The efficacy of WSSA Group 4 herbicides has been reported to vary with dependence on the time of day the application is made, which may affect the value of this mechanism of action as a control option and resistance management tool for Palmer amaranth. The objectives of this research were to evaluate the effect of time of day for application on 2,4-D and dicamba translocation and whether or not altering translocation affected any existing variation in phytotoxicity seen across application time of day. Maximum translocation (Tmax) of [14C]2,4-D and [14C]dicamba out of the treated leaf was significantly increased 52% and 29% to 34% in one of two repeated experiments for each herbicide, respectively, with application at 7:00 AM compared with applications at 2:00 PM and/or 12:00 AM. Applications at 7:00 AM increased [14C]2,4-D distribution to roots and increased [14C]dicamba distribution above the treated leaf compared with other application timings. In phytotoxicity experiments, dicamba application at 8 h after exposure to darkness (HAED) resulted in significantly lower dry root biomass than dicamba application at 8 h after exposure to light (HAEL). Contrasts indicated that injury resulting from dicamba application at 8 HAEL, corresponding to midday, was significantly reduced with a root treatment of 5-[N-(3,4-dimethoxyphenylethyl)methylamino]-2-(3,4-dimethoxyphenyl)-2-isopropylvaleronitrile hydrochloride (verapamil) compared with injury observed with dicamba application and a root treatment of verapamil at 8 HAED, which corresponded to dawn. Overall, time of application appears to potentially influence translocation of 2,4-D and dicamba. Furthermore, inhibition of translocation appears to somewhat influence variation in phytotoxicity across times of application. Therefore, translocation may be involved in the varying efficacy of WSSA Group 4 herbicides due to application time of day, which has implications for the use of this mechanism of action for effective control and resistance management of Palmer amaranth.

Amicarbazone controls annual bluegrass in cool-season turfgrasses but physiological effects that influence selectivity have received limited investigation. The objective of this research was to evaluate uptake, translocation, and metabolism of amicarbazone in these species. Annual bluegrass, creeping bentgrass, and tall fescue required < 3, 56, and 35 h to reach 50% foliar absorption, respectively. At 72 h after treatment (HAT), annual bluegrass and creeping bentgrass translocated 73 and 70% of root-absorbed 14C to shoots, respectively, while tall fescue only distributed 55%. Annual bluegrass recovered ≈ 50% more root-absorbed 14C in shoots than creeping bentgrass and tall fescue. Creeping bentgrass and tall fescue metabolism of amicarbazone was ≈ 2-fold greater than annual bluegrass from 1 to 7 d after treatment (DAT). Results suggest greater absorption, more distribution, and less metabolism of amicarbazone in annual bluegrass, compared to creeping bentgrass and tall fescue, could be attributed to selectivity of POST applications.

The phytotoxicities of soil-incorporated 2,6-dichlorobenzonitrile [dichlobenil] and 2,6-dichlorothiobenzamide [hereinafter referred to as SD-7961] were compared in five soils in the greenhouse. Dichlobenil and SD-7961 were about equally toxic to snap beans (Phaseolus vulgarus L., var. Black Valentine) in each of the five soils. The herbicides were more toxic in Christiana loam than in the other four soils. Phytotoxic residues of both herbicides disappeared most rapidly from Hagerstown silty clay loam and Christiana loam and least rapidly from Sharkey clay. Gas chromatographic analyses of soil extracts confirmed that dichlobenil was most persistent in the clay. Small amounts (0.14 ppmw or less) of 2,6-dichlorobenzoic acid [hereinafter referred to as 2,6-DBA] were detected in soil from greenhouse pot cultures about 9 months after incorporation of dichlobenil. In a field experiment at Beltsville, Maryland, residues of dichlobenil persisted from one season to the next but did not build up in soil sprayed annually for 3 years with 4 or 8 lb/A. Some movement from the surface 6 in into the 6 to 12-in zone occurred on plots sprayed 2 years in succession with 8 lb/A or once with 40 lb/A. Rapid loss of dichlobenil occurred during summer months. When the concentration of dichlobenil on the 40-lb/A plots dropped to about 10%, subsequent loss occurred at a very slow rate.

