Timely results from whole-plant, herbicide-resistant weed screenings are crucial to heighten grower awareness. However, the high degree of physiological dormancy of giant ragweed (Ambrosia trifida L.) seed exacerbates this process. The most effective methods for alleviating dormancy, to date, are either labor-intensive (embryo excision) or require several weeks (soil stratification). This research describes a conditioning process involving clipping and aeration of seed in water that is highly effective at alleviating dormancy and requires less skill and time compared with previous techniques. Ambrosia trifida seeds were collected over 2 yr at two different collection timings (September 25, “early”; October 25, “late”), subjected to various treatments intended to release dormancy, and evaluated for emergence over 18 d in the greenhouse. The use of germination-promoting chemicals (ethephon, gibberellic acid, and thiourea) generally provided no increase in emergence compared with water and occasionally produced seedlings with abnormal growth unsuitable for further experimentation. Conditioning yielded between 30% and 33% emergence for both early and late collections of seeds with no afterripening period compared with 0% emergence for seeds imbibed in water. Following an 8-wk period of dry storage at 4 C, conditioning yielded nearly 80% emergence for both collection timings, while emergence of seeds imbibed in water was 10% and 27% for early and late collections, respectively. Soil stratification in moist soil for 8 wk at 4 C was the second most effective treatment, yielding 46% to 49% emergence across both collections. Parameters of the Weibull function further indicated the conditioning treatment had the fastest rate of emergence and shortest lag phase between planting and first emergence. Methods to germinate A. trifida without an afterripening period have previously been unsuccessful. Therefore, the seed-conditioning method outlined in this work will be useful in expediting the confirmation of herbicide-resistant A. trifida incidences.

Giant ragweed is a highly competitive weed that continually threatens crop production systems due to evolved resistance to acetolactate synthase–inhibiting herbicides (ALS-R) and glyphosate (GR). Two biotypes of GR giant ragweed exist and are differentiated by their response to glyphosate, termed here as rapid response (RR) and non–rapid response (NRR). A comparison of data from surveys of Indiana crop fields done in 2006 and 2014 showed that GR giant ragweed has spread from 15% to 39% of Indiana counties and the NRR biotype is the most prevalent. A TaqMan® single-nucleotide polymorphism genotyping assay was developed to identify ALS-R populations and revealed 47% of GR populations to be ALS-R as well. The magnitude of glyphosate resistance for NRR populations was 4.6 and 5.9 based on GR50 and LD50 estimates, respectively. For RR populations, these values were 7.8 to 9.2 for GR50 estimates and 19.3 to 22.3 for LD50 estimates. A novel use of the Imaging-PAM fluorometer was developed to discriminate RR plants by assessing photosystem II quantum yield across the entire leaf surface. H2O2 generation in leaves of glyphosate-treated plants was also measured by 3,3′-diaminobenzidine staining and quantified using imagery analysis software. Results show photo-oxidative stress of mature leaves is far greater and occurs more rapidly following glyphosate treatment in RR plants compared with NRR and glyphosate-susceptible plants and is positively associated with glyphosate dose. These results suggest that under continued glyphosate selection pressure, the RR biotype may surpass the NRR biotype as the predominant form of GR giant ragweed in Indiana due to a higher level of glyphosate resistance. Moreover, the differential photo-oxidative stress patterns in response to glyphosate provide evidence of different mechanisms of resistance present in RR and NRR biotypes.

The response of potato (Solanum tuberosum L. 'Superior’) to glyphosate [N-(phosphonomethyl)glycine] at rates between 0.28 and 2.24 kg ae/ha was determined in four environments: 24/13 or 13/4 C temperature regimes with light levels of 650 ± 53 or 320 ± 45 μE · m−2 · S−1. Light level did not affect phytotoxicity ratings or shoot fresh weight following glyphosate application. Greater phytotoxicity and reduced fresh weight accumulation were observed from glyphosate at rates above 0.56 kg/ha in a high-temperature regime than for comparable plants grown at low temperature. Studies of 14C-glyphosate uptake and translocation indicated that the differences in phytotoxicity were due to less glyphosate absorption at low temperature and not a shift in distribution patterns within the plant.

