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Effects of nitrogen fertility on Bt endotoxin levels in Bt hybrid maize

H. Arnold Bruns1 and Craig A. Abel2

1Crop Genetics and Production Research Unit, USDA-ARS, Box 345, Stoneville, MS USA 38776, Email abruns@ars.usda.gov
2
Southern Insect Management Unit, USDA-ARS, Box 346, Stoneville, MS USA 38776, Email cabel@ars.usda.gov

Abstract

A study was conducted to determine if Bt endotoxin concentrations during reproductive growth of Bt maize hybrids are affected by different N-fertility rates used to grow the crop. Previous research has shown N-fertility rates positively affect Bt concentrations of young Bt maize plants grown in a glasshouse. Three Bt hybrids, two with Bt event MON-810 (AgriGold brand cv. A 6729Bt and Pioneer brand cv. 33V08Bt) one with Bt event DBT 418 (DeKalb 626Bt) and one non-Bt maize hybrid (DeKalb brand cv. 626) were grown at Stoneville, MS, USA in 2002 and 2003 with N-fertility rates of 0, 112, 224, and 336 kg N/ha. Tissue samples of the outer ear husks and primary ear leaf sheaths were collect at growth stage R3 and analyzed for Bt concentration. Agronomic data were collected at maturity. The concentrations of Bt endotoxin in both sets of tissue were positively correlated with N-fertility rate for the MON-810 Bt hybrids but not the DBT 418 Bt hybrid. Increases in N-fertility increased grain yields. The Bt hybrids had less lodging (0.7%-1.0%) than the non-Bt hybrid (5.1%). Adequate levels of N-fertility are important to MON-810 Bt hybrids not only for yield, but also to ensure sufficient levels of Bt endotoxin for maximum protection from susceptible insect pests.

Media summary

The levels of Bt endotoxins that cause Bt maize hybrids to resist insect damage are increased in some Bt hybrids as levels of nitrogen fertilizer used to grow the crop increase.

Key Words

Maize, Bacillus thuringiensis, nitrogen fertility, Cry proteins

Introduction

Nitrogen is a constituent of proteins and other biological compounds and if deficient, interferes with protein synthesis and growth, in general, in maize (Zea mays L.). The development and release of maize hybrids with a Bacillus thuringiensis (Berliner) (Bt) gene offers control of several important Lepidopteron insect pests common to maize in the USA with little or no chemical pesticides (Williams et al. 1998). The Bt endotoxins are crystalline proteins (Cry proteins) that are solubilized and activated in the highly alkaline mid-gut of these pests (Hofte and Whitely, 1989). Recent research conducted in a glasshouse demonstrated that Bt endotoxin concentrations in early growth maize were associated with levels of N-fertility used to grow the plants (Bruns and Abel 2003). This research was conducted to determine if variation in N-fertility rates in the field would impact Bt endotoxin concentrations in post-anthesis maize plants.

Methods

Three Bt maize hybrids, two with Bt event MON-810 (AgriGold brand cv. A 6729Bt and Pioneer brand cv. 33V08Bt) one with Bt event DBT 418 (DeKalb brand cv. 626Bt) and one non-BT (DeKalb brand cv. 626) were grown in a replicated experiment at the Delta Branch Experiment Station at Stoneville, MS USA in 2002 and 2003. Soil at the experimental site was a Dundee silty clay (fine-silty, mixed, thermic Aeric Ochraqualfs) prepared for planting by forming 50 cm ridges spaced 76 cm apart. Both sites received P and K fertilizer pre-plant each year based on soil tests and yield goals of 12.5 t/ha of grain. The experimental design was a randomized complete block replicated 4 times with the treatments arranged in 4 x 4 factorial. Treatments were hybrids and the N-fertility treatments (0 kg N/ha, 112 kg N/ha, 224 kg N/ha, or 336 kg N/ha). Individual plots were 4 rows 9 m long and consisted of one hybrid and one N-fertility treatment. Plots were furrow irrigated twice each season, once at growth state R2 and again at R5 as defined by Ritchie et al. (1997). Tissues from the outer husk of the ear and the primary ear leaf sheath were collect at growth stage R3 and analyzed by Bt endotoxin concentration using procedures previously described (Bruns and Abel 2003). Grain yield and lodging data were collected approximately 42 d post-growth stage R6. Data were analyzed using PROC MIXED, SAS Institute (2000).