Adsorption and desorption characteristics of fluridone (1-methyl-3-phenyl-5-[3-txifluoromethyl)phenyl]-4-(1H)-pyridinone} on prepared clays and organic matter were studied in unbuffered and buffered aqueous solutions. In unbuffered aqueous solution the adsorption of fluridone decreased in the order: H-saturated organic matter (H-OM) > Ca-saturated montmorillonite (Ca-mont) > Ca-saturated organic matter (Ca-OM) > kaolinite. Based on the chemistry of fluridone, the nature of the adsorbent, and previously reported studies, the major adsorption mechanisms appear to be pH-dependent adsorption of protonated fluridone and direct protonation of the herbicide at acidic surfaces, supplemented by physical adsorption forces such as van der Waals attractions and charge transfer bonds. The adsorption of fluridone on Norfolk sand (Typic Paleudult; fine-loamy, siliceous, thermic), unmodified (CK), or amended with montmorillonite (HC) or organic matter (HM) at pH 4.0, 5.2, and 7.0 was also studied. Over all adsorption was greatest on HC soil, least on CK soil, and intermediate on the HM soil. In each system adsorption was inversely related to pH. These results were interpreted by comparison with adsorption observed on prepared clay minerals and organic matter.

Weed emergence models require biological parameters such as base temperature for germination, determination of which is costly and time-consuming. Transferability of these parameters across different populations may therefore represent one of the main constraints for the development and practical use of emergence models at a large scale. A collaborative project was undertaken to assess the interpopulation variability of base temperature for germination in different European populations of velvetleaf and jimsonweed and evaluate possible applicative consequences in terms of weed control. Seeds were collected in Italy, Portugal, and Spain, and each population was then sown in every country, obtaining nine seed batches named as experimental lots. Base temperature for germination was estimated for each experimental lot to calculate lot-specific accumulation of growing degree days (GDD) under three dissimilar climatic scenarios. Threshold date (TD50) was calculated as the date when GDD accumulation of a given experimental lot surpassed the values corresponding to 50% of cumulative field emergence of seedlings. GDD accumulation and TD50 were then used as indicators to identify differences among experimental lots within each climatic scenario. No significant differences were detected among base temperatures estimated for velvetleaf experimental lots or among their patterns of accumulation of GDD and TD50 values within climatic scenarios. Each value of base temperature determined for a single experimental lot could therefore be adopted to model germination for all the lots regardless of the population of origin or cultivation site. In contrast, the population of origin affected the base temperature for jimsonweed, with significantly higher values for experimental lots of the Portuguese population. From an applicative perspective, differences among patterns of accumulation of GDD and TD50 of several experimental lots within each climatic scenario suggest the need to use population-specific values as base temperature for germination and emergence modeling of jimsonweed.

