“Weedy” red rice is a problematic weed with phenotypic similarities to cultivated rice. Limited herbicide availability has driven a need for nonchemical control options for managing this pest. One preplanting strategy that is being explored is the stale seedbed methodology, which aims to maximize soil seedbank withdrawals via germination. This technique is adapted in rice by flooding a field, waiting for germination and emergence of weed seedlings, and completing the method with a mechanical or chemical control application. Optimization of this process is dependent on maximizing weed seed germination, which is primarily influenced by both temperature and moisture availability. Germinability across a range of these factors is not well understood in California red rice. Thus, this study aimed to determine germinability of California red rice accessions under various temperature and water potential treatments. Previously described red rice accessions 1, 2, 3, and 5, along with ‘M206’, a common California rice cultivar, were exposed to temperatures from 10 to 40 C in 5 C increments in combination with water potentials of 0, −0.2, −0.4, or −0.8 MPa until either germination or weed seed decay occurred. Statistical analysis indicated a three-way interaction between accession, temperature, and water potential. Germination reached 95% or greater when seeds were exposed to temperatures from 20 to 35 C in combination with 0 or −0.2 MPa. Germination was lowest when seeds were water stressed (−0.8 MPa) and when temperatures were colder than 20 C or warmer than 35 C. The ‘M206’ cultivar was utilized for comparison and demonstrated cold tolerance by germinating at 10 C, whereas weedy accessions 1, 2, and 3 did not. When temperatures were at or above 15 C, however, ‘M206’ germinated less often compared with all weedy accessions. Historical preplant temperatures in this region align with those required for weedy rice germination. Thus, the stale seedbed methodology is a viable strategy in years when ample floodwater is available.

Clomazone is a widely used herbicide in California water-seeded rice for control of bearded sprangletop and watergrass. Generally, clomazone is applied to a flooded rice field at day of rice seeding. However, interest exists among growers to delay the clomazone application. Weather variability may encourage growers to practice Leathers’ method. Leathers’ method is the practice of draining the field 1 to 2 d after air seeding to encourage better and more uniform seedling establishment, then reflooding back to a 10- to 15-cm flood 4 to 7 d later. Therefore the objective of this study was to evaluate grass weed control and rice response at four rates of clomazone, applied at two timings: at day of seeding (DOS) in a continuous 10-cm flood and after Leathers’ method. This study was conducted in 2019 and 2020 at the Rice Experiment Station in Biggs, CA. In 2019, there were no difference across clomazone rates on control of bearded sprangletop independent of application timing used; however, in 2020, bearded sprangletop control with clomazone applied after Leathers’ method was 70% to 71% across clomazone rate by 60 d after treatment (DAT), compared to 92% to 97% in the DOS applications. Watergrass control was 100% in 2019 across clomazone rate and application timing. However, in 2020, watergrass control was greater at the DOS application at 54% to 71%. Clomazone applied at the 0.7 kg ha−1 Leathers’ method resulted in 84% bleaching by 14 DAT and was similar across all Leathers’ method clomazone applications and the 0.7 kg ha−1 DOS application. There was no rice grain yield difference among all clomazone-treated plots, with the exception of the 0.7 kg ha−1 Leathers’ method interaction with the DOS applications.

Water seeding is a common cropping strategy in mechanized rice systems. Water seeding of rice can suppress grass weeds, but it can also encourage aquatic weeds and grass ecotypes that escape deep floodwater. In addition, water seeding prevents many cultural methods of weed control and limits available herbicides. Selection pressure from a limited palette of herbicides has resulted in widespread resistance in rice grown in California. This study examined a novel combination of drill seeding and a stale seedbed (“stale-drill”) as a means of using a nonselective herbicide to manage weeds before rice emergence. In 2016 and 2017, rice cultivar ‘M-206’ was drilled at a rate of 120 kg ha−1 to 1.3-cm, 2.5-cm, and 5.1-cm depths. Planting rice deeper than 1.3 cm delayed emergence by 3 to 4 d. A postplant-burndown (PPB) treatment of glyphosate at 870 g ha−1 was applied just prior to rice emergence. Treatment delays had mixed effects on weed control. PPB treatment was more effective at controlling Echinochloa spp. in 2017, reducing density by 30%, 48%, and 73% at 1.3-cm, 2.5-cm, and 5.1-cm seeding depths, respectively. The greatest overall weed control either year was found with applications of glyphosate + pendimethalin followed by penoxsulam + cyhalofop at 1.3-cm planting depth. Rice stand and yield components were more strongly affected by planting depth in 2017 than in 2016, possibly owing to cool weather immediately after seeding. Yields in 2017 were reduced in deeper plantings by up to 72%. Therefore, if the stale-drill method is implemented with higher-vigor cultivars or higher seeding rates, we see potential in this method as a useful tool for reducing herbicide-resistant weeds in rice fields.

