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Published in Agron J 100:163-168 (2008)
DOI: 10.2134/agrojnl2007.0123
© 2008 American Society of Agronomy
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CROP RESIDUES

No Differences in Decomposition Rates Observed between Bacillus thuringiensis and Non-Bacillus thuringiensis Corn Residue Incubated in the Field

R. Michael Lehman*, Shannon L. Osborne and Kurt A. Rosentrater

USDA-ARS-North Central Agricultural Research Lab., 2923 Medary Ave., Brookings, SD 57006

* Corresponding author (michael.lehman{at}ars.usda.gov).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Recent speculation of slower residue decomposition for Bacillus thuringiensis (Bt) corn (Zea mays L.) hybrids compared with non-Bt corn hybrids has prompted investigative study. We evaluated the residue decomposition rates of Bt and non-Bt corn hybrids over a period of 22 mo under field conditions using the litter bag technique. The four corn hybrids used were (i) DKC60–16 (Bt+, Cry1Ab protein active against the leptidopteran European corn borer, event MON810), (ii) DKC60–12 (Bt+, Cry3Bb1 protein active against the coleopteran corn rootworm, event MON863), (iii) DKC60–14 (stacked Bt++, Cry1Ab and Cry3Bb1 proteins) and, (iv) DKC60–15 (Bt–, base genetics). The biochemical and physical properties of the corn residues were determined. No differences in the decomposition rates of the residue from the four corn hybrids were detected. Residue decomposition rate constants were approximately 0.25 d–1 for all four hybrids with predicted residue half-lives of about 200 d. No differences in compositional properties, including lignin content, were observed among the four hybrids. Physical compression testing of the chopped residue failed to detect significant differences in mechanical strength properties among the hybrids. This is the first report regarding decomposition of Bt corn residue under field conditions following ambiguous reports from laboratory studies on the relative susceptibility of Bt corn residue to decomposition.

Abbreviations: Bt, Bacillus thuringiensis • GM, genetically modified

No Differences in Decomposition Rates Observed between Bacillus thuringiensis and Non-Bacillus thuringiensis Corn Residue Incubated in the Field

R. Michael Lehman*, Shannon L. Osborne and Kurt A. Rosentrater

USDA-ARS-North Central Agricultural Research Lab., 2923 Medary Ave., Brookings, SD 57006

* Corresponding author (michael.lehman{at}ars.usda.gov).

Received for publication April 4, 2007.
Recent speculation of slower residue decomposition for Bacillus thuringiensis (Bt) corn (Zea mays L.) hybrids compared with non-Bt corn hybrids has prompted investigative study. We evaluated the residue decomposition rates of Bt and non-Bt corn hybrids over a period of 22 mo under field conditions using the litter bag technique. The four corn hybrids used were (i) DKC60–16 (Bt+, Cry1Ab protein active against the leptidopteran European corn borer, event MON810), (ii) DKC60–12 (Bt+, Cry3Bb1 protein active against the coleopteran corn rootworm, event MON863), (iii) DKC60–14 (stacked Bt++, Cry1Ab and Cry3Bb1 proteins) and, (iv) DKC60–15 (Bt–, base genetics). The biochemical and physical properties of the corn residues were determined. No differences in the decomposition rates of the residue from the four corn hybrids were detected. Residue decomposition rate constants were approximately 0.25 d–1 for all four hybrids with predicted residue half-lives of about 200 d. No differences in compositional properties, including lignin content, were observed among the four hybrids. Physical compression testing of the chopped residue failed to detect significant differences in mechanical strength properties among the hybrids. This is the first report regarding decomposition of Bt corn residue under field conditions following ambiguous reports from laboratory studies on the relative susceptibility of Bt corn residue to decomposition.

