Greenhouse and laboratory experiments were conducted to determine if the synergistic injury to sulfonylurea-tolerant soybean (STS soybean) caused by applications of thifensulfuron plus imazethapyr involves an interaction between the herbicides at the ALS enzyme. In vitro ALS from STS soybean (Asgrow 3200) was 10 times more tolerant to thifensulfuron than ALS from non-STS soybean (Williams 82). In vivo ALS was 30% of maximum activity 6 HAT in STS soybean treated with 4.4 g ha−1 thifensulfuron, but 0% of maximum in similarly treated non-STS soybean. Increasing thifensulfuron from 4.4 to 141 g ha−1 did not increase in vivo ALS inhibition in STS soybean, indicating a highly resistant ALS isozyme. Both cultivars responded similarly to imazethapyr in terms of visual injury, in vitro ALS inhibition, and in vivo ALS inhibition. Soybean injury and in vivo ALS inhibition increased with imazethapyr rate in both cultivars. The relationship between imazethapyr inhibition of ALS in vivo and soybean injury was curvilinear. In vitro ALS assays revealed no synergistic interaction between thifensulfuron and imazethapyr at the enzyme level. In vivo ALS activity was reduced to 7 and 10% of controls 24 and 48 HAT by the tank mixture compared to 56 and 84% for thifensulfuron alone and 20 and 28% for imazethapyr alone. These data show that the synergistic increase in soybean injury caused by tank mixing thifensulfuron with imazethapyr results from the cumulative inhibition of ALS, not a synergistic inhibition of the enzyme.

Greenhouse and laboratory experiments were conducted to determine if the synergistic interaction between imazethapyr and thifensulfuron in sulfonylurea-tolerant (STS) soybean involved enhanced absorption and translocation or reduced metabolism of one or both herbicides. Thifensulfuron at 4.4 g ha−1 and imazethapyr at 70 g ha−1 caused 0 and 28% injury to STS soybean 7 DAT, respectively, while the combination of both herbicides caused 50% injury 7 DAT. Imazethapyr had no effect on absorption of 14C-thifensulfuron into the first trifoliolate. Imazethapyr did not affect absorption, translocation, or metabolism of 14C-thifensulfuron. Metabolism of 14C-thifensulfuron was rapid, with less than 10% remaining after 24 h, and was not affected by imazethapyr. Likewise, thifensulfuron did not affect the absorption, translocation, or metabolism of 14C-imazethapyr. Therefore, the synergism between thifensulfuron and imazethapyr does not involve changes in the absorption, translocation, or metabolism of either herbicide.

Common lambsquarters was treated with metamitron (M) and glyphosate (G) to investigate chlorophyll content as a single, biological parameter for indicating sub-lethal-treated weeds. In three different greenhouse experiments the herbicides were applied at dose rates of 1/8, 1/4 and 1/2 times the M-recommended dose rate (= 0.7 kg ai ha−1), and 1/16, 1/8, and 1/4 times the G-recommended dose rate (= 1.44 kg ai ha−1), respectively. Three weeks after herbicide application the chlorophyll content in leaves of plants treated with 1/2 M was lower than the control. At the same time the chlorophyll content in leaves of plants treated with 1/8 and 1/4 M was higher than the control. The chlorophyll content in the leaves of plants treated with 1/16 and 1/8 G was also higher than the control, but the chlorophyll content in the leaves of plants treated with 1/4 G was equal to the control. A dose of 1/2 M killed 30% of available plants within 29 d, but most of the plants were still alive after 29 d when 1/4 M, 1/8 M, 1/4 G, 1/8 G and 1/16 G had been used. The fresh weight of the biomass reflected the fitness of the plants observed with time and was related to chlorophyll content. The dry weight of the biomass, however, was probably related to the typical action of metamitron and glyphosate. Measurements with a remote sensing technique supported the measurement of chlorophyll with a spectrophotometer confirming a significant discrepancy with time between a herbicide's affect on chlorophyll content and on plant growth. Because differences in chlorophyll content may have caused differences in the progress of plant degradation in sub-lethal- and lethal-treated plants, it is concluded that the chlorophyll content can be used as a single, biological parameter for indicating sub-lethal dose rates of metamitron and glyphosate.

