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Studies were conducted in the field from 2018-2019 in peanut to optimize weed control through application methods and herbicide choices. The herbicide studies were established in Mississippi, Oklahoma, Florida, and Alabama. Across trials, programs that included imazapic POST improved weed control at 28 days after treatment (DAT). The most effective PRE herbicides were flumioxazin, diclosulam, and S-metolachlor, while the use of pendimethalin was less effective. Various nozzles and operating pressures were evaluated for weed control in peanut. Droplet size was measured for each of the nozzles used at all operating pressures tested to determine classification. Classification was determined using the ASABE S572.2 standard. There was no effect on weed control or yield based on nozzle type or application pressure.

Field experiments were conducted in Virginia to evaluate the efficacy of AEF-130060 03 plus AEF-107892 applied postemergence (POST) for the control of Italian ryegrass [Lolium multiflorum (Lam)] in barley [Hordeum vulgare (L.)] in comparison to other herbicides currently registered for use in wheat [Triticum aestivum (L.)] and barley. Laboratory experiments were also conducted to evaluate absorption, translocation and metabolism of AEF-130060 03 plus AEF-107892 in wheat, barley, and Italian ryegrass with or without the addition of dicamba. AEF-130060 03 plus AEF-107892 was applied alone at three POST timings. All of these applications were effective in controlling Italian ryegrass. The third application timing of AEF-130060 03 plus AEF-107892 commonly resulted in lower yields than the first or second application timing, due to increased duration of Italian ryegrass competition, increased barley injury and insufficient time for barley recovery from this injury. In the field experiment, significantly greater barley injury was observed when AEF-130060 03 plus AEF-107892 was combined with 2,4-D and dicamba. Early postemergence (EP) treatments of AEF-130060 03 plus AEF-107892 provided control of Italian ryegrass equivalent to that of delayed preemergence (DPRE) applications of flufenacet plus metribuzin when rainfall was received. However, when rainfall was not received AEF-130060 03 plus AEF-107892 provided superior control. Because the efficacy of AEF-130060 03 plus AEF-107892 is not dependant on rainfall, it should provide more consistent control of Italian ryegrass than DP treatments. Laboratory experiments indicated that Italian ryegrass absorbed greater than 2.5, 2.0, and 1.5 times the amount of applied radioactivity at 24, 48, and 96 hours after treatment (HAT), respectively, than wheat or barley. Metabolism experiments indicated that quantity of parent compound in the three species was greatest in Italian ryegrass, followed by barley and then wheat. However, the sum total of metabolites was not different between species. A higher rate of metabolism of AEF-130060 03 plus AEF-107892 was also observed in wheat and barley than in Italian ryegrass. Thus, lower absorption of herbicide by wheat and barley, coupled with a more rapid rate of metabolism, most likely accounts for differential selectivity between these plant species. No differences in absorption, translocation or metabolism were observed within the three plant species due to the addition of dicamba. An additional experiment was conducted in 2000 and 2001 to evaluate the efficacy of preemergence (PRE) and EP applications of DPX-R6447 for the control of Italian ryegrass in wheat and barley in comparison to other herbicides currently registered for use in these crops. Barley and wheat injury and yield were similar with treatments of DPX-R6447 at rates below 176 g ai/ha and treatments of flufenacet plus metribuzin applied alone in both years. Consistent Italian ryegrass control with DPX-R6447 occurred only with rates of 176 g ai/ha or greater in both years. However, these rates resulted in variable injury in both wheat and barley between years. Rates higher than 176 g ai/ha of DPX-R6447 resulted in unacceptable barley and wheat injury. The lack of consistency with regard to barley and wheat injury could limit the utility of this compound in these crops. Field trials were conducted in Virginia to evaluate herbicide programs for control of Italian ryegrass in no-till corn [Zea mays (L.)] establishment. Herbicide programs using transgenic corn hybrids were compared to standard programs that utilize non-selective herbicides in combination with high rates of triazine herbicides. Italian ryegrass control and corn yields similar to those provided by standard programs could be attained through the use of paraquat plus atrazine, and glyphosate applied in combination with atrazine or rimsulfuron plus thifensulfuron-methyl. In glyphosate-tolerant corn, EP applications of glyphosate controlled Italian ryegrass, but yield did not differ from yields of corn treated with standard PRE applications of glyphosate plus atrazine. The use of glufosinate, imazethapyr plus imazapyr, or sethoxydim with appropriate herbicide-tolerant hybrids did not demonstrate potential for improved control of Italian ryegrass. Field experiments were also conducted to investigate the incidence and severity of maize chlorotic dwarf virus (MCDV) and maize dwarf mosaic virus (MDMV) in response to POST johnsongrass control in two corn hybrids. Recent observations have indicated a lack of virus-tolerance in glyphosate-tolerant corn hybrids in Virginia. The rapidity of virus disease development in corn resulting from application of glyphosate or nicosulfuron was also investigated. The virus-susceptible glyphosate-tolerant hybrid developed significantly higher levels of virus incidence three weeks after treatment than the virus-tolerant, non-transgenic hybrid, and virus incidence and severity increased throughout the duration of the growing season. Little or no disease incidence occurred in the virus-tolerant hybrid. The virus-susceptible hybrid exhibited significant increases in disease incidence in response to any herbicide treatment applied to johnsongrass-containing plots relative to the same treatment applied to weed free plots. Johnsongrass control with nicosulfuron or glyphosate caused similar disease incidence and severity in the virus-susceptible hybrid, regardless of application method. Results of these experiments indicated that growers' choice of hybrid should focus primarily on disease resistance rather than herbicide resistance. / Ph. D.

