Emergence patterns of foxtail in spring wheat following soybean were evaluated for three seeding dates and three tillage regimes. Cumulative foxtail emergence, as a percentage of total plants emerged in the growing season, was generally not influenced by tillage regime throughout most of the emergence period, but when differences occurred, emergence was lower with no-till than with moldboard plow. Foxtail seedling densities were greater in no-till and chisel plow than in moldboard plow. Weed biomass and wheat yields were not affected by tillage regime. Delaying wheat seeding reduced foxtail percent emergence and emerged seedling density. Differences in emergence patterns of foxtail were attributable to thermal accumulation after seeding. Wheat yield was not influenced by seeding date in 2 of 3 yr.

Integration of two or more methods in a weed control strategy may produce a positive interaction. In this study, sequential applications of sublethal rates of 2,4-D and the plant pathogen Sclerotinia minor were assessed for integrated control of common dandelion. S. minor was prepared as a granular treatment of fungal-colonized barley grits. Treatments of 2,4-D (25 or 50% of the recommended field rate) and S. minor treatments (20, 40, or 60 g m–2 rate) were applied alone or sequentially with a 3 wk interval. Fourteen days after inoculation (DAI), sequential applications of either rate of 2,4-D with 40 or 60 g m–2 of S. minor caused greater damage than either treatment alone (P = 0.05). By 21 and 28 DAI, control from 60 g m–2 of S. minor alone was equivalent to any of the sequential treatments (P = 0.05). At all assessment times, the combination of either rate of 2,4-D and 20 or 40 g m–2 of S. minor caused damage equivalent to or greater than that caused by 60 g m–2 of S. minor alone (P = 0.05). According to Colby's test for interactions, 19 of 24 assessments of the sequential treatments were synergistic. Therefore, sequential treatments of sublethal rates of 2,4-D and S. minor can interact positively to increase damage. This synergistic interaction may reduce the rate of either component required for adequate levels of control, possibly decreasing the cost or volume of use of herbicides in traditional weed control strategies.

Field studies were conducted in 1995 and 1996 to determine the rate response of velvetleaf seedling survival, seed production, and shoot biomass to postemergence herbicides in corn and soybean. Dicamba and imazethapyr were applied to corn and soybean, respectively, at 1, ½, ¼, ⅛, 1/16, 1/32, and 0× labeled rates. Velvetleaf mature plant density was linearly related to seedling density, thus indicating that seedling survival was not density dependent, even after seedling densities exceeded 150 plants m−2. Seedling survival as influenced by herbicide was described by a dose–response curve in corn and soybean. In corn, seedling survival ranged from 0 to 48% across herbicide treatments and years. Seedling survival was greater at the ½× or lower herbicide rates than at the 1× rate. In soybean, maximum seedling survival was 61 and 14% in 1995 and 1996, respectively, and minimum seedling survival was less than 2% in each year. Seedling survival was less in 1996 than in 1995 because velvetleaf was infected with wilt in 1996. In soybean, seedling survival was 20 times greater when treated with herbicides at the ½× rate than when treated at the 1× rate in 1995, but seedling survival was similar when herbicides were applied at 1, ½, ¼, and ⅛× rates in 1996. Velvetleaf fecundity (seeds per plant) was dependent on mature plant density in 1995 but was density independent in 1996. Fecundity as influenced by herbicide was described by dose–response curves in corn each year and in soybean in 1995. In 1995, velvetleaf treated with herbicides at ½× and ¼× rates produced 20 to 30 times more seed per square meter than when treated with herbicides at the 1× rate. Differences in seed per square meter were exaggerated by high densities of velvetleaf. Seed per square meter did not differ between velvetleaf treated with herbicides at 1× or ½× rates in corn or soybean in 1996. Wilt infection of velvetleaf in 1996 was the likely cause of differences in herbicide performance between years. Herbicides at reduced rates were not effective at limiting seedling survival and seed production compared to 1× rates in the absence of wilt. As a result, long-term management of velvetleaf with herbicides at reduced rates likely will be difficult, especially in areas with high densities, unless integrated with other management practices.

