In some years, waterhemp will be of little consequence due to good conditions for herbicide activity and unfavorable conditions for survival of late-emerging weeds. In other years, waterhemp will cause major problems due to these two characteristics. There has been considerable discussion concerning the risk of weeds developing resistance to Roundup. Some people stated that it was extremely unlikely that Roundup resistance would appear in weeds, but shortly after these statements were made the first case of Roundup resistance appeared in Australia. The potential for resistance to different herbicides varies widely, and evidence supports the view that Roundup is at a lower risk than many other herbicides.
Some herbicides are at less risk for resistance because they have multiple sites of action.
The likelihood of a biotype having two altered sites of action is much less than that of a biotype having a single altered site of action. Other herbicides may have a low risk of resistance because changes in the site of action that prevent herbicide binding may interfere with the activity of the enzyme. In this situation, most mutations that provide herbicide resistance result in death of the plant since the enzyme is non-functional, even in the absence of the herbicide. This is believed to be the reason for the low risk for resistance with Roundup. The first reported case of resistance to Roundup occurred in Australia in 1995 and involved rigid ryegrass.