Annual treatment with 1 lb/A of 2,4-D (2,4-dichlorophenoxyacetic acid) repeated for several years during mid- to late June gave excellent control of ironweed (Vernonia baldwinii). Later dates of treatment were not as effective. Mowing was not satisfactory as a weed control measure for ironweed. The populations in mowed plots in long-term experiments closely paralleled populations of untreated plots. Where numbers were reduced by spray treatment, recovery or reinfestation was comparatively slow. Quadrat counts were compared with visual estimation as methods of evaluating effects of treatments. The estimation method did not permit quantitative evaluation of year-to-year treatment or environmental effects.

The widespread adoption of glyphosate-resistant corn and soybean in cropping rotations often results in volunteer plants from the previous season becoming problem weeds that require alternative herbicides for control. Corn yield losses due to season-long volunteer soybean competition at several densities in two growing seasons were used to define a hyperbolic yield loss function. The maximum corn yield loss observed at high volunteer soybean densities was about 56%, whereas, the incremental yield loss (I) at low densities was 3.2%. Corn yield loss at low volunteer soybean densities was similar to losses reported for low densities of velvetleaf and redroot pigweed, with 10% yield loss estimated to occur at 3 to 4 volunteer soybean plants m−2. Several herbicides, including dicamba with or without diflufenzopyr applied at the V2 growth stage of volunteer soybean, provided > 90% control, demonstrating several economical options to control volunteer glyphosate-resistant soybean in glyphosate-resistant corn. Reevaluation of control recommendations may be needed with commercialization of other genetically modified herbicide-resistant soybean varieties.

Preemergence applications of 5-bromo-3-sec-butyl-6-methyl-uracil (bromacil) and mixtures containing bromacil controlled both broadleaf and grass weeds best, and increased seed production and quality in side-oats grama [Bouteloua curtipendula (Michx.) Torr.] fields. Other herbicides significantly reduced broadleaf weeds but did not control grass weeds. The most troublesome grasses were foxtails; bristly foxtail [Setaria verticillata (L.) Beauv.] was the most prominent. Weed competition, in plots where herbicides were ineffective, significantly lowered seed production. Excessive rainfall after treatment was probably responsible for decreased weed control and increased crop damage from some of the herbicides. Applications of 2,3,5-trichloro-4-pyridinol (pyriclor) resulted in early crop chlorosis.

Herbicide-application equipment was combined with a sled tool-carrier to plant a test crop and apply a herbicide simultaneously to single rows as surface, subsurface, incorporated, and injected sprays. Results obtained, using the methyl ester of 3-amino-2,5-dichlorobenzoic acid (amiben), indicated the equipment can facilitate the evaluation of the influence of method of application on the activity of herbicides applied to soil.

Visible symptoms of injury in susceptible oat plants were not apparent until 4 days after initial exposure to 1 ppm 2-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine) solution. However, the herbicide affected normal absorption and translocation over this period of time. The expected translocation and uniform distribution of xylem transported atrazine-C14 was stopped after 2 days of exposure to atrazine. The reduction in translocation of atrazine-C14 to the shoot apices in early stages of herbicidal injury was not due to reduced asborption but very likely to some other factor such as reduced transpiration. Reduction in atrazine-C14 absorption reduced translocation to the shoot apices, but this probably was not an important factor in the early stages of injury. Absorption of atrazine-C14 by oat plants was not reduced until 3 days of exposure to atrazine. A reduction in carbohydrate concentration, resulting in injury to root tissues, may be responsible for reduction in total atrazine-C14 absorption.

The search for means of controlling submersed aquatic weeds has emphasized the need for evaluation techniques that can be used for the discovery and development of effective herbicides. Before the development and use of aromatic solvents, submersed aquatic weeds were usually removed mechanically. Mechanical removal is time consuming and costly and contributes little toward lasting control or eradication of the weeds. Recently, a number of herbicides that provide some degree of control have been developed. Many of these materials, however, are toxic to irrigated crops and to fish and other wildlife, and the control period is short. The evaluation techniques reported here were designed for use in the discovery and development of new and more efficient herbicides.

Quackgrass, Agropyron repens, is the major weed problem on at least half of the acreage planted to potatoes in Michigan. The northern counties of the lower peninsula and much of the upper peninsula have an ideal climate for quackgrass, and potato growers must include control measures in their production plans. The work reported in this paper was directed toward development of an effective and economical control program.

