Kriss Lightner of Lohrville, IA, has been practicing precision agriculture for 15 years -before grid sampling, Soilection machines and GPS became available.

"We have always done spot fertilizer spreading based on soil type and fertility maps and what we know about the land from our own experience," he says. "How closely you match your inputs to the soil's needs will make a big difference in your end results."

When variable-rate fertilizer applications became available through his local cooperative three years ago, Lightner was quick to take advantage of the program. Then last year he added 160 acres of variable-rate herbicide applications, with good results. The organic matter levels in his field ranged from 2.1 to 4.2%, and his herbicide application rates varied from 2 to 2.6 pints/acre.

"Our variable-rate strips ended up costing a little less per acre than our constant-rate strips, and control was very good," Lightner says. "The results are certainly good enough that we will continue with the program."

More efficient. Variable-rate technology (VRT) allows more efficient use of most soil-applied herbicides. Product labels specify lower rates for lighter, low-organic soils and higher rates for heavier, high-organic soils. VRT weed control simply reflects these differences within a field, usually based on soil sampling of 1- to 4-acre grids.

The overall amount of herbicide used in a variable-rate program may go up, down or stay about the same, depending on the field, the variable-rate formula used, and how the previous "fixed" rate was determined. But in any field with considerable soil variability, weed control should be more consistent because the herbicide is redistributed to where it is needed most.

Dirk Drost, technical business leader for Zeneca Ag Products, says that Zeneca's variable-rate formulas for Surpass, TopNotch, FulTime and DoublePlay herbicides typically produce a slight savings in overall herbicide used.

"But if you've been setting the fixed rate for a field based on the lighter soils in that field, you might end up using a little more herbicide, overall, with a VRT application," he says. "You should see more consistent control across the field, without the patches of weed pressure that sometimes break through in areas of high organic matter."

No breakouts. That was Harlen Reynolds' experience. The Auburn, IA, farmer has two years of experience with VRT fertilizer and weed-control applications. His field's organic matter levels ranged from3 to 5.1%, and his Surpass application rates varied from 2.1 to 3 pints/acre. "Zeneca's formula looks like it's right on target, and so far it's proving to be cost-effective," he says. "I'm starting to see a difference in weed control in my VRT fields. I can see areas that are clean now that used to have breakouts."

"For long-range planning, it makes sense economically," he adds. Reynolds' cooperative, like many dealers offering precision-farming services, grid-samples fields only every four years, thereby spreading out the cost.

Tom Christensen of Farmer's Co-op in Somers, IA, helped both Reynolds and Lightner develop their VRT weed-control programs. He sees two main advantages to variable-rate weed control. "VRT application of herbicides is positive for the grower because we're putting the correct rate on every particular piece of ground on that farm, so we're likely to see much better weed control," he explains. "It's also environmentally sound in that we're not putting any more herbicide than is required for maximum weed control."

Some growers believe precision farming only fits on large operations. But Jim Sayers of Humboldt, IA, points out that the goal of precision farming is efficient production on a per-acre and per-bushel basis.

"I'm a part-time farmer, and VRT works well for me because I have most everything custom-applied anyway," he says. "It costs a little more, but it's worth it if yield is increased. And it just makes more sense to me to put more fertilizer and herbicide where they're needed, and less where they're not."