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Unintended Effects of Genetic Manipulation
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Bt maize varieties matured more slowly and had on average lower grain yield and higher grain moisture content than conventional varieties.

Manipulated Organism: Maize (Zea mays).

Inserted Transgenes: crylAb gene from Bacillus thuringiensis (Bt). This gene gives transgenic Bt maize (also called Bt corn) the ability to produce insecticidal delta-endotoxin that can kill the larvae of the European corn borer and other insect pests feeding on the corn. The gene was fused to the cauliflower mosaic virus (CaMV-35S) promoter so that the Bt gene would be expressed in all parts of the plant.

Goal of This Study: Investigate whether there are any differences in yield or differential responses to nitrogen fertilizer uptake between conventional commercial hybrid maize varieties and Bt maize varieties. The researchers carried out field experiments over three years and compared three to seven different varieties of conventional hybrid maize with their near-isogenic Bt maize counterparts.

Results of This Study:
  • "Some of the Bt hybrids took 2-3 additional days to reach silking and maturity and produced a similar or up to 12% lower grain yields with 3-5% higher grain moisture at maturity, in comparison to their non-Bt counterpart"(p. 199).
  • "In 2000, no Bt hybrids reached physiological maturity before the killing frost while their corresponding non-Bt hybrids matured or almost reached maturity" (p.204).
  • Bt plants were greener at harvest than their nontransgenic counterparts, which may be connected with their late maturing and higher moisture concentration at maturity.
The researchers conclude: "Our study showed that under the conditions tested, Bt hybrids showed no higher yield potentials and thus added no yield benefits over their near-isolines. . . . In Bt hybrids, Bt gene or related cluster genes and promoters may have acted as a negative impact on dry matter and/or grain yeild when ECB [European corn borer] infestation pressure is low and/or corn crop encounters drought or other environmental stresses. . . . Another potential negative effect associated with the Bt hybrids is greater grain moisture at harvest, which may increase the cost for drying" (p. 210).

Additional Comments: Bt crops are designed with the goal to increase yields by reducing pest damage. However, as this study notes, only a yield increase of about 4% would make up for the increase in costs associated with purchasing Bt crops ($25-30 premium per hectare). Since this study showed that under typical conditions of moderate corn borer infestation Bt crop yield was in part similar to but usually less than in the conventional counterpart, "where there is no severe infestation of ECB [European corn borer], corn producers may not benefit from the use of current Bt hybrids" (p. 210). They warn that "widespread use of Bt hybrids in lower ECB [European corn borer] problem areas not only increases the cost of production, but may also lead to negative impact on non-target organisms in the environment" (p. 210).

Source: Ma, B. L. and K. D. Subedi (2005). "Development, Yield, Grain Moisture, and Nitrogen Uptake of Bt Corn Hybrids and Their Conventional Near-Isolines," Field Crops Research vol. 93, pp. 199-211.

Author Affiliations: Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada.

Funding: Eastern Cereal and Oilseed Research Centre, Canada.

Product Status: The varieties used in this study are commercially planted.

Copyright 2008 The Nature Institute.

This document: http://natureinstitute.org/nontarget/reports/maize_005.php

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