Field studies were conducted at four locations in North Carolina in 1998 and 1999 to evaluate the use of the Herbicide Application Decision Support System (HADSS™) for weed management in nontransgenic, bromoxynil-resistant, and glyphosate-resistant cotton. Weed management systems included trifluralin preplant incorporated (PPI) plus fluometuron preemergence (PRE) or no soil-applied herbicides. Postemergence (POST) options included bromoxynil, glyphosate, or pyrithiobac early POST (EPOST) followed by (fb) MSMA plus prometryn late postemergence–directed (LAYBY) or herbicide recommendations given by HADSS. Glyphosate-resistant systems provided control equivalent to or better than control provided by bromoxynil-resistant and nontransgenic systems for smooth pigweed, Palmer amaranth, large crabgrass, goosegrass, ivyleaf morningglory, and fall panicum. Trifluralin PPI fb fluometuron PRE fb HADSS POST provided equivalent or higher levels of weed control and yield than trifluralin PPI fb fluometuron PRE fb bromoxynil, glyphosate, or pyrithiobac EPOST fb MSMA plus prometryn LAYBY. The trifluralin PPI fb fluometuron PRE fb HADSS POST systems controlled large crabgrass at Goldsboro and fall panicum better than HADSS POST-only systems in nontransgenic cotton. Cotton yield and net returns in the glyphosate-resistant systems were always equal to or higher than the nontransgenic and bromoxynil-resistant systems. Net returns were higher for the soil-applied fb HADSS POST treatments in 8 of 12 comparisons with HADSS POST systems without soil-applied herbicides. Early-season weed interference reduced cotton lint yields and net returns in POST-only systems.

Field and laboratory studies were conducted at Stoneville, MS, from 1996 to 1998 to determine the influence of subsoiling (SS) and conventional tillage (CT) of a Sharkey clay soil on microbial characteristics and herbicide degradation. Soil samples obtained from imazaquin-treated and nontreated plots from the soybean row and interrow position were analyzed. Because only the row position is actually disturbed by SS, a comparison of row and interrow position on the parameter was conducted. Imazaquin (preemergence, 140 g ai ha−1) had no effect on microbial populations, microbial enzyme activity (fluorescein diacetate [FDA] hydrolysis and triphenyl-tetrazolium chloride [TTC] dehydrogenase), and organic carbon content. Estimates of microbial activity based on FDA hydrolysis and TTC dehydrogenase activity indicated greater activity under CT; however, microbial biomass and organic carbon were not affected by tillage or row position. A laboratory study assessed the degradation of carboxyl- and ring-labeled 2,4-D as influenced by tillage and row position. Soils from CT plots had an initially higher mineralization rate of 14C carboxyl-labeled 2,4-D compared to soils from SS plots; however, no effect of tillage or row position was observed on the cumulative amount of 14CO2 evolved 14 d after treatment (DAT) in 1996 and 18 DAT in 1998. In studies with ring-labeled 2,4-D, a higher 14CO2 evolution was detected in soils obtained from SS plots, regardless of row position, whereas a greater amount of radioactivity was observed in the unextractable fraction from CT soils. Because differences in 2,4-D mineralization between tillage regimes were minimal, adoption of SS as a tillage practice for heavy clay soils in the Mississippi Delta may have a limited effect on microbial characteristics and biodegradation of soil-applied herbicides.

Seed from six Australian near-isogenic lines of wild oat were germinated and grown in controlled-environment growth chambers under either ambient CO2 (357 parts per million by volume [ppmv]) or elevated CO2 (480 ppmv) at 20/16 C or 23/19 C. Three soil moisture treatments—−0.01 MPa (field capacity), −0.10 MPa, or −1.00 MPa—were imposed. Wild oat lines grown under elevated CO2 had higher seed production and greater plant dry weights, although the response of these variates involved a complex of interactions with temperature, soil moisture, and line. Plant height varied with wild oat line, and plants grown at 20/16 C were taller than those grown at 23/19 C. At 23/19 C, time taken to mature was reduced for some wild oat lines, and elevated CO2 reduced the time taken to maturity for some lines at 20/16 C. There was no significant difference in the level of dormancy developed in freshly harvested caryopses between the two CO2 treatments, but an effect was present in seed that had been after-ripened for 193 d. These results indicate that the main climate change variables ([CO2], soil moisture, and increased temperature) directly influence the growth and development of wild oat and are likely to affect the population dynamics of this species.

