Treatment of woollyleaf franseria, Franseria tomentosa Gray, with 2,4-dichlorophenoxyacetic acid (2,4-D) at three locations in Nebraska showed that June treatments were much more effective in reducing the stand than those made in July and August. The isopropyl ester and butoxyethanol ester of 2,4-D were somewhat more effective than the triethanolamine salt. Rates of 2 and 4 lb/A gave the most consistent results.

All herbicides were incorporated either as pretransplanting or posttransplanting treatments. Herbicides used were: ethyl N,N–di–n–propylthiolcarbamate (EPTC), propyl N,N–di–n–propylthiolcarbamate (1607), ethyl N,N–ethyl–n–butylthiolcarbamate (2060), propyl N,N–ethyl–n–butylthiolcarbamate (2061), O–(2,4–dichlorophenyl) O–methyl isopropylphosphoramidothioate (DMPA), 2–chloro–4–diethylamino–6–ethylamino–s–triazine (trietazine), and the dimethyl ester of 2,3,5,6–tetrachloroterephthalic acid (DCPA). EPTC, 1607, 2060, and 2061 at 3 lb/A (repeated 3 times), posttransplanting, and DMPA at 20 lb/A, pretransplanting, gave control of the annual weeds present with the exception of annual morningglory, and had no effects on tobacco yield or value. Nicotine, nornicotine, total alkaloids, and reducing sugar content of the cured leaf were not adversely affected by the thiolcarbamates, DMPA, amiben, or DCPA at rates used. Smoke and taste tests of tobacco treated with EPTC, DMPA, amiben, and DCPA revealed no undesirable flavors.

Indoleacetic acid (IAA) stimulates the incorporation of radioactivity derived from methionine-methyl-C14 into a fraction of sesbania hypocotyl that is insoluble in 80 per cent alcohol or 60 percent acetone, but is soluble in hot water. Incorporation of C14 into “hot water soluble material” was stimulated by IAA, 2,4-dichlorophenoxyacetic acid, 2,5-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, naphthaleneacetic acid, 2,3,6-trichlorophenylacetic acid, 2-methoxy-3,6-dichlorobenzoic acid, 4-amino-2,5-dichlorobenzoic acid, and 4-amino-3,5,6-trichloropicolinic acid. All compounds were stimulative at 1 × 10-5M.

Chemical methods offer a quick means of eliminating crabgrass from lawns and other grassed areas. Sodium arsenite, used since 1935, is the oldest of the selective chemical crabgrass killers. In 1947 phenyl mercuric acetate was reported to be effective as a crabgrass killer in the bentgrass type of turf.

Fall treatments with 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) as opposed to cultivation during the growing season reduced hand labor requirements more than 70% for weed control in Japanese holly (Ilex crenata Thumb. Rotundifolia Hort.). The response of Japanese holly to simazine varied from superior growth and quality to severe injury and death. Simazine plus 3-amino-1,2,4-triazole (amitrole) resulted in better control of small weeds than simazine alone in late winter or early spring. Dimethyl 2,3,5,6-tetrachloroterephthalate (DCPA), N,N-dimethyl-2,2-diphenylacetamide (diphenamid), and 2,6-dichlobenzonitrile (dichlobenil) reduced labor requirements for control of annual weeds without any visual response on the Japanese holly. In certain instances, rotations of herbicides were effective for broad spectrum weed control.

Weed control has been a major problem in Hawaiian pineapple production. Hand-hoeing and mechanical cultivation gave way to activated oil emulsion sprays in the 1940's. The 1950's brought the use of preemergence herbicides especially 3–(p–chlorophenyl)–1,1–dimethylurea (monuron). Each method has brought a shift in species distribution. Today, large-seeded species and vegetatively-propagated species are less well-controlled than in the days of contact weeding with oil emulsion sprays. There is need for selective herbicides which will control the weeds which escape the present program.

