Genetic coefficients reflect simulated differences in cultivar growth and development. These are not readily available for most South African maize cultivars. They could either be fitted or determined experimentally. This study showed that the determined values of genetic coefficients are valuable starting points during evaluation and calibration, but that more work is needed to close the gap between determined and fitted values.

A historical data set (soil water content, growth, phenology and yield) consisting of six cultivars and three planting dates was used to evaluate the CERES3 crop growth model. Linear regression and correlation matrix were used to identify algorithms in the model in need of calibration. Results indicated that the model simulates yield and kernel number with low accuracy under local conditions. The number of ears per plant and water stress before and during silking were identified as factors that could explain the low accuracy.'n Historiese datastel (grondwaterinhoud, groei, fenologie en opbrengs) bestaande uit ses cultivars en drie plantdatums is gebruik om die CERES3 gewasgroeimodel te evalueer. Met behulp van linere regressie en korrelasie matriks is algoritmes in die model ge'identifiseer wat gekalibreer moet word. Die resultate toon aan dat die model graan opbrengs en pitmassa met lae akkuraatheid simuleer. Aantal koppe per plant en water stremming voor en gedurende blom is as faktore geTdentifiseer wat die lae akkuraatheid vir beide pit aantal en opbrengs simulasie verklaar.

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Evaluation and calibration of CERES-Maize, 2. Phenology prediction values. Midsummer drought during flowering has a drastic effect on maize yield. By manipulating planting dates this effect can be minimized. Growth simulation models can be used to optimize the planting date of a cultivar. The phenological prediction value of CERES-Maize was evaluated using PAN6363, PAN473 and A1849W. Cultivars were planted, weekly, over a period of five months and detailed phenological measurements were carried out. PAN473 was used to calibrate the model while the other cultivars were used to verify that the adjustments were not cultivar-specific. Emergence, leaf initiation and leaf development rate were calibrated in CERES-Maize to improve the phenological predictability for the western Transvaal. As a result of modifications the systematic error was reduced by four days for emergence, five leaves for leaf initiation and two days for flowering. Midsomerdroogte tydens blom het 'n vernietigende effek op mielie-opbrengs. Deur die plantdatum te manipuleer kan die effek van midsomerdroogte op oesopbrengs verminder word. Gewasgroeisimulasiemodelle kan gebruik word om die optimum plantdatum per cultivar te bepaa/. CERES-Maize se fenologiese voorspellingswaarde is geevalueer met behulp van PAN 6363, PAN473 en A 1849W. Die cultivars is oor 'n tydperk van vyf maande weekliks aangeplant met volledige fenologiese monitering. PAN473 is gebruik vir die model modifikasies, terwyl die ander cultivars gebruik is om te verifieer dat die veranderings nie cultivar spesifiek is nie. Opkoms-, blaarinisiasie-en blaar-verskyningsfunksies is gekalibreer in CERES-Maize, om die model se fenologiese voorspellingswaarde vir die Wes-Transvaal te verbeter. Die veranderings het die sistemiese fout by opkoms verlaag met vier dae, blaarinisiasie met vyf blare en blom met twee dae.