Three herbicides were compared for their ability to reduce both carbon fixation and soil water depletion by yellow nutsedge in a growth chamber study. Whole-plant CO2 exchange and water use were measured for 11 d after herbicide application. MSMA reduced carbon assimilation relative to the untreated control 1 d after treatment, and by 5 d after treatment respiration exceeded carbon assimilation during the photoperiod; however, MSMA had no significant effect on whole-plant water use during the measurement period. Halosulfuron reduced gross carbon assimilation to 30% of the pretreatment rate by the end of the experiment, but in contrast to MSMA it also strongly suppressed water use. Mesotrione never reduced carbon assimilation below 59% of the pretreatment rate and had no measurable effect on water use. Halosulfuron and MSMA reduced shoot regrowth to between 0 and 5% of the control, whereas mesotrione treatment allowed some 58% regrowth. These results indicate that whereas both MSMA and halosulfuron should provide effective control of yellow nutsedge, halosulfuron may be better able to rapidly suppress the weed's ability to compete for available soil water.

A formulation containing 2-chloroethanephosphonic acid, its ethyl ester and anhydride (hereafter referred to as 66-329), caused defoliation and, subsequently, growth of inactive basal buds of honey mesquite [Prosopis juliflora var. glandulosa (Torr.) Cockerell]. With huisache [Acacia farnesiana (L.) Willd.], the same treatments increased the number of branches and leaves per node; however, apical buds often were killed. Application of 66-329 to mesquite increased the release of ethylene to levels in the experimental system which were physiologically active. Bud release by 66-329 appeared to be mediated by ethylene. Treatments that induce growth of basal and lateral buds of woody plants may allow the killing of more plants by subsequent herbicide application.

Barnyardgrass, common lambsquarters, redroot pigweed, and wild mustard are among the most common weeds in cropping systems throughout North America. Crop and weed competition models that predict phenological development across environments are useful research tools for advancing our knowledge of population dynamics or crop and weed competition. Phenological parameter estimates for such models require verification under field conditions. Field studies were conducted in 1999 and 2000 to determine growth and phenological development of these species under noncropped conditions to compare parameters developed previously from controlled environment studies. Weeds were planted on three separate planting dates in each year. Growth and phenological development were recorded. Number of leaves on the mainstem of all weed species, except common lambsquarters, was not affected by planting dates. Rate of leaf appearance described as a function of days after emergence ranged from 0.48 to 0.89, 0.10 to 0.31, 0.33 to 0.65, and 0.24 to 0.29 leaves d−1 for common lambsquarters, barnyardgrass, redroot pigweed, and wild mustard, respectively. When expressed as a function of growing degree days (GDD), rate of leaf appearance for these species ranged from 0.04 to 0.05, 0.01 to 0.02, 0.04 to 0.07, and 0.02 to 0.03 leaves GDD−1, respectively. Planting date had differential effects on the rate of stem elongation and final plant height of each species in the 2 yr. Final plant biomass was also influenced by the time of planting; in general, weeds planted by mid-May had more biomass than those planted later. Parameters developed to describe phenological development under field conditions were comparable to those reported previously from controlled environment studies. We conclude that phenological parameters quantified under controlled environmental studies were comparable to those developed under field conditions for these weed species. Thus, either experimental method can be used to parameterize weed phenological development to initialize crop and weed competition models with reasonable confidence.

Burning and 2,4-D [(2,4-dichlorophenoxy)acetic acid] were used on the early stages of beaked hazel (Corylus cornuta Marsh.) understory to prevent development of a dense, continuous understory. At this early stage beaked hazel understories consisted of seedlings, small clones, and large clones. The application of 1.12 kg/ha of 2,4-D in July and August killed 84 to 100% of the seedlings and small clones, 86 to 98% percent of the large clones; while the application of 0.56 kg/ha of 2,4-D in June, July, or August caused 77 to 98% mortality of seedlings and small clones, but only 57 to 83% of large clones. Effects of burning were much more variable and generally mortality was less than with 2,4-D. One or two burns killed 23 to 100% of seedlings and small clones, and an average of only 13% (range 0 to 93%) of large clones.

