Phytotoxic compounds from plants and microorganisms represent a wide range of chemistries and mechanisms of action that have potential in the design and development of new herbicides. Although several natural products of higher plants have been patented as herbicides, none have been developed commercially. Many microbial products have been patented as herbicides and several have been or are being developed, including bialophos {L-2-amino-4-[(hydroxy)(methyl)phosphinoyl] -butyryl-L-alanyl-L-alanine} and glufosinate [L-2-amino-4-(hydroxyl)(methyl)(phosphinoyl)-butyric acid]. The new tools of molecular biology and biotechnology are making natural products more attractive alternatives in herbicide discovery programs.

The behavior and dissipation of 26 dinitroaniline herbicides in soils are reviewed. Some of the compounds also are referred to as dinitrotoluidines, -benzen(e)amines, -benzenediamines, -cumidines, -benzenesulfonamides, -acetophenones, and sulfonylsulfilimines. All dinitroanilines are sorbed to soil particles, particularly to organic or humic substances, and are nearly immobile in soils. Soil K values range from 7 to 117 and KOC values range from 80 to 471 000. Compounds with vapor pressures greater than 50 × 10-6 mm of Hg at ambient temperatures were reported to volatilize and diffuse through, and out of, the soil in significant amounts, depending on temperature and moisture conditions. Greater losses occurred from warm, moist soils than from cool, dry soils. Photodecomposition of dinitroanilines on soil surfaces was low but occurred when the chemicals were present in the vapor state or in aqueous solutions. Bioavailability of the chemicals decreased as organic matter contents of the soils increased. Dinitroaniline herbicides were degraded primarily by soil microorganisms, and fungi were the major organisms involved. Degradation proceeded faster under anaerobic conditions than under aerobic conditions and faster under warm, moist conditions than under cool, dry conditions. Half-life values for dinitroanilines ranged from 7 to 27 days under anaerobic conditions and from 19 to 132 days under aerobic conditions. The chemicals did not affect most soil microflora; and any significant effects reported were of short duration.

Until recently, phytopathogenic bacteria have not been considered potential biological weed control candidates because they lack the ability to penetrate intact plants. This deficiency can be overcome by providing entry wounds or using surfactants. Spray application of Pseudomonas syringae pv. tagetis (5 × 108cells/ml) in aqueous buffer with a surfactant produced severe disease in Canada thistle, common ragweed, Jerusalem artichoke, sunflower, and certain other members of the Compositae under field conditions. Spray application of the bacterium without surfactant was ineffective on all reported hosts. Xanthomonas campestris pv. poannua controlled annual bluegrass in bermudagrass golf greens when applied by spray during mowing. The bacterium entered through mowing injuries, causing lethal, systemic wilt. Application of the bacterium to annual bluegrass in the absence of fresh mowing injuries failed to produce symptoms. Under field conditions, this previously unknown pathovar's host range was limited to a single subspecies of annual bluegrass, but inundative application to freshly mowed turf resulted in infection of diverse annual bluegrass biotypes. In field trials, six monthly applications resulted in greater than 70% control. The preceding examples are among the first attempts to use foliar phytopathogenic bacteria for biological weed control. Efficacy of these bacterial bioherbicides and of future biocontrol strategies employing bacteria is dependent on facilitated host penetration.

The critical period of weed control for crops grown under conventional tillage systems has been well studied, and the results generated by these studies have been proven to be very useful in developing ecologically and economically sound weed management practices. However, these management systems may not be directly applicable under no-till situations because the species composition, total amount, and temporal pattern of seedling emergence change substantially with tillage. The objective of this study was to identify the critical period of weed control for soybean and corn in fields that had been under no-till management for 1 yr. Although estimates of the critical period for a crop vary from year to year and site to site, some interesting comparisons can be made between no-till and conventional tillage. The start of the critical period in no-till corn was stable, usually beginning at the six-leaf stage. The end of the critical period was more variable ranging from the 9- to 13-leaf stage. The critical period for corn under no-till conditions tended to start and end earlier than under conventional tillage practices. In soybean, we were unable to identify a critical period at one of the sites. At the other location (sandy loam soil), the critical period was estimated to begin at the first or second node developmental stage, whereas the end was determined to be at the R1 stage (early flowering). The critical period in soybean was longer than that observed under conventional tillage.

