Most herbicides that have been used in agriculture for the past 60 years were developed by generating many compounds in the lab and spraying them on weeds in the greenhouse to identify potential candidates for further development. Those that were herbicidal were then tested on crops to assess the crop’s sensitivity. Once researchers had promising candidates for new herbicidal compounds, they then conducted tests to determine whether the herbicide and herbicide resistant crop performed well under field conditions and to assess any impacts on the environment and human health.

A number of decades ago, the number of chemicals that must be tested to develop a new herbicide began to increase exponentially from less than 1,000 in 1950 to approximately one million today. As a result, companies began to consider how they could develop new weed management options by focusing on modifying the crop, rather than finding a new herbicide. Starting with an herbicide already been proven to be effective and then giving the crop plant the capacity to resist that herbicide is more efficient and allows companies that sell herbicides to expand the applicability of their most effective and safest products.

Prior to the advent of modern biotechnology, this approach was not very productive.  Crop developers were constrained by the need to use plants in the same or very closely related species, and most crop plants have very little genetic diversity for herbicide resistance (Green, 1998). An exception was endogenous tolerance of soybeans to metribuzin and 2,4-D, which varied among different cultivars. As a result, soybean breeders were able to use conventional breeding and selection to increase soybean’s endogenous tolerance to these two herbicides and to combine resistance to both in a cultivar that was resistant to one, but not the other. 

Crop developers have used a number of techniques to create novel herbicide resistance traits. Screening and selection of plants with novel genetic variability led to the development of soybeans tolerant to sulfonylurea herbicides (Sebastian et al., 1989).

In the example above, researchers subjected the seeds to mutagens and tested whole plants for herbicide resistance. Screening whole plants for herbicide resistance takes considerable effort, time and space, because the seeds must be planted and allowed to develop before the herbicide is applied. Recent breakthroughs in plant cell and tissue culture associated with modern biotechnology allow researchers to screen cells and undifferentiated plant callus for herbicide resistance, which increases development efficiency. Treatment of corn tissue cultures with imidazolines followed by recurrent selection created imidazoline resistant callus that could then be regenerated into corn plants and bred into a variety of cultivars (Dyer, 1994).

Crop developers have also accessed novel genetic variability by exploiting the capacity of weeds to evolve resistance. For example, a breeding program was initiated to move triazine resistance found in a weed, Brassica rapa, to canola, a crop closely related to the weed (Hall et al., 1996). Triazine resistant canola was created, but unfortunately, the new resistance trait lowered crop productivity, which reduced the cultivar’s commercial value.

The tools of modern biotechnology reduced the impediments imposed by the requirement to cross breed crops with the same or closely related species. An herbicide resistant gene present in a given organism can now be incorporated into virtually any crop. This flexibility has triggered a revolution in weed management by expanding the spectrum of herbicides a crop can tolerate. 

The first transgenic herbicide resistant crop, bromoxynil-resistant cotton, was commercialized in 1995, and glyphosate-resistant soybeans came to market the following year. Glyphosate resistant soybeans, corn, cotton and canola are now grown on millions of hectares around the world (James, 2005).