Substituted quinolinecarboxylic acids, including quinclorac (BAS 514H), are a new class of highly selective auxin herbicides, which are chemically similar to naturally occurring compounds isolated from plants and soils. Quinclorac is used in rice to control important dicot and monocot weeds, particularly barnyardgrass. The herbicide has also been developed for application in turfgrass areas, spring wheat, and chemical fallow. Quinclorac is readily absorbed by germinating seeds, roots, and leaves and is translocated in the plant both acropetally and basipetally. By mimicking an auxin overdose, quinclorac affects the phytohormonal system in sensitive plants. The compound stimulates the induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity and thus promotes ethylene biosynthesis. In susceptible dicots, increased levels of ethylene trigger an accumulation of abscisic acid (ABA), which, as part of the intrinsic auxin activity of quinclorac, plays a major role in growth inhibition and the induction of epinasty and senescence. In sensitive grasses, such as barnyardgrass species, large crabgrass, broadleaf signalgrass, and green foxtail, quinclorac leads particularly to an accumulation of tissue cyanide, formed as a co-product during increased ACC and ethylene synthesis. This causes phytotoxicity characterized by the inhibition of root and particularly shoot growth with tissue chlorosis and subsequent necrosis. These effects were not observed in tolerant rice and a resistant biotype of barnyardgrass. No significant differences in uptake, translocation, or metabolism of quinclorac between resistant and sensitive grasses were found. Hence, a target-site-based mechanism of selectivity is suggested. The induction process of the ACC synthase activity plays the primary role in the selective herbicide action of quinclorac. This is a common effect of auxin herbicides and auxins, which lead to the accumulation of cyanide and/or ABA depending on the plant species and tissues, the compound concentration in the tissue, and their biological activity.

Plant litter that covers the soil surface acts as a layer of insulation moderating temperature and moisture, and creating favorable microsites for germination and the establishment of annual weed species in rangeland communities. Litter cover is an important factor in succession among annual species which culminates in dominance by downy brome (Bromus tectorum L.) and medusahead (Taeniatherum asperum (Sim.) Nevski) in these communities.

Field, laboratory, and greenhouse experiments were conducted to determine the seed production potential and effect of environmental factors on germination, emergence, and survival of texasweed. Texasweed produced an average of 893 seed per plant, and 90% were viable. Seed exhibited dormancy, and prechilling did not release dormancy. Percent germination ranged from 56% for seed subjected to no prechilling to 1% for seed prechilled at 5 C for 140 d. Seed remained viable during extended prechilling conditions, with 80% of seed viable after 140 d of prechilling. Texasweed seed germinated over a range of 20 to 40 C, with optimum germination (54%) occurring with a fluctuating 40/30 C temperature regime. Seed germinated with fluctuating 12-h light/dark and constant dark conditions. Texasweed seed germinated over a broad range of pH, osmotic potential, and salt concentrations. Seed germination was 31 to 62% over a pH range from 4 to 10. Germination of texasweed ranged from 9 to 56% as osmotic potential decreased from − 0.8 MPa to 0 (distilled water). Germination was greater than 52% at less than 40 mM NaCl concentrations and lowest (27%) at 160 mM NaCl. Texasweed seedlings emerged from soil depths as deep as 7.5 cm (7% emergence), but emergence was > 67% for seed placed on the soil surface or at a 1-cm depth. Texasweed seed did not germinate under saturated or flooded conditions, but seed survived flooding and germinated (23 to 25%) after flood removal. Texasweed seedlings 2.5 to 15 cm tall were not affected by emersion in 10-cm-deep flood for up to 14 d. These results suggest that texasweed seed is capable of germinating and surviving in a variety of climatic and edaphic conditions, and that flooding is not a viable management option for emerged plants of texasweed.