The development of fluridone resistance by hydrilla has significantly impacted hydrilla management, and research is ongoing to develop alternate herbicides for effective hydrilla control. We determined the potential cross-resistance in fluridone-resistant hydrilla to other bleaching herbicides norflurazon, mesotrione, and topramezone-methyl. Phytoene, β-carotene, and chlorophyll contents as a function of hydrilla biotype and herbicide treatment were evaluated. Hydrilla shoot tips were collected from fluridone-susceptible (S) and -resistant (R) biotypes and exposed to 5, 25, 50, 75, and 100 µg L−1 of herbicide. The susceptible biotype showed an increase in phytoene and a decrease in β-carotene and chlorophyll contents when treated with 5 µg L−1 fluridone, whereas higher doses of fluridone were required to affect these pigments in the resistant biotype. There was no difference in response by S and R biotypes to mesotrione and topramezone-methyl, with both biotypes showing significant affects on pigment contents at 5 µg L−1. Higher doses of norflurazon were required to affect these pigments in the R compared to the S biotype. The S biotype had EC50 values of 11.7, 12.2, and 4.7 µg L−1, whereas the R biotype had EC50 values of 56.6, 41.1, and 41.7 µg L−1 fluridone for phytoene, β-carotene, and chlorophyll contents, respectively. There was no difference in EC50 for phytoene, β-carotene, and chlorophyll values between the hydrilla biotypes for mesotrione and topramezone-methyl herbicides. In fluridone-susceptible and -resistant hydrilla biotypes, EC50 values for phytoene, β-carotene, and chlorophyll were 12.4 to 11.8, 10.2 to 13.2, and 3.1 to 4.6 µg L−1 mesotrione and 12.6 to 13.5, 13.3 to 11.9, and 4.6 to 5.7 µg L−1 topramezone-methyl, respectively. For norflurazon, S and R biotypes had EC50 values of 33.1, 45.4, and 40.6 µg L−1 and 84.6, 81.0, and 92.7 µg L−1 for phytoene, β-carotene, and chlorophyll, respectively. These studies confirmed negative cross-resistance of fluridone-resistant hydrilla to mesotrione and topramezone-methyl and a positive cross-resistance to norflurazon.

The effects of the fungal protein Nep1 and Pseudomonas syringae pv. tagetis (Pst) applied separately or in combination on Canada thistle, common ragweed, and common dandelion were examined in growth chamber experiments. Experiments examined five treatments: (1) untreated control, (2) Silwet L-77 (0.3%, v/v) control, (3) Nep1 (5 μg ml−1) plus Silwet L-77 (0.3%, v/v), (4) Pst (109 colony-forming units [cfu] ml−1) plus Silwet L-77 (0.3%, v/v), and (5) Pst (109 cfu ml−1) and Nep1 (5 μg ml−1) plus Silwet L-77 (0.3%, v/v). Foliar treatments were applied at 28, 26, and 21 d after planting for Canada thistle, common dandelion, and common ragweed, respectively. For all three species, foliar application of Nep1 alone or in combination with Pst caused rapid desiccation and necrosis of leaves, with the greatest effect on recent, fully expanded (RFE) leaves. Within 4 to 8 h after treatment (HAT), 60 to 80% of RFE leaves of all three species were necrotic. Measured 72 HAT, Pst populations in Canada thistle leaves treated with Nep1 plus Pst were approximately 105 cfu cm−2 compared with 107 cfu cm−2 for leaves treated with Pst alone. Measured 2 wk after treatment, foliar application of Nep1 reduced shoot dry weight of the three weeds by 30 to 41%. Treatment with Pst reduced shoot growth of common ragweed, Canada thistle, and common dandelion by 82, 31, and 41%, respectively. The large suppression of common ragweed shoot growth caused by Pst treatment was associated with a high percentage (60%) of leaf area exhibiting chlorosis. Treatment with Pst plus Nep1 did not result in significant decreases in shoot dry weight for Canada thistle and common dandelion compared with either treatment alone. For common ragweed, shoot growth reduction caused by applying Pst and Nep1 together was not greater than that caused by Pst alone.

Weeds remain the most commonly cited concern of organic farmers. Without the benefit of synthetic herbicides, organic farmers must rely on a host of ecological weed management (EWM) practices to control weeds. Despite EWM’s ability to improve soil quality, the perceived rate of integrated EWM strategy adoption remains low. This low adoption is likely a result of the complexity in designing and evaluating EWM strategies, the tendency for outreach to focus on the risks of EWM strategies rather than their benefits, and a lack of quantitative measures linking the performance of EWM strategies to farmers’ on-farm objectives and practices. Here we report on the development and deployment of an easy-to-use online decision support tool (DST) that aids organic farmers in identifying their on-farm objectives, characterizing the performance of their practices, and evaluating EWM strategies recommended by an expert advisory panel. Informed by the principles of structured decision making, the DST uses multiple choice tasks to help farmers evaluate the short- and long-term trade-offs of EWM strategies, while also focusing their attention on their most important objectives. We then invited organic farmers across the United States, in particular those whose email addresses were registered on the USDA’s Organic Research Integrity Database, to engage the DST online. Results show considerable movement in participants’ (n = 45) preferences from practices focused on reducing weeding costs and labor in the short term to EWM strategies focused on improving soil quality in the long term. Indeed, nearly half of those farmers (48%) who initially ranked a strategy composed of their current practices highest ultimately preferred a better-performing EWM strategy focused on eliminating the weed seedbank over 5 yr.

