PRECIPITATION USE EFFICIENCY AS AFFECTED BY CROPPING AND TILLAGE SYSTEMS

Water is the driving variable in Great Plains agriculture and sustaina bility depends on efficient use of incident precipitation. Spring and winter wheat (Triticum aestivum L.)-fallow (SWF and WWF) farming syste ms, as currently practiced, are not economically sustainable without g overnment subsidies. This paper synthesizes information regarding the water use efficiency (WUE) of intensified cropping systems in cultivat ed dryland agroecosystems and proposes solutions to ensure sustainabli ty. Decreasing tillage and maintaining crop residue on the soil is req uisite to improved efficiency. No-till fallow efficiency, the percenta ge 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 G reat Plains usually are < 3 tons/acre and this probably has capped fal low efficiency near 40%. No-till management of crop residues after spr ing 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 spri ng. Overall system efficiency is best evaluated by calculating grain W UE 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 w inter wheat-sorghum [Sorghum bicolor (L.) Moench]-fallow, increased WU E 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 wh eat = 150 lb/acre per in. Maximum system efficiency depends on choosin g the most efficient plants for a given geographic area. Intensified c ropping 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 environmenta l effects.