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Doctoral Degrees (Crop Science)

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Browsing Doctoral Degrees (Crop Science) by Author "De Jager, James M."

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    Maize crop growth and development related to solar radiation.
    (1976) Kaiser, Heinz Werner.; De Jager, James M.
    The interception and reflection of solar radiant energy by maize crops was measured at Cedara using tube solarimeters. From the measured data the amount of PHAR that was intercepted was related to the L value of the crop. K(v), the extinction coefficient of PHAR thus obtained was found to vary with row spacing, plant population density and cultivar, whereas growth stage, time of day and weather conditions did not appear to effect the K(v) value markedly. On clear days the albedo (R(v)) of a ma1ze crop was of the order of 0,05 to 0,08 during the middle of the day. However, at low sun angles in the early and late hours of the day, R(v) increased to values of the order of 0,2. On overcast days, the diurnal variation was considerably lower. The rate of photosynthesis in a ma1ze crop was measured using the plastic enclosure technique. The rate of CO2 uptake was related to the amount of PHAR absorbed by the crop. A rectangular hyperbola (Eqn. 2. I) was found to describe this relationship well. The photosynthetic response of a crop to population density, plant spacing and temperature at different developmental stages is discussed. The seasonal distribution of dry matter in the maize crop was studied using normal growth analysis techniques. From these data functions describing the partitioning of DM to the various plant organs (e.g. roots, leaves, grain) at different growth stages were developed. The functions describing the absorption of light by the crop canopy, its photosynthetic rate and the partitioning of photosynthate were incorporated in a computer simulation model, which was used to estimate the growth of a maize crop from meteorological radiant flux density and temperature data. The functioning of the model and its possible use are discussed.
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    The use of climatic data for maize grain yield predictions.
    (1972) Mallett, John Bertram.; De Jager, James M.
    The development and testing of a mathematical model for maize grain yield predictions is described. The model is based upon daily considerations of soil moisture, atmospheric evaporative demand and stage of crop development. Final yield predictions depend upon a knowledge of yield decrement due to moisture stress and the number of occasions that stress is recorded. This information was determined in the following manner:- (i) Stress imposed in lysimeters before and after anthesis was found to reduce grain yields by 3,2% and 4,2% per stress day respectively. (ii) A stress day was identified with the aid of mass-measuring lysimeters and a U.S. Weather Bureau Class A evaporation pan for measuring atmospheric evaporative demand. A nomogram constructed in terms of evaporative demand and available soil moisture, which discriminates between stress and non-stress days, was obtained for the Doveton soil used in the lysimeters. The model was applied to Cedara rainfall and evaporation data and yield probability patterns for three planting dates were obtained. It was found that highest yields (8,5 Mg ha(-1)) and least seasonal yield variation, may be expected from the earliest planting data 15/10. The Cedara : Doveton yield prediction model was also applied to climatic records for two other Natal stations (Estcourt and Newcastle) and six stations outside Natal (Bethlehem, Potchefstroom, Hoopstad, Standerton, Ermelo and Krugersdorp). Interesting comparison of the suitability of their respective climates for maize production was obtained. A method which uses the predicted number of stress days and the resultant yield decrement to determine the most effective and economic irrigation scheduling is developed and described. The effect of moisture holding characteristics of various soils upon the shape of the discriminating curve is discussed, and a method of obtaining discriminating curves for other soils by modifying the Doveton curve is described.