Quinclorac controls crabgrass (Digitaria spp.) post-emergence in cool- and warm-season turfgrass. A rate response study revealed that two Mississippi smooth crabgrass (Digitaria ischaemum) species (MSU1 and MSU2) are resistant to quinclorac. Following that, field experiments were carried out to evaluate programmatic approaches to control one of these populations. Despite prior study on quinclorac-resistant weeds, to date, quinclorac-resistant smooth crabgrass and its mechanism of resistance have only been reported once in California. The mechanism of resistance of MSU1 and MSU2 relative to susceptible (SMT) was then investigated. The SMT biotype accumulated three times more cyanide than the resistant populations. Glutathione-S-transferase (GST) activity was evaluated as a possible contributor to non-target site resistance. The GST activity was elevated in the MSU1 and MSU2 populations. These findings suggest a non-target site–based mechanism of resistance involving the accumulation of cyanide. Further research is needed to investigate potential target-site mechanisms of resistance.

This investigation represents one of the first electron microscopic studies on determining the mode of action of the chemical herbicide sethoxydim, 2-[-1-(ethoxydimino)butyl] -5-.2-(ethiothio)propyl]-3- hydroxy-2-cyclohexen-1-one. Samples were excised from the intercalary meristem of johnsongrass (Sorghum halpense) at time intervals of 1,3, 6,12,24,36,48,60, and 72 hours and examined using electron microscopy. Transmission electron micrographs revealed that cellular activity shows alterations or abnormalities as early as six hours. Membranes show deterioration at this time and chloroplasts contain densely staining starch granules. As time intervals increase, cellular degradation becomes more evident. Nuclear membranes extend outward into the cytoplasm in various types of cells. After 72 hours, the protoplasm demonstrates plasmolysis. The cell walls, plasma membranes, and plasmodesmata are also influenced by the sethoxydim treatment. Scanning electron micrographs were studied for the effects on vascular bundles and the surrounding tissue. The results were similar to those found by transmission electron miscroscopy.

Breeding herbicide-tolerant cultivars is a means of extending herbicide utilization. In 1983 and 1984, 20 soybean [Glycine max (L. Merr] cultivars were screened for their reactions to the following herbicides: metribuzin 4-amino-6-(tert)-butyl-3-(methylthio)-astriazin-5-(4H)-one]; vernolate (S-propyl dipropylthiocarbamate); and fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl) aniline]. Each herbicide was applied at the recommended rate X, 2X, and 4X in 1983. An 8X rate was added in 1984. The recommended rates (X) for the herbicides were as follows: metribuzin 0.42, vernolate 2.80, and fluchloralin 1.12 kg/ha. The cultivars were Pella, Pixie, Cumberland, Williams, Essex, Union, Semmes, Tracy, Tracy M; North American Plant Breeders 350, 68225, 420, and 330; Northrup King 8069 and 8047; and Farmers Forage Research 339, 340, 560, and 561. Plant vigor, height, and herbicide injury were the criteria used in estimating herbicide effects. Statistical differences were found among herbicides, rates, and cultivars. Also, there were significant interactions involving herbicides x rates, herbicides x cultivars, rates x cultivars, and herbicides x rates x cultivars. Although not always statistically significant, average vigor ratings decreased with increased herbicide application rate. Pronounced differences in the study results for the two years, likely reflecting rainfall patterns, were evidence of uncontrolled variability. However, a small number of the cultivars exhibited herbicide tolerance.

No-tillage production of crops has developed and become widespread since the late 1950/s. No-till is dependent upon the use of broad spectrum herbicides to kill weeds already growing at planting time. Until recently, this has almost exclusively been paraquat (1,1'-dimethy1-4,4'-bipyridinium ion). Glyphosate [N-(phosphonomethyl)glycine] is a new herbicide which has great potential for use in no -tillage production. Soybeans (Glycine max L.) were planted in the stubble after wheat (Triticum aestivum L.) was harvested. All plots received applications of the residual herbicides alachlor [2-chioro-2',6'-diethyl-N-(methoxymethyl)acetanilide] and linuron [3-(3,4-dichlorophenv1)-1-methoxy-1-methylurea]. Each plot also received either paraquat or glyphosate or was designated as a check. Glyphosate was appiied at three different volumes and paraquat at only one volume. All mixtures were applied at two spraying pressures. Applications were made to areas where straw was undisturbed after combining and to areas where the straw had been removed. Ratings taken 10 days and six weeks after herbicide application indicate that glyphosate performs as well as or better than paraquat. The ratings also indicate that glyphosate performs as well at 187 L/ha as it does at 574 L/ha. Spraying pressure and removal of straw seem to have little effect on the degree of weed control.

