Perhaps no single event in the history of agriculture has changed weed management to the extent that Roundup Ready crops have. In 2005, 89% of the soybean acreage in Missouri was planted with Roundup Ready soybean varieties while approximately 15% of our corn acreage was planted with Roundup Ready corn. Many have speculated that our Roundup Ready corn acreage will continue to increase in the next few years, which would place much of our acreage in a Roundup Ready soybean-Roundup Ready corn rotation. And, even though most growers will be applying a preemergence herbicide treatment in corn (hopefully), it’s probably safe to assume that the vast majority of the postemergence applications in either Roundup Ready corn or soybeans will involve glyphosate. This scenario represents a tremendous selection pressure that will be placed on weeds to develop resistance to glyphosate.

In fact, in recent years a number of glyphosate-resistant weed biotypes have already been identified. These resistant species have been identified primarily in locations where a Roundup Ready crop, such as soybean or cotton, has been planted continuously without rotation and where repeated applications of glyphosate have been made year after year. Some of the more recent examples of these species include a number of glyphosate-resistant horseweed biotypes located throughout the central and eastern United States (including sites in southeastern Missouri), a glyphosate- resistant common ragweed biotype discovered in Missouri and Arkansas in 2004, glyphosate-resistant palmer amaranth biotypes discovered in Georgia, Tennessee, and North Carolina in 2005, and glyphosate-resistant common waterhemp biotypes that we discovered in two separate locations in northwestern Missouri also in 2005. Although we are just beginning our field research on these populations and these biotypes are not "officially" confirmed resistant yet, all research that we have conducted thus far leads us to believe that these waterhemp populations are indeed resistant to standard and even fairly high rates of glyphosate (Figures 1 and 2).

The identification of these glyphosate-resistant waterhemp populations in Missouri should be viewed as a big, flashing "WARNING" sign to growers who have been planting a continuous Roundup Ready crop like soybeans for many years in a row or even to growers who are planting a Roundup Ready soybean-Roundup Ready corn rotation. The last thing we need in our corn and soybean production systems is to lose the profitability of our current weed management programs as a result of the development of glyphosate-resistant weeds.

In our view, two alternatives to this problem exist. Farmers can manage the development of resistance by not spraying glyphosate continuously in the same area over time and by rotating to herbicides with other modes-of-action (proactive resistance management), or farmers can choose to deal with a glyphosate-resistant weed once it occurs by using more expensive weed control measures at that time.

The question boils down to: "do I pay a slightly higher price today for weed management strategies that delay or prevent glyphosate resistance or do I pay a much higher price in the future when a glyphosate-resistant weed requires me to use more expensive control measures?" Costs in the long term (e.g. more than 5 years) do not really cost as much in today’s dollars as costs incurred in the near term (e.g., within 5 years). Discounting all expenses back to today’s dollar allows for a comparison of the cost of managing resistance versus the cost of having resistance.

Recently, some of our colleagues have published an interesting study which attempts to address many of these important issues1. In this study, the authors determined that if it took common waterhemp 29 years or less to develop resistance to glyphosate, then it would be better economically to proactively manage resistance ¹ or use alternative weed control strategies now in an effort to delay resistance. On the other hand, the authors found that if it took more than 29 years for common waterhemp to develop resistance to glyphosate, then it would be better from an economic standpoint to just continue to use glyphosate without rotation (use the tool until it breaks strategy).

This study made a necessary assumption that could affect the predicted costs of developing glyphosate resistance. They assumed that herbicide prices before and after glyphosate resistance develops will be the same. A more likely scenario is that once glyphosate resistance develops, the price of alternative herbicides will increase because the demand for them increases. Such increases in alternative herbicides would make dealing with resistance even more expensive and give added incentive to managing resistance proactively.

Another important aspect of this study is that this was written prior to our discovery of glyphosate-resistant common waterhemp in Missouri. Given that we now have a real-world scenario and glyphosate resistance has been discovered in common waterhemp after about 8 or 9 years of continuous glyphosate use, it seems clear to us that growers should consider adopting some of these proactive weed management strategies in order to prevent higher costs in the future.

Some examples of proactive strategies for the management of common waterhemp, for example, would be to:

1. Rotate to conventional corn hybrids and get out of a continuous soybean rotation if at all possible.
2. Use conventional preemergence and/or postemergence herbicide programs that are effective on common waterhemp in corn, given that most producers will plant Roundup Ready hybrids and utilize glyphosate as the sole herbicide for weed control in soybean.
3. In Roundup Ready soybean, first consider the use of preemergence herbicides that have good activity on common waterhemp (Valor, Spartan, Dual, Intrro, etc.). The use of an effective postemergence (Phoenix, Cobra, Reflex, Flexstar, Ultra Blazer) herbicide other than glyphosate either applied alone or as a glyphosate tank-mix partner is also an option, but these products are generally more expensive than some of the preemergence options like Valor listed above.

¹ Mueller, T. C., P. D. Mitchell, B. G. Young, and A. S. Culpeper. 2005. Proactive versus reactive management of glyphosate-resistant or -tolerant Weeds. Weed Technology 19:924-933.

Kevin Bradley
573-882-4039
Ray Massey
573-884-7788