Field research was conducted to characterize hemp dogbane crown root carbohydrates, to quantify crown root lipids, and to determine seasonal fluctuation of each. The effect of day length on the release of hemp dogbane crown bud dormancy was studied in growth chambers. Total nonstructural carbohydrate (TNC) levels fluctuated with seasonal lows occurring during bud to mid-flowering and seasonal highs in fall predormancy periods. Starch was the primary storage carbohydrate, comprising up to 39.6% of crown root dry weight at the highest seasonal level. Ethanol soluble reducing sugar levels were negatively correlated with seasonal starch levels, and appeared to be maltose and glucose accumulated before conversion to sucrose for translocation. Most lipids were in ethanol soluble fractions and were inversely correlated with seasonal TNC patterns. The maximum level of total lipids in crown roots was 6%. Lipids appeared to be by-products of metabolism in nonarticulated lacticifers and not a significant carbon energy source. An 18-h day length resulted in a 10-fold increase in intact, 2-yr-old crown root dormancy release compared with 10- or 14-h day lengths in growth chamber studies. Seasonal carbohydrate fluctuations in hemp dogbane suggest the most effective control with mowing or tillage would occur when applied at mid- to full flower before root carbohydrates begin to recover. The most effective translocation of phloem mobile herbicides to crown roots would occur after mid- to late flower through leaf senescence when carbohydrates are being stored in the roots.

Weeds exploit underutilized space, causing economic losses in cropping systems. Weed management tactics alter that underutilized space until the crop can mature and efficiently use that space. One tactic is to reduce the weed propagules (e.g., seeds and tubers) that persist quiescently in the soil, which includes minimizing production and addition of new propagules to the soil. Purple nutsedge is a problematic weed around the globe, persisting between growing seasons as tubers in the soil. Imazapic is a peanut herbicide often used in Georgia for control of purple nutsedge. The objective of the experiment was to evaluate the effect of various rates of imazapic on purple nutsedge tuber production. Single presprouted purple nutsedge tubers were transplanted into outdoor microplots and treated after 6 wk of growth with six rates of imazapic (5 to 140 g ai ha−1) POST. A nontreated control was included. All emerged shoots at the time of application were marked with plastic rings; this allowed for classification of tubers at exhumation as (1) tubers attached to shoots that were emerged at time of application, (2) tubers attached to shoots that emerged after application, and (3) tubers without an aerial shoot during the study. At 7 wk after application, the tubers in the microplots were exhumed, classified, and quantified, and their ability to sprout was evaluated. In the nontreated control, there were 544 total tubers, with a log-logistic regression model describing the declining tuber population with increasing imazapic rate. The rate of imazapic that reduced total tuber population 50% (I50) was 36 g ha−1. In the nontreated control, there were 161 tubers attached to shoots that emerged, as when compared with plots that received an imazapic application that had an I50=60 g ha−1. Viability of purple nutsedge tubers was 44% at 70 g ha−1 imazapic, suggesting the action of the herbicide may have rendered the tubers nonviable after new shoots were produced. The final classification of tubers included those that did not have an aerial shoot during the study. These were tubers in which apical dominance suppressed shoot development or were likely the most recent tubers to develop. Of the three classes, the tubers without shoots were the most prevalent in the nontreated control, with 358 tubers and an I50=18 g ha−1. Imazapic controls purple nutsedge foliage but also reduces the number of new tubers produced, and overall tuber viability and is a valuable tool in management of the long-term population density of this weed.

Coevolving interactions between a plant population and its microbiota can potentially yield a rhizosphere enriched in metagenomes containing the blueprints for a vast array of natural products. We describe a method of isolating those metabolites through activity-based screening of soil metagenomic libraries. The method allows for the isolation of small molecules produced in vector–host expression systems containing large-insert DNA fragments extracted from the target plant rhizospheres. Allelopathic activities derived from selected clones were screened against a series of controls. Nonmetric multidimensional scaling (NMS) showed similar effects of the set of controls on lettuce growth, whereas annual bluegrass had a broader range of growth responses. Methanol extracts from clones indicating activity showed distinct patterns in grass seedling growth from the empty vector control, but the same extracts showed no effect on lettuce. The results indicate that the metagenomics method and bioassay screen of clone extracts are tools that can be used for initial determination of allelopathic activity from noncultured soil microbiota.

