School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemSelection of Rhizobium trifolii strains for inoculation of Persian clover (Trifolium resupinatum L.)( 1988)The studies reported in this thesis examined the Rhizobium trifolii (also reclassified Rhizobium leguminosarum biovar trifolii) strain requirements for inoculation of Persian clover (Trifolium resupinatum L.). The agronomic importance of T. resupinatum cv. Maral to agriculture in southern Australia had been recognised and new cultivars of T. resupinatum were being developed, but there had been no objective assessment of the inoculation requirements of this species. A diverse range of R. trifolii strains were examined for symbiotic competence (ability to nodulate) and effectiveness (nitrogen fixation) with T. resupinatum cv. Maral under bacteriologically controlled conditions ideal for plant growth. The results showed that strains varied in effectiveness from 18% to 145% compared to the standard commercial inoculant strain, TA1 (100). A strain selection programme was followed in which three selected strains (CC2483g, CC1317 and CC1305), all superior in effectiveness compared to TA1 (P<0.05) were assessed in glasshouse and field experiments in southwest Victoria. CC2483g was the best strain on T. resupinatum cv. Maral. This strain demonstrated good effectiveness, was highly competitive, persisted in the soil for 18 months under conditions of mild competition from naturalized strains and appeared particularly well adapted to sandy soils of low fertility. In contrast, TA1 was poorly effective, less competitive than other strains and the least persistent strain in the field. Strain differences in symbiotic effectiveness were demonstrated in the field where there was a low population (<5.0 x 102 R. trifolii per gram of soil) of naturalized R. trifolii. In soils with large populations (>1.0 x 104 R. trifolii per gram of soil), of naturalized R. trifolii , improvements in nitrogen fixation and herbage yield by inoculation with effective strains were dependant on the rate of inoculation and the relative symbiotic effectiveness of naturalized strains. T. resupinatum cv. Maral was capable of nodulation by a range of naturalized strains and it was concluded that commercial seed coat inoculation with highly effective strains and lime pelleting would, in general, not improve the nitrogen fixation of T. resupinatum cv. Maral in most areas of southwest Victoria. Host range experiments examined the effectiveness of the three selected strains compared to TA 1 and WU95 on important annual and perennial clovers and showed that CC2483g had a wider host.range that CC1317 and CC1305. The use of TA1 in the T. repens subgroup and WU95 in the T. subterraneum subgroup inoculants were verified in this study. Effectiveness tests with five lines of T. resupinatum var. majus and eight lines of T. resupinatum var. resupinatum suggested that T. resupinatum constituted a seperate symbiotic subgroup within the Trifolium species. The final step in the strain selection programme indicated that all selected strains satisfied the requirements for commercial inoculant production. It was concluded, that under the current policy of using single wide-spectrum strains for commercial inoculation, that a special single-strain inoculant should be produced for T. resupinatum containing CC2483g. Alternative inoculant recommendations to overcome the apparent deficiencies of both Australian commercial clover inoculants are discussed, of which the most practical solution could be mixed-strain culturing.
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ItemEffects of soil and water management on the productivity of irrigated pastures in the Goulburn Valley( 1986)The water status and productivity of perennial pastures were monitored during irrigation cycles. Measurements of leaf water potential reflected the water status of the pastures and when this deteriorated with the developing shortage of water after irrigation, various responses of the pastures were recorded. These included the rate of leaf elongation, canopy conductance, and the rates of net photosynthesis and evapotranspiration. The Parameters of gas-exchange were Measured using open-system, field chambers. The studies were designed to characterise and Compare the responses of these Pastures as water deficit developed, and to quantify their,effects on productivity. Experimental plots included swards of white clover (Trifolium repens), perennial ryegrass (Lolium perenne) and paspalum (Paspalum dilatatum) both as monocultures and as mixtures of the three species. These swards were growing on two soil profiles, both of which were Lemnos loam. The first profile was in its normal state and had been under pasture for fifteen years. The second had been structurally and chemically modified in 1979 to minimise the limitations to plant growth. It had been re--sown to pastures in 1980. The productivity of monocultures and mixed swards of each species on each profile was investigated as water shortage developed. In all species the responses to water shortage were the same, involving a reduction in the rate of development of leaf area,, followed by a reduction in photosynthesis per unit leaf area as the lack of water became more severe. The sensitivity of each species was distinct, with white clover being the most sensitive, ryegrass intermediate and paspalum the least. On modified soil, all species were less affected than on the normal profile but the order of sensitivity was the same. Overall, these experiments showed that after a typical irrigation cycle, which is about 8 days, water shortage reduces the productivity of white clover by 50%, ryegrass by 20% and paspalum by 5-10%. These limitations to productivity were overcome to some extent by modifying the profile, so that after an equivalent period the productivity of white clover was only reduced by about 208 and paspalum was not affected at all. The ability of paspalum to maintain high levels of productivity during an irrigation cycle had the effect of promoting this species in a mixed sward, its dominance becoming greater as the shortage of water became more severe. In order to achieve balanced pastures of maximum productivity, farmers in the Goulburn Valley need to reduce the limits to growth imposed by the physical and hydraulic characteristics of the soils. This may be achieved by irrigating more frequently or by undertaking some form of profile modification.