To identify the most commonly regulated weedy plants in the United States and southern Canada, we compiled a database of noxious weed lists obtained from the 48 continental states and six bordering provinces. The 10 most frequently listed weeds are Cirsium arvense, Carduus nutans, Lythrum spp. (includes purple loosestrife), Convolvulus arvensis, Euphorbia esula, Acroptilon repens, Sorghum spp. (includes johnsongrass and shattercane), Cardaria spp. (includes hoary cress, also called whitetop), Centaurea maculosa, and Sonchus arvensis. When genera are ranked, the top genus is Centaurea, which includes C. maculosa, C. diffusa, and C. solstitalis. Biological control programs have targeted many of the top dicotyledonous weeds of national concern, but none of the weedy grasses and sedges. We recommend that exploratory studies be initiated to determine the feasibility of developing biological control agents for the latter species. The complete database of noxious weed lists is available on the Internet at http://invader.dbs.umt.edu. This information may be useful to resource managers and regulatory officials in assessing which weeds are problematic in adjacent geographic areas and by researchers to help select which weeds to target with new management strategies.

A population of waterhemp was identified in Adams County, Illinois, that survived treatment of several acetolactate synthase (ALS) inhibitors and a postemergence (POST) application of lactofen, a protoporphyrinogen oxidase (PPO)–inhibiting herbicide. Greenhouse studies were conducted to quantify the responses of this waterhemp population, designated ACR, to multiple PPO inhibitors and various other herbicides with different sites of action. Resistance ratios were obtained by comparing herbicide dose–response curves between the ACR population and a herbicide-susceptible waterhemp population. The ACR population was resistant to lactofen (23-fold) and to five other PPO-inhibiting herbicides (ranging from 2.2- to 6.2-fold). Furthermore, the ACR waterhemp population was 17,000-fold and 18,000-fold resistant to imazamox and thifensulfuron, respectively, two ALS-inhibiting herbicides. In response to atrazine, a Photosystem II inhibitor, the ACR population was 38-fold resistant. Plants within the ACR waterhemp population survived treatment of a herbicide mixture containing lactofen at 175 g ai ha−1, imazamox at 44 g ae ha−1, and atrazine at 1,000 g ai ha−1. Thus, individual plants—not just the population as a whole—displayed multiple herbicide resistance. The ACR population was not resistant to glyphosate or paraquat. This is the first reported weed population from the United States with resistances to herbicides inhibiting three unique sites of action. Furthermore, this research identifies a significant reduction in the number of POST herbicide options available for waterhemp control in soybean production.

Integrated weed management (IWM) research has focused on how crop yields and weed interference are affected by changes in management, e.g., tillage, herbicide application timing and rates, cover crops, and planting patterns. Acceptance of IWM will depend on recommendation of specific strategies that manage weeds and maintain crop productivity; such research will and should continue. However, IWM needs to move from a descriptive to a predictive phase if long-term strategies are to be adopted. Linking management changes with crop-weed modeling that includes such components as weed population dynamics and the ecophysiological basis of competition will help predict future weed problems and solutions and the economic risks and benefits of intervention. Predictive approaches would help incorporate IWM into models of the processes that occur in agricultural systems at wider spatial and temporal scales, i.e., in agroecosystems comprised of the interactions among organisms (including humans) and the environment. It is at these larger scales that decisions about management are initiated and where questions about the long-term consequences and constraints of IWM and agriculture are often asked. These questions can be addressed by agroecosystem health, an approach that integrates biophysical, social, and economic concerns and recognizes that agriculture is part of a world with many complex subsystems and interactions. Indicators are used to examine the status of an agroecosystem, e.g., whether or not it contains all that is necessary to continue functioning. Indicators include soil quality, crop productivity, and water quality; all of these are related to the rationale of IWM, hence IWM can be linked to agroecosystem health. Ancillary effects of using IWM relate to other indicators such as diversity and energy efficiency. Linking IWM to agroecosystem health has at least two benefits: (1) predictive models within IWM can be incorporated into larger agroecosystem models to explore hitherto unforseen problems or benefits of IWM, and (2) the relevance and benefits of IWM should become clearer to the public and government agencies who otherwise might not examine how IWM promotes many of the larger social, economic and environmental goals being promulgated.

