Wheat

Distribution

At more than 575 million tons, wheat has the third largest global production of any food crop. The center of origin for wheat is in the Middle East between the Tigris and Euphrates rivers. As a cereal with C3 photosynthesis metabolism, wheat productivity generally declines as temperatures increase above 30°C (Abrol and Ingram, 1996). Thus, wheat is predominantly produced in the temperate regions of the world (table 2.3). Thirty countries produce 90% of all wheat produced globally. A small portion of the wheat production lies in subtropical zones during winter.

Wheat has a dual role in the food system. It provides human nutrition both by direct consumption and through feeding to livestock. Although high temperatures restrict the productivity of wheat in the tropics, the importance of wheat for direct human consumption increases with national development as people develop a taste for breads and pastas. About half of the total world wheat production is traded internationally, with a large portion of that trade occurring from wheat produced in the industrialized nations and exported to developing countries.

Drought and WUE

Although the evapotranspiration WUE of wheat is only 0.010-0.017 Mg ha-1 mm-1 (Zhang et al., 1999), the wheat root system is generally very deep and the crop has a relatively high level of drought tolerance (Musick and Porter, 1990). As is the case for most cereals, wheat is most susceptible to yield reduction when stress occurs during the heading phase. In the North China Plain, wheat was most sensitive to water deficit from stem elongation to heading and from heading to milk dough phases (Zhang et al., 1999). Irrigation two to four times in North China increased evapotranspiration WUE from 0.010 to 0.012 Mg ha-1 mm-1.

There is a wide range of drought tolerance among wheat varieties. Moustafa et al. (1996) found that a 10-day water deficit during heading reduced yield from 0 to 44% among four wheat cultivars. Although the physiological drought adaptations of wheat include osmotic adjustment, desiccation tolerance, and cellular elasticity, the principal contributor to drought resistance appears to be its deep root system (Musick and Porter, 1990). In Mexico, Calhoun et al. (1994) found that selecting wheat

Table 2.3 Global wheat production and area harvested

Country

Area (%)

Production (%)

India

11.7

11.9

China

11.5

16.4

Russian Federation

9.9

7.4

United States

9.3

9.3

Australia

5.2

3.5

Canada

5.2

3.6

Turkey

4.1

2.8

Pakistan

3.8

3.3

Argentina

3.2

3.1

Others

36.1

38.8

World total

212 Mha

576 Tg

germplasm for high yield under full irrigation would lead to increased yield under drought conditions.

Irrigation

Though most wheat is produced under rain-fed conditions, the area of irrigated wheat is increasing. In the subtropical wheat production areas of South Asia, wheat is often grown during the cool season in rotation with irrigated lowland rice and may receive several irrigations during the season. Where declining water quality or water abundance restricts irrigation, some irrigated wheat production areas in the Great Plains of the United States are being converted to rain-fed production, although it is most economical to combine crop rotations (wheat-sorghum-fallow) with limited amounts of irrigation rather than to convert entirely to rain-fed production (Norwood, 1995). Irrigation generally increases wheat yield, but it may also lead to diseases. For example, in Denmark, Olesen et al. (2000) found that irrigation increased wheat yield, but it also increased the incidence of mildew disease.

Where wheat is irrigated, farmers generally apply irrigation amounts below levels of potential evapotranspiration. Schneider and Howell (1997) reported that deficit irrigation at a level of 33 or 66% replacement of the water lost by ET generally increased grain yield per unit irrigation more than did a late start or early termination of irrigation. Stegman and Soderlund (1992) used infrared thermometry to estimate a crop water satisfaction index (CWSI) for spring wheat. They found minimal yield reductions for CWSI less than 0.4-0.5 and maintaining available soil moisture above 50% of total root zone moisture availability. Garrot et al. (1994) showed that a CWSI could be used to schedule irrigation of durum wheat for increased WUE in an arid zone and recommended irrigation to maintain CWSI values less than 0.3-0.37. The relationship between yield and ET changes from year to year, and the CWSI for wheat must be calibrated separately for each variety and location (Stegman and Soderlund, 1992).

Water production functions have been developed widely as guides for the efficient use of irrigation water for wheat. In Saudi Arabia, Helweg (1991) modeled the water-yield relationship of wheat, and in Turkey, Ozs-buncuoglu (1998) tested water production functions to analyze the economic value of water. Though both Helweg (1991) and Ozsbuncouglu (1998) were able to develop accurate water production functions, mathematical relationships differed greatly between the two countries. To be useful, water production functions must be tested and calibrated separately for each location of interest.

Crop Management

Rain-fed wheat must be sown early enough to assure that the root system is well developed before there is a significant probability of water deficit. Saunders et al. (1997) found that sowing wheat in autumn under rain-fed conditions produced yields similar to sowing wheat in spring under irrigation in the Canterbury Plains of the United Kingdom. Thus, autumn sowing could promote the efficient use of scarce water resources. Similarly, delay in the sowing date from November to January consistently reduced yields and responses to inputs of supplemental irrigation and nitrogen for wheat cultivated in Syria (Oweis et al., 1998).

Plant nutrition also affects responses to water deficit. Application of fertilizer nitrogen generally increases the productivity of rain-fed wheat, although in some regions supplemental irrigation may be needed for wheat to fully benefit from the applied nitrogen (Frederick and Camberato, 1995). In contrast, it is important not to apply nutrients at levels so high that shoot growth increases more than root growth. In the Coastal Plain of the United States, increase in the amount of nitrogen applied to winter wheat increased the severity of drought stress during the grain-filling phase and reduced yields (Frederick and Camberato, 1995).

In hot and relatively arid environments such as Sudan and Mexico, mulch and irrigation increased wheat yields, but mulch and irrigation did not affect yields in hot and humid environments such as Bangladesh (Badaruddin et al., 1999). Farmyard manure, which both provides nutrients to the crop and increases the water-holding capacity of soils, increased yields in both arid and humid environments (Badaruddin et al., 1999).

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