As the importance of cotton (Gossypium hirsutum L.) to Virginia crop growers has increased, so has the need for more efficient weed control programs. Current cotton herbicides do not control all broadleaf weeds over the entire growing season, and many cotton herbicides must be applied at specific growth stages in order to reduce crop injury. CGA 362622 is an experimental sulfonylurea herbicide that controls many broadleaf weeds at low use rates. Field, greenhouse, and laboratory studies were conducted to evaluate the potential benefit of CGA 362622 to Virginia cotton growers. Postemergence applications of CGA 362622 resulted in moderate crop response that proved transient in all instances and did not affect cotton yield. Broadleaf weed control from herbicide combinations with CGA 362622 generally controlled weeds more consistently than individual herbicide applications. Timely applications of CGA 362622 controlled common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.), annual morningglory species (Ipomoea spp.), and common cocklebur (Xanthium strumarium L.). However, CGA 362622 applications generally did not control spurred anoda (Anoda cristata (L.) Schlecht.), jimsonweed (Datura stramonium L.), velvetleaf (Abutilon theophrasti Medicus), or annual grass species, but combination treatments of CGA 362622 plus pyrithiobac did control velvetleaf, spurred anoda, and jimsonweed. Combinations of CGA 362622 plus glyphosate controlled common lambsquarters and smooth pigweed (Amaranthus hybridus L.) better than glyphosate alone, and in most instances controlled annual morningglory species better than glyphosate applied alone. Timely applications of CGA 362622 plus bromoxynil controlled velvetleaf, smooth pigweed, common ragweed, common lambsquarters, and common cocklebur. Spurred anoda control was generally not acceptable from CGA 362622, bromoxynil, or CGA 362622 and bromoxynil combinations. In laboratory studies, results supported differential absorption, translocation, and metabolism as the main factors for differential tolerance of cotton, spurred anoda, and smooth pigweed to CGA 362622. Rapid translocation and a slow rate of metabolism likely explains the susceptibility of smooth pigweed to this herbicide, while reduced absorption and translocation plus rapid metabolism contribute to CGA 362622 tolerance in cotton. Limited translocation may also explain the intermediate tolerance of spurred anoda to the herbicide CGA 362622. / Ph. D.

Mesotrione is the first triketone herbicide registered for use in turfgrass. Triketones prevent carotenoid biosynthesis by inhibiting the enzyme p-hydroxyphenylpyruvate dioxygenase (HPPD). Although mesotrione controls many species of grass and broadleaf weeds, it is best know for selective control of perennial grasses like creeping bentgrass (Agrostis stolonifera L.). Field trials conducted at Virginia Tech and Blacksburg Country Club determined that several programs that integrate herbicide treatment and turf seeding effectively transitioned creeping bentgrass contaminated golf roughs back to a tall fescue [Schedonorus phoenix (Scop.) Holub] monoculture. However, mature weeds require multiple mesotrione applications for effective control. This requirement is a major limitation to mesotrione's competitiveness in turfgrass markets. Several greenhouse and laboratory studies were conducted to evaluate scenarios where mesotrione rates were titrated and applied daily to mimic ascending, descending, and intervallic time-release patterns. These patterns were applied following an initial treatment to foliage or soil to mimic a potential sprayable or granular time-release formulation. These scenarios effectively controlled five targeted weed species equivalent to the standard of two broadcast sprays, regardless of initial application placement or release pattern. However, both time-release treatments and the standard injured tall fescue based on leaf counts, plant weights, and visual phytotoxicity ratings. Additional growth chamber studies found that changes in relative humidity from 50 to 90% caused a 4- to 18-fold increase in plant phytotoxicity with a concomitant decrease in photochemical efficiency when mesotrione was applied to foliage of smooth crabgrass (Digitaria ischaemum (Schreb.) Schreb. ex Muhl.). Furthermore, white tissue was found predominately in the two youngest leaves when mesotrione was applied to soil, but in older leaves when applied only to foliage. Laboratory studies were conducted to evaluate interspecific differences in 14C mesotrione absorption and translocation between two plant species when applied to foliage or roots. Annual bluegrass (Poa annua L.) absorbed 2- to 4-fold more radioactivity than Kentucky bluegrass (Poa pratensis L.). Both species absorbed less radioactivity through roots than through foliage and root absorbed radioactivity was more often exuded into Hoagland's solution while foliar absorbed radioactivity was often found in other foliage. / Ph. D.

