Kochia plants resistant (R) to field rates of dicamba were characterized for their frequency of occurrence and levels of resistance and for the physiological fate of applied 14C-dicamba. Of 167 randomly sampled fields and seven fields identified by producers to contain R kochia, 19 contained plants that produced 1% or more R progeny. The maximum percentage of R progeny produced by parental plants from any field was 13%. An inbred R line derived from a field collection was 4.6-fold more resistant to dicamba than an inbred susceptible (S) line. Rates of 14C-dicamba uptake and translocation were similar in R and susceptible (S) plants up to 168 h after treatment (HAT). Concentrations of the primary metabolite, 5-hydroxy dicamba, were similar in R and S tissues up to 60 HAT, although amounts were significantly greater in R treated leaves by 96 and 168 HAT. However, because there were negligible levels of dicamba metabolites in R shoots and because the rate of metabolism was relatively slow, the observed changes were inadequate to account for observed resistance levels. Thus, dicamba resistance in kochia cannot be attributed to differential herbicide absorption, translocation, or metabolism. These findings, together with our field observations on the unusually slow spread of resistance within and among fields may indicate that dicamba resistance is a quantitative trait.

Several inbred lines of acetolactate synthase (ALS)-inhibiting herbicide-resistant (ALS-R) Palmer amaranth and ALS-susceptible (ALS-S) common waterhemp were developed in the greenhouse. Interspecific hybrids were obtained by allowing several ALS-S common waterhemp females to be pollinated only by ALS-R Palmer amaranth in a growth chamber. Putative hybrid progeny were treated with an ALS-inhibiting herbicide, and the hybrid nature verified using a polymorphism found in the parental ALS gene. Polymerase chain reaction (PCR) was used to amplify a region of the ALS gene in both parental plants and putative hybrids. Restriction enzyme digestion of the ALS-R Palmer amaranth PCR fragment resulted in two smaller fragments, whereas the PCR fragment in the ALS-S common waterhemp was not cut. Restriction digestion of the putative hybrid PCR fragment showed a combination of ALS-R Palmer amaranth double fragments and an ALS-S common waterhemp single fragment. Approximately 4 million flowers were present on 11 common waterhemp females and produced about 44,000 seeds that appeared viable. From the approximately 3,500 putative hybrid seedlings that were screened, 35 were confirmed as hybrids using herbicide resistance as a phenotypic and molecular marker. The data collected here verify that interspecific hybridization does occur between these two species, and even at a low rate, it could contribute to the rapid spread of ALS resistance in these species.

Greenhouse experiments were conducted to determine the effects of glufosinate on ammonium accumulation and photosynthetic inhibition in Palmer amaranth. Glufosinate applied at 410 g ha−1 reduced glutamine synthetase activity and enhanced ammonium content 30 min after treatment. Glufosinate application resulted in rapid inhibitions of the photosynthetic rate and stomatal conductance during the first 2 h after treatment (HAT), whereas the ammonium concentration increased over the same time period. Ammonium content 6 HAT was 22 times higher in treated plants than in untreated plants, whereas the photosynthetic rate of treated plants decreased by 63%. At 24 HAT, the ammonium content was 53 times higher in treated plants than in untreated plants; however, no further inhibition of photosynthesis occurred. Photosynthetic inhibition in Palmer amaranth coincided with the rapid accumulation of ammonium and decrease in stomatal conductance shortly after glufosinate application.

The critical period of weed control is the portion of the life cycle of a crop during which it must be kept weed-free to prevent yield loss due to weed interference. The advent of herbicide-resistant canola (Brassica napus L.) varieties in western Canada has meant that there are now more options for postemergence weed control in canola, and this has prompted increased interest in identifying the optimum timing for weed control in this crop. A critical period experiment was conducted at three locations in southern Manitoba in 1998 and 1999, and it consisted of two sets of treatments. In the first set of treatments, the crop was kept weed-free for increasing lengths of time to determine when emerging weeds would no longer reduce crop yield. In the second set of treatments, weeds were permitted to grow in the crop for increasing lengths of time to determine when weeds emerging with the crop began irrevocably to reduce crop yield. Results of the experiments indicated that canola must be kept weed-free in most cases until the four-leaf stage of the crop (17–38 days after crop emergence [DAE]) and, in one early-seeded experiment, until the six-leaf stage of the crop (41 DAE), in order to prevent >10% yield loss. After the four- to six-leaf stage of the canola crop, few weeds emerged, and late-emerging weeds accumulated little shoot biomass. Weeds needed to be removed by the four-leaf stage of the crop (17–38 DAE) to prevent >10% yield loss due to weed interference. In all but the early-seeded experiment, the critical weed-free period and the critical time of weed removal overlapped, such that a single weed removal at the four-leaf stage of the crop would have been sufficient to prevent >10% yield loss. This information will be useful for providing weed control recommendations to canola producers.

