Many conditions need to be satisfied for gene flow to occur between a transgenic crop and its weedy relatives. Flowering overlap is one essential requirement for hybrid formation. Hybridization can occur between canola and its wild relative, wild radish. We studied the effects of wild radish plant density and date of emergence, canola (glyphosate resistant) planting dates, presence of other weeds, and presence of a wheat crop on the synchrony of flowering between wild radish and canola (as a crop and volunteer). Four field experiments were conducted from 2000 to 2002 in St-David de Lévis, Québec. Flowering periods of wild radish emerging after glyphosate application overlapped with early-, intermediate-, and late-seeded canola 14, 26, and 55%, respectively, of the total flowering time. Flowering periods of early-emerging wild radish and canola volunteers in uncropped treatments overlapped from mid-June until the end of July, ranging from 26 to 81% of the total flowering time. Flowering periods of wild radish and canola volunteers emerging synchronously on May 30 or June 5 as weeds in wheat overlapped 88 and 42%, respectively, of their total flowering time. For later emergence dates, few flowers or seeds were produced by both species because of wheat competition. Wild radish density in canola and wild radish and canola volunteer densities in wheat did not affect the mean flowering dates of wild radish or canola. Increasing wild radish density in uncropped plots (pure or weedy stands) hastened wild radish flowering. Our results show that if hybridization is to happen, it will be most likely with uncontrolled early-emerging weeds in crops or on roadsides, field margins, and uncultivated areas, stressing the need to control the early flush of weeds, weedy relatives, and crop volunteers in noncrop areas.

Field and greenhouse experiments were conducted to evaluate the efficacy and safening of metsulfuron applied with dicamba, 2,4-D, clopyralid, and fluroxypyr with and without nonionic surfactant. Greenhouse data showed that 2,4-D and dicamba, but not fluroxypyr, safened grain sorghum from metsulfuron injury. In the field study, grain sorghum injury from metsulfuron was decreased when tank mixed with 2,4-D or dicamba but not when tank mixed with clopyralid or fluroxypyr. Tank mixes of 2,4-D or dicamba with metsulfuron did not reduce ivyleaf morningglory or velvetleaf control. At 4 wk after treatment (WAT), ivyleaf morningglory was controlled 95, 84, 59, and 91%, and velvetleaf was controlled 88, 82, 78, and 95% when metsulfuron was tank mixed with 2,4-D, dicamba, clopyralid, and fluroxypyr, respectively. In a separate field study, differential grain sorghum hybrid responses to a tank mix of metsulfuron + 2,4-D was examined. In general, a tank mix of metsulfuron and 2,4-D caused visible injury to all hybrids at 1 and 2 WAT, but grain sorghum recovered and most hybrids appeared normal at the end of the growing season. Differential hybrid responses to metsulfuron + 2,4-D were observed at 1 and 2 WAT in 2000 and 4 WAT in 2001. The most susceptible hybrid was ‘Mycogen 1506’, and the least susceptible hybrids were ‘NK KS-310’ and ‘Pioneer 87G57’. This study demonstrates the potential for 2,4-D or dicamba to safen metsulfuron injury of sorghum without compromising weed control.

Previous research has shown that flumioxazin, a herbicide being developed as a postemergence-directed spray (PDS) in cotton, has the potential to injure cotton less than 30 cm tall if the herbicide contacts green stem tissue by rain splash or misapplication. In response to this concern, five-leaf cotton plants with chlorophyllous stems and older cotton, 16-leaf cotton plants, with bark on the lower stem were treated with a PDS containing flumioxazin plus crop oil concentrate (COC) or nonionic surfactant (NIS). Stems of treated plants and untreated plants at the respective growth stage were cross-sectioned and then magnified and photographed using bright-field microscopy techniques. More visible injury consisting of necrosis and desiccation was evident in younger cotton. Also, there was a decrease in treated-stem diameter and an increase in visible injury with COC compared with NIS in younger cotton. The effects of plant growth stage and harvest time on absorption, translocation, and metabolism of 14C-flumioxazin in cotton were also investigated. Total 14C absorbed at 72 h after treatment (HAT) was 77, 76, and 94% of applied at 4-, 8-, and 12-leaf growth stages, respectively. Cotton at the 12-leaf stage absorbed more 14C within 48 HAT than was absorbed by four- or eight-leaf cotton by 72 HAT. A majority (31 to 57%) of applied 14C remained in the treated stem for all growth stages and harvest times. Treated cotton stems at all growth stages and harvest times contained higher concentrations (Bq g−1) of 14C than any other tissues. Flumioxazin metabolites made up less than 5% of the radioactivity found in the treated stem. Because of the undetectable levels of metabolites in other tissues when flumioxazin was applied PDS, flumioxazin was foliar applied to determine whether flumioxazin transported to the leaves may have been metabolized. In foliar-treated cotton, flumioxazin metabolites in the treated leaf of four-leaf cotton totaled 4% of the recovered 14C 72 HAT. Flumioxazin metabolites in the treated leaf of 12-leaf cotton totaled 35% of the recovered 14C 48 HAT. These data suggest that differential absorption, translocation, and metabolism at various growth stages, as well as the development of a bark layer, are the bases for differential tolerances of cotton to flumioxazin applied PDS.