Soil lost through wind erosion may transport herbicides to nontarget areas. Shallow incorporation may reduce herbicide concentrations at the soil surface, thereby reducing loss on wind-erodible sediment (particles and aggregates less than 1 mm in diameter). Atrazine, alachlor, and acetochlor concentrations on and dissipation rates from surface wind-erodible sediment and larger size fractions from two soil types in undisturbed and incorporated (5 cm deep) treatments were compared. The surface 1 cm of soil was removed by vacuum 1, 7, and 21 d after herbicide treatment (DAT). This soil was dry-sieved into six size fractions (four fractions considered wind-erodible and two larger size fractions), and herbicide concentrations were determined on each size fraction. About 50% of the recovered material was classified as wind erodible sediment. Incorporation reduced herbicide concentrations on all size fractions and results were similar between soil types. Wind-erodible sediments from undisturbed and incorporated treatments contained about 65 and 8% of the applied herbicides, respectively, 1 DAT. Herbicide concentrations were similar among size fractions within a treatment 7 and 21 DAT; however, incorporation reduced soil herbicide concentrations from 50 to 80% compared to concentrations on soil from undisturbed areas. Shallow incorporation did not affect weed control ratings measured 30 DAT or herbicide dissipation. However, 50% dissipation rates (DT50) for each herbicide were about 15 d for wind-erodible sediments and ranged from 30 to 55 d for size fractions ≥1.68 mm.

As new targets for herbicide action are identified from genomics research, large and diverse chemical collections and high-throughput assays will be required to maximize the probability of identifying compounds with activity at these targets. The new technology of combinatorial synthesis and high-throughput, miniaturized, in vitro screening, which has become an integral part of pharmaceutical discovery, is now being applied to discover new herbicides, insecticides, and fungicides. Depending on the synthesis design, the products of a combinatorial synthesis, referred to as a library, may be either unbiased or biased toward an intended target. Unbiased libraries are generally prepared to maximize chemical diversity around a central core structure or scaffold. Often containing 10,000 to 30,000 compounds each, these libraries are encoded and prepared by a combinatorial methodology known as mix-and-split, which produces compounds as mixtures. The preparation of these large libraries requires robust synthetic methodology that will accommodate reactants (building blocks) with diverse structures. Biased libraries tend to be smaller in size, ranging from 100 to 2,500 compounds. They are prepared using synthetic methodology that produces collections of discrete compounds (parallel synthesis) or pools of five to 10 compounds per pool (mix-and-split synthesis). Compounds in biased libraries are rationally designed to contain structural motifs or pharmacophores that are presumed to be beneficial for activity on the intended target. Screening is conducted in microtiter assay plates containing from 96 to 864 wells per plate. For in vitro assays, high-density formats (864 wells per plate) are preferred. The higher density format allows for testing higher concentrations and fewer compounds per well, which leads to a more rapid identification of the active molecules. For in vivo assays, 96-well formats are preferred. Regardless of the microtiter plate format, multiple beads are distributed into plates by robotic pipetting, and single beads are distributed via robot-controlled suction pipets. Test compounds are cleaved from the beads and transferred in solvent to assay plates. Required reagents are added to the plate to initiate the assay. A wide range of in vitro and in vivo herbicide, insecticide, and fungicide assays can be conducted in microtiter plates.

