Greenhouse and laboratory studies were conducted to examine the effects of site of plant exposure to glyphosate spray on efficacy, absorption, and translocation in pitted morningglory. Absorption of 14C-glyphosate in four-leaf pitted morningglory gradually increased with time from 19% at 1 h after treatment (HAT) to 44% at 192 HAT. The amount of 14C translocated with time ranged from 0.4% at 1 HAT to 25% at 192 HAT. Vining 1-m tall plants were controlled 75 to 100% when the top-, middle-, bottom one-third, or entire plant was treated with 1.38 or 2.76 kg ha−1 glyphosate, with control affected more by glyphosate rate than plant section exposed to glyphosate spray. Absorption of 14C-glyphosate at 96 HAT was similar whether it was applied to the top-, middle-, bottom one-third, or entire plant of 1-m tall pitted morningglory. The amount of 14C translocated out of the treated area (5 to 6%) did not differ whether it was applied to top-, middle-, or bottom one-third plant section. Results indicate that absorption and translocation of 14C-glyphosate in pitted morningglory was rapid and increased with time. Treating any one-third section of pitted morningglory plants was as effective as entire plant exposure, and control with glyphosate is more affected by rate than the degree of plant exposure to glyphosate.

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.

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 1200 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 might have utility for screening other weed populations for resistance to glyphosate.

Clodionafop, an acetyl-coenzyme A carboxylase (ACCase) inhibitor, changed the shape of the chlorophyll fluorescence induction curve (Kautsky curve) in barley and oat in greenhouse experiments. Biomass ED50, based on log-logistic dose–response curves, for barley was considerably higher than that for oat in all experiments. Biomass ED50 and relative potency (ED50 [barley]/ED50 [oat]) were consistent among experiments when sprayed at the same phenological stage of plant development. Especially at high doses, clodinafop changed the shape of the Kautsky curve more for oat than for barley. From the numerous parameters that can be derived from the OJIP steps of the Kautsky curve, we found that (1) F vj, the relative changes at the J step [F vj = (F m − F j)/F m], (2) area between Kautsky curve and maximum fluorescence (F m), and (3) F v/F m, maximum quantum efficiency of Photosystem II [F v/F m = (F m − F 0)/F m], were closely linked to the biomass dose–response relationships for both species. The linkage between biomass and the fluorescence parameters may be used to shorten the screening period for ACCase inhibitors.

The herbicide resistance to dicamba (HRd) biotype of kochia is resistant to several auxinic herbicides and is impaired in shoot gravitropism and other auxin-mediated responses. To better characterize the biotype and investigate its mechanism of resistance, we used messenger RNA (mRNA) differential display to compare patterns of dicamba-induced gene expression in HRd and susceptible (S1) plants. More than 60,000 complementary DNA fragments were generated and examined, 106 of which were isolated and used as probes on Northern blots to confirm gene expression levels. Steady-state levels of > 90% of mRNAs did not change after dicamba application. However, several mRNAs were detected whose levels were decreased, increased, or differentially regulated between the biotypes within minutes of dicamba treatment. The abundance of three mRNAs decreased after treatment, two of which had significant sequence similarity to choline monooxygenase and 5,10-methylenetetrahydrofolate reductase, respectively. Conversely, increased expression levels were observed for a putative chloride channel protein, 1-aminocyclopropane-1-carboxylate synthase, and an unknown gene. Genes differentially expressed between HRd and S1 plants included those similar to a putative translation initiation factor, xyloglucan endotransglycosylase, and a hypothetical protein cloned from several organisms. The results demonstrate that mRNA differential display is a useful technique for discovering genes that are rapidly regulated as part of a physiological response, and that this approach may provide insight into the mechanism of auxinic herbicide resistance in kochia.