Weed control is a critical component in pastures and hayfields in order to ensure maximum forage yields. Typically, broadleaf weed control in pastures and hayfields is achieved through the use of synthetic auxin. However, these herbicides also control desirable broadleaf species such as forage legumes, including white clover. Use of herbicides can lead to severe injury and often complete elimination of white clover, making it difficult for producers to maintain legumes in mixed grass-legume swards while controlling weeds. It is often desirable to have legumes present in the sward due to their high nutritive forage value and ability to fix nitrogen compared to grass only swards. Florpyrauxifen-benzyl + 2,4-D is a new herbicide which is reported to control broadleaf weed species, while preserving white clover. Little published research exists on this herbicide, particularly for use in pastures and hayfields. Research evaluating sward composition indicates that florpyrauxifen-benzyl + 2,4-D is effective in controlling broadleaf weed species while also preserving greater amounts of white clover than any other herbicide treatments. Florpyrauxifen-benzyl + 2,4-D also resulted in significantly more forage grass production than the nontreated control. Florpyrauxifen-benzyl + 2,4-D was less effective than other herbicides when applied via fertilizer impregnation. Additional research assessing the spectrum of broadleaf weed control found that florpyrauxifen-benzyl + 2,4-D is a viable herbicide for the control of several broadleaf weed species including bulbous buttercup, Canada thistle, broadleaf plantain, plumeless thistle, and common ragweed. However, florpyrauxifen-benzyl + 2,4-D was less effective than other herbicides for controlling certain weeds, such as horsenettle. White clover was injured from florpyrauxifen-benzyl + 2,4-D, but was able to fully recover in 90 to 120 days. There were no differences in white clover response between the four varieties tested. When evaluating establishment of forage species, florpyrauxifen-benzyl + 2,4-D did not injure or reduce biomass of tall fescue or orchardgrass plantings, indicating a high level of safety. Florpyrauxifen-benzyl + 2,4-D was also safe to both drilled and frost seeded clover when applied prior to and at planting. Greenhouse trials revealed that flowering white clover is more sensitive to herbicides compared to vegetative white clover, and that safety of white clover to florpyrauxifen-benzyl + 2,4-D is dependent upon use rate. Considerations such as weed species present, and the amount of white clover injury that is considered acceptable will dictate the decision to utilize florpyrauxifen-benzyl + 2,4-D in pastures and hayfields. This research demonstrates the effectiveness and overall utility of florpyrauxifen-benzyl + 2,4-D for use in pastures and hayfields due to the effectiveness of weed species as well as the level of safety to white clover. / Doctor of Philosophy / Pastures and hayfields are a critical component in livestock production. Grazing livestock perform best on highly nutritious forages. Legumes such as white clover are highly nutritious in forage systems and offer other benefits such as the ability to fixate nitrogen. Conversely, weed species negatively impact forage production by competing for resources with desirable forage species. Additionally, many species of broadleaf weeds are toxic to livestock. Because grasses are the backbone of forage systems, the majority of weed control efforts are aimed at controlling broadleaf weed species. However, beneficial forage legumes such as white clover are susceptible to broadleaf herbicides commonly used. This creates a management dilemma for producers who wish to control troublesome weeds, but also have white clover present in their pastures and hayfields. Florpyrauxifen-benzyl + 2,4-D is a herbicide combination which is new for pastures and hayfields. This herbicide is reported to control broadleaf weeds while also preserving white clover. Research trials were conducted in order to determine if florpyrauxifen-benzyl + 2,4-D could be used in forage systems to control weeds, without killing white clover. Several research trials were established to evaluate florpyrauxifen-benzyl + 2,4-D for broadleaf weed control and white clover safety. Research trials were established to determine the effect of florpyrauxifen-benzyl + 2,4-D on the number and overall amount of forage produced and the proportion of weeds and desirable forages as affected by herbicide treatment. Florpyrauxifen-benzyl + 2,4-D resulted in a 140% increase in forage grass production, and more legume production than any other herbicide treatment, while also decreasing the quantity and amount of broadleaf weed species. Because there is little existing research on what weed species florpyrauxifen-benzyl + 2,4-D controls, research trials were established to determine