Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] is a significant weed in winter wheat (Triticum aestivum L.), corn (Zea mays L.), soybean [Glycine max (L.) Merr.], and peanut (Arachis hypogaea L.) crops in Alabama. In response to reports of herbicide failure, field surveys were conducted in these cropping systems across Alabama in 2023. The objectives were to document the distribution of herbicide resistance in the collected L. perenne ssp. multiflorum populations. Populations were evaluated in a greenhouse for sensitivity to herbicides representing three modes of action: an acetolactate synthase (ALS) inhibitor (pyroxsulam), two acetyl-coenzyme A carboxylase (ACCase) inhibitors (fluazifop-butyl and clethodim), and a 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitor (glyphosate). Herbicide screenings were followed by dose–response assays of the most resistant L. perenne ssp. multiflorum population for each herbicide at eight rates (0.5, 1, 2, 4, 8, 16, 32, and 64×) compared with a susceptible population at six rates (0.0625, 0.125, 0.25, 0.5, 1, and 2×). Out of 44 populations evaluated, 21%, 11%, 25%, and 2% were found resistant to glyphosate, fluazifop-butyl, pyroxsulam, and clethodim, respectively. Resistance levels were confirmed to be 192-, 14-, 90-, and 738-fold for glyphosate, fluazifop-butyl, pyroxsulam, and clethodim, respectively. Mutation detection studies revealed specific mutations: Asp-2078-Gly in the ACCase gene, Pro-106-Ser in the EPSPS gene, and a novel Arg-421-Thr mutation in the ALS gene.

Downy brome (Bromus tectorum L.) is a difficult species to control in the dryland wheat (Triticum aestivum L.) production areas of northeastern Oregon. The selection of herbicide-resistant B. tectorum populations has further complicated B. tectorum management. A survey of wheat growers was conducted in 2021 and 2022 to understand B. tectorum management practices. The survey included four questions based on the growing seasons from 2017 to 2022 related to crop rotation, tillage versus no-tillage, irrigation versus dryland, and herbicide programs. To determine the extent of herbicide resistance, seeds were collected from 49 B. tectorum populations in wheat fields in northeastern Oregon and tested for resistance. Herbicides tested were clethodim, glyphosate, imazamox, mesosulfuron, metribuzin, propoxycarbazone, pyroxasulfone, pyroxsulam, quizalofop, and sulfosulfuron. Winter wheat–summer fallow rotation was the most predominant cropping system in the region. Most of the fields were in no-tillage systems, and none were irrigated. Pyroxasulfone applied preemergence and acetolactate synthase (ALS) inhibitors applied postemergence were the most often used herbicides for B. tectorum control in winter wheat. Glyphosate was the most frequently used herbicide for B. tectorum control in summer fallow. Resistance screenings confirmed that 46 of the 49 B. tectorum populations were resistant to ALS inhibitors with different cross-resistance patterns. Two populations were resistant to metribuzin and exhibited multiple resistance to ALS inhibitors. All populations were susceptible to clethodim, glyphosate, pyroxasulfone, and quizalofop. The widespread occurrence of ALS inhibitor–resistant B. tectorum populations limits effective postemergence herbicide options in winter wheat.

