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“We’ve never imagined we had so much variation,” says Gary. “Yields changed 20 to 25 bu. an acre in wheat that looked uniform from the combine. Now we’ll go back to the low-yield areas looking for low fertility, compaction and poor drainage.”
The Wagners worked with Ted Macy of Application Mapping in Frankfort, IL (815-469-7504), to assemble their mapping system. The main component is the Yield Monitor 2000. It measures yields on the go by sensing the strain on a pressure plate in the top of the clan grain elevator. The monitor corrects for moisture and logs yield data to a laptop computer (miniumum 286) with Macy’s MapSight software. At the same time, a receiver logs location data from satellites in orbit as part of the global positioning system (GPS).
“The monitor and receiver log yield and location of data on the computer every second,” explains Macy. “This information is stored on a data card that is then plugged into a mapping program back in the office to generate the yield map.”
Price Tag. The Wagners’ system cost roughly $8,000, including a roving GPS receiver. This does not include a differential GPS base station, which most farmers won’t need. The Department of Defense, which operates the GPS system, puts an error factor in the satellite signals. To restore accuracy, a differential receiver and radio link are needed. Because the Wagners aren’t within range of an operating differential GPS tower, they installed a tower a their shop. However, subscriber networks are springing up around the country to provide differential signals for a fee- generally around $500 a year.
“With the differential tower, location was accurate to within 10’ or 15’,” says Gary. “The biggest error with the yield monitor was 1.2%, so the entire system was extremely precise.”
The Wagners are convinced the system will pay. Their main cash crop is sugar beets, whose quality is easily affected by soil conditions, including fertility. Discounts for poor-quality sugar beets run up to $200 an acre.
Telltale. “We hope to use wheat as an indicator crop to point out low- and high- yielding areas of the field before we plant sugar beets,” says Gary. “This will help us balance fertility in the field and improve the quality of the sugar beet crop.”
Even in wheat, Gary sees payback potential. “An obvious way it will pay is in weed control,” he says. “Wild oats are a major problem for us. Typically, chemical control costs us $13 an acre. However, we can log areas with wild oats as we harvest. Using the field map and a GPS receiver, we can go back and treat only the weedy areas prior to seeding the next crop. On one 80-acre filed we’ll only have to treat 12 acres. That will save $884 in chemical costs in the field alone.”
Gary also thinks he can either increase yields or decrease costs by basing fertilizer rates on the yield map. Researcher are only beginning to assess the economics of variable application. In a study of three fields in Missouri it raised gross returns by $40.89, $16.38 and $9.14 an acre. It increased costs by $10 to $15 an acre, so only one situation was clearly profitable.
Still, yield mapping may play a major role in making variable application more affordable. “Previously, we've had to take a huge number of soil samples to get an accurate map of variability in a field,” say Kansas State agronomist John Havlin. “Soil sampling became a major part of the overall expense. However, with a yield map, we should be able to test only the areas where yield variations occur. This could dramatically lower the cost of variable-rate systems.”
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