Repeated use of sulfonylurea herbicides in continuous, no-till winter wheat selected for a herbicide resistant biotype of prickly lettuce in Idaho. Greenhouse experiments were conducted to compare the relative competitiveness and growth rate of sulfonylurea herbicide resistant (R) and susceptible (S) prickly lettuce. The S biotype of prickly lettuce produced 31% more aboveground biomass than the R biotype averaged over all densities. Both biotypes were equally competitive when analyzed for both inter- and intrabiotype competition. In relative growth rate studies, regression analysis showed that the S biotype accumulated biomass 52% faster than the R biotype. The results of this study showed that the S biotype was superior to the R biotype in biomass production and growth rate, but competitiveness appeared to be equal for both biotypes. Other fitness parameters must be measured before fitness differences between biotypes can be determined.

The process of labeling new herbicides for specialty crops has always been difficult. Progress in solving specialty crop weed control problems will likely be more challenging in the future. Major crops like corn, cotton, rice, soybean, and wheat are planted on millions of hectares, and most of these crops are treated with herbicides. In contrast, specialty crops (i.e., minor crops, e.g., container ornamentals or lettuce) are planted on 122,000 ha or less; thus, the potential value of herbicide sales is limited in these crops by the low number of hectares planted per crop. High crop value, small hectarage per crop, and generally marginal herbicide selectivity results in a high potential of liability for herbicide registrants and little incentive to label herbicides in these crops. The Interregional Project Number 4 (IR-4) program facilitates the registrations of pesticides on minor crops. Work needed to support pesticide tolerance in a given crop is conducted by IR-4 and cooperators. However, to develop new crop tolerances, the IR-4 process requires new herbicides. The success of glyphosate-resistant soybean has resulted in a less profitable herbicide market for all crops. In response, most primary pesticide manufacturers have reduced the size, or even eliminated herbicide discovery programs. As private industry slows or stops herbicide development, there will be fewer new minor-crop herbicides. Many questions face minor-crop weed scientists. For example, what are other practical solutions to control weeds in minor crops besides herbicides? Should research focus on development of competition models and decision thresholds or on weed removal tools such as robotics? What funding sources are available for minor-crop weed scientists? Are grant programs at the Federal level prepared to increase support for minor-crop weed research? Will university administrators replace retiring specialty crop weed scientists, knowing that their funding sources will produce little overhead? These questions require a response from all parties interested in specialty crop weed control.

Fumonisin B1 (FB1) sprayed on intact jimsonweed plants at 10 to 200 μg ml–1 caused chlorosis and necrosis on three- to four-leaf plants and reduced their height and biomass. Excised leaves of jimsonweed were necrotic on both surfaces after treatment with FB1 at 10 to 200 μg ml–1. The degree of damage increased with the concentration of FB1. The minimum amount of FB1 that damaged excised leaves was 0.05 μg. The symptoms of toxicity on sensitive plants varied with concentration of FB1. Symptoms observed included: chlorosis (creeping cucumber), necrosis (jimsonweed and common sunflower), black leaf lesions (sicklepod), curl of tissues (soybean), stunting, defoliation (hemp sesbania and northern jointvetch), and death (prickly sida, spurred anoda, northern jointvetch, and jimsonweed). Monocots (barley, bermudagrass, corn, johnsongrass, rice, sorghum, and wheat) were not visibly affected by FB1. This is the first evidence of the phytotoxic properties of fumonisin B1 against a wide variety of weeds and crop cultivars.

