Daniel M. O'Brien

OPTIMUM LATERAL SPACING FOR SUBSURFACE DRIP-IRRIGATED CORN

A two-year study was initiated in the spring of 1990 on a Keith silt loam soil (Aridic Argiustoll) in northwest Kansas to determine the optimum dripline lateral spacing for irrigated corn (Zea mays L.) using subsurface driplines installed at a depth of 40-45 cm in a direction parallel to the corn rows. Average corn yields were 13.6, 12.8, and 12.2 Mg/ha for dripline spacings of 1.5, 2.3, and 3.0 m, respectively, for a seasonal-irrigation amount of 462 mm. Yields decreased to 10.8 and 9.3 Mg/ha when irrigation was reduced by 33 and 50% for the wider 2.3- and 3.0-m dripline spacings, respectively. The wider dripline spacings resulted in nonuniform horizontal distribution of available soil water. As a result, yields decreased with horizontal distance from the dripline. The highest yield, highest water use efficiency, and lowest year-to-year variation were obtained with the 1.5 m dripline spacing. An economic analysis indicated that because yield reductions were so great, the wider dripline spacings would be justified only at very high dripline costs and or very low corn grain prices.

Crop Production and Economics in Northwest Kansas as Related to Irrigation Capacity

Crop production and economics of corn, grain sorghum, soybean, and sunflower under irrigated and dryland conditions were simulated using 34 years (1972-2005) of weather data in Northwest Kansas. Irrigation system capacities ranged from 2.5 to 8.5 mm/day. The simulated long-term annual average net irrigation requirements for corn, grain sorghum, soybean, and sunflower were 375, 272, 367, and 311 mm, respectively. Assuming a 95% application efficiency (Ea), the average long-term crop yield is approximately 12.9, 8.2, 4.4, and 3.2 Mg/ha for corn, grain sorghum, soybean, and sunflower, respectively. Although corn is currently the predominant irrigated crop in western Kansas, projections for the year 2006 indicate soybean is a more profitable alternative. Net irrigation requirements for soybean are only about 2% lower than corn, so a shift to soybean will not result in significant water conservation. If the price of corn increased just 10% relative to stable prices for the other crops, it would become the most profitable irrigated crop. This indicates that net return projections are very volatile, subject to changes in crop prices and input costs.