Weeds are a fundamental component of agroecosystems and, if not appropriately managed, can cause severe crop yield losses. New perspectives on weed management are required, because current approaches, such as herbicide application or soil tillage, have significant environmental and agronomic drawbacks. We propose the concept of “neutral weed communities,” which are weed communities that coexist with crops and do not negatively affect crop yield and quality compared with weed-free conditions. Management practices that promote neutral weed communities can enable reduced use of herbicides and soil tillage while enhancing ecosystem services and biodiversity. We report scientific evidence of neutral weed communities and survey ecological explanations for why different weed communities have different effects on crop production. We also propose two weed management approaches for attaining neutral weed communities. The first approach aims to maximize weed biodiversity using traditional approaches such as cropping system diversification and integrated weed management. Higher weed biodiversity is associated with lower dominance of competitive weed species that reduce crop yield. The second approach relies on modern tools such as robots and biotechnology to manipulate the density of specific weed species. This approach can remove highly problematic species and minimize niche overlap between the weeds and crops. Given the complexity of interactions among crops, weeds, and other components of the agroecosystem, we highlight the need for multidisciplinary research to illuminate mechanisms that determine the neutrality of weed communities.

Doveweed is becoming more common in agronomic crops in North Carolina. Laboratory and greenhouse experiments were conducted to determine the effect of temperature and seed burial depth on doveweed germination and emergence. Germination of lightly scarified seed at constant temperature was well described by a Gaussian model, which estimated peak germination at 28 C. Similar maximum percentage of germination was observed for optimal treatments under both constant and alternating temperatures. Among alternating temperatures, a 35/25 C regime gave greatest germination (77%). In spite of similar average daily temperatures, germination was greater with alternating temperature regimes of 40/30 and 40/35 C (65 and 30%, respectively) than constant temperatures of 36 and 38 C (4 and 0%, respectively). No germination was observed at 38 C constant temperature or for alternating temperature regimes of 20/10 and 25/15 C. Light did not enhance germination. Greatest emergence occurred from 0 to 1 cm, with a reduction in emergence as depth increased to 4 cm. No emergence occurred from 6 cm or greater depth. This information on seedbank dynamics may aid in developing tools and strategies for management.

Collections of plants of medusahead, Elymus caput-medusae L., were made from natural infestations in Nez Perce County and Washington County in northern and southern Idaho, respectively. The ash and silica content (dry weight basis) decreased as the plant matured. The total ash of the entire plant contained from 72–89 per cent silica. Similar percentages of silica in ash were obtained in the culms, heads, and seeds of the plant. X-ray diffraction patterns and polarizing microscope examinations showed the mineral form of silica to be opal. Heavy deposition of silica was found in the barbs of awns, in the epidermis of the leaves, culms, glumes and seeds, and in strands beneath the epidermis.

An understanding of the physiological action of various herbicides is based in part upon a knowledge of the exact path of movement of the substance in the plant, the rapidity of transfer from one tissue to another, and the relative proportion of the total conductive tissue that is effective in transfer under any given set of conditions. Although the intercellular distribution of radioactive isotopes in both plant and animal tissues has been studied by the use of histoautoradiography, only one brief paper describes the use of this method with plant growth regulators.

The relative tolerance of cucumber and squash to amiben was established for germinating seeds, and for young plants with both foliage and root applications. In all cases, the dosage required for 50% inhibition of growth was 6 to 8 times greater for squash than for cucumber. Root applications of carbon-14 labeled amiben on reciprocal grafts of cucumber and squash plants indicated that the shoots were spared in these experiments by differential blocking of amiben transport from the roots. Shoots of either species appeared tolerant on squash roots and susceptible on cucumber roots. Radioactivity in the shoots of both species was much greater when grafted on cucumber than on squash. Foliage applications of carbon-14 labeled amiben showed more entry into cucumber than squash, but little movement to untreated plant parts in either species.