Experiments were conducted to examine the influence of adjuvant, growth stage, air temperature, and soil moisture on nicosulfuron activity, absorption, and distribution in quackgrass. Foliar absorption of 14C-nicosulfuron by quackgrass was greater in one-leaf than five-leaf plants. Total translocation of 14C-nicosulfuron was similar regardless of growth stage. However, nicosulfuron was more phytotoxic to three- and five-leaf than one-leaf quackgrass. Of the absorbed 14C, at least 71 % remained as 14C-nicosulfuron 168 h after application in three-leaf plants. 14C-Nicosulfuron absorption, translocation, and accumulation increased with increasing temperatures and was independent of soil moisture. Under adequate soil moisture conditions (-0.03 MPa), nicosulfuron activity was not influenced by temperature. Nicosulfuron efficacy was not influenced by soil moisture at a cool (11/6 C) temperature. As plant moisture stress increased, due to low soil moisture and increasing air temperature, nicosulfuron efficacy declined. Differences in nicosulfuron efficacy due to growth stage or environmental conditions could not be explained by differential 14C recovery, absorption, translocation, or accumulation. The addition of 28% urea-ammonium nitrate liquid fertilizer (UAN) to 14C-nico-sulfuron plus petroleum oil adjuvant (POA) provided greater 14C absorption than POA alone. Translocation of 14C with POA alone 168 h after treatment (HAT) was similar to POA plus UAN 24 HAT. However, 168 HAT there was no difference in the total amount of radiolabelled material translocated among the adjuvants.

The preemergence herbicide cinmethylin is a benzyl ether derivative of the monoterpene 1,4-cineole. Other oxygenated monoterpenes (carvone, citronellol, fenchone, geraniol, and pulegone) were previously found to inhibit the germination and growth of several weed species while exhibiting low phytotoxicity to soybean. Benzyl ether derivatives of these oxygenated monoterpenes were synthesized and examined for preemergence and postemergence activity towards corn, soybean, wheat, and velvetleaf. Benzyl pulegyl ether exhibited the most preemergence activity when applied directly to the soil, completely inhibiting wheat and velvetleaf emergence at 1.0 kg ha−1 while reducing soybean emergence 80%. Several of the benzyl ethers were more inhibitory to velvetleaf radicle elongation relative to cinmethylin but less inhibitory to corn and wheat radicle elongation in petri dish bioassays. Several of the benzyl ethers exhibited limited postemergence activity when applied at 1.0 kg ha−1 to seedlings of the test species 10 d after emergence. The benzyl ether derivatives demonstrated altered selectivity and sensitivity as compared to the parent compounds and cinmethylin.

Experiments were conducted to evaluate the occurrence and significance of release of herbicide through subterranean parts of nicosulfuron-treated johnsongrass. In a bioassay, the rooting medium of johnsongrass treated foliarly with 50 or 100 μg nicosulfuron plant−1 was inhibitory to the radicle elongation of sorghum and corn indicating the increased toxicity of the rooting medium of nicosulfuron-treated johnsongrass. The study with 14C-nicosulfuron indicated a basipetal translocation of foliarly applied nicosulfuron in johnsongrass to its roots/rhizomes and also into the rooting medium. By 30 DAT, around 23% of the 14C-label absorbed by johnsongrass was found exuded into the rooting medium. Radiochromatogram scans of thin layer chromatography plates of rooting medium indicated unmetabolized nicosulfuron as the major 14C-labeled compound (56%). The study also revealed a subsequent uptake of exuded 14C by corn roots sharing the medium. On the whole, the amount of 14C-label recovered from untreated corn amounted to 4.3% of the total applied to johnsongrass. In another experiment, the presence of nicosulfuron in the rooting medium was detrimental to corn growth. Reductions in corn growth occurred at concentrations of 10−8 M nicosulfuron or greater in the rooting medium. The sensitivity of corn to root uptake was attributed to greater accumulation of nicosulfuron at a faster rate in the growing parts.

