Through gene biology, the genes of plants can be modified so that they will be disease-resistant and pest-resistant and still produce the same amount. The most common GM crops grown at the moment are those that resist herbicides. The second most common are those crops that kill pests. Some crops have been grown with both these genes. If a crop can resist herbicides, the farmer can spray a field with herbicides without harming the crop. All the weeds and other plant due, but the crop does nit.