The introduction of Clearfield (CL) rice cultivars resistant to imidazolinone herbicides, acetolactate synthase (ALS) inhibitors, has raised concerns of gene flow to weedy rice genotypes collectively called “red rice” that infest rice-growing areas in the southern United States. This experiment was conducted to study hybridization between CL rice and red rice using simple sequence repeats (SSR) markers, identify mutations in the ALS gene of imazethapyr-resistant red rice, and to detect the introgression of the ALS-resistant gene from CL rice into red rice. Natural outcrossing experiments between CL rice and strawhull (SH) red rice were set up in Stuttgart, AR, in 2002 and 2003. Putative red rice hybrids were detected among volunteer plants in the following year. Hybridization was confirmed using SSR markers, and introgression of the resistant ALS gene from CL rice to red rice was detected by ALS gene sequencing. The ALS gene sequences of U.S. rice cultivars ‘Bengal’ and ‘Cypress’, SH red rice, CL rice (CL161), and imazethapyr-resistant red rice/CL rice hybrids were compared. Nucleotide sequences of the ALS gene from the rice cultivars were identical. Three point mutations were present in the SH red rice ALS gene coding region relative to Bengal/Cypress. One of these resulted in the substitution of Asp630 for Glu630. The ALS gene sequences of confirmed hybrids were identical to that of the herbicide-resistant pollen source, CL161. We identified four ALS gene mutations in the herbicide-resistant red rice hybrids relative to the susceptible rice cultivars. One point mutation, resulting in a substitution of Ser653 with Asn, was linked to ALS resistance in callus tissue derived from a Kinmaze rice line from Japan. The other three mutations (Ser186—Pro, Lys416—Glu, and Leu662—Pro) are novel. This experiment confirmed that gene flow from imidazolinone-resistant rice resulted in herbicide-resistant red rice plants.

Available literature indicates that relatively few agricultural leaders and farmers became interested in weeds as a problem before 1200 A.D. or even 1500 A.D. For many centuries, weed control was mostly incidental to tillage for seedbed preparation and growing of crops and to growing and cutting or pasturing of thickly planted crops. Occasional references in literature previous to 1900 mentioned use of mechanical devices and a few inorganic herbicides specifically for weed control.

State weed laws directed at control of plant diseases were enacted during 1721 to 1766, but weed and seed laws involving weeds directly were not enacted until 100 to 200 years later. Only a few extension type publications on weeds were issued in the United States and Canada between 1860 and 1900. There was a rapid increase in such publications after 1900. Research with inorganic chemicals as herbicides was begun in the 1890's in Europe and in a few states and provinces, and was increased at a rapid pace until the early 1940's. New developments in mechanical and biological control of weeds increased steadily during the same period. However, weed control remained a relatively minor phase of agronomy, botany, horticulture, agricultural engineering, and plant physiology until the early 1950's.

About 10 years after the discovery of (2,4-dichlorophenoxy)acetic acid (2,4-D) in 1942–1944, the much increased interest of scientists, federal and state governments, industrial companies, and the general public had begun to bear fruit. The word “weed” or “weeds” began to appear in the titles of college courses and extension specialists. Weed conferences had been organized in six regions of the United States and Canada and in 10 states.

The first meeting of the Weed Science Society of America was held in 1956 and Weed Science was adopted as its official journal. The number of herbicides in general use in the United States and Canada increased from 15 in 1940 to 25 in 1950, and to 100 in 1969. The total support for weed research in 1962 in the United States was six times that in 1950. The number of full-time research and extension workers or their equivalents in part-time workers had increased 20-fold and 13-fold, respectively, over the number in 1940.

The rate of advancement in the art and science of weed control has increased so rapidly that the progress in each of the recent brief periods 1941 to 1968, 1901 to 1940, and 1800 to 1900 is considered greater than that in all previous periods, beginning about 6000 B.C.

