Make Sure You Truly Have Resistant Weeds Before Jumping To Conclusions

Sometimes weeds don't go down due to other factors besides weed resistance.

Published on: Mar 16, 2012

No one is denying that weed resistance too many commonly used herbicides is a growing problem. Resistance to glyphosate has become an issue with several weeds, including marestail. Also known in some areas as horseweed, marestail is most commonly a problem in soybean fields.

It can be a problem in both conventional and no-till systems. However, Barry Fisher, agronomist for the Natural Resources Conservation Service, says that every time you don't get good control of marestail in a field, doesn't automatically mean that you have resistant marestail. There are other factors which can cause escapes or less than ideal control.

Make Sure You Truly Have Resistant Weeds Before Jumping To Conclusions
Make Sure You Truly Have Resistant Weeds Before Jumping To Conclusions

One of those was largely ignored until the past couple of seasons. Most well water in Indiana is hard, meaning it has lots of minerals ion it. Most spraying, at least if you do your own, is done using water from wells as the liquid carrier. Fred Whitford, head of Purdue Pesticide Programs, began to point out recently that hard water can interfere with weed control to the degree that it can really affect performance of a herbicide.

Hard water tends to be associated with higher pH levels, Fisher says. The results is typically that glypohosate, the broad-spectrum weed killer, is not as effective in that solution. The difference can be enough to make or break control on weeds where glyphosate may not be the strongest anyway.

Another complicating factor is that farmers sometimes try to skimp on rates, even of glyphosate. If you skimp on rates and the water is hard, meaning the pH level is high, you're setting yourself up for very poor weed control, Fisher notes. It has nothing to do with whether the weed is resistant or not. Even non-resistant weeds may not be killed effectively if not enough glyphosate reaches the plant. That can happen if you skimp too much on rates, or if the water is hard and you don't buffer the water to raise the pH level.