Herbicides are the primary tool for controlling weeds in peanut (Arachis hypogaea L.) and are crucial to sustainable peanut production in the United States. The literature on chemical weed management in peanut in the past 53 yr (1970 to 2022) in the United States was systematically reviewed to highlight the strengths and weaknesses of different herbicides and identify current research gaps in chemical weed management. Residual weed control in peanut is achieved mainly with dimethenamid-P, ethalfluralin, pendimethalin, and S-metolachlor. More recently, the use of the protoporphyrinogen oxidase inhibitor flumioxazin and acetolactate synthase inhibitors, such as diclosulam, for residual weed control in peanut has increased considerably. Postemergence broadleaf weed control in peanut is achieved mainly with acifluorfen, bentazon, diclosulam, imazapic, lactofen, paraquat, and 2,4-DB, while the graminicides clethodim and sethoxydim are the major postemergence grass weed control herbicides in peanut. Although several herbicides are available for weed control in peanut, no single herbicide can provide season-long weed control due to limited application timing, lack of extended residual activity, variability in weed control spectrum, and rotational restrictions. Therefore, effective weed management in peanut often requires herbicide mixtures and/or sequential application of preplant-incorporated, preemergence, and/or postemergence herbicides. However, the available literature showed a substantive range in herbicide efficacy due to variations in environmental conditions and flushes of weed germination across years and locations. Despite the relatively high efficacy of herbicides, the selection of herbicide-resistant weeds is another area of increasing concern. Future research should focus on developing new strategies for preventing or delaying the development of resistance and improving herbicide efficacy within the context of climate change and emerging constraints such as water shortages, rising temperatures, and increasing CO2 concentration.

Protoporphyrinogen oxidase (PPO)-inhibiting herbicides remain an important and useful chemistry 60 yr after their first introduction. In this review, based on topics introduced at the Weed Science Society of America 2021 symposium titled “A History, Overview, and Plan of Action on PPO Inhibiting Herbicides,” we discuss the current state of PPO-inhibiting herbicides. Renewed interest in the PPO-inhibiting herbicides in recent years, due to increased use and increased cases of resistance, has led to refinements in knowledge regarding the mechanism of action of PPO inhibitors. Herein we discuss the importance of the two isoforms of PPO in plants, compile a current knowledge of target-site resistance mechanisms, examine non–target site resistance cases, and review crop selectivity mechanisms. Consistent and reproducible greenhouse screening and target-site mutation assays are necessary to effectively study and compare PPO-inhibitor resistance cases. To this end, we cover best practices in screening to accurately identify resistance ratios and properly interpret common screens for point mutations. The future of effective and sustainable PPO-inhibitor use relies on development of new chemistries that maintain activity on resistant biotypes and the promotion of responsible stewardship of PPO inhibitors both new and old. We present the biorational design of the new PPO inhibitor trifludimoxazin to highlight the future of PPO-inhibitor development and discuss the elements of sustainable weed control programs using PPO inhibitors, as well as how responsible stewardship can be incentivized. The sustained use of PPO inhibitors in future agriculture relies on the effective and timely communication from mode of action and resistance research to agronomists, Extension workers, and farmers.

False-green kyllinga is a problematic C4 perennial Cyperaceae species. Previous research examined herbicide efficacy but has not integrated nonchemical practices with herbicides. Replicate field experiments were conducted to evaluate late-summer postemergence applications of herbicides in combination with turf-type tall fescue interseeding for false-green kyllinga control. Seven herbicide treatments and a nontreated control formed a complete factorial, with tall fescue interseeding or no seeding (eight-by-two factorial). Six herbicide treatments consisted of single postemergence applications of halosulfuron-methyl (70 g ha–1), imazosulfuron (420 g ha–1), and sulfentrazone + carfentrazone (280 + 30 g ha–1) applied 4 wk before tall fescue interseeding (WBS) and the day of interseeding. Glyphosate (220 g ae ha–1) applied the day of interseeding was the seventh herbicide treatment. Tall fescue was interseeded in September, and false-green kyllinga control was evaluated the following summer. In combination with tall fescue interseeding, imazosulfuron and halosulfuron applied 4 WBS as well as glyphosate applied the day of interseeding controlled false-green kyllinga better than all other treatments. In July, imazosulfuron and halosulfuron treatments applied 4 WBS and combined with interseeding had 1% to 6% false-green kyllinga cover, respectively, compared to 28% and 62% cover, respectively, without interseeding. Interseeding alone controlled false-green kyllinga <50%. Imazosulfuron, halosulfuron, and sulfentrazone + carfentrazone applied the day of seeding severely injured emerging tall fescue seedlings and reduced turfgrass quality the following spring. Applying imazosulfuron or halosulfuron in late summer and interseeding turf-type tall fescue 4 wk later or applying glyphosate the day of seeding are effective strategies for postemergence false-green kyllinga control. Integrating herbicides and the cultural practice of tall fescue interseeding provided more false-green kyllinga control than either practice alone.

