John Shawhan's pointer finger streams down the colored strips as if he's skimming a long list of names in the phone book.

"Look at this," he says with the excitement of a three-year-old displaying her first mud pie.

What Shawhan sees on the multicolored yield map are dramatic differences due to drainage. He knew there were drainage problems in the 85-acre field near his South Charleston, OH, farm. But instead of affecting half an acre, as he first suspected, a silted surface drain slashed yields on 4-5 acres.

"The corn looked perfect," says Shawhan. "But drainage problems were affecting the yields 6, 8, 10 bu/acre on the fringes."

While the value of adequate drainage has long been known, only recently have the specifics surfaced.

"The yield map brings home the importance of drainage," points out Tim Hartsock, Chillicothe, OH. "We knew where the wet spots were in the field before. But the yield map quantifies how big an area and how much drainage is affecting yield."

On Hartsock's farm, drainage improvements have boosted corn yields from 0 to 160-170 bu/acre in spots.

You need to know where the wet spots are and then take action. Making changes and installing tile are dictated by time and economics, Hartsock admits as he looks over yield maps pinned to the corkboard covering his office wall.

At Miles Farm Supply, Owensboro, KY, managers use infrared imaging to uncover drainage problems.

"We take aerial infrared photos and digitize them into a GIS package," explains Phillip Needham, Opti-Crop manager. Then a four-wheeler is driven to potential trouble spots to determine which need attention.

Needham finds that yield maps pinpoint drainage problems and the degree of yield reduction.

"Consultants can simply multiply the crop price by the yield reduction to obtain a financial benefit from tiling that area," he points out.

For example, yields in one field range from 0 to 140 bu per acre. If you increase yields by 140 bu, even with $2 corn the benefits would contribute $280 toward drainage. The total drainage cost for 60' centers would be about $500-600/acre. Nearly half the cost would be paid in just one year.

University of Kentucky ag engineer Scott Shearer suggests using yield maps as scouting guides. First, check the field and make sure excess moisture is the yield-robbing culprit. Next determine if the drainage problem is caused by no tile, a broken tile line, improper tile spacing or undersized mains and outlets.

The changes don't have to involve a $40,000 investment. Shawhan's fields are systematically tiled every 60'. He invested less than $500 to clean the surface drain and eliminate the drainage problem.

"Once you tile a field you might think you are done," he says. "But you have to get those pockets opened up."

Others are also finding drainage pays. Recently, Denny Bell of Soil-Max in Terra Haute, IN, surveyed at least 80 farmers who had bought laser-guided tile plows. Gold Diggers enable farmers to install their own tile. The responses showed the average yield increase was 25 bu/acre for corn and 11 bu/acre for soybeans. As a result, breakeven costs were lowered 75 cents/bu for corn; 25 cents/bu for soybeans.

At Liebrecht Manufacturing, Continental, OH, Junior Liebrecht and his sons manufacture tile plows, waterway ditchers and other equipment. Liebrecht has seen sales increase even with the downturn in the ag economy.

"Yield monitors have really made a difference," Liebrecht says. "Farmers have realized the (economic) difference between drained and undrained soils."

For more information, see the Miles Farm Supply web site at www.opticrop.com. Gold Digger equipment can be located at www.soilmax.com