The effects of ammonium sulfate and pelargonic acid on weed control with glufosinate and glyphosate and safety to glufosinate-resistant and glyphosate-resistant soybean were investigated in the greenhouse and field. Annual and perennial weeds varied in their sensitivity to the herbicides. Based on fresh weight reduction 10 d after treatment (DAT), common milkweed was more tolerant to glufosinate, and horsenettle was more tolerant to glyphosate. Giant foxtail was highly sensitive to both herbicides. The activity of glufosinate on common milkweed and of glyphosate on horsenettle was enhanced with the addition of 5% (wt/v) ammonium sulfate. The addition of pelargonic acid at 3% (v/v) did not enhance the activity of glufosinate or glyphosate on any weed, and it antagonized common lambsquarters and giant foxtail control with glufosinate and with glyphosate. Glyphosate was more effective than glufosinate in suppressing the regrowth of the perennial weeds horsenettle and common milkweed, but addition of ammonium sulfate and pelargonic acid was not beneficial with either herbicide. Under field conditions, the addition of ammonium sulfate or pelargonic acid to glufosinate or glyphosate did not improve efficacy on annual weeds. The addition of pelargonic acid improved yellow nutsedge control with glufosinate, but only at 6 DAT. Glufosinate and glyphosate applied alone or in combination with ammonium sulfate were safe to transgenic soybeans resistant to the respective herbicide. The addition of pelargonic acid to glufosinate or glyphosate in the greenhouse caused a rate-dependent reduction in soybean fresh weight. In the field, slight soybean injury with the addition of pelargonic acid was evident 6 DAT, but not 23 DAT. Addition of ammonium sulfate can increase the efficacy of glufosinate and glyphosate on perennial weeds without negatively affecting soybean yield.

Suppression of grass encroachment of one warm-season grass into another species is an important management tool. Two field studies were conducted in Georgia to determine the timing, rates, and frequency of ethofumesate plus flurprimidol applications needed to suppress three bermudagrass cultivars and determine what effects these treatments have on tolerance of three seashore paspalum cultivars during 1998 and 1999. Tank-mixes of ethofumesate and flurprimidol applied at the 1× rate (1.7 + 0.8 kg/ha) on April 1 suppressed ‘TifEagle’ bermudagrass effectively (≥ 78%) by late September. However, the suppression of ‘Tifway’ bermudagrass (≤ 39%) and ‘common’ bermudagrass (≤ 67%) was not acceptable at the same rates and timing. Ethofumesate plus flurprimidol applied at the 1× rate in April severely injured all paspalum cultivars 61 to 65% within a 2- to 6-wk period, but the cultivars recovered to an acceptable level (≤ 30%) by 10 wk. In most instances, reduced ethofumesate plus flurprimidol rates (¼× and ½×) following the 1× rate in April did not injure the paspalum cultivars as severely as did the initial 1× rate. However, bermudagrass suppression was improved from the multiple treatments. When the chemicals were applied initially at the 1× rate and followed by four timely ¼× rates, bermudagrass suppression by late September was ≥ 72% for all bermudagrass cultivars, with the exception of common bermudagrass in 1998 (52%). Maximum injury to the paspalum cultivars during 1998 and 1999 from the four repeated ¼× rates ranged from 16 to 45% for ‘Sea Isle 1’, to 33 to 37% for ‘Sea Isle 2000’, and 21 to 52% for ‘K-3’. Ethofumesate plus flurprimidol applied initially at the 1× or 2× rate on June 30 and followed by a repeated application on July 25 did not effectively suppress (≤ 70%) common bermudagrass either year or Tifway bermudagrass in 1999. However, the suppression of common bermudagrass (≥ 83%) and Tifway bermudagrass (≥ 90%) was effective when the chemicals were applied initially at the 2× rate on June 30, followed by a 2× rate on July 25 and a 1× treatment on August 14. TifEagle bermudagrass was effectively suppressed (≥ 85%) when applied initially on June 30 and July 25 at the 1× rate. Ethofumesate plus flurprimidol applied once at the 1× rate on June 30 caused ≤ 46% injury to paspalum cultivars. The injury was generally ≥ 50% when the chemicals were applied in two or more applications.