The longevity of brown and black forms of seed of halogeton (Halogeton glomeratus (M. Bieb.) C. A. Mey.), buried at four depths, was determined for a 10-year period in Nevada, Idaho, Utah, and Washington. The black form of seed had almost 100% field germination at all depths by the end of the first year and had completely germinated or was nonviable by the end of the second. Brown seed on the soil surface began field germination the first year at all locations and had germinated or become nonviable at the end of 6 years. Burial at 1, 3, and 6 inches delayed the peak of viability and tended to increase longevity of brown seed. Germination was less than 0.5% at the end of 10 years, and viability was near zero under both field and laboratory conditions. Seed germinated and lost viability most rapidly in Washington, the location where most favorable winter conditions for germination occurred.

Greenhouse and growth chamber studies were conducted to determine conditions for infection of the fungal pathogen Colletotrichum truncatum (Schw.) Andrus and Moore on Florida beggarweed (Desmodium tortuosum (Sw.) DC. # DEDTO and to determine the host specificity of this fungus. Optimum conditions for disease development were 14 to 16 h incubation in 100% relative humidity (RH) at 24 to 29 C. Control of Florida beggarweed with 105 to 107C. truncatum spores/ml was greatest in the cotyledon stage and decreased with plant age. Ten of 18 Desmodium species tested were susceptible to the Florida beggarweed isolate of C. truncatum but 13 other plant species and varieties were resistant and 61 were immune.

An electrical discharge system (EDS) was evaluated in field studies conducted in 1977 through 1979 in western Nebraska for its ability to control weed escapes in sugarbeets (Beta vulgaris L. ‘Mono Hy D2′). Nine weeks after sugarbeets were planted, kochia [Kochia scoparia (L.) Schrad.] had attained a height above sugarbeets sufficient for EDS treatment. Redroot pigweed (Amaranthus retroflexus L.) and common lambsquarters (Chenopodium album L.) generally attained sufficient height above sugarbeets 11 and 13 weeks after sugarbeet planting. Sugarbeet root yields were reduced 40, 20, and 10% from competition by kochia, common lambsquarters, and redroot pigweed, respectively. Treatment of kochia, redroot pigweed, and common lambsquarters with EDS in some cases resulted in a reduction in weed height. The EDS treatments reduced the stand of all weeds 32, 39, and 47% for 1977, 1978, and 1979, respectively. Although the EDS treatments failed to kill many weeds, it did suppress the competitive ability of the three weeds to the extent that sugarbeet yields were higher in areas receiving EDS treatments than areas receiving no EDS treatment.

In a predictable natural selection process, herbicides select for adaptive alleles that allow weed populations to survive. These resistance alleles may be available immediately from the standing genetic variation within the population or may arise from immigration via pollen or seeds from other populations. Moreover, because all populations are constantly generating new mutant genotypes by de novo mutations, resistant mutants may arise spontaneously in any herbicide-sensitive weed population. Recognizing that the relative contribution of each of these three sources of resistance alleles influences what strategies should be applied to counteract herbicide-resistance evolution, we aimed to add experimental information to the resistance evolutionary framework. Specifically, the objectives of this experiment were to determine the de novo mutation rate conferring herbicide resistance in a natural plant population and to test the hypothesis that the mutation rate increases when plants are stressed by sublethal herbicide exposure. We used grain amaranth (Amaranthus hypochondriacus L.) and resistance to acetolactate synthase (ALS)-inhibiting herbicides as a model system to discover spontaneous herbicide-resistant mutants. After screening 70.8 million plants, however, we detected no spontaneous resistant genotypes, indicating the probability of finding a spontaneous ALS-resistant mutant in a given sensitive population is lower than 1.4 × 10−8. This empirically determined upper limit is lower than expected from theoretical calculations based on previous studies. We found no evidence that herbicide stress increased the mutation rate, but were not able to robustly test this hypothesis. The results found in this study indicate that de novo mutations conferring herbicide resistance might occur at lower frequencies than previously expected.

Practical and economical weed control measures are a necessary part of tree fruit and grape production. Weeds and grasses are serious competitors of fruit plants for moisture and nutrients; they provide cover for destructive mice, and interfere with harvest of the crop.