Acetolactate synthase (ALS), a common enzyme in the biosynthesis of branched-chain amino acids, is a target for the sulfonylurea, imidazolinone, triazolopyrimidine, pyrimidinylthiobenzoate, and the sulfonylaminocarbonyltriazolinone classes of herbicides. Widespread resistance to the ALS-inhibiting herbicides has been attributed to single-base mutations in the ALS gene. The objective of this study was to investigate the feasibility of using denaturing high-performance liquid chromatography (DHPLC), a recently developed method of mutation analysis, for the detection of three ALS mutations, Ala122Thr, Leu574Trp, and Ser653Thr, which confer herbicide resistance. The mutated variants of the ALS gene were isolated from herbicide-resistant biotypes of smooth pigweed and Powell amaranth using polymerase chain reaction (PCR). The PCR products were hybridized with a wild-type sample and subjected to DHPLC analysis. All three mutations could be detected using a modified High performance liquid chromatography system; however, the sensitivity of the method was strongly dependent on the melting temperature profile of the analyzed DNA fragment. Once the primers and the DHPLC conditions are optimized, the procedure is economical, rapid, and requires little sample preparation. Because of these favorable features, DHPLC can be used as an alternative to other commonly used mutation detection methods.

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.

Studies were conducted to determine acifluorfen and lactofen absorption, translocation, and metabolism in protox-inhibiting herbicide-susceptible and -resistant common waterhemp. Acifluorfen and lactofen absorption was similar in both biotypes. Herbicide absorption was 12% in both susceptible and resistant common waterhemp 6 h after treatment (HAT). Absorption increased to 32 and 42% in susceptible and resistant plants, respectively, at 72 HAT. Translocation was similar in both biotypes for both herbicides. Herbicide translocation out of the treated leaf ranged between 5 and 15%. In a separate study, resistant common waterhemp plants were treated with acifluorfen or lactofen, alone or with tridiphane. Acifluorfen or lactofen injury to resistant common waterhemp was not altered with the addition of tridiphane. Treatments of 14C-acifluorfen or -lactofen on susceptible and resistant common waterhemp resulted in similar lactofen metabolism in both biotypes, but acifluorfen was not metabolized in either biotype within 24 HAT. This data indicate that differences in herbicide absorption, translocation, or metabolism are not the mechanism of common waterhemp resistance to protox-inhibiting herbicides.

Wild radish is a major weed in winter grain crops in Australia. This weed is poorly controlled by glufosinate. Therefore, factors influencing glufosinate efficacy in this species were examined. Dose–response studies conducted with three populations of wild radish collected from different parts of Australia and one from Europe showed poor control of all populations by glufosinate under Australian winter conditions. Studies conducted in controlled environmental chambers under night/day temperatures of 5/10, 15/20, and 20/25 C and various light intensities demonstrated that wild radish grown under cooler temperatures of 5/10 C were poorly controlled with 1,200 g ai ha−1 glufosinate when the same rate was sufficient to cause 100% mortality under 15/20 and 20/25 C. Light intensity did not significantly influence glufosinate activity at low temperatures. However, under warm temperatures of 20/25 C, glufosinate efficacy was enhanced with low light intensities. Experiments examining absorption and translocation of glufosinate showed that temperature did not have a significant effect on absorption of glufosinate. However, basipetal translocation of glufosinate was greatly increased by higher temperatures. Therefore, the poor control of wild radish by glufosinate at low temperatures is probably because of reduced accumulation of glufosinate in the meristematic regions of the plant.

False cleavers and catchweed bedstraw are problematic weeds of field crops in high-latitude regions of the northern Great Plains of North America. The abundance of these species has been increasing in areas of greater tillage intensity and frequency. Field experiments were established over 4 site-yr in Manitoba, Canada, and results indicated that the recruitment of false cleavers and catchweed bedstraw was strongly promoted by a single shallow tillage operation with a sweep cultivator in the spring. Percent recruitment levels in 2001 (pooled over sites) were 17 and 46% for the untilled and tilled treatments, respectively. In 2002, the percent recruitment levels for the untilled and tilled treatments, respectively, were 28 and 38% for the Komarno site and 13 and 28% for the Petersfield site. Only a few and minor differences in microsite conditions (soil temperature, soil moisture, and bulk density) resulted from the single spring tillage pass. The single tillage pass caused a significant relocation of simulated seeds (plastic beads) to below the soil surface and deeper into the soil profile (2–4 cm). Mean bulk density and volumetric soil moisture increased significantly with soil depth. The results of this study suggest that the positive effect of tillage on cleavers recruitment was not due to the effect of tillage on microsite conditions per se. Rather, the vertical redistribution of seed by tillage moved these seeds to a place in which the microsite conditions differed from those on the surface and were more favorable for recruitment. False cleavers and catchweed bedstraw recruitment is clearly promoted by tillage (even minor tillage) under field conditions, and farmers might be able to limit recruitment by limiting spring tillage.

