Global Population and Food Demand: Global population was 1 billion in 1800 a.d. and it took a whole century and 30 years to double itself by 1930 a.d. However, it took only 30 years to add another billion to reach 3 billion in 1960 a.d. and again in the next 39 years it doubled itself to 6 billion in 1999. It is predicted that it will continue to increase and will double itself by 2100 a.d. to reach 12 billion (TOI, 2001). Most of this increase in population has been and will be in lesser developed countries in Asia, Africa, and South America, with Asia contributing the most. Some short-term projections are available from the World Bank and, according to their 1994-1995 population projections, the world population will increase from 5.7 billion in 1995 to 7 billion in 2020; the increase in China is likely to be from 1.2 to 1.5 billion, in south Asia from 1.3 to 1.9 billion, and in Africa from 0.7 to 1.3 billion (IFA, 1978). Obviously, this large increase in the world population will result in increased demand for food. According to the International Food Policy Research Institute (Pinstrup-Anderson et al., 1997), between 1993 and 2020 a.d. the global demand for cereals is expected to increase by 41%. It has been projected (IRRI, 1998) that annual rice production must increase from 556 million tons in 2000 a.d. to 758 million tons by 2020 a.d., a 36% increase (1.8% year-1). Major rice-growing and rice-eating nations in south and southeast Asia must achieve a higher production growth rate. In addition to direct human consumption, the developing countries' demand for cereals for feeding livestock is expected to double during 1993-2000 a.d. due to increased demand for meat and other animal products, such as milk, butter, and cheese. Some of the factors contributing to the increased demand for animal products are economic growth, rising income, and urbanization. For example, China's per capita annual consumption of grains, meat, and edible vegetable oil was only 97.4, 4, and 1.7 kg, respectively, in 1949 and increased to 377, 42.8, and 21.2 kg, respectively, in 1998 (Jiaguo, 2000). Table I shows that the demand for meat, eggs, milk. and fish in India will almost double by 2020 a.d. from the present (2000). The situation is further complicated by the fact that the increase in the production of cereals and other foods has to be made from the same or even lesser land due to an increased demand for land for housing, industry, railways, roadways, and so on; the pressure of this will be more in populous and predominantly rice-wheat cropping system (RCWS) countries such as China, India, Pakistan, and Bangladesh. Trends in per capita available arable land are shown in Table II. During the period 1961-2000 the per capita arable land in China declined from 0.159 to 0.077 ha and is predicted to decline further to 0.060 ha by 2020; thus it will be only 37.7% of that in 1961. Similarly, in India and Pakistan available arable land per capita by 2020 will be 23 and 21% of that in 1961, respectively. Other RWCS countries are not better off. Rice-Wheat Cropping Systems: RWCS is a long-established grain production system in China; it was reported during the Tang dynasty (617-907 a.d.) and was widely adopted during the Song dynasty (960-1279 a.d.) and spread throughout the Yangtze River Valley in the Ming and Quig dynasties (1368-1911 a.d.) (Lianzheng and Yixian, 1994). However, the wheat yield following rice was only 0.7 to 1.0 tons ha-1 until the 1940s and it increased progressively after the 1950s as a result of improved varieties, better agronomic management, and pest control. Thus, in the Jiangou province, the average yield of wheat after rice was 1.6 tons ha-1 in 1970, 3.3 tons ha-1,in 1980, and 4.0 tons ha-1 in 1988 (Lianzheng and Yixian, 1994). The average wheat yield after rice in the Sichuan province in 1997 was 3.76 tons ha-1, with the highest recorded as 6 tons ha-1 (Jiaguo, 2000). RWCS in the Indian subcontinent is quite new and started only in the late 1960s with the introduction of dwarf wheat from CIMMYT, Mexico, which required a lower temperature (mean below 23 °C) for good germination than that required for traditional tall Indian wheat. Thus, wheat sowings were shifted from mid-October to mid-November, providing a full extra month for the preceding rainy season crop. This provided enough time for rice to mature; high-yielding varieties (HYV) of which such as IR-8 were already available. This set in the RWCS in the Indo-Gangetic plains (IGP) of the Indian subcontinent