Goosegrass control studies were conducted with multiple preemergence (PRE) and postemergence (POST) herbicides on bermudagrass fairways on the tropical island of Guam. Two POST applications of MSMA plus metribuzin, applied 1 wk apart were needed to control the existing goosegrass. Three applications of these herbicides showed a clear trend in goosegrass control, with control lasting 4 wk longer than two treatments. PRE herbicides provided efficacious control only when applied in sequence with a single MSMA plus metribuzin application. The control from sequential applications of PRE herbicides was generally no better than that with two applications of MSMA plus metribuzin. However, application of MSMA plus metribuzin resulted in turf discoloration lasting 5 to 8 wk. A single POST application reduced turf quality from 35 to 41% when compared to the untreated check. Two applications caused an additional 5 to 7% reduction in turf quality. Three applications caused similar discoloration as two applications. Application of PRE herbicides had no effect on turf quality, with ratings being equal to those of plots treated with POST herbicides only.

A study was conducted at three locations in Saskatchewan, Canada, in 1996 and 1997 to determine if increasing the seeding rate of wheat, barley, and lentil by 50% would maintain weed control and crop yield when herbicides are applied at reduced rates or not at all. Three herbicide rates (½ of full, ¾ of full, and full recommended label rate), along with an untreated check, two crop seeding rates (normally recommended and 1.5 times normally recommended rates), and three crops were tested. Increasing seeding rate did not affect weed fresh weights, crop yield, and net return responses to herbicides applied at reduced rates or not at all when averaged across crops, years, and locations. Increased seeding rate, independent of the different herbicide applications, had infrequent and inconsistent effects among the crop by year by location combinations. More broadleaf and grass weed growth, less crop yield, and lower net returns generally occurred when herbicides were not applied or applied at reduced rates. These trends were especially prominent when herbicides were not applied to cereal crops at Saskatoon (40% yield reduction) and when herbicides were applied at ½ the full label rate rather than higher herbicide rates to wheat at the other two locations (16% yield reduction). In 1996, lentil yield and net returns did not respond to herbicide application and rate because of poor grass weed control across all herbicide rates. Lentil yield and net returns decreased by 11% (full vs. ¾), 22% (¾ vs. ½), and 46% (½ vs. none) when herbicides were applied at progressively lower rates in 1997. Reduced herbicide rates did not affect net returns for cereal crops, indicating that herbicide rates lower than the full label rate may be economically viable in certain crops.

Laboratory studies were conducted to determine if electrolyte leakage from either leaf tissue, germinating seeds, or excised roots correlated with previously established soil-applied field response of soybean cultivars and target weeds to sulfentrazone. Sulfentrazone-induced electrolyte leakage from leaf tissue of coffee senna (sensitive), sicklepod (tolerant), and soybean cultivars ‘Asgrow 6785’ and ‘Carver’ (sensitive) and ‘Stonewall’ and ‘DPL 3606’ (tolerant) was monitored over time. Electrolyte leakage from leaf tissues, caused by 25 ppm (65 μM) sulfentrazone, agreed directly with the known response of these weeds, but response of the four soybean cultivars was equivalent. Furthermore, sulfentrazone-induced electrolyte leakage from leaf tissue of Asgrow 6785 and Stonewall was not affected by sulfentrazone concentration as high as 100 ppm (258 μM) nor by light intensity (4 and 120 μmol/m2/s photosynthetically active radiation). For germinating seeds, sulfentrazone-induced electrolyte leakage was also independent of soybean cultivar. In contrast, electrolyte leakage from excised roots of germinal soybean seedlings did concur directly with the previously established cultivar sensitivity to soil-applied sulfentrazone. Results indicate that electrolyte leakage from excised roots of soybean germinal seedlings can be used to assess cultivar sensitivity to soil-applied sulfentrazone.

Field studies were conducted in 1992 and 1993 to evaluate weed control by 15 herbicide treatments in wheat stubble and in the succeeding corn crop. Atrazine at 2.24 kg ai/ha plus several herbicide treatments were applied about 13, 21, and 33 d following winter wheat harvest on separate plots in 1992 and 1993 in a soybean–winter wheat–corn rotation. Atrazine with and without 2,4-D isooctyl ester at 1.46 kg ae/ha or dicamba at 0.36 kg ae/ha did not control barnyardgrass, green foxtail, horseweed, kochia, stinkgrass, tumble thistle, or witchgrass in the wheat stubble 30 d after treatment. Atrazine mixtures containing glyphosate or paraquat with or without 2,4-D or dicamba controlled most summer annual weed species. Atrazine plus paraquat at 0.43 kg ai/ha was more effective on redroot pigweed and tumble thistle than atrazine plus glyphosate at 0.43 kg ae/ha plus 2,4-D at 0.95 kg ae/ha. Atrazine plus glyphosate mixtures were more effective on barnyardgrass for the first and second application date than atrazine plus paraquat. Increasing the glyphosate rate from 0.43 to 0.67 kg/ha was necessary to control barnyardgrass 95% at the first date. With the first date of application, kochia control was greater when 2,4-D or dicamba was added to the atrazine plus paraquat (0.43 kg/ha) mixture. Although annual grass control was generally greater when weeds approached maturity, early applications are a more sound weed control strategy because of soil water conservation and prevention of weed seed production. However, corn yields in 1993 were greater on plots treated at the third application in 1992 because weed biomass in corn was less. In 1994, corn yields were highest for the first application in 1993, probably because of better weed control in the corn. Above average rainfall in 1993 and 1994 aided corn yields.

