The study was undertaken to compare the level of total phenolics and the allelopathic potential of Pluchea lanceolata in soils cultivated once or twice a year. Thirty-five cultivated fields were sampled from seven sites in three different seasons. The leaf area, leaf weight, height, and density of P. lanceolata plants were measured. Leaves of P. lanceolata were analyzed for nine chemical characteristics: leaf ash, total phenolics, phosphate, Cu, Zn, Na, K, Mg, and Ca. Weed-infested topsoil and subsoil were analyzed for pH, electrical conductivity, organic carbon, chloride, bicarbonate, total carbonate, phosphate, total phenolics, Zn, Na, K, Mg, and Ca. Fields cultivated twice a year were designated as Group I, while those cultivated once a year were designated as Group II. Biotic and chemical characteristics of plants of Group I and II were not different. Topsoil and subsoil of Group II had higher values for Ca, while topsoil of Group I had higher values for total phenolics. The higher phenolic content of Group I topsoil was probably due to greater incorporation of weed plant parts into the soils as compared to Group II fields. Seedling growth of crop plants such as radish, carrot, maize, mustard, tomato, turnip, and wheat was affected more by soil from P. lanceolata-infested fields that were cultivated twice a year than those cultivated only once. A correlation between higher total phenolic content of P. lanceolata-infested topsoil with two cultivations, and growth response of crop plants was recorded.

Total foliar fatty acid and hydrocarbon content of sicklepod (Cassia obtusifolia L.) treated with 0, 0.14, 0.28, 0.56, 1.12, 2.24, or 4.48 kg/ha S-ethyl dipropylthiocarbamate (EPTC) was measured by gas-liquid chromatography. Total leaflet fatty acid content increased at sublethal rates of the herbicide, but qualitative composition of the fatty acids was not affected. Percent of long chain alkane (≥ C28) content decreased as EPTC application rates increased. Palmitic (23.9%), stearic (9.1%), oleic (3.8%), linoleic (13.6%), and linolenic (33.2%) acids accounted for 83.6% of the total leaflet fatty acid content of sicklepod. Leaflet cuticle thickness of upper and lower epidermal layers decreased 15 and 20%, respectively, as herbicide concentration increased.

Six herbicides were applied monthly from February to May for control of large crabgrass [Digitaria sanguinalis (L.) Scop.] and goosegrass [Eleusine indica (L.) Gaertn.]. Bensulide [O,O-diisopropyl phosphorodithioate S-ester with N-(2-mercaptoethyl)benzenesulfonamide] applied in February or March controlled at least 70% of large crabgrass, whereas, treatments applied in April resulted in similar control at two of three locations. All herbicides failed to control large crabgrass when applied in May. Oxadiazon [2-tert-butyl-4(2,4-dichloro-5-isopropoxyphenyl)-δ2-1,3,4-oxadiazolin-5-one] and butralin [4-(1,1-dimethylethyl)-N-(1-methylpropyl)-2,6-dinitrobenzenamide] controlled goosegrass for the full season when applied in March or April. Oxadiazon also controlled goosegrass when applied in May.

The effects of oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] were studied on electron transport and phosphorylation in isolated spinach (Spinacia oleracea L.) chloroplasts and on the response of green bean (Phaseolus vulgaris L. ‘Spartan Arrow’) to applications of oxyfluorfen alone or in combination with other herbicides. Coupled non-cyclic electron transfer and phosphorylation through photosystems I and II (H2 O å methyl viologen) were inhibited about 30% and 55%, respectively, by 10−4 M oxyfluorfen. Photosystem II-linked phosphorylation with dimethylbenzoquinone (2,5-dimethyl-p-benzoquinone) as the electron acceptor (H2 O å DMQ) was completely inhibited by 10−4 M oxyfluorfen. Photosystem II electron transport with dimethylbenzoquinone as the electron acceptor was inhibited 60% by 10−4 M oxyfluorfen, whereas photosystem I electron transport with 2,6-dichlorophenol indophenol as electron donor (DCIPH2 å MV) was not susceptible to oxyfluorfen inhibition. Photosystem I-linked phosphorylation and the accompanying electron transport supported by durohydroquinone electron donation (DQH2 å MV) were inhibited about 50% by 10−4 M oxyfluorfen, whereas cyclic phosphorylation was not inhibited at that concentration. Increased conductivity of a solution that contained leaf discs taken from green beans treated with various combinations of foliar-applied herbicides was a measure of membrane damage caused by the herbicides, and revealed that oxyfluorfen has a different site of action than do photosynthesis inhibitor and bipyridilium herbicides. Oxyfluorfen plus dinoseb (2-sec-butyl-4,6-dinitrophenol) injury to green beans was additive, but the two herbicides did not have the same site of action. Oxyfluorfen did not appear to inhibit electron transport in chloroplasts at herbicidal rates, nor was it dependent on electron transport for activation.