This study reports evolved resistance to fenoxaprop-P in a population of sprangletop from a rice field in Thailand (BLC1). After eight applications of fenoxaprop-P, the herbicide appeared no longer effective. To confirm herbicide resistance in the BLC1 population, three experiments were conducted. First, glasshouse experiments revealed that the BLC1 population survived 600 g ai ha−1 of fenoxaprop-P without visual injury. Second, the BLC1 population was treated with fenoxaprop-P and other acetyl coenzyme A carboxylase (ACCase)–inhibiting herbicides (quizalofop-P, cyhalofop-butyl, and profoxydim) under field conditions; BLC1 exhibited resistance to all of these herbicides. Third, seeds of susceptible SLC1 and resistant BLC1 were germinated on 0.6% (v/v) agar across a range of herbicide concentrations. The resistant BLC1 population exhibited 61-, 44-, 9- and 8-fold resistance to fenoxaprop-P, cyhalofop, quizalofop-P, and profoxydim, respectively, compared with a susceptible SLC1 population. At the enzyme level, ACCase from the resistant BLC1 exhibited 30, 24, 11, 4, and 5 times resistance to fenoxaprop, cyhalofop-butyl, haloxyfop, clethodim, and cycloxydim, respectively. The spectrum of resistance at the whole plant level correlated well with resistance at the ACCase level. Hence, the mechanism of resistance to ACCase-inhibiting herbicides in this biotype of sprangletop is a herbicide-resistant ACCase. The specific mutation(s) of the ACCase gene that endows resistance in this population remains to be investigated.

A number of redroot pigweed and Powell amaranth populations from various locations in Ontario, Canada, have distinct patterns of resistance to the acetolactate synthase–inhibiting herbicides imazethapyr and thifensulfuron. This suggested the presence of diverse ALS gene mutations among these populations. Seven polymerase chain reaction primer pairs were used to amplify the gene to obtain full sequence information and to determine the identity of resistance-conferring mutations. There was a high degree of similarity in the ALS gene of the two species with only five nucleotides and one amino acid differing. A total of four herbicide resistance-conferring mutations were identified in the two species. The Ala122Thr, Ala205Val, and Trp574Leu amino acid substitutions were found in redroot pigweed whereas Ala122Thr, Trp574Leu, and Ser653Thr were detected in Powell amaranth. The pattern of resistance known to be conferred by the mutations concurred with the resistance level observed at the whole plant level. Distinct mutations being found in geographically separated populations suggest that selection for resistance occurred simultaneously in different locations. It reinforces the fact that resistance to ALS inhibitors is easily selected and that growers need to take this into account when formulating weed management strategies.

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.

