While weed species of concern may differ from region to region, resistance problems do not. In this article, I would like to share with you a little perspective on the current situation and a few things to keep in mind as you consider weed control options in the future.
Herbicide resistance is not a new problem. For example, atrazine was first registered for use in the U.S. in 1958 and the first incidence of an atrazine-resistant weed, common groundsel, was reported in 1968. For as long as we have been using herbicides, there have been weeds capable of evolving resistance. But in previous decades, there were other factors that helped buffer against the problem that herbicide resistant weeds posed. Tillage was used more often and controlled weeds by burying them and their seeds or uprooting seedlings. Farms were smaller and typically allowed a farmer to spend more time per acre on weed control. In the realm of herbicides, more companies were competing against each other to develop new modes of action and bring products to market. Duke (2012) estimates that before 1990, a new herbicide mode of action was commercialized about every three years. Since 1990, there have been no new modes of action introduced.
The other major game changer was the introduction of Roundup Ready crops. Roundup Ready soybeans were first sold in 1996 and provided a simple, effective option for controlling both broadleaf and grass weeds with little risk of crop injury. This was a major advantage over older chemistries and growers responded by adopting the technology at an unprecedented pace. In 1997, Roundup Ready canola and cotton were introduced and corn followed in 1998. Even in a corn-soybean rotation, the same herbicide could now be used every year. Growing Roundup Ready corn and soybean and relying heavily, or in some cases exclusively, on Roundup homogenized weed control across crops. This in turn reduced the benefits of crop rotation on weed management. The Roundup Ready system was simple, convenient, and facilitated the adoption of reduced-tillage practices. The labor-savings resulting from reduced tillage complemented larger trends of increasing farm size and fewer farmers. But the great success and wide adoption of Roundup-based weed management was its own Achilles' heel. Frequent use and a lack of diversity in control tactics, even across crop rotations, selected for weeds that were Roundup (glyphosate) resistant. Glyphosate resistant weed species are now common in many states and some examples include waterhemp, kochia, Palmer amaranth, common ragweed, and horseweed, also known as marestail. Some of these species have developed multiple herbicide resistance and options for chemical control are running out. There are populations of waterhemp in Illinois and Missouri that have been identified as being resistant to three classes of herbicides: triazine (Group 5), ALS (Group 2), and PPO (Group 14); or glyphosate (Group 9), ALS (Group 2), and PPO (Group 14).
So where do we go from here? We are being presented the option of growing crops with multiple herbicide resistance traits such as Dow's Enlist™ which combines glyphosate and 2,4-D resistance or Monsanto's Roundup Ready 2 Xtend™ which combines glyphosate and dicamba. These options will likely be available in multiple crops which means in-season herbicide applications could again be the same even in different crops. While these options may help control some glyphosate resistant weeds in the short-term, the evidence is not strong for continued control into the future. Even within the constraints of corn-soybean rotations, we have the opportunity to be more creative in how we manage our weeds. Cultural practices such as narrow row spacing in soybean, selecting corn hybrids with fast emergence, planting cover crops, aligning fertilizer application with crop uptake, and selecting a diverse herbicide program across crops are all practices that contribute to more robust weed management programs. As new crop-herbicide combinations become available, a longer-term perspective on weed management is still important. Consider not only what you do within a year, but also what you do across years. Selecting different tactics across time will be key to preserving the value of the technologies you use.
Duke, S.O. 2012. Why have no new herbicide modes of action appeared in recent years? Pest Management Science 68:505-512.