For the Karcher family farm, variable-rate application of inputs means keeping up with almost constant change in software, hardware, GPS and equipment. But the need for basic agronomy knowledge is always the same.

The Karchers, cousins Brandon and Rob Karcher, and Brandon’s father, Ed, farm around 3,000 acres of corn, cotton, soybeans and wheat near Somerville, Tenn. They started experimenting with variable-rate applications of inputs in 1997, with the help of Tim Sharp, a precision ag consultant with Talon Tech. Sharp is Brandon’s former precision agriculture professor at Jackson State University.

Their venture into precision agriculture began in the late 1990s with variable-rate applications of cotton seed to address cotton’s high input costs. “It started out as that, but we found that there was more to it,” Rob said.

Today, nearly all inputs in cotton and corn are applied variable-rate, including seed, nitrogen, Pix for cotton, potash, phosphorus and lime.

“We backed off on the variable-rate seeding and the spraying for a little while after Tim left the area to go to Oklahoma,” Brandon said. “But we got back into it a few years ago, when Tim came back.”

Sharp was essential to the farm’s variable-rate program because he knew how to write prescriptions for seeding rates. “There wasn’t enough research out there for us to figure out how to do it,” Brandon explained. “When we get back into it, we started variable-rate seeding for corn also.”

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The mechanics of variable-rate technology has come a long way since the Karchers began experimenting with it in 1997. They use three Raven Envizio Pros to guide a number of field operations – one for variable-rate seeding and two for variable-rate fertilizer, a GreenSeeker for variable-rate nitrogen and a Raven Cruizer for auto-steer on sprayers. Their John Deere GreenStar 2600 controls variable-rate applications on the sprayer and a John Deere GreenStar 2630 is used for yield mapping in the cotton picker. A Case IH AFS Pro 600 is used for yield mapping in the combine. Tru Count clutches on their planter eliminate overlapping on end rows, which save on seed costs. The Karchers say there is still a lot of incompatibility between various brands.

The key to making variable-rate technology work starts with acquiring data from the field. It’s Sharp’s job to weave the Karchers’ knowledge and experience with geo-referenced information. “I use infrared imagery, GreenSeeker data (for applying nitrogen and other inputs) and yield maps to figure out how everything is interacting in that field.”

They divide each field into three management zones – for high vigor, medium vigor and low vigor. To understand how to make variable-rate applications in those zones can be tricky. For example, a yield map may indicate a low-vigor zone in a cotton field. “But it could be a high vigor zone that has gotten rank from too much nitrogen,” Brandon said. “That’s where you’ve got to know the field. If you’re just looking at the yield map, you wouldn’t know.”

Fix basics first

“Before you do anything with variable-rate applications, you have to have your agronomy right,” Sharp said. “All of your basics, fertilizer, lime, drainage, weed control, have to be fixed first. Don’t do precision ag before you have fixed your problems. You have to do everything in the right order. Then do precision ag.”

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“It’s just old school agronomy,” Brandon said. “The same kind of things that your grandpa was worried about. Get all of that right first.”

Agronomy for the Karchers begins with pH. It’s also one area where they save money with variable-rate applications. For example on one field, a variable-rate application resulted in lime being applied on less than half the field, which led to a significant reduction in costs versus a blanket application.

Each year, Sharp conducts nitrogen-rate plot studies comparing soil type and plant population to determine which corn hybrids perform best on the Karcher’s highly-variable soils. That helps the Karchers in hybrid selection.

Variable-rate nitrogen is applying the input where it’s needed, and for the Karchers, they have found that higher rates in medium vigor soils “is really where you make your money on corn,” Rob said. “That’s what you really push hard.”

The ear flex of a variety can also make a difference, according to Brandon. “In a sand blow, you might cut your seeding rate back to 22,000 plants per acre, so you want a flex-ear hybrid, so if you do get the weather, you could make something in the sand blow.”

It can also help to cut costs during dry years. “It can make zero bushels at 22,000 plants per acre just as well as it could make zero at 30,000 plants per acre and cost a lot less money,” Brandon said.

 Sharp points out that variable-rate Pix on cotton “doesn’t do much good in the absence of variable-rate nitrogen. We set up some variable-rate experiments here on this farm in 2002. We put out a flat-rate of nitrogen, and we had our variable-rate Pix perfect and the field was so uniform. But when we started plant mapping, potential yields in the high vigor zones were disappointing.”

While the plants in the high vigor zones were shorter due to the Pix, they still had too much nitrogen, Sharp explained. The next year, they added variable-rate nitrogen to the management plan “and all of a sudden everything worked.”

Every now and then, someone comes up with a use for precision ag equipment that no one had ever thought of before.

After the cold, wet spring, almost everyone was running late in 2013, including Sharp, who was behind in writing a prescription for variable-rate cotton seed planting. Sharp, who was in route from Oklahoma to the Karcher’s farm in Somerville,  pulled over into a McDonalds parking lot, wrote the prescription, picked up the free Wi-Fi there and emailed the prescription to the Karchers. The Karchers sent the prescription to the planter tractor which was equipped with Raven’s Slingshot technology, which provides connectivity to cell phones and Internet. It gave a whole new meaning to the McDonald's slogan, “You deserve a break today.”

The Karchers and Sharp see variable-rate applications as part of a farming system. “It’s not a set of single practices. You need to variable-rate other inputs,” Sharp said. “You have to tie it all together, variable-rate seeding, fertilizer, nitrogen, Pix and fertility.”

“Generally the cost savings is on the fertilizer,” Brandon said, “especially as expensive as fertilizer and lime is. The savings will pay for the equipment because you’re not applying fertilizer to entire fields. If you can limit an application to 15 acres or 16 acres out of a 60-acre or 70-acre field, you save a lot of money.”

While cost savings from variable-rate applications are debatable among farmers, the Karchers believe strongly in the technology. “Somebody asked me if there was any consideration to going back to the old way of farming,” said Rob. “I don’t see that.”

Neither does Brandon, who sees variable-rate application as a way to fine-tune his management. “Variable-rate is site specific management that takes you from managing fields down to managing acres.”