One of the factors to be considered in the establishment and maintenance of an orchard is the competition from weeds for the available supply of moisture and nutrients. Mechanical elimination of competitive plants is time consuming, expensive, and may not be done as early or as frequently as it is needed each year. The use of herbicides as a supplement to such a program now seems worthy of serious consideration. In some instances, as will be seen below, the use of mechanical methods may be eliminated altogether.

The fungicide, Dyrene [2,4-dichloro-6-(o-chloroanilino)-s-triazine], severely injured var. mite of Canada thistle [Cirsium arvense (L.) Scop.]; and the damage resembled that caused by bipyridylium herbicides. Regrowth from treated plants was abnormal. Other arylamino-s-triazines were from slightly to severely phytotoxic to leaf sections of the var. mite. The var. horridum of Canada thistle was not affected by Dyrene or related compounds. Dyrene and metabolites of Dyrene which were phytotoxic to var. mite leaf disks in bioassay tests were isolated from treated var. mite and horridum plants, but none of these compounds injured the var. horridum. One metabolite was found only in the var. mite; the other five occurred in both varieties. The metabolites were apparently formed from Dyrene by conjugation reactions.

In weed control, soil sterilization is that process whereby the soil is altered so that it will no longer sustain the higher forms of plant life. In most common use the term refers to the application of chemicals to the soil surface. The sterile effect obtained is not instantaneous but develops as the chemical is carried down through the soil with water or after mixing the chemical with the soil by tillage. The sterile effect obtained is not permanent as leaching, fixation, or decomposition by microbes reduce the toxic effect with time. The length of time chemicals will remain active as soil sterilants varies. Highly soluble materials will be removed much more rapidly than will relatively insoluble compounds.

A greenhouse study was conducted to determine the influence of soybean cyst nematode (SCN) –susceptible and –resistant plant combinations on SCN population densities and plant growth. Purple deadnettle, annual ryegrass, SCN-resistant and -susceptible soybean were planted in pots alone or in combination at one plant pot−1. Annual ryegrass and purple deadnettle reduced soybean growth. Pots with SCN-resistant plants had lower numbers of SCN cysts and eggs than pots with SCN-susceptible plants. However, an SCN-susceptible species grown with any of the SCN-resistant plants resulted in higher cyst counts than pots with only SCN-resistant plants. From an SCN management standpoint, this research suggests that there may be no incentive to using annual ryegrass as a cover crop over planting other SCN-resistant crops to reduce SCN population density.

Damping-off of pea (Pisum sativum) seedlings in autoclaved soil subsequently inoculated with Pythium debaryanum ATCC 9998 grown in potato dextrose broth in presence of 75 ppmw O-(2,4-dichlorophenyl) O-methyl isopropylphosphoramidothioate (DMPA) was not different than in soil inoculated with the fungus grown in potato dextrose broth without DMPA. Damping-off of pea seedlings was less in autoclaved soil subsequently inoculated with Pythium debaryanum ATCC 9998 and to which DMPA (100 ppmw) had been added than in inoculated soil with no DMPA present. Damping-off of pea seedlings germinated in soil from plots that had received annual applications of DMPA, 15 lb/A, was nearly as great as that in the control plots.

Data were collected on 36 variables including application, environmental, edaphic, and plant conditions during a 3-year study of aerial brush control treatments at 79 sites. These then were correlated with defoliation responses of blackjack oak (Quercus marilandica Muenchh.) and post oak (Quercus stellata Wangenh.) to (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T). The stage of oak growth was of primary importance in obtaining maximum defoliation with 2,4,5-T; the oaks were most susceptible 6 to 8 weeks after the last killing frost. Air temperatures above 90 F, relative humidity below 60%, poor spray coverage and swath widths greater than 50 ft significantly reduced defoliation by 2,4,5-T. Total rainfall during the month before spray application was positively correlated with defoliation by 2,4,5-T, but correlations with rainfall for other time intervals before and after spraying were not significant. Wind above 6 mph at the time of treatment slightly reduced 2,4,5-T effectiveness, but wind direction and dew at spray time did not influence herbicide activity. Application rates of less than 1.75 lb/A were not as effective as higher rates. Aircraft design, spray-water carrier constituents, soil type, soil depth, soil temperature, and topography had no apparent influence on defoliation of oaks by 2,4,5-T.

