barnyardgrass control and were greatest with cyhalofop-butyl applied EPOST in drained plots. The 0.4 kg/ha rate of cyhalofop-butyl resulted in rice foliar injury and significant reductions in grain yield. Grain qualitative characteristics (total milling yield, grain vitreosity, grain length, and grain length to width ratio) were better with cyhalofop-butyl applied EPOST in drained plots compared with propanil or molinate.

How surfactants modify the characteristics of a spray liquid is now reasonably well understood. Beneficial effects are primarily associated with reduction in surface tension. However, the mechanisms whereby surfactants enhance the diffusion of herbicides across the plant cuticle are less clear. Generally, hydrophilic surfactants with a high hydrophile/lipophile balance (HLB) are most effective at enhancing penetration of herbicides with high water solubility, whereas lipophilic surfactants with a low HLB are most effective for enhancing uptake of herbicides with low water solubility. Both high- and low-HLB surfactants are absorbed into the cuticle, but current theory suggests different mechanisms are involved in enhancing diffusion of hydrophilic and lipophilic herbicides across the cuticle. Surfactants having a high HLB are absorbed into the cuticle and enhance the water-holding capacity (hydration state) of the cuticle. With increased cuticle hydration, the permeance of hydrophilic herbicides into the cuticle is increased, which increases the herbicide diffusion rate at a constant concentration gradient. Surfactants having a low HLB are absorbed into the cuticle and increase the fluidity of waxes, as measured by a small reduction in melting point. This increased fluidity increases the permeance of lipophilic herbicides in the cuticle, which, in turn, increases their diffusion rate at a set concentration gradient.