Herbicide translocation in the phloem is dependent on the production of transport sugar(s) in source tissues and their loading into the phloem. The movement of sugars (primarily sucrose) from mesophyll cells to the sieve element-companion cell complex has been the subject of much investigation in recent years. The current evidence suggests that there are at least two sucrose loading mechanisms in higher plants: one involves sucrose transfer via the apoplasm and subsequent loading into the phloem, while a second mechanism involves movement through continuous symplasmic connections between the mesophyll cells and the phloem. Similarly, both symplasmic and apoplasmic routes may exist at the site of unloading in sink tissue. It is postulated that the mechanism of sucrose loading may influence herbicide entry into the phloem and the likelihood of herbicide transfer to the xylem at the site of loading or unloading.

Ripgut brome has become a problematic weed in Spain both as a consequence of the continuous cropping of winter wheat through minimal tillage systems and its difficult control with selective herbicides. Ripgut brome populations collected in the regions of Castilla-León and Cataluña, two main cereal-growing areas in Spain, were screened in the greenhouse for response to sulfosulfuron, a selective herbicide for the control of brome grasses in wheat, and to glyphosate, often used as a pre-plant knockdown to control bromes in no-till systems. The fresh weight percentage relative to the untreated controls was calculated for each ripgut brome population and herbicide and was used as a measure of the herbicide response. Results showed variation in fresh weight response to both herbicides among populations. Fresh weight of the populations after sulfosulfuron was applied at the two-leaf stage at a rate of 20 g ai ha−1 varied from 3% in the most susceptible population to 35% in the most resistant; the response was similar (6 to 38%) when the herbicide dose was reduced to half. For glyphosate at 800 g ae ha−1, fresh weight varied from 2 to 25% among populations, but the range of variation in fresh weight response increased as herbicide dose decreased to one half, with rates of from 4% to 90% among populations. The location of the collection site (inside the field or in-margin) showed no differences in response to both herbicides, but there was a statistically significant, geographically correlated differentiation for glyphosate response, with a greater resistance in the populations from Castilla-León. Undamaged plants were found after treatments with both herbicides, showing differences in resistance among plants. The study shows inter- and intrapopulation variability for the response of ripgut brome to sulfosulfuron and glyphosate. The implications for resistance development are discussed within the framework of relationships of the structure of the populations relative to their herbicide response.

Greenhouse studies were conducted to determine the prevalence of resistance to acetolactate synthase (ALS)-inhibiting herbicides in 266 Indiana horseweed populations, both glyphosate-susceptible and glyphosate-resistant, and to characterize the response of selected biotypes to combinations of glyphosate and cloransulam. Populations with individuals resistant to ALS inhibitors were more frequent in the northern half (38% of the populations in the NW and 50% of the populations in the NE) of Indiana than in the southern half (26% of the populations in the SW and 5% of the populations in the SE). Only 2% of the populations appeared to be resistant to both glyphosate and ALS inhibitors in an initial greenhouse study. Horseweed populations with resistance to ALS inhibitors exhibited herbicide doses required for 50% reduction in plant growth (GR50) values ranging from 14 to 255 g ai ha−1 of cloransulam. The resistant : susceptible (R : S) ratio for four horseweed populations with suspected resistance to glyphosate and ALS inhibitors ranged from 0.3 to 50 and from 2.5 to 8.1 for cloransulam and glyphosate, respectively. The tank mixtures exhibited an antagonistic response to 3 of the 16 combinations of cloransulam and glyphosate on the susceptible population. The tank mixtures exhibited primarily an additive response to those same combinations in the multiple-resistant populations, but the response was occasionally synergistic for two of the four populations. The additive response between glyphosate and cloransulam indicates that, where the level of resistance is fairly low, combinations of these herbicides should be more effective for control of multiple-resistant populations compared with application of a single herbicide.

Presently accepted methods for calculating the expected response for combinations of agents such as herbicides or other growth regulators from the responses of individual agents can lead to unacceptable results. This may be true even if responses within the linear region of the individual response curves are used in such calculations. A solution to this problem is to assign parameters to response curves of individual agents and to define the expected response function for combinations of agents by the weighted algebraic means of these parameters.