Rice by-products were evaluated in the greenhouse for herbicidal activity on various weeds and crops. Rice by-products and corn gluten meal (CG) were applied at 0, 125, 250, 500, and 750 g m−2 preemergence (PRE) and preplant incorporated (PPI). The efficacy of rice by-products and CG in reducing weed emergence and shoot weight of broadleaf species was in the order of medium-grain fatty rice bran (MF) > long-grain fatty rice bran (LF) > CG > defatted rice bran (DF) > long-grain hull (LH) > medium-grain hull (MH). For reducing grass emergence, MF = CG > LF > DF > LH > MH, and for shoot weight reduction, CG > MF > LF > LH > DF = MH. Palmer amaranth and ivyleaf morningglory were the most susceptible weeds (91 and 82%) followed by sicklepod, hemp sesbania, and prickly sida (65 to 70%). Velvetleaf was the most tolerant broadleaf weed. Grasses were not as susceptible to rice bran as broadleaf weeds. In general, MF was the best material for reducing weed emergence and its efficacy was not affected by application method. Cotton and corn were the most tolerant direct-seeded crops to MF (6% reduction in plant stand), and soybean, Italian ryegrass, tomato, and rice had intermediate tolerances (30 to 86% stand reduction). Mustard, cucumber, and lettuce were the most susceptible crops (71 to 98% reduction in plant survival). The minimum effective rate was 250 g m−2 MF PPI or PRE.

Manipulations of the soil environment can affect the growth and competitive ability of annual weeds because of the large influence that soil conditions exert on seedlings early in the growing season. Our objective was to identify soil nitrogen (N) management systems with weed suppression potential. We hypothesized that competition from wild mustard against sweet corn would be weaker when N was supplied by organic sources (organic) or a split application of NH4NO3 fertilizer applied at planting and 4 wk thereafter (split) than when NH4NO3 fertilizer was applied in a single dose at planting (early). This hypothesis was tested in a 2-yr field experiment conducted in central Maine. Wild mustard's maximal relative growth rate (RGR) was 12% lower (P < 0.05) in 1997 and 1998, and the amount of time needed to achieve maximal RGR was delayed by 0.8 d (P < 0.05) in 1997 in the organic compared to the early treatment. The competitive effect of wild mustard on sweet corn yield was lower in the organic treatment than in the early and split treatments in 1 of 2 yr. In 1997, competition from wild mustard reduced marketable ear yields of sweet corn by 30%, but the magnitude of yield reduction did not differ between the three N addition treatments. In contrast, in 1998, sweet corn yield in the organic treatment was not reduced by weed competition, whereas yield loss in the early and split treatments was 20 and 35%, respectively. The mechanisms underlying selective suppression of weeds, but not crops, by organic N sources require further attention.

Technological advances in molecular biology have contributed substantially to our understanding of plant genetic diversity. Early studies of allozyme variation employing protein electrophoresis revealed that plant populations have high levels of genetic diversity, most of the variation at polymorphic loci is found within populations, and geographic range and breeding system explain the largest proportion of variation in genetic diversity. With the discovery of restriction endonucleases, the first DNA-based markers allowed the detection of variation in DNA sequences in plant population studies. More recently, techniques that utilize the polymerase chain reaction have allowed a more representative assessment of genetic variation in plants by screening multiple loci distributed throughout the genome. The analyses reveal sufficient polymorphism for the examination of fine-scale genetic differences among individuals. Information on plant genetic diversity is also emerging from studies of plant genome structure. Comparative genetic mapping studies of members of the Brassicaceae, Poaceae, and Solanaceae show that gene content is highly conserved between closely related species, although gene order on a chromosomal segment may differ between species. Comparative sequencing studies reveal higher degrees of diversity at the microstructural (less than 1 million base pairs) level than predicted at the genetic map level and suggest that genes are densely packed in gene-rich regions, rather than randomly distributed along chromosomes in species with large genomes. Sequencing of the entire genomes of rice and Arabidopsis thaliana will help identify genes controlling agronomically important traits, improve our understanding of genetic variation for fitness-related traits in wild plant populations including weed species, resolve evolutionary relationships among plant taxa, and potentially revolutionize current ideas on plant diversity and evolution.

An experiment was conducted at two locations in Georgia and one location in North Carolina during 1994 and 1995 to evaluate weed management systems utilizing pyrithiobac applied preemergence (PRE) in conventional-tillage bromoxynil-resistant cotton. Weed management systems evaluated included different combinations of pyrithiobac PRE, bromoxynil or bromoxynil plus MSMA applied early postemergence (EPOST), bromoxynil applied postemergence (POST), and cyanazine plus MSMA applied late postemergence-directed (LAYBY). Pyrithiobac PRE improved control of Florida beggarweed, pitted morningglory, prickly sida, sicklepod, and spurred anoda compared with systems that did not include pyrithiobac PRE. Averaged across locations, pyrithiobac PRE increased cotton lint yields 330 kg ha−1. Bromoxynil applied EPOST or POST increased weed control and cotton lint yield. Bromoxynil EPOST, POST, or EPOST plus POST did not control sicklepod. The addition of MSMA to bromoxynil EPOST improved sicklepod control. Two applications of bromoxynil controlled more pitted morningglory and sicklepod than one application. Control of all dicotyledonous weeds was increased by cyanazine plus MSMA LAYBY, and this treatment increased yields at all locations. Cotton was not injured by pyrithiobac PRE or by bromoxynil applied EPOST or POST, but temporary visual injury was observed with EPOST treatments of MSMA.

