During the course of investigation of the activity of some organic compounds selected for potential herbicidal effects (5), many candidates appeared to have activity equal to that of the standard 2,4–dichlorophenoxyacetic acid (2,4–D), while a few appeared to be more active. One of the latter was p–fluoro–o–toloxyacetic acid, which was first prepared by M. S. Newman (3).

An understanding of the physiological and morphological responses of plant tissues to phytotoxic compounds makes possible a more intelligent approach to the problem of selecting and developing new herbicides for the control of weeds in growing crops. This area of investigation is frequently neglected in the haste to screen the many new chemicals which are reported.

The use of 2,4–dichlorophenoxyacetic acid (2, 4–D) for weed control in irrigated regions, especially on ditch banks, provides possibilities of contaminated irrigation water. A series of investigations has been conducted in the field to determine the response of certain crop plants to applications of 2,4–D in irrigation water, particularly at rates totaling herbicidal dosages. The first investigation dealt with Red Mexican beans and the results were published in 1954. This report deals with sugar beets, a root-type of crop plant.

Swainsona salsula is an unpalatable legume of Asiatic origin which spreads by seed and lateral roots and has become a problem in some poorly drained, marshy, or saline areas of several western states. Its growth characteristics indicate a high potential for establishment along stream banks, irrigation canals, wasteways, pastures, and meadows with high water tables and other poorly drained areas. Its aggressiveness results in its dominance on such areas. 2-(2,4,5-trichlorophenoxy)propionic acid (silvex), 2,4-dichlorophenoxyacetic acid, (2,4-D), or 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) applied at 2 lb/A for 2 or more years in succession controlled swainsonpea satisfactorily on cropped land. 2,3,6-trichlorobenzoic acid (2,3,6-TBA) was also effective.

This herbicide, plus the 2,4 and 2,4,5 compounds, has remained active in the soil for 6 years on a Norfolk fine sandy loam in an area with about 49 inches of rainfall per year. The residue produced formative effects on tobacco leaves throughout this time.

On rich alluvial soils where corn is the principal crop, Johnsongrass is the most serious perennial weed in Ohio at least as far north as the 40th parallel, and it has spread rapidly north of that area, even into southeastern Michigan. Many people still credit the statements of Piper (3), Vinall (6), and Bennett (1) that Johnsongrass behaves as an annual north of the 38th parallel. Pammel and King (2), to be sure, reported Johnsongrass as a perennial weed in southwestern Iowa in 1919 and the junior author's experience with it as a weed in Ohio, from the 40th parellel south, began in 1918. At Columbus these infestations have lived through winters in which the temperature was below 0° F for prolonged periods and the ground was frozen to a depth of 6 to 8 inches. There seems to be no hope for the eradication of Johnsongrass by mere freezing.

Weedy rice (Oryza sativa f. spontanea Rosh.) is an emerging weed of California rice (Oryza sativa L.) that has potential to cause large yield losses. Early detection of weedy rice in the field is ideal to effectively control and prevent the spread of this weed. However, it is difficult to differentiate weedy rice from cultivated rice during early growth stages due to the close genetic and phenotypic relatedness of cultivated rice and weedy rice. The objective of this study is to examine phenotypic variation in weedy rice biotypes from California and to identify traits that could be used to visually identify weedy rice infestations at early growth stages for effective management. Greenhouse experiments were conducted in 2017 and 2018 using five phenotypically distinct biotypes of weedy rice found in California, along with diverse cultivated, weedy, and wild rice types in a randomized complete block design. We measured variation for 13 phenotypic traits associated with weedy rice and conducted principal component analysis and factor analysis to identify important weedy traits. Most weedy rice individuals within a biotype clustered together by phenotypic similarity. Pericarp color, hull color, chlorophyll content, grain length, plant height, leaf pubescence, collar color, and leaf sheath color account for most of the observed variation. California weedy rice biotypes are phenotypically distinct from wild rice and from weedy rice from the southern United States in their combinations of seed phenotypes and vegetative characteristics. In comparison with the locally grown temperate japonica cultivars, California weedy rice tends to be taller, with lower chlorophyll content and a red pericarp. Weedy rice biotypes vary in seed shattering and seed dormancy. For weedy rice management, plant height and chlorophyll content are distinct traits that could be used to differentiate weedy rice from the majority of cultivated rice varieties in California during vegetative stages of rice growth.