the spectrum of weed species that florpyrauxifen-benzyl + 2,4-D controls. Greenhouse trials were also established to evaluate the effect of white clover variety on injury from herbicide. Results showed that florpyrauxifen-benzyl + 2,4-D is effective in controlling several weeds such as bulbous buttercup, Canada thistle, broadleaf plantain, plumeless thistle, and common ragweed. Greenhouse trials showed that white clover variety did not influence the level of injury from herbicide applications. Seedling forages are more vulnerable to weed competition and therefore weed control around the time of planting is critical. However, seedlings are typically very sensitive to herbicides, compared to mature plants. Research trials were established to determine the effect of florpyrauxifen-benzyl + 2,4-D on the establishment of forage grasses tall fescue and orchardgrass, as well as white clover. White clover was established using two commonly used methods: drilling and frost-seeding. Results from the field show that florpyrauxifen-benzyl + 2,4-D is safe use around the time of tall fescue and orchardgrass establishment, as well as white clover planting with either method. Greenhouse trials were also established to determine if white clover's growth stage at the time of herbicide application influences the response. Results show that white clover is more sensitive to herbicides applied to flowering white clover compared to vegetative growth and the level of injury is dependent upon herbicide rate. Overall, our results demonstrate the utility of florpyrauxifen-benzyl + 2,4-D for forage production by controlling weed species and being safer to white clover than commonly used herbicides.

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.

Clomazone is a recently introduced herbicide for the selective control of grass and broadleaf weeds in soybeans. Field studies were conducted in full-season no-till soybeans to determine the efficacy of clomazone as a preplant and preemergence herbicide. Clomazone applied preemergence provided large crabgrass (Digitaria sanguinalis L.) control equivalent to that of oryzalin applied preplant or preemergence and provided better control of several broadleaf weeds. Control from preplant applications of clomazone was not adequate. Preemergence and preplant incorporated applications of clomazone were compared in conventionally-tilled soybeans. Clomazone efficacy at two depths of incorporation was also investigated. Clomazone applied preemergence generally provided control of large crabgrass and several broadleaf weed species equivalent to preplant incorporated applications. The addition of imazaquin or chlorimuron plus linuron improved smooth pigweed (Amaranthus hybridus L.) control over that provided by clomazone alone. These combinations generally did not improve large crabgrass, jimsonweed (Datura stramonium L.), and common lambsquarters (Chenopodium album L.) control over that of clomazone alone. Shallow incorporation (4 cm) of clomazone provided better weed control than deep incorporations (8 cm). Studies were conducted to evaluate efficacy and to quantify volatilization of three clomazone formulations (emulsifiable concentrate, wettable powder, and a microencapsulated formulation) following soil application. Samples were collected at the first, second, and tenth day after clomazone application. The three clomazone formulations provided control of large crabgrass. Clomazone volatilization was greatest 24 h after application from the emulsifiable concentrate and wettable powder formulations and declined at the second and tenth day after application. Volatilization from the microencapsulated formulation was lower than the other two formulations at all sampling times. Clomazone volatilization was greater from preemergence than preplant incorporated applications. Differential selectivity studies were initiated to determine the absorption, translocation, and metabolism of clomazone in tolerant soybean and smooth pigweed and susceptible redroot pigweed and livid amaranth exposed to foliar and root applied clomazone. Redroot pigweed and livid amaranth absorbed more clomazone through the roots than soybean and smooth pigweed. Absorption of foliar-applied clomazone was limited in all species. Of the clomazone absorbed in all species, most was translocated to the leaf tissue. Two metabolites of clomazone were found. One was determined to be a GS-clomazone conjugate. Differences in clomazone metabolism among species examined were not found. Growth and physiological responses of a normal hybrid ('DeKalb XL67'), a dwarf mutant, and an albino mutant of corn (Zea mays L.) to clomazone and interactions of gibberellin with clomazone on normal corn were examined. The dwarf mutant displayed greater tolerance to clomazone than normal corn. Growth measurements suggested that gibberellin was antagonistic with clomazone. / Ph. D.