Starch levels, used as a measure of plant stress, were not consistently reduced in root or crown tissue of purple loosestrife plants after 2 yr of severe Galerucella calmariensis or Galerucella pusilla (Coleoptera: Chrysomelidae) defoliation. Early in the season, defoliation from Galerucella spp. approached 100%, but the majority of Lythrum salicaria plants regrew by the end of August, resulting in an average reduction of 81% of the aboveground biomass compared to the control. The stress imposed by Galerucella spp. defoliation was less than that achieved from more severe stress imposed by mechanical shoot clipping at 2- or 4-wk intervals from June to October. Both shoot-clipping treatments killed the majority of plants after one growing season. Galerucella spp. feeding reduced plant stature, which may reduce competitiveness. However, considering the extensive carbohydrate reserves present in the large woody crowns of Lythrum salicaria, it will require in excess of 2 yr of consistent, severe leaf defoliation to cause plant mortality. A combination of stresses, such as winter crown injury, or other biological control agents in addition to Galerucella leaf defoliation may be required for plant mortality.

Protein damage, as a primary phytotoxic consequence of in vivo lipid peroxidation, induced by the diphenyl ether herbicide oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] at a concentration of 10 μM, was measured with the green alga Scenedesmus acutus. In the light, water-soluble proteins are destroyed by a herbicide-induced peroxidation process that can be detected by production of fluorescent products and loss of specific amino acid residues of proteins. The water-soluble cytochrome c-553 and the membrane-bound cytochrome f-553, components of the photosynthetic electron transport, were specifically used as sensitive markers for protein damage, measured as decrease of redox reactions of the cytochromes. Under peroxidizing conditions, destruction of the algal cytochrome c is significantly higher than destruction of membrane-bound components, such as cytochrome f and chlorophyll. Protection against protein loss is achieved by the nonbiological antioxidant ethoxyquin (1,2-dihydro-6-ethoxy-2,2,4-trimethylquinoline) or the photosynthesis inhibitor diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea].

The study objective was to use demographic information to adjust forage production practices to control the invasive weeds golden chervil and yellow-rattle without herbicides by defining the population dynamics traits that are directly involved in weed responses to farming practices. The principal population traits are capacity for dominance, sensitivity and accessibility of targeted developmental stages, and variation in weed population reactions from year to year. On the basis of demographic surveys of these two weed species when subjected experimentally to various cutting regimes (by date and number), we used matrix simulation models to describe each weed in terms of these traits and to construct species-specific management strategies. Management strategies for golden chervil need to prevent new recruitment by focusing on limiting or eliminating seed production and seedling survival because adult mortality is insensitive to cutting. Grazing to a low residual height is proposed in spring, when seedling emergence is maximal, or when adults reach their apex height to prevent the development of reproductive stems. Cutting before flowering may also efficiently limit seed production. The annual life cycle of yellow-rattle allows more flexibility in its management, even when density fluctuates and is unpredictable. If cutting is scheduled to coincide with peak juvenile height, this can drastically reduce population density the next year, and the population can be eradicated within 3 yr.

Growth of tops and rhizomes of established johnsongrass (Sorghum halepense (L.) Pers.) stopped almost completely in the cool winter season. Rhizome growth started in spring later than topgrowth. Rhizomes constituted more than 90% of the total subterranean weight. In a sward of pure johnsongrass, 60%, 30%, and 10% of the total subterranean weight were found in the 0 to 15-cm, 15 to 30-cm, and 30 to 45-cm soil layer, respectively. Water-soluble sugar content in rhizomes was high in early winter and in early summer and low in early spring and in fall. Less water-soluble sugars were found in shallow than in deeper rhizomes. In winter the sugars accumulated mainly in the deepest rhizomes. No clear relation was detected between the sprouting percentage of rhizome buds and growth, sugar content, or seasonal climatic factors. No case of complete sprouting inhibition was recorded during the year.

An application was developed to project UHF electromagnetic fields into soils containing seeds. The unconfined field (2450±20 Mhz) was effective in killing seeds in the soil. At a fixed energy level the toxicity of the field to wheat (Triticum aestivum L.) and radish (Raphanus sativus L.) was constant or slightly increased as the time of exposure was decreased. Thus, the energy requirement for a given level of toxicity can be met by substituting increased power levels for time of exposure in the range of 100 and 1200 watts.