Herbicides inhibit biochemical and physiological processes or both with lethal consequences. The target sites of these small molecules are usually enzymes involved in primary metabolic pathways or proteins carrying out essential physiological functions. Herbicides tend to be highly specific for their respective target sites and have served as tools to study these physiological and biochemical processes in plants (Dayan et al. 2010b).

Under irrigated field conditions at Prosser, WA, Russian thistle [Salsola kali (L.) var. tenuifolia Tausch. ♯3 SASKR] began to emerge by mid-March each year and usually had completed its emergence by May 1. Common lambsquarters (Chenopodium album L. ♯ CHEAL) was the next to appear, usually during the third or fourth week of March. Barnyardgrass [Echinochloa crus-galli (L.) Beauv. ♯ ECHCG], redroot pigweed (Amaranthus retroflexus L. ♯ AMARE), hairy nightshade (Solarium sarrachoides Sendt. ♯ SOLSA), black nightshade (5. nigrum L. ♯ SOLNI), eastern black nightshade (S. ptycanthum Dun.), and cutleaf nightshade (S. triflorum Nutt. ♯ SOLTR) generally began to emerge during the first 2 weeks of April and emergence generally peaked mid-April to mid-May. Russian thistle and cutleaf nightshade had the most restricted emergence patterns, whereas seedlings of common lambsquarters, redroot pigweed, hairy nightshade, and black nightshade emerged each month from April through September. Shallow tillage at monthly intervals increased the overall emergence of common lambsquarters, redroot pigweed, black nightshade, and eastern black nightshade; decreased the emergence of barnyardgrass; and had no effect on the emergence of Russian thistle, cutleaf nightshade, or hairy nightshade.

Experiments were conducted to determine the effects of temperature on seed germination and growth of redroot pigweed, Palmer amaranth, and common waterhemp. At 15/10 C day and night temperature, respectively, no seed germination was observed in any species. Seed germination increased gradually as temperature increased. Germination peaked at 25/20 C in common waterhemp and at 35/30 C in redroot pigweed and Palmer amaranth. Seed germination of all three species declined when temperatures increased above 35/30 C. All three species produced less biomass at 15/10 C than at 25/20 C and 35/25 C. Redroot pigweed and common waterhemp biomass were similar at 15/10 C and higher than that of Palmer amaranth. However, Palmer amaranth produced more biomass than redroot pigweed and common waterhemp at 25/20 and 35/30 C. At 45/40 C, redroot pigweed, common waterhemp, and Palmer amaranth plants died 8, 9, and 25 d after initiation of heat treatment, respectively. The largest root volume among the three species was in Palmer amaranth grown at 35/30 C, whereas the smallest root volume was produced by Palmer amaranth grown at 15/10 C. Potential quantum efficiency (F v/F max) of Palmer amaranth was higher than that of redroot pigweed and common waterhemp at higher temperature. The greater growth of Palmer amaranth at higher temperatures may be attributed in part to its extensive root growth and greater thermostability of its photosynthetic apparatus.

Hairy vetch was grown as a winter annual cover crop and evaluated for weed suppression when desiccated by paraquat or left alive until natural senescence in a 3-yr field experiment. Total weed density and biomass were variable in the desiccated hairy vetch treatment relative to a bare soil treatment but were consistently lower in the live hairy vetch treatment relative to the desiccated or bare soil treatments. An average of 87% of sites under live hairy vetch compared to 8% of sites under desiccated hairy vetch transmitted less than 1% of unobstructed sunlight. The red (660 nm) to far-red (730 nm) ratio of transmitted light was reduced by 70% under live hairy vetch compared to 17% under desiccated hairy vetch. Daily maximum soil temperature and diurnal soil temperature amplitude were reduced by live hairy vetch > desiccated hairy vetch > bare soil. Soil moisture content was greater under both live and desiccated hairy vetch compared to bare soil during droughty periods. Changes in light extinction, red to far-red ratio, and diurnal soil temperature amplitude were sufficient to explain greater weed suppression by live than desiccated hairy vetch.