With rapid progress being made in deciphering plant genomic sequences, determining the function of these genes is one of the main challenges that plant molecular biologists face today. Herbicidal inhibitors have been very useful for understanding gene function in at least two examples, represented by herbicidal inhibitors of hydroxyphenylpyruvate dioxygenase (HPPD) and deoxyxylulosephosphate reductoisomerase (DXR). In the first, an albino mutant of Arabidopsis isolated during the study of carotenoid biosynthesis was found to have an intact carotenoid biosynthetic pathway. A number of “bleaching herbicides” in development at about the same time (e.g., sulcotrione) produced similar symptoms by strongly inhibiting HPPD, a key enzyme in plastoquinone biosynthesis. Examination of the Arabidopsis mutant revealed that the HPPD gene had been inactivated in the albino plants. Inhibition of the HPPD pathway also led to reduced levels of tocopherol (vitamin E), an end product of the pathway. Further studies and manipulation of the pathway produced plants with significantly higher levels of vitamin E. This result is a clear demonstration of how an herbicidal inhibitor was able to lead to the identification of a gene that was responsible for a particular phenotype. As a second example, identification of fosmidomycin as a specific inhibitor of DXR in the recently elucidated nonmevalonate pathway of isopentenyl pyrophosphate (IPP) biosynthesis was instrumental in furthering the understanding of an important route to synthesis of many important terpenoid products.

Field studies were conducted at Delta Research and Extension Center, Stoneville, MS, in 1996, 1997, and 1998 to assess the effect of tillage systems (conventional tillage and subsoiling) on the environmental fate of imazaquin in a Sharkey clay soil. Imazaquin was applied preemergence at 140 g ai ha−1. Subsoiling in the fall did not affect imazaquin dissipation, total volume of runoff, imazaquin concentration in runoff, or imazaquin concentration in soil, as determined by chemical extraction. A corn root bioassay revealed no differences due to tillage systems in plant-available imazaquin in soil. Imazaquin concentration measured by chemical extraction or bioassay diminished over time, with a half-life ranging from 8 to 25 d. A field bioassay utilizing cotton and corn was conducted in 1997 and 1998 using plots that had received imazaquin the previous year. In 1997, 2 wk after planting, cotton and corn injury ranged from 3 to 15%, whereas no injury was observed in 1998. Injury symptoms declined over time, with no injury 5 wk after planting in either year. Although early-season cotton stunting and slight discoloration of corn was apparent in 1997, imazaquin residues did not affect subsequent vegetative and reproductive growing patterns of cotton or corn. In 1998, corn and cotton height were significantly greater in subsoiled plots compared to conventional tillage.

A framework is presented to consider the value and utility of molecular-based research in weed science. Four themes are used to illustrate why adopting molecular approaches might be helpful. First, the rationale for academic institutions adopting molecular approaches is outlined, including strengths, weaknesses, opportunities, and threats (SWOT) analysis. Second, research strategy and synergies developed into other functions, such as education, consultancy, and business, is considered. Third, project management as a vehicle for integrating technical and personnel skills is examined. Finally, specific examples of outputs such as the application of functional genomics for herbicide discovery are described. The adoption of molecular-based methods can have far-reaching benefits in agriculture and biotechnology. Communicating these benefits within the scientific community and beyond, particularly to end users, is of fundamental importance.

