Transgenic maize (corn) has been deliberately genetically modified to have agronomically desirable traits. Traits that have been engineered into corn include resistance to herbicides and resistance to insect pests, the latter being achieved by incorporation of a gene that codes for the Bacillus thuringiensis (Bt) toxin. Hybrids with both herbicide and pest resistance have also been produced. Transgenic maize is currently grown commercially in a number of countries, including the United States (where over 80% of the maize crop is genetically-modified), South Africa, and, to a limited degree, in Spain, the Czech Republic, Portugal and Germany.
Corn varieties resistant to glyphosate isopropylamine (salt) (Liberty) herbicides and Roundup have been produced. There are also corn hybrids with tolerance to imidazoline herbicides marketed by Pioneer Hi-Bred under the trade mark Clearfield, but in these the herbicide tolerance trait was bred without the use of 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 the United States. A variation of herbicide resistant GM corn was approved for import into the European Union in 2004. Such imports remain highly controversial (The Independent, 2005).
Expressing the toxin was achieved by inserting a gene from the lepidoptera pathogen microorganism Bacillus thuringiensis into the corn genome. This gene codes for a toxin that causes the formation of pores in the 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. (Broderick et al., PNAS 2006) This is contrary to the common misconception that Bt toxin kills the larvae by starvation.
In 2001, Bt176 varieties were voluntarily withdrawn from the list of approved varieties by the United States Environmental Protection Agency (EPA) when it was found to have little or no Bt expression in the ears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. (Current status of Bt Corn Hybrids, 2005)
In May 1999, a laboratory at Cornell University published the results from a laboratory trial that appeared to indicate that the pollen of genetically modified Bt corn presented a threat to monarch caterpillars. Critics claimed that the popular media was wrong to report that monarch butterflies were threatened because this experiment did not duplicate natural conditions under which monarch caterpillars may come in contact with corn pollen. (Cornell News, 1999)
In 2001 the scientific journal the Proceedings of the National Academy of Sciences published six comprehensive studies that showed that Bt corn pollen does not pose a risk to monarch populations for the following reasons:
(Sears, et al., 2001)
Monarch populations in the USA during 1999 increased by 30%, despite Bt corn accounting for 30% of all corn grown in the USA that year. The beneficial effects of Bt corn on Monarch populations can be attributed to reduced pesticide use. (Trewavas and Leaver, 2001).
Numerous scientific studies continue to investigate the potential effects of Bt corn on a variety of nontarget invertebrates. A synthesis of data from many such field studies (Marvier et al. 2007) found that the measured effect depends on the standard of comparison. The overall abundance of nontarget invertebrates in Cry1Ab variety Bt corn fields is significantly higher compared to non-GM corn fields treated with insecticides, but significantly lower compared to insecticide-free non-GM corn fields. Abundance in fields of another variety, Cry3Bb corn, is not significantly different compared to non-GM corn fields either with or without insecticides.
By law, farmers in the United States who plant Bt corn must plant non-Bt corn nearby. These non-modified fields are to provide a location to harbor pests. The theory behind these refuges is to slow the evolution of the pests to the Bt pesticide. Doing so enables an area of the landscape where wild type pests will not be immediately killed.
It is anticipated that 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 non resistant pest. Ensuring that there are at least some breeding pests nearby that are not resistant, increases the chance that resistant pests will choose to mate with a nonresistant one. 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 utilize genetic drift to ensure that any resistance that does emerge does not spread.
However, although mandated by law, compliance data from the EPA for 2008 showed that 25% of Bt corn growers were not in compliance. Data showed that noncompliance climbed to 13.23 million acres or almost 15% of all Bt corn grown, suggesting that 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-born 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 that the pollen is wind-borne rather than another method of transfer. The concentrations in the refuge fields were found to be low-to-moderate.
Possible solutions to the cross-pollination problem are to plant a wider refuge field or plant varieties of corn that bloom at different times than the Bt fields do. (Chilcutt & Tabashnik, 2004)
"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. Herbicide resistant sweet corn has not yet been released for sale.
A 2009 study compared an analysis of blood and organ system data from trials with rats fed three main commercialized genetically modified types of maize which are present in food and feed in the world. Approximately 60 different biochemical parameters were classified per organ and measured in serum and urine after 5 and 14 weeks of feeding. GM maize-fed rats were compared first to their respective isogenic or parental non-GM equivalent control groups, followed by comparison to six reference groups, which had consumed various other non-GM maize varieties. According to the authors, "Our analysis clearly reveals for the 3 GMOs new side effects linked with GM maize consumption, which were sex- and often dose-dependent. Effects were mostly associated with the kidney and liver, the dietary detoxifying organs, although different between the 3 GMOs. Other effects were also noticed in the heart, adrenal glands, spleen and haematopoietic system. We conclude that these data highlight signs of hepatorenal toxicity, possibly due to the new pesticides specific to each GM corn. In addition, unintended direct or indirect metabolic consequences of the genetic modification cannot be excluded."
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 that the reactions 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" 
Aid sent by the UN and the US to Central African nations also contained some StarLink corn. The nations involved refused to accept the aid.
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 positive samples have been found since 2004, showing that it was possible to withdraw this GM crop without leaving traces in the environment once it has been used in the field 
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 maize is still in the research and development phase.