School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemThe effect of applied N and P under mowing and grazing on the yield and botanical and chemical composition of irrigated pasture and on the nutrient status of a red-brown earth at Kyabram( 1978)A split plot experiment was conducted over 4 years to study the effect of method of harvesting (grazing and mowing) and applied N and P (5 rates of nitrogen ranged from 0 to 112 kg N ha -1 cut-1 and 6 rates of superphosphate ranged from 0 to 188 kg P ha 1 year year-1) on the yield, botanical and chemical composition of irrigated phalaris (Phalaris tuberosa x Phalaris arundinacea)/white clover (Trifolium repens L. cv. Irrigation) pasture and nutrient status of a red-brown earth at Kyabram. A second experiment was conducted over 2 years to find out the effect of rates (0 to 112 kg N ha. 1) and frequency of N application (2, 4 or 8 times year ) o n dry matter production, botanical composition and crude protein content of paspalum (Paspalum dilatatum)/white clover pasture. Results: Mowed plots significantly out-yielded grazed plots in the first two years. However, grazed plots significantly out-yielded mowed plots in the following two years. Nitrogen application significantly (P <0.01) increased dry matter production of grass/clover mixtures. The size of response was governed by the growth rate of pasture species, clover percentage in the sward, climatic conditions and the status of other nutrients in the soil. Splitting N rates in smaller but more frequent applications had no effect on total herbage yield or seasonal distribution. Response to N (kg D.M. kg N-1 ) declined as N rates increased. Phosphorus application up to 94 kg P ha -1 year -1 significantly (P,(0.01) increased pasture production and relative response was greater in winter. Occasionally, the P and N requirements for maximum production were higher for grazing than for mowing. Both methods of defoliation and fertilizer applications affected pasture composition and IT, P and K content in the herbage. Nitrogen application decreased N in the herbage before increasing it and P application up to 141 kg P ha- 1 year -1 increased it. The return by the grazing animal increased total soil N by 106 kg ha 1 year 1 and soil P (Colwell) by 1:0 p.p.m. in four years compared with the start of the experiment. The relationship between plant P and soil P and P rates were significant (P < 0.01). The available soil P decreased with P withdrawal or the application of 23.5 kg P ha-1 year -1 under grazing and 47 kg P ha -1 year-1 under mowing. Phosphorus and nitrogen application significantly affected available soil P. 60-80% of the available P accuminulated in the 0-5 cm and the downward movement increased by increasing P rates. The continuous mowing and P application significantly reduced available soil K. Defoliation method affected C/t:N ratio. Phosphorus application increased soil N whereas N application decreased it. P application at the rate of 23.5 kg ha-1 year-1 increased total soil N by 0.068% which is equivalent to 952 kg N ha-1 in five years. It was estimated that 4 to 4.7 p.p.m of P was required to be removed or added to the soil to change the available soil P by 1 p.p.m. The application of 196 kg N ha-1 year -1 as NH4 NO3 did not change the soil pH and 784 kg N ha 1 year 1 reduced soil pH by an average of ').4 unit.
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ItemThe ecology and physiology of two species of Carduus as weeds of pastures in Victoria( 1977)Slender thistles (Carduus pycnocephalus and C. tenuiflorus) were introduced to Australia about the 1880s. They are now important weeds of pastures in much of southern Australia and are difficult to control with present methods. This study was undertaken to investigate several aspects of the ecology and physiology of the plants with the belief that a knowledge of some of these aspects, particularly of seed germination and seedling establishment, might disclose some "weakness" in the plants' growth which could be exploited to improve control measures. Because of confusion over differences between the two species which occur in Australia the initial step was to evaluate the morphological features which have been used to distinguish between the two species. Although they are very similar morphologically, cytological evidence confirmed that the two species were quite distinct and, in fact, had quite different evolutionary origins. Germination of seeds of slender thistles is controlled by three separate forms of dormancy; these are known as innate, induced and enforced dormancy. Dormancy ensures that the plants will survive in a Mediterranean-type climate and also colonize areas with quite different climates and, most importantly, survive natural catastrophes such as drought, fire, and flood. The germination of slender thistles in the field is confined to a very short period (about 6 weeks) after the autumn break in any year. This is a "weakness" in the plants' survival mechanism because they are vulnerable in that year, at least, to any treatment which can kill seedlings. The herbicide, diquat, was found to kill young seedlings of slender thistles and not affect seedlings of desirable pasture plants associated with the thistles in southern Australia. This treatment is economical and leads not only to a reduction in thistles but an increase of about 30% in pasture production. Several other aspects of the plants' growth were investigated. Slender thistles have early growth characters which give them advantages over more desirable components of pastures. They are more competitive than subterranean clover and ryegrass over a wide range of levels of nutrients, and the traditional approach to pasture improvement in southern Australia of applying superphosphate and sowing subterranean clover will encourage, not suppress, slender thistles. Since grazing animals generally do not eat slender thistles the presence of thistles in pastures at densities commonly occurring in Victoria considerably reduces pasture production.
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