In 1963 and 1964, a study was conducted near Bernardo, New Mexico, on control of saltcedar (Tamarix pentandra Pall.), comparing single and combination mowing and spraying with the propylene glycol butyl ether (PGBE) ester of 2-(2,4,5-trichlorophenoxy) propionic acid (silvex), repeated spray applications of silvex, and single applications of mixtures of silvex and 4-amino-3, 5,6-trichloropicolinic acid (picloram). None of the mow-spray combinations initiated in 1963 reduced the stand of saltcedar more than 8%, and there were no significant differences between treatments. A follow-up spray treatment of 4 lb/A of silvex in 1964 increased the plant kill up to 60%. Spraying undisturbed saltcedar in June, 1964, and re-spraying in August with 2 lb/A of silvex, reduced the saltcedar approximately 40%, as did a single application of a mixture of 2 lb/A of silvex and 2 lb/A of picloram in June.

The ribonucleic acid (RNA) content of corn roots and etiolated coleoptiles treated with amitrole (3-amino-1,2,4-triazole) was very similar to that in untreated tissues. Amitrole consistently decreased the acid-soluble nucleotide content of etiolated corn coleoptiles.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is a problematic weed in rice (Oryza sativa L.) fields. Overapplication of herbicides causes environmental pollution and the emergence of resistant weeds, and integrated weed management methods can reduce dependence on herbicides. The growth of E. crus-galli and rice seedlings was shown to be significantly inhibited by high concentrations of fulvic acid (FA, C14H12O8) under flooding conditions (HF, 0.80 g L−1) (P < 0.05). In contrast, seedling growth was promoted by the application of very low concentrations of FA (LF, 0.02 g L−1). The activities of glutathione S-transferase (GST) and antioxidant enzymes, including total superoxide dismutase (T-SOD), peroxidase (POD), and catalase (CAT), in E. crus-galli seedlings were enhanced by the LF treatment; while POD activity decreased and GST, T-SOD, and CAT activity was not significantly altered by the HF treatment. The metabolomic and transcriptomic analyses showed that FA regulated E. crus-galli seedling growth by affecting the synthesis of indole derivatives and flavonoid compounds. Compared with the blank control (CK, 0 g L−1), the levels of four indole derivatives were upregulated under the HF treatment, and the indole derivatives were slightly downregulated under the LF treatment. The flavonoids, including naringenin, naringenin chalcone, eriodictyol, kaempferol, and epigallocatechin, were downregulated under HF treatment, and the growth of E. crus-galli was reduced. In contrast, the metabolism and transcription of flavonoids were not significantly altered by the LF treatment. The addition of 0.80 g L−1 FA obviously inhibited the growth of newly sprouted E. crus-galli, whereas rice growth was significantly promoted 8 d after rice planting (P < 0.05). The application of FA, therefore, might be a potential integrated weed management method to control the damage caused by E. crus-galli in paddy fields.

Due to the looming water and labor crisis, farmers are adopting dry-seeded rice establishment, in which Chinese sprangletop is becoming a major weed. Concerns about the excessive use of herbicides in controlling Chinese sprangletop highlight the need for cultural weed management strategies. Such strategies require an adequate understanding of Chinese sprangletop response to rice plant density, nutrition, and water regime. Therefore, a greenhouse study was conducted to determine the effect of water regime (aerobic and saturated), nitrogen (N) fertilization (50 and 150 kg ha−1), and rice density (0, 160, and 640 plants m−2) on the growth and reproduction of Chinese sprangletop. Chinese sprangletop plants were taller than rice in aerobic conditions than in saturated conditions. All growth parameters (shoot, root, and inflorescence biomass and leaf area, leaf weight, and inflorescence weight ratio) of Chinese sprangletop were higher in aerobic conditions than in saturated conditions when grown without rice. However, no difference was observed for these parameters between water regimes when Chinese sprangletop was grown with rice. Chinese sprangletop growth and seed production was not affected, but rice growth was affected by N rates. Irrespective of N rate and water regime, Chinese sprangletop height (34 to 59%), tiller number (87 to 92%), leaf number (83 to 89%), shoot biomass (93 to 99%), and inflorescence biomass (95 to 99%) decreased as rice density increased from 0 to 640 plants m−2. The ability of Chinese sprangletop to grow taller and produce more plant biomass (107%) and inflorescence biomass (183%) under aerobic than saturated conditions suggests the need for integrated weed management strategies for controlling weeds under water-limited environments. Such strategies might include the use of weed-competitive and drought-tolerant rice cultivars, high seeding rates, and optimum rate of fertilizer application.

