Joel P. Schneekloth

NITRATE LEACHING IN IRRIGATED CORN AND SOYBEAN IN A SEMI-ARID CLIMATE

Nitrate-nitrogen leached from the root zone of land in intensive corn production is a major groundwater contaminant in some of the intensively irrigated regions of the western Cornbelt, including central and western Nebraska. To obtain a clearer understanding of the amount and timing of nitrate leaching losses from irrigated crops, 14 monolithic percolation lysimeters were installed in 1989-1990 in sprinkler irrigated plots at the University of Nebraska’s West Central Research and Extension Center near North Platte, Nebraska. The lysimeters were used to provide a direct measure of leachate depth from continuous corn and a corn-soybean rotation. Both cropping systems were sprinkler irrigated and used current best management practices (BMPs) in the region for water and nitrogen management. Leachate was collected from 1990 through 1998 and analyzed for nitrate-N concentration. Results for the period 1993- 1998 are reported here. In the semi-arid climate of West-Central Nebraska, the interaction of rainfall patterns with the period of active uptake of water by crops played a major role in defining leaching patterns. Careful irrigation scheduling did not eliminate leaching during the growing season. There was no significant difference in drainage depth between continuous corn and the corn-soybean rotation. The average drainage depth among the lysimeters was 218 mm yr–1. This was more than expected, and in part resulted from above normal precipitation during several years of the study. No water quality benefit was found for the corn-soybean rotation as compared to continuous corn. Nitrate-N concentration in the leachate from continuous corn averaged 24 mg L–1, while that from the corn-soybean rotation averaged 42 mg L–1. Total yearly nitrate leaching loss averaged 52 kg ha–1 for continuous corn and 91 kg ha–1 for the rotation. This represents the equivalent of 27% and 105% of the amount of N fertilizer applied over the six years of study. In calculating N fertilizer needs for corn in Nebraska, the recommended legume N credit of 50 kg ha –1 for a preceding crop of soybean may be too low under irrigated production.

Field Scale Limited Irrigation Scenarios for Water Policy Strategies

Approaches to reducing irrigation inputs to crops have been studied for the past 50 to 60 years in research settings. Fewer efforts have been made to document limited irrigation responses over a number of seasons on commercial fields. This study compared farm-based irrigation management (FARM) with best management practices (BMP), late initiation of irrigation (LATE), and a restricted allocation (ALLOC). These irrigation management strategies each occupied 1/8 of a center pivot system in southwest Nebraska in continuous corn production, on four cooperating farms, which were replicated at the same sites for 3 to 6 years. Irrigation variables were achieved by irrigating or not irrigating, or by speeding up or slowing down the center pivot. When the grain yields and irrigation amounts were normalized each year using the FARM treatment as the basis, on average for three of four locations, the BMP treatment yielded equal to the FARM treatment, the LATE treatment yielded 93% of the FARM treatment and the ALLOC yielded 84% of the FARM treatment. At the same time, it took 76% and 57% of the water for the LATE and ALLOC treatments, respectively, to achieve these yields. The adjusted gross returns (yield . price – irrigation treatment costs) of the irrigation treatments were analyzed for each location. When the gross returns were normalized using the FARM treatment as the basis, FARM and BMP returns were equal across combinations of high and low input commodity prices and pumping costs. The LATE treatment gross return was 95% of FARM return. The gross return for the ALLOC treatment was 85% to 91% of the FARM treatment. The higher the water costs, the lower the difference between the highest and lowest returning water treatments. Relationships between evapotranspiration and grain yield were developed for two sites over the limited range of water applications of the projects. Regressions indicated more variability between the commercial field data and research plot environments. Much of this difference may have been due to yearly replication in this study rather than plot-to-plot replication in the research center study. Yield and irrigation data were normalized on the basis of the FARM treatment. Normalized yield – irrigation results over years and locations for three of the four locations showed declining yields as irrigation decreased. The same regression was used to normalize the locations with soil textures from fine sand to sandy loam, which suggested that the three locations behaved similarly with respect to the management treatments.

