Among the four glyphosate-resistant weeds in South Dakota, common waterhemp has the potential to have the highest impact areas where a corn-soybean rotation is the mainstay. Thirty years ago, waterhemp was only found in the very southeast corner of the state. It was a tough weed to control then and still is.
In the eighties and nineties there was a rapid expansion of the corn and bean only rotation to the north and west in South Dakota. These were also very moist years. Additionally, in the nineties, waterhemp moved rapidly to the north and west. Within ten years waterhemp became the most dominant broadleaf weed in row-crops in South Dakota.
Waterhemp has two qualities that helped it spread rapidly. First, it is a very prolific seed producer. Second, it has the ability to germinate all summer long (into August) and still produce viable seed that fall. So each time there is a significant moisture event of an inch or more and the ground is not shaded, a new flush of waterhemp will germinate. Because of these two qualities, glyphosate became the chemical of choice for control of waterhemp.
Before Roundup Ready®, most row crops were started with a pre-emergence herbicide with residual, which was then followed by a late post-emergence application to control the late emerging weeds.
What is the key for the future? Rotating chemicals and the use of timing will help. The addition of a small grain in the rotation will also help a lot, as the small grain will canopy early and shade the ground, keeping the waterhemp from germinating. If there is no resistance (watherhemp) at that time, rotating chemical chemistries could help to prolong the development of resistance in the future.
With the growing characteristics of waterhemp, pre-emergence-only control is not likely to have high success. Currently in the U.S. (but not in South Dakota) waterhemp with four-way chemical resistance has been identified. An example of this resistance is: “No. 5 photo inhibitor like atrazine, No. 2 ALS like Pursuit®, No. 14 PPO inhibitor like Cobra® and No. 9 EPSP like glyphosate.” In that situation, all present post-emergence chemistries used in beans would be ineffective.