Genetically modified maize
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[edit]
Herbicide-resistant maize[edit]
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.[1] Bayer CropScience developed "Liberty Link Corn" that is resistant to glufosinate.[2] 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.[3] Consequently, the regulatory framework governing the approval of transgenic crops does not apply for Clearfield.[3]
As of 2011, herbicide-resistant GM corn was grown in 14 countries.[4] By 2012, 26 varieties of herbicide-resistant GM maize were authorised for import into the European Union,[5] but such imports remain controversial.[6] Cultivation of herbicide-resistant corn in the EU provides substantial farm-level benefits.[7]
Products in development[edit]
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.[28] 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.[29] As of 2014, there have been a few MSV-tolerant cultivars released in Africa. A private company Seedco has released 5 MSV cultivars.[30]
Research has been done on adding a single E. coli gene to maize to enable it to be grown with an essential amino acid (methionine).[31][32]
Refuges[edit]
US Environmental Protection Agency (EPA) regulations require farmers who plant Bt corn to plant non-Bt corn nearby (called a refuge), with the logic that pests will infest the non-Bt corn and thus will not evolve a resistance to the Bt toxin.[33] Typically, 20% of corn in a grower's fields must be refuge; refuge must be at least 0.5 miles from Bt corn for lepidopteran pests, and refuge for corn rootworm must at least be adjacent to a Bt field.[34] EPA regulations also require seed companies to train farmers how to maintain refuges, to collect data on the refuges and to report that data to the EPA.[33] A study of these reports found that from 2003 to 2005 farmer compliance with keeping refuges was above 90%, but that by 2008 approximately 25% of Bt corn farmers did not keep refuges properly, raising concerns that resistance would develop.[33]
Unmodified crops received most of the economic benefits of Bt corn in the US in 1996–2007, because of the overall reduction of pest populations. This reduction came because females laid eggs on modified and unmodified strains alike, but pest organisms that develop on the modified strain are eliminated.[35]
Seed bags containing both Bt and refuge seed have been approved by the EPA in the United States. These seed mixtures were marketed as "Refuge in a Bag" (RIB) to increase farmer compliance with refuge requirements and reduce additional work needed at planting from having separate Bt and refuge seed bags on hand. The EPA approved a lower percentage of refuge seed in these seed mixtures ranging from 5 to 10%. This strategy is likely to reduce the likelihood of Bt-resistance occurring for corn rootworm, but may increase the risk of resistance for lepidopteran pests, such as European corn borer. Increased concerns for resistance with seed mixtures include partially resistant larvae on a Bt plant being able to move to a susceptible plant to survive or cross pollination of refuge pollen on to Bt plants that can lower the amount of Bt expressed in kernels for ear feeding insects.[36][37]
Resistance[edit]
Resistant strains of the European corn borer have developed in areas with defective or absent refuge management.[35][33] In 2012, a Florida field trial demonstrated that army worms were resistant to Bt maize produced by Dupont-Dow; armyworm resistance was first discovered in Puerto Rico in 2006, prompting Dow and DuPont to voluntarily stop selling the product on the island.[38]