Application of 20 to 40 lb/A dichlobenil made prior to emergence accomplished various degrees of control of pondweeds (Potamogeton diversifolius, P. foliosus, P. pectinatus, P. pusillus) and a slender naiad (Najas flexilis). Coontail (Ceratophyllum demersum) was affected only by the higher dosage. Limited control and growth inhibition was achieved on several forms of algae in early spring applications. However, applications made to rooted submersed aquatic plants and filamentous algae (Cladophora, Pithophora and Chara) at postemergence stage of development had little herbicidal effect.

Dichlobenil was not acutely toxic to fish at herbicidal concentrations. The range of median tolerance limits was 10 to 20 ppmw for pumpkinseed (Lepomis gibbosus), bluegill (L. macrochirus), redear sunfish (L. microlophus), and largemouth bass (Micropterus salmoides).

Oats, sesbania (Sesbania macrocarpa Muhl.) and purple nutsedge (Cyperus rotundus L.) were grown as test plants in nine different soil types treated with 0, 0.5, 1, 2, 4, 8, and 16 ppm of ethyl N,N-di-n-propylthiolcarbamate (EPTC). Toxicity of EPTC was negatively correlated with the organic matter of the soil. A positive correlation was found between the sand and silt content of the soil and EPTC toxicity to oats and nutsedge. Toxicity of EPTC was not correlated with soil pH, cation exchange capacity, clay content, general reaction, or parent material. EPTC induced a dormancy of nutsedge tubers, of which the length was inversely related to the organic content of the soil.

Canada thistle was named Carduus arvensis by Tabernaemontanus in 1687, Cirsium arvense by Tournefort in 1700, Serratula arvensis by Linné in 1753, Cirsium arvense by Scopoli in 1772, Carduus arvensis by Robson in 1777 and Cnicus arvensis by Hoffman in 1804. The name Cirsium arvense Tourn. was adopted, but more recently Canada thistle has been referred to as Cirsium arvense (L.) Scop. The cardinal number of chromosomes for the Cirsium genus is 17.

Glyphosate-resistant (GR) crops have been rapidly adopted in the United States and the evolution of GR weeds throughout the world has also been on the rise. With experience, weed scientists and crop advisers develop “intuition” on the basis of field history and current in-field conditions for predicting whether escaped weed biotypes may be herbicide resistant. However, there are no previous reports on the association of in-field crop management factors with the prediction of herbicide resistance. By using in-field survey data, we tested the accuracy of predicting glyphosate resistance in late-season horseweed escapes. We hypothesized that glyphosate resistance in late-season horseweed populations found in soybean fields could be predicted using in-field knowledge of crop residues and the appearance and distribution of weeds in the field. Field survey data were collected to determine the distribution and frequency of GR horseweed populations in Indiana soybean fields during September and October of 2003, 2004, and 2005. After the in-field survey, soil properties for sampled field locations were also collected from the U.S. Department of Agriculture Natural Resources Conservation Service Web Soil Survey. GR horseweed predictions used in-field presence of crop residues and the appearance, abundance, and distribution of weeds in the field. The significance of independent data factors were determined by chi-square statistics. The interactions and relative significance of multiple factors were modeled using classification and regression tree analysis. Our results indicated that the most important factor for predicting GR populations was the identification of an altered plant phenotype after injury from POST glyphosate. This was followed by crop rotation, field distribution, and the presence of other escaped weed species in the field in a model with a classification rate of 0.68.

Pesticide sorption by soil is among the most sensitive input parameters in many pesticide-leaching models. For many pesticides, organic matter is the most important soil constituent influencing pesticide sorption. Increased fertility, irrigation, and mowing associated with highly maintained turfgrass areas result in constant deposition of organic material, creating a soil system that can change drastically with time. Changes in soil characteristics could affect the environmental fate of pesticides applied to turfgrass systems of varying ages. Sorption characteristics of simazine and S-metolachlor were determined on five soils from bermudagrass systems of increasing ages (1, 4, 10, 21, and 99 yr) and compared to adjacent native pine and bare-ground areas. Surface soil (0 to 5 cm) and subsurface soil (5 to 15 cm) from all sites were air-dried and passed through a 4-mm sieve for separation from plant material. Using a batch-equilibrium method, sorption isotherms were determined for each soil. Data were fit to the Freundlich equation, and K d (soil sorption coefficient) and K oc (organic carbon sorption coefficient) values were determined. Sorption and soil system age were directly related to organic matter content in the soil. Sorption of both herbicides increased with age of the soil system and was greatest on the surface soil from the oldest bermudagrass soil system. Herbicide sorption decreased at greater soil depths with lower organic matter. Greater amount of 14C–simazine sorbed to subsurface soil of the oldest turfgrass system compared to 14C–S-metolachlor. Results indicate that as bermudagrass systems age and accumulate higher organic matter levels increased herbicide sorption may decrease the leaching potential and bioavailability of simazine and S-metolachlor.

