A recent trend in the UW Corn Performance trials is the lower frequency of normal dent corn hybrids yielding above the trial average (Figure 1). From 1973 to 1996, normal dent corn hybrids as a group were distributed around the trial average in a 50:50 ratio, i.e. 50% of the normal dent corn hybrids were above the trial average and 50% of the hybrids were below the trial average. Beginning in 1997 and continuing through 2003 the frequency with which normal dent corn hybrids yield above the trial average has been decreasing. In 2003 only 40% of the normal dent corn hybrids yielded above the trial average. There are still quite a few normal dent corn hybrids that have excellent performance and are the top hybrids in a trial, but greater care must be used when selecting normal hybrids.

There are at least two possible reasons that might explain this observation. First, the Bt trait is being incorporated into the best genetics available. Normal dent corn hybrids are being replaced by hybrids with the Bt trait. For the first time more transgenic hybrids were entered into the UW Performance trials than normal hybrids. Most transgenic hybrids that perform well have the Bt-ECB gene (events Mon810, Bt11, or Cry1F-Hx) incorporated into the hybrid (Figure 2). About 53 to 59% of these hybrids perform above the trial average. When stacked with other traits, the stacks perform well also, except for Bt,IMI. Other transgenic events including Roundup Ready (events MonGA21 and Nk603) and Liberty Link (event T25) have had variable performance over the years.

Another possible reason for greater than expected performance of Bt hybrids is that low undetected European Corn Borer pressure may exist in the trials and is reducing the performance of normal dent corn hybrids. The latter reason, although a possibility, is unlikely since the trials are scouted every 2 to 3 weeks and little ECB damage is observed.

For the first time this season we evaluated Bt-CRW (event Mon863) hybrids. These hybrids yielded above the trial average only 36% of the time.

It is imperative that adapted high-performing normal dent corn hybrids remain available to corn farmers. Refugia of 20% of the corn acreage on a farm is required by EPA and must be maintained in order for Bt transgenics to continue to be a farm field tool for controlling ECB. In Wisconsin, 20% of the acreage represents 680,000 acres that must be planted to normal dent corn (USDA, 2001 Statistics).