seal  Purdue News

April 13, 2004

Purdue study drives home benefits of GPS auto guidance

WEST LAFAYETTE, Ind. - Drivers who take their hands off the steering wheel are asking for trouble, but Corn Belt farmers who relinquish the wheel in their tractors may profit handsomely from the maneuver.

Auto guidance, a technology that pilots farm machinery via Global Positioning Systems (GPS) satellites, could help Midwest farmers boost productivity and expand their farm operations, said Jess Lowenberg-DeBoer, director of Purdue University's Site-Specific Management Center.

Although farmers could expect to pay $10,000 or more to adopt auto guidance technology, many could make up their investment through greater use of farm equipment and planting crops on hundreds of additional acres, Lowenberg-DeBoer said.

Lowenberg-DeBoer and Matt Watson, a Purdue agricultural economics graduate student, outlined the technology's advantages in a study, titled "Who Will Benefit From GPS Auto Guidance in the Corn Belt?"

Farmers in high-value crop areas of California and in easily compacted soil regions of Australia already embrace auto guidance. The technology has only recently been introduced in the Midwest, Lowenberg-DeBoer said.

Auto guidance builds on previous GPS-based navigation technology, including light bars. GPS light bars, mounted at the front of a tractor's cab, show farmers how straight the tractor is heading down a field. The farmer then adjusts the steering wheel to bring the tractor into proper alignment.

"Auto guidance is the next step beyond the light bars that have become so common among farmers and custom operators in the Midwest," Lowenberg-DeBoer said. "The technology takes over steering of the farm equipment. The driver still has to turn the tractor at the end of each row, but during the pass in the field the driver can take their hands off the steering wheel, talk on the cell phone or do other things."

There are two basic auto guidance systems, Lowenberg-DeBoer said. They differ in precision and price.

"One of them is a 4-inch accuracy system, known as a differential corrected GPS," he said. "The other is a RTK, real time kinematic, system, which has an accuracy of about 1 inch. Their costs are very different. The 4-inch system starts at around $10,000 to $15,000, while RTK systems are about $40,000 and up."

In their study, Lowenberg-DeBoer and Watson examined the affect of auto guidance on a typical west-central Indiana producer who farms 1,800 acres with a 50/50 corn-soybean rotation using a 12-row planter.

The researchers compared the guidance systems to each other and light bars, and considered the differences in production costs and profits using the technology for various row crops. They also looked at such "spatially sensitive" cropping practices as strip tillage, sidedressing nitrogen fertilizer and controlled traffic - driving on the same wheel track for all field operations - as well as using auto guidance to replace foam markers during spraying and alongside disk markers at planting.

Light bars were found to be the most profitable guidance option for the 1,800-acre farmer who did not plan to expand the farm operation or was not using any "spatially sensitive" GPS technology.

"For a farmer who's at 1,800 acres but would like to go up to 2,000, 2,200 or 2,400 acres, the 4-inch accuracy auto guidance system makes a lot of sense," Lowenberg-DeBoer said. "The reason is that it allows farm equipment to be used for more hours and there is less fatigue on the operator, so they can work longer hours. Also, the farmer has greater flexibility in choosing employees because it requires less skill since the computer is doing a lot of that steering and other detailed work."

In situations where accuracy was an issue, the farmer in the case study was better off with the RTK system.

"An example is controlled traffic," Lowenberg-DeBoer said. "If you want to use only certain tracks through the field and repeat those operation after operation and year after year, then the RTK allows you to stay on those same tracks and limit wheel traffic on that field. Another example is strip tillage. If you want to make those strips in the fall and then come back in the spring and plant on those same strips, then the RTK - with that 1-inch accuracy - is what you need."

The study also found that:

• Benefits from auto guidance equipment were realized only when machinery was driven more accurately, more consistently and/or for longer periods each day.

• Sixteen- or 24-row planters with an auto guidance system provided even greater benefits because the cost of auto guidance was the same regardless of planter size.

• Estimated field time for the 1,800-acre model, not counting harvest, was 496 hours if no GPS guidance system was used. A farmer utilizing light bar technology could cut that time 11 percent, to 439 hours. Replacing the light bar with either auto guidance system trimmed another 6 percent, to 411 hours.

• The 4-inch auto guidance system afforded the largest increase in returns for expanding farms, at $7.36 per acre. The figure was based on an anticipated expansion to 3,100 acres. Light bar guidance, which permitted the operation to expand to 2,600 acres using the same equipment, netted the farmer an extra $6.93 per acre. A farmer using RTK guidance with other equipment remaining the same increased returns $3.41 an acre on 3,100 acres.

• At current equipment prices, RTK guidance was more profitable than foam and disk markers for expanding farm operations, as well as for those farms with soils subject to severe compaction. At current price, light bars and the 4-inch auto guidance system were more profitable than RTK.

The auto guidance study is available online.

Writer: Steve Leer, (765) 494-8415,

Source: Jess Lowenberg-DeBoer, (765) 494-4230,

Ag Communications: (765) 494-2722; Beth Forbes,
Agriculture News Page

Note to Journalists: Jess Lowenberg-DeBoer is traveling overseas through April 28. During his trip, interviews can be arranged through Jody Geller, Purdue University International Programs in Agriculture, at (765) 494-5843,

Related Web site:
Purdue Department of Agricultural Economics

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