Concern about the effects of pesticides on the environment and the desire to reduce purchased inputs are stimuli to reduce herbicide use. Field research was conducted at two locations for two growing seasons to identify integrated weed management practices in corn that are effective and economical. Weed control treatments included PRE application of a herbicide mixture of 1.8 kg ai ha−1 of atrazine plus 2.2 kg ai ha−1 of metolachlor alone, this mixture broadcast or band-applied at a reduced rate in combination with mechanical weeding, and mechanical weeding alone. Weed control, corn yield, and economic returns were generally higher in treatments combining herbicides and mechanical weeding than those using only mechanical measures. Without herbicides, timeliness of rotary hoeing was critical. Herbicide use can be reduced 50 to 75% in integrated systems with minimal risk of yield loss due to uncontrolled weeds.

Integrated weed management (IWM) is a component of integrated pest management (IPM), which is an interdisciplinary practice involving disciplines such as entomology, nematology, plant pathology, weed science, horticulture, agronomy, ecology, economics, and systems science. Most descriptions of IPM mention three elements: a) multiple tactics (for example, competitive varieties, cultural practices, herbicide usage) used in a compatible manner; b) pest populations maintained below levels that cause economic damage, and c) conservation of environmental quality. Integrated weed management was discussed in 1981 during a Weed Science Society of America (WSSA) Symposium entitled Integrated Weed Management Systems Technology for Crop Production and Protection. Topics presented included modeling weed biology, crop manipulation, biological control, herbicide technology, research needs, teaching approaches, and extension implementation. Our paper was presented as part of a symposium on IWM at the 1990 WSSA meeting. This paper describes some of the research that weed scientists have conducted during the past 40 yr. Topics examined include an introductory review of IPM, trends in several categories of weed research, goals and suggestions stated during the 1981 WSSA Symposium on IWM, IWM accomplishments since about 1981, and the future of IWM as a component of IPM.

Field experiments were conducted in 1999 and 2000 to determine the influence of mesotrione postemergence application rate, application timing, and addition of atrazine on corn injury, weed control, and corn grain yield. Corn injury in the form of leaf bleaching ranged from 0 to 15% at 7 d after treatment (DAT). In general, most of the bleaching injury rapidly dissipated with slight (≤ 8%) to no corn injury observed at 28 DAT. Control of common cocklebur with mesotrione at 14 DAT ranged from 79 to 98% for all treatments over both years. Applying mesotrione at 140 g/ha, at the early postemergence (EPOST) timing, or in combination with atrazine provided the greatest control of common cocklebur at 14 DAT. Application rate of mesotrione was the only factor that was significant in both years for control of common cocklebur later in the season at 56 DAT. Control of ivyleaf morningglory with mesostrione at 14 DAT ranged from 60 to 90% for all treatments in both years. Control of ivyleaf morningglory at 14 DAT was enhanced by the addition of atrazine to mesotrione. Control of ivyleaf morningglory at 56 DAT was greater with mid-postemergence and late postemergence than with EPOST applications, and was generally enhanced by the addition of atrazine. Yellow nutsedge control with mesotrione was inconsistent, ranging from 40 to 87% at 14 DAT for all treatments over both years. The addition of atrazine to mesotrione increased yellow nutsedge control from 47 to 87% at 14 DAT in 2000. Increasing the rate of mesotrione from 70 to 140 g/ha, as well as the addition of atrazine, improved control of yellow nutsedge at 56 DAT. Corn grain yield was not affected by corn injury or weed control as there were no significant differences in grain yield between herbicide-treated plots and handweeded plots.

