Isoxaben is a preemergence herbicide used for broadleaf weed control in turf and ornamentals. Although isoxaben can be used on a number of ornamentals, certain species are injured by isoxaben applications. The objectives of this research were: a) to evaluate the tolerance of ajuga, wintercreeper and dwarf burning bush to isoxaben applications, b) to compare the absorption, translocation and metabolism of isoxaben following root and shoot application in these ornamentals, and c) to examine the effect of isoxaben on glucose incorporation in the roots of these species. Greenhouse and lathhouse studies demonstrated that ajuga was the most sensitive species compared to wintercreeper and dwarf burning bush following root and shoot exposure to isoxaben at 0.84, 1.69 and 3.39 kg ai/ha. Following root and shoot application, isoxaben at 3.39 kg/ha caused approximately 50% shoot injury in ajuga at 2 months after treatment compared to approximately 30% in dwarf burning bush in sand culture. Wintercreeper was not visually injured by any isoxaben rate. Isoxaben at 3.39 kg/ha reduced wintercreeper root weight by 15% following root application and shoot weight by 10% following shoot application. Field studies showed that isoxaben applications made one month after bud-break caused 30 to 45% injury to dwarf burning bush. However, the plants outgrew the injury in the following year. Dwarf burning bush was not injured from applications of isoxaben made at the dormant stage or two months after the bud-break stage. Studies with root-applied radiolabeled isoxaben showed that ajuga and dwarf burning bush had absorbed 34 and 41% of the applied radioactivity, respectively, while wintercreeper had absorbed only 21% at 14 days after treatment (DAT). The percent of absorbed radioactivity which translocated was greater in ajuga (58%) and wintercreeper (50%) than in dwarf burning bush (28%). In the root extracts, metabolism of isoxaben was greater in ajuga than wintercreeper or dwarf burning bush at 3, 7 and 14 DAT. Most of the radioactivity recovered from the shoots of the three species appeared to be polar metabolites of isoxaben, possibly conjugates. In studies with shoot-applied radiolabeled isoxaben, radioactivity recovered from the treated leaf of ajuga increased from 46% of applied at 3 days to 64% at 14 days after treatment. In wintercreeper, the most tolerant species, approximately 40% of the applied radioactivity was recovered in the treated leaf at each harvest date. Radioactivity recovered from the treated leaflet increased from 45 at 3 DAT to 70% at 14 DAT in both growth stages of dwarf burning bush. Ajuga and wintercreeper metabolized isoxaben faster than dwarf burning bush. There was no difference in the metabolism of isoxaben between the two growth stages of dwarf burning bush. Incorporation of glucose in the roots of wintercreeper and dwarf burning bush was not inhibited by isoxaben (1 mM). Approximately 10% inhibition of glucose incorporation by isoxaben was observed in the roots of the sensitive species ajuga. / Ph. D.

Yellow and purple nutsedge are difficult to control worldwide. In turfgrass, the availability of herbicides that provide selective control of these weeds is limited. To address this problem, a sulfonylurea herbicide, halosulfuron, is being developed for the control of both yellow and purple nutsedge. To confirm preliminary results, evaluations of this herbicide were performed in both field and greenhouse studies during 1993 and 1994. The objectives of the field studies were to evaluate halosulfuron for turfgrass tolerance (safety to turfgrass) and efficacy for yellow and purple nutsedge control. Greenhouse studies were performed to determine the extent of translocation of halosulfuron in yellow and purple nutsedge. Four species of turfgrass were evaluated for halosulfuron tolerance: Kentucky bluegrass (Poa pratensis L. 'Plush’), tall fescue (Festuca arundinaceae Schreb. 'Confederate'), bermudagrass (Cynodon dactylon (L.) Pers. '419' and 'Vamont') and zoysiagrass (Zoysia japonica Steud. 