Research was conducted in 1998 and 1999 to characterize common lambsquarters photosynthesis and seed production as influenced by biotic (crop environment) and abiotic (climate) factors. Treatments were common lambsquarters in soybean, in corn, and in common lambsquarters monoculture. Common lambsquarters net photosynthesis was variable among treatments and differed between years. In 1998, early-season common lambsquarters net photosynthesis did not differ in soybean, corn, or common lambsquarters monoculture. In 1999, early-season common lambsquarters net photosynthesis was greater in corn than in soybean, but did not differ from that of common lambsquarters in monoculture. By midseason in both years, common lambsquarters net photosynthesis was less in soybean than in corn or in common lambsquarters monoculture. By late season in both years, common lambsquarters net photosynthesis was greater in common lambsquarters monoculture than in soybean or corn. Common lambsquarters seed production per plant was greater in common lambsquarters monoculture than in soybean or corn. Common lambsquarters seed production was variable among plants and between years. Practical applications of models to predict weed fitness that are based on photosynthetic capacity will be limited until variability in net photosynthesis and in seed production are better understood.

Field studies were conducted in 1995 and 1996 to determine the rate response of velvetleaf seedling survival, seed production, and shoot biomass to postemergence herbicides in corn and soybean. Dicamba and imazethapyr were applied to corn and soybean, respectively, at 1, ½, ¼, ⅛, 1/16, 1/32, and 0× labeled rates. Velvetleaf mature plant density was linearly related to seedling density, thus indicating that seedling survival was not density dependent, even after seedling densities exceeded 150 plants m−2. Seedling survival as influenced by herbicide was described by a dose–response curve in corn and soybean. In corn, seedling survival ranged from 0 to 48% across herbicide treatments and years. Seedling survival was greater at the ½× or lower herbicide rates than at the 1× rate. In soybean, maximum seedling survival was 61 and 14% in 1995 and 1996, respectively, and minimum seedling survival was less than 2% in each year. Seedling survival was less in 1996 than in 1995 because velvetleaf was infected with wilt in 1996. In soybean, seedling survival was 20 times greater when treated with herbicides at the ½× rate than when treated at the 1× rate in 1995, but seedling survival was similar when herbicides were applied at 1, ½, ¼, and ⅛× rates in 1996. Velvetleaf fecundity (seeds per plant) was dependent on mature plant density in 1995 but was density independent in 1996. Fecundity as influenced by herbicide was described by dose–response curves in corn each year and in soybean in 1995. In 1995, velvetleaf treated with herbicides at ½× and ¼× rates produced 20 to 30 times more seed per square meter than when treated with herbicides at the 1× rate. Differences in seed per square meter were exaggerated by high densities of velvetleaf. Seed per square meter did not differ between velvetleaf treated with herbicides at 1× or ½× rates in corn or soybean in 1996. Wilt infection of velvetleaf in 1996 was the likely cause of differences in herbicide performance between years. Herbicides at reduced rates were not effective at limiting seedling survival and seed production compared to 1× rates in the absence of wilt. As a result, long-term management of velvetleaf with herbicides at reduced rates likely will be difficult, especially in areas with high densities, unless integrated with other management practices.

