Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
As we move into small grain harvest and considering weed control prior to planting soybeans, we have limited options. And we need to view the options as the best of a difficult situation and be realistic on what we can expect for control. Some things we need to consider are:
Weeds present at time of planting. Horseweed is the one of biggest concern, but horsenettle, ragweed, lambsquarters, grasses, etc are also present. Which of the weeds present can also be controlled after the soybeans have emerged? Are Liberty Link soybeans used, because glyphosate could be used prior to planting and then Liberty (or Ignite) used postemergence. Need to decide what problem is most critical and develop a program to target that species.
What herbicides options are available: product availability, crop rotation constraints, or environmental/soil issues. Liberty/Ignite might be the best product for some of these fields, but it is in very short supply and may not be available. Kixor products may be an option for some fields with medium-texture soils. Always consider what will be planted in the field next season and be sure there is adequate time for the intended crop rotation (it maybe only 9 months until you plant the 2013 crop!)
When glyphosate-resistant horseweed is present and it is the species you are targeting for control, you will need to rely on a herbicide other than glyphosate to control them. No product will consistently control horseweed this late in the season; and we are compounding the problems with cutting the plants off with the combine. If Liberty (or Ignite) is not available and your soil texture prevents use of Kixor, one option to consider is Gramoxone in combination with Canopy, along with crop oil concentrate and nitrogen fertilizer. Canopy is the product I would suggest because it contains metribuzin and chlorimuron. Metribuzin may improve the effectiveness of the Gramoxone. The chlorimuron at the rates used prior to planting, will have some activity on the horseweed as well. The use of crop oil and nitrogen sometimes improve Gramoxone activity, but will also maximize the effectiveness of the chlorimuron. This program is not ideal, but in my experiences it provides the best level of control under many situations. Perennial species (horsenettle or yellow nutsedge) will probably regrow and they will need to be controlled with a later glyphosate application.
If horseweed is not present in the fields then often glyphosate plus a residual herbicide may be the best option for it will control summer annual broadleaves and grasses, as well as start to “work on” the perennials. But since most plants have been damaged during small grain harvest, glyphosate activity may be reduced. But glyphosate will be used again after the soybeans have emerged and should improve overall control.