Weedy rice (Oryza sativa f. spontanea Roshev.) has recently become a significant botanical pest in California rice (Oryza sativa L.) production systems. The conspecificity of this pest with cultivated rice negates the use of selective herbicides, rendering the development of nonchemical methods a necessary component of creating management strategies for this weed. Experiments were conducted to determine the emergence and early growth responses of O. sativa spontanea to flooding soil and burial conditions. Treatment combinations of four flooding depths (0, 5, 10, and 15 cm) and four burial depths (1.3, 2.5, 5, and 10 cm) were applied to test the emergence of five O. sativa spontanea accessions as well as ‘M-206’, a commonly used rice cultivar in California, for comparison. Results revealed that burial depth had a significant effect on seedling emergence. A 43% to 91% decrease in emergence between seedlings buried at 1.3 and 2.5 cm depending on the flooding depth and accession and an absence of emergence from seedlings buried at or below 5 cm were observed. Flooding depth did not affect emergence, but there was a significant interaction between burial and flooding treatments. There was no significant difference between total O. sativa spontanea emergence from the soil and water surfaces regardless of burial or flooding depths, implying that once the various accessions have emerged from the soil they will also emerge from the floodwater. Most accessions had similar total emergence compared with M-206 cultivated rice but produced more dry weight than M-206 when planted at 1.3 cm in the soil. The results of this experiment can be used to inform stakeholders of the flooding conditions necessary as well as soil burial depths that will promote or inhibit the emergence of California O. sativa spontanea accessions from the weed seedbank.

California rice (Oryza sativa L.) production has been recently challenged by the early-season nuisance algae blooms. The algal community in rice is a complex of green algae (Nostoc spongiforme Agardh ex Bornet) and cyanobacteria species that can develop a thick algal mat on the surface of the water and interfere with the emergence and establishment of rice seedlings. The objective of this research was to determine the impact of algal infestation level on rice seedling emergence. A mesocosm study was conducted in 57-L tubs. Three levels of algal infestation (low, medium, and high) were produced by adding fertilizer (N:P) into the tubs at 0:0, 75:35, and 150:70 kg−1 ha. Sixty rice seeds (‘M-206’) were soaked for 24 h and spread into tubs filled with water. Photosynthetic active radiation (PAR), chlorophyll a concentration as the quantitative measure of algae, number of emerged rice seedlings, and their dry biomass were studied during the experiment. Results showed that algal infestation can directly change the amount of light received into the water. Minimum, maximum, and mean percentage of PAR inside the water declined as the algal infestation level increased. As a consequence, rice seedling emergence dropped under the high algal pressure. At very high algal infestation (i.e., chlorophyll a concentration of above 500 µg ml−1), rice seedling emergence was reduced up to 90%. Furthermore, rice seedling emergence was delayed under algal infestation. When algal infestation was low, time to 50% rice seedling emergence (t50) ranged between 5 and 10 d, while at high algal infestation, t50 ranged between 12 and 20 d. Moreover, individual rice seedling biomass was reduced from 1 g to 0.01 g as algal infestation increased. The results from this study indicate that uncontrolled algae at the beginning of the rice-growing season could reduce rice seedling emergence, establishment, and rice stand. Given that algal infestation in fields has a patchy pattern, loss of rice stand in these patches could provide empty niches for other weeds to grow.