The current assays to confirm herbicide resistance can be time- and labor-intensive (dose–response) or require a skill set/technical equipment (genetic sequencing). Stakeholders could benefit from a rapid assay to confirm herbicide-resistant weeds to ensure sustainable crop production. Because protoporphyrinogen oxidase (PPO)-inhibiting herbicides rapidly interfere with chlorophyll production/integrity; we propose a new, rapid assay utilizing spectral reflectance to confirm resistance. Leaf disks were excised from two PPO-inhibiting herbicide-resistant (target-site [TSR] and non–target site [NTSR]) and herbicide-susceptible redroot pigweed (Amaranthus retroflexus L.) populations and placed into a 24-well plate containing different concentrations (0 to 10 mM) of fomesafen for 48 h. A multispectral sensor captured images from the red (668 nm), green (560 nm), blue (475 nm), and red edge (717 nm) wavebands after a 48-h incubation period. The green leaf index (GLI) was utilized to determine spectral reflectance ratios of the treated leaf disks. Clear differences of spectral reflectance were observed in the red edge waveband for all populations treated with the 10 mM concentration in the dose–response assays. Differences of spectral reflectance were observed for the NTSR population compared with the TSR and susceptible populations treated with the 10 mM concentration in the green waveband and the GLI in the dose–response assay. Leaf disks from the aforementioned A. retroflexus populations and two additional susceptible populations were subjected to a similar assay with the discriminating concentration (10 mM). Spectral reflectance was different between the PPO-inhibiting herbicide-resistant and herbicide-susceptible populations in the red, blue, and green wavebands. Spectral reflectance was not distinctive between the populations in the red edge waveband and the GLI. The results provide a basis for rapidly (∼48 h) detecting PPO-inhibiting herbicide-resistant A. retroflexus via spectral reflectance. Discrimination between TSR and NTSR populations was possible only in the dose–response assay, but the assay still has utility in distinguishing herbicide-resistant plants from herbicide-susceptible plants.

Cover crop residue left on the soil surface as organic mulch in no-tillage crop production provides several environmental benefits, including weed suppression. Thus, many farmers who use cover crops attempt to reduce the use of agricultural inputs, especially herbicides. Therefore, our objectives were to study the potential of different cover crop species to suppress weeds and produce an in situ organic mulch and to evaluate the effect of the organic mulch with and without spraying glyphosate on weed suppression for vegetable (tomato [Solanum lycopersicum L.] and broccoli [Brassica oleracea L. var. botrytis]) growth and yield. Five cover crop treatments (sunn hemp [Crotalaria juncea L.], jack bean [Canavalia ensiformis (L.) DC.], pearl millet [Pennisetum glaucum (L.) R. Br.], forage sorghum [Sorghum bicolor (L.) Moench ssp. bicolor], and no cover crop [control]) were used in the main plots; and glyphosate was or was not sprayed on the flattened cover crop in the subplots of this split-plot experimental design. Organic mulch from pearl millet, sorghum, and sunn hemp resulted in lower weed biomass during the early seasons of both tomato and broccoli than mulch from jack bean and no cover crop (control). Spraying glyphosate after roller-crimping reduced weed biomass by 103 g m−2 and 20 g m−2 by 45 and 60 d after transplanting (DATr) of tomato, respectively, and resulted in a better tomato yield compared with not spraying. Glyphosate reduced weed biomass by 110 g m−2 in the early season of broccoli (30 DATr) but did not affect yield. Terminating high-biomass cover crops with a roller-crimper is a promising technique for weed management in vegetable crops that has the potential to reduce or even eliminate the need for herbicide.