Sulfonylurea (SU)-resistant monochoria has recently been found in rice paddies in Korea. A quick and accurate means of confirming herbicide resistance is necessary to take timely management decisions. This article describes a rapid and reliable assay to detect SU-resistant biotype of monochoria. The techniques tested include seed germination, in vivo and in vitro acetolactate synthase (ALS; EC 4.1.3.18) activity, leaf, and whole-plant bioassays. In the whole-plant bioassay, shoot dry weight of the resistant (R) biotype was 3,200-fold less affected by imazosulfuron and sevenfold less affected by pyrazosulfuron-ethyl than the susceptible (S) biotype. Although the whole-plant bioassay is reliable, it is expensive, requires a lot of infrastructure, and takes a few months to complete. The germination rate of the R biotype in petri dish bioassays was > 200-fold less inhibited by imazosulfuron and 100-fold less inhibited by pyrazosulfuron-ethyl than that of the S biotype. Seed germination bioassays in petri dishes do not require as much infrastructure as whole-plant bioassays do and can be completed in a shorter time. Leaf bioassays showed that leaf color of the R biotype was > 1,600- and 300-fold less affected by imazosulfuron and pyrazosulfuron-ethyl, respectively, compared with that of the S biotype. This assay takes about 6 d to complete. In vivo ALS assays showed lower levels of resistance to ALS herbicides than did in vitro ALS assays, where the R biotype was about 200- and 30-fold less sensitive to imazosulfuron and pyrazosulfuron-ethyl, respectively, than the S biotype. All assays successfully distinguished the R from the S biotype, but in vitro ALS assays are the simplest and the quickest. The in vitro ALS assay was chosen as the standard procedure for future confirmation of resistance in monochoria populations. Caution is needed because the in vitro assay is not appropriate in cases wherein the resistance mechanism is increased metabolism of the herbicide or overexpression of the target enzyme. Results should be interpreted in relation to field history and field observations. Follow-up studies also are needed to verify that other resistance mechanisms do not confound the in vitro assay.

In Asia, a significant area under rice is affected by salinity. Salt stress can affect growth of crops as well as weeds. A study was conducted in a greenhouse to determine the effect of salinity (electrical conductivity [EC] of 1, 6, 12, 18, and 24 dS m−1) on growth of barnyardgrass, horse purslane, junglerice, and rice. Growth variables were analyzed using regression analysis. The tested levels of EC influenced leaf production of barnyardgrass and junglerice but not that of horse purslane. As compared with the control treatment (EC of 1 dS m−1), shoot biomass of barnyardgrass decreased by only 24% at 12 dS m−1, whereas rice biomass declined by 59% at this level of EC. At EC of 24 dS m−1, barnyardgrass still produced 4% of the biomass of the control treatment, whereas rice did not survive at this level of EC. Junglerice shoot biomass decreased by 73% at 18 dS m−1 EC compared with the control treatment, whereas rice shoot biomass declined by more than 86% at 18 dS m−1 EC. An EC of 10 dS m−1 was required to inhibit 50% shoot biomass of rice, whereas the EC to inhibit 50% shoot biomass of barnyardgrass and junglerice was 15 and 13 dS m−1, respectively. Shoot biomass of horse purslane was not influenced by the tested levels of EC. At the highest EC (24 dS m−1), at which rice did not survive, horse purslane shoot biomass was similar to that of the control treatment. In all weed species, data for root biomass showed trends similar to those of shoot biomass. The results of this study suggest that weeds were more tolerant to salt than rice, and horse purslane was the most tolerant species among the weeds.

The process of biological nitrification in soil is a two-step oxidation of the ammonium ion to nitrite and thence to nitrate and is largely caused by the autotrophic bacteria in the genera Nitrosococcus, Nitrosomonas and Nitrobacter. The biochemical activities of these soil microorganisms can be studied by using the percolation technique of Lees. When studying the nitrifying organisms, an ammonium salt solution is bubbled under air pressure through a lift tube to the surface of a soil sample in a suitable container from whence it percolates downward through the soil and drains back into the reservoir from which it came. Air and moisture are thus constantly replenished in the soil and water logging is avoided. The value of the method lies in the fact that any substance introduced into the percolating fluid which affects the metabolism of the nitrifying organisms causes a change in the rate of conversion of the ammonium ions to nitrate ions. When the nitrate in the percolating fluid is in equilibrium with the nitrate in the soil, measurements of the nitrate nitrogen in aliquots of the percolating fluid are a measure of the rate of nitrification in the soil.

Downy brome is one of the most troublesome winter annual weeds in winter wheat-fallow rotations in the central Great Plains. A 3-yr seed burial study was initiated to determine how long downy brome seed remained germinable when placed on the soil surface or 2.5 cm deep at four different times in four environments. Only 1 to 7% of the downy brome seed survived after 1 yr on the soil surface in chemical fallow and stubble mulch when deposited in August, but survival varied in September, October, and November. In 1970, a year with low fall and winter precipitation, 36 to 46% of the seed placed on the soil surface of chemical fallow in September, October, and November survived, compared with 1 to 8% for stubble mulch tillage. Early spring tillage covered more seed with soil, and downy brome seed survival decreased. When fall and winter precipitation was normal, stubble mulch and chemical fallow had 1 to 20% germinable seed remaining. Induced dormancy existed in some years. More downy brome seed survived when placed on the soil surface of crested wheatgrass sod (14 to 50%) than on smooth brome sod (0 to 36%). No differences existed among environments when downy brome seed was buried 2.5 cm deep. Only 0.4% of downy brome seed buried 2.5 cm survived after 1 yr when averaged across all environments.