Wheat cultivars resistant to imazamox will facilitate selective chemical control of many winter annual grass weeds, including jointed goatgrass, downy brome, and feral rye. These three weed species respond differently to imazamox postemergence treatments with feral rye, demonstrating more tolerance than jointed goatgrass or downy brome; therefore, growth chamber studies were conducted to evaluate imazamox absorption in all three weed species and translocation and metabolism in jointed goatgrass and feral rye. Adding nonionic surfactant (NIS) or methylated seed oil increased absorption in jointed goatgrass and feral rye but not in downy brome, compared to imazamox applied alone. Imazamox applied with NIS and urea ammonium nitrate resulted in the highest absorption in each species: 97, 91, and 92% of applied 14C for jointed goatgrass, downy brome, and feral rye, respectively, 48 h after treatment (HAT). Imazamox translocation from the treated leaf was similar for jointed goatgrass and feral rye across seven harvest intervals between 0 and 96 HAT. Shoot tissues of jointed goatgrass and feral rye accumulated 17 and 14% of applied 14C, respectively, by 96 HAT. Differential translocation of imazamox into root tissue was observed within 12 HAT; by 96 HAT, 20% of applied 14C translocated to jointed goatgrass roots compared to 27% for feral rye. Imazamox was readily metabolized in both weed species. At 96 HAT, 73 and 98% of the applied 14C was metabolized in the treated leaves of jointed goatgrass and feral rye, respectively. Metabolism was consistently higher in feral rye than in jointed goatgrass in all plant parts 96 HAT. On a whole-plant basis, metabolism was 25% greater in feral rye than in jointed goatgrass. The differential response of jointed goatgrass and feral rye to foliar applications of imazamox appears to be related to differences in translocation and metabolism but not in absorption.

A mechanistic model was constructed for common ragweed growth and development based on the generic plant model CROPSIM. Adaptations were made to CROPSIM's growth and development subroutines to enable common ragweed growth to be simulated. Data from field studies using a single-source common ragweed grown in monoculture and from the literature were used to parameterize the model. The influences of varying environmental conditions across years, densities, and emergence timing on leaf number, leaf area, leaf weight, height, and biomass accumulation were taken into account by the model. Deviations between simulated and measured values generally fell within a relatively narrow range. Deviations outside this range tended to be associated with common ragweed growth shortly after emergence, particularly during temperature and moisture extremes. Future versions of the CROPSIM model may need to include more detailed algorithms for upper soil surface layer temperature and moisture conditions and improved germination and emergence algorithms to reduce these deviations.

The influence of herbicide placement and plant growth stage on the absorption and translocation patterns of 14C-glyphosate in glyphosate-resistant cotton was investigated. Plants at four growth stages were treated with 14C-glyphosate on a 5-cm2 section of the stem, which simulated a postemergence-directed spray (PDS) application, or on the newest mature leaf, which simulated a postemergence (POST) application. Plants were harvested 3 and 7 d after treatment and divided into the treated leaf or treated stem, mature leaves, immature leaves and buds, stems, roots, fruiting branches (including the foliage on the fruiting branch), squares, and bolls. The PDS versus POST application main effect on absorption was significant. Absorption of 14C-glyphosate applied to stem tissue was higher in PDS applications than in POST applications. Plants receiving PDS applications absorbed 35% of applied 14C-glyphosate, whereas those receiving POST applications absorbed 26%, averaged over growth stages at application. Absorption increased from the four-leaf growth stage to the eight-leaf stage in POST applications but reached a plateau at the eight-leaf stage. Plants with PDS applications showed an increase in absorption from the four- to eight- to twelve-leaf stages and reached a plateau at the 12-leaf stage. Translocation of 14C-glyphosate to roots was greater at all growth stages with PDS treatments than with POST treatments. Herbicide placement did not affect translocation of 14C-glyphosate to squares and bolls. Squares and bolls retained 0.2 to 3.7% of applied 14C-glyphosate, depending on growth stage. Separate studies were conducted to investigate the fate of foliar-applied 14C-glyphosate at the four- or eight-leaf growth stages when harvested at 8- or 10-leaf, 12-leaf, midbloom (8 to 10 nodes above white bloom), and cutout (five nodes above white bloom, physiological maturity) stages. Thirty to 37% of applied 14C-glyphosate remained in the plant at cutout in four- and eight-leaf treatment stages, respectively. The concentration of 14C-glyphosate in tissue (Bq g−1 dry weight basis) was greatest in mature leaves and immature leaves and buds in plants treated at the four-leaf stage. Plants treated at the eight-leaf stage and harvested at all growth stages except cutout showed a higher concentration of 14C-glyphosate in squares than in other plant tissue. Accumulation of 14C-glyphosate in squares reached a maximum of 43 Bq g−1 dry weight at harvest at the 12-leaf stage. This concentration corresponds to 5.7 times greater accumulation of 14C-glyphosate in squares than in roots, which may also be metabolic sinks. These data suggest that reproductive tissues such as bolls and squares can accumulate 14C-glyphosate at higher concentrations than other tissues, especially when the herbicide treatment is applied either POST or PDS during reproductive stages (eight-leaf stage and beyond).

