Variable-rate planting, or changing populations within a field, has been done successfully on farms and in research plots for several years. Now variable-variety planting - switching hybrids and varieties within a field - is starting to sprout.

And it may be more valuable than changing seeding rates on the go.

Two years of Pioneer research throughout the Corn Belt showed that variable-variety planting can bump up returns by $10-25/acre for corn and beans.

"There is some potential for variable-variety planting," says Tom Doerge, Pioneer's precision farming agronomist in Des Moines, IA.

However, Doerge cautions that the payoff depends on several factors. First, soil properties and yield potential in the field must vary. Conditions within fields that cause significant yield differences include the presence or absence of pathogens that cause white mold, stem rot or phytophthora root rot, and differences in drainage or topsoil depth. For example, a drought-tol erant hybrid or variety could be planted in the drier part of a field to maximize yield.

Another example of predictable differences within fields is the western Corn Belt's alkaline soils. Alkaline soils, above pH 7.0, can induce iron chlorosis. But University of Nebraska research shows that benefits from variable-variety planting to ward off chlorosis may be difficult to achieve, Doerge says. Chlorosis severity depends on seasonal weather patterns, which aren't predictable.

Researchers are also examining ways to identify phytophthora and soybean cyst nematode (SCN) hot spots and match varieties to those areas of a field. In 1998, Purdue researchers tried to find out if they could maximize yield by varying varieties in an SCN-infected field.

Although resistant varieties can reduce yield losses, some performance trials show they have a yield lag in the absence of SCN.

The researchers' goal was to plant a resistant variety in infested areas and a susceptible variety in the rest of the field. They used an experimental drill, says Terry West, Purdue research agronomist. The drill had two hopper boxes, each feeding 12 rows. The susceptible variety was Pioneer 9352; the resistant variety was Pioneer 9362. Assisting in the project were Gary Steinhardt, extension agronomist, and Ellsworth Christmas, extension soybean specialist.

With good growing conditions in '98, there were no significant yield differences between the resistant and susceptible varieties.

"Under stress, like we had last year, we'd have observed greater yield differences," says Christmas.

Also, the resistant variety may not have had resistance to the particular race or biotype in the field, Christmas speculates. The field was rotated to corn in '99 and the experiment wasn't repeated.

"Planting resistant varieties in the hot spots appears to be beneficial, but it's not clear that planting susceptible varieties in the rest of the field can maximize yield potential," says West. "The take-home message is that you have to sample for SCN. In our research case, the whole field was infested at some level."

Another challenge is mapping the field for SCN levels. "Do you grid sample, visually observe or base it on history?" asks West. A visual observation isn't accurate. "Soybeans can be infected without any visual effects," he says.

Interest in variable-variety planting is spreading. Jim Bell, Clarksville, TN, sees potential advantages in his fields, which range from rich bottom ground to rocky hills.

"If the technology was available to vary varieties on the go, and the seed was in the same maturity group, there are possibilities of some real benefits," says Bell.

He has been using a yield monitor and variable- rate planting in corn for several years. "Certain varieties and hybrids perform better on the different classes of ground," says Bell.

The problem is proving which hybrids and varieties work best where.

"By the time we get three years of data, the variety is gone," says Greg Lyon, of Opti-Crop, a crop consulting firm in Owensboro, KY. Instead, Lyon relies on seed company recommendations.

Pioneer's Doerge admits that once farmers become familiar with seed performance in their own fields, the hybrid or variety may no longer be available. His advice is to work closely with your seed company rep.

And before you start digging into variable-variety planting, know and understand why the yields differ, experts advise.