A reduced-till (RT) experiment determined the effect of rye seeding density and method of kill on rye biomass persistence, weed suppression, and tomato yield. ‘Wheeler’ rye was seeded at 56, 110, and 170 kg ha−1 in the fall of 1988 and 1989 in Champaign, IL; Lexington, KY; and Lafayette, IN. One wk before transplanting tomatoes, rye was desiccated either by applying glyphosate at 1.1 kg ha−1 and mowing (RT-glyphosate) or by mowing alone (RT-mowed). Both methods left rye residues on the soil surface as a mulch. Seeding density did not affect rye biomass, weed control, or tomato yield. Rye biomass differed depending on location and year, ranging from 320 to 1150 gm−2. Rye was completely killed by glyphosate, but regrew following mowing. RT-glyphosate suppressed weeds for 4 to 8 wk, which was similar to the suppression from conventional tillage (fall plowing with spring disking and harrowing) with trifluralin and metribuzin (CT-herbicide). Weed communities varied, but redroot pigweed and giant foxtail were present at all three locations. All treatments required supplemental weed management for commercially acceptable control. Except for IL in 1990, processing tomato yields (kg ha−1) in RT-glyphosate with hand-weeding were equal or better than yields in the CT treatments.

Tissue and cell culture offer weed scientists many opportunities to research herbicide effects on plants. This review will discuss examples in which plant cells grown in vitro have been used to study herbicide action. Plant cell and tissue culture have many advantages over the use of whole plants; however, several disadvantages that exist are discussed. Cell cultures can be established for most plant species and provide a relatively homogeneous system for studying herbicide action. Responses of plant cells to herbicides are usually correlated with responses at the whole plant level, and cells have the advantage of posing fewer physical barriers to herbicide uptake and translocation. Cell culture techniques discussed include: screening candidate herbicide compounds; investigating herbicide efficacy, mechanism of action, metabolism, and uptake; and ascertaining mechanisms of herbicide resistance, selecting for resistance, and regenerating crops.

Two biotypes of field bindweed differing in their susceptibility to glyphosate were used to determine if diluent or carrier volume and additional surfactant could overcome differences in intraspecific response to glyphosate. In greenhouse studies, glyphosate (formulated product) was applied at 1.68 kg/ha in three diluent volumes (142, 189, and 237 L/ha), with and without 1 % (v/v) additional amphoteric surfactant. Nonparametric and ordinal categorical analyses indicated that field bindweed biotype, diluent volume, and surfactant significantly increased glyphosate phytotoxicity 7 DAT. Only biotype and volume were significant 21 DAT. The tolerant biotype was less injured at the 189 and 237 L/ha volumes than the susceptible biotype. Field bindweed injury was similar at a diluent volume of 142 L/ha for both biotypes. These greenhouse studies suggest that control of field bindweed may be improved with glyphosate by using low spray volume in concert with additional surfactant.

Weed control in tomato production systems is difficult because few are registered. The use of rye for weed control and its influence on transplant tomato yields was investigated during 1986 and 1987 at two locations in IN to determine if cover crops can provide an alternative weed management technique. ‘Wheeler’ rye was sown in the fall of 1985 and 1986, and mowed or desiccated with glyphosate at various times before planting ‘IND 812'tomatoes. At the time of glyphosate application, rye residues reduced the growth of overwintering weeds by 93% or more compared to bare ground (no cover crop) areas. The time of desiccating rye prior to planting tomatoes affected the extent of weed suppression by rye residues. In 1986, rye treated 4 wk before planting (WBP) tomatoes provided up to 89% suppression of weed growth at 2 wk after planting (WAP) tomatoes, but no measurable weed suppression 5 WAP tomatoes. Rye treated 2 WBP tomatoes provided up to 97% weed suppression up to 5 WAP tomatoes. In 1987, weed suppression varied between locations and differed from 1986. At Lafayette, rye treated 2 and 1 WBP tomatoes provided greater than 81% suppression of weed growth up to 8 WAP tomatoes. Rye mowed and the residues placed into a plot at a known density also reduced weed growth (60%) 8 WAP tomatoes. At Vincennes, however, rye treated 2 and 1 WBP in 1987 did not reduce weed growth later than 4 WAP tomatoes compared to the unweeded, bare ground treatment. The mowed rye residues at Vincennes suppressed weed growth (96%) up to 8 WAP tomatoes. Tomato yield was correlated to weed suppression. In 1986, tomato yield in the rye treated 2 WBP tomatoes was comparable to yield in the bare ground, weeded controls. However, tomato yield in rye plots treated 4 WBP tomatoes was similar to yield in the bare ground, unweeded control. In 1987, tomato yields in all rye plots (mowed, treated 2 and 1 WBP tomatoes) were similar to tomato yields in the bare ground, weeded control at Lafayette. At Vincennes, only the mowed rye treatment yielded comparably to the bare ground, weeded control. In general, rye plots that were weeded yielded similar to or up to 28% more than a bare ground, weeded control. Tomato yields were not reduced by rye residues. Tomato yields in rye residues that provided effective suppression of weed growth (greater than 80%) for a minimum of 4 to 5 WAP tomatoes were comparable to bare ground, weeded controls.

