The prevalence of Roundup Ready crops has not caused weeds to become resistant to glyphosate; rather, resistance is a function of how glyphosate has been used in RR crops and in other areas. In general, resistance to an herbicide is related to how it is used and the potential for resistance to evolve. Because of the recognized safety and environmental benefits of glyphosate, many farmers relied on it as the sole herbicide to control weeds, and we now know that this practice was not sustainable. Importantly, in cooperation with leading academics, we are making progress in educating growers to use other herbicides and to incorporate non-chemical weed-control practices into their overall weed management program. This increased diversity will lead to a reduction in future resistance.
Additionally, we know that when a weed is resistant, increasing the herbicide rate will not then control it, so more glyphosate will not be effective to control-resistant populations.
Dow AgroSciences is asking the government to approve 2,4-D-tolerant crops, and Monsanto is asking for approval of dicamba-tolerant crops. Both herbicides are effective for controlling weeds that are resistant to glyphosate and for which there are currently few other options. This is why groups like the National Soybean Growers Association, National Cotton Council, Weed Science Society of America and others have written letters to the government in support of both 2,4-D and dicamba. This is important to farmers and to environmentalists who appreciate the benefits of conservation tillage systems, such as reduced pollution of our rivers and streams due to soil erosion. To continue to realize these important benefits (see answer to previous question on benefits), farmers need new herbicide options for managing hard-to-control weeds; otherwise, they would have to consider incorporating tillage operations for weed control and give up some of the benefits already gained by switching to conservation tillage. From a health-and-safety standpoint, both 2,4-D and dicamba have been thoroughly tested and have been approved for agricultural and homeowner uses for over 30 years.
Canadian regulators have recently authorized the planting of soybeans and feed corn tolerant to the herbicide 2,4-D. Similar approvals are being sought in the United States. Approvals for the import of 2,4-D-tolerant corn grain have also been granted recently by many other nations. While cultivation of 2,4-D-tolerant crops is expected in coming growing seasons, thus far no commercial planting of these crops has occurred.
The reason crops are being stacked with 2,4-D tolerance is to undercut the weed resistance now developing in North America (and elsewhere) as a result of over-reliance on glyphosate as a single mechanism of control. Exclusive reliance on any single means of weed control, whether mechanical, chemical or genetic modification, will foster weed shifts and weed resistance. Using multiple methods of control (including herbicides with different modes of action) helps reduce the natural selection processes that drive weed resistance.
Over the past 15 years, glyphosate-tolerant cropping (which requires less plowing) has made important contributions to agricultural sustainability in terms of reductions in soil erosion, field runoff, soil compaction and air emissions from tractor exhaust. Farmers want to retain glyphosate-tolerant cropping because of its many benefits, which include increased productivity and flexibility in farm operations. To do this, however, they need new crop technology to break current weed resistance dynamics that are reducing the effectiveness of glyphosate as a farm tool, which in turn is resulting in increased herbicide use (in terms of both increased use rates and frequency of application). Incorporating 2,4-D tolerance traits into glyphosate-tolerant crops can help sustain the benefits of current farm methods by making it convenient for farmers to use multiple modes of control in their operations.
2,4-D is currently one of the most widely used herbicides in the world. It has been used for more than 60 years and is currently approved for use in more than 70 countries worldwide—among them Canada, the United Kingdom, Germany, France, Japan, Australia and the United States. Based on evaluations of extensive research, both regulators and health and safety organizations worldwide have reported little risk for adverse effects when 2,4-D is used in accordance with directions on the product label.