Irrigation research in the Mid-South over the last 25 years shows soybeans will almost always need irrigation to achieve maximum seed yield, even in years when rainfall during reproductive development is well above normal and drought stress is minimal.
Since irrigation fixed costs are incurred even when irrigation is not applied, only a very small yield increase will pay the variable costs for irrigation. This was especially true in years like 2004, when anticipated prices were higher than normal.
Research indicates that once irrigation is started, it must be continued on a regular basis. An initial irrigation that recharges the soil profile will essentially kill deep roots on clay soil, and the plant, therefore, depends on a shallow root system from that point on.
Thus, continued irrigation will be necessary to insure adequate water for the remainder of the season. Research also indicates irrigation reapplied when soil water deficit is no more than 2 inches will result in the greatest yield increase.
Weather station data show that pan evaporation during June through August in the Mid-South is generally between 0.25 and 0.30 inch per day. During the irrigation period for soybeans, which encompasses these months, evapotranspiration by the crop will be 80 to 100 percent of pan evaporation.
If an average 0.28 inch per day pan evaporation is used as a baseline, and it is assumed that evapotranspiration will be 0.9 of pan evaporation, then a 2-inch deficit will be reached eight days after an irrigation. This has been confirmed by many years' data collected from soil water measuring devices, and by measuring irrigation water applied to soybean research plots.
Thus, in the absence of rain, furrow irrigation of soybeans should be scheduled every eight days. If rain occurs between irrigations, the amount for each rain can be added to the 2 inch deficit. For example, a 0.5-inch rain will equal two days of water, so irrigation can be scheduled 10 days from the previous event.
Deciding when to end irrigation of soybeans in a season is a simple matter. When beans have completely filled the pod cavity, full seed stage has been reached. This is later than the “pods touching” criterion that is often used.
Having wet soil or applying a last irrigation to dry soil at or just before the full seed stage will insure that enough soil moisture is available to completely fill the many seeds that result from a well-watered environment. This will insure maximum yield.
Stopping irrigation too soon will not result in fewer seed, but will result in smaller seed. A properly irrigated crop with a 70-bushel-per-acre yield potential will have 12.5 million (3,000 seeds per pound) to 14.5 million (3,500 seeds per pound) seeds per acre. Thus, even very small reductions in seed size can translate into large reductions in maximum yield.
A last irrigation of 1.5 to 2 inches of water costing about $4 to $5 per acre that is not applied when it should have been will be the most expensive mistake an irrigator can make. These points are especially pertinent when it is considered that the full seed stage in an early soybean production system occurs during mid-July through late August, when drought stress conditions are greatest.
Larry G. Heatherly is a retired USDA-ARS Research Agronomist and current crop consultant. e-mail firstname.lastname@example.org