Biotypes of field bindweed (Convolvulus arvensis L. ♯3 CONAR) identified in Indiana varied widely in susceptibility to glyphosate [N-(phosphonomethyl)glycine] but not to 2,4-D [(2,4-dichlorophenoxy)acetic acid] or bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] in field tests. Significant differences in injury to two of the biotypes occurred with glyphosate applied at 1.12 to 4.48 kg ai/ha in greenhouse tests. Differences of greater than 70% in injury rating, root and shoot dry weight, and shoot regrowth dry weight occurred between the two biotypes at 2.24 kg/ha glyphosate. The susceptibility of the tolerant biotype at 2.24 kg/ha glyphosate was decreased by 40% as it increased in age, while the susceptible biotype sustained complete foliar necrosis when treated at all plant ages tested. Susceptibility differences between the two biotypes could not be correlated to differences in leaf stomatal or epidermal cell number. These studies suggested that the variable control of field bindweed observed in the field may be due to the occurrence of biotypes within a given population of this weed which differ in their susceptibility to glyphosate.

Field experiments were conducted from 1995 to 1997 to test approaches to managing atrazine-resistant Datura stramonium. Eight weed management programs in a Zea mays and Glycine max rotation were evaluated for their effects on the dynamics of atrazine-resistant and -susceptible D. stramonium populations. Overall D. stramonium density and relative abundance of resistant (R) biotypes were greatly reduced in no-till Secale cereale L. (rye) cover crop management programs without triazine compared to conventional-tillage systems with the application of triazine herbicides. The negative effects of no-till on D. stramonium were greater under a G. max–Z. mays—G. max (SCS) rotation than under a Z. mays–G. max—Z. mays (CSC) rotation. A cropping system involving more G. max phases under no-till reduced both the resistant and susceptible D. stramonium populations. Results from this study support the use of soil management, crop rotation, and negative cross-resistant herbicides to manage atrazine-resistant biotypes.

Field experiments conducted over a 2-yr period demonstrated that common bermudagrass [Cynodon dactylon (L.) Pers. # CYNDA] inhibited growth of newly planted peach (Prunus persica L. ‘Norman’) trees. Common bermudagrass densities of 100, 75, 50, and 25% ground cover reduced tree fresh weight by 86, 64, 43, and 19%, respectively, the first year (1978) and 87, 62, 44, and 28%, respectively, the second year (1979) after planting. Tree trunk diameter relative growth rate (RGR) was reduced by 75 and 100% common bermudagrass ground cover densities at all measurement dates only in 1978. Tree leaf N and K were reduced in both years by common bermudagrass; however, only at the 100% common bermudagrass density in 1978 was N at a deficient level. Leaf chlorophyll was reduced in trees grown in all densities of common bermudagrass only in 1978. Reduced tree growth cannot be explained entirely by competition for essential nutrients; thus an allelopathic effect of the bermudagrass on young peach roots is suspected.

