Genetically modified maize
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Herbicide resistant corn
Corn varieties resistant to glyphosate herbicides (Liberty and Roundup) were first commercialized in 1996 by Monsanto. Pioneer Hi-Bred has since marketed corn hybrids with tolerance to imidazoline herbicides under the trademark "Clearfield" - though in these hybrids, the herbicide-tolerance trait was bred using tissue culture selection and the chemical mutagen ethyl methanesulfonate not genetic engineering. Consequently, the regulatory framework governing the approval, use, trade and consumption of transgenic crops does not apply for imidazoline-tolerant corn.
Herbicide-resistant GM corn is grown in 14 countries. By 2012, 26 varieties herbicide-resistant GM maize were authorised for import into the European Union,. but such imports remain controversial even though in 2012 the EU was reported to import 30 million tons a year of GM crops and cultivation of herbicide-resistant corn in the EU provides substantial farm-level benefits.
Bt corn is a variant of maize that has been genetically altered to express the bacterial Bt toxin, which is poisonous to certain insect pests. In the case of corn, the pest is the European corn borer. In recent years, traits have also been added to ward off Corn ear worms and Root worms.
Expressing the toxin was achieved by inserting a gene from the microorganism Bacillus thuringiensis into the corn genome. This gene codes for a toxin that causes the formation of pores in the Lepidoptera larval digestive tract. These pores allow naturally occurring enteric bacteria, such as E. coli and Enterobacter, to enter the hemocoel, where they multiply and cause sepsis. This is contrary to the common misconception that Bt toxin kills the larvae by starvation.
In 2004, Bt176 varieties were voluntarily withdrawn from the list of approved varieties by the United States Environmental Protection Agency (EPA) when it was found to have little or no Bt expression in the ears and was not found to be effective against second generation corn borers.
Debate over the effects of Bt corn on nontarget insects
In May 1999, a laboratory at Cornell University submitted the results from a laboratory trial to Nature. The trial appeared to indicate the pollen of genetically modified Bt corn presented a threat to monarch caterpillars. Unlike many pesticides, the Bt-corn has been shown to have no effect on many "nontarget" organisms—pollinators such as honeybees or beneficial predators of pests like ladybugs. But the Bt-modified corn produces pollen containing crystalline endotoxin from the bacterium genes. When this corn pollen is dispersed by the wind, it lands on other plants, including milkweed, the exclusive food of monarch caterpillars and commonly found around cornfields. In the laboratory tests, monarchs fed milkweed leaves dusted with so-called transformed pollen from a Bt-corn hybrid ate less, grew more slowly and suffered a higher mortality rate, the researchers report. Nearly half of these larvae died, while all of the monarch caterpillars fed leaves dusted with nontransformed corn pollen or fed leaves without corn pollen survived the study. Several factors make monarch caterpillars particularly likely to make contact with corn pollen, Losey says. Monarch larvae feed exclusively on milkweed because it provides protection against predators. The plant contains cardenolides, which are toxic, bitter chemicals that the monarch caterpillar incorporates into its body tissues, rendering it unpalatable to predators. Milkweed grows best in "disturbed" habitats, like the edges of cornfields, Losey notes.
Greenpeace was one of the first to pick up the information and launch a campaign against Bt corn in Europe. The responses to the poor science were too late and too poorly framed to be effective.
Preventing Bt resistance in pests
By law, farmers in the United States who plant Bt corn must plant non-Bt corn nearby. These unmodified fields are to provide a location to harbor pests. The theory behind these refuges is to slow the evolution of the pests' resistance to the Bt pesticide. Doing so enables an area of the landscape where wild type pests will not be immediately killed.
It is anticipated resistance to Bt will evolve in the form of a recessive allele in the pest. Because of this, a pest that gains resistance will have an incredibly higher fitness than the wild type pest in the Bt corn fields. If the resistant pest is feeding in the non-Bt corn nearby, the resistance is neutral and offers no advantage to the pest over any nonresistant pest. Ensuring there are at least some breeding pests nearby that are not resistant increases the chance the resistant pests will choose to mate with nonresistant ones. Since the gene is recessive, all offspring will be heterozygous, and the offspring from that mating will not be resistant to Bt and therefore no longer a threat. Using this method, scientists and farmers hope to keep the number of resistant genes very low, and use genetic drift to ensure any resistance that does emerge does not spread.
