One of the questions that comes up once a grower decides to kill marestail with tillage instead of burndown herbicides is – does he still need residual herbicides? The thinking here apparently is that: A) tillage takes care of marestail for this crop year; B) marestail is the only weed for which residual herbicides are necessary; and C) POST applications of glyphosate are adequate to control the rest of the weeds that are going to emerge after planting. Only “A” is correct. A brief list of reasons why we use residual herbicides may be helpful.
Using a program consisting of only glyphosate is the reason we have glyphosate-resistant populations of marestail. We also have glyphosate-resistant giant and common ragweed, waterhemp, and Palmer amaranth, and apparently also redroot pigweed based on our latest greenhouse screens. Early-season control from residual herbicides reduces the number of weeds that will be treated with glyphosate POST, which reduces the risk of additional glyphosate-resistant populations.
Giant ragweed and other large-seeded broadleaf weeds are most easily managed with a combination of residual and POST herbicides. The residual herbicides can reduce the population and slow the growth of remaining plants, so that the field is more likely to need just one POST application. Omitting residual herbicides pretty much ensures that two POST applications will be needed if the field has more than a few giant ragweed.
Remember lambsquarters - easily controlled with residual herbicides and tolerant to many POST herbicides? Back in 2005 or so, before the use of residual herbicides rebounded, we received lots of questions about problems with POST control of lambsquarters with glyphosate. We decline to answer those questions now because they indicate a failure to use a simple solution.
Remember the two other major reasons we use residual herbicides, to start with some early-season weed control so that: 1) not every field has to be sprayed POST at the same time; and 2) crop yield is not reduced by weeds if the POST is sprayed too late.
To be completely candid and not tactful at all, all of us who provide guidance to growers on weed control issues – dealers, CCA’s, consultants, extension – don’t know what else to do to communicate the importance of residual herbicides. They are a first line of defense against poor weed control, herbicide resistance, and yield loss due to weeds. Omit them from your weed management program at your own risk.