The efficacy of the safeners naphthalic anhydride (NA), R-29148, benoxacor, and dichlormid as protectants of corn against injury from the methyl ester of tribenuron was evaluated under greenhouse conditions. Tribenuron applied alone reduced the root length of both ‘Pioneer 3180’ and ‘Northrup-King 9283’ corn. The concentration of tribenuron causing 50% reduction of corn root length (GR50) was 1 μM. NA applied as a seed dressing at 0.5 and 1.0% (w/w) was the most effective of the four safeners tested and provided excellent protection to both corn hybrids against 1, 2, and 4 μM tribenuron. Soaking corn seeds in an aqueous solution of R-29148 containing 0.5,1, or 2 mM protected ‘Pioneer 3180’ against 1 μM of tribenuron. The safening efficacy of R-29148 against tribenuron injury to ‘Northrup-King 9283’ corn was limited. Dichlormid and benoxacor, were less effective as tribenuron safeners and offered partial protection to both corn hybrids. Individual applications of the synergist piperonyl butoxide (PBO) at 0.1 and 0.2 mM did not affect the growth of corn seedlings. PBO synergized the activity of all concentrations of tribenuron, but only on the ‘Northrup-King 9283’ hybrid. These results demonstrate that the tolerance of corn to tribenuron can be enhanced with the use of safeners such as NA and R-29148, and that oxidative reactions may be involved in the metabolism of tribenuron by corn.

Greenhouse and laboratory experiments were conducted to determine the basis of imazethapyr tolerance in cowpea. Cowpea required more than 700 g ha−1 imazethapyr to reduce shoot growth 50% while corn required 70 g ha−1 to reduce growth 50% in greenhouse experiments. Extractable acetohydroxy acid synthase activity from cowpea 24 h after foliar imazethapyr treatment was only 20% of that of untreated cowpea but increased to control levels 72 h after treatment (HAT). The unifoliate leaves of cowpea absorbed 25% of the 14C-imazethapyr applied 72 HAT. Movement of 14C-imazethapyr out of the treated zone increased gradually to approximately 47% of the absorbed 14C 168 HAT. Parent imazethapyr decreased from 41 to 27% of the recovered 14C between 4 and 24 HAT, and continued to decrease to 14% by 168 HAT. Polar metabolites increased from 27 to 55% of the recovered 14C from 24 to 168 HAT. The polar metabolites of imazethapyr included hydroxyimazethapyr and conjugates of hydroxyimazethapyr. Tolerance of cowpea to imazethapyr is based on the plants ability to metabolize the herbicide to nontoxic metabolites.

Laboratory experiments were conducted to determine the mechanism of resistance to diclofop in two wild oat biotypes (designated ‘B’ and ‘C’ biotypes) from the Willamette Valley of Oregon. Resistance could not be attributed to differential absorption, translocation, or metabolism of diclofop. Resistance was not correlated with membrane plasmalemma repolarization following diclofop acid treatment. Compared to a susceptible (’S') wild oat biotype, acetyl CoA carboxylase from the B and C biotypes showed a 10.3 and 4.5 fold increase in the level of resistance, respectively, to diclofop acid. Cross-resistance to fenoxaprop acid was 5.5 and 7.3 times higher in the B and C biotypes, respectively than the S biotype. Correlation between resistance at the whole plant level and at the ACCase level was good for diclofop and fenoxaprop in the B biotype. For the C biotype, this correlation was not as good. Possible reasons for the discrepancy are given.

Research was conducted to elucidate mechanism(s) of nicosulfuron and primisulfuron selectivity in corn, woolly cupgrass, and shattercane. Corn absorbed less than one half the nicosulfuron and primisulfuron that woolly cupgrass and shattercane absorbed. Acetolactate synthase (ALS) isolated from three species responded similarly to nicosulfuron and primisulfuron. Values for 50% (I50) inhibition of ALS for nicosulfuron were 36.9, 37.2 and 29.5 nM for corn, woolly cupgrass and shattercane, respectively. Primisulfuron I50 values were 13.8, 11.5 and 10.0 nM for corn, woolly cupgrass and shattercane, respectively. Shattercane's I50 was different from corn's but not from that of woolly cupgrass. Corn rapidly metabolized nicosulfuron and primisulfuron, with a half-life of less than 4 h. Shattercane metabolized the herbicides more slowly, with a half-life greater than 72 h for nicosulfuron and 36 h for primisulfuron. Nicosulfuron and primisulfuron half-lives were greater than 72 h and less than 4 h, respectively, in woolly cupgrass. Therefore, selectivity with nicosulfuron and primisulfuron is likely based on metabolism to nonphytotoxic compounds. Corn tolerance to nicosulfuron and primisulfuron was also attributed to reduced herbicide penetration and translocation below the treated leaf.

