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Corn rootworm-resistant hybrids became available to growers in 2003. Favorable yields and reduced need for insecticides led growers to flock to the technology. In 2010, USDA estimates 63% of corn acres across the U.S. were planted to hybrids offering some type of insect resistance — corn rootworm resistance, corn borer resistance or both.

However, little is known about the nutrition needed to optimize yields of hybrids with the rootworm-resistant gene. That’s why researchers at the University of Illinois-Urbana-Champaign in collaboration with The Mosaic Co. compared the nutritional needs of rootworm-resistant corn hybrids to their non-resistant counterparts.

Protected from rootworm feeding, these hybrids develop more intact roots and greater root mass than their non-resistant cousins.

“We expected the rootworm-resistant hybrids would have higher nutrient uptake than their conventional counterparts,” says Fred Below, Ph.D., professor of Plant Physiology, University of Illinois.

“Results of our initial trials show that the per acre removal rates of nutrients (nitrogen, phosphorus, potash, sulfur, zinc) are from 14% to 27% greater for hybrids with the rootworm-resistant gene, which is significant,” says Below. “In fact, both the yield and the concentration of nutrients in the grain were higher for the transgenic hybrids.”

“These findings would suggest that some nutrient response curves we are using today to formulate fertility recommendations may not be valid for today’s modern genetics,” explains Dan Froehlich, Ph.D., agronomist with Mosaic.

Impact On Nutrient Recommendations

What does information like this mean as agronomists work with growers to maximize yields from new technologies such as corn-rootworm resistance? To most, the study’s findings make sense. However, selling growers on the concept varies.

“From an agronomic standpoint, the findings make sense,” says Scott Dettmann, vice president of agronomy with Ag Partners in Albert City, IA, who has read about the findings, but not seen the research data. “The assumption is you’re working with a better corn hybrid with a more massive root structure.

“It is going to be healthier. It is going to yield more, and as a result, it is going to remove more nutrients from the soil,” Dettmann relates.

Ag Partners employs a progressive nutrient-management philosophy, so Dettmann doesn’t anticipate making wholesale changes based on research such as this.

“We evaluate research findings for what they’re worth. To me, it doesn’t matter how the plant produces the additional yield. In any case, the more yield you pull off an acre, the more nutrients required to produce that yield. You’re going to have to replace those nutrients in the soil by applying fertilizer,” he says.

The Ag Partners program encourages soil sampling every four years, and more than 90% of soil tests are completed on a 2.5-acre-grid basis. Nutrient recommendations are then based on the soil test levels, crop removal, soil type and increasing yield goals.

Dettmann believes steadily increasing production levels in his northwest Iowa trade area have helped put Ag Partners ahead of the curve on nutrient recommendations and in helping growers understand the need for more and better crop nutrition programs. Most customers in his area would be disappointed with yields below 200 bushels per acre.

“When yield averages were at 160 bushels per acre, we were preparing nutrient recommendations for 180 and when they got to 180, we were pushing for 200 bushels per acre. Our customers realize the importance of crop nutrition as yields increase year after year,” he relates. In the Ag Partners’ program, increasing levels of nitrogen, phosphorus and potash are givens, while more and more attention is being paid to the need for micronutrients such as zinc and sulfur.

Greater Yield Potential

Greg Nelson, sales agronomist and chemical department manager for Larsen Coop in New London, WI, has seen high adoption of insect-resistant technology among his customers in east central Wisconsin. Nelson estimates 85% to 90% of customers plant some rootworm-resistant hybrids, and as many as 30% of customers plant the maximum acreage allowed while still complying with insect resistance management (IRM) requirements.

While he also agrees the University of Illinois research makes sense, he believes the message is challenging to deliver because of his position selling fertilizer.

“I agree if you are fertilizing for 200 bushels per acre and a corn rootworm-resistant hybrid has the potential to reach 240 bushels per acre, there obviously is a need for more nutrition that will either be drawn from the soil or required as additional fertilizer,” Nelson says. “Our customers usually want to hear this type of message from someone besides me, because I’m the one trying to sell them fertilizer. As the research becomes more widely recognized and recommendations are developed based on this information, it will be easier to convince growers to increase their nutrition programs.”

To convince skeptical customers to change their fertility programs, Nelson has often turned to on-farm side-by-side trials. He also is working to increase the number of growers who are soil testing on 2.5-acre grids and as a result applying nutrients on a variable-rate basis.

“Our soils are fairly variable and with grid-based soil test levels and yield maps we can really dial it in so we’re not wasting fertilizer on a sandy knoll or low area of the field with low production ability,” Nelson explains.

Fertility Decisions Based On Economics

Moss Fertilizer agronomist Nick Musser recognizes the potential corn-rootworm resistant hybrids have for increased nutritional needs, but in his area, he believes economics are driving the fertilizer-application decision.

“It definitely makes sense that these hybrids will have a much larger root system that will more aggressively uptake nutrients, but here fertilizer decisions are being driven by economics because of the uncertainty in the market on where the price of corn and soybeans will be for the 2011 crop,” explains Musser. “This uncertainty, plus rising cash rents and the potential of losing rented acres have some growers cutting corners again, even though they need to commit to stronger fertility programs.”

Fortunately, attractive fertilizer prices in 2010 saw 95% of Musser’s customers follow newly established yield goals and apply much needed phorphorus (P) and potash (K) to keep soil tests above critical levels and at levels high enough to support 180- to 200-bushel-per-acre yields.

“Two years ago, we saw consistently higher yields, so we made the decision with our customers to set our yield goals at minimums of 185 for corn and 60 for soybeans,” Musser relates. “Grid sampling along with yield mapping is also helping us sell higher fertility rates. It really changes attitudes when they see the soil test levels of an area that may be 40% or 50% below the field average on yield.”

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