The primary goal of this research was to determine the inheritance of cross-resistance to several groups of auxinic herbicides through classical genetic approaches using auxinic herbicide–resistant (R) and –susceptible (S) wild mustard biotypes obtained from western Canada. F1 progeny were raised from crosses between homozygous auxinic herbicide–R and –S wild mustard parental lines. The F1 and F2 populations were assessed for picloram (pyridine group) and 2,4-D (phenoxyalkanoic group) resistance or susceptibility. Analyses of the F1 as well as the F2 progeny indicate that a single dominant gene confers the resistance to picloram and 2,4-D similar to an earlier report of dicamba-based (benzoic acid group) resistance in this wild mustard biotype. Furthermore, analyses of backcross progeny in this species indicate that resistance to all three auxinic herbicides, i.e., picloram, dicamba, and 2,4-D, is determined by closely linked genetic loci. With this information on inheritance of resistance to several auxinic herbicide families, the R biotype of wild mustard offers an excellent system to isolate and characterize the auxinic herbicide–resistance gene.

Previous research revealed that resistance to cloransulam in at least one population of common ragweed was conferred by an altered herbicide target site, specifically, by a tryptophan-to-leucine amino acid substitution at position 574 (W574L) of acetolactate synthase (ALS). In this study, 22 common ragweed populations, several of which were suspected cloransulam resistant, were assayed to determine if the W574L ALS substitution was correlated with resistance to ALS inhibitors. From each population, 16 greenhouse-grown plants were treated with cloransulam, and another 16 were treated with imazamox. Plant dry weights were recorded 20 d after treatment and individual plants were considered resistant if their dry weight exceeded 50% of that of nonherbicide-treated controls. For each herbicide-treated plant, allele-specific primers were used in polymerase chain reactions to determine whether the ALS alleles contained leucine or tryptophan codons at position 574. Of the 352 plants treated with cloransulam, 70 were determined to be resistant, and all but two contained one or more Leu574 alleles. The frequency of imazamox resistance was much higher than that of cloransulam in the populations, with 149 of 352 plants identified as imazamox resistant. However, only about half (80) of the imazamox-resistant plants contained one or more Leu574 alleles. Correlation of imazamox resistance and Leu574 alleles was population dependent. ALS activity assays confirmed that imazamox resistance in plants from at least one population was due to an altered target site, even though plants from that population did not have a W574L substitution. These results lead to the conclusion that a Leu574 allele is the predominant basis for cloransulam resistance in common ragweed; however, other mechanisms of resistance to ALS inhibitors exist in some populations.

We have isolated both a genomic and near full length cDNA clone for a D-class cyclin gene from the perennial weed leafy spurge. Sequence analysis indicates that this gene has the highest similarity to CYCLIN D3-2 of Arabidopsis. This gene is preferentially expressed in growing shoot apices and is up-regulated in adventitious buds on resumption of growth following loss of correlative inhibition (apical dominance). CYCLIN D3-2 is also induced in nongrowing adventitious buds of plants treated with gibberellic acid or after removal of leaves—treatments known to up-regulate expression of G1 to S phase transition–specific genes such as HISTONE H3 in adventitious buds. CYCLIN D3-2 was not induced on removal of the apical and axillary buds. Expression of CYCLIN D3-2 is down-regulated in adventitious crown buds during initiation of ecodormancy in early winter. Sequence comparisons of CYCLIN D3-2 with its putative orthologue from Arabidopsis identified several conserved motifs in the promoter region and a conserved region capable of forming a stable hairpin loop in the 5′ untranslated region. Conservation of these noncoding sequences across species strongly suggests they have a regulatory function.

