Fall armyworm, Spodoptera frugiperda (J. E. Smith), and corn earworm, Helicoverpa zea Boddie, perennially cause leaf and ear damage to corn for silage production in the southeastern United States. Transgenic hybrids expressing the Cry1Ab (MON810 event) insecticidal endotoxin from Bacillus thuringiensis (Bt) were evaluated for management of fall armyworm and corn earworm in central Georgia during 2006 and 2007. Hybrids with a temperate or semitropical background were planted at the recommended time in mid-April and in late June to simulate a double-crop corn planting. Whorl infestation and damage by fall armyworm was significantly reduced in hybrids with the Bt trait when infestations were large. Fall armyworm infestation levels and damage were similar in both temperate and semitropical types. Hybrids with the Bt trait also had a small reduction in ear infestation and less kernel damage in ears infested by corn earworm than susceptible hybrids in most trials. Corn earworm infestation level were less in the semitropical than the temperate hybrids in 2006 but were not different in 2007. Silage yield was not significantly different among hybrids with and without the Bt trait in the first planting in both years. The Bt trait prevented significant yield loss of 17.1% during the second planting in 2006 when fall armyworm whorl infestations exceeded 39% in susceptible hybrids, but did not significantly affect silage yield in the late planting in 2007 when fall armyworm infestations were low. Temperate hybrids yielded more than semitropical hybrids in the early planting, but the semitropical type tended to perform better in the late planting. Corn silage quality as measured by neutral detergent fiber, acid detergent fiber and crude protein content did not differ among hybrids with or without the Bt trait indicating silage dry matter yield was the main silage component affected by the Bt trait and insect damage.
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