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To deal with rising resistance to existing herbicides, Bayer CropScience has been developing various genetically modified crops resistant to HPPD inhibitors: in one version, the crops are resistant to both HPPD inhibitors and glyphosate, and in collaboration with Syngenta, crops that are resistant to HPPD inhibitors and glufosinate. [12] [13]
One major complication to the use of herbicides for weed control is the ability of plants to evolve herbicide resistance, rendering the herbicides ineffective against target plants. Out of 31 known herbicide modes of action, weeds have evolved resistance to 21. 268 plant species are known to have evolved herbicide resistance at least once. [59]
A weed that develops resistance to one herbicide typically has resistance to other herbicides with the same mode of action (MoA), so herbicides with different MoAs, or different resistance groups, are needed. Preventative weed resistance management rotates herbicide types to prevent selective breeding of resistance to the same mode of action.
Genetically modified crops resistant to glufosinate were created by genetically engineering the bar or pat genes from streptomyces into the relevant crop seeds. [ 4 ] : 98 [ 7 ] In 1995 the first glufosinate-resistant crop, canola, was brought to market, and it was followed by corn in 1997, cotton in 2004, and soybeans in 2011.
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]
The majority of GM crops have been modified to be resistant to selected herbicides, usually a glyphosate or glufosinate based one. Genetically modified crops engineered to resist herbicides are now more available than conventionally bred resistant varieties; [20] in the USA 93% of soybeans and most of the GM maize grown is glyphosate tolerant. [21]
Weeds resistant to multiple herbicide modes of action are also on the rise. [7] Before glyphosate, most herbicides would kill a limited number of weed species, forcing farmers to continually rotate their crops and herbicides to prevent resistance. Glyphosate disrupts the ability of most plants to construct new proteins.
This reduction in pesticide use has been ecologically beneficial, but benefits may be reduced by overuse. [4] Yield gains and pesticide reductions are larger for insect-resistant crops than for herbicide-tolerant crops. [5] Yield and profit gains are higher in developing countries than in developed countries. [3]