Continuous use of heavy machinery in fields and frequent farm traffic sometimes result in soil compaction. Soil compaction reduces the oxygen (O2) concentration in soil capillaries and hence lowers the O2 availability for germinating seeds. We investigated how reduced O2 levels changed germination behavior of weeds to elucidate their potential to adapt to O2-deficient soils (compacted, compressed, and waterlogged soils and soils with hard surfaces). Two similar laboratory experiments were conducted with five O2 treatments (20.9%, 15%, 10%, 5%, and 2.5%). The germination percentage of the invasive weed hairy fiddleneck [Amsinckia menziesii (Lehm.) A. Nelson & J.F. Macbr. var. menziesii] and the common weeds common lambsquarters (Chenopodium album L.) and Persian speedwell (Veronica persica Poir) was not significantly reduced at 15% O2. The germination of scarlet pimpernel (Anagallis arvensis L. ssp. arvensis), silky windgrass [Apera spica-venti (L.) Beauv.], catchweed bedstraw (Galium aparine L.), and knawel (Scleranthus annuus L.) was significantly reduced at 15% O2. The highest germination was obtained at 20.9% O2 for blackgrass (Alopecurus myosuroides Huds.), A. spica-venti, G. aparine, annual bluegrass (Poa annua L.), wild mustard (Sinapis arvensis L.), scentless chamomile [Tripleurospermum inodorum (L.) Sch. Bip.], field violet (Viola arvensis Murray) and the less common weeds A. arvensis and S. annuus. Distribution of flora in the landscape may change on O2-deficient soils by reducing germination of some species such as A. arvensis and S. annuus and favoring others like A. menziesii and C. album. The ability to germinate at 2.5% and 5% O2 may contribute to explain why A. myosuroides and A. menziesii have become successful as weeds on O2-deficient soils, as they maintained a germination percentage between 34% and 58% at 2.5% O2.

Gene flow is an important consideration in the adoption of crops with novel traits or transgenes when sexually compatible relatives occur in the landscape. Unfortunately, gene flow and its long-term environmental impacts are very difficult to predict without releasing and studying the novel genotype. This project uses a retrospective population genetics approach to characterize the relationship between cultivated creeping bentgrass (CB) on a golf course and the same species in five feral populations nearby. CB plants were collected from an 8-yr-old golf course, five weedy populations up to 1,020 m from the golf course, and four modern CB cultivars. Using microsatellite markers and Bayesian inference, two major genetic clusters were distinguished: (1) CB cultivars and individuals from the golf course (cultivar genotype), and (2) the majority of individuals (62%) from the five feral populations (feral genotype). Two feral CB individuals (3.3% of all feral plants) were partially assigned to the cultivar genotype. Principal coordinates analysis agreed with this assignment, suggesting that an intraspecific hybridization event may have occurred. Plants in four feral populations showed a high degree of genetic similarity, but one feral population (Reservoir) was heterogeneous indicating that genetically complex CB populations can develop in cultural landscapes. While recognizing the limitations inherent in a single study of CB population genetics, these results add to the relevant knowledge for predictive ecological risk assessment.

Shade avoidance alters the way plants grow, usually causing them to grow taller at the expense of placing resources into leaves, roots, seeds, and other harvestable materials. Sugar beet (Beta vulgaris L.) is a rosette-forming biennial species that has limited capacity to grow tall in the first year of growth. In the context of crop–weed competition, it is mostly unknown to what extent shade avoidance reduces yield in sugar beet relative to other effects like resource competition. To determine the extent of yield loss due to shade avoidance in a field-relevant situation, sugar beets were grown alongside Kentucky bluegrass (Poa pratensis L.) sod in a field study. Roots were separated with a steel root barrier placed into the ground between the grass and beets. Four treatments included a weed-free control (no root barrier or grass), a root barrier control (with root barrier but no grass), shade avoidance (with root barrier and grass), and full competition (with grass but no root barrier). The presence versus absence of grass was the primary driver of effects on measured sugar beet growth and yield parameters, regardless of whether a root barrier was present. Leaf number and root length were also impacted by the presence of the root barrier. These results suggest that shade avoidance is at least as important as root interactions and resource depletion in the context of early-season sugar beet yield loss due to weeds.