Late watergrass is a competitive weed of rice that is well adapted to both aerobic and anaerobic environments. Cultural controls such as a stale-seedbed and alternating from wet- to dry-seeding have been proposed as management options. However, the effects of these systems on its emergence and early growth are unknown. The objective of this study was to modify a previously developed population-based threshold model (PBTM) to predict emergence and early growth under field conditions. In 2013, a series of experiments were conducted at the California Rice Experiment Station (CRES) in Biggs, CA, to evaluate emergence and early growth of multiple herbicide–resistant and -susceptible late watergrass at four burial depths (0.5, 2, 4, and 6 cm) under three irrigation regimes: continuously flooded (CF), daily flush (DF), and intermittent flush (IF). Resistant plants emerged at a significantly higher rate under the IF treatment (P < 0.05). Both biotypes showed decreasing emergence with increasing depth, and no plants emerged from the 4- or 6-cm depths in the CF treatment. Using the Gompertz growth curve, resistant plants had greater predicted growth rates (k), lower predicted maximum heights (h max ), and a shorter time to predicted maximum growth rate (t m ) than susceptible plants under the CF and DF treatments. Under the IF treatment, the susceptible plants had greater k, lower h max , and shorter time to predicted t m . Information about burial depth and irrigation was incorporated into a previously developed PBTM for late watergrass, and validated at the CRES in a field with a susceptible late watergrass population in 2013 and 2014, under two irrigation systems, CF and IF. Model fit was best in the CF treatments (average Akaike information criteria [AIC] = 199.05) compared to the IF treatments (average AIC = 208.6).

Weedy rice (Oryza sativa f. spontanea Rosh.) is an emerging weed of California rice (Oryza sativa L.) that has potential to cause large yield losses. Early detection of weedy rice in the field is ideal to effectively control and prevent the spread of this weed. However, it is difficult to differentiate weedy rice from cultivated rice during early growth stages due to the close genetic and phenotypic relatedness of cultivated rice and weedy rice. The objective of this study is to examine phenotypic variation in weedy rice biotypes from California and to identify traits that could be used to visually identify weedy rice infestations at early growth stages for effective management. Greenhouse experiments were conducted in 2017 and 2018 using five phenotypically distinct biotypes of weedy rice found in California, along with diverse cultivated, weedy, and wild rice types in a randomized complete block design. We measured variation for 13 phenotypic traits associated with weedy rice and conducted principal component analysis and factor analysis to identify important weedy traits. Most weedy rice individuals within a biotype clustered together by phenotypic similarity. Pericarp color, hull color, chlorophyll content, grain length, plant height, leaf pubescence, collar color, and leaf sheath color account for most of the observed variation. California weedy rice biotypes are phenotypically distinct from wild rice and from weedy rice from the southern United States in their combinations of seed phenotypes and vegetative characteristics. In comparison with the locally grown temperate japonica cultivars, California weedy rice tends to be taller, with lower chlorophyll content and a red pericarp. Weedy rice biotypes vary in seed shattering and seed dormancy. For weedy rice management, plant height and chlorophyll content are distinct traits that could be used to differentiate weedy rice from the majority of cultivated rice varieties in California during vegetative stages of rice growth.

Weedy rice is an emerging problem of cultivated rice in California. Infestations of weedy rice in cultivated rice result in yield loss and reduced grain quality. In this study, we aimed to evaluate growth and yield components of a widely grown cultivated rice variety in California in response to weedy rice competition. Greenhouse competition experiments in an additive design were conducted in 2017 and 2018 to determine the growth and yield components of ‘M-206’ rice and five weedy rice biotypes found in California at varying weed densities. M-206 rice initially grew at a faster relative growth rate of 0.53 cm−1 wk−1 under competitive conditions compared with 0.47 cm−1 wk−1 in the absence of weedy rice, but absolute and relative growth rates declined more rapidly under competitive conditions as plants approached maturity. At harvest, M-206 plant height was reduced 13% under competitive conditions, and M-206 tiller number was reduced 23% to 49%, depending on the weedy rice biotype it was competing with. Except for 100-grain weight, the growth traits and grain yield components of M-206 rice were reduced with increasing density of weedy rice. At the highest weed density measured, 40 plants m−2, M-206 rice had yield losses of 69% grain yield plant−1, 69% panicle weight, 59% fresh and dry biomass, 55% grain yield panicle−1, and 54% panicle number. The five evaluated weedy rice biotypes varied widely in early growth rates, height, biomass production, and grain yield, indicating differing competitive strategies. Most weedy rice biotypes produce plants with greater plant height, tiller number, panicle number, and above- and below-ground biomass compared with cultivated rice. Weedy rice biotypes produced 45% to 57% higher grain yield per plant than M-206 rice under competitive conditions.