Littleseed canarygrass, a troublesome wheat weed, has developed resistance against isoproturon in India. Laboratory, pot, and field experiments were conducted to quantify the level of isoproturon resistance in various littleseed canarygrass biotypes and to evaluate the response of isoproturon-resistant littleseed canarygrass to alternate herbicides. For quantification of resistance, 10 littleseed canarygrass biotypes were collected from fields with a varied history of cropping and herbicide use. Pot studies indicated that 6 out of the 10 biotypes tested were resistant to isoproturon. The resistant (R) biotypes had 50% growth reduction values, 5 to 10 times greater than that of the most susceptible biotype. The R biotypes were from areas where isoproturon had been used continuously for more than 10 yr in wheat under a continuous rice–wheat cropping system. The isoproturon resistance was not detected where crops and herbicides were rotated. Under field conditions, postemergence isoproturon at 1,000 g/ha 32 d after sowing failed to control littleseed canarygrass and as a result wheat grain yield decreased by 65% compared with the weed-free control. The herbicides observed to be effective in wheat against isoproturon-resistant littleseed canarygrass were CGA 184927 (50 to 60 g ai/ha), fenoxaprop-p-ethyl (100 to 120 g/ha), ICIA 0604 (350 g ai/ha), and MON 37503 (25 g ai/ha). The use of these herbicides increased the grain yield by 200% or more when compared with the weedy check. Among the herbicides tested, MON 37503 at 25 g ai/ha was the best because of complex weed flora control and produced wheat yield similar to that of the weed-free control. Based on this study, the absence of resistance when herbicides and crops were rotated indicates that growers should follow herbicide and crop rotation in order to avoid development of herbicide resistance in the near future. The integration of this system (crop and herbicide rotation) with cultural and mechanical means of weed control could be further targeted as a long-term strategy for resistance management.

The use of POST herbicides has been limited in sweet corn because of the narrow spectrum of weed control or potential crop injury. Field experiments were conducted to evaluate the 4-hydroxyphenyl pyruvate dioxygenase (HPPD)-inhibiting herbicides mesotrione, tembotrione, and topramezone applied POST in sweet corn at three locations. Efficacy of mesotrione, tembotrione, and topramezone applied alone or mixed with atrazine was compared to other labeled POST herbicides following PRE S-metolachlor. Giant foxtail control was greater with tembotrione or topramezone than mesotrione alone or mixed with atrazine. Common lambsquarters, velvetleaf, and common ragweed were controlled 98% or greater with the HPPD-inhibiting herbicides when mixed with atrazine. Tolerance of six sweet corn hybrids was determined in the field when treated with 1× and 2× rates of these herbicides mixed with atrazine. Tolerance of six sweet corn hybrids to these herbicides was determined in the greenhouse when treated with 0.5, 1, 2, 4, 8, and 16 times the labeled rate. Differential hybrid tolerance to each herbicide was observed in both the field and greenhouse evaluations. Tembotrione killed ‘Merit’ in both evaluations. Excluding Merit, hybrids generally had good tolerance to tembotrione and topramezone in the field, but had differential tolerance to mesotrione. With the exception of Merit, hybrids generally had greater tolerance to tembotrione than topramezone and less tolerance to mesotrione in the greenhouse. These HPPD-inhibiting herbicides provide POST weed control, but the potential for sweet corn injury varies among the herbicides and hybrids and warrants further characterization.

Field studies were conducted in 1990 and 1991 to determine the effects of corn planting date and hairy vetch control method on the efficacy of fall-planted hairy vetch as a weedsuppressive cover crop for no-till corn. Glyphosate controlled hairy vetch when applied at the early bud growth stage (April), but hairy vetch residue provided no weed control compared to the weedy check. Mowing was not an effective means of suppressing hairy vetch at the early bud stage. Untreated hairy vetch reduced weed biomass 96% in 1990 and 58% in 1991 but reduced yield over 76% in April-planted corn. There was no competition of untreated hairy vetch with corn when corn planting was delayed until May or June (mid- or late-bloom growth stages of hairy vetch). Corn planted in May into untreated hairy vetch yielded similarly to corn planted in a no-cover weed-free check.

Forty-three Spanish populations of hairy fleabane, sampled from perennial crop locations, were studied under controlled and field conditions to confirm and characterize glyphosate resistance. In the initial screening, under controlled conditions, significant differences in glyphosate response between locations and among plant progenies within location were observed. From the initial screening, six populations (five potentially resistant [R] and one susceptible [S]) were selected, and a dose–response experiment was conducted to determine the resistance factor. The resistance factor was close to 10× for the most resistant population. In addition, the glyphosate response of R and S populations was dependant on phenological stages: the glyphosate rate required for control increased as a function of plant age. Finally, the resistance was confirmed with field trials conducted in five locations (one S and four R previously studied under controlled conditions). The field trials were also used to find alternative solutions for Spanish farmers to control resistant hairy fleabane populations.

