Maize

Distribution and Use

The center of origin for maize lies in Central America. It is grown widely throughout the world including both tropical and temperate countries, with the largest total production estimated at more than 604 million tons (table 2.1). Industrialized countries generally grow maize for livestock feed. Among the top five maize producers, significant amounts of maize are consumed directly by humans only in Mexico. In the developing countries of Africa and Asia, maize is mostly consumed directly by humans.

Drought and Water Use Efficiency

Maize has a C4 photosynthetic mechanism, which gives it high potential productivity with record yields near 20 Mg/ha (Rhoads and Bennett, 1990). Estimates of seasonal WUE for maize range from 0.012 Mg ha-1 mm-1 in a relatively arid environment (Musick and Dusek, 1980) to 0.030 Mg ha-1 mm-1 in a humid environment (Hook, 1985).

With an intermediate level of drought tolerance as compared with other cereal crops, the majority of global maize production is under rain-fed conditions. In contrast, because maize has a high water requirement during tasseling and silking phases, drought during these phases may significantly reduce productivity. Thus, maize crops are often irrigated in areas that have frequent rainfall deficit during the early reproductive phase (Rhoads and Bennett, 1990).

Rather than applying water to maintain soil moisture at a high level, some farmers allow soils to dry appreciably before irrigation and then do not replenish the full soil reservoir, a practice called deficit irrigation. If properly maintained, a deficit irrigation schedule may improve irrigation WUE of maize. For example, Steele (1994) found that irrigation after soil

Table 2.1 Global maize production and

area harvested

Country

Area (%)

Production (%)

United States

2G.3

39.6

China

17.G

18.3

Brazil

9.G

6.9

Mexico

5.3

3.1

Argentina

2.G

2.5

France

1.4

2.7

Others

45.G

26.9

World total

138 Mha

604 Tg

matric potential at 25 cm depth reached -0.1 MPa produced the greatest irrigation WUE in North Dakota, United States. Because the WUE depends on the water-holding capacity of the rooted soil layer, irrigation schedules must be optimized for each location.

Optimum WUE also depends on crop nutrient status. Sylvia et al. (1993) also found that they could improve maize grain yield at an average of 0.8 Mg/ha across a range of irrigation levels though inoculation with my-corhizae (Glomus etunicatum Becker and Gerdemann), a soil fungus that functions to increase the absorptive area of roots.

Mycotoxins

For maize, it is especially important to recognize that drought reduces crop quality as well as productivity. Under drought, fungi often attack maize ears and may produce mycotoxins, especially aflatoxins and fumonisins, which are among the most carcinogenic natural substances known. My-cotoxins have serious health implications for humans and livestock. They may cause liver cancer, thyroid cancer, reduced immune system function, slowed growth of livestock, and exacerbation of other illnesses (Galvano et al., 2002). Wherever and whenever maize crops are exposed to drought during grain filling, a special effort should be made to prevent consumption of these mycotoxins.

Crop Management

In western Kansas, United States, farmers grow rain-fed sorghum but generally do not grow maize because they believe it is insufficiently drought and heat tolerant for rain-fed production. In a five-year field experiment, Norwood and Currie (1997) found that no-till cultivation increased maize yields by 28% and net returns by 69% compared with conventional tillage. In contrast, no-till cultivation increased sorghum yields by only 11% and did not increase net return. Moreover, no-till maize yielded 28% more grain with 169% greater net return than no-till sorghum over the course of the experiment. The challenge is whether farmers are willing to accept low yields during dry years for the greater potential yields of maize during the wet years.

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