A set of universal and degenerate primers has been designed to clone (by polymerase chain reaction [PCR]) the conserved domains of the acetolactate synthase (ALS) gene where mutations confer resistance to ALS herbicides in plants. These primers were successful in cloning conserved domains of ALS in all monocotyledonous and dicotyledonous plants tested to date, as well as that of bacteria. Total genomic DNA was used as the source of target DNA because no introns were found in the sequences to be amplified. The design of the universal primers was performed after subtle modifications of the consensus degenerate hybrid oligonucleotide primers approach, which implies the synthesis of hybrid oligonucleotide primers containing fixed clamp 5′ and degenerate core 3′ sequences. Optimizations of PCR reactions were done according to a Taguchy approach described for the first time with degenerate oligonucleotides. This method optimizes a PCR reaction using four variables (deoxynucleoside triphosphate, DNA, primers, and Mg2+) under three different concentrations per variable using just nine reactions. The ALS herbicide-binding domains from many susceptible and resistant plants can be cloned and sequenced in a few hours by using only 100 mg of starting plant material, like one leaf or several small seedlings or seeds.

Selective control of Egyptian broomrape is extremely difficult because the close association between host crop and parasite limits the use of most mechanical and herbicidal approaches. However, this host–parasite interaction can also form the basis of the simplest control strategy: parasite-resistant crops. Although much work has been conducted to identify and characterize mechanisms of parasite resistance, varieties with stable resistance are still unavailable for most affected crops. The development of resistant crops can be accelerated by genetic engineering to the extent that important aspects of the host–parasite interaction are understood. In this study, we characterize a variety of gene promoter elements with respect to parasite induction and expression pattern. Studies were conducted using transgenic plants expressing fusions of the β-glucuronidase reporter gene with promoter elements from several genes. Promoters from genes known to have increased expression in response to pathogen attack or wounding showed localized, induced expression after parasitism. These included phenylalanine ammonia lyase, chalcone synthase, sesquiterpene cyclase, and HMG1 (3-hydroxy-3-methylglutaryl CoA reductase). In contrast, the systemic acquired resistance–associated gene PR-1a was not induced by parasitism. Non–defense-related genes varied in response, with squalene synthase being repressed, whereas farnesyltransferase was highly expressed in the region of parasite attachment. These results demonstrate a range of expression, both in intensity and tissue specificity, in response to parasitism.

Leafy spurge can be detected during flowering with either aerial photography or hyperspectral remote sensing because of the distinctive yellow-green color of the flower bracts. The spectral characteristics of flower bracts and leaves were compared with pigment concentrations to determine the physiological basis of the remote sensing signature. Compared with leaves of leafy spurge, flower bracts had lower reflectance at blue wavelengths (400 to 500 nm), greater reflectance at green, yellow, and orange wavelengths (525 to 650 nm), and approximately equal reflectances at 680 nm (red) and at near-infrared wavelengths (725 to 850 nm). Pigments from leaves and flower bracts were extracted in dimethyl sulfoxide, and the pigment concentrations were determined spectrophotometrically. Carotenoid pigments were identified using high-performance liquid chromatography. Flower bracts had 84% less chlorophyll a, 82% less chlorophyll b, and 44% less total carotenoids than leaves, thus absorptance by the flower bracts should be less and the reflectance should be greater at blue and red wavelengths. The carotenoid to chlorophyll ratio of the flower bracts was approximately 1:1, explaining the hue of the flower bracts but not the value of reflectance. The primary carotenoids were lutein, β-carotene, and β-cryptoxanthin in a 3.7:1.5:1 ratio for flower bracts and in a 4.8:1.3:1 ratio for leaves, respectively. There was 10.2 μg g−1 fresh weight of colorless phytofluene present in the flower bracts and none in the leaves. The fluorescence spectrum indicated high blue, red, and far-red emission for leaves compared with flower bracts. Fluorescent emissions from leaves may contribute to the higher apparent leaf reflectance in the blue and red wavelength regions. The spectral characteristics of leafy spurge are important for constructing a well-documented spectral library that could be used with hyperspectral remote sensing.

