All plants require water for growth and reproduction. Crop plants require water in an amount that will allow them to produce an economical yield of grain, forage, or fiber.
Drought is an extended period of dryness that results in stress to crop plants which is manifested in irreversible yield loss. In the Mid-South, moderate to severe drought stress in crops is common during the summer growing season.
Drought resistance is the ability of a plant to survive and reproduce during drought. Survival in the face of drought is enough for a desert cactus or shrub. However, mere survival is not enough for crop plants. They must produce a yield with a value that surpasses the cost to produce that yield.
The water required to produce an economical yield is significantly greater than that required for survival. Hence, the effect of drought stress on crop plants and their ability to cope with it has been, and always will be, a major concern for agriculture.
Relative drought tolerance is the ability of some varieties or hybrids within a particular crop species to suffer less yield loss than other varieties or hybrids within that same species as a result of drought stress. This is a desirable and sought characteristic for Mid-South crops grown in dryland systems.
Soybeans grown in the Mid-South are significantly affected by drought. Current methods employed to reduce the effects of drought are alleviation of stress by irrigation and avoidance of stress by manipulation of production practices, such as earlier planting of early-maturing varieties.
Most soybean breeding programs produce soybean varieties that have been selected and tested under irrigated conditions. This allows breeders to assess the true production potential of a variety without the confounding effect of drought stress. However, this procedure does not allow varieties to be tested for tolerance to drought, which often occurs in Mid-South dryland production environments.
State soybean variety trials offer little help in selecting varieties for drought tolerance. In fact, most of the trials in the Mid-South region are conducted in an irrigated environment (See my Feb, 2 and 16, 2007, articles in Delta Farm Press).
Often, results from nonirrigated trials in the region are not reported because of low yields resulting from drought stress. Therefore, yields that are reported from nonirrigated trials generally do not reflect how varieties perform under true drought conditions.
Also, the amount of drought stress that occurs in nonirrigated trials cannot be categorized accurately, so response of varieties to drought cannot be accurately measured. Consequently, data from nonirrigated trials are of little value for picking soybean varieties more suitable for dryland production.
Some soybean lines show a smaller yield decrease under drought, but they also have a lower yield in the absence of drought. Current information indicates that a drought tolerant variety is by nature a low-yielding variety. In fact, the genes that contribute to a lower sensitivity to drought also lead to low yield potential.
Dryland producers in the Mid-South have to guess about which soybean variety to plant. Do they plant a variety that produces a high yield under irrigation, or do they attempt to select a variety with so-called drought tolerance that may be a low yielder when adequate water is available?
As things now stand, varieties that are high yielders under irrigated conditions are the best choice for both nonirrigated and irrigated production systems in the Mid-South. Thus, reported results from nonirrigated variety trials are of little value and probably should not be used as a tool in selecting soybean varieties for dryland production. Resources currently used to conduct nonirrigated trials should be re-allocated to conduct optimally-managed irrigated trials.
My next article will discuss promising soybean traits that may contribute to higher yields under drought conditions.