Glyphosate, the active ingredient in Roundup and other trade name herbicides, is now used on 85% of Wisconsin’s soybean crop, an increasing percentage of the corn crop, a large number of no-till acres for pre-plant burndown control, and to terminate alfalfa stands in the fall or spring.  Soon to come will be Roundup Ready wheat and alfalfa.  To summarize, everybody’s doin’ it. 

        Through all of this “glyphosate mania”, much has been learned and put into practice, learned and not put into practice, and there remain some unanswered questions.  Perhaps it is an appropriate time for some review of the knowledge base, a Glyphosate 101 if you will.  Here’s the course outline: 

A.    Less is sometimes more

        If you go to the well too many times, it will eventually go dry.  The same is true with glyphosate.  The opportunity exists to use glyphosate on every field in just about every year, but this is one time when opportunity knocks you don’t want to answer the door.  Development of weed resistance from frequent glyphosate applications is the OVERWHELMING number one concern with this herbicide.  Use glyphosate in the crop rotation where it clearly makes the most sense from a cost-return perspective, not just because it’s the easiest.  In many cases, this will mean soybeans. Using glyphosate every other year is probably too much. Vigilant use of glyphosate will result in long term effectiveness and that’s good for everyone involved. 

B.    Controlling weeds versus preserving yield         

        Fact of the matter is that glyphosate can knock down some pretty big weeds.  It’s also a fact that early season weed competition can take the edge off yield potential.  Glyphosate offers a wider window to kill weeds than many other herbicides but the window to preserve yield remains the same.  Numerous research studies have shown that crop yields begin to decline when annual grass weeds reach about 4 inches in corn and 6 inches in soybeans.  Hence, late season crop appearance may not always translate to full yield potential if glyphosate applications are made too late. 

C.    Variety is the spice of life

        In corn and soybean Roundup Ready systems, there are often good reasons to use other herbicide products in combination with glyphosate.  The obvious reason is that this will delay or prevent the establishment of glyphosate resistant weeds.  However, there are sometimes other good agronomic reasons as well.  Pre-emergence herbicides provide early season and residual activity that can help control weed species that are more difficult to control with glyphosate (e.g. lambsquarters in soybeans) and reduce early season weed competition.  Similarly, residual soil activity is sometimes needed at the time of the post-emerge glyphosate application and calls for tank mixing other herbicide products with the glyphosate (especially in corn).

D.    No-till......a different animal

        The introduction of Roundup Ready crops offered no-tillers a tempting choice to skip the pre-plant burndown treatment and simply ride the one-pass herbicide horse.  Research and grower experience have told us that this is not a good choice to make.  Again, small weeds become big weeds very quickly in the spring.  Early emerging weeds become too large, too competitive, and may even produce seed by the time later emerging annuals are prime for control.  These larger weeds both offer competition to the crop and make for even more difficult weed problems to deal with in the future.

E.     Not ready for Roundup crops

        Many growers have both Roundup Ready  and conventional crops.  A potential problem exists in the fact that only one sprayer is owned.  This makes it imperative that spray tanks and lines are thoroughly cleaned between spraying these different genetic types.  Seeing a conventional field that has been sprayed with glyphosate (even a small amount) is not a sight conducive for the cover of Farm Journal.  Worse yet is the scenario of spraying a full rate of glyphosate on a conventional field because of a miscommunication between the person doing the spraying and the person in the “know.” 

F.     The “D” word

        The good thing about glyphosate is that it will kill virtually anything that hasn’t been genetically modified for resistance.  On a windy day, it’s also the bad thing about glyphosate.  Be it your own field, a neighbor’s field, or Grandma’s garden, herbicide drift is raised to a whole new level with glyphosate.  Advances in spraying technologies now offer growers some tools that can be used to reduce drift.  These include both retardant tank additives and low-drift nozzles.  At the end of the day, however, reducing or eliminating drift mostly just involves proper equipment set-up and some common sense decision making. 

G.    The key word is “contact”

          Glyphosate is a post-emerge herbicide.  Target weeds are not killed unless contacted with the spray droplet, and because there is no soil activity, weeds that have not yet emerged will be unaffected.  We discussed not waiting too long to kill weeds but there is also a yield penalty for spraying too early.  By doing so, weeds emerging after the spray treatment can reduce yield potential.  Any factor that reduces spray contact with target weeds will hinder overall weed control.  As with spraying too early, herbicide contact may also be an issue when spraying too late as larger weeds (or a larger crop) will shield spray droplets from smaller weed seedlings. 

        Glyphosate is an important crop management tool.  However, it does not cover-up for poor management decisions and it will not remain effective unless growers strive to preserve its effectiveness.