In 1949, Bruns and Rasmussen initiated studies to determine the length of time certain weed seeds would maintain viability when held in fresh running water. Seeds of white top, Russian knapweed, Canada thistle, morning glory, and poverty weed were placed in Lumite screen bags and submersed at 12- and 48-inch depths in the Prosser Power Canal.

Mixtures of CDAA + atrazine or propazine applied pre-emergence gave excellent grass and non-grass weed control and no sorghum injury. Such mixtures offer promise of more consistent weed control than either component alone and less soil residue than a higher rate of atrazine or propazine alone. CDAA + trichlorobenzyl chloride preemergence was a less effective mixture but gave better grass weed control and poorer non-grass control than propazine or atrazine pre- or postemergence. Atrazine or propazine postemergence following CDAA preemergence gave excellent results. The best time to spray sorghum with atrazine or propazine between planting and the 11-leaf stage was when the weeds were in the very early postemergence stage. Sorghum was tolerant at all times of application. Atrazine over all trials averaged superior to propazine as an herbicide for grain sorghum.

Anionic and cationic surfactants differentially enhanced the phytotoxicities of four herbicides to corn and soybeans in relation to variations in their structure. Thirteen ionic and two related nonionic surfactants were studied in sprays containing water-soluble forms of 2,2-dichloropropionic acid [dalapon], 2,4-dichlorophenoxyacetic acid [2,4-D], 4,6-dinitro-o-sec-butylphenol [DNBP], and 3-amino-1,2,4-triazole [amitrole]. Effects of surfactant structure, type, and concentration on two other spray properties—surface tension and specific conductivity—were measured at three pH conditions.

Appreciable enhancement of herbicide toxicities occurred only at the highest concentration, 1.0 g per 100 ml (1.0 percent). With straight-chain isomers of an alkylbenzenesulfonate (ABS) anionic surfactant, enhancement of all herbicides increased as the position of the benzene ring was moved to the mid-chain (C5 and C6) carbons of an n-dodecyl alkyl group. DNBP toxicity to both species and surfactant toxicities to soybeans were increased progressively in separate odd- and even-carbon series, i.e. a C1,3,5-series and a C2,4,6-series. Other ABS variations which provided marked enhancement were use of a highly-branched dodecyl alkyl and substitution of an ester oxygen atom for the benzene ring. Among ethoxylated alkylamine cationic surfactants, variations in alkyl structure affected differentially the toxicities of the various herbicides at alkaline, neutral, and acid pH. Surfactants caused marked modification of physical-chemical properties of sprays in relation to their structure and concentration. However, no one of the properties studied could be correlated consistently with the herbicidal effectiveness of a spray. Results suggested that spray performance of herbicide formulations can be improved by using more effective surfactants to achieve other desired formulation characteristics. The increased toxicity of 2,4-D in certain alkaline sprays also suggested that the conditions necessary for absorption of 2,4-D should be examined more critically.

The compound, 3–amino–1,2,4–triazole (amitrole), has been used as an herbicide against different weed species with varying degrees of success. In the field, Canada thistle (Cirsium arvense (L.) Scop.) has been found relatively susceptible while field bindweed (Convolvulus arvensis L.) has been found relatively resistant to the action of amitrole. The reason(s) for this difference in sensitivity is not known. Because penetration of amitrole from the surface of the leaf to the interior is the first step in the chain of events leading to a response from a chemical by the treated plant, the nature of this process was examined.

Prohexadione calcium is an experimental turfgrass growth regulator that selectively controls or suppresses annual bluegrass in desirable turfgrass such as creeping bentgrass, Kentucky bluegrass, and perennial ryegrass. To help explain interspecific differences in turfgrass and weed response to prohexadione calcium, two laboratory trials were conducted to measure 14C-prohexadione calcium absorption, translocation, and metabolism in these four species. Annual and Kentucky bluegrass absorbed more prohexadione calcium than creeping bentgrass and perennial ryegrass when averaged over harvest timing and trial. Radioactivity partitioning to other foliage did not differ between species but annual bluegrass and Kentucky bluegrass translocated more radioactivity to roots and evolved more radioactive CO2 than creeping bentgrass and perennial ryegrass. Thin-layer chromatographic separations indicate radioactivity was translocated predominately as prohexadione calcium. When averaged over species and trial, 25 and 16% of recovered prohexadione calcium was metabolized within 1 h after treatment in treated leaves and other foliage, respectively. The rate of metabolic degradation was 0.7% h−1 in treated leaves and 0.4% h−1 in other foliage. Previous research indicates that annual and Kentucky bluegrass growth is suppressed more by prohexadione calcium than is growth of creeping bentgrass and perennial ryegrass. Increased prohexadione calcium absorption partially explained these interspecific differences in growth suppression.

