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Bt Corn Hybrid Makes Top Silage Yield in Virginia Trial

Crop and Soil Environmental News, July 1997

Rod Youngman, Curt Laub, and Tom Kuhar
Department of Entomology
Virginia Tech
Blacksburg, VA 24061

The European corn borer is one of the most serious pests of corn grown in the United States. Although this pest occurs throughout Virginia, serious economic damage varies from one season to the next and tends to be sporadic across the corn growing regions of the state. As many as three European corn borer generations occur per year in Virginia; however, the second generation typically causes the greatest economic damage to corn. The larvae of second generation European corn borer damage corn primarily by tunneling in the stalks and ear shanks. Stalk tunneling reduces nutrient uptake and leads to lodged or broken stalks and ear loss. For more information on the life history and damage associated with this pest, please refer to European corn borer fact sheet, Virginia Cooperative Extension publication 444-232.

In 1996, we compared stalk damage, silage yield, and nutritional characteristics of 'Mycogen 6821' (a Bt corn hybrid) with two non-Bt hybrids under natural European corn borer infestation at the Virginia Tech Dairy Farm. The two non-Bt corn hybrids in our study included 'Pioneer 3394,' a popular, high-yielding corn hybrid for Virginia, and 'Mycogen 5790.' 'Mycogen 5790' is genetically similar to 'Mycogen 6821,' but without the Bt gene construct NatureGardTM. We recorded European corn borer feeding damage (entrance holes and tunnels), larval and pupal numbers, and silage yield data. Nutritional analyses also were conducted on both fresh forage and silage corn.

All varieties were planted 20 May 1996 in 10 x 60 ft plots (4 rows wide) and replicated four times in a randomized complete block design. An Almaco, 2-row Max-Emerge planter was used to plant the plots at a population density of 26,100 seeds/acre. Force 1.5G insecticide was applied at 0.13 lb AI/acre in a T-band at planting. On 21 June, Accent 75% was applied at 0.031 lb AI/acre to control johnsongrass and pigweed. On 20 August, soft dough stage corn was sampled for damage and presence of European corn borers. Twenty stalks from the outer 2 rows of each plot were removed and examined for European corn borer entrance holes. Stalks with holes were dissected and tunnels were measured for length and inspected for larvae and pupae. On 24 September, 60 row feet of corn was hand harvested from the inner two rows of each plot. Stalks were weighed in the field and then chopped into 1- to 2-inch pieces using a Troy Bilt® Chipper/Shredder. For each plot, a 25-lb subsample of chopped plant material was fermented for 28 days in a miniature silo; and a 2.2-lb subsample was dried at 140oF for 48 hours to determine percent dry matter. The dry matter (greenchop) and fermented subsamples were analyzed at the Virginia Tech Forage Testing Laboratory for crude and soluble protein, acid and neutral detergent fiber, lignin, and minerals (Ca, P, Mg, K, Na, S, Zn, Cu, Mn, and Fe).

Marked differences in European corn borer pressure were detected between the Bt hybrid, 'Mycogen 6821', and the non-Bt hybrids: 'Mycogen 5790' and 'Pioneer 3394' (Table 1). The Bt hybrid experienced the fewest entrance holes and tunnels, and no larvae or pupae were found in any of the examined stalks. The differences in European corn borer infestation among the hybrids certainly contributed to the observed differences in yield. The Bt hybrid produced over 47% more silage dry matter (9.9 tons/ac) compared to 'Pioneer 3394' which yielded an average 6.7 tons/ac. Silage dry matter yield in the other non-Bt Mycogen hybrid (i.e., '5790') was intermediate at 8.0 tons/ac.

Table 1. European corn borer (ECB) damage evaluation and silage yield of one Bt and two non-Bt corn hybrids in Virginia.

Mean numbers per 20 corn stalks Mean silage
dry matter

(approx. 1" long)
(> or = 2" long)

Pioneer 3394 (non-Bt)31.3 a 21.3 a 5.3 a 2.8 a 1.06.7 b
Mycogen 5790 (non-Bt)12.3 b 4.8 b 3.5 a 0.3 b 0.38.0 ab
Mycogen 6821 (Bt)2.3 b 0.3 c 0.0 b 0.0 b 0.09.9 a

Means within a column followed by different letters are significantly different (LSD, alpha = 0.05).

Analysis of the dry greenchop forage indicated no significant differences among varieties for proteins, ADF, Lignin, NDF, or any minerals except copper, which was significantly higher in the Bt hybrid 'Mycogen 6821' (5.2 ppm) compared to 'Mycogen 5790' (3.0 ppm). The 5.0 ppm copper level detected in 'Pioneer 3394' was not significantly different from that detected in 'Mycogen 6821'. Analysis of the ensiled forage material indicated significantly higher soluble protein in 'Pioneer 3394' compared to both Mycogen lines (Table 2), and significantly higher lignin in 'Pioneer 3394' compared to the Bt hybrid. Several minor differences in Ca, Mg, and S also were detected (Table 3). Although we cannot be certain, the lower soluble protein level in the Bt hybrid may in part be due to the physiological cost of producing the Bt toxin in the resistant corn hybrid. Nevertheless, the yield advantage of the Bt hybrid under the conditions of our test more than compensated for the differences in soluble protein levels.

Table 2. Soluble protein and lignin analysis of ensiled corn hybrids.

Corn hybrid Soluble protein (percent) Lignin (percent)

Pioneer 3394
2.7 a 5.4 a
Mycogen 5790
2.5 b 5.0 ab
Mycogen 6821 (Bt)2.4 b 4.5 b

Table 3. Mineral analysis of ensiled corn hybrids.

Corn hybrid Ca (percent) Mg (percent) S (ppm)

Pioneer 3394 (non-Bt)0.10 b 0.11 b 995 a
Mycogen 5790 (non-Bt)0.14 a 0.13 a 933 b
Mycogen 6821 (Bt)0.14 a 0.12 ab 1,000 a

These results indicate that the Bt corn hybrid, 'Mycogen 6821,' did a superior job of suppressing European corn borer compared to the other corn hybrids in our test. The very low number of entrance holes suggests that Bt corn is killing corn borer larvae before they move from leaves and try to enter the stalk. Surprisingly, fewer entrance holes in the 'Mycogen 5790' non-Bt hybrid compared to 'Pioneer 3394' indicates some natural resistance to European corn borer in 'Mycogen 5790.' The reduced amount of corn borer damage to the Bt corn appears to translate into a substantial silage yield improvement over the non-Bt varieties tested. Additional research on the efficacy of the Bt transgenic technology in commercial corn hybrids at managing European corn borer in Virginia is ongoing, and is being supported by the Virginia Corn Board.

Trade names are used in this publication for information purposes only. Virginia Cooperative Extension, Virginia Polytechnic Institute and State University, and Virginia State University do not warrant those mentioned nor do they intend to imply discrimination against those not mentioned.

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