Proso millet occurs both as a crop and a weed in North America. In 1970, an olive-black seeded biotype called ‘wild proso millet’ was found as an aggressive weed in row crops in Minnesota and Wisconsin and has since spread over a large area. We used Random Amplified Polymorphic DNA (RAPD) to assess genetic relationships among biotypes, measure genetic variation within wild proso millet across its range, and detect hybridization between wild proso millet and crop biotypes of proso millet. We found 97 RAPD genotypes among 398 individuals: 69 wild proso millet genotypes, 26 crop and crop-like weed genotypes, and two hybrid genotypes. Five RAPD markers consistently differentiated wild proso millet from crop cultivars and crop-like weeds. About 10% of the genotypes had at least one marker of the other type, suggesting possible hybridization between wild proso millet and crop biotypes. Most genotypes occurred in only one or two of the over 100 populations tested. The most widespread wild proso millet genotype occurred in 12 populations distributed in North Dakota, Minnesota, Illinois, and Wisconsin. More genetic variation exists among populations of wild proso millet than expected for a plant that presumably experienced a severe genetic bottleneck only 20 generations ago. Hypermutation rates and crossing between wild proso millet and crop cultivars could not account for the degree of genetic variation found in wild proso millet. The pattern of genetic variation among wild proso millet populations suggests multiple introductions of wild proso millet to North America.

Atrazine [2-chloro-4-(ethylamino)-6-isopropylamino)-s-triazine] carryover under reduced or no-till row crop production systems was measured by planting oats (Avena sativa L.) the following year as a field bioassay during 1970 through 1976 at Lincoln and North Platte, Nebraska. Oat yields indicate that soil persistence of normal-use rates of atrazine into the subsequent year is only a minor residue problem under reduced tillage cropping systems. Atrazine carryover in soil was less of a problem under these reduced tillage systems as compared with prior experiments with conventional tillage systems across Nebraska.

Inclusion of any one of several ethoxylated nonionic surfactants in aqueous spray mixtures of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) greatly increased its toxicity to crabgrass (Digitaria sanguinalis (L.) Scop.). Phytotoxicity of sprays made up from commercially formulated diuron was greater than when technical diuron was used. The toxicity of diuron was also increased by the addition of any of several low-molecular-weight alcohols to the spray mixture.

The turbidity of diuron suspensions was greatly decreased by addition of any of several nonionic, anionic, or cationic surfactants which contained a fatty acid or fatty acid derivative as the hydrophobic portion of the molecule. Phytotoxicity of diuron applied with ethoxylated nonylphenol surfactants was associated with ethylene oxide (hydrophil) content. Surfactants containing nonylphenol, alkylthiol, or tridecylphenol as hydrophobes appeared essentially equal in their abilities to increase diuron toxicity, but dodecylphenol surfactants were slightly less effective.

Irrigated sandy soils have a high potential for leaching of agricultural chemicals to ground water. Objectives of this study were to determine the movement and persistence of metolachlor applied annually to a Hubbard loamy sand soil (Udorthentic Haploboroll) under irrigated potato production. Metolachlor was applied to the plots at rates of 1.7 and 3.4 kg ai ha-1. A KBr tracer was applied to the same plots at the rate of 130 kg ha-1. Intact soil cores were collected before and after herbicide application during the cropping season. Water samples were collected weekly from suction samplers, 90 and 150 cm deep in each plot, during the growing season. Potato yields of 60 and 59 Mg ha-1 in 1989 and 1990, respectively, were above Minnesota state averages. Bromide tracer data confirmed the movement of water through the soil profile to at least a 150-cm depth. No detectable quantities of metolachlor were found below 30 cm in soil during either year. Time for dissipation of 50% of metolachlor ranged from 85 to > 106 d. Dissipation in 1990 appeared slower than in 1989 and residues remaining at the end of the season, averaged across application rates, were 44 and 64% in 1989 and 1990, respectively. No water samples contained metolachlor in 1989. Metolachlor was detected in 18 of 409 water samples collected during and after the growing season in 1990. Median detectable concentration of metolachlor was 0.4 μg L-1 in water samples from suction samplers at 90- and 150-cm depths in eight of 16 plots. Metolachlor Kd and Koc values, 3 and 220, respectively, were similar to reported values. Metolachlor sorption in the soil profile was correlated, in order of contribution, to organic carbon content, cation exchange capacity, percent sand, and percent clay.