Reduced herbicide inputs can diminish pesticide movement into water supplies, enhancing environmental quality. A 3-yr study was conducted to evaluate the efficacy and economic viability of reducing herbicide inputs by using ultra-low rates (ULRs), consisting of one-eighth the normal use rates of nicosulfuron plus thifensulfuron, delayed planting (DP), or both in corn and soybean. We compared the ULR treatment at the assigned cost of $12.35/ha with three other traditional types of weed management systems in both chisel plow and no-till production schemes. The ULR weed management system suppressed weeds enough to allow economical soybean production all 3 yr of the study, but this system proved viable only during the wettest year for corn. DP was economically competitive with the best systems in both crops under chisel-plowed tillage only in the driest year of the study.

Field experiments were conducted to study the competition effect of winter wheat planted in a square arrangement and Italian ryegrass planted randomly on biomass yields of both species, ryegrass seed yield, N use efficiency, and progeny seed germination. Increases in wheat density up to 800 plants/m2 reduced ryegrass seed yield by 87% but increased its harvest index up to 42% compared to its monoculture yield. Species densities and their interactions accounted for 66 to 73% of the total variation in per-unit area biomass of species, and their association was more favorable to ryegrass biomass than wheat. Seeds of each species had three times greater nitrogen percentage than did shoots. Intra- and interspecific competition increased nitrogen percentage in wheat seeds. In Italian ryegrass, only interspecific competition increased N percentage in seeds. Although total nitrogen uptake by winter wheat was three times greater than in Italian ryegrass, Italian ryegrass was two times more efficient than wheat at producing biomass per unit of N taken up and specific leaf area at heading stage in mixture. Germination percentages of progeny seeds of both species in mixtures were greater in presence of high densities of the companion species than in their monocultures. Nitrogen was not the main limiting factor for competition between winter wheat and Italian ryegrass in this study.

The influence of four tillage systems, varying from intensive to zero tillage, on weed populations and the vertical distribution of weed seeds in the soil was determined at Alliance, Hairy Hill, and Wainwright in northeastern Alberta. The soil was sampled at two depths (0 to 5 and 5 to 10 cm) in fall. Weed seedling emergence in the greenhouse over the winter was assumed to represent the type and amount of weed seeds present in the soil seedbank. Emerged weed seedlings were also identified and counted in the field in spring. In the zero-tillage system, most of the weed seeds were close to the soil surface (0 to 5 cm) at Alliance and Wainwright but were deeper (5 to 10 cm) at Hairy Hill. The winter annuals, field pennycress, shepherd's-purse, and flixweed, and the summer annuals, wild buckwheat and common lambsquarters, increased in the soil seedbank as tillage was reduced, but the higher populations in the soil seedbank did not always result in higher spring seedling populations under zero tillage. In contrast to the seedbank, spring seedling populations of common lambsquarters at Alliance and field pennycress and ball mustard at Hairy Hill were lowest in the zero-tillage system, suggesting that the requirement for herbicides for controlling these weeds in the crop may be least under zero tillage. Both soil seedbank and spring seedling populations of shepherd's-purse at Wainwright and Alliance and of flixweed at Alliance were highest in the zero-tillage system. At Alliance, wild buckwheat seedling emergence in the spring tended to be highest in the minimum-tillage system (one tillage operation prior to seeding). Both soil seedbank and spring seedling populations of green foxtail decreased as tillage was reduced, indicating that green foxtail should become less of a problem under reduced tillage.

Differential response of wild carrot to 2,4-D was found in seeds collected from 10 locations in Michigan, three in Ohio, one in Illinois, and one in Ontario, Canada. Greenhouse studies were conducted on plants grown from the collected seeds to confirm resistance of wild carrot to 2,4-D, and to study variations among and within populations. The differential response of wild carrot to 2,4-D in field research was due to resistant individuals. Among the 14 locations, wild carrot control with 2,4-D ranged from 18 to 91%. Wild carrot varied in its response to 2,4-D among and within populations as well as within individual umbels. In 69% of the tested samples, at least one wild carrot plant was resistant to 2,4-D.

