Browsing by Author "Nixon, D. J."

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    A comparison of the responses of mature and young clonal tea to drought.
    (Cambridge University Press, 2001-01-01T00:00:00Z) Nixon, D. J.; Burgess, Paul J.; Sanga, B. N. K.; Carr, M. K. V.
    To assist commercial producers with optimising the use of irrigation water, the responses to drought of mature and young tea crops (22 and 5 years after field planting respectively) were compared using data from two adjacent long-term irrigation experiments in Southern Tanzania. Providing the maximum potential soil water deficit was below about 400-500 mm for mature, and 200-250 mm for young plants (clone 6/8), annual yields of dry tea from rainfed or partially irrigated crops were similar to those from the corresponding well-watered crops. At deficits greater than this, annual yields declined rapidly in young tea (up to 22 kg (ha mm)-1) but relatively slowly in mature tea (up to 6.5 kg (ha mm)- 1). This apparent insensitivity of the mature crop to drought was due principally to compensation that occurred during the rains for yield lost in the dry season. Differences in dry matter distribution and shoot to root ratios contributed to these contrasting responses. Thus, the total above ground dry mass of well-irrigated, mature plants was about twice that for young plants. Similarly, the total mass of structural roots (>1 mm diameter), to 3 m depth, was four times greater in the mature crop than in the young crop and, for fine roots (<1 mm diameter), eight times greater. The corresponding shoot to root ratios (dry mass) were about 1:1 and 2:1 respectively. In addition, each unit area of leaf in the canopy of a mature plant had six times more fine roots (by weight) available to extract and supply water than did a young plant. Despite the logistical benefits resulting from more even crop distribution during the year when crops are fully irrigated, producers currently prefer to save water and energy costs by allowing a substantial soil water deficit to develop prior to the start of the rains, up to 250 mm in mature tea, knowing that yield compensation will occur later.
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    Evaluation of simple hand-held Mechanical systems for harvesting tea (Camellia Sinensis)
    (Cambridge University Press, 2006-04-01T00:00:00Z) Burgess, Paul J.; Carr, M. K. V.; Mizambwa, F. C. S.; Nixon, D. J.; Lugusi, J.; Kimambo, E. I.
    Over an eight-year period, harvesting methods based on simple mechanical aids (blade and shear) were evaluated against hand harvesting on mature morphologically contrasting tea clones in Southern Tanzania. The effects of shear step height (5-32 mm) and the harvest interval (1.8-4.2 phyllochrons) were also examined. Except in the year following pruning, large annual yields (5.7- 7.9 t dry tea ha[minus sign]1) were obtained by hand harvesting at intervals of two phyllochrons. For clones K35 (large shoots) and T207 (small shoots), the mean harvested shoot weights were equivalent to three unfurled leaves and a terminal bud. The proportions of broken shoots (40-48 %) and coarse material (4- 6 %) were both relatively high. Using a blade resulted in similar yields to hand harvesting from K35 but larger yields from T207 (+13 %). The yield increase from clone T207 was associated with the harvest of more shoots and heavier shoots, smaller increases in canopy height, and a higher proportion (7-9 %) of coarse material compared to hand harvesting. On bushes, which had been harvested by hand for two years following pruning, using flat shears (no step) supported on the tea canopy resulted, over a three year period, in yields 8-14 % less than those obtained by hand harvesting and, for clone K35, a reduction in the leaf area index to below 5. The development of a larger leaf area index is made possible by adding a step to the shear. However, since annual yields were reduced by 40-50 kg ha[minus sign]1 per mm increase in step height, the step should be the minimum necessary to maintain long-term bush productivity. As mean shoot weights following shear harvesting were about 13 % below those obtained by hand harvesting, there is scope, when using shears, to extend the harvest interval from 2 to 2.5 phyllochrons.
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    A preliminary assessment of climate change impacts on sugarcane in Swaziland
    (Elsevier Science B.V., Amsterdam., 2010-02-01T00:00:00Z) Knox, Jerry W.; Rodriguez Diaz, J. A.; Nixon, D. J.; Mkhwanazi, M.
    The spatial and temporal impacts of climate change on irrigation water requirements and yield for sugarcane grown in Swaziland have been assessed, by combining the outputs from a general circulation model (HadCM3), a sugarcane crop growth model and a GIS. The CANEGRO model (embedded with the DSSAT program) was used to simulate the baseline and future cane net annual irrigation water requirements (IRnet) and yield (t ha-1) using a reference site and selected emissions scenario (SRES A2 and B2) for the 2050s (including CO2-fertilisation effects). The simulated baseline yields were validated against field data from 1980-1997. An aridity index was defined and used to correlate agroclimate variability against irrigation need to estimate the baseline and future irrigation water demand (volumetric). To produce a unit weight of sucrose equivalent to current optimum levels of production, future irrigation needs were predicted to increase by 20-22%. With CO2-fertilisation, the impacts of climate change are offset by higher crop yields, such that IRnet is predicted to increase by 9%. The study showed that with climate change, the current peak capacity of existing irrigation schemes could fail to meet the predicted increases in irrigation demand in nearly 50% of years assuming unconstrained water availability.