Reliable mechanical weed control requires knowledge of the achievable levels of weed control and crop damage when using certain implements in specific conditions. Quantitative methods that use weed, crop, soil, and cultivator characteristics to predict weed control and crop damage need to be developed. To that end, the relative susceptibility of weeds and crop plants to mechanical weeding and the selective ability of cultivators need to be quantified separately. The method presented in this study uses measured plant anchorage forces to quantify crop and weed sensitivity to being uprooted by a weed harrow and predicts the relationship between weed and crop uprooting by mechanical weeding. Uprooting and anchorage force of young perennial ryegrass and garden cress plants were measured in laboratory harrowing experiments on sandy soil. A nonlinear equation was introduced to describe the relationship between weed uprooting and crop uprooting. The parameters representing the selective potential of the actual crop–weed condition (K pot) and the implement selective ability (K cult) did not depend on crop uprooting. The relationship between potential weed and crop uprooting that could theoretically be obtained by a perfectly selective implement (i.e., pulling each plant with equal force) was calculated from plant anchorage force distributions measured before harrowing. The observed uprooting percentages achieved by harrowing were lower than the potential uprooting percentages. With K cult accounting for imperfect weeder selective ability, prediction accuracy was satisfactory. Field validation is required to confirm whether this method improves comparison and prediction of weeding performance of different weeding implements in different crop–weed situations.

Effects of ethylene, gibberellins, kinetin, nitrate, light, and temperature, each alone or combined with other factors, were studied on the germination of dormant seed of field pennycress (Thlaspi arvense L. # THLAR). Treated seed were incubated at a range of constant temperatures, at temperatures alternating daily between 12 and 22 C (12 h each), or at 12 C followed by transfer to 22 C. No germination ocurrred in water under any conditions. When applied in the dark KNO3 had no effect on germination under any of the temperature regimes. A combination of KNO3, light, and a shift in temperature from 12 to 22 C induced 100% germination. Four days of incubation at 12 C induced maximal seed sensitivity towards KNO3 + light, and the sensitivity remained unchanged if the incubation at 12 C was extended to 15 days. Upon transfer to 22 C, the effect of low-temperature incubation was lost within 3 days. Of the two seed lots studied, one had a relatively lower chilling requirement, but both had similar requirements for light and KNO3. Gibberellic acids4+7 (GA4+7) induced 100% germination in the dark or light at all temperature regimes. Gibberellic acid3 (GA3) was promotive under isothermal conditions, but its effectiveness was enhanced by a shift in temperature from 12 to 22 C. Kinetin or ethephon, each applied alone, had little effect on germination. Ethephon and GA3 together induced 100% germination.

Research was conducted to determine whether resistance to glyphosate among Palmer amaranth (Amaranthus palmeri S. Watson) populations within the U.S. state of Arkansas was due solely to increased EPSPS gene copy number and whether gene copy number is correlated with resistance level to glyphosate. One hundred and fifteen A. palmeri accessions were treated with 840 g ae ha−1 glyphosate. Twenty of these accessions, selected to represent a broad range of responses to glyphosate, underwent further testing. Seven of the accessions were controlled with this dose; the rest were resistant. The effective dose to cause 50% injury (ED50) for susceptible accessions ranged from 28 to 207 g ha−1. The glyphosate-resistant (GR) accessions had ED50 values ranging from 494 to 1,355 g ha−1, a 3- to 48-fold resistance level compared with the susceptible standard (SS). The 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene relative copy number was determined for 20 accessions, 4 plants accession−1. Resistant plants from five GR accessions (38% of resistant plants tested) did not have increased EPSPS gene copies. Resistant plants from the remaining eight GR accessions (62% of resistant plants tested) had 19 to 224 more EPSPS gene copies than the SS. Among the accessions tested, injury declined 4% with every additional EPSPS copy. ED50 values were directly correlated with EPSPS copy number. The highly resistant accession MIS11-B had an ED50 of 1,355 g ha−1 and 150 gene copies. Partial sequences of EPSPS from GR accessions without EPSPS amplification did not contain any of the known resistance-conferring mutations. Nearly 40% of GR accessions putatively harbor non–target site resistance mechanisms. Therefore, elevated EPSPS gene copy number is associated with glyphosate resistance among A. palmeri from Arkansas.

