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Progress in Breeding for Low Nitrogenand Drought Stress in Thailandy. Chantaclllllne 1 ,", P. Grudloyma", S, Norradachanon 2 and T. Manupeerapall JI Corresponding author" Mai;.e breeder. Nakhon Sawan Field Crops Research Center (NSW FCRC).Takfa, Nakhon Sa>van, Thailand 60190Tel.' 66-56-241019. Fax: 66-56-241498. E-mail: yodsapet@asianet.co,thJ Mai;;e breeder. Field Crops Research Institute, Bangkok. Thailand 10900AbstractThe project of maize breeding for tolerance to low N was established at NSW FCRC in 1995with the objectives of developing inbred lines, hybrids and populations tolerant to low N. A block ofland of approximately 0.7 ha was used as a testing block for low N tolerance. Before using the block.two successive crops of maize were planted with no fertilizer in 1995 early rainy season (ER) and1995 late rainy season (LR) to create the low N condition. The evaluation for tolerance to low N wasstarted in 1996ER. The breeding scheme used was screening of inbred lines and early generation linesfrom NSW FCRC hybrid maize development project and the selected lines recombined into apopulation (NSLNIC) which will be used as a source population for tolerance to low N. Theseselected lines were also crossed with two elite inbred lines Nei 9202 and Nei9008 and then thetopcross hybrids were evaluated under low N condition. In conclusion, twenty three topcross hybridswere selected based on tolerance to low N and will be tested in various environments in 1998 ER.Breeding for drought stress tolerance was initiated in 1977. The objectives of this project are todevelop inbred lines, hybrids and populations for tolerance to drought stress. Two populations namelyKK-DR(S)C5 and NS-DR(S2)C I were developed using S1 and S2 recurrent selection, respectively.The 5 I and 52 progenies were evaluated both under artificial stress and normal water conditionsduring cycles of the population improvement and the tolerant progenies were selected based on adrought index plus some other important agronomic traits under stress condition such as anthesis andsilking interval, number of ears per plant, wilt score and shelling percentage, etc. Three syntheticvarieties were developed from each of the two populations. Commercial hybrids and inbred linesevaluations for drought stress tolerance were also conducted. There were approximately 150 topcrosshybrids developed from the selected tolerant lines crossing with 2 elite inbred lines Nei 9202 and Nei9008 and these topcross hybrids will be evaluated for tolerance to drought during the 1998 dry season.There were at least 2-3 sets of drought and low N trials received from CIMMYT each year forevaluation of the stress tolerance of inbred lines and hybrid maize.IntroductionDrought and low N are two of the major limiting factors of maize production in Asia. Reportfrom the 6 1h Asian Regional Maize Workshop in India in 1995 indicated that about half of the totalmaize production area in Asia has been affected by both stresses. The estimated losses of yield due todrought and low N stress were in the range of 10-75% and 10-50%, respectively (Logrono andLothrop, 1996). In Thailand, during 1975 to 1994, the estimated ,affected planting area and yield lossdue to the two stress conditions were approximately 0.1-0.2 million hectares and 0.2-0.3 million metrictons per year, respectively, which is equivalent to 30-50 million U5$ per year (Office of AgriculturalEconomics, 1985-1995).Genetic variation in response to N supply of maize population (Lafitte and Edmeades, 1994) andinbred lines (Balko and Russell, 1980) was observed, and it seem, therefore. possible to breed varietiesor hybrids with tolerance to low N. Gain in grain yield under drought from four recurrent selectionstudies in lowland tropical maize have averaged about 100 kg/ha/year or about 5% per year (Edmeadeset a!., 1995). Development of maize germplasm tolerant to both stresses should give a long term