Bearded sprangletop is a problematic weed in California rice production and few herbicides provide effective control. As control of bearded sprangletop has declined, grower suspicion of resistance to clomazone has increased, because of the continuous rice cropping system and herbicide dependence in the region. The objectives of this research were to confirm clomazone resistance in bearded sprangletop populations and determine the level of resistance. Seed from 21 suspected clomazone-resistant populations was collected from the California rice growing region. A greenhouse experiment was conducted to determine population sensitivity to clomazone. Clomazone was applied into the water to emerging seedlings. Plant ht and control of bearded sprangletop were recorded weekly for 3 wk, plants were then harvested, and dry weight was measured. Of the populations tested, 17 were susceptible and four (5%) were resistant to clomazone. A dose-response assay was conducted using eight doses ranging from an eighth of the full rate to 12 times the full rate. The three most resistant populations had resistant-to susceptible ratios of 1.25×, 2×, and 5× the labeled rate of clomazone. The use of clomazone in California rice production is beneficial; however, it should be used at the appropriate timing and as part of an herbicide program to prevent further development of clomazone resistance.

The repetitive use of ALS inhibitors for smallflower umbrella sedge (Cyperus difformis L.) control has selected for herbicide-resistant (R) populations that threaten the sustainability of rice (Oryza sativa L.) production and demand alternative control measures be developed. A better understanding of seedling recruitment patterns at the field level is required to optimize the timing and efficacy of control measures. Therefore, a population-based threshold model was developed for optimizing germination prediction in multiple acetolactate synthase (ALS)-R and ALS-susceptible (ALS-S) C. difformis biotypes and applied to field-level emergence predictions. Estimated base temperatures (Tb) ranged from 16.5 to 17.6 C with no clear pattern between biotypes; such values are higher than Tb values of other important rice weeds, as well as for rice. Germination rates increased linearly from 16 to 33.7 C. ALS-R seeds germinate faster due to smaller median thermal times to germination (θT(50)) while also displaying lower germination synchronicity across water potentials. Interestingly, ALS-R biotypes were capable of germinating under lower moisture availability, as indicated by their lower (more negative) base water potential values (Ψb(50)) for seed germination; Ψb(50) values ranged from −0.24 to −1.13 MPa. In-field soil germination measurements found thermal times to emergence varied across three water regimes (daily water, flooded, or saturated). Seedling emergence under the daily water treatment was fastest; however, total seedling density was lower than for the other water regimes. In order to optimize springtime C. difformis seedling emergence, soil moisture should be kept around field capacity, as germination is hindered at lower moisture contents. By predicting when most of the seed population germinates, the thermal-time model can address issues regarding the optimal timing for herbicide applications, thereby allowing for improved C. difformis management in rice fields.

Bearded sprangletop is a problematic weed in California rice production. The objective of this research was to determine the response of two bearded sprangletop biotypes (clomazone-susceptible [S] and -resistant [R]) to flooding depth. A study was conducted in 2017 and 2018 at the California Rice Experiment Station in Biggs, CA, to evaluate the flooding tolerance of the two biotypes against 5-, 10-, and 20-cm continuous flooding depths. Plant emergence, plant height, panicles per plant, seed per panicle, 100-seed weight, and seed per plant data were collected. At the 5-cm flood depth, neither biotype was controlled, and the R biotype had 260% more emergence, produced 475% more panicles per plant, and 455% more seed per plant than the S biotype. With a 10-cm flood, only the R biotype survived flooding and produced more panicles per plant and seed per plant than any other flood depth–biotype combination evaluated. There was no emergence of either bearded sprangletop biotype at the 20-cm flood depth. Continuous flooding can still be used as a management tool to control bearded sprangletop; however, the depth of flooding appears to limit emergence of S biotypes at 5 cm and R biotypes at 10 cm, and completely inhibits growth of both biotypes at 20 cm. The results of this study indicate that clomazone-resistant bearded sprangletop is more likely to spread throughout the Sacramento Valley because this biotype can survive clomazone applications and can tolerate a standard 10-cm flood.

Smallflower umbrella sedge is a prolific C3 weed commonly found in rice fields in 47 countries. The increasing infestation of herbicide-resistant smallflower umbrella sedge populations threatens rice production. Our objectives for this study were to characterize thermal requirements for germination of smallflower umbrella sedge seeds from rice fields in California and to parameterize a population thermal-time model for smallflower umbrella sedge germination. Because the use of modeling techniques is hampered by the lack of thermal-time model parameters for smallflower umbrella sedge seed germination, trials were carried out by placing field-collected seeds in a thermogradient table set at constant temperatures of 11.7 to 41.7 C. Germination was assessed daily for 30 d, and the whole experiment was repeated a month later. Using probit regression analysis, thermal time to median germination [θT(50)], base temperature for germination (Tb), and SD of thermal times for germination [σθT(50)] were estimated from germination data, and model parameters were derived using the Solver tool in Microsoft Excel®. Germination rates increased linearly below the estimated optimum temperatures of 33.5 to 36 C. Estimated Tb averaged 16.7 C, whereas θT(50) equaled 17.1 degree-days and σθT(50) was only 0.1 degree-day. The estimated Tb for smallflower umbrella sedge is remarkably higher than that of japonica and indica types of rice, as well as Tb of important weeds in the Echinochloa complex. Relative to the latter, smallflower umbrella sedge has lower thermal-time requirements to germination and greater germination synchronicity. However, it would also initiate germination much later because of its higher Tb, given low soil temperatures early in the rice growing season in California. When integrated into weed growth models, these results might help optimize the timing and efficacy of smallflower umbrella sedge control measures.

