Not many growers who've planted Bt corn would dispute its effectiveness at controlling European corn borers.

Nationally, some reports say Bt corn yields have averaged about 7 bu/acre higher than the average of non-Bt hybrids. Even at $1.50/bu, that's a $10.50/acre advantage before the technology fee is deducted.

But that's not the entire story, says Clint Pilcher, an Iowa State University research entomologist.

In 1997, Pilcher and Iowa State extension entomologist Marlin Rice planted 16 Bt hybrids from five seed companies at 14 locations around the state. They planted only adapted hybrids at each location, so not every hybrid was planted at each site. To measure the value of the Bt gene, they also planted seed from most of the same hybrids without the Bt gene.

When yield data from all sites was averaged, the Bt hybrids had a 9.6-bu/acre advantage.

The highest Bt corn yield in the plots was 186.7 bu/acre, compared with the highest non-Bt yield of 173.3 bu/acre. The lowest Bt yield was 119.6 bu/acre, compared with the lowest non-Bt yield of 112.7 bu.

After looking at all the yield data from the 14 sites, Pilcher and Rice isolated data from northwestern Iowa, where corn borer pressure was highest in 1997, and from northeastern Iowa, where pressure was the lowest. In northwestern Iowa, the Bt hybrids averaged about 11 bu more than their non-Bt counterparts. In northeastern Iowa, the difference was slightly more than 2 bu.

Pilcher says what growers need to take away from this is that not all Bt hybrids have the same genetic yield potential.

Dale Farnham, Iowa State extension corn specialist, says the study reaffirms the best advice corn growers have been given for years: "The first thing a grower needs to consider is yield."

He recommends checking state and regional yield trial results from universities, independent groups like corn grower associations, and seed companies.

"After you've chosen a few of the top yielders for your maturity group, look at individual traits important to you," suggests Dale Hicks, University of Minnesota extension corn specialist.

Hicks says a grower doesn't necessarily need to try a hybrid on his own farm before planting a sizeable acreage to it.

"On-farm test plots can be helpful, but growers need to look at more data than they can get from one plot in one year," Hicks says.

Most seed companies can provide information on how hybrids have performed according to tillage, planting systems, severity of leaf and stalk diseases, insect pressures, weed pressures and soil types.

Hicks says growers who insist on planting Bt hybrids, or hybrids with resistance to specific broad-spectrum herbicides, may be passing up the highest yielding hybrids adapted to their areas.

"You may be missing a hybrid that could put more money in your pocket," he cautions.

Hicks and Farnham agree that there are some disease- and insect-resistance traits growers need in their corn hybrids. If you often end up treating for European corn borer, or frequently suffer serious yield losses from the pest, Bt is probably one of them.

"If, on your list of high-yielding hybrids, there are one or two with the Bt gene, it's probably worth planting some, particularly if you know you're losing some yield to European corn borer," Farnham states.