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., proso millet (Panicum miliuceum 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. Research QuestionWater is the driving variable in Great Plains agriculture and sustainablity depends on efficient use of precipitation. If Great Plains agriculture is to be economically and environmentally sustainable, systems must be developed that maximize water storage efficiency and grain water use efficiency (WUE). The main objective of this paper was to synthesize existing information regarding the WUE of intensified cropping systems in the dryland environment of the Great Plains. Literature SummaryFor more than 80 years research scientists across the Great Plains have experimented with methods of improving water storage efficiency during summer fallow periods with the ultimate goal of increasing ensuing spring and winter wheat yields. It has been clearly demonstrated that reducing tillage and maintaining residue on the soil surface increases the percentage of the precipitation stored as soil water during fallow; this percentage is termed fallow eflciency. From 1916 until 1970, fallow efficiency increased from 19% to 35% as sweep tillage replaced plows and discs. By 1975, scientists had increased fallow efficiency to about 40% by using no-till practices. Wheat yields increased proportionately as fallow efficiency increased. Recent efforts to increase fallow efficiency in wheat-summer fallow systems beyond 40% have not been successful, and in fact wheat yields in no-till fallow systems have not increased much beyond yields obtained with a reduced till system. The expense of storing the water during the summer fallow months with no-till methods has not been recovered in added grain yield. Since 1980, there has been renewed effort to intensify cropping systems to improve overall WUE, a concept that was first explored in the 1960s. The most common of these attempts has been with 3-yr systems like winter wheat-corn-fallow (WW-C-F) or winter wheat-sorghum-fallow (WW-S-F). Study DescriptionWe assembled published data from Montana, North and South Dakota, Nebraska, Kansas, Colorado, and Texas and synthesized the findings of experiments that compared intensified cropping systems with spring wheat-fallow (SW-F) and winter wheat-fallow (WW-F). The literature we reviewed usually reported a WUE for a particular crop in a particular system, but did not evaluate systems as a whole. Using the available data, we calculated a WUE for the entire system, which allowed us to directly compare the 2-yr WW-F with 3-yr systems containing corn or sorghum or with continuous spring wheat. We also calculated WUE for 4-yr systems and even continuous cropping when data were available. Applied QuestionsHave we learned how to increase fallow water storage efficiency in spring and winter wheat-fallow systems by refining our no-till technology? No-till summer fallow efficiencies have not increased since the mid 1970s and it appears they are capped at about 40% because of the limited amount of residue we have available in our SW-F and WW-F systems. How efficient are the 3-yr cropping systems compared with winter wheat-fallow in the Central and Southern Great Plains? The 3-yr systems, like WW-C-F and WW-S-F, produced 20 to 100 lb more grain per inch of water than did WW-F (Fig. 1). Intensified cropping systems produced an average of 55 lb more grain per inch of water than did WW-F. How efficient is continuous spring wheat compared with spring wheat-fallow in the Northern Great Plains? Continuous spring wheat grown in a no-till system produced 18 lb more grain per inch of water than did SW-F. This represents a 17% increase in WUE. How do crop plants differ in their individual water use efficiencies? Corn, sorghum, proso millet, and forage all produced more product per inch of water than did winter wheat (Fig. 2). For example, corn and sorghum produced an average of 85 lb more grain per inch of water than did winter wheat. Spring wheat in the Northern Plains produced about the same amount of grain per inch of water as did corn in the Central Plains. RecommendationProducers currently using either SW-F or WW-F systems should consider adopting intensified systems. In the Central and Southern Plains, WW-C-F and WW-S-F are good alternatives to increase production efficiency. In the Northern Plains, continuous spring wheat or spring wheat-sunflower rotations are good choices. Alternate crop-fallow systems cannot take advantage of the excellent water storage that occurs in the first part of the summer fallow period using properly managed minimum and no-till systems. By planting a spring crop, the producer can effectively use the stored water and avoid the expense of keeping fields weed-free during the long summer fallow period associated with either SW-F or WW-F. Each producer must develop a particular cropping system to fit climatic and marketing situations. A general rule in developing the most profitable cropping system is to choose crops that have the highest potential WUE and that are adapted to a given climatic zone and local markets. Fig. 1Water use efficiency (WUE) comparison of winter wheat-summer fallow (WW-F) and intensified cropping systems at various Central and Southern Great Plains locations. WUE is pounds of grain produced per inch of water used during the growing season. Fig. 2Water use efficiencies (WUE) for five agronomic crops.