Abbreviations: Bt, Bacillus thuringiensis • GM, genetically modified


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE ADOPTION OF GENETICALLY MODIFIED (GM) crops has occurred rapidly in the United States, especially in the corn–soybean [Glycine max (L.) Merr.] rotations of the upper Midwest. The near-term economic advantages of GM crop lines such as pest-resistance, herbicide resistance, and quantity and quality of crop yield have been studied and represented to producers by seed manufacturers. Potential adverse effects relating to the differences between engineered organisms and their protein expression as compared with their naturally-recombinant counterparts are not well understood. Although some human health and ecological risk assessments have been performed before release of GM varieties, full consideration of potential unintended or long-term effects of this technology has not occurred (NRC, 2000, 2002; Rombke et al., 2003; Wolfenbarger and Phifer, 2000). Several insect-resistant, transgenic corn hybrids are being cultivated that contain genes derived from the bacterium, Bacillus thuringiensis (Bt), that produce proteins possessing selective insecticidal activity.

The possibility that some Bt-containing corn hybrids have higher lignin content than their non-GM counterparts (Saxena and Stotzky, 2001a; Stotzky, 2004), represents an unintended effect with no known mechanism. Compositional differences between plants should translate into different rates of residue decomposition, apart from any direct effects due to the presence of the endotoxin in the residue or exuded into the soil. There is wide-spread anecdotal information within the agricultural community that plant residue from some GM corn hybrids may be resistant to degradation, and implement manufacturers now market improved or alternative tillage machinery to deal specifically with "tough Bt corn residue." If more aggressive tillage is required to handle a growing amount of tougher residue, then decades-long gains in soil and water quality achieved through conservation tillage may be at risk.

Additional studies comparing Bt and non-Bt corn residue composition and evaluating residue decomposition under laboratory conditions have produced ambiguous results. Jung and Sheaffer (2004) thoroughly examined the lignin content in Bt (Cry1Ab protein active against some leptidoptera) and non-Bt corn near-isolines and found no consistent differences. In contrast, Poerschmann et al. (2005) reported that the stems of Cry1Ab-containing maize hybrids possessed greater lignin content and different molecular composition of their lignin compared to near-isogenic non-Bt hybrids. In a study that showed Cry1Ab-containing corn residues performed equal to their non-Bt near isolines as feed for beef and dairy cattle (Bos taurus), slightly higher lignin contents were reported for the transgenic residues (Folmer et al., 2002). Mungai et al. (2005) found no consistent differences in either residue composition or laboratory N mineralization rates between Bt (Cry1Ab) and non-Bt corn varieties. Hopkins and Gregorich (2003) compared the decomposition rates of lepidopteran-active Bt corn leaves (Cry1Ab, Pioneer 38W36) with that of the non-GM isoline in laboratory studies and found no significant difference between the two varieties. In contrast, Flores et al. (2005) recently presented data showing that Bt (Cry1Ab-containing hybrids) corn residue decomposed significantly slower than their non-GM isolines in laboratory soil microcosms. Experimental data suggested that the slower decomposition rate was due to increased lignin content in the Bt corn, and not to inhibition of soil microorganisms.

At present, there are no published experimental data comparing the decomposition of Bt and non-Bt corn residue under field conditions. Additionally, all the studies cited previously have used residue from GM corn expressing the cry1Ab transgene active against some leptidoptera, whereas the decomposition of plant residue with anti-coleopteran varieties (Cry3 proteins) of GM corn or varieties with stacked traits have not been studied. Longer-term, sufficiently replicated field studies involving several Bt-containing hybrids may aid in clarifying equivocal observations regarding the relative susceptibility of Bt-corn to decomposition. We undertook a 22-mo field study using the litter-bag approach to compare residue decomposition rates of four corn near-isolines with the following genetics: (i) cry1Ab transgene active against European corn borer (Ostrinia nubilalis Hübner); (ii) cry3Bb1 transgene active against corn rootworms (Diabrotica spp); (iii) stacked with both transgenes; and, (iv) base genetics with no genetic modification. Experimental procedures were used to approximate the exposure of the residue when it is chopped and buried during combining and tillage operations conducted in the fall. We also report on the compositional and physical properties of the residue used for the decomposition study.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Collection and Preparation of Corn Residue
The four corn hybrids used were (i) DKC60–16 (Bt+, Cry1Ab protein active against European corn borer, event MON810), (ii) DKC60–12 (Bt+, Cry3Bb1 protein active against corn rootworms, event MON863), (iii) DKC60–14 (stacked Bt++, Cry1Ab and Cry3Bb1 proteins) and, (iv) DKC60–15 (Bt–, the non-GM near-isoline containing the base genetics). These four hybrids were cultivated in 0.76 m-spaced rows under no-till conditions with fertilizer application based on soil test recommendations in randomly arranged triplicate test plots (6.1 by 237.8 m) under irrigation in consecutive years (2003, 2004) at the South Dakota State University's Dakota Lakes Field Station, 27 km southeast of Pierre, SD along the Missouri River. For each hybrid, five fully mature plants were collected in November 2004, from the center of each triplicate 12-row plot for a total of 15 plants per hybrid. There was no observable insect pressure at this site during the 2004 growing season (Dwayne Beck, personal communication, 2005). The plants were dried at 60°C for 1 wk. Plant residue was processed for each hybrid by removing cobs, and chopping stalks and leaves together with a chipper. The resulting chopped plant residue (nominal size <10 cm) was manually mixed and maintained at 60°C during the subsequent period of litter-bag construction (about 1 wk).

