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.”