'Meyers'). Over a two-year period, injury to these turfgrass species did not exceed 10% and in most cases was non-existent. In these studies, yellow nutsedge control with halosulfuron at 0.14 kg ai/ha averaged 90% after six weeks in the four turfgrasses. However, after six weeks, yellow nutsedge regrowth did occur. Purple nutsedge control was evaluated only in Kentucky bluegrass and was uniformly transplanted into the study area. Purple nutsedge control averaged 96% in Kentucky bluegrass at 6 weeks after treatment. Yellow and purple nutsedge contains a well-developed rhizome/tuber system, and as was seen in several of the studies, have the ability to regrow after herbicide treatment. Two greenhouse studies were designed to determine halosulfuron translocation into the tuber and connecting shoot and through a rhizome into another shoot. In the first study, a tuber was only allowed to develop two shoots from separate buds on the tuber. After a month, one of the shoots was treated with halosulfuron, and control ratings were taken on both shoots. In the second study, a plant was placed in one of two connected pots and allowed to grow. A rhizome from this plant (mother plant) was guided into the connecting pot where a new plant developed. After a month, the mother plant was treated with halosulfuron, and control ratings were taken on both mother and new plant. From both of these tests, there is statistically significant evidence that translocation was occurring through both the tuber and the rhizome. This translocation occurred not only at rates used for nutsedge control but at rates well above and below 0.14 kg ai/ha. More work, however, needs to be performed using radiolabeled tracers or immunological techniques to confirm the movement of halosulfuron in yellow and purple nutsedge. Although regrowth of yellow and purple nutsedge was seen in both field and greenhouse studies, halosulfuron does provide good initial control of both species. Sequential applications of halosulfuron are desirable in poorly established turfgrass. / Master of Science

While cover crops are actively growing, they compete with winter annual weeds. Studies were conducted to determine the ability of early planted cover crop monocultures and mixtures and a fall applied residual herbicide to compete with winter annual weeds. Cereal rye containing cover crops provided the greatest control of winter weeds in May. Flumioxazin, as a fall applied herbicide, controlled winter weeds in December, but control did not persist until May. Once cover crops are terminated, their residue suppresses summer annual weeds. A greenhouse experiment studying the effects of cereal rye biomass on common ragweed and Palmer amaranth control and light penetration and two field experiments to determine the effects of cereal rye and wheat cover crop biomass terminated with a roller crimper or left standing on summer weed control and light penetration were conducted. For summer weed control, as cover crop biomass increased, weed control increased, light penetration decreased, soil temperature decreased, and soil moisture increased. Standing cover crop residue provided greater control of common ragweed compared to rolled residue until 8400 kg ha⁻¹ of cover crop biomass. As cover crop biomass increased, rolled cover crop residue reduced light penetration compared to standing residue. Wheat cover crop residue increased soil moisture more compared to cereal rye residue. Cover crops must become established to produce adequate biomass to compete with weeds. Herbicide carryover has the potential to reduce cover crop establishment. A study was conducted to evaluate the potential for 30 different residual herbicides applied in the cash crop growing season to carryover to 10 different cover crops. While visible injury was observed, cover crop biomass was similar to the nontreated check in all cases, indicating that herbicide carryover to cover crops is of little concern. Herbicide carryover has few biological effects on establishment of cover crops, under the conditions and herbicides evaluated, that are effective at competing with winter annual weeds and suppressing summer annual weeds. / Master of Science in Life Sciences / Cover crops are grown after the cash crop has been harvested and terminated before another is planted. They are grown for environmental and agronomic benefits and not harvested. Cover crops improve soil health, reduce erosion, prevent nutrient loss, and control weeds. Cover crops can compete with weeds while they are actively growing. Their residue can create a mulch layer to reduce weed establishment and limit the amount of light reaching weed seed to reduce germination and establishment. As winter cover crops are growing, they compete with winter weeds for sunlight, nutrients, and water. Fall applied herbicides that remain active in the soil are also utilized to control winter weeds in between cash crop growing seasons. Experiments evaluated the ability of cover crop monocultures and cover crop mixtures compared to a fall applied herbicide to compete with winter annual weeds. Monocultures and mixtures of cereal rye, crimson clover, hairy vetch, and forage radish were utilized. Cereal