Traditional breeding has been used to develop grain sorghum germplasm that is tolerant to acetolactate synthase (ALS)-inhibiting herbicides (Inzen Technology, DuPont). Inzen sorghum carries a double mutation in the ALS gene (Val560Ile and Trp574Leu), which confers high level of tolerance to ALS-inhibiting herbicides. Overreliance on ALS-inhibiting herbicides for weed control during the 1990s resulted in the evolution of ALS inhibitor–resistant shattercane populations in Nebraska. According to a survey conducted in 2013, ALS inhibitor–resistant weedy Sorghum populations persist in Nebraska. The objectives of this research were to determine whether the ALS mutations present in Inzen sorghum were present in the ALS inhibitor–resistant shattercane and johnsongrass populations detected in Nebraska and northern Kansas, and whether these populations evolved ALS resistance independently. Primers specific to the Val560 and Trp574 region of the ALS gene were used to screen the populations with PCR. The Trp574Leu mutation was present in one ALS inhibitor–resistant johnsongrass population. The Val560Ile was detected in three ALS inhibitor–resistant shattercane, one susceptible shattercane, one ALS inhibitor–resistant johnsongrass, and one susceptible johnsongrass population. Moreover, Val560Ile was present in resistant and/or susceptible individuals within johnsongrass and shattercane populations that were segregating for ALS resistance, indicating that by itself the Val560Ile mutation does not confer resistance to ALS-inhibiting herbicides. None of the populations presented both mutations simultaneously, as does Inzen sorghum. A shattercane population containing the Ser653Thr mutation was also detected. This research indicates that the ALS mutations present in Inzen sorghum already exist individually in weedy sorghum populations. Moreover, our results present strong evidence that ALS resistance in these populations evolved independently. Thus, widespread overreliance on ALS-inhibiting herbicides prior to adoption of glyphosate-tolerant crops in the 1990s exerted sufficient selective pressure on shattercane and johnsongrass populations for resistance to evolve multiple times in the Midwest. Finally, a survey of the 5′ portion of the ALS gene in more diverse wild and weedy Sorghum species was hampered by limited coverage in genomic resequencing surveys, suggesting that refined PCR-based methods will be needed to assess SNP variation in this gene region, which includes the Ala122, Pro197, and Ala205 codons known to confer ALS resistance in other species.

Ryegrass (Lolium spp.) is a troublesome weed in major wheat (Triticum aestivum L.) production regions in the United States. High diversity and adaptive potential are known to contribute to its success as a weed species and also create difficulties in correct species identification in fields. The objective of this research was to characterize diversity for 16 different morphological traits among 56 Lolium populations collected from wheat production fields across the Texas Blackland Prairies region and identify Lolium species based on taxonomic characteristics. Populations were highly diverse (both at inter- and intrapopulation levels) for the traits studied, and a taxonomic comparison with USDA-GRIN reference samples revealed that all the populations were variants of Italian ryegrass [Lolium perenne L. ssp. multiflorum (Lam.) Husnot] with a few offtypes of perennial ryegrass (Lolium perenne L.) or probable hybrids between the two species. Hierarchical clustering grouped the populations into six clusters based on their similarities for the morphological traits investigated. Principal component analysis showed that the variability for yield traits greatly contributed to the total diversity. Pre-flowering plant height (stage 10 on Feekes scale) was positively correlated with tiller count, shoot biomass, and spike count, but not with total seed count per plant, whereas plant height at maturity (stage 11.3 to 11.4 on Feekes scale) was highly correlated with total seeds per plant. Further, basal node color was positively correlated with plant growth habit, regrowth rate, and leaf color. Leaf blade width was positively correlated with survival to pinoxaden and multiple herbicides, whereas, spike count was negatively correlated with survival to mesosulfuron. The high levels of intra- as well as interpopulation variability documented in this study indicate the potential of this species to rapidly adapt to herbicides and emphasize the need for implementing diverse management tactics, including the integration of harvest weed seed control.

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.

Growth habit is an important determinant of competitive ability in legume crops. Erect ‘Iron-Clay’ (IC), semi-erect ‘IT89KD-288’ (288), and prostrate ‘UCR 779’ (779) cowpea genotypes were planted with four densities of common sunflower or common purslane to determine which cowpea growth habit is more competitive to these weeds having markedly different statures. Both sunflower and purslane reduced cowpea relative growth rate (RGR) but at different phases of the growing season. Low-growing purslane had an effect in the earlier part of the season, and sunflower decreased cowpea RGR in the middle of growing season. RGR of genotype IC was the least affected and genotype 779 the most reduced by sunflower. Cowpea apparently compensated for early season purslane competition by increasing RGR after purslane flowering, with RGR for the IC genotype increasing the most and 779 increasing the least. All cowpea genotypes caused a similar reduction in sunflower biomass accumulation; but purslane biomass was most reduced by genotype IC and least affected by genotype 288. Erect genotype IC is more competitive due to its taller stature, greater height growth rate, and higher position of maximal leaf area density, despite a lower photosynthetic rate and light use efficiency than the other cowpea genotypes. Our results suggest that erect cowpea growth habit may be generally more competitive with weeds compared to semi-erect or prostrate growth habit.