The lack of adequate weed control is one of the major problems encountered in soybean production. Johnsongrass presents one of the most serious weed control problems in the Southeastern United States. numerous herbicides are used in an effort to control johnsongrass, and research is still being conducted to find new herbicides for this purpose. Two of these herbicides are BAS 9021 [6,6-dimethyl-2,4-dioxo- 3[1-[2-(propenyloxy)amino]butylidene]-cyclohexane] and BAS 9052 [2-(n-ethoxybutyrimidoyl)-5-(2-ethylthiopropy1)-3- hydroxy-2-cyclohexen-1-one]. In 1978, johnsongrass control with BAS 9021 at 1.1 and 1.7 kg/ha in sim-le early postemergence or late postemergence applications, or in each of two split applications, was evaluated using the herbicide alone, with a nonionic surfactant, or a nonphytotoxic crop oil. The split applications gave johnsongrass control ranging from 49, to 81,. Late postemergence applications of 1.7 kg/ha of BAS 9021 were as Food as split applications. As a group, the addition of oil or surfactant gave increased control over the herbicide alone. In 1979 BAS 9021 and BAS 9052 were compared with mefluidide [N-[2,4-dimethyl-5[[(trifluoromethyl)sulfonyl]amino]phenyl]acetamide]and alachlor (2-chloro-2'6'-diethyl-N-(methoxymethyl)acetamide] for seedling johnsongrass control. The BAS materials were compared with mefluidide, trifluralin]α, α, α trifluoro-2,6-dinitro-N,N-dipropyl-p-toliuidine], fluchloralin [N-(2-chloroethyl)-2,6-dinitro-n-propyl- 4-(trifluoromethyl)aniline] and trifluralin plus vernolate [S-propyl dipropylthiccarbamate] for rhizomic johnsongrass control. In both experiments BAS 9021 was applied at 1.1 and 1.7 kg/ha in each of two applications. BAS 9052 was applied at rates of 0.6 to 1.1 kg/ha early postemergence, late postemergence or in split applications. Mefluidide was applied at rates of 0.3 to 0.6 kg/ha either early postemergence or in split applications. All postemergence herbicide applications contained an oil concentrate at a rate of 2.4 L/ha. Alachlor for seedling control was applied preemergence at 3.4 and 4.5 kg/ha. Preplant incorporated applications of trifluralln and fluchloralin were made at rates of 2.2 kg/ha and vernolate at 2.8 kg/ha was combined with —1 and 2.2 kg/ha of trifluraiin for rhizomic johnsongrass control. In the seedling area, all BAS treatments gave 88% or better johnsongrass control. In the rhizomic area split applications of BAS 9052 tended to result in the best johnsongrass control. Ratings were taken both 8 and 11 weeks after planting. Soybean yields were generally better with higher johnsongrass control. Mefluidide caused apparent soybean injury, resulted in poor johnsongrass control and lowered soybean yields. All preplant incorporated treatments gave poor johnsongrass control and low soybean yields. Alachlor gave fair johnsongrass control but resulted in high soybean yields.

With the introduction of herbicide resistant Palmer amaranth into Kentucky during the past 10 years there has been an increasing concern for effective control measures in grain production. Field trials were performed in 2016 and 2017 near Barlow and Paris, KY to determine efficacy of chemical control programs targeting herbicide resistant Palmer amaranth. Percent visual control, effects on plant density and plant height were measured in 2016 to determine treatment effectiveness. Treatments containing four different sites of herbicide activity achieved an average of 98% control. Treatments containing only 3, 2 or 1 site of activity only achieved 64%, 45% and 33% control, respectively. Within the long-chain fatty acid inhibitors herbicides in this study, pre-emergent applied pyroxasulfone provided greater control than S-metolachlor or acetochlor. Pyroxasulfone also provided greater control than the photosystem II herbicides atrazine and metribuzin. In 2017 PRE treatments consisting of three-way mixtures of flumioxazin + pyroxasulfone + chlorimuron or S-metolachlor + metribuzin + fomesafen followed by a POST herbicide treatment provided > 90% suppression of Palmer amaranth 4 weeks after trial initiation. Post-emergence treatments containing glyphosate + dicamba or glyphosate + 2,4-D following a soil-applied pre-emergent treatment achieved the most effective season-long control of Palmer amaranth.

