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Drought-Tolerant Crops for Drylands

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The vast majority of the poor in drylands depend on agriculture. And drought is the principal constraint of crop production in these areas. It may be defined as periods in the natural cycle of stress and renewal during which the amount of moisture in the soil no longer meets the needs of a particular crop . Drought occurs frequently in drylands, partly because average rainfall is low, ranging across locations and years from an average of about 300 to 800 millimeters per annum, but also because it may be highly erratic, with torrential storms during the cropping season, followed by long dry spells.

Photo: ICRISAT. Sorghum in Africa.

The regions most vulnerable to drought are located in Sub-Saharan Africa and Central Asia. Over the last four decades, the African continent has suffered seven major drought episodes. In two regions- the Sahel and the Horn of Africa- droughts in 1972-74 and 1981-84 caused massive social disruption and human suffering. And the problem is likely to become worse, according to recent studies, if trends in climate change play out as expected.

Given the severity of drought in dry areas, a central challenge for researchers is to devise technologies that lend greater resilience to agricultural production under this stress. One way in which they have responded successfully to the challenge is by developing varieties of major food crops that are drought tolerant or escape drought through early maturity.

Various staples have withstood harsh dryland conditions for thousands of years. Exploiting the drought-tolerance genes they possess, national and international plant breeding programs have scored important gains in improving some crops for this trait, as described below. Nonetheless, progress has been slow, and much remains to be done.

To speed the progress of breeding for drought tolerance, scientists are actively employing two sets of cutting-edge tools. One is derived from molecular biology and involves the use of molecular markers to better understand the genetic basis of drought tolerance and to select more efficiently for this trait. The other, called participatory plant breeding, offers a more active role to farmers, who are keen observers of plant performance and can contribute importantly to selection for better drought tolerance.

Selected Highlights from Research for Dryland Development

Millet and sorghum, hardy dryland staples: Pearl millet, the most inherently drought-tolerant of all the major staples, together with sorghum, are key cereal grain crops in the drylands, providing food, feed and, in the case of millet, fuel and construction material as well. Despite formidable obstacles to improvement of these crops for drylands, plant breeders at the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) and in national partner organizations have made important gains, and farmers are adopting new varieties. In southern Africa, for example, about 34 percent of the total millet area is now planted to improved materials and 23 percent of the sorghum area.

Early maturing varieties of these crops have proved especially useful for helping dryland communities get through the "hungry season." This is the period before harvest, when the previous year's grain supplies have been exhausted. The millet variety 'Okashana 1', for example, which was selected by farmers in Namibia and matures 4-6 weeks earlier than traditional varieties, spread in just a few years during the mid-1990s to cover half the country's millet area. The US$3 million investment required to develop and disseminate the variety was estimated in 1998 to be yielding annual benefits worth $1.5 million. At about the same time in southern Chad, an improved sorghum variety, which shows a 50 percent yield advantage over local materials, also spread quickly, generating benefits worth almost $4 million annually.

Multipurpose grain legumes: These crops are vital sources of low-cost protein in drylands, and the sale of excess grain generates significant farm income. Grain legumes also help restore soil fertility, since their roots fix nitrogen from the air in forms that can be used by subsequent crops. In addition, the stems and stalks of these crops are valued as livestock feed. Cowpea is the most widely grown grain legume in the dry areas of Africa, while chickpea and pigeonpea predominate in much of the Asian drylands. More than 60 countries have released improved cowpea varieties with support from the International Institute of Tropical Agriculture (IITA). Chickpea and pigeonpea varieties resulting from the work of ICRISAT are having a major impact in India, Nepal, Pakistan and increasingly in China.

Farmer participation in barley improvement: Barley is the world's fourth most important cereal crop, and drought affects its yields severely. During 2000 in Syria, for example, when rainfall dropped 20 to 30 percent below the long-term average, the crop produced little or no grain in some areas. Nonetheless, at four dry experimental sites, a few barley lines were able to produce grain. Based on these materials, a new drought-tolerant barley variety was developed, with the participation of farmers, through a plant breeding program coordinated by the International Center for Research in the Dry Areas (ICARDA). The economic benefit so far of participatory barley improvement in Algeria, Egypt, Ethiopia, Iraq, Jordan, Morocco, Tunisia and Syria was estimated at about US$91 million in 1997.

Maize and molecular biology: Maize is the world's third most important cereal crop (after rice and wheat), and drought is second only to soil infertility as a constraint of maize production in developing countries. The International Maize and Wheat Improvement Center (CIMMYT) has achieved important progress in developing drought-tolerant maize through conventional plant breeding. And the Center has widely tested and disseminated improved materials in drought-prone areas of Africa. CIMMYT scientists believe they can make further gains by using tools from molecular biology. With the aid of a genomic map that combines data for different types of tropical maize in diverse environments, they are identifying genetic "hot spots" in maize, that is, areas of the crop's chromosomes that confer drought tolerance.

This work is critical in light of a recent study that examined the likely impact of climate change on maize yields in Africa and Latin America during the coming decades. The study was conducted jointly by the International Center for Tropical Agriculture (CIAT) and International Livestock Research Institute (ILRI). In parts of southern Africa, the study predicted drastic yield declines, requiring major adjustments in maize-based systems.