CROP ROTATIONS WITH FULL AND LIMITED IRRIGATION AND DRYLAND MANAGEMENT

ABSTRACT Irrigated cropping systems need to maximize the economic value of both rainfall and irrigation water, especially in areas of declining groundwater. This study compared water management systems in a winter wheat (Triticum aestivum, L.)-corn {Zea mays, L.)-soybean (Glycine max, L.) (W-C-S) and continuous corn (CC) rotation in west central Nebraska for dryland, limited irrigation (150 mm/yr), and full irrigation. Crop yield, evapotranspiration, and soil water storage were determined from field studies conducted at North Platte, Nebraska, on a Cozad silt loam (Fluventic HaplustoU) soil. Dryland com used 21.5% more evapotranspiration (ET) in the W-C-S rotation compare to CC. ET for the limited and full irrigation com was 4.6% and 4.9% more for the W-C-S rotation compared to the CC and was statistically significant at the P > 0.08 level. Water use efficiency, defined by the slope of the linear relationship between grain yield and ET (3Y 3ET-^), was the same for com in the W-C-S and CC rotations. Com grain yield response to irrigation and ET was more than the yield response of winter wheat and soybean. The W-C-S rotation increased com grain yields in two out of three years at this location for dryland management and increased the seasonal ET of corn compared to continuous corn. Full irrigation management did not consistently increase winter wheat and soybean grain yields above the limited irrigation treatments. Soil water storage for the full irrigation management was greatly reduced compared to dryland and limited irrigation management for both rotations.

Long-Term Response of Corn to Limited Irrigation and Crop Rotations

Dwindling water supplies for irrigation are prompting alternative management choices by irrigators. Limited irrigation, where less water is applied than full crop demand, may be a viable approach. Application of limited irrigation to corn was examined in this research. Corn was grown in crop rotations with dryland, limited irrigation, or full irrigation management from 1985 to 1999. Crop rotations included corn following corn (continuous corn), corn following wheat, followed by soybean (wheat-corn-soybean), and corn following soybean (corn-soybean). Full irrigation was managed to meet crop evapotranspiration requirements (ETc). Limited irrigation was managed with a seasonal target of no more than 150 mm applied. Precipitation patterns influenced the outcomes of measured parameters. Dryland yields had the most variation, while fully irrigated yields varied the least. Limited irrigation yields were 80% to 90% of fully irrigated yields, but the limited irrigation plots received about half the applied water. Grain yields were significantly different among irrigation treatments. Yields were not significantly different among rotation treatments for all years and water treatments. For soil water parameters, more statistical differences were detected among the water management treatments than among the crop rotation treatments. Economic projections of these management practices showed that full irrigation produced the most income if water was available. Limited irrigation increased income significantly from dryland management.

FURROW IRRIGATION MANAGEMENT WITH LIMITED WATER

Dwindling water supplies in the Great Plains are a major concern to the economics of the region. Water management studies on limited irrigation usually focus attention on sprinkler irrigation for the delivery system. Furrow irrigation has more management factors that can influence water application including water advance and water distribution over the furrow length. This study compared water application strategies to reduce the amount of water delivered to corn during the vegetative and late grain fill growth periods from 1998 to 2000. In addition to a “full irrigation” and “late initiation of irrigation” treatment, two water allocation treatments were imposed, which limited seasonal water applications to 150 and 250 mm, respectively and “rainfed” treatment. Average grain yield over three years for “late” treatment was 0.2 Mg ha-1 or 2% less than that of “full” treatment. Yields for the 150- and 250-mm treatments were 1.1 and 0.4 Mg ha-1 less than “full.” The reduction in gross water applied was 21%, 36%, and 51% for “late,” 250 mm, and 150 mm as compared to “full”. Soil water was measured to a depth of 1.8 m at quarter points along the furrow to evaluate the distribution of the irrigation. Soil water profiles tended to dry slightly during the season, but there was no statistical difference among the limited and full irrigation treatments. Best management cropping practices were applied, including ridge tillage/planting, furrow packing, irrigation scheduling, and surge irrigation techniques to keep water in the soil profile and target water applications to critical crop growth stages.. These practices maximized the potential success of all irrigation treatments.

Retiring Land to Save Water: Participation in Colorado’s Republican River Conservation Reserve Enhancement Program

Agricultural land retirement is increasingly used to manage water resources. This study uses well-level enrollment data to explore the factors that influence landowner participation in the Colorado Republican River Conservation Reserve Enhancement Program. An empirical model of enrollment is informed by a theoretical model of participation that incorporates aquifer and soil characteristics in addition to financial incentives. Our results reveal that enrollment is predicted to increase by 0.087 percentage points with a

Comparison of irrigation strategies for surface-irrigated corn in West Central Nebraska

10 increase in the incentives offered. The probability of enrollment is also influenced by the aquifer’s saturated thickness and the soil characteristics that impact land productivity. (JEL Q25)