Tuesday, 11 July 2006 - 10:15 AM

Dryland Agriculture Challenges and Opportunities.

B.A. Stewart, West Texas A&M University, Dryland Agriculture Institute, WTAMU Box 60278, Canyon, TX 79016

Dryland agriculture is a commonly used term although there is not a common definition. To many, dryland agriculture and rainfed agriculture are synonymous while others draw clear distinctions between them. Dryland agriculture in this presentation is considered with all phases of land use under semiarid conditions except where irrigation is practiced. Therefore, livestock systems, crop-livestock systems, and crop systems are all important components of dryland agriculture. There are also different systems for defining semiarid areas but the aridity index where a semiarid area is one where the ratio of annual precipitation to annual potential evapotranspiration is > 0.20 but < 0.50 is used here. Dryland cropping is the growing of cultivated crops in dryland areas relying entirely on precipitation. However, harvesting water from part of the land and concentrating it on another part of the land is still considered dryland cropping. The first and most important decision for producers in dryland regions is to decide whether or not land should be used for growing crops. Clearly, many lands in semiarid regions have been utilized for cropping that resulted in serious land degradation. Although these lands were usually not highly productive in their native state, they were sustainable with proper management. In many cases, cropping of these lands could not be sustained because of insufficient and highly variable precipitation that resulted in a rapid decline of soil organic matter and increasing wind and water erosion. In some cases, when cropping was terminated and these lands were returned to grazing lands, they were less productive than prior to cultivation. Water management is the most important factor for dryland cropping. Every growing season is vastly different from the previous one in terms of growing season precipitation and temperatures, but severe water stress is common in all years. Yields are highly variable and water supply usually limits yields to <40% of full (water-unlimited) potential with grain yields commonly ranging from zero to three times the average. The most important opportunity for improving water management in dryland cropping systems is increasing the use of crop residues as surface mulch, but this is also the most serious constraint. It is a constraint not only because there is not sufficient residue produced in dry years, but in many developing countries, crop residues are commonly removed from the land and used for animal feed or household fuel. In many developing countries, the rapid increase in population is closely linked to soil degradation and this is particularly true in dryland regions. The increased soil degradation, most notably the loss of soil organic matter, is also linked to an increased carbon dioxide concentration in the atmosphere that is generally recognized as a factor of climate change and global warming. The loss of soil organic matter makes the severe lack of water for cropping even more limited because soil quality characteristics such as soil structure, infiltration rate, and plant available water capacity are all negatively impacted. Maintaining or increasing the soil organic matter levels in dryland soils is critical. Some scientists, particularly modelers, suggest that soil carbon can be restored to precultivation levels, and in certain circumstances to above them. However, long-term studies in Oklahoma and Oregon showed that soil carbon in cultivated soils were increased only by additions of manure. A drastic reduction in tillage is an essential part of any strategy for increasing soil organic matter in dryland regions. No-tillage systems require added inputs, particularly herbicides and insecticides, and these are often too expensive to use in developing countries. The possibility exists that concerns over global warming will result in incentives for producers to adopt practices that lead to increased amounts of carbon sequestration. Such incentives might not be applied to dryland regions because the expected benefits would not be sufficiently large enough. However, it may be advantageous for the world community to invest in carbon sequestration in dryland regions. Dryland regions are home to large numbers of poor people and they are custodians of globally important environmental resources. Degradation of these lands will have a negative impact on the environment and the agricultural and biodiversity of ecosystems. An increase in soil carbon storage could increase soil fertility, land productivity for food production and security, and prevention of land degradation.

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