rye containing treatments provided the greatest control of winter weeds in the spring. The fall applied herbicide provided adequate control of winter weeds in December, but control did not persist until the May, indicating that a fall applied herbicide will not provide control of winter weeds from cash crop harvest to the next cash crop planting. As cover crop biomass increases, summer annual weed control increases. Cover crops are usually terminated with herbicide and left standing in Virginia, but the use of a roller crimper lays down residue and creates a mulch layer. Experiments compared the effects of cereal rye and wheat cover crops at different biomass rates terminated with herbicide only (left standing) or a roller crimper and herbicide on summer weed control, light penetrating the cover crop canopy and reaching the soil surface, soil moisture, and soil temperature. As cover crop biomass increased, weed control increased, light reaching the soil surface decreased, soil temperature decreased, and soil moisture increased. Standing cover crop residue provided greater weed control until 8400 kg ha⁻¹ of cover crop biomass was reached. After 8400 kg ha⁻¹ rolled cover crop residue provided greater control, but control from standing and rolled were both greater than 80% compared to the no cover control. Cereal rye intercepted more light than wheat cover crop residue at less than 6000 kg ha⁻¹ of cover crop biomass was achieved. Rolled cover crop residue intercepted more light than standing residue. Established cover crops most produce adequate biomass to effectively control weeds. Herbicides applied during the cash crop growing season to control weeds can remain active in the soil and reduce the establishment of subsequently planted cover crops. Experiments evaluated the potential for different herbicides applied during the cash crop, such as corn, cotton, or soybeans, to remain in the soil at high enough concentrations to injure cover crops commonly utilized in the Mid-Atlantic region. Cover crops utilized were wheat, barley, cereal rye, oats, annual ryegrass, forage radish, Austrian winter pea, crimson clover, hairy vetch, and rapeseed. Results suggest that little potential exists for the herbicides utilized to persist in the soil to injure the five grass cover crop species utilized. There is the potential for some herbicides to injure forage radish, crimson clover, and rapeseed, but no reduction in cover crop biomass was observed, indicating there is little concern for herbicide carryover to cover crops. Residual herbicide carryover has little effect on establishment of cover crops and does not reduce cover crop biomass, under the conditions and herbicides tested in this study. Cover crops can effectively compete with winter weeds, and as cover crop biomass increases, summer annual weed control increases.

Weed control and desiccation are routinely executed with herbicides. Potato vine desiccation facilitates harvest, improves skin set, and regulates tuber size. Saflufenacil, glufosinate, saflufenacil plus glufosinate, and carfentrazone plus glufosinate were compared to diquat applied at 43, 31, and 17% B potatoes; similar vine desiccation (14 days after treatment), skin set, and yield were noted amongst treatments. Residual herbicides are routinely used for weed control in pumpkin. Fluridone and acetochlor formulations applied preemergence were evaluated in direct-seeded pumpkin compared to other labeled herbicides. Fluridone resulted in total crop loss following heavy rainfall immediately after planting; less rainfall resulted in transient injury. Acetochlor formulations resulted in significant pumpkin injury (34 to 39%) 14 days after planting. S-metolachlor controlled weeds similar to acetochlor without significant injury. Palmer amaranth has developed resistance to six different herbicide modes of action. The weed grows rapidly and is best controlled <10 cm in height. To control glyphosate and ALS- resistant biotypes, fomesafen plus dicamba were applied at first postemergence (POST) to small Palmer amaranth (<5 cm, 0 d) and at simulated delays of 7, 14, 21, and 28 d. All plots received lactofen plus dicamba 14 days after first POST. Palmer amaranth control 14 days after first POST was 100% when delayed 0 or 7 d and 62% at the 28 day delay; control increased to 88% following lactofen plus dicamba applied second POST. Yield was significantly reduced when first POST was delayed 28 days at one location. / Master of Science in Life Sciences / Herbicides effectively control weeds by either applying them to the soil prior to emergence or applying them to