Several years ago, it developed resistance to a couple of herbicide families. Today, growers worry the same thing could happen to glyphosate. No documented cases of glyphosate-resistant waterhemp exist, but plenty of speculation does. Here's a look at the latest developments.

No need to be embarrassed. The neighbor has the same problem. Stemmy outcrops of waterhemp clutter what would be an enviably clean soybean field. But mere aesthetics are the least of problems for growers facing waterhemp. Heavy infestations steal yield, and even rogue season-long survivors shower the soil with seed for next year. As if that isn't troubling enough, growers using Roundup Ready technology keep in the back of their minds the fact that waterhemp managed to develop resistance to popular ALS-inhibitor herbicides. They hope that a weed shift won't turn into a weed's resistance.

One tough weed
Of course, one reason the neighbors have the same problem with waterhemp is they probably used the same herbicide regimen. In the 2001 growing season, glyphosate-tolerant soybeans covered some 68 percent of all acres planted to soybeans in the United States. For that, there is a reason: The herbicide is cheap and it works. At least it works up to a point. But the nature of waterhemp, with its season-long emergence, can foil non-residual herbicides. Moreover, growers may be hoping for more than they'll get when they attack weeds larger than the label's target size.

"If you get outside that size window, there is a lot of plant to kill," said Reid Smeda, a University of Missouri weed scientist. "Waterhemp can form a carpet, and it grows at different rates so you get a coverage issue that can minimize the amount of Roundup a plant receives."

Resistance reports
Aside from problems with the timing or mechanics of herbicide application, there have been news-making reports of possible glyphosate resistance among waterhemp populations. However, those early reports of possible resistance have been scaled back.

Smeda is currently studying progeny from a suspect population of waterhemp found near Monticello, Mo., along with seed of a separate population from Illinois.

His ongoing findings shine light on the complexity of understanding waterhemp and why scientists are reserving judgment about resistance.

Plants from both fields survived an initial application of Roundup. The suspect plants were collected in 1999 and in the greenhouse have generated three subsequent generations of the weed. So far, each of the those generations has been largely controlled by a 1-quart application of Roundup. But "largely controlled" is the interesting part: In each generation, 3 to 5 percent of the plants survive the 1-quart (or sometimes higher) rate.

Yet when seeds from those surviving plants are grown and treated with Roundup, the result is another population that is largely controlled (except for 3 to 5 percent of the plants) by the 1-quart rate.

Smeda says that one tenet of weed resistance is that weeds survive rates of herbicide that were previously toxic and that plants pass the characteristic that lends to their survival to their progeny. When waterhemp adapted to ALS-inhibitors, for example, progeny from the resistant plants delivered the trait in their seed and larger populations were created.

In Smeda's experiments, the population that survives the glyphosate application is consistent but not growing. He has dubbed these plants "insensitive."

"Farmers may say that 95 percent control is great, but to us it is a nagging question. Right now it might be more of an academic pursuit than a problem out there," said Smeda. "But we want to know why those plants aren't dying."

Possible, not likely
One thing Smeda would like to make clear: When weeds become resistant to a herbicide it isn't because the chemical used in the herbicide caused some mutation in the genetic makeup of the plant. Instead, a small population with natural resistance already existed or the plant naturally mutates at the physiological point affected by the herbicide. Smeda said that rule-of-thumb estimations for natural mutations that can lead to resistant plants are one in one million to one in 100 million. In either case, selection pressure via continous use of a herbicide chemistry causes the resistant weed population to increase.

Along those lines, Smeda outlines four paths toward herbicide resistance.

1. Continuous use of a herbicide with the same mode of action. Think ALS-inhibitors in the late 1980s.
2. High selection pressure--a very effective herbicide will effectively kill off any non-resistant weed seed sources.
3. Genetic variation within the weed plant. For example: Waterhemp plants are either male or female; thus any progeny is the result of cross-pollination.
4. Use of herbicides with only one mode of action.

Across the board
Of course, glyphosate fits all four categories. Affordable glyphosate prices and thin commodity margins likely will continue to make glyphosate-resistant soybeans a popular choice, but Smeda offers a familiar caveat:

"I've tried to advocate that the way to prevent resistance in weeds is to rotate the chemistry of herbicides used.

"Right now we want to understand the physiology of those 3 to 5 percent that survive a quart (application rate of glyphosate). If it is something going on that is complex and involves more environmental factors, we might have some insensitive populations that pop up, and we have to manage them at that level. Or, it could be the tip of the iceberg."