Engineering Crop Resistance to Herbicides

Herbicides are the method of choice to control weeds in most broadscale agricultural systems. They play a major role in maximising crop yields by reducing competition from weeds for space, light, water and nutrients and help control erosion by enabling weed control where crops are drilled directly without ploughing. Weeds can also act as a reservoir for crop pathogens.
Because herbicide resistance genes are also effective selectable marker genes in culture, herbicide-tolerant crop varieties were the first major transgenic trait to be produced and commercialised and herbicide tolerant varieties are still the most widely grown transgenic crops.
Based either on expression of a herbicide insensitive gene, degradation of the herbicide or overexpression of the herbicide target gene product, engineered resistance is now available to a range of herbicides,including: glyphosate (‘Roundup Ready’, ‘Touchdown’), glufosinate (‘Liberty Link’), imidazalonones (IMI), protoporphyrinogen oxidase inhibitors (‘Acuron’), bromoxynil, triazines, 2,4-D, chlorsulfuron/sulfonylureas and isoxazoles.
There is now good evidence that far from increasing the application of herbicides, the control that transgenic herbicide resistant crops provide to the farmer has resulted in a reduction in glyphosate usage of 33% on Roundup Ready soybeans and a reduction in glufosinate usage of about 20% for Liberty Link canola.