Research was conducted from 2011 to 2014 to determine weed population dynamics and frequency of glyphosate-resistant (GR) Palmer amaranth with herbicide programs consisting of glyphosate, dicamba, and residual herbicides in dicamba-tolerant cotton. Five treatments were maintained in the same plots over the duration of the experiment: three sequential POST applications of glyphosate with or without pendimethalin plus diuron PRE; three sequential POST applications of glyphosate plus dicamba with and without the PRE herbicides; and a POST application of glyphosate plus dicamba plus acetochlor followed by one or two POST applications of glyphosate plus dicamba without PRE herbicides. Additional treatments included alternating years with three sequential POST applications of glyphosate only and glyphosate plus dicamba POST with and without PRE herbicides. The greatest population of Palmer amaranth was observed when glyphosate was the only POST herbicide throughout the experiment. Although diuron plus pendimethalin PRE in a program with only glyphosate POST improved control during the first 2 yr, these herbicides were ineffective by the final 2 yr on the basis of weed counts from soil cores. The lowest population of Palmer amaranth was observed when glyphosate plus dicamba were applied regardless of PRE herbicides or inclusion of acetochlor POST. Frequency of GR Palmer amaranth was 8% or less when the experiment was initiated. Frequency of GR Palmer amaranth varied by herbicide program during 2012 but was similar among all herbicide programs in 2013 and 2014. Similar frequency of GR Palmer amaranth across all treatments at the end of the experiment most likely resulted from pollen movement from Palmer amaranth treated with glyphosate only to any surviving female plants regardless of PRE or POST treatment. These data suggest that GR Palmer amaranth can be controlled by dicamba and that dicamba is an effective alternative mode of action to glyphosate in fields where GR Palmer amaranth exists.

Herbicide resistance, and in particular multiple-herbicide resistance, poses an ever-increasing threat to food security. A biotype of junglerice [Echinochloa colona (L.) Link] with resistance to four herbicides, imazamox, fenoxaprop-P-ethyl, quinclorac, and propanil, each representing a different mechanism of action, was identified in Sunflower County, MS. Dose responses were performed on the resistant biotype and a biotype sensitive to all four herbicides to determine the level of resistance. Application of a cytochrome P450 inhibitor, malathion, with the herbicides imazamox and quinclorac resulted in increased susceptibility in the resistant biotype. Differential gene expression analysis of resistant and sensitive plants revealed that 170 transcripts were upregulated in resistant plants relative to sensitive plants and 160 transcripts were upregulated in sensitive plants. In addition, 507 transcripts were only expressed in resistant plants and 562 only in sensitive plants. A subset of these transcripts were investigated further using quantitative PCR (qPCR) to compare gene expression in resistant plants with expression in additional sensitive biotypes. The qPCR analysis identified two transcripts, a kinase and a glutathione S-transferase that were significantly upregulated in resistant plants compared with the sensitive plants. A third transcript, encoding an F-box protein, was downregulated in the resistant plants relative to the sensitive plants. As no cytochrome P450s were differentially expressed between the resistant and sensitive plants, a single-nucleotide polymorphism analysis was performed, revealing several nonsynonymous point mutations of interest. These candidate genes will require further study to elucidate the resistance mechanisms present in the resistant biotype.