Tribenuron antagonized diclofop control of wild oat in greenhouse studies. Wild oat shoot fresh weight was reduced 79% when plants were treated with diclofop at 1.12 kg ha−1, but only 46% when treated with diclofop at 1.12 kg ha−1plus tribenuron at 18 g ha−1. Increasing diclofop rate increased control of wild oat but did not overcome the antagonism. Separating the applications of diclofop and tribenuron by as little as 12 s reduced the antagonism by 40%, indicating the importance of close proximity of the two herbicides on the leaf surface for the antagonistic response. The inhibitory effect of diclofop at its target site (acetylCoA carboxylase) was not reduced by tribenuron. Metabolism of 14C-diclofop by wild oat leaves was not altered quantitatively or qualitatively by tribenuron. Tribenuron had no effect on spray retention or absorption of 14C-diclofop by wild oat leaves, and only slightly decreased the total amount of radiolabel translocated out of the treated zone on the leaf. However, tribenuron decreased basipetal translocation of diclofop from the treated zone by approximately 20%. The ability of tribenuron to reduce basipetal translocation of diclofop to meristematic regions of wild oat may be a factor in antagonism.

Consistent with field observations, sicklepod exhibited considerable tolerance to sulfentrazone, and coffee senna showed relatively high sensitivity to this herbicide in greenhouse tests. Germination was not inhibited in either species at up to 12.9 μM of the herbicide. However, the chlorophyll content of herbicide-treated coffee senna cotyledonary leaves was greatly reduced, and seedlings died within 10 d after treatment, while sicklepod seedlings were not visibly affected. Shoot height of coffee senna was inhibited 90% by sulfentrazone at 0.5 kg ai ha−1, while the growth of sicklepod was not affected up to 2.0 kg ai ha−1. Root uptake of radiolabeled sulfentrazone was 74% greater in coffee senna than sicklepod, but the amount of radioactivity recovered from the shoots of both species after 12 h was not different. Eighty-three percent of the parent compound remained in coffee senna leaf tissue after 9 h root exposure to the herbicide. In contrast, sicklepod took up relatively less sulfentrazone through the root and metabolized sulfentrazone in the foliage more rapidly than coffee senna, with 91.6% of the herbicide being metabolized during the first 9 h of exposure. These results suggest that the tolerance of sicklepod to sulfentrazone is primarily due to a relatively high rate of metabolism of the herbicide compared to coffee senna.

Laboratory experiments were conducted to identify adjuvants that improve absorption of imazethapyr, 2,4-D amine, and picloram by leafy spurge. Adjuvants (0.25% v/v) included crop oil concentrate (COC), methylated seed oil (MSO), nonionic surfactant (NIS), organosilicones (Silwet L-77®, Sylgard® 309, Silwet® 408), 3:1 mixtures of acetylinic diol ethoxylates (ADE40, ADE65, ADE85) with Silwet L-77, ammonium sulfate (2.5 kg ha−1), and 28% urea ammonium nitrate (UAN, 2.5% v/v). Adjuvants were combined with 14C-herbicide and commercially formulated herbicide product. Leaves were harvested 2 DAT, rinsed with 10% aqueous methanol to remove surface deposits of herbicide, and dipped in 9:1 hexane:acetone to solubilize cuticular waxes. Imazethapyr absorption increased by 38 to 68% when UAN was combined with COC, NIS, or MSO. Total absorption of imazethapyr plus COC, MSO, or NIS exceeded 86% 2 DAT when UAN was added. Urea ammonium nitrate reduced the amount of imazethapyr associated with the cuticular wax by 2.0%. Imazethapyr absorption was similar on both the abaxial and adaxial leaf surface when UAN was not added; however, 12% more imazethapyr was absorbed from the abaxial leaf surface than from the adaxial leaf surface when UAN was combined with Sylgard 309. Uptake of 2,4-D ranged from 54 to 78% and was greatest with Silwet 408 and 3:1 mixture of ADE40: Silwet L-77. Picloram absorption ranged from 3 to 19%. Buffering picloram treatment solutions to pH 7 and including 2.5 kg ha-1 ammonium sulfate increased picloram absorption to 37%.