Two-chloro-4-ethylamino-6-isopropylamino-s-triazine (atrazine), 5-bromo-3-sec-butyl-6-methyluracil (bromacil), and 6,7-dihydrodipyrido-(1,2-a:2’,1′-c)-pyrazidiinium salt (diquat) significantly inhibited photosynthetic C14O2-fixation to varying degrees in corn (Zea mays L., var. Dixie 18), cotton (Gossypium hirsutum L., var. Deltapine), and soybeans (Glycine max (L.) Merr., var. Lee). Atrazine treatments of 10 ppm overloaded the protective mechanisms in corn and cotton. Both 1 and 10 ppm of atrazine reduced C14O2-fixation to less than 5% of controls in soybeans, a sensitive species. One ppm of bromacil and of diquat reduced C14O2-fixation in all three crops, but diquat was less inhibitory than either bromacil or atrazine. Except for diquat on corn, herbicides had no significant effect on C14O2-fixation in the dark. Treatments in which photosynthetic C14O2-fixation was 5% or more of controls had little effect on the kinds or relative amounts of C14-labeled compounds produced. Only the total amount fixed was reduced. Treatments, in which photosynthetic C14O2-fixation was less than 5% of controls, caused a significant reduction in the relative amounts of sucrose and alanine produced; however, the relative amounts of malic, aspartic, and glutamic acids increased.

Adsorption of ethylene in four North Carolina soils and on montmorillonite, kaolinite, and organic matter was studied. Rate of ethylene diffusion through soils in the field and laboratory and its rate of loss from field soil were also examined. Ethylene was adsorbed in greater quantities on two soils which were limed as compared to their field pH. Ethylene adsorption by another surface soil and a subsoil were unaffected by pH changes. The order of adsorption of several hydrocarbon gases by two of the soils was ethylene = propylene ≫ methane ≥ ethane ≥ propane. Montmorillonite adsorbed more ethylene than did kaolinite or organic matter. Soils which adsorbed ethylene also had slower diffusion rates when ethylene was injected into a homogeneous soil column. Ethylene diffused 2.1 m horizontally and 0.7 m vertically to cause at least 50% witch weed (Striga lutea Lour.) seed germination in a Lakeland soil in the field. Witchweed seed germination as high as 90% was obtained at soil depths of 0.3 m and horizontal distances of 0.7 m from the point of ethylene injection.

Laboratory experiments were conducted to assess the influence of surfactants applied with or without nitrogen on MON 37500 foliar absorption by Bromus tectorum, Bromus japonicus, Aegilops cylindrica, Triticum aestivum, Chorispora tenella, and Lactuca serriola. MON 37500 absorption in B. tectorum and B. japonicus increased from 40% 24 h after treatment (HAT) to 48% 48 HAT, averaged across surfactants with no added nitrogen. Averaged across nitrogen source and species, nonionic surfactant, ethylated seed oil, and organosilicate provided comparable enhancement of MON 37500 absorption (56 to 68%), whereas crop oil concentrate provided only 27 to 29% absorption under the same conditions. Averaged across species and surfactant class, urea ammonium nitrate had the greatest effect on MON 37500 absorption (68%), compared to ammonium sulfate (59%) or no nitrogen (40%). Nitrogen, regardless of the type, significantly improved foliar absorption of MON 37500. MON 37500 absorption by species was 71, 63, 57, 57, 49, and 38% in C. tenella, B. japonicus, T. aestivum, A. cylindrica, B. tectorum, and L. serriola, respectively, when averaged across surfactants and nitrogen. Densely pubescent B. japonicus leaves did not retain significant amounts of MON 37500 following a primary leaf wash.

