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.

Aquatic nuisance plants (ANS), like watershield (Brasenia schreberi), negatively affect water chemistry, ecosystem ecology, and human uses of waterbodies if not properly managed. Understanding ecological predictors as well as efficient control strategies is needed for successful control of ANS. This research assessed 1) potential environmental and ecological predictors of watershield prevalence along with 2) submersed injections of flumioxazin herbicide for watershield control. Twenty-one lakes in the state of Mississippi were surveyed for watershield presence. Watershield prevalence was not correlated to water clarity or plant diversity suggesting these are not drivers of watershield prevalence. A series of mesocosm and field trials were conducted to assess flumioxazin rate reductions for watershield control. In the mesocosm study, 100 ppb flumioxazin injections reduced water shield biomass to >70% 8 weeks after treatment (WAT). This was validated on a field population (>88% reduction) and provides a new control measure for Mississippi resource managers and landowners.

Differences in cropping practices, including tillage, inputs and crop rotations are the driving factors affecting weed dynamics (weed abundance, composition and crop-weed competition), which can ultimately affect crop yields. Several experiments were carried out to assess the impact of long-term organic and conventional cropping systems on weed abundance, weed community composition, crop yield and yield loss using a long-term (18 year) alternative cropping systems study (ACS) at Scott, Saskatchewan, Canada. The ACS study consisted of three input systems, namely high (conventional tillage), reduced (no-till conventional) and organic input systems and three crop rotation diversities (low diversity, diversified annual grains and diversified annual-perennials). A statistical analysis of the 18-year rotation revealed that the organic rotations have four and seven times higher weed density and 32% and 35% lower crop yields than the reduced and the high input systems respectively. Weed community composition was consistently different in organic rotations compared to the two conventional rotations throughout the years, but year to year random variations were more profound. All cropping systems showed an increase in weed density, weed biomass and crop yields over time, probably due to an increase in rainfall over time. Increasing the crop rotation diversity with annual and perennial crops did not reduce weeds, but decreased crop yields in all systems. A two-year micro-plot experiment with four additional weed competition treatments on the ACS study revealed that the wheat yields were lower in the organic rotations even in the absence of weeds, implying that lower crop yields were due to soil fertility related factors. A greenhouse pot experiment from soils obtained from both organic and reduced rotations revealed that wheat yields were still lower in organic compared to the reduced input systems, even after excess mineral N and P were added. Furthermore, no differences in crop yield loss due to weed competition among cropping systems were identified. Overall, this study revealed that eliminating tillage and reducing inputs are possible without long-term changes in weed abundance, weed community composition or affecting crop yields. However, eliminating synthetic inputs as was done in the form of organic crop rotations resulted in increased weed abundance, changed community composition and decreased crop yields.

3 pp. / Revised / The most effective weed control practice in alfalfa is maintaining a healthy crop and dense stand. Cultural practices that promote a vigorous stand can reduce the need for chemical weed control. Cultural practices that promote a healthy stand reduce the need for chemical weed control, but herbicides are sometimes necessary even in well-managed alfalfa.

The tolerance of seedling alfalfa to norflurazon applied at various times after planting was evaluated at the University of Arizona Maricopa Agricultural Center (MAC) and in Glendale, AZ during 1998 and 1999. At each application date, rates of 0, 1, 1.5, 2, 3, or 4 lbs a.i./A of norflurazon formulated as a 5% sand granule (Zorial 5G) were applied using a ground driven Valmar granule applicator. Zorial 5G at rates ranging from 1.5 to 4.0 lbs a.i./acre applied as early as 25 days after planting (DAP) had no effect on alfalfa seedling emergence and stand establishment at (MAC). Significant alfalfa height reductions were observed as Zorial 5G rate was increased when Zorial was applied at 25 and 62 DAP (MAC) and 64 DAP (Glendale). Alfalfa fresh weight yield for the MAC location was reduced at the second cutting after application as Zorial 5G rate increased for the at planting and 25 DAP treatments. Alfalfa yields approximately one year after planting were not affected by applications of Zorial 5G at 25 DAP or later. Results suggest that Zorial applications at rates of 1 to 2 lbs a.i./A applied at the 3 to 4 trifoliate leaf stage (approximately 2 months after planting) have little effect on yield at first and second cuttings of alfalfa.