The efficiency of cotton production has been materially increased since 1949 through the use of several pre- and post-emergence herbicides. In the high-rainfall areas, combinations of a pre-emergence herbicide such as isopropyl N–(3–chlorophenyl)carbamate (CIPC) or 3–(3,4–dichlorophenyl)–1,1–dimethylurea (diuron) with one of several post-emergence oils have given more dependable control than the use of single herbicides. In the irrigated areas, early-season weeds have been less of a problem and more attention has been given to herbicides for the control of late-appearing grasses and vines. The substituted ureas, 3–(p–chlorophenyl)–1,1–dimethylurea (monuron) and diuron, have been the most successful herbicides for this use.

One of the major prerequisites to a study of the vegetative buds on the rhizomes of quackgrass is the development of a method in which the rhizomes may be grown where bud growth can be observed easily. Since buds on the rhizomes of quackgrass are subterranean and since immediate observations of growth responses are virtually impossible, it was concluded that a method should be devised which would provide in vitro conditions favorable for growth. Changes in bud activity or dormancy could then be followed throughout the growing season. The method should also be such that observations of and treatments to the rhizomes could be made without disturbing them. Several methods have been used previously, but they do not lend themselves readily to observation or treatment.

Techniques were developed for evaluating herbicides on submersed, floating, and emersed aquatic weeds. Among 75 herbicides evaluated on three species of submersed weeds only 1:1′-ethylene-2:2′-dipyridylium (diquat); 1:1′-dimethyl-4:4′-dipyridylium (paraquat); di-N,N-dimethylcocoamine salt of 3,6-endoxohexahydrophthalic acid (endothal); and acrylaldehyde (acrolein) gave 85 percent or better control at 1 ppmw in still-water tests. Only acrolein and the di-N,N-dimethylcocoamine salt of endothal were effective on submersed weeds at 1 ppmw in the limited-exposure test. The evaluation of 17 herbicides on three species of floating aquatic weeds indicated that diquat was effective when applied at rates of 1 or 2 lb/A. The tertiary fatty acid amines of 2,4-dichlorophenoxyacetic acid (2,4-D) were more effective on floating weeds than an ester of 2,4-D. Many of the 78 herbicides evaluated on alligator-weed (Alternanthera philoxeroides (Mart.) Griseb.) killed the tops, but only two retarded the sprouting of underwater nodes longer than 6 weeks. Applications of 20 lb/A of 2-(2,4,5-trichlorophenoxy)-propionic acid (silvex) in two formulations retarded sprouting of underwater nodes for 8 weeks.

Differential tolerances to dalapon (2,2-dichloropropionic acid), amitrole (3-amino-s-triazole) plus ammonium thiocyanate, and glyphosate [N-(phosphonomethyl)glycine] of 3- to 10-week-old seedlings of creeping red fescue (Festuca rubra L. ‘Boreal’) and redtop (Agrostis alba L.), which are desirable ditchbank grasses, and reed canarygrass (Phalaris arundianacea L.), a troublesome ditchbank weed, were evaluated in the field. Within each of four treatment dates (growth stages), dalapon affected all species similarly; effects among treatment dates ranged from none to severe. Amitrole at 4.5 kg/ha plus ammonium thiocyanate at 4.1 kg/ha reduced the stand of 3-week-old seedlings of redtop, creeping red fescue, and reed canarygrass by 0, 42, and 94%, respectively, but had little effect on older seedlings. Glyphosate at 1.1 kg/ha did not significantly reduce the stand of 5- to 10-week-old creeping red fescue seedlings, but killed nearly all reed canarygrass seedlings. Redtop was intermediate in its tolerance to glyphosate. Applications of amitrole plus ammonium thiocyanate or glyphosate should be effective in converting ditchbank vegetation from an undesirable stand of reed canarygrass to a desirable stand of redtop and creeping red fescue, respectively.