Ludwigia prostrata is a problematic weed in rice fields in China, where acetolactate synthase (ALS)-inhibiting herbicides (e.g., bensulfuron-methyl) are widely used for the management of broadleaf weeds. Recently, an L. prostrata biotype (JS-R) that failed to be controlled with ALS-inhibiting herbicides was found in Jiangsu Province, China. This study aims to determine the level and molecular mechanism of resistance to bensulfuron-methyl in this JS-R biotype and to evaluate its spectrum of cross-resistance to other ALS-inhibiting herbicides. The dose–response assays indicated that the JS-R L. prostrata biotype had evolved 21.2-fold resistance to bensulfuron-methyl compared with the susceptible biotype (JS-S). ALS gene sequencing revealed that a nucleotide mutation (CCA to TCA) at codon 197, resulting in a Pro-197-Ser mutation, was detected in the resistant plants. Moreover, while the JS-R biotype contained the Pro-197-Ser resistance mutation and showed cross-resistance to pyrazosulfuron-ethyl (12.0-fold), it was sensitive to penoxsulam, bispyribac-sodium, and imazethapyr, which may serve as alternative herbicides to control the resistant L. prostrata biotype. This is the first confirmation of an L. prostrata biotype resistant to bensulfuron-methyl due to a Pro-197-Ser resistance mutation in the ALS gene.

A survey of the prevalence of rigid ryegrass (Lolium rigidum) resistant to ACCase and ALS herbicides was conducted in major-cereal growing regions in the north of Tunisia. Randomly collected ryegrass populations were assessed, using the Syngenta RISQ® test, for resistance to clodinafop-propargyl, iodosulphuron + mesosulphuron and pinoxaden. Of the 177 tested populations, 58% exhibited resistance to clodinafop-propargyl and 52% to iodosulphuron + mesosulphuron, with 40% exhibiting resistance to both herbicides. Significant variations in the frequencies of rigid ryegrass resistant to clodinafop-propargyl and/or iodosulphuron + mesosulphuron were observed between surveyed regions which may be the result of differences in the history of herbicide use. Over 50% of resistant populations contained 60% of resistant plants or more, indicating the extent of resistance evolution in these regions. Our study demonstrates that the extent of resistance to ACCase and ALS-inhibiting herbicides in rigid ryegrass is widespread in major cereal-growing regions of Tunisia. Therefore, weed management must be focused on reducing the frequency of herbicide application, using multiple herbicide mechanisms of action, rotating different modes of action and integrating alternative control options.

We conducted research to evaluate various herbicides for POST false-green kyllinga control in cool-season turfgrass (primarily creeping bentgrass). In a preliminary evaluation, single and sequential applications of halosulfuron-methyl (70 g ai ha−1), mesotrione (175 g ai ha−1), and sulfentrazone (140 g ai ha−1), as well as a single application of imazosulfuron (740 g ai ha−1), were evaluated in New Jersey. Imazosulfuron and sequential applications of halosulfuron-methyl controlled false-green kyllinga >93% at 9 and 18 wk after initial treatment (WAIT). Sulfentrazone and mesotrione controlled false-green kyllinga <50%. Additional experiments were conducted to evaluate single and sequential applications of halosulfuron-methyl (70 g ha−1), imazosulfuron (420 and 740 g ha−1), and sulfentrazone (140 g ha−1) in New Jersey and Indiana at two locations in each state. At 12 WAIT, imazosulfuron generally controlled false-green kyllinga more effectively than other treatments at all locations. Sequential applications of imazosulfuron controlled false-green kyllinga 100% at 12 WAIT. Halosulfuron-methyl was less effective in Indiana than in New Jersey. Sulfentrazone controlled false-green kyllinga <40% at 12 WAIT. This research demonstrates that imazosulfuron is more effective than halosulfuron-methyl and sulfentrazone for POST false-green kyllinga control in cool-season turf.

