Genetically modified maize (corn) is a genetically modified crop. Specific maize strains have been genetically engineered to express agriculturally-desirable traits, including resistance to pests and to herbicides. Maize strains with both traits are now in use in multiple countries. GM maize has also caused controversy with respect to possible health effects, impact on other insects and impact on other plants via gene flow. One strain, called Starlink, was approved only for animal feed in the US but was found in food, leading to a series of recalls starting in 2000.
Marketed products
Herbicide-resistant maize Corn varieties resistant to glyphosate herbicides were first commercialized in 1996 by Monsanto, and are known as "Roundup Ready Corn". They tolerate the use of Roundup. Bayer CropScience developed "Liberty Link Corn" that is resistant to glufosinate. Pioneer Hi-Bred has developed and markets 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 of transgenic crops does not apply for Clearfield. As of 2011, herbicide-resistant GM corn was grown in 14 countries. By 2012, 26 varieties of herbicide-resistant GM maize were authorised for import into the European Union, but such imports remain controversial. Cultivation of herbicide-resistant corn in the EU provides substantial farm-level benefits.
Insect-resistant corn
Bt maize/corn Bt maize/Bt corn is a variant of maize that has been genetically altered to express one or more proteins from the bacterium Bacillus thuringiensis including Delta endotoxins. The protein is poisonous to certain insect pests. Spores of the bacillus are widely used in organic gardening, although GM corn is not considered organic. The European corn borer causes about a billion dollars in damage to corn crops each year. In recent years, traits have been added to ward off corn ear worms and root worms, the latter of which annually causes about a billion dollars in damages. The Bt protein is expressed throughout the plant. When a vulnerable insect eats the Bt-containing plant, the protein is activated in its gut, which is alkaline. In the alkaline environment, the protein partially unfolds and is cut by other proteins, forming a toxin that paralyzes the insect's digestive system and forms holes in the gut wall. The insect stops eating within a few hours and eventually starves. In 1996, the first GM maize producing a Bt Cry protein was approved, which killed the European corn borer and related species; subsequent Bt genes were introduced that killed corn rootworm larvae. The Philippine Government has promoted Bt corn, hoping for insect resistance and higher yields. Approved Bt genes include single and stacked (event names bracketed) configurations of: Cry1A.105 (MON89034), CryIAb (MON810), CryIF (1507), Cry2Ab (MON89034), Cry3Bb1 (MON863 and MON88017), Cry34Ab1 (59122), Cry35Ab1 (59122), mCry3A (MIR604), and Vip3A (MIR162), in both corn and cotton. Corn genetically modified to produce VIP was first approved in the US in 2010. A 2018 study found that Bt-corn protected nearby fields of non-Bt corn and nearby vegetable crops, reducing the use of pesticides on those crops. Data from 1976 to 1996 (before Bt corn was widespread) was compared to data after it was adopted (1996–2016). They examined levels of the European corn borer and corn earworm. Their larvae eat a variety of crops, including peppers and green beans. Between 1992 and 2016, the amount of insecticide applied to New Jersey pepper fields decreased by 85 percent. Another factor was the introduction of more effective pesticides that were applied less often.
Sweet Corn GM sweet corn varieties include "Attribute", the brand name for insect-resistant sweet corn developed by Syngenta and Performance Series insect-resistant sweet corn developed by Monsanto.
Cuba While Cuba's agriculture is largely focused on organic production, as of 2010, the country had developed a variety of genetically modified corn that is resistant to the palomilla moth.
Drought-resistant maize In 2013 Monsanto launched the first transgenic drought tolerance trait in a line of corn hybrids called DroughtGard. The MON 87460 trait is provided by the insertion of the cspB gene from the soil microbe Bacillus subtilis; it was approved by the USDA in 2011 and by China in 2013.
Health Safety In regular corn crops, insects promote fungal colonization by creating "wounds," or holes, in corn kernels. These wounds are favored by fungal spores for germination, which subsequently leads to mycotoxin accumulation in the crop that can be carcinogenic and toxic to humans and other animals. This can prove to be especially devastating in developing countries with drastic climate patterns such as high temperatures, which favor the development of toxic fungi. In addition, higher mycotoxin levels leads to market rejection or reduced market prices for the grain. GM corn crops encounter fewer insect attacks, and thus, have lower concentrations of mycotoxins. Meta-analyses of long-term field studies are reporting that Bt maize contains significantly lower concentrations of mycotoxins with carcinogens than non-GM maize, mainly due to reduced insect damage to the kernels. Fewer insect attacks also keep corn ears from being damaged, which increases overall yields.
Products in development In 2007, South African researchers announced the production of transgenic maize resistant to maize streak virus (MSV), although it has not been released as a product. While breeding cultivars for resistance to MSV isn't done in the public, the private sector, international research centers, and national programmes have done all of the breeding. As of 2014, there have been a few MSV-tolerant cultivars released in Africa. A private company Seedco has released 5 MSV cultivars. Research has been done on adding a single Escherichia coli gene to maize to enable it to be grown with an essential amino acid (methionine).
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