Results

Concentrations of Bt endotoxin in both the husk and ear leaf sheath tissue of the MON-810 Bt hybrids increased with increasing levels of N-fertility (Figures 1 & 2). The linear relationships between N-fertility rates and Bt endotoxin were significant (P<0.05) and positive.

Figure 1. The concentration of Bt endotoxin in the primary ear leaf sheaths of two MON-810 Bt maize hybrids (AgriGold brand cv. A 6729Bt and Pioneer brand cv. 33V08Bt) grown with varying rates of N-fertility in Stoneville, MS, USA.

Figure 2. The concentration of Bt endotoxin in the outer husk of the ear of two MON-810 Bt maize hybrids (AgriGold brand cv. A 6729Bt and Pioneer brand cv. 33V08Bt) grown with varying rates of N-fertility in Stoneville, MS, USA.

The hybrid DeKalb 626Bt with the DBT 418 Bt event however showed no significant changes in Bt endotoxin concentration with changes in N-fertility and averaged 0.187 mg/kg Bt endotoxin in the primary ear leaf sheaths and 0.146 mg/kg Bt endotoxin in the outer husks. These data differ from those previously reported on maize plants at growth stage V5 grown in a glasshouse where plants from both Bt events showed an increase in Bt endotoxin with increased N-fertility (Bruns and Abel 2003). Data on the relative amount of stalk lodging just prior to harvest showed significantly (P<0.01) more broken stalks in the non-Bt hybrid (DeKalb 626) (5.1%) than all of the Bt hybrids. No significant differences in relative amounts of stalk lodging were noted among the Bt hybrids, which ranged from 0.7% to 1.0%. No significant differences were noted among N-fertility treatments for stalk lodged plants. Grain yields were not significantly different among the hybrids. Grain yields did significantly (P<0.01) increase with increases in N-fertility from 0 kg N/ha to 112 kg N/ha both years (Table 1). In 2002 more grain was produced by plots receiving 224 kg N/ha and 336 kg N/ha than those receiving 112 kg N/ha. However, no differences in grain yield were noted for 2003 among plots receiving supplemental N-fertility.

Table 1. Grain yields among four maize hybrids grown with different rates of N-fertility in 2002 and 2003 at Stoneville, MS, USA. †

N-Fertility

Grain Yield
t/ha

kg/ha

2002

2003

0

5.4

7.3

112

8.5

9.8

224

10.2

10

336

10.8

9.9

†Means of four hybrids (AgriGold brand cv. A 6729Bt, Pioneer brand cv. 33V08Bt, and DeKalb brand cv. 626Bt, and cv. 626), and four Reps. Means within a column LSD@ P<0.01=1.0 and within a row=1.1.

Conclusions

The concentrations of Bt endotoxin in MON-810 Bt maize hybrids during reproductive growth are positively affected by increases in N-fertility. However, this may not be true for all Bt events based on the data from the hybrid DeKalb 626Bt that has the DBT 418 Bt event. Maize growers who choose to produce Bt hybrids with the MON-810 Bt event, as a means of avoiding feeding damage by susceptible insect pests should apply sufficient N-fertilizer to ensure adequate levels of Bt endotoxin for maximum protection and produce profitable yields.

References

Bruns HA and Abel CA (2003). Nitrogen fertility effects on Bt δ-endotoxin and nitrogen concentrations of maize during early growth. Agron. J. 95:207-211.

Hofte H and Whitely HR (1989). Insecticidal crystal proteins of Bacillus thuringiensis. Microbiol. Rev. 53:242-255.

Ritchie SW, Hanway JJ, and Benson GO (1997). How a corn plant develops. Spec. Rep. 48. Iowa State Univ. of Sci. and Technol. Coop. Ext. Serv. Ames, IA.

SAS Institute (2000). User’s manual. Version 8.1. SAS Inst. Cary, N.C.

Williams WP, Buckley PM, Sagers JB, and Hanten JA (1998). Evaluation of transgenic corn for resistance to corn borer (Lepidoptera: Noctuidea), fall armyworm (Lepidoptera: Noctuidea), and southwestern corn borer (Lepidoptera: Cambidae) in laboratory bioassay. J. Agric. Entomol. 15:105-112.

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