The ether extract of small everlasting (Antennaria microphylla Rydb.) was phytotoxic to lettuce seed root growth and leafy spurge (Euphorbia esula L. # EPHES) root elongation and cell culture growth. Three phenolic compounds (hydroquinone, arbutin, and caffeic acid) were chromatographically isolated and characterized from the ether extract. Arbutin and caffeic acid were moderately phytotoxic to leafy spurge root growth at 300 ppm (w/v), and hydroquinone was strongly phytotoxic to leafy spurge root growth at 50 ppm (w/v). The observed phytotoxicity of hydroquinone and the high-yield natural occurrence of arbutin, a water soluble, easily hydrolyzed monoglucoside of hydroquinone, in small everlasting is consistent with the participation of these two compounds in the observed allelopathy of small everlasting against leafy spurge.

Greenhouse studies were conducted to determine the response of 200 corn hybrids and 29 inbreds to metolachlor applied at 4.5 kg ai ha−1. Both hybrids and inbreds varied in their response to the herbicide. The distribution of injury revealed a normal distribution curve with most of the hybrids having a midlevel of tolerance. Some hybrids were very tolerant, while others were quite sensitive. Laboratory studies were conducted to evaluate absorption and metabolism of 14C-metolachlor for a subset of tolerant and sensitive hybrids. There was no observed difference in the product of metolachlor metabolism in the tolerant and sensitive hybrids. The observed variability in metolachlor tolerance among hybrids appeared due to differences in the amount of metolachlor absorption and metabolism and differences at the site of metolachlor action. The tolerant ‘Great Lakes 584’ hybrid absorbed significantly less 14C-metolachlor than did the sensitive ‘Pioneer 3744’, while the tolerant ‘Cargill 7567’ metabolized significantly faster more 14C-metolachlor than the other hybrids. The internal concentrations of available 14C-metolachlor were the same for the tolerant Cargill 7567 and the sensitive ‘Northrup King 9283’ after 8 h, indicating differences at the site of action of metolachlor for these two hybrids.

Adsorption of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine], terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine], and GS-14254 [2-(sec-butylamino)-4-(ethylamino)-6-methoxy-s-triazine] decreased on natural and limed soils as the soil pH increased to pH 8. More reduction in adsorption occurred with GS-14254 than with the other two herbicides. Increased toxicity to barley from GS-14254 was proportional to decreased adsorption by soils. Adsorption-desorption studies with Chehalis sandy loam (Cumulic Ultic Haploxeroll) indicated that extent and strength of adsorption were equivalent for GS-14254 (pKa 4.36) and terbutryn (pKa 4.32) at a soil pH of 5.20. Two soils, Boyce silt loam (pH 9.6, Cumulic Haplaquoll) and Crooked silt loam (pH 8.5, Xerolic Durorthid), adsorbed the s-triazines readily in contrast to the soils below pH 8. The high adsorption values were substantiated by the GR5 0 values on barley for atrazine and terbutryn, 10.2 and 13.2 kg ai/ha, respectively.

Pronamide [3,5-dichloro-N-(1,1-dimethyl-2-propynyl)benzamide] inhibited root and shoot growth, but had little effect on the germination of oat (Avena sativa L.) or cucumber (Cucumis sativus L.), and sprouting of quackgrass [Agropyron repens (L.) Beauv.] buds. Root growth was more sensitive than shoot growth. Pronamide decreased the longitudinal growth rate and caused radial enlargement in the elongation region of the roots of oat seedlings within 4 hr. Dry weights of the swollen root tips increased 6% over the controls after 4 hr. Pronamide stopped the growth of established oat plants within 24 hr when added to nutrient culture medium. It caused a slight increase in 86Rb uptake by excised oat roots. Pronamide stopped normal mitosis in treated oat roots within 0.5 hr. Examination of treated root tips under a microscope showed numerous cells in arrested metaphase, c-pairs, multinucleate cells, polyploidy and swollen and misshapen nuclei. Incorporation of 14C-glucose into oat root cell walls and 14C-leucine into protein were increased by 26 and 20% respectively, following 4-hr incubations in pronamide. The primary mode of action of pronamide is mitotic poisoning.