Wild mustard (Sinapis arvensis L.) is a weed that frequently infests winter wheat (Triticum aestivum L.) fields in Golestan province, Iran. Tribenuron-methyl (TM) has been used recurrently to control this species, thus selecting for resistant S. arvensis populations. The objectives were: (1) to determine the resistance level to TM of 14 putatively resistant (PR) S. arvensis populations, collected from winter wheat fields in Golestan province, Iran, in comparison to one susceptible (S) population; and (2) to characterize the resistance mechanisms and the potential evolution of cross-resistance to other classes of acetolactate synthase (ALS)-inhibiting herbicides in three populations (AL-3, G-5, and Ag-Sr) confirmed as being resistant (R) to TM. The TM doses required to reduce the dry weight of the PR populations by 50% were between 2.2 and 16.8 times higher than those needed for S plants. The ALS enzyme activity assays revealed that the AL-3, G-5, and Ag-Sr populations evolved cross-resistance to the candidate ALS-inhibiting herbicides from the sulfonylureas (SU), triazolopyrimidines (TP), pyrimidinyl-thiobenzoates (PTB), sulfonyl-aminocarbonyl-triazolinone (SCT), and imidazolinones (IMI) classes. No differences in absorption, translocation, or metabolism of [14C]TM between R and S plants were observed, suggesting that these non-target mechanisms were not responsible for the resistance. The ALS gene of the R populations contained the Trp-574-Leu mutation, conferring cross-resistance to the SU, SCT, PTB, TP, and IMI classes. The Trp-574-Leu mutation in the ALS gene conferred cross-resistance to ALS-inhibiting herbicides in S. arvensis from winter wheat fields in Golestan province. This is the first TM resistance case confirmed in this species in Iran.

Tall fescue is susceptible to injury from many acetolactate synthase (ALS) inhibitors used for broadleaf weed control in turfgrass. Florasulam is an ALS inhibitor that selectively controls broadleaf weeds in tall fescue, but the mechanisms for selectivity are not well understood. The objective of this research was to evaluate the physiological basis of tall fescue tolerance to florasulam. In greenhouse experiments, florasulam rates required to injure tall fescue 20% (I20) and white clover 80% (I80) measured 320 and 65 g ai ha–1, respectively. The I20 and I80 values of another ALS inhibitor, flucarbazone, on these species measured 33 and 275 g ai ha–1, respectively. In laboratory experiments, the time required to reach 50% foliar uptake for 14C-florasulam and 14C-flucarbazone measured 23 and 62 h for white clover, respectively, and >72 h for both herbicides in tall fescue. The half-lives of florasulam and flucarbazone in tall fescue were 15 and 40 h, respectively, whereas the half-life in white clover was >72 h for both herbicides. The concentrations of florasulam and flucarbazone required to inhibit ALS enzymes 50% in excised leaves of tall fescue measured >1,000 and 32 μM, respectively. The selectivity of florasulam for white clover control in tall fescue is associated with differential levels of absorption and metabolism between species. Tall fescue has faster metabolism and less ALS enzyme inhibition from florasulam as compared to a more injurious ALS inhibitor, flucarbazone, which contributes to the differential tolerance levels between these herbicides.

Eight triazolopyrimidine sulfonanilides were tested for metabolic stability in a number of crop and weed species. These data, along with in vitro determinations of activity (I50) against acetolactate synthase, successfully described the in vivo activity of these compounds in a two-parameter model. Whole plant activity increased with increasing compound stability and decreasing I50 (r2 =.78, N = 36). The difficulty in obtaining metabolic stability data during a structure optimization program prompted a study with substituent parameters in models of in vivo activity. Models describing whole plant activity in jimsonweed were developed using a series of 5-methyl triazolopyrimidine sulfonanilides that differed only in ortho and meta substituents on the aniline ring. The I50 term and clogP were most important to jimsonweed activity. Hence, in vitro activity (I50) may be a useful component of whole plant structure activity models to aid in identification of barriers to in vivo performance.