Nemaha oats and 3-amino-2,5-dichlorobenzoic acid (amiben) were effective in reducing weed growth during sweetclover establishment. Hubam and Madrid sweetclover, as well as Nemaha oats, were tolerant of amiben rates necessary for annual weed control. Second-year Madrid sweetclover hay yields were increased from amiben applications the previous year.

Studies conducted near Bernardo, New Mexico, from 1962 to 1964 involved the influence of environment on weekly shoot growth and carbohydrate storage in roots and stems of undisturbed saltcedar (Tamarix pentandra Pall.) and of regrowth (plants regrown from having been mowed the previous winter). Environmental variables were air and soil temperatures, relative humidity, wind, solar radiation, barometric pressure, evaporation from a free-water surface, depth to water table, and water quality. No variable or combination of variables could be used to predict accurately the maximum shoot growth period or the minimum carbohydrate storage. An analysis of the individual weekly growth patterns of plants showed considerable week-to-week variation in the rate of growth. Wide variations in total carbohydrate storage in roots and stems within weeks also were evident, and probably reflected the variation in growth.

Experiments were conducted in the field, greenhouse, and controlled environmental chambers to determine the injury to sunflower (Helianthus annuus L.) from diclofop (2-[4-(2,4-dichlorophenoxy) phenoxy] propanoic acid} as influenced by diclofop rate, sunflower line and growth stage, and temperature/relative humidity (RH). Eighteen sunflower lines varied widely in response to diclofop applied postemergence at 2.2 kg/ha with visual injury ratings ranging from 5 to 46%. Neither diclofop-tolerant (Hybrids 894 and 903) nor diclofop-susceptible (Hybrid 923 and Inbred HA290) sunflower lines were injured by diclofop at 10 C. Diclofop injured all four sunflower lines at 30 C. Sunflower injury following diclofop application at 30 C was greater at 90% RH than at 40% RH; and the diclofop-susceptible lines were injured more than the diclofop-tolerant lines. Diclofop injury to sunflower at the two- to four-leaf stage was greater than at any other growth stage in the field and greenhouse. Seed yields of sunflower (Hybrid 894) treated with diclofop in the two- and four-leaf stage was reduced 15 and 12% compared to that of untreated weed-free sunflower, respectively, averaged over diclofop rates.

Trichoderma polysporum (Link) Rifai HZ-31 fermentation broth was separated and purified by extraction, column chromatography, and high-performance liquid chromatography. Four monomer compounds with strong herbicidal activity were obtained: p-hydroxyphenyl-2,3-dihydroxypropyl ether, o-hydroxy-3-carbonyl-1-phenylpropanol, 1,8-propanediol o-xylene, and 2-3-dihydroxypropyl propionate. The biological activity verification test indicated that the four monomer compounds could inhibit the germination of wild oat (Avena fatua L.) and canola (Brassica napus L.) seeds. Of the four, compound 3 (1,8-propanediol o-xylene) had obvious inhibitory effects on the germination of A. fatua and B. napus seeds, with inhibition rates of 83.33% and 86.67%, respectively. Therefore, the identification of this monomer compound lays a foundation for the further development of a novel microbial herbicide by directly utilizing it and developing new derivatives with herbicidal functions as lead compounds.

