A concern about what affect genetically modified material could have on human health. For example, transgenic crops have been suggested to cause allergies in some people, although it is uncertain whether transgenic crops are the source of this reaction .Furthermore the antibiotic resistance genes placed in these crops has been suggested to cause resistance to antibiotics leading to super bugs that cannot be killed with antibiotic treatments . The idea of a population being uncomfortable with ingesting DNA that originated from another source, such as a virus or bacteria, must also be considered when thinking about producing transgenic crops. However, to date, there is no evidence of the DNA from transgenic crops being any different from the DNA ingested from conventional crops.
Several studies have used data collected by the USDA or industry sources to compare the amount of insecticides or herbicides applied to GE crops compared to conventional crops. An aspect that is obscure in most analyses is the correlation of effects on production with actual changes in the amount or type of pesticides applied. For example, changing pesticide applications can affect yields based on the effectiveness of pest control, but yield changes could also be due to some other production factor.
Although not indisputable, several studies have offered these general conclusions:
• A USDA-ERS econometric model that attempts to control for other variables suggests that, overall, a reduction in pesticide use in the U.S. was associated with the adoption of GE insecticide resistant and herbicide tolerant crops.
• Most comparisons of insecticide use have shown small or not statistically significant reductions attributable to use of Bt corn compared to conventional corn varieties overall. Reasons for this may be because many U.S. Corn Belt corn acres are not actually sprayed specifically for European corn borer (the primary pest targeted by current Bt corn varieties), since outbreaks of this pest are difficult to control and are extremely variable. Also, insecticides used against the European corn borer are also used to control other insect pests and generally would still be applied independently of European corn borer pressure. Some studies have attributed regional increases in yield to better control of European corn borer in Bt corn. In cases where this is true, although the total amount of pesticides released into the environment may not decrease, yield per unit of pesticide applied may increase. (Note that these estimates do not count the Bt toxin produced by the plants as a pesticide application.)
• When considering insecticides directed at pests targeted by Bt cotton (cotton bollworm, tobacco budworm, and pink bollworm), both the number of insecticide applications and the pounds of insecticide used on cotton were significantly lower in 1998 and 1999 in six cotton growing states compared to applications in 1995, prior to the introduction of Bt cotton. These reductions are substantial, representing about 10-14% of the total amount of pesticides used in those states. It is unclear precisely how much of this reduction is directly attributable to the use of Bt cotton. Reductions of insecticide applications (acre-treatments, adjusted for changes in acreage planted) for Bt-targeted pests and significant decreases in yield loss due to Bt-targeted pests were reported in twelve of sixteen cotton producing states in the U.S. in 1998 and 1999 compared to 1995.
• Herbicide applications to soybeans, quantified as total pounds of herbicide active ingredient applied, have increased slightly overall with the adoption of herbicide-tolerant GE varieties, largely because the increased number of pounds of glyphosate applied to Roundup-Ready® soybeans (the most widely adopted type of GE crop in the U.S.) exceeded the reduction in the number of pounds of other herbicides replaced by glyphosate. It has been proposed that the substitution of glyphosate for other herbicides is environmentally beneficial since glyphosate has lower toxicity to mammals, fish, and birds, is less likely to leach, and is less persistent in the environment than the herbicides it replaces.
• Antibiotic resistance genes are frequently used at several stages in the creation of genetically engineered plants as convenient "selectable markers". Bacteria or plant cells without a gene for resistance to the antibiotics used can be killed when the antibiotic is applied to them. So when scientists link the gene for the desired trait being introduced into a plant with an antibiotic resistance gene, they can separate cells carrying the desired gene from those that don't by exposing them to the antibiotic. The antibiotic resistance genes end up in the genetically engineered plants as excess baggage whose function is no longer required after the process of making them is complete.
• Concern has been raised about the possibility that antibiotic resistance genes used to make transgenic plants could be transferred to microorganisms that inhabit the digestive tracts of humans or other animals that eat them, and therefore might contribute to the already serious problem of antibiotic resistant pathogens. Transfer of DNA from one microbe to another (horizontal gene transfer) is known to occur in nature and has been observed in some laboratory experiments under specific conditions, but the likelihood of DNA being transferred from plant material in the digestive system to microbes has not yet been experimentally determined. It is thought that for such a transfer to be possible, it would have to come from consumption of fresh food since most processing would degrade the plant's DNA. Also, there is evidence that most DNA is rapidly degraded by the digestive system. However, results of one recent experiment have suggested that horizontal transfer of DNA from genetically engineered plants can occur in the human digestive tract under some circumstances. But overall, the risk of antibiotic resistance genes from transgenic plants ending up in microorganisms appears to be low.
• A second concern about the use of some antibiotic resistance genes is that they could reduce the effectiveness of antibiotics taken at the same time transgenic food carrying the resistance gene for that antibiotic was consumed. In cases where this has been identified as a risk based on the mechanism of resistance, studies have suggested the chance of this happening was probably very low due to rapid digestion of the inactivating enzymes produced by the transgenic resistance gene. Most transgenic plants do not carry resistance genes for antibiotics commonly used to treat infections in humans.
• While the risk of creating additional problems of antibiotic resistance in microorganisms from the use of the resistance genes in transgenic plants appears to be low, steps are being taken to reduce the risk and to phase out their use. The FDA recommends that developers of transgenic crops use only antibiotics that are not commonly used for treatment of diseases in humans. Scientists are developing and using different selectable markers, and are also experimenting with methods for removing the antibiotic resistance genes before the plants are released for commercial use.
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