and the northwestern states of India [Punjab, Haryana, western Uttar Pradesh (UP)] and the Punjab and Sind province of Pakistan, which were traditionally wheat regions, were transformed into rice-wheat regions. The reverse of this happened in Bihar and West Bengal states of India and parts of Bangladesh, which changed from traditional rice regions to rice-wheat regions. In RWCS two to three crops are grown during a span of 12 months or a crop year (July-June), as it is termed in India. In RWCS belt Indian subcontinent rice is grown during rainy season (July to November) when 700-1000 mm rainfall is received, while wheat is grown during the winter season (November to May) on stored soil moisture with supportive irrigation. In China, almost the same months are occupied by rice and wheat, although the rainfall pattern differs and, in some parts, quite a bit of rain is received during the wheat-growing season. Many farmers in India take a third crop of potato or toria in between rice and wheat or rice/mungbean/cowpea/green manure (GM)/sunflower in between wheat and rice. Some of the rice-wheat cropping systems are listed. 1Rice (Oryza sativa L.)-wheat (Triticum aestivum)2Rice-potato (Solanum tuberosum)-wheat3Rice-toria (Brassica campestris)-wheat4Rice-wheat-mungbean (Vigna radiata)5Rice-wheat-cowpea (Vigna unguiculata)6Rice-wheat-green manure (Sesbania spp., Crotolaria spp.)7Rice-potato-wheat-green manure8Rice-wheat-sunflower (Helianthus annuus)9Rice-wheat-rice10Rice-vegetable peas (Pisum sativum)-wheat11Rice-vegetable peas-wheat-green manure12Rice-wheat-maize (Zea mays). There could be many more variants involving vegetables and other short-duration crops. Most rice in RWCS is transplanted and rice varieties grown are of 90-140 days duration (seed to seed) of which 25-45 days may be spent in nursery; more aged seedlings (50 to 60 days old) are transplanted in some parts of China (Gupta et al., 2000). Wheat in the cropping system takes 120 to 160 days; its maturity is determined by temperatures above 35 °C, bright sunshine, and high wind velocity. Thus sown in mid-November (optimum for India), wheat matures by the end of March in eastern India, by the end of April in western Uttar Pradesh and Haryana, and by the first fortnight of May in Punjab and Himachal Pradesh. In China, wheat matures in June/July and thus receives quite a bit of rainfall asking for adequate drainage. It also delays rice transplanting, which is why the tradition of using older rice seedlings is used in some parts of China. A calender of RWCS in China for different regions is given in Fig. 1. The estimates of area under RWCS in the world vary considerably. Paroda et al. (1994) reported 22.4 million ha (m.ha), whereas Ladha et al. (2000) reported 24 m.ha under RWCS. However, adding up the estimates available for different countries, it totals 28.8 m.ha: 13 m.ha in China (Jiaguo, 2000), 12.3 m.ha in India (Kumar et al., 1998), 2.2 m.ha in Pakistan, 0.5 m.ha in Nepal (Paroda et al. 1994), and 0.8 m.ha in Bangladesh (Ladha et al., 2000). These 5 RWCS countries are not just any 5 of the more than 200 countries of the world, they represent 43% of the world's population on 20% of the world's arable land (Singh and Paroda, 1994). Also, more than half of the world's malnourished people are in these countries. In the Indian subcontinent, RWCS is predominant in the Indo-Gangetic Plains (Fig. 2) (Woodhead et al., 1994), although there are pockets of this cropping system in several other states of India. The IGP are spread from 67° to 96°E longitude and from 20° to 33°E latitude (Schwartzberg, 1978). It extends from Assam and the Bay of Bengal on the east to the Afghan border and Arabian sea in the west and covers India, Bangladesh, and Pakistan. It has Himalayas in the north and minor hills or plateau in the south and covers about 2400 km from east to west and about 160 km wide in the east and 500 km in the west. In China, RWCS is predominantly in the Yangtze River Valley (Fig. 3). In India, as well as in China, areas under RWCS have spread over time. Data on the spread of the area under RWCS in India in 1983 and a decade later in 1993 under RWCS are given in Table III. In the states of Punjab, Haryana, and UP, which are predominantly wheat-growing states, almost all rice is under RWCS. However, in West Bengal, which is a predominantly rice-growing state, only 4% of the area under rice is under RWCS. The reverse is true when the area under RWCS is expressed as the percentage of total wheat area in these states.
