Tuesday, July 15, 2014

Battling Weed Resistance



Burrus believes that resistance is the single greatest agronomic issue facing growers, and we also believe resistance will continue to be the greatest single threat to productivity.  We have often noted that agriculture faces the uncomfortable dilemma of having only a limited number of pest management tools, many of which are being lost to resistance (pests adapting to the point that such products no longer prove effective).  If the issue of resistance has been displayed anywhere throughout the Burrus footprint, it has been dramatically displayed in the arena of weed management (especially in the arena of Roundup resistance).
Managing resistance is a very complex topic.  However, managing weed resistance essentially comes down to a five step approach.  Growers need to ambitiously scout their fields, and growers need to “Error on the Side of Caution.”  Growers need to engage in a “Lockdown” strategy.  Growers need to give herbicides a fighting chance, and growers need to move beyond “Number 9.”
Growers need to ambitiously scout their fields.  The weed resistance battle is a war.  It is a war that, if lost, will dramatically increase the cost of production while decreasing yields.  It is not dramatic to say that its’ outcome will define the future of agriculture.  As with any war, winning the battle requires good intelligence.  You have to know the nature and status of your enemy.  Weeds are that enemy.  Growers need a clear picture of historic weed pressure in each field.  Detecting a problem requires such a reference point.  They need to know the spectrum of weeds they have to deal with, and they should know if recent history has changed any part of that story.  Windshield observations cannot provide that type of insight.  Professional crop scouting can help, but surveying the field must also become part of the grower’s daily routine (if it is not already).  Grower “boots on the ground” are essential.
Growers need to error on the side of caution.  One of the most unfortunate things about resistance is that once officially confirmed, management strategies are already too late.  Rather than waiting to battle resistance, growers should be battling resistance before it ever gains a foothold.   We might call this being proactive rather than reactive.   Here are a few signals that might indicate resistance is beginning or ready to appear:  1) Weeds that escape minus obvious applicator errors.  2) The rise of a single “hard to control” species rather than multiple “hard to control species.”  3) Plant herbicide exposure symptoms minus plant death.  4)  Historic good control followed by a recent change toward more escapes.  5) Frequent or continuous reliance upon one herbicide mode of action.  Some might say that this sounds like “nervous pest management,” but Burrus would say this is an “error on the side of caution.”
Growers need to engage in a “lockdown” strategy.  “Lockdown” is defined as “a state of isolation or restricted access instituted as a security measure.”  We have already stated that resistance represents one of the most significant threats to 21st century agriculture.  It might be a little dramatic to say that resistance is an “agricultural security” issue, but with that said ….. the statement really is not too far from the mark.  When an issue of security arises, a first logical step toward winning the battle is to limit access.  In this case, the enemy is resistant weeds and we need to restrict current and/or future access to the field.  First, we need to remove suspect weeds where present to eliminate seed production (even if only a few and even by hand if necessary).  Second, we need to remove suspect plants that reside near a field (elimination of suspect weeds along road sides comes to mind via hand removal or by mowing the field previous to seed production).  Third, we need to avoid those areas infested with suspect plants during harvest (removing those plants from the field soon after harvest or mowing them down minus incorporation to increase seed mortality).  Fourth, we need to avoid using products, materials, or equipment that could harbor seed from resistant plants (no more hitching a ride into fields).
Growers need to give herbicides a fighting chance.  One of the quickest ways to encourage herbicide resistance is to expose a plant to “less than lethal” amounts of herbicide.  Skimping on rates is one example of “less than lethal”/resistance encouraging herbicide exposure.  However, there are many ways to create “less than lethal” exposure.  Applying an incorrect rate for the size of plant, applying a product with the knowledge that rain may soon wash a lot of it from the plant, poor coverage, etc. all create a “less than lethal” exposure event. 
Growers need to move beyond number 9.  While the list of herbicide resistance concerns is long, we would lie if we did not admit that glyphosate/Roundup resistant weeds are “the” weed resistance topic at the moment.  Glyphosate is cheap and until recently, it has been very affective across fields.  This resulted in repeated high use of Roundup which created lots of selection/resistance pressure.  Glyphosate is classified as a HRAC 9 herbicide (HRAC stands for Herbicide Resistance Action Committee).  A truly affective resistance management program must move beyond reliance upon only HRAC 9 (or any single HRAC group for that matter).  Other modes of action should be used in place of HRAC 9 or other frequently used modes of action.  Other modes of action should also be used before or along with HRAC 9 (glyphosate) or other frequently used modes of action as well.

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