Residue Compositional Analyses
Subsamples of residue from each hybrid were reduced in size using a Wiley mill ( <2 mm) for analysis of acid detergent fiber, acid detergent lignin, and neutral detergent fiber using an Ankom Fiber Analyzer and AOAC Method 973.18 (AOAC, 2003). Ground residue was milled further (Udy mill, <1 mm) for total C and N analysis by dry combustion (LECO CN 2000analyzer, Leco Corp., St Joseph, MI) (Nelson and Sommers, 1996).

Decomposition Rates
The decomposition rates of residue prepared from the aboveground biomass of the four corn hybrids were evaluated using the litter-bag technique according to the recommendations of Harmon et al. (1999). Nylon mesh (3.2-mm, Memphis Net and Twine) and poly thread were used to construct the litter-bags (20 by 20 cm). Each bag was filled with 10 g of dried plant residue and closed with rust-resistant staples. Sixty-three bags were prepared for each of the four hybrids for a total of 252 litter bags. Completed bags were kept at 60°C ( <1 wk) until burial. The decomposition study was performed at the Eastern South Dakota Soil and Water Research Farm, Brookings, SD (44°19', 96°46' W). On 15 Nov. 2004, nine bags per hybrid were buried in each of seven groups in Barnes clay loam (fine-loamy, mixed, superactive, frigid Calcic Hapludoll). The soils at this site are reported to have clay content of about 280 g kg–1 (Pikul et al., 2007) with smectite the predominant clay mineral followed by mica, kaolinite, and quartz (Soil Survey Staff, 2007). Each group consisted of a total of 36 bags, buried in a repeated sequence of the four hybrids. The litter bags were buried in vertical slits on the ridges so that the top of each bag was 5 cm below the soil surface. The litter-bags were buried into incorporated spring wheat stubble. Weeds were controlled by hand spraying area on 1 July 2005 with an herbicide mixture of Isooctyl (2-ethylhexyl) ester of 2,4-dichlorophenoxyacetic acid, 672 g L–1 active ingredient and Sethoxydim: 2-[1-(ethoxyimino)butyl]-5-[2-(ethythio)propyl]-3-hydroxy-2-cyclohexen-1-one, 130 g L–1 active ingredient at rates of 7.4 x 10–6 L m–2 and 3.7 x 10–6 L m–2.

Groups of 36 bags were randomly selected and excavated at these intervals following burial: 134 d (overwinter), 168 d, 217 d, 262 d, 337 d, 559, and 658 d. Soil loosely adhering to the exterior of the bags was brushed off and each litter-bag was placed in a resealable, polyethylene bag which was kept on ice for transport to the laboratory (10 min) and immediate processing. The plant residue remaining in each bag was dried (60°C, 48 h) in a forced air oven and then combusted (450°C, 12 h) in a muffle furnace to obtain an ash-free dry weight.

The percentage residue remaining was calculated from the difference between the initial and final dry masses, corrected for the organic matter content of the soil entrained in each bag (ash weight x % soil organic matter, SOM) (Harmon et al., 1999).

Formula

Soil Properties
At the time of litter-bag burial, four soil samples were collected by soil probe (2.5 cm diam.) from the 5 to 25 cm depth interval and analyzed for total C and N, inorganic N, pH (1:1 with deionized water), and electrical conductivity using standard methods (Rhoades, 1996). At each time interval when litterbags were retrieved, four soil samples were collected by soil probe (5–25 cm depth) for percentage soil organic matter determination using weight loss on ignition (450°C, 12 h) (Nelson and Sommers, 1996).

