Precipitation use efficiency as affected by cropping and tillage systems

Water is the driving variable in Great Plains agriculture and sustainability depends on efficient use of incident precipitation. Spring and winter wheat (Triticum aestivum L.)-fallow (SWF and WWF) farming systems, as currently practiced, are not economically sustainable without government subsidies. This paper synthesizes information regarding the water use efficiency (WUE) of intensified cropping systems in cultivated dryland agroecosystems and proposes solutions to ensure sustainablity. Decreasing tillage and maintaining crop residue on the soil is requisite to improved efficiency. No-till fallow efficiency, the percentage of the precipitation stored during fallow, reached 40% in the early 1970s. However, scientists in the 1980s and 1990s still report fallow efficiencies no greater than 40%, indicating that other major system changes must occur if progress is to continue. Residue levels in the Great Plains usually are < 3 tons/acre and this probably has capped fallow efficiency near 40%. No-till management of crop residues after spring or winter wheat harvest increases soil water storage in the first portion of the fallow (July to May) compared with conventional fallow management, but the soil in the late fallow period (June to September for winter wheat and June to May for spring wheat) gains no more water, and may even lose water relative to the quantity present in the spring. Overall system efficiency is best evaluated by calculating grain WUE values. Modern no-till wheat-fallow (WF) systems, even with maximum fallow efficiencies, only had average grain WUE of 104 lb/acre per in. for spring wheat and 140 lb/acre per in. for winter wheat. WUE for 3-yr cropping systems, like winter wheat-corn (Zea mays L.)-fallow or winter wheat-sorghum [Sorghum bicolor (L.) Moench]-fallow, increased WUE in Central and Southern Great Plains. Three year system WUE averaged 180 lb/acre per in., a 28% increase compared with WF. In the Northern Plains, continuous spring wheat systems averaged 122 lb/acre per in., a 15% increase compared with SWF. Individual crops within systems had the following potential WUE values: corn = 245 lb/acre per in., grain sorghum = 225 lb/acre per in., prose millet (Panicum miliaceum L). = 195 lb/acre per in., spring wheat = 216 lb/acre per in., and winter wheat = 150 lb/acre per in. Maximum system efficiency depends on choosing the most efficient plants for a given geographic area. Intensified cropping systems improve our ability to use precipitation efficiently. However, adoption of higher intensity cropping systems depends more on economic outcomes and government programs than on WUE or environmental effects.