Absorption and translocation of three commercial formulations of glyphosate, the isopropylamine salt formulated as Roundup Ultra™ (IPA1) and Roundup UltraMax™ (IPA2) and the diammonium salt formulated as Touchdown™ IQ (DA), were compared in three- to five-leaf velvetleaf, common waterhemp, and pitted morningglory. Absorption of 14C-glyphosate in velvetleaf was not significantly different among the three formulations up to 50 h after treatment (HAT). More absorption of 14C-glyphosate occurred in the IPA1 (26.0%) vs. the IPA2 (17.7%) formulation over 74 h. Of the total 14C-glyphosate absorbed, 20 to 35% was translocated from the treated leaf to the rest of the plant. Initial absorption of 14C-glyphosate was rapid in common waterhemp with the IPA1 (42.7%) and IPA2 (30.7%) formulations; both were higher compared with absorption of the DA formulation (11.5%) by 2 HAT. These differences continued up to 26 HAT, but no differences were evident by 74 HAT. Up to 65% of the 14C-glyphosate absorbed was translocated out of the treated leaf by 74 HAT, with roots the primary sink. Initial absorption of 14C-glyphosate was slow in pitted morningglory compared with the other species. More foliar absorption occurred in plants treated with the DA (13.6%) vs. the IPA2 formulation (4.9%) by 6 HAT. Absorption beyond 26 HAT was not different among the three glyphosate formulations. Translocation of 14C-glyphosate to roots was 27% greater as the DA salt than IPA1 and IPA2 by 74 HAT. The distribution pattern of glyphosate was similar in all species; phosphorimages demonstrated movement both acropetal and basipetal, with accumulation in roots greater than in any other plant parts. An efficacy study parallel to the 14C study showed no difference among the three glyphosate formulations on the species investigated at both 74 HAT and 2 wk after treatment.

The weed science discipline has changed significantly since 1970. New herbicides have been introduced, many herbicide-resistant weeds have been documented, herbicide-resistant crops have been introduced, federal laws affecting pesticides and weeds have been modified, the number of companies discovering and developing herbicides has been drastically reduced, basic studies on weed biology have received more emphasis, and integrated methods of controlling weeds with nonchemical as well as chemical methods have received increasing attention.

Characterizing the long-term effect of agricultural management systems on weed communities will aid in developing sustainable weed management practices. Weed seedbanks and aboveground biomass were measured within a corn–soybean–wheat crop sequence from 1990 through 2002 at Hickory Corners, MI. Four management systems were compared: conventional (CONV; full rates of N fertilizer and herbicides, moldboard tillage), no till (NT; same as CONV with no primary tillage), reduced input (RI; reduced rates of N fertilizer and herbicides, moldboard tillage, mechanical weed control, wheat underseeded with red clover), and organic (ORG; same as RI but no synthetic inputs). Multivariate ordinations of weed seedbanks showed a divergence of the CONV and NT systems from the RI and ORG systems. The CONV and NT seedbanks were dominated by grass species (mainly fall panicum and large crabgrass), whereas the RI and ORG systems were dominated by common lambsquarters and common chickweed. Within a single growing season, weed seedbanks in the RI and ORG systems were positively correlated with weed biomass whereas seedbanks in the CONV and NT system had little predictive value. Weed biomass from 1990 through 2002 showed a strong association of grass weed species with the corn phase of the CONV and NT system and common lambsquarters and redroot pigweed with the corn and soybean phases of the RI and ORG systems. Weed biomass diversity measures were negatively correlated with soybean yields in RI and ORG and wheat yields in NT, RI, and ORG. It is not clear whether crops were less competitive in the NT, RI, and ORG treatments, allowing new weed species to enter the plots, or whether less effective weed management in the NT, RI, and ORG treatments resulted in increased species richness, causing reduced crop yields. Mechanistic studies are needed to elucidate the relationship between weed community diversity and crop performance.

Row spacing and the relative time of velvetleaf emergence affects the time of soybean canopy closure relative to velvetleaf, influencing the growth and development of velvetleaf. Field studies were conducted in northeastern Nebraska in 2002 and 2003 to describe velvetleaf growth as influenced by soybean presence or absence (velvetleaf grown with soybean or in monoculture), soybean row spacing (19 and 76 cm), and relative time of velvetleaf emergence. Velvetleaf seed production, leaf area (LA), and total dry matter (TDM) were greater in 76-cm- than in 19-cm-wide soybean rows. LA, TDM, and seed production of velvetleaf were reduced with later emergence times in both monoculture and with soybean. Velvetleaf LA, TDM, and seed production decreased when grown with soybean compared with when grown in monoculture. Practical implications of this study suggest that narrowing crop row spacing and controlling early-emerging velvetleaf in soybean can be an effective part of an integrated weed management strategy.