Limited and Full Irrigation Comparison for Corn and Grain Sorghum
by Kevin Larson, Research Scientist; Dennis Thompson, Technician;
and Deborah Harn, Research Associate, Plainsman Research Center, Walsh, Colorado

The importance of limited irrigation (supplemental irrigation) has traditionally been associated with very low capacity irrigation wells. The current high fuel prices and associated pumping costs places new emphasis on limited irrigation as a replacement for full irrigation when commodity prices are low. At the Plainsmen Research Center, we define limited irrigation on corn and grain sorghum as applying one in-season, furrow irrigation of less than 10 a-in./a, or a similar amount or water applied with a sprinkler. Applying less than 10 a-in./a of in-season irrigation assumes that the soil water profile is full from sufficient winter moisture, or, if winter moisture is lacking, the soil water profile is filled by pre-irrigation.

Limited irrigation becomes a more profitable choice, as fuel costs increase and commodity prices remain low. Our research (http://www.colostate.edu/depts/prc/pubs/LimitedandFullIrrigationComparisonforCorn.pdf) suggests that the decision point for conversion from full irrigation to limited irrigation with our current costs, and the target price ($2.63/bu for corn and $2.57/bu for grain sorghum) as the expected grain price, is $5.00/a-in. pumping cost for corn and $6.00/a-in. pumping cost for grain sorghum (Figure 1). This past season, research the pumping cost in the Southern High Plains Groundwater District was between $6.50 a-in./a to $7.50 a-in./a for a typical 600 gpm, 300 ft. well with a natural gas power plant.

An economic comparison between corn and grain sorghum under full and limited irrigation is dependent on commodity price and input costs. Having a similar target price for corn ($2.63/bu) and grain sorghum ($2.57/bu) provides corn with higher net income than grain sorghum under limited irrigation and nearly identical net income under full irrigation. However, when corn and grain sorghum commodity prices are above the target price, corn frequently has a $0.30/bu price advantage compared to grain sorghum in the local market. Corn priced $0.30/bu higher than grain sorghum provides corn with higher net income than grain sorghum under both full and limited irrigation.

With the current commodity prices, full irrigation has the income advantage over limited irrigation even with relatively high pumping costs. The current demand for corn and grain sorghum for ethanol production has increased the new crop contract price to over $4.00/bu. With these high commodity prices ($4.00/bu for corn and $3.70/bu for grain sorghum), pumping costs would have to exceed $9.50/a-in. for grain sorghum and $11.00/a-in. for corn in order to make limited irrigation more profitable than full irrigation.

When input costs, particularly pumping costs, are high and commodity prices are low, limited irrigation is more profitable than full irrigation. Inputs such as fertilizer and seed, which differ between full and limited irrigation regimes, favor limited irrigation when these input costs increase. When commodity prices are high and input costs are low, full irrigation is more profitable than limited irrigation. Currently, full irrigation is more profitable than limited irrigation because the high commodity prices more than compensate for the moderately high input costs.

Figure 1. Full and limited sprinkler irrigation comparison of net income for corn and grain sorghum. Assumptions: yield: 151 bu/a for full irrigation grain sorghum, 123 bu/a for limited irrigation grain sorghum, 182 bu/a for full irrigation corn, and 144 bu/a for limited irrigation corn; grain price: $2.63/bu and $4.00/bu for corn, and $2.57/bu and $3.70/bu for grain sorghum; irrigation: 17 a-in./a for full irrigation corn and grain sorghum, 8 a-in./a for limited irrigation grain sorghum, and 9 a-in./a for limited irrigation corn; production costs: pumping cost varies from $4 to $11/a-in., all other costs remain constant.

Kevin Larson: Kevin.Larson@Colostate.edu
CSU Plainsman Research Center, Walsh