Giant sensitiveplant interference at different population densities in cassava established at 10,000 plants ha−1 was investigated on a Ferric Luvisol in a humid tropical environment. Interference for 12 mo was compared at 0, 10,000, 20,000, 30,000, and 40,000 plants ha−1 and at natural populations (averaging 630,000 plants ha−1) in four randomized complete blocks. Results showed that the order of cassava growth parameter response to giant sensitiveplant interference for 12 mo was leaf number > height > stem girth > leaf size = petiole length. The natural population density of giant sensitiveplant reduced growth faster and more than populations of 10,000 to 40,000 plants ha−1 in cassava. All giant sensitiveplant populations from 10,000 plants ha−1 and higher reduced storage root yield in cassava 12 mo after planting. Yield reduction increased as giant sensitiveplant population increased and the highest reduction of 85% occurred in the natural population of giant sensitiveplant.

The objectives of this study were to model the influence of herbicides, wilt disease, and mechanical treatments on velvetleaf population dynamics, annualized net return (ANR), and economic optimum threshold (EOT) in a 20-yr rotation involving alternate years of corn and soybean. Mechanical treatments were interrow cultivation in corn and rotary hoeing in soybean. Herbicides at a quarter (¼×) rate or lower did not reduce velvetleaf seed banks without mechanical treatments in the absence of wilt. Herbicides at full (1×) and half (½×) rates decreased velvetleaf seed banks 95% within 6 and 20 yr, respectively, when there was no wilt. Herbicides at ½× rates with mechanical treatments reduced the seed bank 95% in only 10 yr, but mechanical treatments did not increase the rate of seed bank decline with 1× rates. Wilt infection had to occur annually to reduce velvetleaf seed banks as effectively as herbicides at 1× rates alone. ANR was maximized with herbicides at reduced rates, even though they were not as effective at reducing seed banks as were 1× rates. The herbicide rate required to maximize ANR increased as the initial velvetleaf seed bank density increased. Mechanical treatments and wilt decreased the herbicide rate required to maximize ANR. In fact, wilt infection increased the ANR of herbicides at reduced rates. The EOT was 0.55 and 0.4 seedlings m−2 when velvetleaf was managed with herbicides at 1× and ½× rates, respectively. Mechanical treatment had no effect on EOT, but wilt increased the EOT. Herbicides at reduced rates should only be used to manage velvetleaf in fields with a low seed bank density when integrated with mechanical treatments or when the field has a history of wilt. Herbicides should be used at 1× rates when fields have a large velvetleaf seed bank and when integrated management practices are not used.

Research was conducted to characterize the phenology of common lambsquarters growth parameters as influenced by climatic variation among years. Treatments included soybean or corn grown alone, common lambsquarters with soybean or corn, and common lambsquarters grown alone. Common lambsquarters leaf area and plant height phenology differed among years and was variable within treatments. Conversely, crop leaf area and plant height phenology did not differ among years and was less variable within a treatment than common lambsquarters. Weed relative leaf area and relative volume differed among years because of differences in crop and common lambsquarters leaf area and plant height phenology. Differences in common lambsquarters relative leaf area and relative volume among years may explain differences in previously reported crop yield responses to weed infestations between sites and years. Although common lambsquarters relative leaf area and relative volume differed among years, variability as indicated by regression coefficients of determination was also high within year and treatment. Crop leaf area and plant height phenology were well described by regression equations, with r2 values greater than 0.68. Therefore, low coefficients of determination for relative leaf area and relative volume models were attributed to variability in common lambsquarters within a treatment.

Disulfoton and phorate applied in the seed furrow greatly reduce clomazone phytotoxicity to cotton in the field, whereas aldicarb does not. An experiment was conducted to determine the effect of aldicarb, disulfoton, and phorate on 14C-clomazone absorption, translocation, and metabolism by cotton grown in a sandy loam soil. Clomazone at 0.87 μg g−1 of soil alone or in combination with aldicarb at 0.6 μg g−1 of soil reduced cotton root and shoot growth 26 to 33%. Root and shoot growth were not reduced by clomazone plus disulfoton or phorate at 0.6 μg g−1 of soil. Protection of cotton against injury by clomazone was not explained by reduced absorption or translocation of clomazone or a metabolite to the shoot. Clomazone metabolism was reduced by disulfoton and phorate, thus indicating a clomazone metabolite may be more toxic to cotton.

