the Symposium on Wheats for More Tropical Environments - cimmyt

260Agronomic Management Issues forWheat Production in More TropicalEnvironments of Sou<strong>the</strong>ast AsiaD.A. Saunders, Wheat Program, CIMMYT, Bangkok, ThailandAbstractWheat is not intended as a competitor to established crops in Sou<strong>the</strong>ast Asia;ra<strong>the</strong>r, due to its relative water efficiency, it is expected to produce crops wherenone are presently grown. Where some irrigation is available. managementmust be directed towards avoidance of waterlogging. particularly duringestablishment. Under rainfed conditions. <strong>the</strong> correct timing ofsowing and <strong>the</strong>conservation ofsoil moisture are keyfactors. Strategies can involve reducedcultivation depth. reduced or zero tillage and straw mulching. Weed populationsare high in both irrigated and rainfed areas. necessitating excessive labor inputsfor control by hand weeding: chemicals to control major weed species have beenidentified. Response to nitrogenfertilizer is normally obtained. but responses tophosphorus have not been consistent.Wheat has been cultivated on restrictedareas. mainly at high altitudes. inSou<strong>the</strong>ast Asia for many years; it isnow proposed that it might be grown atlower elevations where a larger landpotential exists. This interest has beenstimulated by <strong>the</strong> increasingconsumption of wheat products(approximately 12% per annum)throughout <strong>the</strong> region.There are basically two situationswhere wheat may be fitted into existingfarming systems without competingwith o<strong>the</strong>r well-adapted crops. The firstis in areas where dry-season irrigationis not aVailable, and <strong>the</strong> crop would begrown on residual moisture plus anyrainfall during <strong>the</strong> crop cycle. Thisencompasses rainfed paddy. after rice,and upland areas. usually follOWingmaize. The o<strong>the</strong>r situation is in lowlandsoils. follOWing rice. where <strong>the</strong> quantityor availability of irrigation water cannotsustain ano<strong>the</strong>r rice crop. Presentmanagement aims to maximize <strong>the</strong>yield of relatively unadapted varietieswithin <strong>the</strong> climatic and croppingsystemsrestraints.Soil and Water ManagementPreliminary estimates of crop-water usesuggest that a successful rainfed cropmay be grown on about 350 mm ofwater in nor<strong>the</strong>rn Thailand. and that <strong>the</strong>greatest chance of success is where <strong>the</strong>upper 1.5 meters of soU has an availablewater content (AWe) of approximately250 mm (14). This recommendationwould be affected by <strong>the</strong> expectation ofrainfall dUring <strong>the</strong> crop cycle and <strong>the</strong>influence of a shallow water table (22).The objective in <strong>the</strong> rainfed situation isto commence <strong>the</strong> crop in a full soilprofile. while avoiding periods ofprolonged waterlogging. On upland soilsthis presents few difficulties; <strong>the</strong> farmerhas sufficient time to prepare <strong>the</strong>seedbed (due to early harvest of <strong>the</strong> priorcrop in <strong>the</strong> rotation) and merely seedsinto a full profile when <strong>the</strong> probability ofcontinuous, heavy rainfall hasdecreased.On rainfed paddy. wheat seeding wouldoften be delayed beyond <strong>the</strong> optimaltime (in relation to rainfall probabilities).due to <strong>the</strong> utilization of long growingseason rice varieties (8); cropestablishment systems should be