Two rapid, nondestructive assays were developed and tested for their potential in differentiating glyphosate-resistant from glyphosate-susceptible biotypes of horseweed. In one assay, leaves of glyphosate-resistant and -susceptible corn, cotton, and soybean plants, as well as glyphosate-resistant and -susceptible horseweed plants, were dipped in solutions of 0, 300, 600, and 1,200 mg ae L−1 glyphosate for 3 d, and subsequent injury was evaluated. In the second assay, plant sensitivity to glyphosate was evaluated in vivo by incubating excised leaf disc tissue from the same plants used in the first assay in 0.7, 1.3, 2.6, 5.3, 10.6, 21.1, 42.3, and 84.5 mg ae L−1 glyphosate solutions for 16 h and measuring shikimate levels with a spectrophotometer. The leaf dip assay differentiated between glyphosate-resistant and -susceptible crops and horseweed biotypes. The 600 mg L−1 rate of glyphosate was more consistent in differentiating resistant and susceptible plants compared with the 300 and 1,200 mg L−1 rates. The in vivo assay detected significant differences between susceptible and glyphosate-resistant plants of all species. Shikimate accumulated in a glyphosate dose–dependent manner in leaf discs from susceptible crops, but shikimate did not accumulate in leaf discs from resistant crops, and levels were similar to nontreated leaf discs. Shikimate accumulated at high (≥ 21.1 mg ae L−1) concentrations of glyphosate in leaf discs from all horseweed biotypes. Shikimate accumulated at low glyphosate concentrations (≤ 10.6 mg L−1) in leaf discs from susceptible horseweed biotypes but not in resistant biotypes. Both assays were able to differentiate resistant from susceptible biotypes of horseweed and could have utility for screening other weed populations for resistance to glyphosate.

Studies were conducted to evaluate uptake, translocation, and metabolism of root-absorbed 14C-sulfentrazone in peanut, prickly sida, and pitted morningglory. Peanut absorbed more than five and three times greater 14C-sulfentrazone than pitted morningglory and prickly sida, respectively. All plant species translocated appreciable amounts (≥ 39%) of radioactivity to the leaves. The three plant species had some capacity to metabolize 14C-sulfentrazone. At 3 h after treatment, 7, 29, and 71% of the radioactivity in the shoots of peanut, prickly sida, and pitted morningglory, respectively, was sulfentrazone. Sulfentrazone levels in the shoots at 3 and 6 h after treatment correspond to reported tolerance levels, with peanut being the most tolerant of the three species, whereas prickly sida and pitted morningglory are moderately tolerant and completely susceptible to sulfentrazone, respectively. Levels of metabolites varied among species, plant part, and harvest timing. On the basis of these data, tolerance in peanut is largely due to its ability to rapidly metabolize sulfentrazone.

Glyphosate-resistant (GR) crops are produced over large areas in North America. A study was conducted at six western Canada research sites to determine seed date and tillage system effects on weed populations in GR spring wheat and canola cropping systems from 2000 to 2003. Four-year wheat–canola–wheat–pea rotations were devised with varying levels of GR crops in the rotation. Weed populations were determined at pre– and post–in-crop herbicide application intervals in 2000 and 2003. Early seeding led to higher and more variable in-crop wild oat and wild buckwheat populations. High frequencies of in-crop glyphosate wheat in the rotation usually improved weed management and reduced weed density and variability. Canonical discriminant analysis (CDA) across all locations revealed that by 2003, green foxtail, redroot pigweed, sowthistle spp., wild buckwheat, and wild oat, all associated with the rotation lacking in-crop glyphosate. Similar CDA analyses for individual locations indicated specific weeds were associated with 3 yr of in-crop glyphosate (Canada thistle at Brandon, henbit at Lacombe, and volunteer wheat, volunteer canola, and round-leaved mallow at Lethbridge). However, only henbit at Lacombe and volunteer wheat at Lethbridge occurred at significant densities. Although excellent weed control was attained in rotations containing a high frequency of GR crops, the merits of more integrated approaches to weed management and crop production should also be considered. Overall, rotations including GR spring wheat did not significantly increase short-term weed management risks in conventional tillage or low soil-disturbance direct-seeding systems.

Combine harvesters have the potential to disperse weed seeds great distances. Reducing this dispersal may be important in an integrated weed management system. The objectives of this study were to determine the distance that wild oat seeds are dispersed by a combine harvester and the effect of chaff collection on combine harvester seed dispersal. This was measured by sampling wild oat seeds at varying distances behind a combine equipped with a removable chaff collection system after it passed through a wild oat patch. Chaff collection consistently reduced the amount and distance that wild oat seeds were dispersed. This occurred because more than 74% of the total wild oat seed that were ejected from the combine were in the chaff. Because most of the chaff falls in a row directly behind the combine, chaff collection only affected dispersal in this area. In 1996, chaff collection reduced wild oat seed dispersal past the wild oat patch to less than 10 seeds m−2 at 45 m, whereas without chaff collection, there was greater than 10 seeds m−2 up to 145 m. At distances beyond 145 m, chaff collection had no significant effect on seed dispersal. Chaff collection may be an important tool in an integrated weed management program because it may slow weed invasions and reduce the expansion of weed patches.