N fertilizers suppress witchweed plant growth and development, thus reducing the severity of parasite attack and increasing host yield simultaneously. However, the underlying physiological mode of N action occurring within the parasite cells remains largely unknown. This study aims at screening for the effects of N forms and different growth conditions on some N assimilation enzymes in witchweed seedlings grown aseptically without host plant, and in pots with host plants. Results show that supply of N in NH4 + or urea forms resulted in 83 to 92% reduction in nitrate reductase activity (NRc), compared with control. Increasing NO3 − concentrations from 0 mM to 100 mM, led to a corresponding increase in NRc in giant witchweed. NRc of giant witchweed seedlings grown under light and dark cycles were about 270 times higher than seedlings grown in continuous darkness. A combination of NH4 + and NO3 −, resulted in increased giant witchweed NRc, compared with NH4 + or NO3 − supplied singly. Highest shoot development and NRc was at NH4 + and NO3 − ratio 1:1, followed by ratios 1:3, 3:1, 0:1, and 1:0, respectively. Addition of N in soils resulted in increased NRc, followed by rapid deterioration and death of giant witchweed plants. NRc, GSc, and GDHc in witchweed, maize, cowpea, and tobacco were affected by diurnal fluctuations with higher enzyme activities occurring during the day than at night. Higher GSc than GDHc suggests that NH4 + assimilation occurs mainly through the GS pathway in witchweed plants. NRc and GDHc were two and four times higher in giant witchweed grown in aseptic media without host plant, than that grown in potted soils with host plants. These findings provide insight into the physiological mode of N action and their implications on witchweed control.

Poisonous plants and noxious weeds are often chemically examined to determine concentrations of secondary metabolites which are responsible for their toxic or biological activity. This study examined sample size requirements and sample methods necessary to quantify accurately the concentrations of individual and total toxic alkaloids in two tall larkspur populations. A high performance liquid chromatography analytical method was utilized to determine toxic alkaloid concentrations in all leaves from three individual plant stems and leaves from the remaining stems (remainder) from each of 50 plants in each population. To obtain high precision in quantifying toxic alkaloids in the larkspur populations (within 2.5 to 5% of the population mean, 0.95 confidence), very large numbers of samples (>50–200) were required. However, lower precision (within 10% of the population mean, 0.90–0.95 confidence) required only 20 samples. Similarly, testing parameters relating to toxin concentration in tall larkspur populations within 5 or 10% of the population mean also required hundreds of samples at power levels of 0.95 and α-levels of 0.05. Relaxing power and α-level requirements to 0.80 and 0.1 respectively, reduced sample size to about 30. The means obtained by four different sampling methods were similar (P>0.05). Alkaloid concentrations in leaf samples from single stems were highly correlated to whole-plant leaf (remainder) samples (r2≥0.76), indicating that harvesting leaves from single stems provided representative samples of the entire plant. The results indicate the difficulty in obtaining accurate information about toxins in poisonous plant populations for risk assessment by livestock producers or extension agents and demonstrate the necessity for efficient analytical methodology. Researchers evaluating concentrations of plant compounds in other weeds or toxic plants should consider variability, sampling procedure, and sample size before experiments begin.

Experiments were conducted to determine the productivity, inter-, and intraspecific competitive ability of giant foxtail accessions resistant (PCW1) and susceptible (AC1) to acetyl-coenzyme A carboxylase (ACCase) inhibitors. Under noncompetitive conditions in the field, shoot dry biomass, plant height, and leaf area over time were similar between the PCW1 and AC1 accessions. The instantaneous relative growth rate and instantaneous net assimilation rate did not differ between the accessions; however, the instantaneous leaf area ratio was slightly greater for the AC1 accession than the PCW1 accession. The seed yield was similar between the PCW1 and AC1 accessions. Addition series experiments were conducted in the field to determine the intraspecific competitive ability of the PCW1 and AC1 accessions. Regression surface analysis of reciprocal mean shoot dry biomass and seed yield indicated that the relative competitive ability of the PCW1 and AC1 accessions was similar. Replacement series experiments were conducted in the greenhouse with or without corn to determine the interspecific competitive ability of the accessions. The relative growth rate, shoot dry biomass, and seed yield of the PCW1 and AC1 accessions were greater without than with corn competition. The relative competitive ability of the PCW1 and AC1 accessions was similar with or without corn competition based on shoot dry biomass or seed yield. These results indicated that the intra- and interspecific competitive ability of the PCW1 and AC1 giant foxtail accessions are similar and suggest that resistance to ACCase inhibitors has not reduced the relative fitness of the PCW1 accession.

The incidence of weed resistance to acetolactate synthase (ALS) inhibiting herbicides has increased in the United States. In 1993, a population of ALS-resistant common waterhemp was discovered after two confirmed applications of an imidazolinone herbicide. Following another imazethapyr application in the glasshouse, the resistant biotype demonstrated 130-fold resistance to imazethapyr at the whole plant level. The concentration of imazethapyr required to inhibit the ALS activity by 50% was 520 times greater for the resistant biotype than the susceptible. Plants also demonstrated cross-resistance to the sulfonylureas, chlorimuron and thifensulfuron, at the whole plant and enzyme levels. This particular discovery is of concern due to the low number of applications of the selection agent (imazaquin 1989, imazethapyr 1992, and imazethapyr in the greenhouse) and the high degree of cross-resistance eliminating several options for weed control.