All actively metabolizing cells have an electrical potential difference, negative on the interior, across their membranes. This electrochemical potential gradient is generated primarily by proton-pumping ATPases and provides the driving force for the transport of various ionic and neutral solutes. It is a key element in the energy metabolism of cells. Such factors as alteration of transport processes, energy metabolism, cytoplasmic pH, and membrane permeability have a direct effect on the magnitude of the membrane potential. In a brief survey, diclofop-methyl, diclofop, hydroxydiclofop, CGA 82725, haloxyfop-methyl, haloxyfop, bentazon, dinoseb, 4-hydroxy CIPC, and 2-hydroxy CIPC caused rapid depolarizations of the membrane potential of oat coleoptiles. Chlorsulfuron, dimethipin, propham, CIPC, dicamba, alachlor, metolachlor, napthalic anhydride, and paraquat had no measurable effects. The depolarizing effects of diclofop-reported earlier are used to illustrate the methods and interpretation of plant cell membrane potential measurements. Diclofop and diclofop-methyl affect the membrane properties of sensitive plant cells. Diclofop irreversibly depolarized the membrane potential and increased the proton permeability of sensitive cells but not resistant cells. It also increased the ATPase activity of isolated membrane vesicles. The mechanism through which diclofop exerted its effect is not fully understood. The equipment and techniques required for the intercellular recording of membrane potentials and resistance are described as well as the limitations of the techniques. A method not used in herbicide studies but with great potential for studies of herbicide interactions with membranes is patch clamp. A brief introduction to the methods will be given.

Field and greenhouse studies were conducted in 2002 and 2003 to evaluate mechanically stimulated compensatory growth response of ivyleaf morningglory, common waterhemp, and giant ragweed. Compensatory growth was initiated by the physical removal of the apical shoot to break apical dominance. The amount of apical shoot removed had an effect on mechanically stimulated compensatory growth of common waterhemp and giant ragweed. With these species, the more of the apical shoot removed from the plant, the less compensatory growth occurred. Removal of the shoot from above the cotyledons resulted in giant ragweed that were 48% shorter and weighed 41% less than control plants at 6 wk after shoot removal. However, the amount of apical shoot removed had no effect on the growth of ivyleaf morningglory compared with control plants at the completion of the study. The influence of plant height at the time of shoot removal on compensatory growth was specific to each weed species. Ivyleaf morningglory exhibited less compensatory growth when the plants were 10 cm at the time of shoot removal compared with 20-, 30-, and 40-cm-tall plants. Removal of the shoot when common waterhemp plants were 30 or 40 cm in height reduced plant weight by 23 and 21%, respectively, compared with control plants. However, no reductions in plant weight were observed when common waterhemp were 10 or 20 cm tall at the time of shoot removal. Giant ragweed subjected to shoot removal was smaller in most growth parameters than control plants, regardless of plant height at the time of shoot removal.

Increasing the germination potential of dormant seeds in a population over time generally requires afterripening. Research was conducted to study the relationship between temperature and seed moisture content on afterripening of dormant leafy spurge seeds. Germination of nonafterripened seeds was 59 and 36% after 21 d for the Harwood and Fargo populations, respectively. Germination of 85 to 87% and 58 to 62% was obtained for the Harwood and Fargo populations, respectively when afterripened for 12 to 24 wk under the most effective conditions of 30 C and 2.6% seed moisture; increasing the afterripening temperature to 45 C did not increase germination. Germination decreased slightly at 30 C as the seed moisture content increased to 5.6%, but germination still exceeded that of nonafterripened seeds. Afterripening at 30 C with a seed moisture content of ≥9% greatly decreased germinability due to seed ageing. A temperature of 5 C was effective for afterripening when the moisture content was 3.3%, but germination was still slightly less than for the low moisture content seeds afterripened at 30 C. Afterripening seeds with 6 to 13% moisture at 5 C generally did not increase germination compared with the control, but did not result in seed ageing.