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3. Maize Agro-ecosystems in Asia3.1 Maize Production EnvironmentsMaize (Zea mays L.) is a versatile crop that adaptseasily to a wide range of production environments.Maize grows at latitudes ranging from the equator toslightly above 50 o North and South, from sea level toover 3,000 meters above sea level (masl), under heavyrainfall and in semi-arid conditions, in temperate andtropical climates. The maize growing cycle can rangefrom three months to more than a year (Dowswell etal. 1996). Maximum grain yields have been recordedin locations where temperatures reach 30-32 o C duringthe day and drop to 11-18 o C at night, but the crop cantolerate wide deviations from this ideal temperaturerange (Dayanand 1998).There is no universally recognized system forclassifying maize production environments. CIMMYT,which holds a global mandate for maize improvementin developing countries, has developed a classificationsystem based on the concept of mega-environments(CIMMYT Maize Program 1988a, Hartkamp et al.2000). 6 CIMMYT maize breeders commonly distinguishfour major mega-environments: the lowland tropics(or tropical lowlands), tropical highlands, subtropics/mid-altitude zones, and temperate zones (Table 3.1).These classifications do not correspond precisely togeographic definitions of the tropics, subtropics, andtemperate regions, but rather are based on agroclimaticcriteria that include minimum and maximum meantemperatures during the growing season, elevation,and, to a lesser extent, latitude (Dowswell et al. 1996).Steps are being taken to refine this classification toalso consider, among other factors, adaptability ofCIMMYT germplasm and incidence of maize insectpests and diseases.Across all developing countries and in Asia, maizeis grown mainly in tropical lowland and temperateenvironments. However, while tropical lowlandproduction environments are found in all theworld’s regions and in all Asian countries, temperateproduction environments are found mostly in EastAsia (87%), particularly in northern China (Table 3.1;CIMMYT 1988b as cited in Dowswell et al. 1996; Vasal1998). Within a given country, maize production maybe concentrated within a single mega-environmentTable 3.1. Major maize mega-environments.Tropical lowlands Tropical highlands Subtropics/mid-altitude zones Temperate zonesGeneral characteristicsLatitude 0-25º North and South 0-25º North and South 26º-36º North and South > 36º North and SouthElevation (masl) < 1,000 > 1,800 1,000 – 1,500 All elevationsDay length (hours) 11 – 12.5 11 – 12.5 12.5 – 13.4 > 13.4Mean temperature (ºC) > 24 < 18 18 – 24Description Largely high humidity, Equatorial highlands, Typically less than Highest latitude regionsrainfed systems. Includes typically over 1,800 masl. Usually where maize productionsome winter season 2,000 masl. rainfed but with large is possible.regions at higher latitudes.variation in rainfall.% of total maize area, all developing countries in Asia, late 1990sIncluding China 35.4 2.5 11.2 50.8Excluding China 53.6 3.8 16.9 25.6Source: Vasal (1998); CIMMYT (1989); Hartkamp et al. (2000).6A mega-environment (ME) is a broad, not necessarily contiguous production area, occurring in more than one country and frequently trans-continental,delineated by certain ecological conditions (e.g., temperature, rainfall, soils), crop characteristics (e.g., maturity cycle, grain color, grain texture), biotic andabiotic constraints, and socio-economic factors (e.g., production systems, cropping patterns, consumer preferences). MEs are also defined as the largestsubunits of a crop’s growing or target environment within which a particular variety or related practice is useful. Hartkamp et al. (2000) present a GISbasedapproach by CIMMYT to defining mega-environments for maize research.13