Continued use of these older chemicals, ones that tend to persist in the soil and may move into groundwater, could cause environmental problems. Herbicide resistance of many different lines may be tested using cell culture techniques. The corn hybrid on the left was found to have resistance to herbicide, while the plants on the right are susceptible to herbicide. Associated with the exclusive use of one herbicide for weed control in each crop in a rotation is the possible development of herbicide-resistant weeds. There are already over 100 species of weed plants known to have developed resistance to one herbicide or another.
Few herbicides are available for control of broadleaf weeds in cotton.
Genetic engineering has made it possible to develop herbicide-resistant cotton. This possibility has not received much attention in the United States because in most cases our weed species are not closely related to crop species. An exception is the vegetable industry in California because many of the vegetable crops have closely related weed relatives in the wild. On the other hand, the possibility of outcrossing may not be very likely, considering the lengths to which plant breeders had to go to accomplish interspecific hybridization before genetic engineering was introduced. Disease Control Control of disease is a subject of great interest for biotechnologists.