Five presumed biotypes were identified among field bindweed (Convolvulus arvensis L. ♯3 CONAR) clones collected from a field population near Lafayette, IN. Consistent variations in leaf morphology, floral characteristics, and accumulation of shoot and root biomass were found between biotypes when grown in a controlled environment. The biotypes also differed in their flowering capacity. The earliest flowering biotype formed flowers 23 days before the latest and produced 19 times more flowers per plant, which indicated further differences in seed production potential between biotypes. Pollination studies helped to differentiate biotypes within the population and showed that the presumed biotypes were self-incompatible. Vegetative reproduction potential of the biotypes varied from 1.8 to 74.5% in the number of root buds that developed into shoots. The variability in growth and reproduction observed between these field bindweed biotypes may explain the survival and adaptability of a population of this weed as environmental conditions and control practices change.

Biotypes of field bindweed that vary in sensitivity to glyphosate were studied to determine the physiological or biochemical bases for these differences. Studies con ducted using whole plants and in vitro cultured shoots identified several potentially important differences between the most tolerant (biotype 4) and most susceptible (biotype 1) biotypes. Biotype 4 plants had greater 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase (DAHPS) activity and higher concentrations of phenolic compounds, indicating greater shikimate pathway activity than biotype 1. This may reflect a higher growth ability of biotype 4, as observed in shoot cultures. 5-Enolpyruvylshikimate-3-phosphate synthase (EPSPS) activity in whole plants increased in all parts of biotype 4 by 7 d after a glyphosate treatment of 1.1 kg ae ha−1, whereas activities in biotype 1 plants did not increase at any time. However, this may not be the only mechanism of glyphosate tolerance because EPSPS activity in cultured shoots of both biotypes increased equally in response to glyphosate, even though biotype 4 shoots were able to survive and grow on a glyphosate-containing medium that inhibited growth of biotype 1 shoots. We propose that multiple mechanisms operating at a cellular/metabolic level combine to enable biotype 4 to tolerate higher glyphosate rates than biotype 1.

Field studies were conducted to determine if oxyfluorfen alone or combined with other herbicides could be applied between black polyethylene mulch without injuring muskmelon. Oxyfluorfen at rates up to 1.1 kg ai/ha did not injure muskmelon provided adequate rain or irrigation occurred to incorporate the oxyfluorfen thereby minimizing vapors that could injure muskmelon. Using this practice, muskmelon yields from oxyfluorfen-treated plots were the same as yields from untreated cultivated plots.

Field bindweed biotypes varying in susceptibility to glyphosate were studied to determine the role of glyphosate absorption and translocation in susceptibility differences. Absorption and translocation of radiolabeled glyphosate were measured after application to leaves of glyphosate-tolerant and -sensitive field bindweed biotypes grown under ambient, low, or high relative humidity (RH). Glyphosate injury and cuticle mass were also determined at both low and high RH. To avoid potential differences in susceptibility due to differential foliar absorption, glyphosate was fed to roots of hydroponically grown plants, and uptake, translocation, and injury patterns were also determined. Under ambient RH, no differences in foliar-applied glyphosate absorption or translocation were detected between field bindweed biotypes. The RH under which plants were grown influenced cuticle amounts, glyphosate absorption, and injury. Plants of both biotypes had less cuticular material when grown at high RH (80%) than at low RH (30%); however, more radioactivity was absorbed by the susceptible biotype at low RH. Regardless of RH regime, glyphosate injury was greater in the susceptible biotype than in the tolerant biotype. Glyphosate fed to roots of hydroponically grown plants was absorbed and translocated equally in the biotypes, yet relative differential glyphosate sensitivities were retained. These results suggest that differences in glyphosate susceptibility between field bindweed biotypes cannot be fully accounted for by differences in glyphosate absorption or translocation.

The growth regulator herbicides 2,4-D and dicamba are used to control glyphosate-resistant horseweed before crops are planted. With the impending release of 2,4-D–resistant and dicamba-resistant crops, use of these growth regulator herbicides postemergence will likely increase. The objective of this study was to determine the effectiveness of various growth regulators on Indiana horseweed populations. A greenhouse dose–response study was conducted to evaluate the effectiveness of 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of four populations of horseweed in the greenhouse. Population 66 expressed twofold levels of tolerance to 2,4-D ester and diglycolamine salt of dicamba. Population 43 expressed an enhanced level of tolerance to diglycolamine salt of dicamba but not to the other herbicides. Diglycolamine salt of dicamba provided the best overall control of populations 3 and 34. Additionally, a field study was conducted to evaluate standard use rates of 2,4-D amine, 2,4-D ester, diglycolamine salt of dicamba, and dimethylamine salt of dicamba on control of various sized glyphosate-resistant horseweed plants. Control of plants 30 cm or less in height was 90% or greater for all four herbicides. On plants greater than 30 cm tall, diglycolamine salt of dicamba provided 97% control while 2,4-D amine provided 81% control. Diglycolamine salt of dicamba provided the highest level of control of glyphosate-resistant horseweed, followed by dimethylamine salt of dicamba, 2,4-D ester and 2,4-D amine, respectively. This research demonstrates that horseweed populations respond differently to the various salts of 2,4-D and dicamba, and it will be important to determine the appropriate use rates of each salt to control glyphosate-resistant horseweed.

