Transgenic Corn Response to Nitrogen Rates under Corn Rootworm Pressure

CORE IDEAS: Bacillus thuringiensis rootworm traits and corn rootworm feeding affected N uptake and root mass. N uptake by DK44‐61 SSX at V6 lagged DK46‐60 VT3 and DK 44‐92 RR2. DK44‐92 RR2 had a larger root system at V6 than either DK46‐60 VT3 or DK44‐61 SSX. Rootworm injury limited N uptake, root mass of DK44‐92 at V12 compared to Bt hybrids. Root mass, trait efficacy, and root injury interplay with N rate have insect resistance management implications. Recommended N rates for corn (Zea mays L.) typically consider yield, economic, and environmental dimensions, but not optimal protection by Bacillus thuringiensis (Bt) Berliner‐based traits against corn rootworm (CRW) (Diabrotica spp.). This research explored N rate effects on CRW trait performance under CRW pressure. The experiment featured a split‐plot randomized complete block design with factorial treatment combinations of N rates and hybrids differing in CRW traits. Root mass, N uptake, CRW efficacy, and grain yield data were collected from two locations in 2011. Root mass at growth stage V6 (before CRW injury) increased with N rate and was greater in the Roundup Ready (RR2) hybrid (Monsanto, St. Louis, MO) by 30.6%. Nitrogen uptake was similar between VT Triple (VT3) (Monsanto, St. Louis, MO) and the RR2 hybrids, which exceeded the SmartStax (SSX) (Monsanto, St. Louis, MO: Dow AgroSciences Indianapolis, IN) hybrid by 12.5%. By growth stage V12, CRW injury in the RR2 vs. Bt hybrids reduced N uptake by 42.0%, Beetle emergence was reduced 66 to 91% by Bt hybrids. Yield response to N differed between hybrids at only one site: the CRW‐injured RR2 hybrid was unable to utilize increasing N rates while the SSX hybrid did not plateau. Recommended N rates (134–185 kg N per ha⁻¹) were sufficient to optimize root mass, N uptake, root protection, and yield under a low stress, modest CRW‐pressure environment: however, low N resulted in poor performance of these hybrids, suggesting further exploration of N rate effects when N loss or denitrification is present.