foliage. Herbicides are used for desiccation of potato vines to facilitate harvest, improve skin set, and regulate tuber size. Potatoes with tougher skin have a longer shelf life and are more resistant to disease. Potato grade classifications include size chef, A, and B potatoes. Size B potatoes hold the greatest value for redskinned potatoes. Experiments were conducted in Virginia to evaluate saflufenacil, glufosinate, saflufenacil plus glufosinate, and carfentrazone plus glufosinate as desiccants compared to diquat applied at 43, 31, and 17% B potatoes. All desiccants resulted in similar vine desiccation 14 days after treatment, skin set, and yield. This research demonstrates that glufosinate and saflufenacil are effective alternatives to diquat for potato vine desiccation; however, further research is needed to evaluate the safety of saflufenacil applied to potatoes prior to harvest. Soil applied herbicides are commonly used in pumpkin production. Fluridone and two acetochlor formulations, herbicides that effectively control troublesome weeds in other crops, were evaluated for pumpkin production in addition to fomesafen, ethalfluralin, clomazone, halosulfuron, and S-metolachlor. Fluridone and acetochlor formulations resulted in significant pumpkin injury early in the growing season and total crop loss was observed by fluridone in 2018. Fomesafen significantly reduced pumpkin iv stand and yield. S-metolachlor, a member of the same chemical family as acetochlor, provided similar weed control without significant pumpkin injury. This research demonstrates that fluridone and acetochlor formulations are poor candidates for pumpkin production. Palmer amaranth is a troublesome weed in soybean that grows rapidly and is resistant to many herbicides. Palmer amaranth is best controlled at a height of 10 cm or less, but timely applications are not always feasible. Fomesafen plus dicamba were applied to small Palmer amaranth (<5 cm, 0 day) and at simulated delays of 7, 14, 21, and 28 days. All treatments received lactofen plus dicamba 14 days after the initial postemergence. Palmer amaranth control 14 days after first postemergence was 100% when application was delayed 0 or 7 day whereas Palmer amaranth control was 62% when first postemergence was delayed 28 days. Lactofen plus dicamba applied second postemergence increased control to 88% when the first postemergence was delayed 28 days. Compared to nontreated plots, Palmer amaranth biomass was reduced 99% by all treatments. This research demonstrates that fomesafen plus dicamba followed by lacofen plus dicamba can be effective for rescue control of Palmer amaranth.

Cover crops have become an important part of cropping systems in the United States, especially in the Mid-Atlantic region. Rapeseed is a popular choice due to its deep growing taproot which creates soil macropores and increases water infiltration. If not properly terminated rapeseed can become problematic due to its pod-shattering tendency and its difficulty to terminate with herbicides once it enters reproductive growth. Results indicate termination of rapeseed is most effective when the cover crop is small. Combinations that successfully terminated rapeseed include glyphosate plus 2,4-D and paraquat plus 2,4-D. Halauxifen-methyl is a new Group 4 herbicide marketed for preplant burndown horseweed (Conyza canadensis L.) control. Previous research indicates that halauxifen effectively controls glyphosate-resistant horseweed. However, little is known about control of other common winter annual weeds by halauxifen. Results indicate halauxifen has a narrow spectrum of control providing adequate control (>80%) of horseweed, henbit (Lamium amplexicaule L.), and purple deadnettle (Lamium purpureum L.), while failing to control cutleaf evening-primrose (Oenothera laciniata Hill), curly dock (Rumex crispus L.), purple cudweed (Gamochaeta purpurea L. Cabrera), common chickweed (Stellaria media L.), and mousear chickweed (Cerastium L.). Little is known of cotton (Gossypium hirsutum L.) tolerance to halauxifen applied preplant burndown. Results indicate cotton is more tolerant to halauxifen than 2,4-D or dicamba when the interval between preplant application and cotton planting is less than 30 days. / Master of Science in Life Sciences / Cover crops are an important part of cropping systems in the United States, especially in the Mid-Atlantic region. Producers utilize cover crops to aid in weed suppression, reduce soil erosion, as well as to increase soil health. Cereals, legumes, and Brassicaceae species are popular cover crops planted either as monocultures or mixtures. Rapeseed