Two rates of Raptor herbicide were evaluated for late winter weed control in alfalfa in combination with various types of surfactancts, the insecticide Furadan, and other alfalfa herbicides with known limited control spectrums. Herbicides that were combined with Raptor were also evaluated separately, as was Pursuit. Effects of treatments on wild oats, littleseed canarygrass and annual sowthistle were obtained. Treatments containing the active ingredient clethodim (Select/Prism) reduced canarygrass height and reproduction, while Raptor treatments increased numbers of inflorescences. No treatment provided effective control of sowthistle although some activity was noted from the Raptor treatments when numbers of reproductive structures and height were examined. Surfactants/ adjuvants greatly increased Raptor activity. Wild oat control was noted in treatments containing clethodim and several Raptor treatments when utilizing a surfactant/adjuvant.

The response of alfalfa regrowth, yield and plant populations to halosulfuron applied following cuttings and irrigations in the summer and fall of 1999 was studied in an experiment conducted at the University of Arizona Maricopa Agricultural Center. A single application of halosulfuron applied when there was little alfalfa foliage slightly reduced plant heights for several cutting cycles with increasing rate decreasing plant height. The cumulative forage fresh weight yields for the October 4th, November 15th, and February 22nd harvests for treatments receiving no halosulfuron or 0.032, 0.047, or 0.063 lb a.i./A were (means ± std. dev.): 15.94 ± 0.91, 14.99 ± 0.66, 14.80 ± 1.74, and 14.46 ± 0.97 tons/A, respectively. The trend of decreasing cumulative forage fresh weight with increasing halosulfuron rate was significant (Adj. R2 = 0.178, P = 0.015) indicating that for the three harvests after August 25th, halosulfuron had a small but negative effect on forage fresh weight. The harvest on April 5, 2000, the fourth harvest following the halosulfuron applications on August 25, 1999, indicated that there was no longer any residual effect of halosulfuron on alfalfa growth. Plant populations measured on April 10, 2000 were not affected by either one or two halosulfuron applications in this experiment. A set of sequential halosulfuron treatments applied when there was substantial alfalfa foliage (about 80% of the ground surface covered) severely suppressed alfalfa regrowth. Little regrowth occurred in these plots in October or November after the sequential applications compared to the untreated control or to the plots that received only the initial application of halosulfuron. The change in mean percent yield loss with successive harvests on November 15th, February 22nd and April 5th of 85, 40 and 14% indicated that the alfalfa plants were recovering from the halosulfuron applications. The cumulative forage fresh weight yields for treatments receiving sequential halosulfuron treatments (0.032+0.032, 0.047+0.047, or 0.063+0.063 lb a.i./A) were (means ± std. dev.): 11.67 ± 1.46, 10.85 ± 1.06, and 10.44 ± 0.98 tons/A, respectively, and were much less than the cumulative yield of 18.97 ± 1.17 tons/A from the untreated plots. The data suggest that the critical factor in determining the degree of alfalfa injury caused by halosulfuron is the amount of foliage present at the time of application.

Doctor of Philosophy / Department of Agronomy / J. Anita Dille / Integrated weed management (IWM) is an ecological approach to weed control that reduces dependence on herbicides through understanding of weed biology and involves using multiple weed control measures including cultural, chemical, mechanical and biological methods. The critical period of weed control is the duration of the crop life cycle in which it must be kept weed-free to prevent yield loss from weed interference. Eight experiments were conducted throughout Kansas between October 2010 and June 2012 to identify this period in winter wheat grown under dryland and irrigated conditions. Impact of henbit and downy brome density on winter wheat yields were evaluated on four farmer’s fields with natural populations and on a research station with overseeded populations. Henbit density up to 156 plants m-2 did not affect winter wheat yield, while downy brome at a density of 40 plants m-2 reduced yield by 33 and 13% in 2011 and 2012, respectively. In the presence of downy brome, winter wheat should be kept weed-free approximately 30 to 45 days after planting to prevent yield loss; otherwise, weeds need to be removed immediately following release from winter dormancy to prevent yield loss due to existing weed populations. Flumioxazin and pyroxasulfone are herbicides registered for use in winter wheat, soybean and corn for control of broadleaf and grass weeds. Flumioxazin and pyroxasulfone were evaluated for plant response to localized herbicide exposure to roots, shoots, or both roots and shoots utilizing a novel technique. Two weed species, ivyleaf morningglory and shattercane, as well as two crops, wheat and soybean, were evaluated for injury after localized exposures. The location and expression of symptoms from the flumioxazin and pyroxasulfone herbicides were determined to be the shoot of seedling plants. The utilization of preemergence herbicides in winter wheat is not a common practice, although application may protect winter wheat from early season yield losses as determined by the critical weed-free period. Kansas wheat growers should evaluate the presence and density of weed species to determine which weed management strategy is most advantageous to preserving winter wheat yield.

