Evaluation of transgenic corn (Zea mays L.) resistant to both glyphosate and western corn rootworm (Diabrotica virgifera virgifera LeConte) in Michigan

Annual weeds and western corn rootworm (Diabrotica virgifera virgifera LeConte) (WCR) can limit corn (Zea mays L.) grain yield. With the failure of a corn and soybean [Glycine max (L.) Merr.] rotation as an effective control program for WCR, growers are relying more heavily on conventional insecticides to control WCR. Glyphosate has been used as a postemergence (POST) weed control program since the introduction of glyphosate-resistant corn hybrids in 1998. The adoption of glyphosate-resistant corn has increased. The glyphosate-resistant trait is often stacked with other resistance traits. The use of herbicide and insect resistance traits gives producers new options in pest control program. Many studies have looked at the agronomic and economic considerations of using either using insect or herbicide resistance traits in corn hybrids. However, no study has been conducted on corn hybrids containing resistance traits to both herbicides and insects. The objective of this study was to determine the consistency of conventional programs and programs using transgenic corn for control of WCR and annual weeds and to examine the profitability of these programs under a range of Michigan conditions. Field studies were conducted in 2004, 2005, and 2006 at four locations in Mid-Michigan. Sites were selected to reflect a range of annual weed and WCR densities. Treatments consisted of conventional weed management and a management program using glyphosate-resistant corn in combinations with management programs. Good weed control resulted in increased corn yields at all locations all years. Good weed control was obtained with both glyphosate-based and conventional herbicide programs. Under low WCR densities, the use of any of the WCR control programs tested increased corn yields in one of six environments. Under high WCR densities, the use of the transgenic Bt corn hybrid resulted in increased corn yields in three of six environments compared to no WCR control. In those years where WCR damage was high, all control programs resulted in corn yields greater than when left untreated, with the transgenic Bt corn hybrid consistently providing the greatest corn yields. Weed control costs were economically justified under both low and high weed densities, based on gross margins over weed control costs. Gains in gross margins relative to no weed control were reported for all weed control strategies at all locations all years. The presence and intensity of WCR larvae feeding on corn roots varied by year and was less predictable than weed density. The overall gross margin of the no WCR control program at the low WCR sites was often higher than the gross margins of the WCR programs. This indicated that, unlike weed control, the costs associated with the control of WCR in many instances was not justified. At the high WCR sites, the cost of WCR control via either Bt-corn or conventional seed or soil insecticide treatment was justified in two of six environments. In those two environments, the Bt-hybrid consistently had the highest gross margin gains relative to no WCR control. The adoption of stacked transgenic corn hybrids will likely be related to economic return associated with the control of weeds and WCR. Since gross margins were positively correlated with corn yield (r2 = 0.98), growers should focus on yield potential by choosing high yielding hybrids adapted to local growing conditions.