can become problematic due to its difficulty to terminate once it enters reproductive stage, as well as its podshattering characteristic. Experiments were conducted to evaluate various herbicides and herbicide combinations for rapeseed termination two application timings. At three locations where rapeseed averaged 12 cm in height at early termination, and 52 cm in height at late termination, glyphosate + 2,4-D was most effective, controlling rapeseed (96%) 28 days after early termination (DAET). Paraquat + atrazine + atrazine (92%), glyphosate + saflufenacil (91%), glyphosate + dicamba (91%), and glyphosate (86%) all provided at least 80% control 28 DAET. Paraquat + 2,4-D (85%), glyphosate + 2,4-D (82%), and paraquat + atrazine + mesotrione (81%) were the only treatments to provide at least 80% control 28 days after late termination (DALT). At one location where rapeseed was much taller (41 cm early termination; 107 cm late termination), herbicides were much less effective, as no herbicide treatments provided greater than 80% control. Results indicated that rapeseed size at time of termination was more critical to successful termination than herbicide choice. Prior to the development of glyphosate-resistant horseweed, producers were able to control horseweed and other weeds with glyphosate applied preplant burndown. Producers now rely on auxin herbicides tank mixed with glyphosate and a residual herbicide to control horseweed and other winter weeds prior to cash crop planting. Experiments were conducted to evaluate halauxifen-methyl, a new Group 4 herbicide, for control of horseweed and other commonly encountered winter annual weeds. Halauxifen (89%) controlled small horseweed (<5 cm in height at time of application) similar to dicamba (91%), while providing better control of large horseweed (79%) (>15 cm in height at time of application) than either dicamba (77%) or 2,4-D evaluated (64%). Halauxifen provided adequate control (>80%) of henbit (Lamium amplexicaule L). and purple deadnettle (Lamium purpureum L.), while failing to effectively control of cutleaf evening-primrose (Oenothera laciniata Hill), curly dock (Rumex crispus L.), purple cudweed (Gamochaeta purpurea L. Cabrera), common chickweed (Stellaria media L. Vill.), and mousear chickweed (Cerastium L.). Results indicate that halauxifen has a narrow spectrum of control and should be tank mixed with 2,4-D or glyphosate in order to control weeds other than horseweed and henbit. Glyphosate plus dicamba or 2,4-D plus a residual herbicide is typically applied prior to cotton planting. Previous research has shown that as long as rainfall requirements and rotation intervals are met, no adverse effects on cotton is observed from 2,4-D or dicamba herbicides. Little is known of cotton tolerance to halauxifen applied preplant burndown. Experiments were conducted to determine if halauxifen applied sooner than the labeled 30-day rotation interval would injure cotton. Very little injury was observed from halauxifen (9%) applied at-planting, however dicamba (26%) and 2,4-D (21%) applied at the same timing did injure cotton. Auxin herbicides applied earlier in the season resulted in little injury (<2%). Early season injury was transient as cotton recovered later in the season and seedcotton yield was unaffected.

Palmer amaranth (Amaranthus palmeri) and common ragweed (Ambrosia artemisiifolia) can cause detrimental soybean yield loss. Due to widespread resistance to glyphosate and ALS-inhibiting herbicides, growers rely on protoporphyrinogen oxidase inhibiting herbicides (PPO) such as flumioxazin applied preemergence (PRE) and fomesafen postemergence (POST) to control both weeds. Experiments were conducted with the overarching goal of reducing PPO selection pressure for Palmer amaranth and common ragweed. Flumioxazin alone PRE controlled Palmer amaranth near 100%. However, sulfentrazone combined with pyroxasulfone or pendimethalin provided similar control to flumioxazin. Acetochlor and linuron controlled common ragweed <74%, yet controlled Palmer amaranth >96%. Glufosinate applied POST controlled Palmer amaranth and common ragweed 74-100%, regardless of PRE treatment. Flumioxazin PRE followed by fomesafen POST controlled common ragweed well; however, several non-PPO herbicide treatments or programs with only 1 PPO-inhibiting herbicide provided similar common ragweed control as the 2 PPO system (flumioxazin followed by fomesafen). Treatments consisting of a PRE and POST herbicide controlled Palmer amaranth at least 80% and common ragweed 95%. To reduce PPO selection pressure, soybean producers growing glufosinate-resistant