A resistência de plantas daninhas é um grande problema que afeta a produtividade das culturas em nosso país, ao passo que quando é identificada, deve ser estudada visando desenvolver estratégias de manejo que evitem a expansão e seleção em novas áreas, além de seleção de resistência múltipla em áreas onde está presente. Nesta temática o objetivo desta pesquisa foi determinar manejo alternativo ao glyphosato para as espécies Chloris elata e Digitaria insularis no período de entressafra da cultura da soja e identificar características morfológicas da superfície foliar de Chloris elata que possam interferir sobre o controle com herbicidas. Foram estudadas alternativas ao glyphosate para manejo em áreas de sistema de produção soja/milho, em dois experimentos com as plantas daninhas Chloris elata e Digitaria insularis perenizadas e com duas condições de roçada (15 e 30 cm). Foram avaliados o controle visual das plantas daninhas, redução de matéria fresca e seca. Para morfologia das folhas da espécie Chloris elata foi realizada análise histológica com recursos de microscopia eletrônica de varredura, visando caracterizar a morfologia da folha, além de determinar o índice estomático, densidade estomática e densidade de tricomas para as superfícies adaxial e abaxial das folhas, nos estádios 4 e 6 folhas totalmente expandidas. Digitaria insularis se mostrou menos suscetível as técnicas de manejo empregadas para plantas perenizadas, necessitando duas aplicações sequenciais para conter a rebrota dos tratamentos. O uso da maior dose do herbicida haloxyfop associado aos herbicidas glyphosate ou amônio-glufosinato se mostrou eficaz no controle de C. elata e D. insularis. O manejo de roçagem potencializou a ação dos herbicidas, e a planta daninha D. insularis se mostrou mais suscetível a está técnica em relação ao C. elata. Para este manejo a associação da maior dose de haloxyfop ao glufosinato foi eficaz para as duas gramíneas. As alturas de roçagem somente se diferenciaram nas primeiras avaliações. O biótipo resistente de C. elata apresentou menor densidade estomática, no estádio de 4 folhas completamente expandidas e maior densidade de tricomas no estádios de 6 folhas completamente expandidas, o que pode contribuir para a resistência ao glyphosate. Está espécie apresentou formações de cristais de cera que circundam o ostíolo, no estádio de 6 folhas completamente expandidas, podendo estar relacionada a a menor suscetibilidade ao glyphosate desta planta daninha perenizada. / Weed resistance is a major problem that affects crop productivity in our country, whereas when it is identified, it must be studied in order to develop management strategies that avoid expansion and selection in new areas, as well as multiple resistance selection in areas where it is present. In this subject the objective of this research was to determine alternative management of glyphosate for the Chloris elata and Digitaria insularis species during the off - season of the soybean crop and to identify the morphological characteristics of the leaf surface of Chloris elata that could interfere the control with herbicides. It was studied alternatives to glyphosate for management in areas of soybean / maize production system in two experiments with weeds Chloris elata and Digitaria insularis perennial and with two mowing conditions (15 and 30 cm). The visual control of weeds, fresh and dry matter reduction were evaluated. For the morphology of the leaves of the Chloris elata species, a histological analysis was carried out using scanning electron microscopy to characterize the leaf morphology, as well as to determine the stomatal index, stomatal density and trichome density for the adaxial and abaxial surfaces of the leaves, stages 4 and 6 fully expanded leaves. Digitaria insularis was less susceptible to the management techniques used for perennial plants, requiring two sequential applications to contain the regrowth of treatments. The use of the highest dose of the herbicide haloxyfop associated with the herbicides glyphosate or ammonium glufosinate proved to be effective in the control of C. elata and D. insularis. Herbicide management potentiated herbicide action, and D. insularis weed was more susceptible to this technique than C. elata. For this management, the association of the highest dose of haloxyfop to glufosinate was effective for both grasses. The cutting heights differed only in the first evaluations. The resistant C. elata biotype showed lower stomatal density at the stage of 4 fully expanded leaves and greater density of trichomes in the stages of 6 fully expanded leaves, which may contribute to resistance to glyphosate. This species presented waxy crystal formations that surround the ostrich, in the stage of 6 fully expanded leaves, and may be related to the natural tolerance to the glyphosate of this perennial weed.