All horizontal underground structures were roots and not rhizomes. The small, light-colored feeder roots are short-lived and consist mainly of primary tissue. Only one transition zone per clone was found. This was found in the collet (lower hypocotylary swelling). The transition from exarch, radial stele of the root, to endarch, collateral stele of the upper collet, was complete in approximately 2500 microns. All root primordia examined had endogenous origin from pericycle and pericyclic tissues. Buds arose endogenously in roots but exogenously in the region of the hypocotyl with endarch protoxylem. Primordia were often identified as roots or shoots prior to their emergence from the main axis.

Plots in dense quackgrass (Agropyron repens) sods were treated with 5-bromo-3-isopropyl-6-methyl uracil (isocil) or 5-bromo-3-sec-butyl-6-methyl uracil (bromacil) at rates of 0.5 to 4.0 lb/A in the fall or in the early spring and then planted to corn. Quackgrass was virtually eliminated by all rates but 0.5 lb/A. Corn was severely injured on plots treated at 2 lb/A but made fair growth at rates of 1.0 lb/A or less. The year following corn, soil residues of isocil were more toxic to oats than those of bromacil. Carbohydrate analyses of rhizomes from treated plots indicated that depletion of carbohydrate reserves contributed to the elimination of the quackgrass.

Field experiments were conducted at Lacombe and Ellerslie, Alberta, Canada, to study the interaction of tillage with glyphosate and sethoxydim on quackgrass shoot and rhizome biomass, rhizome bud viability, and crop yield. Glyphosate was most effective in the conventional tillage regime, whereas sethoxydim was effective only in conventional tillage. Tillage reduced viable rhizome buds more than shoot or rhizome biomass. The large tillage effect on quackgrass control with either herbicide would likely be diminished in zero tillage systems that employ preseeding burn-off or preharvest application of glyphosate. Under zero and conventional tillage, glyphosate at 220 g ha−1 often was as effective as glyphosate at 880 g ha−1.A split application of glyphosate at 220 g ha−1 in the fall and spring was as effective as a single 880 g ha−1 treatment in conventional tillage, and sometimes more effective than 880 g ha−1 in zero tillage. Glyphosate followed by sethoxydim was usually no better than glyphosate alone. Short-term split applications of sethoxydim offered no consistent advantage over single applications.

Several differences between the mode of action of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and triallate [S-(2,3,3-trichloroallyl)diisopropylthiocarbamate] were observed on wild oats (Avena fatua L.) in growth chamber experiments. Trifluralin exhibited little or no postemergence effect when applied to the soil surface. Surface applications of triallate interfered with deposition of epicuticular wax on leaf surfaces, and caused necrotic lesions, leaf break, and the abortion of the first leaf through the coleoptile. Trifuralin, but not triallate, incorporated in the soil reduced extension of the wild oat coleoptile. Soil-incorporated treatments involving trifluralin induced more extensive swelling of the mesocotyl, coleoptile node, and and coleoptile than triallate applied similarly. Triallate appeared to exert its major effect on cell elongation of the wild oat foliar material enclosed in the coleoptile. Trifluralin caused its greatest phytotoxic effect on the meristematic tissue at the region of the coleoptile node.

The effect of dimethyl tetrachloroterephthalate (DCPA) applied during the establishment of Tifgreen bermudagrass (Cynodon dactylon (L.) Pers. X C. transvaalensis Burtt-Davy) on subsequent winter injury was studied. DCPA treatments to plots sprigged in July or later increased winter injury. DCPA applied in July, August, or September to plots planted in June resulted in approximately 15% of the turf being killed during the winter; however, DCPA applied before the middle of June did not significantly influence winter injury. Significant increases were found in total nitrogen, nitrate, amino acids, and protein of stolons harvested from DCPA-treated plots. These data suggest that DCPA treatments applied during establishment resulted in a less winter hardy condition by maintaining vegetative growth into the normal period of winter hardening.

On Tifton loamy sand, N-1-naphthylphthalamic acid (naptalam) controlled weeds much better, and injured soybeans [Glycine max (L.) Merr.] much less, when mixed with 2-sec-butyl-4,6-dinitrophenol (dinoseb) and applied as a delayed preemergence treatment, than when it was applied alone immediately after planting. Differences attributable to either timing of application or to mixing with dinoseb were not as marked for 3-amino-2,5-dichlorobenzoic acid (amiben) as for naptalam. Neither amiben, naptalam, nor mixtures of these two herbicides with dinoseb adversely affected germination, weight per seed, or total oil and protein content of the harvested soybean seed.