Compression Testing of Chopped Residue
To determine if potential differences among hybrids in composition (e.g., lignin content, (Saxena and Stotzky, 2001a) or structural properties (e.g., lignin molecular structure, (Poerschmann et al., 2005) might be manifested in physical resistance of the plant residue, the mechanical strength of chopped residue was measured with an Instron compression tester (Model 5564, Instron Corp., Canton, MA), using a 1 kN loadcell and the methods of Van Pelt (2003). Two residue subsamples were analyzed from each hybrid. Approximately 35 g of well-mixed residue was placed in a fixed (101.6 mm diam. by 101.6 mm height) cylindrical receiving vessel, and the plunger (98.09 mm diam.) was lowered at a constant rate of 0.1 mm/min. During testing, the force applied to the plunger was measured, as was progressive travel distance; thus stress (MPa) and strain (%) were determined over time via the compression tester's computer control software. During the testing of each subsample, at least 4,000 individual stress-strain data points were collected.

Statistics
Decomposition rates and residue half-lives were calculated by linear regression of the percentage residue remaining over the time interval when this percentage declined in a linear fashion (up to, and including, the 262 d time point following burial). The slopes of the regression lines for each of the four hybrids were tested for equality by analysis of variance (Systat 11). Residue composition, decomposition rates, residue decomposition half-lives, and the amount of residue remaining at each time point were tested for significance differences among hybrids at the 0.05 alpha level by one-way ANOVA (Systat 11). Compression data were analyzed by nonlinear regression using Gauss-Newton iterative convergence (SAS 8.0). Differences in mechanical stress at 50% strain among the hybrids were tested by one-way ANOVA (SAS 8.0).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Residue Composition
Carbon/N ratios for freshly collected plant residue were about 54– 56:1, with no significant differences among hybrids (P = 0.83, one-way ANOVA) (Table 1 ). Lignin contents (dry weight basis) for freshly collected plant residue were all between 5 and 6%, with no significant difference among hybrids (P = 0.29, one-way ANOVA) (Table 2 ). The equivalent residue compositions of our Bt-containing hybrids and their non-Bt near isoline are similar to findings of two other studies that only examined Cry1Ab-containing hybrids (Jung and Sheaffer, 2004; Mungai et al., 2005). While Folmer et al. (2002) reported a slightly higher lignin content for Cry1Ab-containing residue; other researchers have found that several Cry1Ab-containing hybrids contained significantly higher lignin contents (Poerschmann et al., 2005; Saxena and Stotzky, 2001a; Stotzky, 2004). At present, there is no consensus explanation for the disparate findings, although methodological differences among the studies were discussed by Jung and Sheaffer (2004) and Poerschmann et al. (2005).


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  		Table 1. Carbon and N content of residue used in decomposition study.{dagger}

 

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  		Table 2. Macromolecular composition of residue used in decomposition study.{dagger}

 
Residue Decomposition
The first set of litter-bags was retrieved on 29 Mar. 2005, 134 d after burial in soils with a pH of 5.2 and about 37 g kg–1 soil organic matter, among other properties (Table 3 ). During this "over-winter" period, the ground had been continuously frozen from shortly after burial to shortly before retrieval. An average of 30 to 34% of the residue had decomposed or was lost from the bags during the over-winter period (Fig. 1 ). There were no significant differences in the amounts of residue remaining among the four corn hybrids (n = 9 for each hybrid) after the overwintering period (Table 4 ). This initial approximately 30% dry weight lost should include the highly soluble, lower molecular weight fractions that can be lost by leaching. Through about 262 d, decomposition rates were fairly linear, and by the 1-yr mark when decomposition rates were slowing, 75 to 80% of the dry weight had been lost from the litter bags. During the second year of the study, little additional weight loss was recorded. No significant differences in the percentage of residue remaining were detected among any of the hybrids at any of the sampling points (Table 4). From inspection of the plots of residue remaining vs. time (Fig. 1) and plots of the log residue remaining vs. time (data not shown), decomposition appeared to be conforming to zero order kinetics up to 262 d. Between 262 d and the end of the study at 658 d, there seemed to be two different (nonzero order) kinetic phases of residue disappearance. Calculated (using only data through 262 d) litter half-lives were approximately 200 d and decomposition rate constants were about –0.25 d–1 for all four hybrids (Table 5 ). There were no significant differences (P = 0.14) among the slopes of the linear regression (residue by time) among the four hybrids.