Herbicides are powerful chemical agents that exert strong biological activity on plants. The release of new formulations of dicamba and 2,4-D and their use in transgenic agronomic crops will probably result in many more applications during the time of year when sensitive nontarget vegetation will be present. The use of herbicides is regulated by the U.S. Environmental Protection Agency, and there are usually no negative effects on nontarget species. One negative aspect of herbicide use occurs when the application moves away from the target area and causes unwanted plant injury on susceptible species. Interest in herbicide drift is increasing, as evidenced by the number of refereed articles that investigate the mitigation or potential for herbicide drift (Figure 1). Although the topic of herbicide drift is broad, in this manuscript I will focus on an overview of off-site movement from a historical perspective and then discuss specific research protocols to examine vapor drift.

Greenhouse studies were conducted to evaluate the influence of wild radish–amended soil on tomato, bell pepper, and yellow nutsedge growth. In addition, yellow nutsedge interference with tomato and bell pepper was evaluated with and without the wild radish amendment. Leaf margins of tomato and bell pepper plants were necrotic for approximately 2 wk after transplanting into soil amended with 1% (wt/wt) wild radish biomass. Injury to both crops was transient, but bell pepper biomass through 9 wk after transplanting was negatively affected, whereas tomato was not. In a replacement series study, tomato was more competitive than yellow nutsedge in nonamended soil and the competitiveness of tomato further increased at the expense of reduced yellow nutsedge growth in wild radish–amended soil. Bell pepper was less competitive than yellow nutsedge in nonamended soil, but in wild radish–amended soil, bell pepper held a competitive advantage over yellow nutsedge. In addition, yellow nutsedge tuber production was reduced as much as 88% when soil was amended with wild radish, and tuber weight in nonamended soil was 0.32 g tuber−1 compared with 0.05 g tuber−1 in wild radish–amended soil. This research shows that competitiveness of tomato and bell pepper is increased over yellow nutsedge when soil is amended with wild radish, in addition to reducing yellow nutsedge tuber production and size.

Maximizing cereal rye biomass has been recommended for weed suppression in cover crop–based organic no-till planted soybean; however, achieving high biomass can be challenging, and thick mulch can interfere with soybean seed placement. An experiment was conducted from 2012 to 2014 in New York to test whether mixing barley and cereal rye would (1) increase weed suppression via enhanced shading prior to termination and (2) provide acceptable weed suppression at lower cover crop biomass levels compared with cereal rye alone. This experiment was also designed to assess high-residue cultivation as a supplemental weed management tool. Barley and cereal rye were seeded in a replacement series, and a split-block design with four replications was used with management treatments as main plots and cover crop seeding ratio treatments (barley:cereal rye, 0:100, 50:50, and 100:0) as subplots. Management treatments included high-residue cultivation and standard no-till management without high-residue cultivation. Despite wider leaves in barley, mixing the species did not increase shading, and cereal rye dominated cover crop biomass in the 50:50 mixtures in 2013 and 2014, representing 82 and 93% of the biomass, respectively. Across all treatments, average weed biomass (primarily common ragweed, giant foxtail, and yellow foxtail) in late summer ranged from 0.5 to 1.1 Mg ha−1 in 2013 and 0.6 to 1.3 Mg ha−1 in 2014, and weed biomass tended to decrease as the proportion of cereal rye, and thus total cover crop biomass, increased. However, soybean population also decreased by 29,100 plants ha−1 for every 1 Mg ha−1 increase in cover crop biomass in 2013 (P=0.05). There was no relationship between cover crop biomass and soybean population in 2014 (P=0.35). Soybean yield under no-till management averaged 2.9 Mg ha−1 in 2013 and 2.6 Mg ha−1 in 2014 and was not affected by cover crop ratio or management treatment. Partial correlation analyses demonstrated that shading from cover crops prior to termination explained more variation in weed biomass than cover crop biomass. Our results indicate that cover crop management practices that enhance shading at slightly lower cover crop biomass levels might reduce the challenges associated with excessive biomass production without sacrificing weed suppression in organic no-till planted soybean.