Drunken horse grass [Achnatherum inebrians (Hance) Keng] is a perennial poisonous weed in western China. A comprehensive understanding of the ecological response of A. inebrians germination to environmental factors would facilitate the formulation of better management strategies for this weed. Experiments were conducted under laboratory conditions to assess the effects of various abiotic factors, including temperature, light, water, pH, and burial depth, on the germination and seedling emergence of A. inebrians. The seeds germinated at constant temperatures of 15, 20, 25, 30, and 35 C and in alternating-temperature regimes of 15/5, 20/10, 25/15, 30/20, 35/25, and 40/30 C, and the germination percentages under constant and alternating temperatures ranged from 51% to 94% and 15% to 93%, respectively. Maximum germination occurred at a constant temperature of 25 C, and germination was prevented at 45/35 C. Light did not appear to affect germination. The germination percentage of seeds was more than 75% in the pH range of 5 to 10, with the highest germination percentage at pH 6. The seeds germinated at osmotic potentials of 0 MPa to −1.0 MPa, but decreasing osmotic potential inhibited germination, with no germination at −1.2MPa. After 21 d of low osmotic stress, the seeds that did not germinate after rehydration had not lost their vitality. The seedling emergence percentage was highest (90%) when seeds were buried at 1 cm, but declined with increasing burial depth, with no emergence at 9 cm. Deep tillage may be effective in limiting the germination and emergence of this species. The results of this study provide useful information on the conditions necessary for A. inebrians germination and provide a theoretical basis for science-based prediction, prevention, and control of this species.

Weed invasion is a prevailing problem in modestly managed lawns. Less attention has been given to the exploration of the role of arbuscular mycorrhizal fungi (AMF) under different invasion pressures from lawn weeds. We conducted a four-season investigation into a Zoysia tenuifolia Willd. ex Thiele (native turfgrass)–threeflower beggarweed [Desmodium triflorum (L.) DC.] (invasive weed) co-occurring lawn. The root mycorrhizal colonizations of the two plants, the soil AM fungal communities and the spore densities under five different coverage levels of D. triflorum were investigated. Desmodium triflorum showed significantly higher root hyphal and vesicular colonizations than those of Z. tenuifolia, while the root colonizations of both species varied significantly among seasons. The increased coverage of D. triflorum resulted in the following effects: (1) the spore density initially correlated with mycorrhizal colonizations of Z. tenuifolia but gradually correlated with those of D. triflorum. (2) Correlations among soil properties, spore densities, and mycorrhizal colonizations were more pronounced in the higher coverage levels. (3) Soil AMF community compositions and relative abundances of AMF operational taxonomic units changed markedly in response to the increased invasion pressure. The results provide strong evidence that D. triflorum possessed a more intense AMF infection than Z. tenuifolia, thus giving rise to the altered host contributions to sporulation, soil AMF communities, relations of soil properties, spore densities, and root colonizations of the two plants, all of which are pivotal for the successful invasion of D. triflorum in lawns.

Multiple applications of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] during the winter, spring, and summer were evaluated for bahiagrass (Paspalum notatum F.) control in centipedegrass [Eremochloa ophiuroides (Munro) Hack.]. Single and repeated atrazine, metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)-one], and methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione] treatments were applied in the spring for common lespedeza [Lespedeza striata (Thunb.) H. & A. ‘Kobe’] control in centipedegrass. Atrazine at 2.2 kg/ha in two (July + August) or three (January + April + May, April + May + July, or May + July + August) applications controlled bahiagrass without injuring centipedegrass turf. In most instances repeated treatments were needed a second year for consistent control of 98 to 100%. A single treatment of atrazine at 2.2 kg/ha, metribuzin at 0.6 kg/ha, or methazole at 1.1 kg/ha satisfactorily controlled emerged common lespedeza in centipedegrass turf. None of the treatments injured centipedegrass. When atrazine was applied to actively growing centipedegrass, the turf had a darker green color than untreated turf. This usually occurred within a month after treatment and lasted for 3 to 4 weeks.

