Big Weeds in Soybean. What Can You Do?

Published: August 1, 2008

There hasn't been a lot of variety to the calls I've been getting lately. They have to do with essentially one issue; controlling big weeds in soybean. Either these weeds never got sprayed because of the wet season we've had, or the weeds have already been sprayed one or more times with glyphosate and they are still living. Most of the complaints I've been hearing have to do with waterhemp (as usual), but running a close second this year is giant ragweed. Last year we conducted a phone survey, which indicated that we have at least 200,000 acres of soybean in Missouri infested with glyphosate-resistant waterhemp. At the time, I thought it was a very conservative estimate. Now I know it is.

Soybean fields like this one with a waterhemp population that is 4 to 5 feet in height are not an uncommon site this year.

My answer to these calls has been pretty simple-if you have a fi eld where the weeds (including waterhemp) have gotten tall simply because you haven't been able to get a sprayer across the fi eld and you DON'T suspect you have any glyphosate-resistant weeds present, then our research shows that increasing the rate of glyphosate will generally provide as good or better weed control than adding a herbicide tank-mix partner to glyphosate in Roundup Ready soybeans. Unless you have glyphosate- resistant weeds present, most of the available research indicates that the number one factor that infl uences the level of weed control you will get with glyphosate is the rate you apply. Simply put, the higher the rate, the better the weed control in most cases.

If, however, you suspect that you do have a glyphosate-resistant weed like waterhemp present, then a tank-mix partner can be very benefi cial. Increasing the rate of glyphosate in this case will rarely provide better weed control and will almost certainly cost you more money. In our research, we found that a tank-mix of Ultra Blazer at 1.5 pts/A, Flexstar at ¾ pt/A, or Phoenix at 8ozs/A with glyphosate provided from 77 to 85% control of glyphosate-resistant waterhemp six weeks after treatment. It is important to recognize that this level of control was achieved when these herbicides were applied to 6- to 8-inch tall waterhemp. It is unlikely that you will get this kind of control if you are spraying waterhemp that is much taller than this. Regardless, these herbicides are essentially the last option for suppression or control of glyphosate-resistant waterhemp at this point in the season.

My answer is the same if you suspect you have other glyphosate-resistant weeds like common or giant ragweed. Although for the most part Firstrate and some of the other ALS-inhibiting herbicides still have good activity on common and giant ragweed in Missouri, these herbicides are perhaps even more sensitive to weed height. Th is means that when the ragweeds get over one foot or so in height, our likelihood of controlling them with these herbicides goes down dramatically. So, we are still left with PPO-inhibiting herbicides like Phoenix in combination with glyphosate and hope we get "burn-back" of these weeds and good canopy closure. Not ideal by any stretch of the imagination, but about the best we can hope for with the conditions we have experienced this year.