The problems associated with the use of white clover in pastures in the eastern high potential areas of South Africa i.e. high P requirements, low tolerance to high Al levels and low pH in the soil as well as a limited survival time of approximately 30 months, were identified and found to be related to the inadequate root system of white clover cultivars. During the improvement programme cultivars available on the world market were introduced and evaluated under dryland conditions. Selections were made from these introductions on the basis of root conformation in high AI, low pH soils, their response to grazing and induced moisture stress. A laboratory technique for the improvement of Al tolerance was developed and the tolerance of white clover plants to high levels of Al was improved but due to the complexity of pasture plant improvement it was decided that the selection for tolerance to Al could be more effectively carried out in the field. The effectiveness of vesicular arbuscular mycorrhizas as phosphate gatherers indicated that local strains of mycorrhizas combined as effectively with white clover as the imported strains. As no seed production of white clover is undertaken in South Africa guidelines for local seed production were also established. As a result of the improvement programme, Trifolium repens cv. DUSI was developed as an open pollinated synthetic variety, based on thirty eight selected mother lines. DUSI has a greater tolerance to high AI, low pH, low P in the soil and due to an improved root system with a high percentage of secondary taproots produces better under dryland conditions and has a longer stand life than any of the cultivars of white clover available on the local market. Plant Breeders Rights were obtained for cv. DUSI and the cultivar was inscribed on the South African variety list. Limited amounts of Breeders seed have been made available to the South African Forage Seed Association for commercial seed production. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1988.

Thesis (Ph. D.)--Oregon State University, 1989. / Typescript (photocopy). Includes bibliographical references. Also available on the World Wide Web.

No description available.

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.

Bibliography: leaves 213-229. / xxiii, 229, 26 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Plant Science, 2000

Thesis (M.Sc.) -- University of Western Sydney, Hawkesbury, 1999. / Thesis submitted for the degree of Master of Science. Includes bibliographical references (leaves 113-131).

Dissertation (PhD (Agric))--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

No description available.

Both red (Trifolium pratense L.) and white (Trifolium repens L.) clover seed yields can be highly variable and low in western Oregon. The objectives of this study were to: i) determine crop water requirements and supplemental irrigation timing, and ii) quantify the effects of soil and water status on inflorescence production, seed yield and seed yield components for red and white clover seed crops. In each species, five supplemental irrigation treatments were applied in 1990 and 1991 to first and second year seed crops grown on a Woodburn silt loam (fine-silty, mixed, mesic Aquultic Argixeroll) near Corvallis, OR. Non-irrigated controls were also maintained. In red clover, increased plant water stress reduced the duration of the season-long bud and flower production, stem length, potential floral capacity (PFC), and seed yield (SY). Root rot index (RRI) increased with increasing levels of plant water stress, indicating that supplemental water applications reduced second-year root rot severity. The reduction in SY from increasing plant water stress was primarily caused by a decrease in floral fertility, and less conclusively by reductions in inflorescence number per unit area. One irrigation to fill the soil active profile during peak flowering provided adequate water to maintain efficient seed production. In white clover, SY was maximum in 1990 when water application was delayed until 68% of the available soil-water was used by the crop which maintained an even flush of flowers and restricted vegetative growth. In 1991, all irrigation treatments yielded the same or less than the non-watered control. This was due to the excessive vegetative growth from stolons that had grown between the planted rows the previous and present crop year. In both years excessive amounts of irrigation water favored profuse vegetative growth and reduced SY. Inflorescence density was increased by constraining soil-water in 1990 and was the yield component that most affected SY both years. Crop water stress index (CWSI) was a useful indicator of plant stress status and can be used to schedule irrigations in red and white clovers grown for seed under typical climatic conditions of western Oregon. / Graduation date: 1993

Characterization of the most frequently isolated virus found during a survey of eastern Ontario red clover fields, which was designated as the Ottawa isolate, was undertaken. Determination of the biological and physico-chemical properties of the virus showed that the virus studied differed in some respects from other WCMV isolates. An ultrastructural examination of virus localization of cytopathological changes in infected red clover and pea plants revealed various types of viral inclusions, including a membrane-bound mass of tubules, not usually associated with infection by potexviruses, in red clover leaves, and severe organelle disorganization within pea leaf tissues. WCMV infection significantly reduced several processes relevant to symbiotic nitrogen fixation, including plant growth, nodulation, nitrogenase activity, leghemoglobin content, and Rhizobium population, but nitrate reductase and acid phosphatase specific activities were increased. The correlation between leghemoglobin content and nitrogenase activity shown at ten-leaf and pre-blooming stages was absent during the flowering and senescence stages. Infectivity assays, immunosorbent electron microscopy, and an ultrastructural examination showed the presence of the virus in nodular tissues; the cytopathological changes seen in bacteroids suggested that virus infection accelerated nodule senescence. Application of a cytochemical technique using diaminobenzidine showed that the leghemoglobin was located in the peribacteroidal space.