A 2-yr experiment assessed the potential for using soil degree days (DD) to predict cumulative weed emergence. Emerged weeds, by species, were monitored every 2 wk in undisturbed plots. Soil DD were calculated at each location using a base temperature of 9 C. Weed emergence was fit with logistic regression for common ragweed, common lambsquarters, velvetleaf, giant foxtail, yellow foxtail, large crabgrass, smooth pigweed, and eastern black nightshade. Coefficients of determination for the logistic models fit to the field data ranged between 0.90 and 0.95 for the eight weed species. Common ragweed and common lambsquarters were among the earliest species to emerge, reaching 10% emergence before 150 DD. Velvetleaf, giant foxtail, and yellow foxtail were next, completing 10% emergence by 180 DD. The last weeds to emerge were large crabgrass, smooth pigweed, and eastern black nightshade, which emerged after 280 DD. The developed models were verified by predicting cumulative weed emergence in adjacent plots. The coefficients of determination for the model verification plots ranged from 0.66 to 0.99 and averaged 0.90 across all eight weed species. These results suggest that soil DD are good predictors for weed emergence. Forecasting weed emergence will help growers make better crop and weed management decisions.

Trifloxysulfuron controls green and false-green kyllinga more effectively than halosulfuron. Studies were conducted to evaluate the absorption, translocation, and metabolism of 14C-halosulfuron and 14C-trifloxysulfuron when foliar applied to green and false-green kyllinga. No differences were observed between the two kyllinga spp. with regard to absorption, translocation, or metabolism of either herbicide. The majority of 14C-halosulfuron and 14C-trifloxysulfuron was absorbed by 4 h, with an accumulation of 63 and 47% radioactivity, respectively. Accumulation of both herbicides occurred in the treated leaf and the primary shoot from whence the treated leaf was removed, with minor accumulation occurring in the roots and newly formed rhizomes. Of the total amount of 14C-halosulfuron absorbed into the plant, 77% remained in the form of the parent compound compared with 61% of 14C-trifloxysulfuron. The parent compound was distributed mainly in the treated leaf and primary shoot, whereas polar metabolites were concentrated in the roots and rhizomes. Nonpolar metabolites of 14C-trifloxysulfuron accumulated in the treated leaf and primary shoot. These data indicate that absorption, translocation, or metabolism could not explain the variation in green and false-green kyllinga control between halosulfuron and trifloxysulfuron.

Fifty-three fungal strains belonging to 15 mainly Fusarium species were isolated from branched broomrape plants. Their virulence was assessed using a plastic bag system, and they were grown both in liquid and on solid media, extracted, and the extracts were chemically analyzed and biologically assayed to find new metabolites that inhibit germination of branched broomrape to estimate the production of fusaric and dehydrofusaric acids by Fusarium strains and evaluate their possible involvement as virulence factors and their practical use as biomarkers to make the selection of potential mycoherbicides easier, and to ascertain whether toxins affected mammals. Nine strains proved to be highly virulent and 18 strains produced fusaric and dehydrofusaric acid at concentrations from 4 to 165, and from 9 to 204 mg L−1 respectively. Fifteen extracts from solid cultures caused high mortality when assayed on brine shrimps. Five extracts from liquid cultures caused total inhibition of seed germination. Some strains may be considered as sources of new biocontrol agents but virulence was not always positively correlated to the production of phytotoxic compounds.