Invasive species are often successful and problematic because of their ability to persist in disturbed and undisturbed environments despite weed management practices. Understanding reproduction and dispersal strategies in these species can aid in developing management approaches to help control their spread. Centaurea melitensis, a nonnative invasive with European origins, is found in disturbed areas of southern California. It produces three different types of flower heads that develop at different times and at different locations on the plant during the growing season. The chasmogamous (CH) flower heads are located at the top of the plant, the initial cleistogamous (iCL) heads are located at the base of the plant and at some branch and axillary points, and the final cleistogamous (fCL) heads are located along the stem and at some branch points. This pattern differs from that in Centaurea solstitialis, its most closely related congener, which develops one type of flower head with two morphologically distinct achenes within each flower head: peripheral and center achenes. The overall objective of this study was to examine potential differences between peripheral and center achenes of Centaurea melitensis, including morphological differences in dispersal features, potential differences in response to temperature, tendency to disperse, dormancy and viability. The approach involved selecting whole plants from the field and separating peripheral and center achenes from each of the three head types. Morphological differences were assessed by measuring mass, fruit length, fruit width, pappus length, pappus width, and elaiosome features. Temperature response patterns were evaluated by exposing peripheral and center achenes to temperatures ranging from 5 C to 30 C and measuring germination. Tendency of different achene types to remain in the flower heads was assessed by comparing ratios of peripheral to center achenes in heads immediately after maturation (March to June) to the ratios remaining in heads in early fall (September). Viability and dormancy testing was performed using a cut test and tetrazolium chloride tests in conjunction with germination tests. Results of these studies indicate that peripheral achenes of Centaurea melitensis were lighter and narrower, with shorter pappi and smaller elaiosomes than center achenes. Peripheral achenes responded similarly to their center counterparts in germination response to temperature, but the pattern differed among head types. Broad temperature optima were observed within the fCL and CH heads and a narrow optimum was observed in iCL. There was no evidence that the peripheral achenes remained in the heads longer than the center achenes. Ratios of peripheral to center achenes were 3.8:1 in CH heads, 2.23:1 in iCL heads, and 1.94:1 in fCL heads. Peripheral achenes were more likely to be dormant while maintaining viability than center achenes were. The results of this study, therefore, indicate that peripheral and center achenes of Centaurea melitensis differ morphologically. Dispersal features, such as pappi and elaiosomes, were more highly developed in center achenes than in peripheral achenes, but these differences were not reflected in differences in behavior to the extent we could measure it (i.e., the tendency to remain in heads or the germination responses to temperature). Differences between peripheral and center achenes of Centaurea melitensis trended in the same direction as differences seen in its closely related congener, Centaurea solstitialis, which has center achenes that disperse more readily than peripheral achenes. However, the differences observed in Centaurea melitensis were not as pronounced as those seen in Centaurea solstitialis.

Weed management options in agronomic crop production have been severely limited by widespread populations of weeds resistant to herbicides, including waterhemp [Amaranthus tuberculatus (Moq.) Sauer (syn. rudis)] resistant to foliar applications of herbicides that inhibit protoporphyrinogen oxidase (PPO; EC 1.3.3.4) activity (PPO-R). Herbicides within this site of action (WSSA site of action #14) remain efficacious when soil-applied to PPO-R waterhemp populations. Therefore, the continued use of these herbicides for soil-residual control of PPO-R waterhemp, especially in soybean production, is paramount with limited postemergence herbicides that remain effective. An improved understanding of the selection for PPO-R waterhemp would provide information to help minimize future loss of residual PPO-inhibiting herbicide activity. Five studies, consisting of 14 experiments, were conducted to improve our understanding of the selection for herbicide-resistant individuals. Soil-residual herbicides have been suspected to select for herbicide-resistant individuals; however, this phenomenon has never been observed experimentally in field conditions. This dissertation provides direct evidence from greenhouse and field experiments that significant selection pressure can occur from soil-residual herbicides; however, this selection for resistance could be delayed when using full commercial herbicide rates and effective herbicides from multiple sites of action. Also, the frequency of heterozygous individuals (RS) and PPO-inhibiting herbicide efficacy on RS individuals is a factor in the selection for herbicide resistance; however, current information on the these topics is limited. To provide additional information on RS individuals, a large-scale genotypic and phenotypic screen of multiple PPO-R waterhemp populations was conducted. It was determined that RS individuals were less frequent than expected and PPO-inhibiting herbicide efficacy on RS individuals was population-dependent. Finally, the hormetic effects of soil-residual herbicides have been paradoxically implicated as a means of both mitigating and exacerbating the selection for herbicide resistant biotypes; however, limited information was available on the hormetic effects of soil-residual PPO inhibitors. Greenhouse and growth chamber experiments were conducted to improve understanding of hormesis and soil-residual PPO-inhibiting herbicides. Experiments indicated that PPO-inhibiting herbicides may exert a limited hormetic effect on waterhemp germination below doses that cause a phytotoxic effect of the emerging seedling, indicating this effect may exacerbate the issue of selection for PPO-R waterhemp. Overall, data presented in this dissertation provides important information on the under-studied interaction between PPO-inhibiting herbicides and PPO-R waterhemp to safeguard the sustained efficacy of herbicides within this site of action.