soybean may use flumioxazin PRE followed by glufosinate POST whereas non-glufosinate-resistant soybean growers should reduce PPO herbicide use by using a non-PPO herbicide PRE. Alternatively, these producers can effectively reduce PPO selection pressure by implementing residual combinations of a PPO-inhibiting herbicide + non-PPO with spectrums of weed control that overlap at either Palmer amaranth or common ragweed. / Master of Science in Life Sciences / Soybean producers planted 35.4 million hectares in the US during 2018. Palmer amaranth (Amaranthus palmeri) and common ragweed (Ambrosia artemisiifolia) are both common and problematic in soybean production. The introduction of a glyphosate-resistant soybean cultivars coupled with glyphosate allowed soybean producers to easily control these weeds along with many other broadleaf and grass weeds. However, over reliance on glyphosate selected for biotypes of common ragweed and Palmer amaranth resistant to the herbicide. In response, soybean producers have reverted to preemergence (PRE) herbicides and alternative modes of action postemergence (POST) to control these herbicide-resistant weeds. One such herbicide mode of action is inhibition of protoporphyrinogen oxidase (PPO). Flumioxazin and fomesafen are both PPO-inhibiting herbicides and have been widely used in soybean, however increasing reliance on PPOs has selected for resistant common ragweed and Palmer amaranth biotypes. This research focused on reducing risk of PPO-inhibiting herbicide resistance development (“selection pressure”) by finding alternatives to or combinations with PPOinhibiting herbicides that would effectively control both weeds and thus preserve effectiveness of a valuable herbicide group. Of PRE herbicides applied alone, flumioxazin was the only treatment to control Palmer amaranth >79% 14 DA-PRE at Painter 2017. However, combination of PRE herbicides such as sulfentrazone or metribuzin in combination with pyroxasulfone, and pendimethalin + sulfentrazone, all controlled Palmer amaranth well. While metribuzin and pendimethalin alone did not provide as much control, a POST application of glufosinate coupled with these residual herbicides adequately controlled Palmer amaranth. Soybean producers can effectively control Palmer amaranth with a non-PPO PRE herbicide followed by glufosinate postemergence (POST) or residual combinations of a PPO + non-PPO while reducing risk of herbicide resistance development. Several PRE herbicide treatments adequately controlled common ragweed. During 2017, residual herbicides that controlled common ragweed at least 90% included flumioxazin, flumioxazin + clomazone, linuron, or metribuzin, fomesafen + linuron, and linuron + clomazone. All treatments controlled common ragweed greater than 94% during 2018, except metribuzin, linuron, and clomazone, which controlled the weed 75, 86, and 90%, respectively. Fomesafen alone or in combination with metribuzin controlled common ragweed 80 to 84%. Regardless of PRE, glufosinate POST controlled common ragweed 99% 56 and 70 days after planting (DAP). In fields infested with common ragweed yet to develop PPO resistance, growers should use a non-PPO herbicide in combination with flumioxazin PRE. Additionally, tank mixtures of effective MOAs PRE followed by glufosinate rather than a PPO POST may reduce herbicide selection pressure. The final study set out to determine which was more critical to controlling herbicideresistant Palmer amaranth and common ragweed in soybean, a PPO-inhibiting herbicide applied PRE or POST. Flumioxazin applied PRE controlled both weeds almost completely. Acetochlor and linuron did not control common ragweed as well, but controlled Palmer amaranth >96%. Both metribuzin and clomazone were weaker on common ragweed and Palmer amaranth. However, all PRE herbicide treatments followed by glufosinate or fomesafen controlled Palmer amaranth and common ragweed at least 80 and 95%, respectively. To reduce PPO selection pressure, soybean producers growing glufosinate-resistant soybean may use flumioxazin PRE followed by glufosinate POST whereas non-glufosinate-resistant growers should reduce PPO herbicide use by using a non-PPO herbicide PRE. Alternatively, these producers can effectively reduce PPO selection pressure by implementing residual combinations of a PPO-inhibiting herbicide + non-PPO with spectrums of weed control that overlap at either Palmer amaranth or common ragweed. Results from these experiments suggest PPO-inhibiting herbicides are critical for common ragweed and Palmer amaranth control. Previous research has shown effective tank mixtures