The tolerances of wheat and wild buckwheat (Polygonum convolvulus) to various rates of 2-methoxy-3,6-dichlorobenzoic acid (dicamba) were determined under field conditions and supplemented by studies with wheat in the growth chamber. Wheat height and yield were inversely proportional to rate, but response to various rates was dependent on stage of growth at application. Greatest reductions in wheat height and yield resulted from treatment at the late tiller and boot stages, respectively. The weight per 100 wheat seeds was increased from treatment at the boot and flower stages but was decreased at the early tiller stage.

The ratio of peduncle length: length of the flag leaf-sheath was decreased most from 24 oz/A applied preemergence and 12 oz/A applied at the late tiller stage.

Growth rate of wheat was decreased for a 2-week period after treatment. Excellent kill of wild buckwheat was observed from all rates applied through the 12- to 15-leaf stage of growth.

Application of 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine) generally reduced dry weight of tops and roots of corn and Pinus radiata seedlings. Simazine always increased top-root ratios of corn but generally decreased those of pine seedlings. Simazine increased significantly the uptake of N by corn in all soils; of Mg and P in limed, and of K in acidified soils. Aluminum sulfate decreased injury from simazine. Lime at 3 tons per acre mixed into the soil exerted a synergistic effect on the activity of 12 pounds per acre of simazine and killed all pine seedlings in the 7th or 8th week after sowing. The death of the seedlings was. caused by excess calcium ions and not by pH.

The influence of temperature, light, solution pH, water stress, salt stress, and burial depth on seed germination and seedling emergence of catchweed bedstraw and the sensitivity of that weed to commonly available herbicides in China were studied in laboratory and greenhouse. Germination occurred at day/night temperatures from 5/0 C to 30/25 C, with optimum germination at 15/10 C. Catchweed bedstraw germinated equally well under a 12-h photoperiod and continuous darkness; however, a 24-h photoperiod inhibited seed germination. Catchweed bedstraw seed is moderately sensitive to osmotic potential and salt stress, with 15 and 3% germination rates at an osmotic potential of −0.5 Mpa and salinity level of 120 mM, respectively. Maximum seed germination was observed in near neutral pH; germination was greater than 80% over a broad pH range from 5 to 8. Seedling emergence of the seeds buried at a depth of 1 cm was higher (74%) than those placed on the soil surface (20%), but declined with burial depth increasing. Few (10%) seedlings emerged when seeds were placed at a depth of 5 cm. Bensulfuron-methyl, and ethametsulfuron-methyl applied PRE and tribenuron-methyl, fluroxypyr, and florasulam applied POST can be used to provide greater than 80% control of catchweed bedstraw. The results of this study have contributed to more complete understanding of the germination and emergence of catchweed bedstraw.

2-Methyl-4-chlorophenoxyacetic acid (MCPA) is widely used to control broad-leaved weeds in pea fields. Occasionally, peas are severely injured by MCPA. This study was conducted in 1959 and 1960 to determine the relationships, if any, of environmental factors and of chemical constituents of plants at the time of spraying to the tolerance of peas to MCPA. Plants were equally susceptible to MCPA at either the 5- or 10-node stages. The time of day at which plants were sprayed appeared to be the most important factor associated with the tolerance of pea to MCPA. Fresh weights of vines and yields of shelled peas from plants sprayed at noon and 4:00 pm were significantly lower than those from unsprayed plants or plants sprayed at other times of the day. The carbohydrate content of pea plants varied with the time of day (exposure to light). The carbohydrate content was associated with susceptibility to injury by MCPA. A high carbohydrate content at the time of spraying was associated with a high degree of susceptibility to MCPA. High temperature at the time of spraying tended to increase the rate of translocation of MCPA. However, in the absence of prolonged exposure to light prior to treatment, high temperature alone did not predispose peas to injury by MCPA.

