the Symposium on Wheats for More Tropical Environments - cimmyt

267Rice varietiesAs mentioned above, early planting ofwheat is essential; it follows that <strong>the</strong>preceding rice crop must be harvestedearly. The range of maturity that existsin rice germplasm now available innor<strong>the</strong>rn Thailand is considerable (1).There are traditional, photosensitivevarieties which seem to respond tophotoperiod changes while <strong>the</strong> days arestill much longer than reqUired toinduce flowering in full-season varieties.These early varieties, which can beginto be harvested in early October, havelong been used in rice-based croppingsystems which require early riceharvest, such as rice-shallot, rice-garlicand rice-tobacco. New IRRI-type RDvarieties which are not sensitive tophotoperiod, e.g., RDl, RD7 and RDIO,offer more alternatives for earliness inrice. Although all of <strong>the</strong>se early ricesare somewhat inferior to full-seasonvarieties, experience from <strong>the</strong> villageprogram of <strong>the</strong> Multiple CroppingProject has shown that farmers arewilling to sustain this loss in <strong>the</strong> ricecrop if it can later be compensated forby <strong>the</strong> following crop.SoliRice-based multiple cropping involVingupland crops is somewhat ofananomaly. During <strong>the</strong> rice season, <strong>the</strong>main objective in soil management is tokeep <strong>the</strong> water in <strong>the</strong> field. This isachieved with puddling, whicheffectively destroys any structure that<strong>the</strong> soil may have; <strong>the</strong> original structuremay be regenerated only in certain soiltypes. For o<strong>the</strong>r soils, management oftillage, water and organic manure iscrucial in <strong>the</strong> modification of soilstructure, especially during <strong>the</strong> earlyestablishment of <strong>the</strong> crop.Unfortunately, <strong>the</strong> majority of rice soilsin nor<strong>the</strong>rn Thailand belong to thisgroup, and <strong>the</strong> management of tillageand water are extremely critical towheat yield. Organic manure is nowextensively used for high-value crops,but will not be <strong>the</strong> solution for wheat.Satisfactory germination andestablishment have been achieved bysowing into moist soil and floodirrigating in subsequent applications. Asuccessful practice is that of sowingPresent SystemsA./R _ic _e,.,7 A./ S_O_y_b_e_an /,/ R_ic_e/JA./ Ri_c_e /Proposed Rice-WheatWheat/Rice7/7Jul Aug Sepi i i I i I iOct Nov Dec Jan Feb Mar Apr May JunMonthFigure 2. The growing season of a rice-wheat cropping system comparedwith <strong>the</strong> ezisting rice-soybean and rice-rice systems. nor<strong>the</strong>rn Thailand