Resistance to aryloxyphenoxypropionate and cyclohexanedione herbicides (ACCase inhibitors) was characterized in five green foxtail populations from western Canada. Field histories indicated that these populations were exposed to up to seven applications of diclofop-methyl and sethoxydim over the previous ten years. Major differences occurred among the populations in their levels of resistance and patterns of cross-resistance. With one exception, R/S ratios calculated from GR50 values derived from growth room experiments varied from 2 to 54 depending on the population and herbicide. The exception was UM131 which was at least 75 times more resistant to sethoxydim (R/S > 2981) than the other populations. UM59 was not resistant to sethoxydim. It was also the least resistant to the other herbicides with R/S ratios ranging from 2 for fenoxaprop-p-ethyl to 8 for diclofop-methyl. In comparison, UM8 was resistant to diclofop-methyl, fenoxaprop-p-ethyl, sethoxydim, and tralkoxydim with R/S ratios of 11, 5, > 39, and 27, respectively. In field experiments, shoot dry weights of UM8 treated at two times the recommended rates of these herbicides were reduced by 9, 45, 74, and 49%. In contrast, the susceptible population, UM7, was completely controlled. UM7 and UM8 did not differ in their response to trifluralin, ethalfluralin, quinclorac, propanil and TCA.

White light-grown seedlings of smallseed dodder were (a) provided with unilateral far-red (700 to 800 nm) at photon irradiances ranging from 20 to 110 μmol m−2 s−1 against a background of cool white light (400 to 700 nm) from above at 77 μmol m−2 s−1, or (b) transferred to darkness and provided with unilateral white light at 20 μmol m−2 s−1, unilateral blue light (400 to 500 nm) at 10 μmol m−2 s−1, unilateral red light (600 to 700 nm) at 10 μmol m−2 s−1, unilateral far-red at 50 μmol m−2 s−1, or (c) in experiments utilizing bilateral irradiations, provided with unilateral far-red perpendicular to unilateral white light. Positive phototropic curvature was induced by unilateral white light and by unilateral blue light in otherwise darkness and by unilateral far-red in a background of cool white light. Seedling vines were also phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Phototropism to unilateral white light was inhibited in seedlings treated with 200 μM norflurazon and 50 mM potassium iodide. Norflurazon- and potassium iodide-treated seedlings remained phototropic toward unilateral far-red when provided with unilateral white light perpendicular to unilateral far-red. Seedling vines were not phototropic to unilateral red or to unilateral far-red in otherwise darkness, and seedlings in cool white light were neither skototropic (i.e., tropic toward unilateral darkness) nor tropic to or from infra-red (radiation with wavelengths greater than 900 nm). Phototropism toward regions of lowered red:far-red may aid smallseed dodder in chlorophyllous host location and attachment.

The objectives of this study were to determine the inheritance of aryloxyphenoxypropionate (APP) resistance in the wild oat population UM33 and to determine the genetic relationship between resistance in UM33 and another population, UM1, which has a different cross-resistance pattern. Reciprocal crosses were made between UM33 and a susceptible population UM5, and between UM33 and UM1. Initial screenings of F1 and F2 Is populations derived from crosses between UM33 and UM5 were conducted over a range of fenoxaprop-P rates to determine a discriminatory dosage. F2 populations and F2-derived F3 families were then screened at this dosage (1200 g ai ha−1) to determine segregation patterns. Results from reciprocal UM33 x UM5 F1 dose-response experiments, and F2 and F2-derived F3 segregation experiments indicated that UM33 resistance to fenoxaprop-P was governed by a single, partially dominant nuclear gene system. To determine if resistance in UM1 and UM33 results from alterations at the same gene locus, 584 F2 plants derived from reciprocal UM33 x UM1 crosses were screened with 150 g ha−1 fenoxaprop-P. This dosage was sufficient to kill susceptible plants (UM5), but was not sufficient to kill plants with a resistance allele from either parent. None of the treated F2 plants exhibited injury or death, indicating that UM1 and UM33 resistance genes did not segregate independently. From this it was concluded that resistance in both populations is encoded at the same gene locus.