A field study was established to evaluate symptoms, growth, yield, and nut quality of walnut trees subjected to multiple exposures of simulated bispyribac-sodium drift. Nut yield the year following simulated drift treatment was also evaluated because tissue differentiation for future fruiting position occurs in the prior season. Bispyribac-sodium was applied four times, at weekly intervals, at 0.5% and 3% of the use rate in rice (45 g ai ha-1). Injury from the 0.5% rate exceeded 5% after three applications. In general, the severity of the symptoms peaked 14 d after last application (23% and 40% injury for 0.5% and 3% rate, respectively) and subsequently remained nearly constant over the duration of the study. Growth of shoots treated with the 0.5% rate was initially delayed during the treatment regime but recovered after treatments ended; however, walnut shoots exposed to the higher rate had fewer internodes than nontreated trees at the end of the season. No measurable reduction in walnut yield or average nut weight either in the year of exposure or in the subsequent year was observed. However, both rates negatively affected walnut kernel color in the year of exposure.

A field study was established to study symptoms, growth and yield of 2-year-old walnut trees exposure to simulated drift of several herbicides commonly used in rice production. Bispyribac-sodium, bensulfuron-methyl, and propanil were applied at four rates representing 0.5%, 1%, 3% and 10% of the normal use rate in rice (45, 70, and 6725 g ai ha−1, for the three herbicides respectively). Symptoms started to appear approximately 7 days after application (DAT) and peaked 28 DAT. At that time, bispyribac-sodium caused greater injury at low drift rates (6% and 15% visual injury for 0.5% and 1% rate, respectively) compared to bensulfuron-methyl and propanil. Bispyribac-sodium also appeared to slow walnut shoot elongation compared to nontreated trees; however, no yield reductions were observed either in the year after drift exposure. The effect of bispyribac-sodium simulated drift on the yield and nut quality in the year of drift exposure was evaluated in a separate study on 3-year-old walnut trees. While no yield or nut quality reductions were observed, a linear correlation between rate of bispyribac-sodium and color, an important quality factor, was found: higher herbicide rates tended to be associated with darker kernel color. Bispyribac-sodium may damage nearby walnut orchards if drifted at significant amounts. However, it is unlikely that in a field situation bispyribac-sodium would drift at high enough levels to cause the symptoms observed from the 10% use rates in this study.

Chlorsulfuron, thifensulfuron, bromoxynil, 2,4-D, glyphosate, and a combination of 2,4-D plus glyphosate were applied on newly planted and established ‘Lemberger’ wine grape at 1/3, 1/10, 1/33, and 1/100 of the maximum labelled rate in wheat or fallow to simulate exposure to drifted herbicides. All herbicides produced symptoms on grape but the most severe symptoms were with 2,4-D and the least severe with bromoxynil. Newly planted grape was more sensitive to herbicides than established grape. Although established grape recovered from injury caused by all treatments except 2,4-D and the highest rate of chlorsulfuron and glyphosate, newly planted grape recovered only from lower rates of bromoxynil. All herbicides resulted in diagnostic symptoms, but other symptoms were very similar to those caused by other stresses.

Broadleaf weed control ranged from 80% with clomazone at 0.14 kg/ha to 100% with clomazone at 1.12 kg/ha. Broadleaf weed control was higher with clomazone than naptalam or ethalfluralin. The combination of clomazone at 0.14 kg/ha and naptalam at 5 kg/ha or ethalfluralin at 1.25 kg/ha gave more than 90% broadleaf weed control. Clomazone caused chlorosis and bleaching on cucumber leaves but plants rapidly recovered. Cucumber yields were higher in plots treated with clomazone alone at 0.14 to 0.56 kg/ha than with ethalfluralin or naptalam alone. Clomazone dose response studies were conducted in weed-free plots with five cucumber varieties. The 0.28 kg/ha rate caused low levels of visible injury and did not decrease yields. Cultivar differences were more pronounced at higher clomazone rates. The five cucumber cultivars were, in order of increasing clomazone tolerance, ‘Sunre 3537,’ ‘Pioneer,’ ‘Quest,’ ‘Prince,’ and ‘Calypso.’ We concluded that clomazone is an effective and selective herbicide for broadleaf weed control in pickling cucumber.