Horseweed can be a problematic weed in no-till soybean fields and populations can vary in their response to 2,4-D. The objective of this study was to evaluate the growth and seed production of four horseweed populations after exposure to 2,4-D. 2,4-D amine was applied at 0, 140, 280, and 560 g ae ha−1 to 5- to 10-cm-tall horseweed plants. An additional treatment of 280 g ha−1 of 2,4-D + 840 g ae ha−1 of glyphosate was included in the study. At 2 wk after treatment (WAT), injury ranged from 47 to 98%, but by 6 WAT the injury ranged from 89 to 100% for all four populations. Between 6 and 12 WAT some individual horseweed plants started to recover. No differences in dry weights were observed between the four populations in the untreated checks at 0, 2, 6, and 12 WAT. At 280 g ha−1 of 2,4-D amine, seed production was reduced by greater than 95%. However, three of the four horseweed populations had plants that survived and produced seed after exposure to 840 g ha−1 of glyphosate + 280 g ha−1 of 2,4-D. One plant produced seed after exposure to 560 g ha−1 of 2,4-D. These results suggest that horseweed can evolve resistance to 2,4-D and no fitness penalities were observed in populations that had higher levels of tolerance to 2,4-D.

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.

A segment of the debate surrounding the commercialization of genetically engineered (GE) crops, such as glyphosate-resistant (GR) crops, focuses on the theory that implementation of these traits is an extension of the intensification of agriculture that will further erode the biodiversity of agricultural landscapes. A large field-scale study was conducted in 2006 in the United States on 156 different field sites with a minimum 3-yr history of GR corn, cotton, or soybean in the cropping system. The impact of cropping system, crop rotation, frequency of using the GR crop trait, and several categorical variables on emerged weed density and diversity was analyzed. Species richness, evenness, Shannon's H′, proportion of forbs, erect growth habit, and C3 species diversity were all greater in agricultural sites that lacked crop rotation or were in a continuous GR crop system. Rotating between two GR crops (e.g., corn and soybean) or rotating to a non-GR crop resulted in less weed diversity than a continuous GR crop. The composition of the weed flora was more strongly related to location (geography) than any other parameter. The diversity of weed flora in agricultural sites with a history of GR crop production can be influenced by several factors relating to the specific method in which the GR trait is integrated (cropping system, crop rotation, GR trait rotation), the specific weed species, and the geographical location. The finding that fields with continuous GR crops demonstrated greater weed diversity is contrary to arguments opposing the use of GE crops. These results justify further research to clarify the complexities of crops grown with herbicide-resistance traits, or more broadly, GE crops, to provide a more complete characterization of their culture and local adaptation.

In 2010, a grower survey was administered to 1,299 growers in 22 states to determine changes in weed management in the United States from 2006 to 2009. The majority of growers had not changed weed management practices in the previous 3 yr; however, 75% reported using weed management practices targeted at glyphosate-resistant (GR) weeds. Growers were asked to rate their efforts at controlling GR weeds and rate the effectiveness of various practices for controlling/preventing GR weeds regardless of whether they were personally using them. Using the herbicide labeled rate, scouting fields, and rotating crops were among the practices considered by growers as most effective in managing GR weeds. Sixty-seven percent of growers reported effective management of GR weeds. Between the 2005 and 2010 Benchmark surveys, the frequency of growers using specific actions to manage GR weeds increased markedly. Although the relative effectiveness of practices, as perceived by growers, remained the same, the effectiveness rating of tillage and the use of residual and POST herbicides increased.