Laboratory and greenhouse studies were conducted to determine the effect of temperature, soil moisture, light, planting depth, and rhizome water content on shoot emergence and vegetative growth of alligatorweed. Optimum shoot emergence and growth occurred at constant 30 C, and no shoot emergence was found below constant 5 C. A maximum shoot emergence of 93% occurred at constant soil moisture of 30% with temperatures of 10 to 35 C. Shoot emergence and growth decreased as rhizome water content decreased, and shoot emergence did not occur below a rhizome water content of 20%. Shoot emergence and growth decreased with burial depth; shoot emergence was above 90% when rhizomes were buried 0.5 to 1.0 cm deep compared to 16% when they were buried 18 cm deep. Alligatorweed shoot emergence and vegetative growth were not significantly affected by light. In the fields, shoot emergence began in late March and culminated in May and June. These data help explain why this species is most commonly found in crop fields, orchards, roadsides, rivers, lakes, ponds, and irrigation canals. This information may aid in the development of more effective management measures, such as bringing alligatorweed rhizomes to the surface or below 20 cm deep to restrain its emergence and growth at winter or summer plowing.

Velvetleaf is a troublesome annual weed in many cropping systems of the United States and Canada. Differences in the growing environment of parent plants can influence the number, structure, germinability, and viability of seeds produced. Thus, the effects across a range of competitive environments and corn planting dates on velvetleaf seed production, germination, and seed coat weight were examined under field conditions. Seed production of velvetleaf increased with increasing biomass. Total velvetleaf reproductive output was reduced in competition with corn compared with monoculture stands. Corn planting date had no effect on the dormancy status of seeds, but increased competition from corn resulted in up to a 30% decrease in the proportion of seeds that were dormant. Seed and seed coat weights also decreased for plants of velvetleaf grown in competition with corn compared with those grown in monoculture. These findings suggest that velvetleaf plants growing in relatively noncompetitive environments, such as along field edges or in field areas with poor crop stands, are likely not only to produce a greater number of seeds but also a greater proportion of seeds that are dormant. This alteration in the dormancy status of velvetleaf seeds in the absence or presence of a crop provides unique opportunities for effective long-term management of the soil seedbank in this species, especially for velvetleaf individuals bordering fields or growing in fallow areas that might require more stringent control because of increased seed dormancy.

Since 1995, canola cultivars with herbicide resistance (HR) have been readily adopted by Canadian producers. Gene flow between these cultivars with different HR traits has led to the occurrence of double herbicide–resistant (2HR) volunteers. To evaluate the fitness of canola volunteers with double HR, we compared three 2HR combinations to each of their parent single-HR plants (1HR: glufosinate-R, imidazolinone-R, glyphosate-R) commercial canola lines in separate greenhouse experiments. The replacement series design included five ratios of 2HR vs. 1HR plants at a single density of 129 plants m−2 and three stress treatments: herbicide application with either glufosinate, imazethapyr, or glyphosate; competition with a wheat crop; and a control without herbicide or wheat competition. Fitness indicators included aboveground biomass at 5 and 12 to 16 wk, seed production, and reproductive allocation. The 2HR plants showed delayed reproductive growth but were generally as competitive as 1HR commercial lines. Plant biomass of 2HR canola was comparable to or greater than 1HR canola, whereas seed biomass of 2HR canola was less than that of 1HR canola in half of the cases, likely because of delayed reproductive growth and early harvesting. Glufosinate–glyphosate 2HR was the fittest combination. Herbicide application had little effect on 2HR biomass at harvest, except for imazethapyr, which reduced the biomass and seed production of 2HR plants with imidazolinone-glyphosate resistance by 30%. The latter effect could have been from the unsuspected presence of 2HR plants with only one of the two acetolactate synthase mutations conferring resistance to imidazolinones. Wheat competition reduced fitness values of both 2HR and 1HR canola similarly, but seed production was still 64% that of the controls. Overall, there was little indication of reduced fitness in 2HR canola compared with commercial 1HR varieties.

