Irrigation of soybeans in the southern US is a management option for increasing yields and ensuring consistent production from year to year. Its use is governed by its cost in relation to expected increase in returns.
The 2002 Census of Agriculture reveals a marked difference between southern regions in the use of irrigation for soybeans. In the midsouth or Delta states of Arkansas, Louisiana, and Mississippi, 43% of the soybean acres were irrigated. In the southeastern states of Alabama, Georgia, North Carolina, and South Carolina, only 2% of soybeans were irrigated.
The limited irrigation in the southeast is through center pivot systems. The University of Georgia enterprise budget estimates the total cost of irrigating soybeans at $137.50 per acre.
Georgia Extension Soybean Specialist Philip Jost states that irrigating soybeans isnít cost effective at projected yield levels in the southeastern US. "If a producer installs a new overhead irrigation system in Georgia, I seriously doubt he would put beans under it," said Jost. "With a projected 50 bushels per acre soybean yield from an irrigated production system, our budget analysis estimates the total cost breakeven price to be $7.21 per bushel. Conversely, the total cost breakeven yield is over 60 bushels per acre."
Jim Dunphy, North Carolina Extension Soybean Specialist, concurs with Jostís assessment. "In the 1980's, we conducted 38 tests across the state," Dunphy said. "Our check (nonirrigated) yields were greater than 50 bushels per acre, and we got only a 1 bushel per acre yield increase with each irrigation. That was not economical."
Irrigation in the midsouth uses surface methods (furrow, flood, and border). Mississippi State University enterprise budgets estimate irrigation costs at between $71 and $74 per acre. Profits from properly irrigated soybeans consistently exceed those from nonirrigated soybeans primarily because of early planting and lower costs for surface irrigation.
John Gourlay border irrigates nearly 1700 acres of soybeans in Bolivar County, MS. He applies water down every other furrow starting at about R1 (beginning bloom), and continues until beans are "squared"or fill the pod cavity (R6). "We realize about 20 bushels per acre increase in yield from irrigation," said Gourlay. "In 2005, all of our irrigated fields were in the 65 to 70 bushels per acre range."
Obtaining maximum yields from irrigation in the southern US requires planting early-maturing varieties before May. For these early plantings, irrigation in an average year should be started no later than R3 (beginning podset). Rain and soil moisture should be monitored to ensure that the effective rooting zone (18 to 24 inches) has adequate water (>50% available) to maintain optimum growth before R3, especially for sandy soils such as those of the southeastern Coastal Plain.
For May and later plantings of later-maturing varieties, irrigation should be started at or near R1. Soybeans planted behind small grain in a doublecrop system should be monitored for drought stress conditions (<50% available water in rooting zone) before R1 to ensure that rain and soil moisture provide adequate water for vegetative growth.
During the irrigation period, soybeans typically use about 0.25 inch of water per day. Soil-recharging surface irrigation should be scheduled to meet this requirement when no more than 2 inches of soil water has been depleted. This will require an 8 to 10 day re-irrigation schedule in the absence of rain.
Re-irrigation with overhead systems is typically timed to system capabilities rather than to available soil water. Thus, a system designed to apply a net 1 inch per acre per day should be scheduled to irrigate every four days in the absence of rain to replace depleted soil water.
Irrigation should be terminated at or near R6. A soil-recharging irrigation or wet soil at this time will supply enough water to finish filling seeds and maximize yield.