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Soil residuals… have a backup plan?

In my last column I ended on the topic of “whole farm” weed control and resistance management. Like it or not, weed management has become more complicated — even if you have not seen the first incidence of weed resistance on your farm.

I quoted Steve Powles: “If your weed management program is working, change it.” I have often used the common saying in articles through the years, “if it isn’t broken, don’t fix it.” The rapid increase in herbicide-resistant weeds has now changed that saying to “if it isn’t broken, you need to fix it.”

Resistance management is not as simple as making a change in one field where you suspect a problem may be developing. It is also not as simple as just using more soil residual herbicides in your program. It is going to require a long-term plan to get more crop, herbicide and technology diversity across the entire farm.

I have attended several meetings where the primary emphasis on Palmer pigweed management has been to increase the use of soil residual herbicides in cotton and soybeans. This is a step in the right direction, but that alone is not enough — especially where you know you already have glyphosate-resistant pigweeds.

I am all for increasing the use of soil residual herbicides, but I am much more interested in the backup plan for when they do not work. Before the introduction of Roundup Ready soybeans, we were using soil residual herbicides followed by conventional postemergence herbicides and were steadily losing ground to Palmer pigweed in some areas of the state.

Surface-applied soil residual herbicides are erratic in nature due to our erratic rainfall patterns in the Mid-South. In the heyday of soil residual herbicides, most of those applied in Arkansas were incorporated for that reason. There is an increased interest in incorporated herbicides. However, with the widespread use of conservation tillage, it is doubtful there will be a big increase in the use of these treatments.

Perhaps, the best way to make my point about the erratic nature of surface-applied residual herbicides is with a slide Bob Scott, University of Arkansas Extension weed scientist, is using in his soybean weed control talks. On the slide Bob has the 2009 results of several pre-emergence herbicides from Dick Oliver’s Palmer pigweed work at the Pine Tree Experiment Station. In 2009 all of the herbicides provided Palmer pigweed control above 80 percent. That makes the point of why you need to use them.

However, also on the slide is the three-year average control from the same herbicides. Those averages were in the 40 to 50 percent range, which means in at least one of the other years the control was much lower than the average.

There has been an increased emphasis on early preplant, surface-applied residual herbicides — especially those containing Valor. Applying a surface applied treatment 14 or so days prior to planting can increase the chances for activation before pigweed emergence and help the consistency. It also allows the option of applying another residual treatment at planting. However, you need a solid postemergence backup plan.

Soil residual herbicides worked great in 2009 and I recommend using them regardless of whether you grow conventional, Roundup Ready or LibertyLink soybeans. However, they have always been erratic and that is not going to change. In the days before Roundup Ready we would hit a dry spell at planting and the residual herbicides would fail. Then if the grower missed the timing by a few days (often as little as a day or two) with Blazer- and Reflex-type herbicides, or if the weeds were drought-stressed a little, it was over.

Maybe things have changed since the 1980s and early 1990s, but I doubt it. Somewhere write down that Ford said, “Overdependence on soil residual herbicides for controlling resistant pigweeds will result in a lot of grown up messes.”


TAGS: Management
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