Guiding Right
April 2, 2010 
For years growers have realized the importance of accuracy in planting. If you can put a seed down in the same spot year after year, you’re going to achieve a more fertile, less compact seed bed with more organic activity.
And with the introduction of a cover crop into many fields across the Mid-South and Southeast, those seed beds are becoming even more fertile. Growers who plant legumes ahead of their cotton are familiar with the benefits of residual nitrogen, for instance.
But all of those benefits are lost on a cotton plant if the cotton seed is planted inaccurately. And the presence of stubble at planting can complicate that process.
“If you’re planting into cover-crop stubble, it doesn’t matter what you’re planting, the fact is you can’t really see where you’ve been,” says Randy Taylor, an Extension specialist at Oklahoma State University. And while it’s true that farmers planted into stubble long before anyone was able to bounce a signal off of a satellite in outer space, Taylor says the benefits of a GPS system are still evident.
“(A GPS guidance system) is not required, but there would definitely be an incentive to having one,” says Taylor.
Precision Pays
Testing to put a concrete dollar amount on that incentive began a decade ago. Back then, growers were looking for a better way to go about planting after having strip-tilled their fields.
“You basically have to do something like that in a lot of our Coastal Plains soil in the southeast,” says Randy Raper with USDA-ARS at the Dale Bumpers Small Farms Research Center in Booneville, AR. “You do a strip-till operation and then you want to come back in and plant directly on top of where you strip-tilled.”
Raper and his colleagues set out to determine exactly how much value a farmer stood to gain by working with a real time kinematic (RTK) GPS automatic guidance system, as opposed to simply “eye-balling” his or her way through planting into stubble.
The latter option, Raper says, is a difficult proposition.
“You’ve got all that residue out there and it’s rather difficult to pick out exactly where you were. If you did a decent job of strip-tilling, that means you didn’t disturb a lot of residue in the field.”
To get a gauge of how much an inaccurate planting job could cost a grower, Raper and his colleagues tested in the same field for three years. The group started out planting precisely over the path that they had strip-tilled. But as the planter moved through the field, they would deviate from that path. First they planted two to three inches off of the initial path. Eventually they would plant up to eight or nine inches off of the original course.
After harvesting and collecting data for three cotton crops, the results of the test were conclusive: precision pays.
A Return on Investment
“What we found was that when you got off the row about two inches, you had about a 16% drop in yield. And then when you get off the row, basically about eight or nine inches, then you see a big drop. It’s up around 29%,” Raper says.
The drop in yield for a nine-inch error is pretty easily explained. Raper believes that at this range, the seed starts to fall under the edge of the tractor’s tire.
“And that yield drop makes sense because the edge of the tire is what causes your compaction. It’s not necessarily the center of your tire, for farmers who are properly inflating their radial tires.”
But a 16% drop in yield for even a two-inch deviation in planting range is as curious as it is alarming. Researchers are still studying the disparity in yields for such a small error in planting width, but Raper believes that drop can be attributed to the plant’s taproot not being in its optimal position.
Raper’s research also found the added benefit of savings on fuel costs. In operations that consistently rely on strip-tillage, there are considerable energy savings to be had by putting the plow in precisely the same spot, year after year.
“Instead of trying to go over and rip out where you ran those tires, where it’s going to be really packed and really solid, if you just go back into that loosened area, the energy savings is quite significant as well,” he says.
In addition to those savings on fuel costs, Raper’s research showed a clear need for precise accuracy during planting. He was able to take his study a step further by working with an agricultural economist, as well. Their findings suggested that a grower with 1,000 acres of cotton could see an (RTK) GPS guidance system pay for itself in one season.
The general rule for GPS guidance systems, according to Taylor, is that you pay for what you get.
“Generally if you spend more money, you get more accuracy. And so now the question is, how much accuracy do you need? And that depends on the individual,” says Taylor.
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