Nicosulfuron efficacy varies with surfactant, natural salts in the spray water carrier, and added nitrogen fertilizer salts. Scanning electron micrographs (SEM) were taken of nicosulfuron spray droplet residue on large crabgrass in the greenhouse. Spray residue characteristics differ for nicosulfuron applied with surfactants alone and with specific salts. Uniform deposits with close contact to the leaf epicuticular surface generally related positively to nicosulfuron efficacy. Ammonium salt enhancement of nicosulfuron phytotoxicity when with surfactant X-77® related to a change from a ring to a uniform deposit. Spray mixtures containing Tween 20 or Atplus 300F surfactants that gave distinct dark amorphous deposits over anticlinal cell walls generally related to effective nicosulfuron treatments. Salts that were antagonistic to nicosulfuron phytotoxicity left a large amorphous deposit, including ammonium nitrate antagonism of nicosulfuron applied with Pluronic® P85 surfactant and general antagonism from sodium bicarbonate. The SEM information indicates that the effect of surfactants and salts on spray deposit characteristics influence nicosulfuron efficacy.

Field research was conducted to determine effects of application rate, spray adjuvants, and spray carriers on visible control of downy brome, jointed goatgrass, and cheat and potential injury to wheat by MON 37500. MON 37500 at 24 and 46 g/ha with and without a methylated seed oil or nonionic surfactant and carriers of water, urea ammonium nitrate (UAN), and a 1:1 water/UAN combination were applied to the weeds at the one- to four-leaf stage. Cheat was the most susceptible weed to MON 37500, with control consistently above 87% with all treatments except MON 37500 at 24 g/ha in water 26 weeks after treatment. Downy brome control was more variable, with ratings ranging from 50 to 99% among treatments. For jointed goatgrass, only moderate stunting was observed from all MON 37500 applications. Both wheat varieties showed early season injury after MON 37500 was applied with UAN and either adjuvant; however, no visible injury or yield reduction to either wheat variety was noticed at harvest.

Applying glyphosate relative to the growth stage of soybean is important for maximizing weed control and profits in glyphosate-resistant soybean under no-till systems. A study was conducted in Ontario for 4 yr to evaluate the effectiveness and gross return on the timing and sequence of applications of glyphosate in glyphosate-resistant no-till soybean. Percent control of various weed species varied among years due to environmental conditions. Timing of glyphosate was critical relative to weed emergence and determined the success of the treatment in terms of optimum soybean yield and gross return. Soybean yield and gross return approximated that the critical period for weed control in glyphosate-resistant no-till soybean was the unifoliolate to the one- to three-trifoliolate stage. Sequential applications of glyphosate provided higher soybean yield and gross return than a single preplant application of glyphosate. Glyphosate applied preplant or at the unifoliolate stage followed by a second application at the one- to three-trifoliolate stage consistently provided maximum average soybean yield and gross return. Gross return of the sequential glyphosate treatments was also more consistent across variable soybean price scenarios. Competition from uncontrolled later emerging weeds resulted in soybean yield loss with the single preplant application of glyphosate. Competition from uncontrolled early-emerging weeds reduced soybean yields when glyphosate was applied only at the one- to three-trifoliolate stage of soybean. Overall, two weed control strategies were identified: (1) two applications of glyphosate, the first at preplant to the unifoliolate stage, followed by a second application at the one- to three-trifoliolate stage of soybean, (b) first application of glyphosate at the unifoliolate stage followed by a second application at the one- to three-trifoliolate stage of soybean if later emerging weeds exceeded threshold densities.

Adjuvants have proven very useful materials when added to a herbicide formulation or added to the spray tank to improve herbicidal activity of application characteristics. Adjuvants are so closely linked to the agrochemical markets that an understanding of the trends occurring in those markets is critical to understanding trends occurring in the adjuvant markets. The worldwide crop protection market is estimated to be about $31 billion. Herbicides are estimated to represent about 50% of the total, with insecticides (25%), fungicides (20%), and other (5%) making up the balance. The U.S. crop protection market is estimated to be in the $7 to 8 billion range. Herbicides are estimated to represent about 70% of the total, with insecticides (17%), fungicides (9%), and other (4%) making up the balance.

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.

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 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.

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.