During a 12-hr incubation low concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) enhance the metabolic breakdown of ribonucleic acid (RNA) whereas high concentrations of the auxin almost completely inhibit RNA breakdown after 4 hr of treatment. The data, from experiments where the incorporation of adenosine-8-C14 into RNA was followed, clearly show that the role of auxin in this system is not to appreciably alter net synthesis of RNA, but rather to alter some process(s) related to the degradative metabolism of RNA.

Although drought-stressing green foxtail before and up to 1 d after herbicide application did not reduce control with fluazifop-P in growth chamber trials, extending the drought stress for 2 to 4 d after herbicide application reduced control of green foxtail with fluazifop-P by 40 and 57%, respectively. Absorption and translocation of 14C-fluazifop-P 1, 4, and 10 d after herbicide application were similar between nonstressed and drought-stressed plants. Drought-stressed green foxtail converted slightly more of the parent acid of fluazifop-P to polar conjugates 4 d after herbicide application than did nonstressed plants, but these differences in metabolism were probably not adequate to explain large differences in efficacy.

Methods for evaluating the efficacy of potential classical biocontrol agents were outlined for a model biocontrol agent-weed-crop system. A proposed biocontrol agent (the fiddleneck flower gall nematode), its weed host (coast fiddleneck), and wheat were used as representative organisms. An additive experimental design (inverse linear model) was used. Regression of the reciprocal of the average plant biomass of each species onto the density of itself and the other plant species yielded competitive indices that measure the competitive ability of the plants. The results of 2 yr of field experiments revealed a dramatic change in the competitive interaction between fiddleneck and wheat due to the nematode. During the 1986–87 season in the absence of the nematode, fiddleneck intraspecific competition was 33 times stronger than interspecific competition with wheat. In the presence of the nematode, intra- and interspecific competition of fiddleneck were nearly equal. Only the coefficients that measure interspecific competition changed significantly in the presence of the nematode while the coefficients for intraspecific competition did not.

Cotton was grown alone (one plant/pot), in intraspecific competition (two plants/pot), and in interspecific competition with spurred anoda and velvetleaf in controlled-environment chambers, subjected to water stress by withholding water, and allowed to recover before the final harvest. Minimum leaf water potential reached during the drought period ranged from −1.87 MPa in cotton grown alone to −2.63 MPa in velvetleaf grown in intraspecific competition. Water stress reduced plant height, total dry weight, and leaf area in all species, compared to well-watered controls. Reductions in cotton growth due to intra- or interspecific competition were evident by 11 days following the onset of competition. The competitive impact of both weeds tended to increase with time. Growth reductions in cotton due to competition were associated primarily with reductions in leaf area duration; during drought the net assimilation rate also was reduced. Relative yields of cotton confirmed that interspecific competition from the weeds was more severe than intraspecific competition. Imposed drought did not affect the relative competitive abilities of the three species or the impact of the weeds on cotton.