Large proportions of nonhybrid progeny result from controlled crosses between Palmer amaranth and common waterhemp, both dioecious weeds. Agamospermy was proposed as an explanation for this phenomenon, and here we provide evidence in support of this hypothesis. We evaluated 60 nonhybrid offspring from two interspecific crosses, and all plants were females and had DNA content values similar to the female (Palmer amaranth) parent. Among nine hybrids resulting from these crosses, eight were nonviable (lethal or neuter), and only one hybrid allowed for continued gene movement. Cytogenetic evaluation of this hybrid revealed triploidy, further supporting the occurrence of unreduced gametes in these species. In light of this new evidence, we examine earlier data regarding Palmer amaranth by common waterhemp hybridization and suggest some prior conclusions may be premature.

Genetic markers have been used in horticulture and conservation biology to identify breeding lines, assess genetic diversity, and examine gene flow. In weed science, analysis of genetic markers is not as common, as the focus often lies in the development of control methods. This is unfortunate, because advances in genetic marker techniques may lead to innovative methods in controlling weedy plants. Microsatellites, random amplified polymorphic DNA (RAPDs), intersimple sequence repeats (ISSRs), amplified fragment length polymorphisms (AFLPs), DNA sequences, single nucleotide polymorphisms (SNPs), and derived cleaved amplified polymorphic sequences (dCAPS) have been used in studying genetics of weedy plants. Beyond assessing genetic diversity of weeds, markers have been used to examine gene flow, patterns of dispersal, ploidy levels, and relationships of weedy and nonweedy species, as well as identifying founder populations. Identification of closely related species may indicate the potential for hybridization, cross reactivity of chemical applications, or nontarget biological control effects. Furthermore, markers have been used in identification of mutations conferring herbicide resistance and identification of regions correlating fitness of weedy and hybrid plants in new habitats. Knowledge from genetic markers provides fundamental information that can be used in advancing chemical and biological control in weed management. This article serves as a review of these marker types and their application to weed science.

The effect of stage of growth of field bindweed (Convolvulus arvensis L.) on absorption and translocation of 14C-labeled 2,4-D and picloram was studied. Three stages of growth investigated were s-week old seedlings, and 7-week old and 16-week old vegetatively propagated plants. One middle leaf of each plant was treated with either 14C-labeled (2,4-dichlorophenoxy)-acetic acid (2,4-D) or 4-amino-3,5,6-trichloropicolinic acid (picloram). Treated plants were harvested after 48 hr and assayed for radioactive 14C. The percent of 2,4-D applied which was absorbed by seedlings was less than that absorbed by adult plants. Picloram absorption was not different in seedlings and 7-week old adult plants, but the 16-week old adult plants absorbed more picloram than either the 5-week old seedlings or 7-week old adult plants. More picloram than 2,4-D was absorbed at all stages of growth. Translocation of both 2,4-D and picloram was greater in seedlings than in older plants. No difference was found in amounts of 2,4-D and picloram translocated by adult field bindweed plants. More 2,4-D than picloram was translocated by seedlings.

Cogongrass is widespread in the moist savanna and forest zones of West Africa, where recurrent fires, tillage, weeding, and other farm activities continuously disturb land. Field experiments were conducted in the forest–savanna transition zone of Nigeria from 1996 to 2000 to evaluate the potential of two cover crops (velvetbean and tropical kudzu) for reclaiming land that had been abandoned to cogongrass. Cover crops were grown on the same plots for 3 consecutive yr (1996 to 1998). The control was natural fallow dominated by cogongrass in 1996 to 1998. Corn was planted in all treatments in 1999 and 2000. Total dry matter of cogongrass before the treatments were imposed was 9,000 kg ha−1, and rhizomes contributed 49% of this. At all subsequent sampling dates, plots with cover crops had lower cogongrass shoot and rhizome dry matter than plots without cover crops. Shoot dry matter was reduced to zero 65 wk after planting in both cover crops; rhizome dry matter was reduced to zero after 97 wk in velvetbean plots and after 105 wk in tropical kudzu plots. Corn grain yield was 60% higher in plots with tropical kudzu and 102% higher in plots with velvetbean than in control plots without any previous cover crops. Further research is required to integrate use of cover crops with other control methods for improved cogongrass management.