The incidence of auxinic herbicide resistance in plants has increased worldwide. Auxinic herbicides were the first selective organic herbicides developed and have been used in agriculture for over 50 yr, primarily for the selective control of broadleaf weeds in cereal crops. However, the mode of action of auxinic herbicides and the molecular basis of auxinic herbicide resistance remain unknown, although an auxin-binding protein (ABP) is proposed to be the primary target site. Using auxinic herbicide-resistant (R) and -susceptible (S) biotypes of wild mustard as a model system, we have extensively studied the mode of action of auxinic herbicides and the resistance mechanisms at the physiological, biochemical, and molecular genetic levels. There are no differences in uptake, transport, and metabolism of auxinic herbicides between the R and S biotypes. Based on these results, as well as the studies on the role of auxin-enhanced ethylene biosynthesis and calcium in mediating the auxinic herbicide resistance, we hypothesize that resistance of the R biotype to auxinic herbicides is due to an altered target site, possibly an auxin receptor. We have identified and characterized a small ABP gene family as well as their cDNAs from both R and S of wild mustard. Amino acid changes were found in the ABP of the R biotype. Functional and mutational analyses of these genes are underway to determine the role of ABP in mediating auxinic herbicide resistance. In this review, we focus on the mode of action of auxinic herbicides and the molecular basis of auxinic herbicide resistance in wild mustard.

Many advances in disciplines such as chemistry, biochemistry, plant breeding, genetics, engineering, and others have been applied in a positive manner to improve knowledge in weed science. The emerging field of genomics is likely to have a similar positive effect on our understanding of weeds and their management in various plant agriculture systems. Genomics involves the large-scale use of molecular techniques for identification and functional analysis of complete or nearly complete genomic complements of genes. Commercial application of genomics has already occurred for improvement in certain crop input and output traits, including improved quality characteristics and herbicide and insect resistance. Additional commercial applications of genomics in weed science will be identification of genes involved in a crops' competitive ability. Genes controlling early crop shoot emergence, rapid early-season leaf and root development for fast canopy closure, production of allelochemicals for natural weed control, identification of novel herbicide target sites, resistance mechanisms, and genes for safening crops against specific herbicides can and will be identified. Successful crop improvement in these areas using the tools of genomics will dramatically affect weed–crop interactions and improve crop yields while reducing weed problems. In relation to improved basic knowledge of weeds and the resulting ability to improve our weed management techniques, genomics will offer the weed science community many new and exciting research opportunities. Scientists will be able to determine the genetic composition of weed populations and how it changes over time in relation to agricultural practices. Identification of genes contributing to weediness, perennial growth habit, herbicide resistance, seed and vegetative structure dormancy, plant architecture and morphology, plant reproductive characters (outcrossing and hybridization, introgression), and allelopathy will be identified and utilized with high-throughput DNA sequencing and other genomics-based technologies. Using genomics to improve our understanding of weed biology by determining which genes function to affect the fitness, competitiveness, and adaptation of weeds in agricultural environments will allow the development of improved management strategies. This review provides a summary of the various plant genomic research methods being used. Information is provided concerning the current state of molecular research in various areas of weed science and specific genomic research currently being conducted at Purdue University using transfer DNA (T-DNA) activation tagging to generate large populations of mutated plants that can be screened for genes of importance to weed science.

The discovery of the first systemic or hormone herbicides, 2,4-D, 2,4,5-T, and MCPA, initiated an agricultural revolution and modern weed science. The finding of these herbicides was a striking case of multiple independent discovery by four groups of workers in two countries, the United Kingdom and the United States: William G. Templeman and associates at Imperial Chemical Industries; Philip S. Nutman and associates at the Rothamsted Agricultural Experiment Station; Franklin D. Jones at the American Chemical Paint Company; and Ezra Kraus, John Mitchell, and associates at the University of Chicago and the U.S. Department of Agriculture. Because of wartime and commercial secrecy, the usual procedures of scientific publication and patent disclosure were not followed; instead, the first scientific report on these herbicides occurred in a publication by workers who were not original discoverers. Considerable confusion consequently resulted concerning the discovery and the discoverers. This confusion has not been completely dispelled in subsequent years. The present report summarizes the complete story, clarifies the chronology of the discoverers and their publications, and makes the case that all four groups of workers deserve credit for this revolutionary advance. The scientific background of the discovery and events in its immediate aftermath, especially the ticklish patent situation, are also briefly chronicled.