Greenhouse and laboratory studies were conducted to determine the degree of dominance of the monogenic sulfonylurea herbicide resistance trait in diploid sugarbeet by comparing the response of homozygous and heterozygous resistant sugarbeet to primisulfuron, thifensulfuron, and chlorimuron on the whole plant and acetolactate synthase (ALS) enzyme level. Progeny tests suggested that the monogenic sulfonylurea herbicide resistance was semidominant. Subsequently, heterozygous resistant (R-1) and homozygous resistant (R-2) sugarbeet lines were sprayed with increasing rates of primisulfuron, thifensulfuron, and chlorimuron, and herbicide rates required for 50% growth reduction (GR50) were determined. GR50 values were also determined for homozygous susceptible sugarbeet lines (S-1 and S-2). The GR50 values indicated that the R-2 sugarbeet was 377, 269, and 144 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than susceptible S-2 sugarbeet. In contrast, R-1 sugarbeet was only 107, 76, and 57 times more resistant to primisulfuron, thifensulfuron, and chlorimuron, respectively, than S-1 sugarbeet, indicating at least a twofold difference in the magnitude of resistance between homozygous resistant and heterozygous resistant sugarbeet lines. ALS enzyme activity analysis were consistent with whole plant results. Thus, based on these two, maximum crop resistance can be obtained by developing homozygous resistant cultivars.

Five biotypes of common cocklebur that were not controlled with acetolactate synthase (ALS)-inhibiting herbicides were tested in greenhouse and laboratory studies to determine the magnitude of resistance and cross-resistance to four ALS-inhibiting herbicides. In vivo inhibition of ALS was also evaluated. Based on phytotoxicity, all five ALS-resistant biotypes of common cocklebur were > 390 times more resistant than the susceptible biotype to imazethapyr. However, only four of these biotypes were also resistant to another imidazolinone, imazaquin. Two biotypes were cross-resistant to the sulfonylurea, chlorimuron, and the triazolopyrimidine sulfonanilide, NAF-75. One biotype demonstrated intermediate susceptibility to imazaquin, chlorimuron, and NAF-75. In all cases, the resistance exhibited at the whole plant level was associated with an insensitive ALS.

Synergistic interaction between the insecticide terbufos and the herbicide nicosulfuron may result in severe injury to corn. Greenhouse and laboratory experiments were conducted to determine if using the imidazolinone-resistant corn ‘Pioneer-3343 IR’ (P-3343 IR) or coating corn seeds with naphthalic anhydride (NA) would reduce herbicidal injury imposed by the nicosulfuron-terbufos interaction. Greenhouse experiments showed nicosulfuron-terbufos interactions resulting in herbicidal injury in both P-3343 IR and ‘DeKalb 689’ (D-689) corn varieties, but the D-689 was more sensitive than the P-3343 IR corn. The greenhouse experiments also demonstrated protection against the nicosulfuron-terbufos interaction by NA seed treatments. Studies with radiolabeled nicosulfuron showed that terbufos inhibited the metabolism of nicosulfuron, but pretreatment of D-689 and P-3343 IR corn seed with NA decreased the inhibition in excised corn leaves. The differences in sensitivity to nicosulfuron in the two corn varieties resulted in part from the differential metabolism and primarily from the differential sensitivity of the target enzyme, acetolactate synthase, to the herbicide.