Integrated Weed Management (IWM), a long time practice by farmers has become more commonly discussed as a total weed management system. Whether an off shoot of Integrated Pest Management (IPM) or a further recognition of integrating weed control measures within the cropping and farming system, it has become more widespread. IWM is being practiced using many of the same components, from croplands to forests and rangeland. A weed management hierarchy has been developed by degree of diversity of management practices. IWM researchers and educators should invite other pest management specialists to join us in striving for Integrated Crop Management systems.

The effect of johnsongrass [Sorghum halepense (L.) Pers.] on soybean [Glycine max (L.) Merr.] yields, grades, and net returns were determined by regression analysis of data from 252 field research plots from 1971 through 1976. Johnsongrass control from 0 to 100% was achieved by various combinations of summer fallow, dalapon (2,2-dichloropropionic acid) as a preplanting application at 8.3 kg/ha, or trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 0.8 to 1.7 kg/ha incorporated into the soil before planting. Repeated usage of trifluralin at 1.7 kg/ha, either with or without use of summer fallow, provided the most effective and most profitable soybean production. Foreign material in soybean seed samples was about 0.8% with 100% johnsongrass control and nearly 6% with no control. At least 70% johnsongrass control was required to avoid deductions from gross harvested weights caused by seed moisture levels exceeding 13%. Levels of johnsongrass control in excess of 60% tended to increase soybean test weights. With 100% johnsongrass control, about 1.2% damaged soybean seed occurred; whereas, without johnsongrass control, about 1.7% seed damage occurred. The level of johnsongrass control had little effect on the level of split soybean seed at harvest. Failure to control johnsongrass resulted in predicted soybean grades of 4.1; whereas, nearly 100% johnsongrass control was necessary to provide U.S. No. 1 grade soybeans. Soybean yields were increased 4.8 to 6.2% for each 10% increase in johnsongrass control when plots were not fallowed or after fallowing, respectively. Net returns in soybean production were nearly twice as great with 100% johnsongrass control as when johnsongrass was not controlled.

An organic muck and a montmorillonite clay were incorporated into the surface 7.6 cm of a structureless sandy soil at rates ranging from 26,880 to 89,600 kg/ha. Prometryne [2,4-bis(isopropylamino)-6-(methylthio)-s-triazine] and fluometuron [1,1-dimethyl-3-(α,α,α-trifluoro-m-tolyl)urea] were surface-applied and trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) were incorporated at two rates each in 1968, 1969, and 1970. Organic matter greatly reduced the biological activity of all three herbicides. Montmorillonite clay greatly reduced the activity of prometryne, slightly decreased the activity of fluometuron, and had no significant effect on trifluralin activity in all but the first year of the study. Both soil additives increased the cation exchange capacity of the soil. Muck decreased soil pH while montmorillonite increased it.

Greenhouse cage studies were conducted to determine the influence of shoot morphology and genetic variation on establishment of Spurgia esulae gall midge on seven leafy spurge genotypes. The genotypes were collected from South Dakota, North Dakota, Nebraska, Montana, Wyoming, Manitoba, and Austria. Genotypes from South Dakota and Nebraska were most susceptible to gall formation and had the highest larvae survival, while the genotypes from Montana and Manitoba were most resistant. Morphological characteristics of the leafy spurge stem tips, such as stem diameter, leaf length, width, and area did not correlate with gall formation or larvae survival. Chloroplast DNA restriction fragment length polymorphism analysis of the genotypes identified six chloroplast types among the seven leafy spurge genotypes. The two genotypes most resistant to galling by S. esulae, Manitoba and Montana, had the same chloroplast genotype, but also were closely related to the two most susceptible genotypes. Because eggs were laid on all genotypes, it appears that adult females were not preferentially selecting appropriate host genotypes, but that egg and larvae survival was strongly influenced by genotype.