The differences in tolerance, morphology, and physiological response of diclofop-resistant and -susceptible wild oat biotypes collected from fields in Saskatchewan, Canada, were investigated under growth room and field conditions. Under herbicide-free conditions the resistant biotype had more upright leaves with about 12% less leaf area and 50% less leaf width than the susceptible biotype. A marked difference in the level of tolerance to diclofop was observed. Photosynthesis was initially significantly reduced in both biotypes after treatment with diclofop at the rate of 0.70 kg ai ha−1, but the resistant biotype was able to recover. Injury to the susceptible biotype was reduced by coating the seeds with 1,8-naphthalic anhydride. Differential foliar retention was not an important factor in selectivity of diclofop among the two biotypes.

Germination and longevity of purple witchweed seeds stored in nylon gauze bags in the soil in situ were tested in northern Bénin over a 2-yr period, covering the rainy seasons in 1994 and 1995. The seeds were collected at Ina Station in November 1993 from corn and sorghum fields. It appeared that germination percentages of the seeds, which were stimulated by GR24 to germinate, as well as their viability according to a tetrazolium test, decreased steadily in wet soil. During the 1994 rainy season, germination percentage of seeds, which reached maximum values of 30 to 74%, decreased to values of 11 to 17%. During the 1995 rainy season, the number of germinating seeds decreased further, and at the end of this season the germination percentage approached zero. Seed viability also decreased in line with the decrease in germination. In addition to the study on longevity under field conditions, seeds also were exposed to various water regimens in pots. In the pot experiment, purple witchweed seed viability and germination declined in moist soil treatments. The dying-off process observed contradicts the common opinion on longevity of Striga seeds in their natural environment. “Wet dormancy” was not observed in the course of the rainy season.

Growth and the chlorophyll and carotenoid contents were measured in greenhouse-grown yellow nutsedge (Cyperus esculentus L. ♯ CYPES), following treatment with glyphosate [N-(phosphonomethyl)glycine]. Herbicide was applied as a foliar spray at concentrations of 0.1, 1.0, 5.0, and 10.0 mM. After 2 weeks, growth was inhibited, and chlorosis and leaf apex necrosis were observed. Plant height was reduced, leaf fresh weight was decreased by 40%, and leaf dry weight was slightly affected. Rhizome, tuber, and secondary shoot formation was strongly inhibited, but root development was not affected by glyphosate treatment. With the 10-mM treatment, dry weight of the underground system was reduced by 80%. Chlorophyll and carotenoid levels were decreased by 52 and 54%, respectively, following glyphosate treatment.

The effect of ammonium sulfate (AMS) on efficacy and on absorption and translocation of sethoxydim in wild oats and barley was investigated under controlled environmental conditions. Increases in efficacy in the presence of AMS were relatively small. The rate of sethoxydim required to reduce dry weight 50% was reduced from 0.08 kg a.i. ha−1, for both species, to 0.07 and 0.06 kg ha−1 for wild oats and barley, respectively, in the presence of added AMS. In short-term experiments, uptake of 14C from 14C-labeled sethoxydim 0.5 and 1 h after application was increased approximately twofold with the addition of AMS. AMS had little effect on sethoxydim absorption beyond 1 h after application. Translocation at 1 h after application was unaffected by AMS. However, translocation 12 and 24 h after application was increased with AMS. It is postulated that the translocation increase results indirectly from the increase in absorption. The observed increase in absorption with the addition of AMS is evident only when crop oil concentrate (COC) is also present In model systems, the addition of AMS enhanced sorption of sethoxydim into Isolated cuticles but had no effect on movement into excised roots.