© 2010 Plant Management Network. Accepted for publication 2 September 2010. Published 22 October 2010. Assessing Bt Silage Corn in Maine John Jemison, Extension Professor, University of Maine Cooperative Extension, Orono, Maine, ME 04469; and Chris Reberg-Horton, Assistant Professor, Department of Crop Science, North Carolina State University, Raleigh, NC 27695 Corresponding author: John Jemison. jemison@maine.edu Jemison, J., and Reberg-Horton, C. 2010. Assessing Bt silage corn in Maine. Online. Crop Management doi:10.1094/CM-2010-1022-01-RS. Abstract Over 90 percent of the corn grown in Maine is grown for silage, yet most research has focused on the effectiveness of Bacillus thuringiensis (Bt) corn as grain corn. In response to grower interest, a seven site-year-location (SYL) study was conducted across the dairy production region of Maine to evaluate the effect of Bt corn on insect feeding damage, silage corn yield, mycotoxin content in chopped silage, and forage quality. Black cutworm damage over the course of the study was generally less than 1%, and European corn borer (ECB) damage was light to moderate with stalk tunneling between 2% for Bt hybrids and 11% based on non- Bt hybrids. Although Bt significantly reduced ECB feeding in leaves and stalks, this did not lead to yield, silage mycotoxin, or forage quality differences. Based on these results, potential yield increases and reduced mycotoxins in silage would be expected only in years with greater insect pressure than that found in this study. Introduction Several types of transgenic silage and grain corn hybrids that express Bacillus thurin g iensis (Bt) insecticidal proteins have been used in the USA in most states since 1996; Maine approved their use in 2006. The primary Maine pests that Bt proteins are used to control include European corn borer ( Ostrinia nubilalis ), black cutworm ( Agrotis ipsilon ), and to a lesser extent corn rootworm ( Diabrotica sp). In previously published research, corn yield was influenced primarily by intensity of insect pest pressure, environmental conditions, and crop rotation p atterns (7,12,13). While most research has shown Bt corn to be economically effective with grain corn under moderate to high p est pressure (4,6,7,10), less research has been conducted evaluating the effectiveness of Bt corn grown for silage corn. Over 90 percent of the corn grown in Maine (and much of northern New England) is grown for silage, and combined with hay it makes up the primary feed source for dairy cattle ( Bos taurus L.). European corn borer (ECB) overwinters in Maine. The intensity of pest pressure and the amount of damage to yield and quality from larvae tunneling in the stalk or feeding in leaf tissue varies from year to year (1). Leaf holes and stalk tunneling may also allow plant pathogen entry, possibly leading to mycotoxin concentrations toxic to cows (8,17). Black cutworm (BCW) does not overwinter in Maine; adults moths move into Maine on storms from the south. For this reason, BCW pressure is variable from year to year, but damage can be sufficiently high to force growers to replant fields. While northern corn root worms Diabrotica barbari (NCR) and western corn rootworm ( Diabrotica virgifera virgifera ) (WCR) are found in Maine, the weaker and less damaging NCR is much more common. Damage is generally less than that found in the mid-Atlantic or midwestern states (9,15). Grower decisions to adopt Bt corn are based on seed cost, yield benefit, and forage quality compared to non-Bt hybrid lines (2). No studies on the effectiveness of Bt corn in silage systems have been produced from the New En gland region. Based on grower interest, seven exp eriments were conducted in the dairy production region of Maine from 2006 through 2009 to evaluate Bt corn hybrids and their near isolines for plant damage, yield, quality, and mycotoxin content. 22 October 2010 Crop Management