Five rigid ryegrass populations suspected of being resistant to both glyphosate and oxyfluorfen were collected in southern Spain and tested under laboratory-controlled conditions. Four populations (Depuradora, Condado, AlamoRasilla, and Portichuelo) were treated with glyphosate for at least 15 consecutive years, and treatments during the last 5 yr were mixed with oxyfluorfen. The fifth population (4alamos) followed the same glyphosate treatment, although oxyfluorfen was never used to control it. Dose–response assays confirmed glyphosate resistance in all populations, with resistance indexes ranging from 11.7 to 37.5 (GR90). Shikimate accumulation assays consistently supported these data, as the most glyphosate-resistant populations (Depuradora and Condado) displayed the lowest shikimate levels. Surprisingly, four populations (Depuradora, Condado, AlamoRasilla, and Portichuelo) displayed 7.93- to 70.18-fold more resistance (GR90) to oxyfluorfen, despite limited selection pressure, showing a similar resistance pattern as that for glyphosate. The 4alamos population displayed oxyfluorfen GR90 values that were similar to those observed in susceptible plants; however, this population was significantly more resistant in terms of plant survival (LD90). Protoporphyrin IX accumulation assays supported the results of dose–response assays, in that the most oxyfluorfen-resistant populations accumulated less protoporphyrin IX. Although more studies are needed, it seems that these five glyphosate-resistant weed populations display a natural tendency to easily develop resistance to oxyfluorfen, with the populations that have higher resistance to glyphosate also having higher resistance to oxyfluorfen.

Histological abnormalities were found in Canada thistle (Cirsium arvense (L.) Scop.) roots after plants received foliar treatments of 4-amino-3,5,6-trichloropicolinic acid (picloram). These abnormalities included destruction and distortion of cortical cells and cambial and phloem disintegration. Some or all of these conditions were present, depending upon which portion of the root was sectioned.

Amitrole (3-amino-1,2,4-triazole) was applied to purple nutsedge (Cyperus rotundus L.) clones newly established in the field in the Piedmont Plateau. Applications were made one week after initial foliar emergence, then at biweekly intervals through the 20th week. The principal results were: (1) Nutsedge was most susceptible to amitrole applied 4 weeks after initial emergence. The next most susceptible stages were at 1 or 2 weeks; (2) Six weeks after emergence, nutsedge appeared resistant to amitrole; (3) All applications after 10 weeks were relatively ineffective; (4) Chemical analyses of tubers and bulbs indicated a high ash content early in the season, particularly at 6 weeks after emergence.

In an attempt to improve the metering of herbicides applied to soil in horticultural plantings, loosely knitted cellulose acetate cloth was impregnated with herbicides, using appropriate acre rates as determined by those used in spray applications. Treated cloth was placed on the soil and the edges anchored or the cloth was lightly covered with soil. Adjacent plots were sprayed with the same herbicides. The herbicide-impregnated cloth was as effective as spray applications of 12 of the 15 herbicides used.

Response of wild oat (Avena fatua L.) at the three-leaf stage to five postemergence herbicides, asulam (methyl sulfanilylcarbamate), barban (4-chloro-2-butynyl m-chlorocarbanilate), diclofop {2-[4-(2,4-dichlorophenoxy)phenoxy] propanoic acid}, difenzoquat (1,2-dimethyl-3,5-diphenyl-1 H-pyrazolium), and flamprop [N-benzoyl-N-(3-chloro-4-fluorophenyl)-DL-alanine] was measured and compared in nine physiological and biochemical processes (chlorophyll content, photosynthesis, photosynthate translocation, and changes in ATP, lipids, DNA, RNA, amino acids, and proteins). Content of chlorophyll a and b was reduced by the application of diclofop and difenzoquat. Photosynthesis was inhibited with all five herbicides, but photosynthate translocation from the leaves to the roots was reduced only with diclofop, difenzoquat, and flamprop. ATP production was inhibited by all herbicides except flamprop, which increased the ATP content. Incorporation of 32P into lipids was reduced by all herbicides except asulam. The incorporation of 32P into nucleic acids (DNA and RNA) was inhibited by all five herbicides. Serine and threonine, especially the latter, accumulated in large quantity with all herbicides except difenzoquat. Protein content (water-soluble,-insoluble, and total amount) was reduced by asulam, diclofop, and flamprop, but barban and difenzoquat reduced water-insoluble and total proteins only. Apparently, all herbicides examined affected more than one physiological function and biochemical process in wild oat.