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  		Table 3. Soil properties.

 

Figure 1
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  		Fig. 1. Percentage residue remaining in litter-bags for each of four corn hybrids through 658 d of burial. RW = Bt+ containing Cry3Bb1 protein active against corn rootworm (DKC60–12); CB+RW = BT++ stacked, Cry1Ab proteins active against European corn borer and Cry3Bb1 protein active against corn rootworm (DKC60–14); Base = Bt–, base genetics containing no Bt genes (DKC60–15), CB = Bt+ containing Cry1Ab protein active against European corn borer (DKC60–16). Error bars represent one standard deviation for n = 9 replicate litter-bags per hybrid.

 

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  		Table 4. One-way ANOVA for main effects of hybrid on residue remaining at each sampling point.

 

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  		Table 5. Residue decay rates and half-lives calculated from linear regression analysis.{dagger}

 
These data are the first (to our knowledge) on the relative decomposition of Bt corn stover under field conditions. Our approach using chopped litter in buried litter bags to simulate the effect of chopping and tillage operations on the stover showed that Bt-containing hybrids decomposed at rates similar to their non-Bt near isoline under these conditions. We were unable to measure any losses of the final 20% of the residue, presumably the most recalcitrant, lignin-containing material. In fact, after about 1 yr, little further loss of residue weight was detected with our approach, perhaps because the increasing proportion of soil to residue in the litter bags made smaller changes in residue weight difficult to resolve.

The lack of difference in decomposition rate that we observed in the field between the cry1Ab-containing hybrid (DKC60–16) with its non-Bt near isoline (DKC-6015) was consistent with results of a laboratory study on a different cry1Ab-containing hybrid (Pioneer 38W36) (Hopkins and Gregorich, 2003), but contrasts with another lab study that used the cry1Ab-containing NK4640Bt (Flores et al., 2005). Both laboratory studies used evolved carbon dioxide as an indicator of residue decomposition, rather than residue weight loss, and both laboratory studies presented differences in methodological details, including the residue particle sizes used. There are no published data from plant residue decomposition studies using cry3Bb1-containing maize hybrids for comparison with ours.

Immuno-detectable endotoxins (Cry1Ab proteins) have been shown to persist in corn residue for periods that may exceed several seasons (Baumgarte and Tebbe, 2005; Hopkins and Gregorich, 2003; Zwahlen et al., 2003). If there are no differences in the breakdown of the residues in the field as observed in our study, it is unlikely that toxin within the residue is negatively affecting the biological decomposing community. On the other hand, Cry1Ab exuded by growing plants into soil is reported to persist, sorbed to clays or organic matter and may retain some insecticidal activity (Saxena and Stotzky, 2000; Saxena et al., 1999, 2002). Although the sorbed endotoxin is not bioavailable to microorganisms (Chevallier et al., 2003; Koskella and Stotzky, 1997) or toxic to selected organisms (Saxena and Stotzky, 2001b), reports of residual insecticidal activity mean that soil reservoirs of endotoxin may influence litter decomposition activities of soil insects where Bt-corn has been recently cultivated. Thus, soil clay content and mineralogy may influence residue fate at a given location.

Residue Mechanical Strength
Even without differences in lignin concentrations, it is possible that the GM plants may possess a different cellular structure (an unintended effect) (Poerschmann et al., 2005) that confers increased resistance to physical disruption. Differences in cellular structure may or may not be manifested in differential decomposition of the chopped residue used in the current study. Compression testing is one methodology often used to quantify a material's mechanical strength. Compression of biological materials often results in a nonlinear, and often exponential, relationship between applied strain and resulting stress within the material matrix (Al-Widyan and Al-Jalil, 2001; Munoz and Herrera, 2002; Stroshine, 2001; Van Pelt, 2003). For the residue samples in this study, it was determined that the ideal relationship could be described using:

Formula
where A and B are empirical coefficients determined by nonlinear regression analysis.

