Some BT hybrids have shown powerful control of European corn borer (ECB) and outstanding protection against yield loss. In 1997 university trials, Bt hybrids yielded 5 to 40 bu./acre more than non-Bt counterparts. But buying the right hybrid is key.

What the universities report "We had a non-Bt hybrid that beat most of the other Bt corns with a 170 bu./acre yield. That same hybrid with the Bt gene yielded 200 bu./acre," says Kevin Steffey, University of Illinois entomologist. "It's absurd to say Bt corn is better than non-Bt corn. It depends on the hybrid and the growing environment each season."

In University of Minnesota trials, Bt hybrids performed well, yielding 12.6 bu./acre more than their non-Bt counterparts on average across multiple locations. In the western part of the state at Morris and Barnesville where ECB pressure was high, Bt corn hybrids yielded 20 to 35 bu./ acre more than non-Bt isolines. However, in two southern locations, one Bt hybrid yielded from 19 to 20 bu./acre less than the average of all non-Bt hybrids in the trials. "Not all Bt hybrids offer the same yield advantage," says Ken Ostlie, extension entomologist for the university.

John Foster, University of Nebraska extension entomologist, agrees: "Bt hybrids were among the highest-yielding and also among the lowest-yielding hybrids in our study of 21 different hybrids. Some of the highest-yielding hybrids did not have the Bt gene." The study included Bt hybrids, their near isolines and conventional corn. ECB levels were low. In another strip trial study conducted on Nebraska farmers' fields, Bt hybrids consistently yielded more than their non-Bt counterparts. At one farm in St. Edwards, the yield advantage for Bt hybrids ranged from19 to 44 bu./acre.

Foster advises growers to select from the top tier of hybrids. "Growers should choose hybrids as they always have, based on a whole package of traits. Disease resistance, relative maturity, dryland vs. irrigated performance and insect resistance all play a role," says Foster. "No one hybrid is the best in every field or every year. There are a lot of good hybrids to choose from, both Bt and non-Bt."

Steffey adds, "Look at a whole lot of data from as many sources as possible. Don't rely only on seed company data and don't rely only on one university trial."

One problem, however, is that few companies have entered their Bt hybrids in the standard state variety trials, instead preferring separate trials comparing only Bt hybrids and their isolines. Companies are concerned that, in small plot trials, Bt pollen will drift onto adjacent rows of conventional corn, giving it insect protection that it would not otherwise have and narrowing the yield advantage for the Bt corn. Regardless, farmers and university agronomists hope that more companies will enter their Bt varieties in the standard yield trials in 1998 so they can make better hybrid decisions in 1999.

What the companies report Novartis Seeds the most trials and years of testing Bt hybrids. With more than 4,100 comparisons between 1994 and 1997 across all environments, the yield advantage of NK Bt hybrids compared to near isoline non-Bt hybrids was 8.6 bu./acre. Under low ECB pressure, the NK Bt yield advantage was 6.8 bu./acre; under moderate pressure, it was 10.4 bu./acre; and under severe pressure, it was 17.6 bu./acre. (These data include both YieldGard and KnockOut Bt hybrids. See sidebar.)

Monsanto reports that in side-by-side comparisons by seed companies in 1997, YieldGard hybrids yielded on average 15.7 bu./acre more than non-Bt hybrids, nationally. "Even in areas with light infestation of ECB, YieldGard Bt protection resulted in improved yields of six or more bu./acre, more than enough to pay for the cost of the seed," notes Monsanto entomologist Paula Davis.

Pioneer Hi-Bred International reports that, in its 1997 research trials, its hybrids with the YieldGard gene provided an average yield advantage of 14 bu./acre over untreated nonresistant hybrids if at least one corn borer cavity was present in the untreated hybrids. With two cavities per plant, the advantage was more than 20 bu./acre. Even with only 1/2 cavity per plant, the advantage for the resistant hybrids more than paid for the additional cost of Bt seed.

Mycogen Seeds reports yield advantages for its NatureGard Bt hybrids ranging from 5 bu./acre to 20 bu./acre in 1997 strip trials.

Dekalb Genetics says its Bt hybrids (including both Bt-Xtra and YieldGard hybrids) yielded 8% more (121/2 bu./acre) than their non-Bt counterparts in 1,000 side-by-side comparisons in 1997. "It varied by area and corn borer pressure," says Mark Herrmann, Bt marketing manager. "In southwestern Minnesota, we had a 10% yield advantage and in southern Michigan, it was just 4% better."

"You can't predict where corn borer pressure will be the next year. It's a good management tool to use some Bt in your hybrid package as long as it's in leader hybrids that have the agronomic characteristics you need," he adds. What the farmers report .