The movement of chemicals in the soil caused by water percolation was treated in a manner analogous to the development of materials through chromatographic columns. A “slotted tube test” was developed, which simulated a miniature field plot, utilizing a chromatographic soil column and the growth of either seeds or an indicator organism in agar as the detector. It was found that the median effective dose was higher for the more strongly sorptive soils. When correlations were made between soil compositions and the corresponding median effective doses for nonionic organic molecules, virtually all of the variations in required dosages were accounted for by percent organic matter and water content. The “active fraction” of organic matter, that fraction concerned with sorption equilibria, appeared to be of the same nature in soils of various compositions from various sources. This provided a technique for calculating the percent “active” organic matter of soil.

Management of perennial weeds is a major concern in reduced-tillage cropping systems. Field research was conducted at Nashua, IA, from 1977 through 1990 to evaluate the long-term impacts of tillage and cropping patterns on perennial weed populations in corn and soybean production. Continuous corn and a corn/soybean rotation were conducted utilizing moldboard plow, chisel plow, ridge tillage, and no-tillage systems. The research area was free of established perennial weed species at the initiation of the experiment in 1977. Hemp dogbane was observed by 1980, with the greatest densities in no-tillage. By 1990, continuous corn had greater hemp dogbane densities with no-tillage than other tillage system by crop rotation treatments. American germander densities were not affected by tillage systems in 1980 and 1981, but by 1990, corn/soybean rotations had greater densities in moldboard plow than other tillage systems. Field bindweed developed primarily in the corn/soybean rotations with the greatest densities occurring in no-tillage. Greater and more diverse populations of perennial weeds developed in reduced-tillage systems than in the moldboard plow system. However, practices used to control annual weeds and environmental factors interacted with tillage to regulate perennial weed populations.

Field and greenhouse experiments were conducted to determine the level of glyphosate resistance in common waterhemp populations from Platte County (MO1) and Holt County, Missouri (MO2), and to determine the level and distribution of resistance to glyphosate, acetolactate synthase (ALS)–inhibiting herbicides, and protoporophyrinogen oxidase (PPO)–inhibiting herbicides across the MO1 site. Results from greenhouse experiments revealed that the MO1 and MO2 waterhemp populations were 19 and 9 times more resistant to glyphosate, respectively, than a susceptible waterhemp population. In field experiments, greater than 54% of waterhemp at the MO1 site survived 1.7 kg glyphosate ae ha−1 (twice the labeled rate) 6 wk after treatment. Tank-mix combinations of ALS- and PPO-inhibiting herbicides with glyphosate also failed to provide complete control of the waterhemp population at the MO1 site. Collection and screening of seed from individual female waterhemp accessions revealed multiple resistance to glyphosate, ALS-, and PPO-inhibiting herbicides across the MO1 site. All 14 waterhemp accessions collected across the MO1 site exhibited greater than 65% survival to 2× rates of glyphosate and thifensulfuron, and these accessions were spread across a 5-km2 (503-ha) area. Four waterhemp accessions collected across a 0.9-km2 (87-ha) area also exhibited 26 to 38% survival to 2× rates of lactofen. The results from these experiments provide evidence and confirmation of the first glyphosate-resistant waterhemp population in the United States and reveal that multiple resistance to glyphosate, ALS-, and PPO-inhibiting herbicides can occur in waterhemp.

Hard-water cations, such as Ca+2 and Mg+2, present in the spray solution can greatly reduce the efficacy of glyphosate. These cations potentially compete with the isopropylamine in the formulation for association with the glyphosate anion. 14C-Glyphosate absorption by sunflower was reduced in the presence of Ca+2. The addition of ammonium sulfate overcame the observed decrease in 14C-glyphosate absorption. Nuclear Magnetic Resonance (NMR) was used to study the chemical effects of calcium and calcium plus ammonium sulfate (AMS) on the glyphosate molecule. Data indicate an association of calcium with both the carboxyl and phosphonate groups on the glyphosate molecule. Initially, a random association of the compounds occurred; however, the reaction progressed to yield a more structured, chelate type complex over time. NH4+ from AMS effectively competed with calcium for complexation sites on the glyphosate molecule. Data suggest that the observed calcium antagonism of glyphosate and AMS reversal of the antagonism are chemically based.