Florida beggarweed is native to the Western Hemisphere but is naturalized around the world. During the last century, the mechanization of agriculture has transitioned Florida beggarweed from an important forage component to a weed of significance in the coastal plain of the southeast United States. This herbaceous annual is naturalized and found in fields and disturbed areas throughout the southern United States. The characteristics that made Florida beggarweed a good forage crop also make it a formidable weed. This review describes the importance of Florida beggarweed as a weed in the southern United States and the taxonomy of this species and details the distribution throughout the world and within the United States. The ecology of Florida beggarweed and its interactions with crop plants, insects, nematodes, and plant pathogens also are summarized. Finally, management of Florida beggarweed in agricultural systems using cultural practices and herbicides is reviewed.

Populations of wild radish were collected from two fields in the northern Western Australian wheatbelt, where typical herbicide-use patterns had been practiced for the previous 17 seasons within an intensive crop production program. The herbicide resistance status of these populations clearly established that there was multiple-herbicide resistance across many herbicides from at least four modes of action. One population exhibited multiple-herbicide resistance to the phytoene desaturase (PDS)–inhibiting herbicide diflufenican (3.0-fold), the auxin analog herbicide 2,4-D (2.2-fold), and the photosystem II–inhibiting herbicides metribuzin and atrazine. Another population was found to be multiply resistant to the acetolactate synthase–inhibiting herbicides, the PDS-inhibiting herbicide diflufenican (2.5-fold), and the auxin analog herbicide 2,4-D amine (2.4-fold). Therefore, each population has developed multiple-herbicide resistance across several modes of action. The multiple resistance status of these wild radish populations developed from conventional herbicide usage in intensive cropping rotations, indicating a dramatic challenge for the future control of wild radish.

Herbicides currently registered for use near water have been ineffective for control of perennial pepperweed. Previous research has demonstrated that mowing followed by an application of glyphosate at 3.33 kg ae ha−1 to resprouting tissue can enhance the control of perennial pepperweed. The objectives of this study were to determine the mechanism(s) responsible for the enhanced effectiveness of glyphosate in combination with mowing. Mowing plants altered the leaf area distribution within the canopy. In mowed areas, the majority of leaf area was in the basal third of the canopy, whereas the bulk of the leaf area was in the top third of the canopy in unmowed plots. This change in plant architecture affected the deposition pattern of the spray solution. Unmowed plants retained 49 to 98% and 42 to 83% of a dye solution within the middle and top thirds of the canopy at the Colusa and Woodland sites, respectively, with only 1.9 to 6.0% dye deposited on the basal third of the canopy at both sites. In contrast, mowed plants had 18 to 34% and 26 to 70% of the dye retained in the basal third of the canopy at the Colusa and Woodland sites, respectively. Greenhouse studies showed that 14C-glyphosate applied to basal leaves of mowed plants translocated significantly more to belowground tissue. Unmowed plants accumulated 0.37% of the applied 14C-glyphosate in belowground tissue 48 h after labeling. In contrast, mowed plants accumulated 6.7% 14C-glyphosate in the belowground tissue. In field studies, estimates of basipetal seasonal translocation rates using total nonstructural carbohydrate pools of roots indicate that mowing did not change the translocation rate. However, the delay in application timing to allow plants to resprout appeared to synchronize applications with maximal translocation of carbohydrates to belowground structures. We hypothesize that the change in the canopy structure of perennial pepperweed after mowing results in fewer aboveground sinks and greater deposition of herbicide to basal leaves where it can preferentially be translocated to the root system. Furthermore, the delay between mowing and resprouting synchronized maximal belowground translocation rates with herbicide application timing. These factors all appear to be involved in the observed enhanced control of perennial pepperweed when combining mowing and glyphosate.

Foramsulfuron (AE F130360) is a sulfonylurea herbicide for postemergence control of grasses and some broadleaf weeds in corn. Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of corn hybrids to foramsulfuron. Differences in corn tolerance were quantified by determining the herbicide rate required to cause injury and reduce corn height by 15% (GR15). Seven hybrids were screened in the greenhouse where GR15 values indicated that the some of the corn hybrids were sensitive to foramsulfuron. All but one of these hybrids exhibited an increase in tolerance when the safener, isoxadifen-ethyl, was applied with foramsulfuron. Experiments using 14C-foramsulfuron were conducted to determine whether isoxadifen-ethyl affected foramsulfuron absorption, translocation, and metabolism in two of the corn hybrids. There was no difference in absorption between a sensitive hybrid Novartis 58D1 and a more tolerant hybrid Novartis 59Q9, but the addition of isoxadifen-ethyl increased foramsulfuron absorption in both hybrids 24 h after treatment. Less than 1 and 3% of the 14C-foramsulfuron was translocated to plant portions above and below the treated leaf, respectively. Differences in hybrid tolerance were primarily due to differential herbicide metabolism. Foramsulfuron metabolism, with and without isoxadifen-ethyl, was similar at 4 h after treatment for both the sensitive and tolerant hybrids. However, by 24 h after treatment the more tolerant hybrid metabolized foramsulfuron to more polar compounds to a greater degree than the sensitive hybrid.