Synthetic auxin herbicides have long been utilized for the selective control of broadleaf weeds in a variety of crop and noncrop environments. Recently, two agrochemical companies have begun to develop soybean with resistance to 2,4-D and dicamba which might lead to an increase in the application of these herbicides in soybean production areas in the near future. Additionally, little research has been published pertaining to the effects of a newly-discovered synthetic auxin herbicide, aminocyclopyrachlor, on soybean phytotoxicity. Two field trials were conducted in 2011 and 2012 to evaluate the effects of sublethal rates of 2,4-D amine, aminocyclopyrachlor, aminopyralid, clopyralid, dicamba, fluroxypyr, picloram, and triclopyr on visible estimates of soybean injury, height reduction, maturity, yield, and yield components. Each of these herbicides was applied to soybean at the V3 and R2 stages of growth at 0.028, 0.28, 2.8, and 28 g ae ha−1. Greater height reductions occurred with all herbicides, except 2,4-D amine and triclopyr when applied at the V3 compared to the R2 stage of growth. Greater soybean yield loss occurred with all herbicides except 2,4-D amine when applied at the R2 compared to the V3 stage of growth. The only herbicide applied that resulted in no yield loss at either stage was 2,4-D amine. When applied at 28 g ae ha−1 at the V3 stage of growth, the general order of herbicide-induced yield reductions to soybean from greatest to least was aminopyralid > aminocyclopyrachlor = clopyralid = picloram > fluroxypyr > triclopyr > dicamba > 2,4-D amine. At the R2 stage of growth, the general order of herbicide-induced yield reductions from greatest to least was aminopyralid > aminocyclopyrachlor = picloram > clopyralid > dicamba > fluroxypyr = triclopyr > 2,4-D amine. Yield reductions appeared to be more correlated with seeds per pod than to pods per plant and seed weight. An 18- to 26-d delay in soybean maturity also occurred with R2 applications of all synthetic auxin herbicides at 28 g ae ha−1 except 2,4-D. Results from this research indicate that there are vast differences in the relative phytotoxicity of these synthetic auxin herbicides to soybean, and that the timing of the synthetic auxin herbicide exposure will have a significant impact on the severity of soybean height and/or yield reductions.

Field experiments were conducted near Chickasha, OK, in 1999 and 2000 and near Perkins, OK, in 2000 to evaluate the effects of Palmer amaranth on harvest efficiency of grain sorghum and full-season competition with that crop. Weed densities were 0 (the weed-free check), 1, 2, 4, 6, 9, 12, and 18 plants/15 m of row. In the harvest efficiency experiments, each additional weed per 15 m of row increased grain moisture before cleaning by 0.7 and 0.2% at Chickasha in 1999 and at Perkins, respectively. After cleaning, it increased moisture by 0.2% in both the experiments for each weed. Foreign material increased 67, 2, and 3 kg/ha at Chickasha in 1999, Chickasha in 2000, and Perkins, respectively. At Chickasha in 2000, sorghum seed loss through the combine increased 11 kg/ha for each additional weed per 15 m of row. Grain grades improved at Perkins at higher weed densities. In the competition experiments, grain yield decreased by 1.8 to 3.5% for each increase of 1 weed/15 m of row. More weeds resulted in higher weed dry weight. Each kilogram of Palmer amaranth dry weight per plot reduced grain yield by 5.3 to 9.1%. In 2000, sorghum seeds per panicle were reduced by 27 to 50 for each weed. Grain grades generally decreased as weed density increased at Chickasha in 2000 but not at Perkins.