Characterizing the long-term effect of agricultural management systems on weed communities will aid in developing sustainable weed management practices. Weed seedbanks and aboveground biomass were measured within a corn–soybean–wheat crop sequence from 1990 through 2002 at Hickory Corners, MI. Four management systems were compared: conventional (CONV; full rates of N fertilizer and herbicides, moldboard tillage), no till (NT; same as CONV with no primary tillage), reduced input (RI; reduced rates of N fertilizer and herbicides, moldboard tillage, mechanical weed control, wheat underseeded with red clover), and organic (ORG; same as RI but no synthetic inputs). Multivariate ordinations of weed seedbanks showed a divergence of the CONV and NT systems from the RI and ORG systems. The CONV and NT seedbanks were dominated by grass species (mainly fall panicum and large crabgrass), whereas the RI and ORG systems were dominated by common lambsquarters and common chickweed. Within a single growing season, weed seedbanks in the RI and ORG systems were positively correlated with weed biomass whereas seedbanks in the CONV and NT system had little predictive value. Weed biomass from 1990 through 2002 showed a strong association of grass weed species with the corn phase of the CONV and NT system and common lambsquarters and redroot pigweed with the corn and soybean phases of the RI and ORG systems. Weed biomass diversity measures were negatively correlated with soybean yields in RI and ORG and wheat yields in NT, RI, and ORG. It is not clear whether crops were less competitive in the NT, RI, and ORG treatments, allowing new weed species to enter the plots, or whether less effective weed management in the NT, RI, and ORG treatments resulted in increased species richness, causing reduced crop yields. Mechanistic studies are needed to elucidate the relationship between weed community diversity and crop performance.

The adsorption of herbicides from aqueous solution by muck (organic soil), bentonite, an anion exchanger, and a cation exchanger was studied. The nature of the adsorbent, pH, and temperature all influenced adsorption, and different herbicides responded differently to changes in these factors. Lowering of the pH resulted in increased adsorption by bentonite of all of the herbicides studied, except 6,7-dihydrodipyrido (1,2-a: 2’, 1′-c) pyrazidiinium salt (diquat) which was completely adsorbed at both a high and a low pH. The influence of pH was greatest for 4,6-dinitro-o-sec-butylphenol (DNBP) and 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine). The DNBP was adsorbed by an anion exchanger, but not by a cation exchanger, while isopropyl N-(3-chlorophenyl)carbamate (CIPC), 3-(p-chlorophenyl)-1,1-dimethylurea (monuron), and atrazine were adsorbed by both. Diquat was completely adsorbed by the cation exchanger and only slightly by the anion exchanger. All were adsorbed by muck but to varying extents as follows: diquat > CIPC > DNBP > monuron > atrazine. No relationship was found between water solubility and adsorption. Adsorption by bentonite (pH 8.5) was much greater at 0C than at 50C, but adsorption by muck was the same at both temperatures.

Desorption studies were conducted using C14-labeled atrazine, monuron, 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 3-amino-2,5-dichlorobenzoic acid (amiben), and 2,4-dichlorophenoxyacetic acid (2,4-D). Repeated extractions with distilled water desorbed these herbicides from bentonite and muck and their desorption was more readily accomplished from bentonite than from muck.