Field research was conducted in Arkansas to determine the effect of selected rice pesticides on the microbial herbicide Colletotrichum gloeosporioides (Penz.) Sacc. f. sp. aeschynomene (henceforth designated C.g.a.) and to integrate C.g.a. into control programs for northern jointvetch [Aeschynomene virginica (L.) B.S.P. # AESVI] in rice (Oryza sativa L.). Water suspensions of dry-formulated spores of C.g.a. at 1.9 × 1011 spores/ha were applied midseason in sequence with normal use rates of other pesticides. Carbofuran (2,3-dihydro-2,2-dimethyl-7-benzofuranyl methyl-carbamate) at 0.56 kg ai/ha, propanil (3’4’-dichloropropio-anilide) at 2.2 kg ai/ha, acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid} at 0.14 kg ai/ha, or fentin hydroxide (triphenyltin hydroxide) at 0.56 kg ai/ha did not reduce disease development of C.g.a. on northern jointvetch. Sequential applications of benomyl [methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate] at 0.56 kg ai/ha reduced activity of the fungus on northern jointvetch when fungicide applications began 1 week after C.g.a. application but had no effect on mycoherbicide activity when applications began 2 weeks after C.g.a.

Thirteen selections of kochia (Kochia scoparia (L.) Schrad.), self-pollinated for four generations, varied in response to (2,4-dichlorophenoxy)acetic acid (2,4-D) and 3,6-dichloro-o-anisic acid (dicamba). The most susceptible selection was injured more by 0.35 kg/ha of the dimethylamine salt of 2,4-D than the most tolerant by 0.70 kg/ha based on visual injury rating, growth in plant diameter relative to untreated controls, and seed production. The 0.28 kg/ha rate of dicamba injured the most susceptible selection as much as 0.54 kg/ha injured the most tolerant based on visual injury rating and change in plant diameter. Plant size and maturity influenced the response of kochia to herbicides, but the differences in response among selections could not be explained by variations in size or maturity. All kochia selections were tolerant to 4-amino-3,5,6-trichloropicolinic acid (picloram) at 4.28 kg/ha.

Field and greenhouse experiments were established to determine the phytotoxicity of Canada thistle [Cirsium arvense (L.) Scop.] residue on crop growth. Field examination of infestations showed that as Canada thistle shoots increased in an area, the number of kochia [Kochia scoparia (L.) Schrad.], marshelder (Iva xanthifolia Nutt.) and foxtail barley (Hordeum jubatum L.) plants decreased. Conversely, as Canada thistle shoots decreased, the aforementioned annual and perennial plants increased. In greenhouse studies, roots and shoots of Canada thistle that were mixed with soil reduced the growth of sugarbeet (Beta vulgaris L. ‘Mono Hy D2’), wheat (Triticum aestivum L. ‘Centurk’), alfalfa (Medicago sativa L. ‘Dawson’), and Canada thistle seedlings. Corn (Zea mays L. ‘Jacques No. 1004’) and dry edible beans (Phaseolus vulgaris L. ‘Great Northern No. 59’) grown in soil treated with Canada thistle residue were affected to a lesser extent. When Canada thistle residue was mixed with soil, crop growth was inversely proportional to the amount of Canada thistle residue added to the soil. Both roots and shoots of Canada thistle were toxic to crops when mixed with the soil. The effects of Canada thistle residue on crop growth lasted for about 60 days. Neither autoclaving residue and soil nor fertilization of soil had any effect on residue toxicity. Canada thistle leaf leachate inhibited the growth of sugarbeets watered daily with the leachate.