Surviving rigid ryegrass plants were collected from a cropping field at Pindar, Western Australia (population WALR 50), after inadequate control by glyphosate applied at the normal field rate. Plants were grown to maturity in pots and seeds were collected. Glyphosate dose–response experiments with known susceptible and resistant control populations confirmed the resistant status of the WALR 50 population. The glyphosate rate resulting in 5% mortality (LD50) and GR50 (the glyphosate rate required to reduce mean growth of individuals to 50% of the untreated control) values for this population were 1,069 and 217 g ae ha−1, respectively, corresponding to R:S ratios of 3.4 and 1.9 for mortality and growth. In addition, a novel root growth–based assay of glyphosate resistance was developed and validated, giving a root growth GR50 R:S ratio of 3.4. A resistance profile was established by assessing population-level survival of WALR 50 after applications at recommended rates of a range of herbicides commonly used for rigid ryegrass control in Australia. High levels of resistance to the acetolactate synthase (ALS)–inhibiting sulfonylurea herbicides chlorsulfuron and sulfometuron, moderate resistance to the acetyl coenzyme A carboxylase (ACCase)–inhibiting herbicide diclofop, and low levels of resistance to the imidazilinone herbicide imazethapyr were found. More detailed dose–response experiments confirmed resistance to chlorsulfuron, sulfometuron, and diclofop. In vitro enzyme-inhibition studies demonstrated that ALS resistance in WALR 50 is due to an insensitive target enzyme and that ACCase resistance is due to a nontarget site–based mechanism. WALR 50 is the first glyphosate-resistant weed population with confirmed resistance to ACCase- and ALS-inhibiting herbicides.

Common waterhemp seeds were collected from two Missouri soybean fields where biotypes were not controlled by acifluorfen. Plants grown from these seeds were tested for resistance to the diphenyl ether herbicides acifluorfen and lactofen. Resistance to susceptibility (R/S) ratios, calculated as the ratio of the dose required to inhibit dry weight accumulation by 50% (GR50) in resistant plants to that for susceptible plants, were 9.5 and 11 for the Meadville biotype and 28 and 44 for the Bethel biotype exposed to acifluorfen and lactofen, respectively. Electrolyte leakage assays determined that light-induced lipid peroxidation by acifluorfen was greatest on a control population (Bradford), intermediate for the Meadville biotype, and lowest for the Bethel biotype. Levels of the photodynamic pigment protoporphyrin IX (Proto) accumulating in leaf disks exposed to acifluorfen were much lower in the resistant biotypes than in the susceptible wild type, and the level of Proto accumulation was significantly correlated to the degree of membrane disruption. Although the binding of acifluorfen to protoporphyrinogen oxidase in chloroplasts may have been altered in the resistant biotypes, the molecular and biochemical factors involved in the mechanism of resistance remain to be fully characterized. However, this study establishes that the physiological basis for the evolved resistance to diphenyl ethers in common waterhemp rests on the reduction of Proto accumulation.

Tomato and pepper differ in their whole-plant tolerance to sulfonylurea (SU) herbicides rimsulfuron and halosulfuron despite both being members of the Solanaceae family. This study examined whether tomato's tolerance to SU herbicides rimsulfuron and halosulfuron was due to insensitivity of the target enzyme acetolactate synthase (ALS). Rimsulfuron and halosulfuron inhibited ALS from both tomato and pepper leaves. Enzyme inhibition and kinetic analyses showed that extractable ALS from tomato was more sensitive to rimsulfuron and halosulfuron than ALS from pepper. ALS from both species were inhibited with a mixed inhibition pattern. Thus, results indicate that enzyme insensitivity is not the reason why tomato is more tolerant than pepper to these herbicides. Tomato tolerance to rimsulfuron at the whole-plant level was reduced in the presence of terbufos, a known inhibitor of cytochrome P450 enzymes. Rimsulfuron applied at 0.018 and 0.035 kg ha−1 with 1.1 kg ha−1 of terbufos reduced tomato shoot weight 69 and 66%, respectively, compared with a 4 and 29% reduction when rimsulfuron was applied alone. The reduction of tomato tolerance to rimsulfuron by terbufos suggests that the sensitivity differences between these species may reflect their differences in SU herbicide metabolism.