Although sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] is the fifth most important grain crop in terms of global production, no commercial hybrids carry genetically engineered (GE) traits for resistance to insect pests or herbicides due to regulatory concerns about gene flow to weedy relatives. However, non-GE herbicide resistance currently is being developed in grain sorghum and will likely transfer to related weeds. Monitoring the impact of this new nuclear technology on the evolution and invasiveness of related weeds requires a baseline understanding of the population biology of grain sorghum genes once they transfer to in situ weed populations. We previously characterized the rate of gene flow from grain sorghum to shattercane [Sorghum bicolor (L.) Moench nothosubsp. drummondii (Steud.) de Wet ex. Davidse], a conspecific weed relatively common in North America; as well as the ecological fitness of an F1 population when S. bicolor nothosubsp. drummondii was the maternal parent. Here we report the ecological fitness of a S. bicolor nothosubsp. drummondii × S. bicolor ssp. bicolor F2 population relative to its crop and weed parents. Parental and F2 populations were grown in two Nebraska environments in 2012 and 2013. Traits evaluated included overwinter survival, field emergence, biomass production and partitioning at anthesis, total seed production, and 100-seed weight. Results indicated that F2 traits were generally intermediate between the parents, but more similar to S. bicolor nothosubsp. drummondii than to grain sorghum. The one exception was overwinter survival, which was nearly 0% for both the F2 and the grain sorghum parent in these northern environments. Thus, the frequency of crop alleles stably introgressed into S. bicolor nothosubsp. drummondii populations appears to primarily depend on overwinter survival of the F2 and which selective pressures are imposed upon it by the cropping system. These data provide needed baseline information about the environmental fate of nuclear genetic technologies deployed in this important global crop.

Cultivation of lowbush blueberry (Vaccinium angustifolium Aiton), an important crop in the eastern part of North America, is unique, as it is carried out over the course of two consecutive growing seasons. Pest management, particularly weed management, is impacted by this biennial cultural practice. The choice of methods to control weeds is narrow, and such a system relies heavily on herbicides for weed management. Availability of unique herbicide active ingredients for weed management is limited, and available herbicides are used repeatedly, so the risk of developing resistance is acute. Hair fescue (Festuca filiformis Pourr.), a perennial grass weed, has evolved resistance to hexazinone, a photosystem II inhibitor frequently used in lowbush blueberry production. We show that substitution of phenylalanine to isoleucine at position 255 is responsible for a decreased sensitivity to hexazinone by a factor of 6.12. Early diagnosis of resistance based on the detection of the mutation will alert growers to use alternative control methods and thus help to increase the sustainability of the cropping system.

Glyphosate’s efficacy is influenced by the amount absorbed and translocated throughout the plant to inhibit 5-enolpyruvyl shikimate-3-phosphate synthase (EPSPS). Glyphosate resistance can be due to target-site (TS) or non–target site (NTS) resistance mechanisms. TS resistance includes an altered target site and gene overexpression, while NTS resistance includes reduced absorption, reduced translocation, enhanced metabolism, and exclusion/sequestration. The goal of this research was to elucidate the mechanism(s) of glyphosate resistance in common ragweed (Ambrosia artemisiifolia L.) from Ontario, Canada. The resistance factor for this glyphosate-resistant (GR) A. artemisiifolia biotype is 5.1. No amino acid substitutions were found at positions 102 or 106 of the EPSPS enzyme in this A. artemisiifolia biotype. Based on [14C]glyphosate studies, there was no difference in glyphosate absorption or translocation between glyphosate-susceptible (GS) and GR A. artemisiifolia biotypes. Radio-labeled glyphosate metabolites were similar for GS and GR A. artemisiifolia 96 h after application. Glyphosate resistance in this A. artemisiifolia biotype is not due to an altered target site due to amino acid substitutions at positions 102 and 106 in the EPSPS and is not due to the NTS mechanisms of reduced absorption, reduced translocation, or enhanced metabolism.

The herbicide dicamba has injured millions of hectares of sensitive plant species in the United States since 2017. This injury has coincided with the commercialization of dicamba-resistant soybean [Glycine max (L.) Merr.] and cotton (Gossypium hirsutum L.). We quantified atmospheric deposition and mass flux of dicamba in 12 soybean production regions of Missouri. Dicamba was routinely detected in weekly deposition samples collected during agriculturally intensive spray periods. Observed concentrations were indicative of both local (<1 km) and long-distance transport (>1 km) of airborne dicamba. High-deposition events (>100 µg m−2) occurred annually in southeast Missouri, and peak dicamba deposited at these sites (12.5 to 84.0 µg m−2) was sufficient to injure non–dicamba resistant soybean. Adoption rate of dicamba-resistant crops and atmospheric stability explained much of the variance, and it is difficult for a herbicide product label to address these variables. Overall, these results demonstrated that dicamba was commonly deposited from the atmosphere during the growing season, and observed concentrations and fluxes were strongly related to the timing and magnitude of rainfall events and the amount of dicamba usage near collection sites.

