Introduction

Maize or corn (Zea mays L.) and sorghum (Sorghum bicolor (L.) Moench.) are, respectively, the third and fourth most important food crops globally in terms of sources of energy and protein in human nutrition; only wheat (Triticum aestivum L.) and rice (Oryza sativa L.) are more important. Among all uses of grains, maize and sorghum rank third and fifth in terms of global production, with wheat first, rice second, and barley (Hordeum vulgare L.) fourth. Maize and sorghum are the only major food crops that have C4 photosynthesis (Table 6.1). Over the past 40 years the total area sown to sorghum has changed little, while that sown to maize has increased by c. 40% (Fig. 6.1a). In the same period, average grain yields have risen linearly from less than 2 t ha-1 to 4 t ha-1 for maize. By contrast, average sorghum yields showed a modest increase, from 1 tha-1 in 1960 to about 1.4 t ha-1 in 1980, and no increase over the last 20 years (Fig. 6.1b). Globally, the lower yields for sorghum may reflect that the crop is commonly grown in semi-arid areas where maize cannot be grown, particularly in the regions bordering the Sahara (Purseglove, 1972; Maiti, 1996). The USA and Canada account for more of the global maize production than any other area, while sub-Saharan Africa accounts for the largest proportion of sorghum production (Fig. 6.2). Because yields vary tremendously among regions, total production is a poor indication of the area sown to the crop in a region. For example, 30 Mha were sown to maize in the USA and Canada in 1998 compared with a slightly smaller 24 Mha in sub-Saharan Africa (FAO, 1998). However, the total production in the USA and Canada was more than eight times that of sub-Saharan Africa (Fig. 6.2).

┬ęCAB International 2000. Climate Change and Global Crop Productivity

Table 6.1. Total global production of indicated C3 and C4 crops and the areas that they occupied in 1997 (FAO, 1998).

Crop Area (106 km2) Total production (Mt) Average yield (t ha-1)

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