One-fourth of Tim Tungland's southwestern Minnesota corn ground was planted to Bt hybrids last year. "I wish it had all been planted to Bt corn. Second brood corn borer was real tough on us," he says. "The three Bt hybrids I planted yielded 20 bu./acre better than my conventional corn on average. We did have one conventional variety that did well without being a Bt corn, too." He plans to plant 75% of his corn acreage to six Bt hybrids in 1998. "It would be great if one variety would work for every field, but it doesn't work that way. We have to evaluate them ourselves in our own fields and plots," he adds.

The Ken Beaver family - Ken, Leila and their son Chris - regularly pit their crop production skills against the nation's best in the NCGA annual yield competition. And they've taken home many national titles over the years. But, in 1997, European corn borer populations were at an all-time high, thwarting their chances. "In my non-Bt corn field, the corn borers kept coming all season long, paving the way for stalk rot and standability problems," Ken explains. "But there wasn't a borer alive in my Bt field."

The Beavers ultimately captured first place in the Ridge-Till Irrigated Division of the contest with NK Brand N7333Bt and a whopping 319.9 bu./acre yield. They also won the largest cash award ever offered by a NCGA yield contest sponsor: a $100,000 purse from Novartis Seeds for national first-place NCGA titles won with an NK Bt hybrid. The Beavers only planted 20 acres of Bt corn in 1997 but will plant 75% of their acreage to Bt corn this spring. "This was the worst corn borer infestation I've seen," Beaver says. "I was totally impressed by the Bt hybrids. Still, I'm carefully selecting the best hybrids. Having Bt in a mediocre hybrid won't do me any good. Plus, we'll be running yield checks to confirm what's best for my farm."

Mark Kies, Allen, MI, tried some Mycogen NatureGard Bt hybrids in 1997, but ECB pressure was too low to notice a yield difference. He plans to plant 15% of his corn acreage to Bt corn this spring. "I'll plant Bt corn on my early planted fields and on lighter soils where a variety of stresses usually affect the crop. We'll also plant a hybrid with native ECB resistance that performed great in 1996 when we had more corn borer pressure." In Estherville, IA, Novartis dealer David Pelzer planted 1,200 acres of Bt corn. "There was not one stalk leaning in the Bt fields. It was amazing," he says. "My Bt corn yielded 10 to 15 bu./acre more than my conventional corn."

Why BT corn differs To date, there are four EPA-registered Bt events. "Event" is the term used to refer to a specific transformation event. Events differ in their gene construction and where the gene is inserted into the plant. The four Bt events and the respective companies are: 176 (Novartis and Mycogen), Bt 11 (Novartis Seeds), MON810 (Monsanto) and DBT418 (Dekalb). Events Bt 11 and MON 810 both use the YieldGard trademark. Event 176 is trademarked as KnockOut by Novartis Seeds and NatureGard by Mycogen. Event DBT418 is trademarked Bt-Xtra.

Monsanto has licensed its YieldGard technology to Cargill, Golden Harvest, Novartis Seeds (Bt 11), Pioneer, and Dekalb, and to Holden's, which provides foundation seed to many regional seed companies.

Bt 11 and MON810 (YieldGard) events express the Bt protein throughout the entire plant and offer season-long protection against multiple generations of ECB and Southwestern Corn Borer (SWCB). Event 176 (KnockOut and NatureGard) expresses the Bt protein almost exclusively in the green tissues and pollen of the plant. It offers excellent control of first-generation ECB and SWCB, very good control of second-generation ECB and no control of second-generation SWCB. Event DBT418 (Bt-Xtra) expresses a different Bt protein than the other events. It expresses it throughout all plant tissues except pollen. It offers excellent control of first-generation ECB and good control of first-generation SWCB. It provides very good control of second-generation ECB.

"We've found that hybrids with the YieldGard technology consistently control corn borers better than events without season-long protection. Whether they yield differently is a more complex question," says Kevin Steffey, University of Illinois entomologist. Monsanto claims that YieldGard hybrids consistently yield more than other Bt events - on average 9.8 bu./acre better. Mycogen, Novartis and Dekalb, companies that market other Bt events instead of, or in addition to, the YieldGard event, say their research shows slight to no yield difference between their events and YieldGard. "We aren't concerned with whether the plants look pretty late in the season, but whether yield is protected," says Dave Isenhour, DeKalb's entomologist. "Our Bt-Xtra hybrids provide yield protection equal to our YieldGard Bt hybrids."

Novartis Seeds' corn product manager Dan Hinderliter acknowledges a 1 to 2- bu./acre yield difference in yield protection between its NK YieldGard Bt hybrids and its KnockOut Bt hybrids. Hinderliter explains, "In 3,152 comparisons, NK's YieldGard Bt hybrids yielded 9 bu./acre better than isoline hybrids. The KnockOut Bt hybrids yielded 7.4 bu./acre better than isoline hybrids in 1,017 comparisons. The yield protection difference between the two Bt events averages 1.6 bu./acre across all locations and infestation levels."