Does the Rootworm Bt Trait Improve N Use and Productivity of Corn?.

Corn hybrids possessing transgenic resistance to corn rootworm (Diabrotica spp.) feeding are becoming increasingly valuable in corn production. Although the grain yield differences between rootworm protected hybrids and their null counterparts can be quite dramatic, the specific physiological mechanisms by which these yield increases result have not yet been fully characterized. Clearly, suppression of rootworm larval feeding protects the investment that the plant has made in the root system and maximizes critical root functions such as water uptake and nutrient acquisition. Increased uptake of nitrogen (N) in particular might explain the increased yield attributable to transgenic rootworm protection strategies.  Our hypothesis is that rootworm protected transgenic hybrids exhibit increased N uptake, resulting in higher grain yield and improved N use efficiency (NUE) relative to their non-protected counterparts.

In 2008, two transgenic rootworm resistant (RW) hybrids along with their near-isogenic (non-RW) hybrids were evaluated at Champaign, IL.  These hybrids were grown under five levels of fertilizer N availability ranging from 0 kg ha^-1 – 270 kg ha^-1.   Averaged across N rates, the RW hybrids produced approximately 1.4 Mg ha^-1 more grain relative to their near-isogenic counterparts.  Rootworm pressure in 2008 was generally low; root node injury scores for the hybrids were approximately 0.10 (Iowa State University 0-3 rating scale).  The remarkable yield improvements along with apparently minor rootworm feeding suggest that even slight root damage can have a profound influence on grain yield.  Harvest indices for non-RW and RW hybrids were equivalent indicating that increased grain yields resulted from greater total biomass accumulation.  Averaged across N rates, RW hybrids increased total biomass accumulation at R6 by approximately 10% relative to their near-isogenic counterparts.  There was a marked improvement in fertilizer N use efficiency from the RW trait in both hybrid pairs that was primarily a result of an improvement in N uptake efficiency which averaged 55% in non-RW hybrids and 71% in RW hybrids.  Although the RW trait protected grain yield at all levels of N fertilizer, the N rate needed for maximum grain yield was not necessarily lower than that needed for non-RW hybrids suggesting that rootworm protected hybrids protect the investment in N fertilizer but might not reduce its use.