Previous research has examined the extent to which red rice affects both yield and grain quality of cultivated rice. However, this research was conducted over 15 yr ago. Modern long-grain rice cultivars have the potential to produce yields above 10,000 kg ha−1; however, it is unknown whether modern rice cultivars sacrifice competitiveness to achieve higher yields, or if, in fact, they are more competitive. Field studies were conducted in 2002 and 2003 at the Southeast Research and Extension Center near Rohwer, AR, and at the University of Arkansas Pine Bluff Research Farm near Lonoke, AR, to investigate the effect of red rice density on interference between red rice and five rice cultivars (‘CL161’, ‘Cocodrie’, ‘LaGrue’, ‘Lemont’, and ‘XL8’). White rice yield reductions were between 100 and 755 kg ha−1 for every red rice plant m−2. The hybrid rice, XL8, had higher yields than the conventional cultivars. Red rice contamination in milling samples increased linearly as a function of red rice density at Lonoke and Rohwer in 2003. Dockage for each cultivar was calculated on the basis of the relationship between red rice density and red rice contamination. Semidwarf Lemont was the most contaminated and hybrid XL8 the least contaminated by the various densities of red rice.

A stochastic simulation modeling framework was developed for measuring the impact of weed management technologies in terms of their risk and efficacy. The framework explicitly accounted for the variability of environmental conditions, which underpins risk, and its effect upon the weed population dynamics and crop yields. It was applied to wild oat and wild radish in spring wheat as case studies. Technologies considered included a postemergence herbicide, preseeding tillage, increased crop density, and a selective spray-topping (seed-head sterilizing) herbicide. All stages of the weed life cycle were specified as random variables on the basis of triangular probability distributions, which either were derived from daily environmental conditions or specified as a subjective probability distribution. By using probability density functions the study identified the risks to changes in the weed seed bank and weed density associated with various integrated weed management strategies. This approach not only quantified the probabilities associated with the different outcomes, but also identified how the probability distributions of outcomes were changed as a result of different technology combinations used in weed management strategies. For instance, strategies involving a selective spray-topping herbicide to control seed rain not only resulted in lower seed banks, but the range in possible values was considerably reduced, implying lower likelihood of a population increase. The use of such a risk framework not only benefits weed scientists in terms of designing more effective weed management technologies, but can also assist farmer adoption by being able to quantify the probabilities of success and failure of a technology.

A greenhouse experiment was conducted to evaluate the herbicidal activity of five aliphatic (ethyl, propyl, butyl, allyl, and 3-methylthiopropyl) and three aromatic (phenyl, benzyl, 2-phenylethyl) isothiocyanates on Texas panicum, large crabgrass, and sicklepod. All isothiocyanates were applied to soil at 0, 10, 100, 1,000, and 10,000 nmol g−1 of soil and incorporated. Weed emergence was generally stimulated at the lower isothiocyanate concentrations, but all isothiocyanates provided 37% or more suppression of each species at the highest concentration. Propyl and allyl isothiocyanate were most effective in suppressing Texas panicum, with 50% effective dose (ED50) values of 345 and 409 nmol g−1 of soil. All aliphatic isothiocyanates reduced Texas panicum density by at least 98%. Allyl and 3-methylthiopropyl isothiocyanate were the most effective aliphatics on large crabgrass, with density reductions of 98 and 100%, respectively. All aromatic isothiocyanates reduced large crabgrass density by 86 to 96%. Sicklepod was generally the most tolerant of the three species evaluated, with ED50 values for ethyl, propyl, and butyl isothiocyanate being greater than the evaluated concentrations. Maximum reduction in sicklepod density was 72, 68, 65, and 62%, which was achieved with allyl, benzyl, 3-methylthiopropyl, and phenyl isothiocyanate, respectively. This research shows that soil-applied and incorporated isothiocyanates are effective in suppressing some important weeds of the southeastern United States, but effectiveness of each isothiocyanate varies among species. Application techniques that minimize loss of volatile isothiocyanates may further improve their potential as an effective means of controlling these and other troublesome weeds.