Herbicide resistance is often viewed as a complex problem in need of innovative management solutions. Because of the transboundary mobility of many weeds, resistance to herbicides is also viewed as a community-scale issue. Consequently, the idea of greater coordination among resource users—especially growers—is often promoted as a management approach. Recently, scholars have framed herbicide resistance as a commons problem in need of collective action. Specifically, social scientists have explored the utility of adopting bottom-up, community-based approaches to help solve the growing problem of herbicide resistance through a framework for interpreting the commons known as common pool resource theory. This article analyzes how herbicide resistance fits—and fails to fit—within common pool resource theory and offers an updated conceptual framework from which to build future work. We argue that the application of common pool resource theory to herbicide-resistance management is underdeveloped, and approaches based on this theory have shown little success. The relevance of common pool resource theory for informing herbicide-resistance management is less settled than existing scholarship has suggested, and other frameworks for approaching transboundary resource problems—such as co-production of knowledge and participatory action research—warrant consideration.

Narrow row spacing (<76 cm) could improve crop competitiveness, suppress weeds and might provide yield advantage. Many studies have been conducted to evaluate the impact of narrow row spacing; however, no quantitative synthesis of these studies exists. The objectives of this meta-analysis were to (1) quantify the overall effect of narrow row spacing (<76 cm) on weed density, biomass, control, weed seed production, and yield in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] compared with 76-cm row spacing, and (2) assess the influence of agronomic management decisions (tillage type, weed management, herbicide application frequency and time) on effect of narrow row spacing on weed suppression and corn and soybean yield. We compiled 1,904 pair-wise observations from 35 studies conducted in 12 states in the United States during 1961 to 2018. Averaged across individual observations, narrow row spacing suppressed weed density by 34%, weed biomass by 55%, and weed seed production by 45%, while it improved weed control by 32% and crop yield by 11% compared with 76-cm row spacing. Narrow row spacing in soybean suppressed weed density by 42%, weed biomass by 71%, and increased crop yield by 12% compared with 76-cm row spacing. Although narrow row spacing had a nonsignificant effect on response variables in corn, the number of studies (n = 1 to 6) and observations (n = 1 to 59) addressing each response variable were limited. Tillage type (conventional and reduced) did not influence the response of weed density, control, and seed production in narrow row spacing; however, weed biomass and weed seed production were more greatly reduced with the sequential application of herbicides compared with a single application. Thus, narrow row spacing in soybean can be integrated with other options for management of herbicide-resistant weeds.

Due to increased food demand, the use of herbicides is both necessary and on the rise. Several herbicide classes target photosynthetic electron transport: Herbicide Resistance Action Committee (HRAC) Groups 5, 6, and 22. These herbicides are used in large amounts in many different cropping systems to control several species of broadleaf and grass weeds. This article provides a comprehensive review of what these photosynthesis inhibitors are, how they are used, and their modes of action. Presently, commercial herbicides only inhibit electron flow at two different sites: photosystem II (PSII) and photosystem I (PSI). Herbicides that inhibit electron flow at PSII block the movement of electrons down the electron transport chain, while those that inhibit electron flow at PSI accept electrons. Necrosis developing on the leaves of plants treated with PSII and PSI inhibitors is due to the accumulation of reactive oxygen species. Evolution of resistance, toxicity concerns, and other limitations of these herbicides call for the exploration of new chemistries that can be used to target this pathway.

During 1962, barnyardgrass (Echinochloa crusgalli (L.) Beauv.) seedlings were infested by the larvae of Hyperodes humilis Gyll. The larvae attacked the growing points and the youngest tissues in the crown as well as meristematic tissues of intercalary growth. These infestations produced considerable decreases of barnyardgrass growth and yields, suggesting the possibility of effective biological control of barnyardgrass.

The possibility of using 2,4-D as a soil sterilant was investigated by Crafts in 1949. He concluded that 2,4-D may be useful under very dry conditions. In 1954, Shafer reported several years work using soil applications of 2,4-D for control of perennial weeds. Since that time many research workers have evaluated 2,4-D and other phenoxy herbicides for the control of bindweed and other perennial weeds. Results of these trials varied from excellent to poor.