European corn borers were almost a non-event in most areas last summer. Yet Bt hybrids still showed a yield advantage on many farms.

Most company and university trials showed a yield benefit for Bt corn, although in some cases not enough extra corn was harvested to pay the technology fee that comes with Bt seed.

In Iowa State University comparisons covering 16 counties across the state, Bt hybrids averaged 162.6 bu/acre vs. 159.7 bu/acre for non-Bts of similar genetics. That rounds off to a 3-bu/acre average benefit.

Other comparisons revealed a range from negative up to a 10-bu/acre edge for Bt corn.

"Our 1998 data from 580 Midwestern locations showed an average yield benefit of 4.6 bu/acre for Bt hybrids over their isolines (non-Bt counterparts)," reports Dave Dornbos, director of product development for Novartis. "The range was from 0.8 to 9.5 bu/acre."

Monsanto carried out 1,120 comparisons of hybrids with YieldGard technology vs. their isolines. The tests were in all Midwestern states, plus Texas, Delaware, Pennsylvania, Maryland and Ontario.

"Overall, there was a 2.4-bu/acre yield advantage for the Bt hybrids," says Monsanto's Dan Holman.

The range was from -3 bu/acre in Ontario to +7.7 bu/acre in South Dakota.

Randy Ganschow, a certified crop adviser affiliated with Meriden Grain, Mendota, IL, says a seed company in his area did wide-scale comparisons of Bt hybrids vs. non-Bts (not isolines). The Bt corns averaged about a 7-bu/acre advantage.

Why the Bt yield bump in a year with light corn borer pressure?

"There were two reasons," answers Novartis' Dornbos.

"First, corn borers were out there - especially later generations - at a lower intensity than the two previous years. And because it was a lighter infestation, they were less obvious."

A second factor was that certain Bt hybrids also can control or suppress other insects, such as Southwestern corn borers, corn earworms and fall armyworms, Dornbos explains.

Last year's results show that the yield drag present in early Bt hybrids is disappearing, points out Iowa State agronomist Dale Farnham.

"Bt hybrids outperformed their non-Bt counterparts in 61% of our comparisons. That data clearly shows that, even without corn borer pressure, Bt hybrids yield as well if not better than their non-Bt counterparts."

Although Bt hybrids did not always produce a positive return on investment in 1998, they should more than pay for themselves on a long-term basis. That's according to an extrapolation by Monsanto.

The company's representatives used University of Illinois corn borer population figures for each year from 1986 through 1996. They assumed a yield level of 150 bu/acre, a 5.5% yield loss per borer per plant from first-generation corn borers, and a 3% loss per borer per plant from second-generation borers.

They used a YieldGard technology fee of $10 an acre and a $2.50 corn price.

Based on those numbers, they concluded that YieldGard technology would have had an 11-year average return of $9.78/acre.