It has been suggested that soil treated with a herbicide and subsequently carried by wind and deposited on plant foliage can cause crop injury. This study compared foliar uptake and translocation of herbicides applied to plants as an aqueous solution or in herbicide-treated soil. Leaves of 3-wk-old seedling alfalfa, grape, and pea were treated with 14C-labeled thifensulfuron, chlorsulfuron, glyphosate, 2,4-D, and bromoxynil. Significant amounts of all herbicides were absorbed by pea, alfalfa, and grape from the aqueous solutions, whereas very limited absorption occurred from herbicide-treated soil. Prolonged and multiple exposure to herbicide-treated soil did not increase herbicide uptake. High relative humidity enhanced herbicide absorption from aqueous solutions but not from herbicide-treated soil. All herbicides except bromoxynil were readily translocated in alfalfa, grape, and pea. Limited quantities of herbicides were absorbed from herbicide-treated soil by plant foliage, and this small amount is unlikely to cause crop damage.

Bentazon efficacy was evaluated in early, normal, and late plantings of green pea with the addition of four adjuvants: crop oil concentrate (COC), LI-700, Sylgard 309, and X-77. Weed control was improved by all adjuvants in normal and late plantings of green pea, but only by Sylgard 309, X-77, and LI-700 in the early planting. In normal and late plantings, weed control with bentazon applied at 0.56 kg/ha with any adjuvant was equal to weed control with bentazon applied alone at 1.12 kg/ha. Bentazon injury on green pea plants varied from slight to severe, with the lowest injury resulting from bentazon applied with COC and LI-700 and highest injury from bentazon applied with Sylgard 309. Bentazon injury was greater in normal and late plantings than in the early planting.

Broadleaf weed control and tulip, daffodil, and iris response to alachlor, dithiopyr, diuron, isoxaben, napropamide, oryzalin, oxadiazon, oxyfluorfen, pendimethalin, pronamide, and thiazopyr applied preemergence were evaluated. All herbicides controlled broadleaf weeds for five months after application, but control with oxyfluorfen, pendimethalin, alachlor, pronamide, and napropamide decreased six months after application. The greatest weed control was with oxadiazon, isoxaben, thiazopyr, dithiopyr, and diuron, whereas the least weed control was with napropamide and alachlor. In general, tulip was more sensitive to herbicides than daffodil and iris. All herbicides injured tulips at early growth stages but bulb yield of tulip was reduced only by oxadiazon, oxyfluorfen at 0.28 kg ai/ha, pronamide, and dithiopyr. Tulip flower quality was reduced only by oxadiazon, oxyfluorfen at 0.28 lb/ha, and pronamide. Daffodil and iris were tolerant to thiazopyr, isoxaben, pronamide, alachlor, oryzalin, napropamide, and diuron.

Imazethapyr resistance in common sunflower (Helianthus annuus) was confirmed in 1996 in a field near Rossville, KS. In 1997, common sunflower achenes were collected within a 20-km radius of the field with known resistance to determine if resistance was present in nearby fields or if resistance had spread to the native population on the roadside. Collections were made from 14 soybean (Glycine max) fields, one corn (Zea mays) field, and 11 roadsides. Achenes from Konza Prairie Research Natural Area, a prairie that had received no herbicide applications in the past 25 yr, served as the susceptible control. Common sunflower seedlings were treated in a greenhouse with 71 g ai/ha imazethapyr and 11 g ai/ha chlorimuron. In all 15 fields sampled, at least 1% of the common sunflower exhibited an intermediate response to imazethapyr or chlorimuron. In 13 fields, at least 1% of the plants were resistant to imazethapyr, and in all 15 fields, at least 1% of the plants were resistant to chlorimuron. Ten roadsides had common sunflower that showed intermediate response to imazethapyr or chlorimuron. At least 1% of the plants from seven roadsides were resistant to imazethapyr or chlorimuron. Common sunflower collected from fields with repeated applications of imazethapyr showed more resistance to imazethapyr than to chlorimuron.