Cover crop residues and other biologically based approaches often provide incomplete and inconsistent weed control. This research was conducted to evaluate interactions between hairy vetch residue on the surface of soil and the herbicide metolachlor. Herbicide was applied and incorporated with simulated rainfall before residue placement, residue was applied to the soil surface at precise rates, and potentially confounding variables such as nitrogen and soil moisture were controlled in a greenhouse experiment. Emphasis was placed on the use of suboptimal rates of both residue and metolachlor to explore the potential synergistic interactions between these factors. Deviation from a multiplicative model that included a quadratic response to hairy vetch residue and a log-logistic response to metolachlor was used to demonstrate the presence or absence of synergism. This model effectively showed that emergence of smooth pigweed, common lambsquarters, giant foxtail, and velvetleaf and early growth of smooth pigweed and common lambsquarters were reduced synergistically by the combination of hairy vetch residue and metolachlor. For example, smooth pigweed emergence was reduced 13% by 500 g m−2 of hairy vetch residue alone and was reduced 16% by 10 g ha−1 of metolachlor alone, but together, they reduced smooth pigweed emergence by 86%. This model could be used to determine synergistic interactions between any combination of a phytotoxin and a biologically based weed management approach that could be expressed in quantitative units.

Development of more comprehensive and cost-effective integrated weed management systems is required to facilitate greater integrated weed management adoption by farmers. A field experiment was conducted at two locations to determine the combined effects of seed date (April or May), seed rate (recommended or 150% of recommended), fertilizer timing (applied in fall or spring), and in-crop herbicide dose (50% or 100% of recommended) on weed growth and crop yield. This factorial set of treatments was applied in four consecutive years within a spring wheat–spring canola–spring wheat–spring canola rotation in a zero-till production system. Both wheat and canola phases of the rotation were grown each year. Weed biomass was often lower with May than with April seeding because more weeds were controlled with preplant glyphosate. However, despite fewer weeds being present with May seeding, wheat yield was only greater in 1 of 7 site-years, and canola yield was never greater with May compared with April seeding. Higher crop seed rates had a consistently positive effect on reducing weed growth and the weed seedbank. Crop yield was sometimes greater, and never lower, with higher seed rates. Fertilizer timing did not have a large effect on crop yield, but applying N in the spring compared with fall was less favorable for weeds as indicated by lower weed biomass and a 20% decrease in the weed seedbank. In-crop herbicides applied at 50% compared with 100% doses often resulted in similar weed biomass and crop yield, especially when higher crop seed rates were used. Indeed, the weed seedbank at the conclusion of the 4-yr experiment was not greater with the 50% compared with 100% herbicide dose at one of two locations. This study demonstrates the combined merits of early seeding (April), higher crop seed rates, and spring-applied fertilizer in conjunction with timely but limited herbicide use to manage weeds and maintain high crop yields in rotations containing wheat and canola.

Field experiments were conducted at Ashland Bottoms in northeastern Kansas and at Hays in western Kansas in 2001, 2002, and 2003 to determine the response of soil microbial and nematode communities to different herbicides and tillage practices under a glyphosate-resistant cropping system. Conventional herbicide treatments were a tank mixture of cloransulam plus S-metolachlor plus sulfentrazone for soybean and a commercially available mixture of acetochlor and atrazine for corn. Glyphosate was applied at 1.12 kg ai ha−1 when weeds were 10 or 20 cm tall in both corn and soybean. Soil samples were collected monthly at Ashland Bottoms during the growing period for soil microbial biomass (SMB) carbon determination. In addition, substrate-induced respiration (SIR) and BIOLOG substrate utilization were determined at the end of the growing season each year at Ashland Bottoms, and nematode populations were determined at the beginning and the end of the growing season at both sites. Direct effects of glyphosate rates on soil microbial and nematode communities were also studied in a controlled environment. Values for SMB carbon, SIR, and BIOLOG substrate utilization were not altered by glyphosate. Nematode community response to the glyphosate treatment was similar under both conventional tillage and no-till environments. Total nematode densities were similar with the glyphosate and conventional herbicide treatments. SMB carbon and BIOLOG substrate utilization did not differ between tillage treatments. Nematode densities were greater under conventional tillage than in the no-till system. This study showed that soil health when glyphosate was applied in a glyphosate-resistant cropping system was similar to that of cropping systems that used conventional herbicides.