Greenhouse and laboratory studies were conducted to determine the effects of dicamba, atrazine, and bentazon on efficacy, foliar absorption, and translocation of MON 12000 or CGA-152005 applied to velvetleaf. The efficacy of MON 12000, CGA-152005, and a combination of CGA-152005 plus primisulfuron applied at 4.5 g ai ha−1 was similar when applied alone or with 140 g ha−1 of dicamba. However, applying these herbicides in combination with 840 or 560 g ha−1 of atrazine or bentazon, respectively, reduced velvetleaf control. Increasing the rate of MON 12000, CGA-152005, or the combination of CGA-152005 plus primisulfuron to 9 g ai ha−1 or replacing crop oil concentrate (COC) with methylated seed oil (MSO) increased velvetleaf control of the atrazine and bentazon combinations but not to levels equal to these herbicides applied alone. Dicamba had no effect on the foliar absorption and translocation of 14C from MON 12000 or CGA-152005. Atrazine had little effect on foliar absorption of 14C from MON 12000 or CGA-152005, but bentazon reduced the foliar absorption of 14C from MON 12000. Replacing COC with MSO increased the foliar absorption of 14C from MON 12000 or CGA-152005 applied alone or with dicamba or atrazine, but not with bentazon. Translocation of 14C from MON 12000 or CGA-152005 out of the treated leaves was 11 and 12%, respectively, averaged across adjuvants and sampling times. These values were reduced to an average of 3 to 4% for both MON 12000 and CGA-152005 when applied in combination with atrazine or bentazon. The majority of 14C from MON 12000 or CGA-152005 was translocated acropetally. Atrazine and bentazon significantly reduced the acropetal translocation of 14C from MON 12000 at 24 and 72 h and for CGA-152005 at 12, 24, and 72 h. The physiological basis for the observed antagonism of MON 12000 and CGA-152005 by atrazine and bentazon appears to be due to reductions in acropetal translocation of MON 12000 and CGA-152005 to velvetleaf meristems.

More force was required to induce leaf slippage in coleoptiles from corn (Zea mays L. ‘Tenn 5009′) plants grown in the presence of 25 ppmw alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] than in the controls, or in coleoptiles from plants grown in the presence of 25 ppmw alachlor in combination with 150 ppmw 1,8-naphthalic anhydride (hereafter referred to as NA). NA alone reduced the force required to induce leaf slippage below that of all treatments. Incubation for 16 hr in 10 ppmw IAA (indoleacetic acid) did not lower the force required to induce leaf slippage in coleoptiles from plants grown in the presence of 25 ppmw alachlor even though the length of the coleptiles increased 30% more than coleoptiles not receiving the IAA treatment. Percent dry matter, dry matter per 100 plants, and percent protein increased in coleoptiles from alachlor treated plants, while percent cellulose and lignin decreased. The extent of variation in these parameters was lessened when NA was applied in combination with alachlor, with the exception of percent lignin which was further decreased by the combination treatment. Percent hemicellulose was not affected by alachlor, NA, or their combination.

The influence of weeds on crop yield is not only dependent on weed-related factors such as density and time of emergence, but also on environmental and management factors that affect both the weed and crop through time. This study was undertaken to develop the first physiologically based dry bean model that would account for the influence of weed competition. The specific objective was to develop a model that would account for the influence of weed competition on crop yield, and to use this model to test the hypothesis that crop yield losses resulted from competition for photosynthetically active radiation (PAR). To this end, a model that simulated the growth and development of dry bean was developed. The model performed daily calculations and simulated the phenology, leaf area expansion, dry matter production and distribution, and grain yield of dry bean based on weather and management information, but assumed adequate water and nutrients. The model was calibrated without weed competition at two locations and yr, and for these situations, adequately described the growth and development of the crop. Simulations were then run for five common ragweed densities and two times of emergence. Common ragweed leaf area was read into the model from input files and used to simulate weed shading. Shading of the dry bean canopy by common ragweed accounted for about 50 to 70% of the yield losses observed in field studies when weeds emerged with the crop. Weed shading did not account for the yield reduction measured from weeds that emerged at the second trifoliate stage of crop growth. The agreement between model predictions and field studies was consistent with the hypothesis that competition for PAR was a principal factor in weed-crop interaction. The ability to account for differences in weed densities, management, and environmental conditions suggested that modeling was a useful tool for evaluating the interaction among weeds and crops.