Among the four glyphosate resistant weeds in South Dakota, common waterhemp has the potential to be the most problematic in areas where corn-soybean rotation is a 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," said Paul O. Johnson, SDSU Extension agronomy field specialist. 

Johnson explains that in the 1980s and 1990s there was a rapid expansion of corn and bean only rotations to the north and west in South Dakota.

"These were also very moist years," he said. "Additionally, in the 1990s, waterhemp moved rapidly to the north and west. Within 10 years waterhemp became the most dominant broadleaf weed in row-crops in South Dakota."

Waterhemp has two qualities which helped it spread rapidly Johnson explained. "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."

Because of this, Johnson said 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," he said.

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, Johnson said. "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 watherhemp resistance at that time, rotating chemical chemistries could help to prolong the development of resistance in the future," he said.

With the growing characteristics of waterhemp, Johnson said 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," Johnson said. "In corn, this situation would limit effective chemistries to only three which are readily used."

He added that there is also known resistance to the No. 27 Hppd Bleacher like Callisto and the synthetic auxins like 2, 4-D. "Thus, it is not unreasonable to speculate waterhemp could develop a six way resistance. If that happened, there would only be one viable post-emergence chemistry would remain for use in corn," Johnson said.