with various effective MOAs has reduced the risk of herbicide resistance development. Protoporphyrinogen oxidase herbicides should be used sparingly and in combination with effective non-PPO herbicides to reduce selection pressure. / Soybean producers planted 35.4 million hectares in the US during 2018. Palmer amaranth (Amaranthus palmeri) and common ragweed (Ambrosia artemisiifolia) are both common and problematic in soybean production. The introduction of a glyphosate-resistant soybean cultivars coupled with glyphosate allowed soybean producers to easily control these weeds along with many other broadleaf and grass weeds. However, over reliance on glyphosate selected for biotypes of common ragweed and Palmer amaranth resistant to the herbicide. In response, soybean producers have reverted to preemergence (PRE) herbicides and alternative modes of action postemergence (POST) to control these herbicide-resistant weeds. One such herbicide mode of action is inhibition of protoporphyrinogen oxidase (PPO). Flumioxazin and fomesafen are both PPO-inhibiting herbicides and have been widely used in soybean, however increasing reliance on PPOs has selected for resistant common ragweed and Palmer amaranth biotypes. This research focused on reducing risk of PPO-inhibiting herbicide resistance development (“selection pressure”) by finding alternatives to or combinations with PPOinhibiting herbicides that would effectively control both weeds and thus preserve effectiveness of a valuable herbicide group. Of PRE herbicides applied alone, flumioxazin was the only treatment to control Palmer amaranth >79% 14 DA-PRE at Painter 2017. However, combination of PRE herbicides such as sulfentrazone or metribuzin in combination with pyroxasulfone, and pendimethalin + sulfentrazone, all controlled Palmer amaranth well. While metribuzin and pendimethalin alone did not provide as much control, a POST application of glufosinate coupled with these residual herbicides adequately controlled Palmer amaranth. Soybean producers can effectively control Palmer amaranth with a non-PPO PRE herbicide followed by glufosinate postemergence (POST) or residual combinations of a PPO + non-PPO while reducing risk of herbicide resistance development. Several PRE herbicide treatments adequately controlled common ragweed. During 2017, residual herbicides that controlled common ragweed at least 90% included flumioxazin, flumioxazin + clomazone, linuron, or metribuzin, fomesafen + linuron, and linuron + clomazone. All treatments controlled common ragweed greater than 94% during 2018, except metribuzin, linuron, and clomazone, which controlled the weed 75, 86, and 90%, respectively. Fomesafen alone or in combination with metribuzin controlled common ragweed 80 to 84%. Regardless of PRE, glufosinate POST controlled common ragweed 99% 56 and 70 days after planting (DAP). In fields infested with common ragweed yet to develop PPO resistance, growers should use a non-PPO herbicide in combination with flumioxazin PRE. Additionally, tank mixtures of effective MOAs PRE followed by glufosinate rather than a PPO POST may reduce herbicide selection pressure. The final study set out to determine which was more critical to controlling herbicideresistant Palmer amaranth and common ragweed in soybean, a PPO-inhibiting herbicide applied PRE or POST. Flumioxazin applied PRE controlled both weeds almost completely. Acetochlor and linuron did not control common ragweed as well, but controlled Palmer amaranth >96%. Both metribuzin and clomazone were weaker on common ragweed and Palmer amaranth. However, all PRE herbicide treatments followed by glufosinate or fomesafen controlled Palmer amaranth and common ragweed at least 80 and 95%, respectively. To reduce PPO selection pressure, soybean producers growing glufosinate-resistant soybean may use flumioxazin PRE followed by glufosinate POST whereas non-glufosinate-resistant growers should reduce PPO herbicide use by using a non-PPO herbicide PRE. Alternatively, these producers can effectively reduce PPO selection pressure by implementing residual combinations of a PPO-inhibiting herbicide + non-PPO with spectrums of weed control that overlap at either Palmer amaranth or common ragweed. Results from these experiments suggest PPO-inhibiting herbicides are critical for common ragweed and Palmer amaranth control. Previous research has shown effective tank mixtures with various effective MOAs has reduced the risk of herbicide resistance development. Protoporphyrinogen oxidase herbicides should be used sparingly and in combination with effective non-PPO herbicides to reduce selection pressure.