Percent germination and germination rates of seeds of atrazine (6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine)-resistant and -susceptible biotypes of redroot pigweed (Amaranthus retroflexus L. # AMARE) and Powell amaranth (A. powellii S. Wats. # AMAPO) from southern Ontario were compared in petri dishes using a 100-cell germination plate. The plate provided 100 separately controlled, alternating temperature combinations ranging from 0 to 45 C in increments of 5 C. The experiment was conducted under alternating light and dark conditions for 21 days and under constant dark conditions for 7 days. Initial seed viabilities exceeded 90% for all four biotypes. Under alternating light and darkness, the resistant biotype of Powell amaranth had significantly lower total germination and a slower rate of germination than the susceptible biotype, whereas the biotypes of redroot pigweed did not differ in germination response. Germination rate and total germination were positively correlated for all four biotypes. In constant darkness, the atrazineresistant biotypes of both species had significantly lower germination percentages compared to the susceptible biotypes. In general, percent germination of Powell amaranth was greater than that of redroot pigweed for day and night temperatures <25 C, whereas redroot pigweed had greater germination at higher temperatures. Greater seed dormancy of the resistant biotypes under some light and temperature combinations may have implications for the persistence of resistant populations in the field.

Weed management is a major constraint in organic cropping systems. In 2004, the Cornell Organic Vegetable Cropping Systems Experiment was established in central New York state using a split-plot randomized complete block design with two crop rotation entry points (split-plot factor). Four organic vegetable cropping systems that varied in cropping intensity and tillage (main plot factor) were compared: (1) intensive, (2) intermediate, (3) bio-extensive, and (4) ridge tillage. The basic crop rotation was cabbage, lettuce, potato, and winter squash, with additional short-season crops in the intensive system and with cover crops and fallow substituted for cabbage and potato in the bio-extensive system. In 2014, two uniformity trials were conducted in which oat and then a mixture of sorghum-sudangrass plus Japanese millet were grown uniformly over the entire experiment. Prior to sowing oat, soil samples were collected from each plot and an emergence bioassay was conducted to assess the soil weed seedbank. Crop biomass, weed density, and weed biomass were sampled in the uniformity crops. Soil weed seedbank density was three to four times greater in the intensive, intermediate, and ridge-tillage systems than in the bio-extensive system. The bio-extensive system also had lower weed density and weed biomass in the oat uniformity trial compared with the other three systems. Oat biomass did not differ between the cropping systems. Weed density and biomass in oat were also affected by the crop rotation entry point. Cropping system legacy effects on weed abundance and community composition were greater in the oat than in the sorghum-sudangrass plus Japanese millet uniformity trial. Our results illustrate the effects of different organic vegetable production practices on weed community structure and highlight the value of tilled fallow periods, cover crops, and prevention of weed seed rain for reducing weed populations.

Waterhemp is a weed indigenous to the midwestern United States and is problematic in agronomic crop production. This weed is well suited to inhabit minimally tilled environments and is increasing in prevalence across many agricultural production areas and systems. A common garden experiment was established in Indiana in 2014 and 2015 with waterhemp populations from Indiana, Illinois, Missouri, Iowa, and Nebraska to compare the growth and development of waterhemp from these regions. Three establishment dates (May, June, and July) were used each year to simulate discontinuous germination. Mean biomass accumulations from the May (1,120 g plant−1) and June (1,069 g plant−1) establishment dates were higher than from the July (266 g plant−1) establishment date. There were no differences in biomass accumulations between the five populations in the May and June establishments, but biomass accumulations ranged from 195 to 338 g plant−1 in the July establishment. Mean seed yields were higher from the May (926,629 seeds plant−1) and June (828,905 seeds plant−1) establishment dates compared with the July (276,258 seeds plant−1) establishment. In the May and June establishments, seed yields ranged from 469,939 seeds plant−1 to 1,285,556 seeds plant−1. The Illinois population flowered the latest of all the populations yet also grew the tallest. The July establishment flowered the most rapidly after establishment, accumulated less biomass, and also had the largest seeds. This study demonstrated differences among waterhemp populations when grown in a common environment and the effect of establishment timing on waterhemp growth and development.