Chlorimuron antagonized the activity of quizalofop on broadleaf signalgrass when applied in greenhouse studies as a postemergence tank mix. In vitro leaf disc assays utilizing 14C-acetate or 14C-pyruvate as substrates were conducted to ascertain the effect of clorimuron and quizalofop on fatty acid biosynthesis and to determine if antagonism between the two herbicides occurs at the biochemical sites of action. Incorporation of 14C-acetate in control treatments was linear with time to 120 min. Acetate incorporation in the presence of quizalofop (1.1 μM) was also linear but was inhibited 30 min after initialization of the reaction. The concentration of quizalofop that inhibited 14C-acetate incorporation 50% (I50) was 0.54 μM. Chlorimuron, up to 155 μM, had no effect on 14C-acetate incorporation. A mixture of quizalofop (1.1 μM) and chlorimuron (4.8 μM) inhibited 14C-acetate incorporation similar to that of quizalofop alone at 1.1 μM. Quizalofop I50 for incorporation of 14C-pyruvate was 1.1 μM, and clorimuron at 4.8 μM decreased incorporation 15%. Excess unlabeled pyruvate (5 μM) had no effect on either 14C-acetate or 14C-pyruvate incorporation in the presence of both herbicides. It is believed that antagonism of quizalofop by clorimuron is not due to an excess pool of pyruvate resulting from inhibition of acetolactate synthase by chlorimuron.

Chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl]-benzenesulfonamide} applied preemergence or postemergence as a soil drench retards the growth of four corn (Zea mays L.) genotypes in soil with 50% inhibition in each case at a rate of approximately 5 g ai/ha. R-25788 (N,N-diallyl-2,2-dichloroacetamide), applied at 1.0 kg ai/ha to the soil preemergence with chlorsulfuron or postemergence 2 days prior to chlorsulfuron treatment, reverses shoot growth inhibition induced by the herbicide in varying degree in ‘XL25A’ and ‘XL72B’ corn hybrids and ‘FRM017’ and ‘FR619’ inbreds, but not in soybean [Glycine max (L.) Merr., var. ‘Amsoy-71’]. Under similar conditions, flurazole [2-chloro-4-(trifluoromethyl)-5-thiazolecarboxylic acid (phenylmethyl ester)] and CGA-92194 [α-(1,3-dioxolan-2-yl-methoxy)-imino-benzeneacetonitrile] are less effective than R-25788 in protecting corn genotypes from chlorsulfuron injury. R-25788 also partially reverses chlorsulfuron-induced root growth inhibition in XL25A and FR619 corn but not in pea (Pisum sativum L. var. ‘Dwarf Gray Sugar’). Acetohydroxyacid synthase (AHAS) is higher in both activity and the proportion that is very sensitive to in vitro chlorsulfuron inhibition when prepared from XL25A shoots than from the roots. R-25788 does not reverse the in vitro inhibition of corn acetohydroxyacid synthase (AHAS) by chlorsulfuron. Pretreatment of 5-day-old XL25A corn with R-25788 at 24 or 48 μM significantly increases AHAS activity by approximately 25% in both shoots and roots but does not change its in vitro sensitivity to chlorsulfuron. This antidote also increases the glutathione (GSH) content in roots and etiolated shoots. The R-25788-induced elevation of AHAS activity may contribute to its antidotal effect in corn.

Field observations indicate that sulfonylurea-resistant kochia may germinate at lower soil temperatures and/or germinate more rapidly than susceptible kochia in the absence of herbicide. To investigate this possibility, seeds from three resistant and two susceptible kochia accessions were germinated at temperatures ranging from 4.6 to 13.2 C on thermal gradient plates. At 4.6 and 13.2 C, germination rates of all resistant accessions were higher than susceptible accessions, while germination rates of one resistant accession were higher than susceptible accessions at 7.2 and 10.5 C. Percent germination of all resistant accessions was significantly higher than susceptible accessions after 48 h at 4.6 C. At higher temperatures, percent germination of some resistant accessions was higher after 12 or 24 h, but germination of all accessions was similar at later times. HPLC analysis revealed that seeds from resistant accessions contained about 2-fold higher free levels of branched chain amino acids than seeds from susceptible accessions. The results indicate that mutations conferring resistance to sulfonylurea herbicides in these kochia accessions may concomitantly reduce or abolish acetolactate synthase sensitivity to normal feedback inhibition patterns, resulting in elevated levels of branched chain amino acids available for cell division and growth during early germination.

