Crop Manipulations for Efficent Use of Water in the U.S. Southern Great Plains.

The yields (Y) of grain and fiber crops depend on the amount of water used for evapotranspiration (ET), the portion of ET used as transpiration (T), the units of T required to produce a unit of aboveground biomass (TR), and the harvest index (HI) which is the weight of the harvested product divided by the aboveground biomass of the crop. These factors can be mathematically expressed as Y = ET x T/ET x 1/TR x HI. The ET for dryland crops is limited to growing season rainfall plus plant available water stored in the soil profile. Remarkable progress has been made in recent years to manage crop residues in ways that increase the amount of plant available water stored in the soil at the time of seeding and to increase the T/ET factor. ET is the most important factor because increasing ET generally affects the other factors in a positive manner so yields increase significantly. For example, a doubling of ET can result in a four-fold yield increase which is why irrigation is so productive in dryland areas. Large yield increases occur only when all factors are positively impacted. With limited ET, however, changing a cropping practice can cause factors to move in opposite directions. A common belief is the ideal plant spacing for any crop is where each plant is the same distance from each of its neighbors. While uniformity may be ideal where resources are not limiting, a useful generalization is where a resource like water or fertility is limited, non-uniformity is advantageous. Opportunities for using increasingly available precision agriculture technologies to manipulate plants in ways that limited water and fertilizer resources can be used more efficiently will be discussed.