After four consecutive years of drought, most Georgia cotton producers would agree that it pays to irrigate. The uncertainties lie in how, when and in what amounts the water should be applied.
Even irrigated fields have fallen well below yield expectations during years of severe, persistent drought, causing researchers to take a new look at our understanding of plant water use, irrigation timing and irrigation efficiency.
The USDA-ARS-National Peanut Research Laboratory has initiated a comprehensive, long-term, multi-crop research project to quantify the impact of irrigation methods and amounts on Southeastern agriculture. The Multi-Crop Irrigation Research Farm, located in southwest Georgia, involves collaboration with other USDA laboratories and agencies, land-grant universities, the Flint River Water Planning and Policy Center, the Georgia Environmental Protection Division, the Georgia Soil and Water Conservation Commission and numerous other partners.
In addition to cotton, other crops planted in the farm's rotation system include peanuts, corn, soybeans, wheat and grain sorghum.
Beginning in 2001, researchers began looking at four irrigation scenarios on cotton at the Multi-Crop Irrigation Research Farm. These included sprinkler irrigation, subsurface drip irrigation, surface drip irrigation and no irrigation.
“Within these irrigation systems, we have nozzle and pressure regulators to put out different amounts of water,” says Marshall Lamb, economist with the National Peanut Research Laboratory (NPRL). “If we tell the system to put out a full amount, it'll put out 100 percent. At the same time, we have spans that put out two-thirds or 66 percent and one-third. We also have a non-irrigated check.”
Seven rotation sequences are used, consisting of peanut, cotton, corn and grain sorghum, he adds. “Basically, we use conventional-tillage and follow university recommendations on all tillage and other cultural practices. We use the University of Georgia irrigation scheduling system, and it has worked very well for us,” says Lamb.
The subsurface drip irrigation system is on a 3-foot spacing, with drip lines on every row or every 36 inches. Researchers also are looking at subsurface drip irrigation on a 6-foot spacing, with drip lines between every other row, or one drip line services two rows.
“That's a capital investment issue because you'll have half the investment in drip tape if you go with the 6-foot spacing,” says the economist.
The irrigation research site received good rainfall in 2001, notes Lamb. “We received almost 21 inches of rain during the 2001 cotton production year. But in 2002, we missed much of the rainfall and received only 12 inches or rain during the production year,” he says.
In 2002, about 12 inches of irrigation was applied on the 100 percent scenario, 6.9 inches was applied on the two-thirds irrigation and about 4 inches was applied on the one-third.
“On the subsurface drip systems, we actually used more water, which is contrary to what we normally would think. We put out 14 inches on the 100-percent subsurface drip, 9 inches on the two-thirds and about 6 inches on the one-third system. We were forced to irrigate aggressively in 2002.” On the 100 percent scenario, 6.9 inches was applied on the two-thirds irrigation and about 4 inches was applied on the one-third.
“On the subsurface drip systems, we actually used more water, which is contrary to what we normally would think. We put out 14 inches on the 100-percent subsurface drip, 9 inches on the two-thirds and about 6 inches on the one-third system. We were forced to irrigate aggressively in 2002.”
Researchers developed an irrigation model designed to parallel the University of Georgia's recommendation for a yield goal of 1,200 to 1,500 pounds of cotton per acre, says Lamb.
In 2002, the 100-percent irrigated cotton received about 24 inches of water - 12 inches of rain and about 11.8 inches of irrigation, he says.
“Summing up the data from 2001, we averaged over 1,000 pounds per acre on the 100-percent sprinkler irrigated cotton. However, that was not significantly different from the two-thirds irrigated or even the one-third. And remember, this was a year of plentiful rainfall. “The 3-foot and 6-foot subsurface drips systems also made good cotton in 2001, but there was no significant difference in yields when comparing the irrigation amounts. All irrigation treatments were significantly better than dryland in 2001, with the dryland cotton making just over one bale per acre. So, irrigation did seem to pay in 2001.”
In 2002, with much less rainfall than in 2001, the 100-percent sprinkler irrigated cotton made about 987 pounds per acre, says Lamb. The one-third sprinkler irrigated cotton made about 700 pounds per acre.
“We saw a bigger response to irrigation this past year. But what bothered us was the lack of response we saw from drip irrigation. We're not sure if it's a management problem or if it just didn't respond well in 2002.”
The 100-percent irrigated 3-foot drip made about 700 pounds per acre in 2002. The two-thirds 3-foot drip made about 600 pounds and the one-third made just over one bale per acre. The same trend was seen in the 6-foot subsurface drip treatments. With only 12 inches of rainfall, the dryland cotton made about 189 pounds per acre.
“Drip irrigation may have a place for us in small fields, even though we saw a yield reduction in our tests. If you look at the investment cost per-acre, comparing center pivot and subsurface drip, subsurface drip is very much tailored to small fields where we don't have the ability to spread the fixed costs associated with center pivot irrigation.
“The break-even point of subsurface drip irrigation is cheaper in terms of investment costs, with the break-even point being about 55 acres. A field less than 55 acres — from an investment cost standpoint — is more cost-effective towards subsurface drip irrigation. As the field gets larger, the benefits accrue towards the center-pivot systems.”
The recent price of cotton, says Lamb, has affected producers' return from irrigation. “On the 100-percent sprinkler irrigated cotton, we generated about $566 per acre, based on a price of 55 cents. In our cost-monitoring system, we had variable production costs of about $340, an irrigation cost of about $66 and picking costs of 9 cents per pound, giving us total variable costs of $507.
“This leaves a net return above variable costs of only about $59, and this doesn't include returns to land, fixed assets, management or overhead. Fifty-nine dollars isn't enough to cover all of that. Even a 1,000-pound-per-acre crop isn't enough to show a profit at these prices.”
Each time a grower adds an extra input such as irrigation to his crop, a certain yield gain is expected in return, says Lamb. “You want to increase that input to a point to where the value of the yield gain is equal to the cost of that additional input. At that point, you've maximized your profit on that input.
“In looking at the three levels of irrigation in our tests, we were $190 short of reaching the profit maximization level on the 33 percent or one-third irrigated. On the two-thirds irrigated, we were $50 short. At 100-percent irrigation, the marginal revenue equals the marginal costs, which means we were at the profit maximizing level of irrigation.”
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