The spontaneous flow of genes from wheat to jointed goatgrass is of great concern to breeders intending to release herbicide-resistant wheat. The objectives of this research were to study how genes could flow from wheat to jointed goatgrass through crossing and backcrossing between these two species and, based on this knowledge, to propose possible ways to minimize the chance of gene flow between them. Results showed that the wheat × jointed goatgrass hybrid can only serve as a female parent to produce the BC1 generation. The BC1 generation was found to have 1.8% male fertility and 4.4% female fertility, indicating that it could serve as either the male or female parent to produce a BC2 generation. The fertility of the resultant BC2 generation further increased. The average male, female, and self-fertility was 8.9, 18.0, and 6.9%, respectively. After the BC2 generation, the backcross progeny has three possible ways to reproduce: to pollinate jointed goatgrass, to be pollinated by jointed goatgrass, or to pollinate itself. Restoration of the chromosome number of jointed goatgrass continues as the BC2 generation is selfed, but some plants can contain an alien chromosome over generations. The possible ways to reduce the chance of gene flow between these two species are (1) prevent the production of hybrids, (2) prevent the production of the BC1 generation, and (3) put a herbicide-resistant gene on the A- or B-genome of wheat.

This study examined pollen morphological variation among Amaranthus species and interspecific hybrids. Ten weedy Amaranthus species, a cultivated grain species, and several putative hybrids resulting from interspecific mating between common waterhemp and Palmer amaranth were grown in a greenhouse. Mature pollen was collected, viewed, and photographed with a scanning electron microscope (SEM). The pollen grains were spherical shaped with polypantoporate, or golf ball-like, aperture arrangement. Differences were observed between the monoecious and dioecious Amaranthus species. Pollen grains of the dioecious species had a greater number of apertures on the visible surface. One exception to these trends was the dioecious species, Palmer amaranth, whose pollen was similar to that of the monoecious species spiny amaranth. However, pollen grain diameters did not differ between the monoecious and dioecious plants. Significant differences also were noted between the pollen from the putative common waterhemp × Palmer amaranth hybrids and the parental-type pollen grains. Pollen of the hybrids was similar in size to the maternal parent but had an aperture number that was intermediate between parents. This indicates that pollen characteristics may be controlled by the female and that hybrids may be more prevalent than originally thought.

All red rice found in commercial rice in the United States has traditionally been classified as Oryza sativa ssp. indica. This assumption was tested by analyzing red rice samples collected from across the southern United States rice belt with 18 simple sequence length polymorphism (SSLP) markers distributed across all 12 chromosomes. The results clearly demonstrate that the traditional classification of red rice is inadequate. Some red rice is closely related to O. sativa ssp. indica cultivated rice. However, other red rice is more closely related to O. sativa ssp. japonica. Most importantly, some red rice samples collected from Arkansas, Louisiana, Mississippi, and Texas form a distinct group that includes a number of Oryza nivara and Oryza rufipogon accessions from the National Small Grains Center. In particular, red rice samples from three states were identified that for all 18 markers are identical to the O. rufipogon accession IRGC 105491. These different classes of red rice are intermingled across the southern U.S. rice belt and within individual production fields. Oryza sativa ssp. indica-like red rice and O. rufipogon-like red rice have been found within a single 9-m2 collection site. While the classification of red rice as O. sativa ssp. indica, O. sativa ssp. japonica, or O. rufipogon using DNA markers is generally in agreement with classification based on simple morphological traits, readily observed morphological traits alone are not sufficient to reliably classify red rice. Because red rice is much more diverse than previously assumed, this diversity must be considered when developing red rice management strategies.

The partitioning coefficient is defined as the proportion of new dry matter partitioned among different plant parts. Partitioning coefficients can be used to model plant dry matter accumulation. In 1994 and 1995, field studies were conducted at two locations near Manhattan, KS, to determine the influence of density and relative time of emergence of redroot pigweed on dry matter partitioning to stem, leaves, and reproductive parts throughout the season. Redroot pigweed was grown with sorghum and in monoculture at densities of 2, 4, and 12 plants m−1 of row each year at each location. Dry matter partitioning during vegetative growth was not influenced by plant density. However, partition coefficients during the reproductive growth stage changed as a linear function of the time of pigweed emergence relative to the sorghum leaf stage. The later the emergence time relative to sorghum leaf stage, the higher the partitioning coefficient values for leaf (PCleaf) and stem (PCstem) and the lower the partitioning coefficient values for reproductive parts (PCrp). The observed differences in partitioning coefficients due to relative emergence time are valuable information to those interested in simulating growth of competing plant species, especially with reference to their seed production.