The compression curves (Fig. 2 ) and parameter estimates (Table 6 ) indicate that there were variations in the mechanical strength among the shredded residue samples. As strain (i.e., deformation) increases, the internal stress developed to resist this deformation is an indication of material strength. The steeper the stress-strain curve, the higher the internal resistance and, thus, mechanical strength. Often, this type of behavior is due to differences in microscopic tissue structure. The Bt hybrids all had greater strength (i.e., allowable mechanical stress) than the non-Bt hybrid. Among these, the stacked Bt hybrid had the greatest resistance to deformation compared with the other Bt hybrids. At 50% strain, the stacked Bt hybrid (DKC6014) had a maximum stress of 0.1138 (±0.0011) MPa (mean ± one standard deviation); the rootworm-protected hybrid (DKC6012) had a strength of 0.0750 (±0.0346) MPa; the corn borer-protected hybrid (DKC6016) had a strength of 0.0365 (±0.0021) MPa; and the non-Bt-hybrid (DKC6015) had a maximum stress of 0.023 (±0.0050) MPa. Statistical analysis on the mechanical strength values indicated that they were not, however, statistically significant (P = 0.1040; F = 4.08). Similar analyses of intact stalks should determine if differences in mechanical strength among hybrids may have existed before chopping the stalks.


Figure 2
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  		Fig. 2. Nonlinear regression curves of the stress-strain measures for each residue type, two subsamples per hybrid, >4,000 stress-strain data points per hybrid. RW = Bt+ containing Cry3Bb1 protein active against corn rootworm (DKC60–12); CB+RW = BT++ stacked, Cry1Ab proteins active against European corn borer and Cry3Bb1 protein active against corn rootworm (DKC60–14); Base = Bt–, base genetics containing no Bt genes (DKC60–15), CB = Bt+ containing Cry1Ab protein active against European corn borer (DKC60–16).

 

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  		Table 6. Nonlinear regression coefficients for mechanical compression testing.

 
Alternative Explanations
It remains possible that the apparent persistence of Bt corn residue observed by agricultural producers may be related to (i) the overall amount of residue produced, regardless of residue compositional qualities, or (ii) the relative sturdiness of the Bt+ residue compared to non-Bt residue where insect pressure is present. No consistent differences in silage yield between Bt+ and Bt– hybrids were reported by Folmer et al. (2002). Similarly, Mungai et al. (2005) found no differences between Bt+ and Bt– hybrids with respect to above- or below-ground biomass in a 2-yr study. No insect pressure was reported at the Dakota Lakes Field Site where all four corn hybrids used in the current study were cultivated (Dwayne Beck, personal communication, 2005). Thus it is possible that where insect pressure is present, non-Bt-protected corn stalks will be weaker and Bt-protected stalks will be relatively more resistant to decay. In this case, research studies which examine the decomposition of shredded or ground residue may not find any differences correlated with the presence of the transgene.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
We found no differences in the decomposition of chopped Bt and non-Bt corn residue under the field conditions existing at our research site. We also found no difference in compositional properties among Bt and non-Bt corn residues, but physical testing suggested differences in residue mechanical strength. As yet, insufficient data exists to explain the perceived resistance of some GM corn residues to decay. The relative persistence of corn residue is important because of potential effects on conservation tillage practices and carbon budgets.


   ACKNOWLEDGMENTS
 
Corn plants were cultivated and provided by Dwayne Beck, SDSU Dakota Lakes Field Station. Technical assistance in the field or laboratory was provided USDA-ARS employees Max Pravacek, Deb Hartman, Kurt Dagel, Dave Harris, Kendra Jensen, and Amy Christie. Mark West (USDA-ARS) consulted on the statistical testing of regression slopes.


   NOTES
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 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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R. M. Lehman, S. L. Osborne, and K. A. Rosentrater
No Evidence That Bacillus thuringiensis Genes and Their Products Influence the Susceptibility of Corn Residue to Decomposition
Agron. J., November 7, 2008; 100(6): 1687 - 1693.
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