the Symposium on Wheats for More Tropical Environments - cimmyt

287can be used. The computer is a usefultool to reduce <strong>the</strong> time needed forcalculations, but simulations can alsobe performed with a desk calculator(18,20).At Production Levell. irradiation andtemperature are two factors ofimportance for dry-matter production ofa wheat crop during development fromemergence to maturity. Biotic andabiotic factors o<strong>the</strong>r than globalradiation and temperatures areassumed to be nonlimiting. Simplesimulation models and calculationmethods can define <strong>the</strong> phenologicalstate of <strong>the</strong> crop, making use of heatsums expressed in degree days. Thisnot only applies to Production Levell,but can also be applied to levels 2 and 3(17,18,19,20,21). Rate of plantdevelopment is governed by geneticproperties and environmentalconditions. Genetype accounts for <strong>the</strong>distinction in short, medium and longmaturationtypes (Table 1), whileenvironmental factors cause variationsin growth duration for one genotypebetween locations and seasons.Presently, <strong>the</strong> effects of day length aredifficult to describe quantitatively. Theheat sum for <strong>the</strong> period from an<strong>the</strong>sisto maturity appears to be constant forwheat genotypes (22).Potential yields (root, shoot and grain)can be calculated for crops sown atdifferent dates, in different years and atdifferent locations, as was demonstratedwith a model developed for Zambia (21).Thus, simple simulation models canhelp, among o<strong>the</strong>r things, to eliminateareas unsuitable for production, toidentify potentially suitable areas forresearch and production and to plan forSOWing-date experiments at one or morelocations. The influence of <strong>the</strong> durationof <strong>the</strong> vegetative period (to an<strong>the</strong>sis of<strong>the</strong> plant) on calculated yields for wheatsown at a given location at differentdates can be evaluated (21). This mayhelp in deciding whe<strong>the</strong>r to use short,medium or long-maturation cultivars.Using <strong>the</strong> heat sum, <strong>the</strong> maturation ofvarious phenological development stagesof wheat, as well of rice, maize andTable 1. Heat sum in degree days (oCd) above a base temperature of OOC forwheat and riceGenotypeDegree days (oCd)Emergence An<strong>the</strong>sisto an<strong>the</strong>sis to maturity TotalWheatShort maturation~./Emergence January 1Emergence July 1Medium maturatioF~/Long maturatio#82095011001175-13509009009009001720185020002075-2250Ric~./Short maturationMedium maturation~a503400Sources:!.f Van Keulen and De Milliano (21)~/ Van Keulen and Seligman (22)£. VanKeulen (18)