Title : Why is the mid-southern United States an epicenter for the development of herbicide resistant weeds?
The intensity of agriculture varies across the globe. This report discusses one aspect of plant biology as it relates to the control of unwanted vegetation in a highly managed, intensively farmed area within the United States. While the biological principles apply in all environments, a combination of factors has resulted in a rapid development of herbicide resistant weeds based upon the selection pressure applied. The general area I’m describing is the mid-southern United States approximately 500 km in a circle drawn around Memphis Tennessee.The list of currently confirmed glyphosate resistant weeds in this area is lengthy, and includes Conyza canadensis, Palmer amaranth, Ambrosia trifida, Amaranthus tuberculatus, Lolium multiflorum, Sorghum halepense, Poa annua, Eleusine indica and Echinochloa species. Many of these also have resistance to other different modes of action. Why are there so many weeds that develop resistance in this area? While no meta-analysis has been performed to identify the most important causal agent, a listing of the factors would include 1. A long growing season, 2. Intensive agricultural production 3. Imperfect crop rotation sequences, 4. No tillage systems being commonplace, 5. A warm climate, 6. Rapid adoption of new technology, and other factors. A long growing season is important because there are multiple niches available for different crops to grow and thus develop and complete their lifecycles. More days of plant growth allow more opportunities for different species to thrive here. Intensive agriculture implies that there is no grazing and no perennial crop such as grass that would provide long-term competition for the various weeds. It is rare to have major herbicide resistant issues within grazed systems. The agriculture in this area is cotton, corn, soybeans and some rice. The frequency of farmers changing what crops they are growing, or rotating their crops, is variable among the different operations. Some farmers grow soybeans each year due to annual river flooding of bottom land fields. Unfortunately, some farmers will grow the same crop multiple years in a row which leads to the situation to select a given weed given a specific herbicide regime. The loess-based rolling topography of this area coupled with intense thunderstorms in the early summer impart the propensity for massive soil erosion losses, so many of the fields are under no tillage production systems. This removes tillage, both before and after planting, as a viable weed control strategy. This puts even more pressure on the herbicides to provide effective weed control. A warm climate has several factors besides having more crops growing here. Crops can germinate earlier in the year and later in the year. Additionally, herbicides that may last and persist to provide greater residual control under cooler environments may degrade more rapidly and thus provide less residual control in a warm climate such as this area. Given the combination of all the above aforementioned aspects of this region, many growers rapidly adopt new herbicides and new herbicide technologies. Thus they are the first ones to begin using them and as such are the first where resistance may develop. As the colloquial phrase goes, “the pioneers take all the arrows”. That being said, even though they may rapidly adapt the new technology, there is still a culture where there is some resistance to change. Thus, the attitude, “I will continue to use my identical herbicide program as long as it works” is a prevalent attitude. Another aspect of why so many herbicides have been confirmed with resistance in this area has to do with scientific discovery. In this region there are several highly active weed science research communities or groups. These diligent and dedicated scientists pursue the weed populations that are not adequately controlled and conduct research upon them to confirm and provide alternate control strategies to the producers. In the absence of this research activity, the existence of some of these populations would only be known in a very small local area.