With herbicide efficacy declining as weeds continue to evolve and resist key modes of action, long-term, multi-faceted control practices need to be investigated. Two studies, involving cover crops and tillage, were implemented to understand how management practices influence the weed seed bank. The objectives of both studies were to examine long-term changes in the weed seed bank in response to tillage methods, fertility, and cover crops, and to analyze differences in distribution and community composition between individual species in the weed seed bank. To assess the weed community present in both seed banks, soil samples were collected, and a soil grow out was conducted in the greenhouse. To examine the field-emerged seed banks, percent coverage was collected for each weed species, crop residue, bare ground, and cover crop if present. The first study was established in 1970, evaluating four tillage systems: moldboard plow (CT), chisel plow (RT), alternative (AT), and no-tillage (NT). From 1970 to 1990, this study was continuous corn (Zea mays L.), but in 1991, soybeans (Glycine max L.) were added into the rotation, marking the beginning of the current corn-soybean rotation. Fertilizer treatments (no-fertilizer, nitrogen only, and NPK) were also evaluated. Each tillage and fertility treatment were replicated four times in the field in 6 m by 8 m plots. Weed seeds were found to be distributed within the soil profile differently by tillage treatment. No-till treatments maintained most of the seed bank near the surface of the soil. Based on the response of individual species to fertility treatments, community shifts in seed bank composition were found. LAMAM, STEME, and SIBVI had the greatest richness in NPK treatments compared to no fertilizer and nitrogen only. CERVU tended to favor treatments without any fertilizer. Tillage and fertility were also found to interact and influence species presence and community composition. The second study was established in 2013, to examine changes and differences in distribution and composition between individual species in the weed community in response to cover crop rotations and tillage. A split-plot design with three crop rotation systems was implemented: 1) corn (Zea mays L.) – cereal rye (Secale cereale L.) – soybean (Glycine max (L.) Merr.) – hairy vetch (Vicia villosa Roth) [CcrShv], 2) corn-cereal rye-soybean-oats + radish (Avena sativa L. + Raphanus sativus L.) [CcrSor], and 3) corn-no cover crop-soybean-no cover crop [NOCC], and two tillage treatments: conventional tillage and no-till. This field study also supported previous findings of higher weed diversity in no-till systems. ANOVA performed in R suggested species richness was significantly higher in no-tillage treatments in comparison to tillage treatments. For the field-emerged weed community, a pairwise comparisons test suggested cover crop treatments have significantly lower weed richness compared to plots with no cover crop present, but there was no interactive effect of tillage. 3-Way ANOVAs suggested time, tillage, and crop rotation influenced each weed species differently. Due to individual weed species having different requirements for germination and seed longevity, these data suggest the importance of developing and implementing a quality, integrated weed management program to maintain low levels of weed emergence and seed credits to the seed bank.

Herbicide resistance development among weed populations in cotton and peanut is becoming increasingly difficult to manage. If resistant populations continue to persist, weed control practices for producers will become less efficient and more costly. The objective of this research was to evaluate alternative weed control techniques designed to mitigate herbicide resistance development for their agronomic and economic impact on weed management systems. Studies were conducted in 2019, 2020, and 2021 at multiple locations in Mississippi and Arkansas investigating multiple techniques including the addition of soil surfactants in herbicide tank mixtures, increasing SOAs utilized in peanut herbicide programs, applying non-labeled herbicides to cotton with post-directed spray placement, and applying complete residual herbicide programs in cotton. Our results suggests that some novel strategies incorporated into existing weed management programs, can provide sufficient control of troublesome weed species and conserve crop yield and profit returns. For example, the use of post-directed application placement allowed for non-labeled herbicides to be applied to cotton without detrimental effects, thus increasing potential options for POST weed control within that crop. Additionally, weed control, seed cotton yield, and net returns were not affected when only residual herbicides were applied in season-long weed control programs as opposed to the standard of mixed, foliar and residual programs. This indicates that high selection pressure associated with foliar chemistries which leads to resistance development, can be alleviated through the adoption of alternative strategies.