A field study was conducted over 2 yr to compare efficacy and economics of glyphosate-resistant, sulfonylurea-tolerant, and conventional soybean (Glycine max) weed control programs. Herbicide programs in the three soybean systems provided at least 90% control of browntop millet (Brachiaria ramosa), prickly sida (Sida spinosa), yellow nutsedge (Cyperus esculentus), pitted morningglory (Ipomoea lacunosa), and hemp sesbania (Sesbania exaltata) in most cases and postemergence (POST)-only programs were as effective as preemergence (PRE) followed by POST programs. Control of hyssop spurge (Euphorbia hyssopifolia) ranged from 93 to 100% in glyphosate-resistant soybean and from 88 to 100% in conventional soybean, but control was 60 to 100% in sulfonylurea-tolerant soybean. Sicklepod (Senna obtusifolia) control was at least 91% in glyphosate-resistant and sulfonylurea-tolerant soybean but was 81% for the standard SAN 582 plus imazaquin PRE and acifluorfen plus bentazon early POST treatment in conventional soybean. In glyphosate-resistant soybean, glyphosate applied sequentially resulted in an average yield of 3,020 kg/ha with a net return of $407/ha. In sulfonylurea-tolerant soybean, chlorimuron applied sequentially yielded 2,500 kg/ha with a net return of $271/ha. Conventional soybean yield with the standard herbicide program was 2,770 kg/ha with a net return of $317/ha. Yields for the cultivars were equivalent when the same standard herbicide program was used. When weed control is satisfactory and herbicide costs relatively comparable, yield potential of the cultivar and seed cost, including any technology fee, would be key factors in selecting a weed management system.

Experts have long sought to understand the factors that underlie farmer decision making for weed management. The majority of this interest has been in relation to the weak adoption of integrated management approaches and more recently, herbicide resistance strategies. In order to increase adoption in these contexts there is a need to understand better the underlying drivers for weed management decisions. The objective of the research reported here was to probe farmers' understanding of weed management to establish a baseline understanding of these key drivers. Thirty Ohio farmers participated in an in-depth interview where they were asked to reflect on how weeds are introduced and spread, what risks and benefits weeds pose, and what management strategies farmers are familiar with and which they prefer. Their responses were mapped, coded, and analyzed for dominant beliefs and major decision-making influences. The results indicate that farmers largely attribute the introduction and movement of weeds to factors outside their control (e.g., the environment, plant characteristics). They frequently cite diverse and integrated management, but their focus is on control as opposed to prevention. In general, they tend to receive messages about integrated and preventive approaches, but do not always put them into practice because of underlying beliefs about the inevitability of new weed introductions and spread.

Weed control in organic vegetable production systems is challenging and accounts for a large portion of production costs. Six methods to prepare a stale seedbed were compared on certified and transitional organic land in Salinas, CA, in 2004. Weed control operations occurred on raised beds 2 to 3 d before planting baby spinach or a simulated vegetable planting. A flamer and an herbicide application of 10% v/v of a clove oil mixture (45% v/v clove oil) at 280 L/ha (iteration 1) or 15% v/v of a clove oil mixture (45% clove oil) at 467 L/ha (iterations 2 and 3) were used to control weeds without disturbing the soil. Top knives on a sled, a rolling cultivator, and a rotary hoe were used to control weeds while tilling the bed top. A bed shaper–rototiller combination was also used, which tilled the entire bed. Broadleaf weed control was 36% with clove oil, 63% with the rotary hoe, and significantly higher (87 to 100% control) with the remaining treatments in iteration 1. Broadleaf weed control was consistently lower (72 to 86% control) with the flamer than all other treatments (95 to 100% control) in iterations 2 and 3. The difference between sites can probably be attributed to differences in weed size. The flamer and the clove oil herbicide had the lowest number of weeds emerging with the crop following stale seedbed formation. The most expensive technique was clove oil at $1,372/ha. The estimated cost of forming the stale seedbed with the remaining weed management tools ranged from $10 to $43/ha.

Indigenous to tropical and East Asia, 20 species comprise the genus Pueraria in the Leguminosae (pea or bean family) (Mabberley 1989). The generic name commemorates Swiss botanist Marc Nicolas Puerari (1766–1845). The specific epithet lobata, from the Greek lobos, refers to kudzu's three-lobed leaves (Brown 1956).