Bibliography: leaves 157-174. This study examined the fate of three common sulfonylurea herbicides in highly alkaline soils through a series of laboratory and field experiments to determine if existing leaching models could be used to describe their field behaviour under Australian climatic conditions. A liquid chromatographic method was developed to simultaneously determine levels of triasulfuron, metsulfuronmethyl, and chlorsulfuron in soil and water. The investigation of base hydrolysis for the herbicides in aqueous buffer and soil solutions determined that it was unlikely to be a major loss pathway for sulfonylureas in alkaline soils. The herbicides were found to have low sorption, very little retardation and high mobility, moving at a marginally slower rate than water. Degradation did not follow first-order kinetics, but rather a two-stage process appeared to be involved. Both VARLEACH and LEACHM models predicted the measured concentration of the herbicides reasonably well in profile under low rainfall conditions but were less adequate under high rainfall. Forecasts with the LEACHP model predicted levels of the herbicides for a dominant soil type of the cereal belt of southern Australia with median rainfall after a year.

No abstract available. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2001.

Bibliography: leaves 157-174. / xx, 192 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This study examined the fate of three common sulfonylurea herbicides in highly alkaline soils through a series of laboratory and field experiments to determine if existing leaching models could be used to describe their field behaviour under Australian climatic conditions. A liquid chromatographic method was developed to simultaneously determine levels of triasulfuron, metsulfuronmethyl, and chlorsulfuron in soil and water. The investigation of base hydrolysis for the herbicides in aqueous buffer and soil solutions determined that it was unlikely to be a major loss pathway for sulfonylureas in alkaline soils. The herbicides were found to have low sorption, very little retardation and high mobility, moving at a marginally slower rate than water. Degradation did not follow first-order kinetics, but rather a two-stage process appeared to be involved. Both VARLEACH and LEACHM models predicted the measured concentration of the herbicides reasonably well in profile under low rainfall conditions but were less adequate under high rainfall. Forecasts with the LEACHP model predicted levels of the herbicides for a dominant soil type of the cereal belt of southern Australia with median rainfall after a year. / Thesis (Ph.D.)--University of Adelaide, Dept. of Soil Science, 1999

La contamination simultanée des écosystèmes aquatiques par des polluants et des nutriments est un problème majeur dont les effets sur les macrophytes sont encore méconnus. L'objectif de cette thèse était d'étudier les effets de deux types de polluants : herbicides et éléments trace métalliques (ETM), en combinaison avec des ressources variables (azote, phosphore, ou carbone) sur Myriophyllum spicatum, en utilisant des paramètres provenant de l'écologie chimique et de l'écotoxicologie. Les tests ont montré des effets contrastés des herbicides sur la plante, mais pas d'effet des ETM. Dans tous les cas, les modifications de ressources (nitrate, ratio N:P ou saccharose) ont induit des effets importants sur la physiologie et la stœchiométrie de la plante, pouvant altérer ses réponses à un stress supplémentaire comme les polluants. Ces résultats suggèrent qu'un déséquilibre des ressources en présence de polluants pourrait modifier leurs effets sur les macrophytes, et soulignent également la nécessité d'ajouter des paramètres plus informatifs dans les tests afin de répondre aux enjeux du stress multiple et améliorer l'évaluation du risque environnemental / Simultaneous contamination of aquatic ecosystems by pollutants and nutrients is a major problem whose effects on macrophytes are still unknown. The objective of this thesis was to study the effects of different pollutants, herbicides and trace metal elements (TME), in combination with varying resource availability (nitrogen, phosphorus, or carbon) on Myriophyllum spicatum, using parameters from chemical ecology and ecotoxicology. Tests showed contrasting effects of herbicides, but no effects of the TME. In all cases, resource modifications (nitrate, N:P ratio or sucrose) induced significant effects on the physiology and stoichiometry of the plant, which might alter its response to additional stress like pollutants. These results suggest that an imbalance of resources in the presence of pollutants may lead to unforeseen changes in the combined effects on macrophytes, and also highlight the need to add more informative parameters in tests to meet the challenges of multiple stress and improve environmental risk assessment.