Research studies rootworm-resistant hybrids' nitrogen use

Three years ago, several researchers began a trial at the University of Wisconsin's Arlington Agricultural Research Station to determine if higher nitrogen rates are needed to achieve the greater yield potential associated with corn rootworm-resistant hybrids.

Carrie Laboski, a University of Wisconsin soil fertility specialist, explained to farmers and crop consultants at the Aug. 31 Agronomy Field Day that many agronomists believe that higher rates of nitrogen are needed by the transgenic corn rootworm-resistant varieties. This research trial was designed to look at the nitrogen-use efficiency of the corn rootworm- (CRW) resistant hybrids compared to non-resistant hybrids.

The theory is that with a bigger root system - undamaged by rootworms - the plant would also grow larger and would require more nitrogen to feed itself. The study was run at Arlington in 2008, 2009 and 2010 where the corn plots were planted after corn year on year, in an effort to increase rootworm pressure, she said.

Each hybrid was treated with insecticide in a band and all border areas were planted to non-genetically modified corn using no insecticide as part of the design of the study, she said. The various hybrids were then fertilized with varying amounts of nitrogen - from zero to 200 pounds.

Laboski said she and her colleagues spent a lot of time selecting the hybrids for the trial and chose eight hybrids - four hybrids with and four without the rootworm resistance. The trial was aimed at measuring the relationship between yield and nitrogen need of a given hybrid. (Even with careful selection, some of the hybrids were unavailable by the end of the study, indicating how quickly they are changing, she said.)

There was some variation in yield levels and the optimum nitrogen rate between years and between hybrids, she said. The hybrids with the CRW trait had greater yields when no fertilizer nitrogen was applied, suggesting a greater ability for those corn hybrids to use mineralized soil nitrogen. However, the efficiency of these hybrids in a low-nitrogen environment did not translate into greater efficiencies when nitrogen was applied.

"When averaged over all three years, the yield at zero nitrogen for the non-rootworm hybrids is significantly less than the CRW hybrids," she said. "The rootworm-resistant hybrids were more efficient at converting soil nitrogen and more efficient at our no-nitrogen rates, but that doesn't convert to greater efficiency at higher nitrogen rates."

There was no significant difference between all the CRW hybrids and those without the trait above the zero nitrogen part of the trial, Laboski said. When specific isolines - versions of the same hybrid with and without the CRW trait - were tested, there was no clear trend for the traited hybrids to have great nitrogen use efficiency.

"There was no difference in any measure of nitrogen use efficiency when we compared all the CRW versus non-rootworm resistant hybrids. Not a lick," Laboski said.

There were some significant differences in some years, but nothing that translated into a trend. Most often those differences were not significant, she added. When differences were found, they flip-flopped between the hybrids in subsequent trials.

Laboski said the trial began in 2008 when the growing season featured a wet June and cool weather, with rain that came in deluges. In 2009 the weather was somewhat dry and cold and then in 2010 June and July offered nearly perfect growing conditions with timely rains and warmer temperatures.

The results of this study show that the CRW hybrids don't translate to significantly greater yields when nitrogen varies. "The value of true precision nitrogen management will be limited until we can more accurately estimate crop nitrogen need and soil nitrogen mineralization," she said.

"We can't keep throwing nitrogen at it all the time, we have to be conscious of the consequences to the environment," she added.

During the course of this study Laboski said that it became obvious how often hybrids change. Some of the initial hybrids they selected for the research were unavailable by the third year of the study. The research group talked about how they could have purchased all the seed necessary for all the years of the trial, but then there would have been questions about seed viability or age of the seed, she added.

Laboski, who worked with corn specialist Joe Lauer and soil scientist Todd Andraski on this trial, said the research was supported by the Wisconsin Fertilizer Research Program and the Fluid Fertilizer Foundation.