Although mandated by law, compliance data from the EPA for 2008 showed, however, 25% of Bt corn growers were not in compliance. The data showed noncompliance climbed to 13.23 million acres (53,500 km2), or almost 15% of all Bt corn grown, suggesting in some areas ample acreage does not exist to support pests without resistance to mate with any resistant pests that survived the Bt corn.
The non-Bt pesticide status of the refuges is being compromised by wind-borne pollen drifting into the non-Bt corn fields. Corn harvested from the supposed Bt-free zones has shown traces of Bt toxin. The levels found in the non-Bt corn decreases with distance from the Bt-corn fields, indicating the pollen is wind-borne rather than another method of transfer. The concentrations in the refuge fields were found to be low-to-moderate.
Sweet corn for human consumption
"Attribute" is the brand name for a line of Bt sweet corn. Seed is available only to large professional farmers who sign a stewardship agreement. The farmer must agree to not repackage or resell Attribute seed. Growers also must grow the corn exactly as directed.
The French High Council of Biotechnologies Scientific Committee reviewed the 2009 Vendômois et al. study and concluded that it "..presents no admissible scientific element likely to ascribe any haematological, hepatic or renal toxicity to the three re-analysed GMOs." However, French government applies a principle of precaution against GMOs. A review by Food Standards Australia New Zealand and others of the 2009 Vendômois et al. study concluded that the results were due to chance alone.
A 2011 Canadian study looked at the presence of CryAb1 protein (BT toxin) in non-pregnant women, pregnant women and fetal blood. All groups had detectable levels of the protein in blood, including 93% of pregnant women and 80% of fetuses at concentrations of 0.19 ± 0.30 and 0.04 ± 0.04 mean ± SD ng/ml, respectively. The paper did not discuss safety implications or find any health problems. The paper has been found to be unconvincing by several authors and organizations. In a swine model, Cry1Ab-specific antibodies were not detected in pregnant sows or their offspring and no negative effects from feeding Bt maize to pregnant sows were observed.
StarLink corn controversy
U.S. regulatory authorities permitted the commercial sale of StarLink seed with the stipulation that crops produced must not be used for human consumption. This restriction was based on the possibility that a small number of people might develop an allergic reaction to the Bt protein used in StarLink that is less rapidly digested than the version used in other Bt varieties.
StarLink corn was subsequently found in food destined for consumption by humans. An episode involving Taco Bell taco shells was particularly well publicized. This led to a public relations disaster for Aventis and the biotechnology industry as a whole. Sales of StarLink seed were discontinued. The registration for Starlink varieties was voluntarily withdrawn by Aventis in October 2000.
28 people reported apparent allergic reactions related to eating corn products that may have contained the Starlink protein. However, the US Centers for Disease Control studied the blood of these individuals and concluded there was no evidence the reactions these people experienced were associated with hypersensitivity to the Starlink Bt protein.
A subsequent review of these tests by the Federal Insecticide, Fungicide, and Rodenticide Act Scientific Advisory Panel points out that while "the negative results decrease the probability that the Cry9C protein is the cause of allergic symptoms in the individuals examined ... in the absence of a positive control and questions regarding the sensitivity and specificity of the assay, it is not possible to assign a negative predictive value to this"
The southern portion of the U.S. corn belt planted the greatest amount of StarLink corn. It is this portion of the U.S. where corn borer damage creates the greatest economic loss to farmers.
The US corn supply has been monitored for the presence of the Starlink Bt proteins since 2001. No verified positive test for the protein for dry milling have been found since 2003.
In 2007, researchers from South Africa announced the production of transgenic maize resistant to maize streak disease (MSD), caused by maize streak virus (MSV). This variety of maize is still in the research-and-development phase.
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