Although the effectiveness of vegetative filter strips (VFS) for reducing herbicide runoff is well documented, a comprehensive review of the literature does not exist. The objectives of this article are to denote the methods developed for evaluating herbicide retention in VFS; ascertain the efficacy of VFS regarding abating herbicide runoff; identify parameters that affect herbicide retention in VFS; review the environmental fate of herbicides retained by VFS; and identify future research needs. The retention of herbicide runoff by VFS has been evaluated in natural rainfall, simulated rainfall, and simulated run-on experiments. Parameters affecting herbicide retention in VFS include width of VFS, area ratio, species established in the VFS, time after establishment of the VFS, antecedent moisture content, nominal herbicide inflow concentration, and herbicide properties. Generally, subsequent transport of herbicides retained by VFS is reduced relative to adjacent cultivated soil because of enhanced sorption and degradation in the former.

New and improved glyphosate-resistant (GR) crops continue to be rapidly developed. These crops confer greater crop safety to multiple glyphosate applications, higher rates, and wider application timings. Many of these crops will also have glyphosate resistance stacked with traits that confer resistance to herbicides with other modes of actions to expand the utility of existing herbicides and to increase the number of mixture options that can delay the evolution of GR weeds. Some breeding stacks of herbicide resistance traits are currently available, but the trend in the future will be to combine resistance genes in molecular stacks. The first example of such a molecular stack has a new metabolically based mechanism to inactivate glyphosate combined with an active site-based resistance for herbicides that inhibit acetolactate synthase (ALS). This stack confers resistance to glyphosate and all five classes of ALS-inhibiting herbicides. Other molecular stacks will include glyphosate resistance with resistance to auxin herbicides and herbicides that inhibit acetyl coenzyme A carboxylase (ACCase) and 4-hydroxyphenyl pyruvate dioxygenase (HPPD). Scientists are also studying a number of other herbicide resistance transgenes. Some of these new transgenes will be used to make new multiple herbicide-resistant crops that offer growers more herbicide options to meet their changing weed management needs and to help sustain the efficacy of glyphosate.

Absorption and translocation of 14C-fluazifop-butyl {butyl ester of (±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl] oxy] phenoxy] propanoate} was compared in susceptible quackgrass [Agropyron repens (L.) Beauv. ♯3 AGRRE] and tolerant soybean [Glycine max (L.) Merr.]. Foliar absorption was more rapid in soybeans with 75% of the recovered 14C absorbed 6 h after treatment compared to 36% in quackgrass. Translocation of the radiolabel occurred in both soybean and quackgrass with no significant difference between the species. Differential absorption and translocation did not contribute to the selectivity of fluazifop-butyl. Herbicidal activity was lower on quackgrass plants at the five- to six-leaf stage when compared to the two- to three-leaf stage. Greater quackgrass control was observed at 30 than at 20 C. Foliar absorption of 14C-fluazifop-butyl was significantly greater at 30 than at 20 C. Translocation of the radiolabel was greater in plants exposed to full light as compared to shade. Moisture stress significantly reduced quackgrass control with fluazifop-butyl.

Barnyardgrass is a serious weed problem in cornfields in Ontario. Field experiments were conducted at two locations in 1994 and 1995 to determine the influence of emergence time and barnyardgrass density on corn yield loss, leaf area at 50% silking, and barnyardgrass seed production. Selected barnyardgrass densities up to 200 plants m−1 were established within 12.5 cm on either side of the corn row. Barnyardgrass seeds were planted concurrently with corn and at the 3- to 5- or 1- to 2-leaf stage of corn growth in 1994 and 1995, respectively. Barnyardgrass density and seedling emergence relative to corn influenced the magnitude of corn yield loss. Maximum corn grain yield loss ranged from 26 to 35% for early emerging barnyardgrass, and less than 6% yield loss occurred from barnyardgrass seedlings emerging later than the 4-leaf stage of corn growth. Changes in corn leaf area index at 50% silking reflected the level of barnyardgrass competition in corn. Maximum leaf area reduction ranged from 21 to 23%. Barnyardgrass seed production varied with time of seedling emergence and density. Ten barnyardgrass plants emerging up to the 3-leaf stage of corn growth produced 14,400 to 34,600 seeds m−2 compared to only 1,200 to 2,800 seeds m−2 from plants emerging after the 4-leaf corn stage. The results of this study are essential in the development of an integrated weed management strategy for corn.