Extra income of $60-87-plus/acre. That's what Grant Webber gains by using variable rate application (VRA) to get fertilizer where it belongs on corn, based on $3.50-bu. corn.

The Sublette, KS, grower combines VRA fertilizer treatments and seeding with an auto-steer GPS system to get the most out of his strip-tilled corn, which is mostly under center pivot-irrigation.

“I get a 10% or more increase in yields from the VRA fertilizer program,” says Webber, who also uses VRA for planting. “Our corn performs better with it.”

SITE-SPECIFIC applications of chemicals have been around since the late 1920s. Variable-rate technology has emerged along with everything else electronic on the farm. Yield monitors are a must for most growers. GPS auto-steer systems, whether low-range units with a 4-in.-span accuracy or RTK's (real time kinematic) sub-inch accuracy, are more affordable to more growers than they were three or four years ago.

Shotgun spraying techniques are less efficient than precision applications. University of Illinois agricultural engineer Lei Tian, who helped develop an early VRA sprayer in the late 1990s, says along with fertilizer, growers are seeing the advantages of herbicide and other applications with sensor-run sprayers.

“A sophisticated application delivery system, which applies herbicide where weeds exist and shuts off where there are no weeds, can place herbicides more effectively,” Tian says.

Webber uses an Ag Leader Technology Insight VRA system. He just finished his second year with it, as well as his first year using a Trimble RTK auto-steer system after using a 2-4-in. accuracy GPS system for three years on his strip-tilled acres.

He's also used a yield monitor for eight years on a John Deere 9760 series combine. He produces field maps from those eight years of data, divided into management zones. Based on the yield results and detailed soil samples in 2½-acre grids, he can use the maps to develop prescription fertilizer programs.

“When we prepare the management zones and fertilizer recommendations, we load the data onto a computer card,” he says, “which then goes into the Insight monitor.”

Attached to the Insight monitor are a flow meter and servo control valve controlled by the data card. They distribute nitrogen (N) in the form of anhydrous ammonia and/or a liquid blend of N, phosphorus, sulfur, zinc and other micronutrients during strip-tilling.

THE INSIGHT SYSTEM also controls a centrifugal pump that provides the correct amount of pressure to different portions of a field, again based on the data loaded into the system.

The system sounds complex, but Webber keeps it simple with three basic levels for fertilizer applications. “We go with a low, medium and high level, based on yields from different parts of a field,” he says.

For example, his low-yielding corn normally produces 180-200 bu./acre. Medium yields are about 225 bu. High yields are in the 250-260-bu. range.

“We usually apply 1 lb. of N for every bushel we expect to yield per acre,” says Webber. “Soil samples tell us how much residual N is there and how much we need to apply, according to the average yield of any part of a field.” The previous crop usually leaves 30-50 lbs. residual N, he says.

Webber has not figured a specific dollar advantage for the VRA program, but sees yield increases of 10% or more.

“That's because we are more efficient in getting our fertilizer where it can benefit our corn the most,” he says, adding that the “first field where we had variable rate happened to be the highest yield ever at the time (over 260 bu.).”

At a price of $3.50/bu., a 10% increase in the 180-200/bu. yield generates an additional $60-70/acre, based on 18-20 more bu. Add 10% to 250 bu. and the extra return is $87-plus.

Webber plants in 30-in. rows. And just like with fertilizer, he counts on VRA for seeding.

“We use the same information on yields and soil fertility for planting,” he says. “The low-yielding areas have a seeding rate of 29,000/acre. The medium-yielding areas receive 32,000. The high-yielding ones receive 34,000.”

The Insight system features a 10.4-in. color touch-screen display, which allows Webber to view on-the-go information as he moves through the field. If needed, he can generate color maps while planting to better monitor the location of each hybrid or variety. It displays maps of prescriptions and resulting applied rates.

Webber notes that he could create more than three management zones if needed. “We've tried doing more zones,” he says. “But there comes a point where there is just so much data you can look at. We feel that the low, medium and high system works well enough for us.”

The Ag Leader system is among several VRA systems on the market. The InTime system (www.gointime.com) is among those that enable growers to use aerial imagery to develop prescription application programs.

The TeeJet products (www.teejet.com) provide a wide range of precision application tools. John Deere, Case IH and other companies also offer VRA systems.

University of Illinois' Tian says that for weed control, VRA systems can help growers prevent having to broadcast herbicides on an entire field without regard to the variability of weed populations.

“This practice results in areas being sprayed where no or few weeds exist,” he says. “They receive just as much herbicide as areas with a high weed population. But a VRA practice would result in a lower environmental burden and increase agricultural profitability.”