Agriculture and Food Systems - Theses

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    Root-shoot interactions in the growth of irrigated white clover
    Blaikie, Samuel James ( 1993)
    White clover pastures support the dairy industry in the irrigated area of northern Victoria. However, pasture production is low because conditions for root growth are sub-optimal, particularly under flood irrigation. This thesis investigated the possibility that the growth of white clover can be increased by reducing the limitations to root growth. A series of experiments examined the response of white clover plants to various soil-based treatments and quantified relationships between root and shoot growth. Plants were grown in intact soil cores in the greenhouse with shoot and root growth measured by destructive harvest. The cores were collected from a range of field sites that were characterised by their different soil physical properties and the variation in pasture yield they supported. Other cores contained a sand-based potting mix in which the conditions for root growth were superior to the most productive field soil. Despite the large effects of soil treatment on white clover production, the growth of shoots and roots was highly correlated (R2>0.95). A prerequisite of high shoot yield is, therefore, a large root system. In one experiment, soil drying or defoliation perturbed the correlation but this disruption was only temporary. In another,experiment, the repeated cycles of drought stress that accompanied a series of extended irrigation intervals had no effect on the relationships between shoot and root growth. In field soils, the restrictions to root growth could not be overcome by intensive irrigation and fertiliser management. However, plants in the treatments in which the soil physical properties had been modified produced 4.0 - 6.5 times as much shoot DM compared with the least productive treatment. This suggests that the potential to improve pasture yield by amelioration of the soil physical properties is very large. Two further experiments were conducted in which either the soil texture or the frequency of irrigation varied between the upper and lower sections of the soil cores. In both cases the production of shoots was correlated with total root production. However, when `unfavourable' conditions restricted the growth of roots in one layer, extra growth of roots in the `favourable' layer was not sufficient to compensate. As a consequence, both total root and shoot growth were reduced. Taken together, these results suggest that there is a large scope to improve the yield of white clover by removing the restrictions to root growth that currently exist in field soils. This will probably entail both amelioration of the soil physical properties and careful management with respect to water and fertiliser applications. However, if the experiments reported here accurately reflect the field situation, then the growth of white clover pastures can only be maximised if the entire root zone is modified.
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    Effects of soil and water management on the productivity of irrigated pastures in the Goulburn Valley
    Blaikie, Samuel James ( 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.