The relative competitive abilities of yellow starthistle accessions that are resistant (R) and susceptible (S) to picloram were compared using a replacement series experiment. With no herbicide treatment, total shoot dry weights at vegetative and early reproductive stages of plant growth were similar for the two accessions, although S plants accumulated more total shoot dry weight by the late reproductive stage, mainly as a result of a greater contribution of vegetative growth. Without herbicide, relative yield of total biomass or reproductive structures did not differ from theoretical competitive equivalence at any accession ratio, thereby indicating that interaccession interference was similar. For picloram-treated plants, R plants accumulated more total, vegetative, and reproductive dry weight than did S plants at the early and late reproductive stages, and there was no difference between S and R plants at the vegetative growth stage. Seed production by R plants was 10-fold greater than that observed in S plants, but seed size remained unchanged, regardless of accession ratio. With herbicide present, the relative yield of S plants differed from theoretical competitive equivalence as S:R accession ratios decreased, but relative yield of R plants did not. Therefore, only in the presence of picloram will R plants have a competitive advantage over S plants. Some of the progeny from mixed populations of S and R plants that were cross-pollinated, even at low R frequency (25%), expressed resistance to picloram.

Giant ragweed exhibits a high degree of polymorphism among individual plants in seed size, shape, spininess, and color. These features may play an important role in giant ragweed seed survival and predation avoidance; however, they are difficult to evaluate because of lack of quantification methods. A computer imaging technique was developed for describing and classifying giant ragweed seeds using digital images of the seed top and side views. Seed samples collected from 20 different giant ragweed plants (classes) were mounted and digitally scanned. Quantitative features were extracted from the seed images, including color, width, height, area, and seed perimeter. A polygon (convex hull) of the seed image based on the seed outline was constructed, from which spininess indices were developed. Fisher's linear discriminant with normalized nearest neighbor classification was used to classify randomly selected images of individual seeds according to class (maternal origin), using the extracted features as a database. The best classification rate achieved was 99%, with 138 out of 140 seeds correctly matched using data from both the top and side views. Seed features were easily extracted and varied from 1.2- to 4.5-fold among classes. Area and perimeter measurements varied least within classes but varied most among classes, suggesting that these features discriminate effectively among seeds from different plants in giant ragweed. Convex hull area : seed area ratio, using the seed top view images, was the best index of seed spininess, aligning well with visual assessment and providing greatest discrimination among classes. This experiment shows that in the case of giant ragweed, seeds from different plants are distinguishable in an objective and quantitative manner. This imaging technique can be applied to identification of seeds from different species and to studies on variable seed morphology within a species.

Acetyl-coenzyme A carboxylase (ACCase) assays and absorption, translocation, and metabolism experiments were conducted to investigate the mechanism(s) responsible for resistance in a johnsongrass biotype that exhibited low levels of resistance to the cyclohexanedione (CHD) herbicide sethoxydim and the aryloxyphenoxypropionate (APP) herbicides quizalofop-P and fluazifop-P. The rate of [14C]quizalofop-ethyl absorption was significantly higher in the resistant compared to the susceptible biotype 8, 24, and 48 h after treatment (HAT), but by 72 HAT, there was no significant difference in the amount of [14C]quizalofop-ethyl detected in either biotype. Additionally, little or no differences in the translocation of [14C]quizalofop-ethyl were observed in the resistant and susceptible biotypes at any time interval after application. In [14C]quizalofop-ethyl metabolism experiments, similar levels of quizalofop-ethyl and quizalofop metabolites were observed in the resistant and susceptible biotypes 8, 24, 48, and 72 HAT, but slightly higher levels of quizalofop acid were detected in the resistant biotype 48 and 72 HAT. In ACCase assays, the concentrations of quizalofop-P, clethodim, and sethoxydim that inhibited ACCase activity by 50% (I50) were statistically similar in the two biotypes, indicating that the resistant johnsongrass biotype contains an ACCase that is sensitive to the APP and CHD herbicides. In the absence of APP or CHD herbicides, however, the specific activity of ACCase in the resistant biotype was two to three times greater than that of the susceptible biotype. The specific activity of ACCase in the resistant biotype was also significantly greater than that of the susceptible biotype in the presence of all concentrations of quizalofop-P and sethoxydim and in the presence of 0.1, 1, and 10 µM clethodim. These results suggest that resistance to quizalofop-P and sethoxydim is conferred by an overproduction of ACCase in the resistant johnsongrass biotype.