The mechanism of glyphosate resistance in horseweed was investigated. Eleven biotypes of putative sensitive (S) and resistant (R) horseweed were obtained from regions across the United States and examined for foliar retention, absorption, translocation, and metabolism of glyphosate. Initial studies used spray application of 14C-glyphosate to simulate field application. When S and R biotypes were compared in the absence of toxicity at a sublethal dose, we observed comparable retention and absorption but reduced root translocation in the R biotypes. S and R biotypes from Delaware were further examined at field use rates and results confirmed similar retention and absorption but reduced root translocation in the R biotypes. Application of 14C-glyphosate to a single leaf demonstrated reduced export out of the treated leaf and lower glyphosate import into other leaves, the roots, and the crown in R relative to S biotypes. Examination of the treated leaf by autoradiography showed that glyphosate loading into the apoplast and phloem was delayed and reduced in the R biotype. Our results consistently showed a strong correlation between impaired glyphosate translocation and resistance. Tissues from both S and R biotypes showed elevated levels of shikimate suggesting that 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) remained sensitive to glyphosate. Analysis of tissue shikimate levels demonstrated reduced efficiency in EPSPS inhibition in the R biotypes. Our results suggest that resistance is likely due to altered cellular distribution that impaired phloem loading and plastidic import of glyphosate resulting in reduced overall translocation as well as inhibition of EPSPS.

A biotype of green foxtail found in Spain exhibited cross-resistance among acetyl-CoA carboxylase (ACCase)–inhibiting herbicides. Field doses that totally inhibited shoot fresh weight in the susceptible (S) biotype were determined for six aryloxyphenoxypropionates (clodinafop, diclofop, fenoxaprop-P, fluazifop-P, haloxyfop-P, and propaquizafop) and six cyclohexanediones (clefoxydim, clethodim, cycloxydim, sethoxydim, tepraloxydim, and tralkoxydim). The resistant (R) biotype showed cross-resistance to all herbicides except fenoxaprop-P, propaquizafop, clefoxydim, and tepraloxydim. There were no differences in the absorption, translocation, and metabolism of [14C]diclofop between the S and R biotypes. On the basis of herbicide dose that inhibited ACCase activity by 50% (I50 values), ACCase of the R biotype was 5.8-, 13.9-, 20.0-, 102.4-, 416.7-, and 625.0-fold less sensitive to clethodim, haloxyfop, diclofop, fluazifop, cycloxydim, and sethoxydim, respectively, than that of the S biotype. Two multifunctional ACCase isoforms (ACCase I and ACCase II) were purified partially and separated. ACCase II was highly resistant to diclofop acid in both biotypes, with I50 values ranging between 92 and 95 μM. However, the I50 values observed for ACCase I revealed that the R biotype was 30.8-fold less sensitive to diclofop than the S biotype. These results suggest the mechanism of resistance in green foxtail to diclofop relates to an altered ACCase I isoform.

Broom and threadleaf snakeweed are major rangeland weeds in the western United States, and picloram is the major herbicide used for their management. Previous work has shown that these species are most susceptible to picloram applied in autumn or when precipitation is high and that differences in herbicide absorption and tissue sensitivity as measured by picloram-induced ethylene production do not fully explain variation in seasonal response. Therefore, the role of picloram metabolism in seasonal susceptibility to picloram was examined. Because snakeweed is characterized as highly genetically variable, picloram metabolism was evaluated monthly for 3 yr among populations from two species as well. Picloram metabolism was examined monthly for 3 yr among two populations of threadleaf and nine populations of broom snakeweed grown in a common garden. Metabolism ranged from 30 to 70% of picloram applied, and picloram was converted to two metabolites more polar than picloram regardless of species or population. Although metabolism was greatest in the year with the most precipitation, rate of metabolism was unrelated to precipitation received in the 7-d period before treatment. Application timing as defined by a given month or specific phenological stage was not related to the level of metabolism. We conclude that variation in picloram metabolism is not involved in differential susceptibility across season or population.