Greenhouse and laboratory studies were conducted to determine the extent of cross-resistance of chlorsulfuron-resistant sugarbeet (CR1-B) to other herbicides that inhibit acetolactate synthase (ALS) and to determine the physiological basis of resistance. Cross-resistance to metsulfuron, imazaquin, and imazethapyr was not evident, while only marginal cross-resistance was observed to triasulfuron, DPX-L5300, and nicosulfuron. CR1-B was moderately resistant to chlorsulfuron and chlorimuron and was highly cross-resistant to thifensulfuron and primisulfuron. Further greenhouse studies demonstrated that CR1-B was not significantly injured by thifensulfuron and primisulfuron applied at or exceeding the field use rate. Studies with 14C-primisulfuron showed that differential absorption or metabolism of primisulfuron could not account for the observed resistance. ALS enzyme assays showed that the CR1-B ALS enzyme activity was 66, 26, and 13 times less sensitive to chlorsulfuron, thifensulfuron, and primisulfuron inhibition, respectively, compared to ALS enzyme extracted from sensitive sugarbeets. An altered ALS enzyme, which is less sensitive to sulfonylurea herbicide inhibition, appears to be the physiological basis of resistance.

Variability in weed control following pyrithiobac applications has been observed under field conditions. The influence of temperature on this variability was investigated. Results from field studies performed over two growing seasons identified plant and air temperatures at the time of herbicide treatment that correlated with whole-plant efficacy differences. Based on the field data, weed control with pyrithiobac was acceptable at application temperatures of 20 to 34 C. To investigate a potential source of thermal limitations on pyrithiobac efficacy, the thermal dependence of in vitro inhibition of acetolactate synthase (ALS), the site of action for pyrithiobac, was examined. A crude leaf extract of ALS was obtained from Amaranthus palmeri. Relative inhibitor potency (I50) values were obtained at saturating substrate conditions for temperatures from 10 to 50 C. Regression analysis of field activity against I50 values showed the two data sets to be highly correlated (R 2 = 0.88). The thermal dependence of enzyme/herbicide interactions may provide another means of understanding environmental factors limiting herbicidal efficacy and predicting herbicide inhibition at the whole-plant level.

Greenhouse and laboratory studies were conducted to determine kochia resistance to a spectrum of acetolactate synthase (E.C.4.13.18) (ALS)-inhibiting herbicides. The chlorsulfuron-resistant biotype plants were resistant to six herbicides: triflusulfuron, thifensulfuron, halosulfuron, imazamethabenz, chlorsulfuron, and nicosulfuron. But, the resistant biotypes showed sensitivity similar to the susceptible biotypes to three herbicides: metsulfuron, imazethapyr, and imazaquin. The resistant biotypes were slightly less sensitive to primisulfuron, chlorimuron, and flumetsulam than the sensitive biotypes. The I50 values for 50% inhibition of the ALS enzyme indicated that the resistant biotype was 22, 18, and 16 times more resistant to primisulfuron, chlorsulfuron, and thifensulfuron than the susceptible biotype. In contrast, the I50 ratios (resistant/susceptible) were 3, 2, and 1 for flumetsulam, nicosulfuron, and imazethapyr, respectively. The altered ALS enzyme system of the resistant biotype showed a differential response for the ALS-inhibiting herbicides. Addition of a mixed function oxidase inhibitor, piperonyl butoxide (PBO) at 2 kg ha−1, to primisulfuron and thifensulfuron increased visual injury and reduced plant height of the chlorsulfuron-sensitive kochia biotype plants. The addition of PBO to primisulfuron enhanced visual injury of the resistant biotype at low rates of primisulfuron.