Ag Instructor Vic Martin: Is Resistance Inevitable?

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Great Bend Tribune
Published September 22, 2019

Part I

First, the drought monitor is unchanged for this week.  Second, slowly, very slowly corn harvest is starting; some grain sorghum fields are nearing harvest; soybean plants are dropping leaves and pods are turning color.  While the hot weather hasn’t been terribly peasant for people, it has aided in summer crop maturation and preparing fields for winter wheat.  One note, last week a column in the paper discussed fall planting of alfalfa.  The information was accurate, however, one caveat is in order, actually two.  Fall establishment is preferred as it allows for better production the first year.  But, unless under irrigation, it tends to be riskier especially with hot air and soil temperatures due to the shallow planting depth required.  This means it is best to wait until temperatures are more moderate with adequate soil moisture for establishment.  And you must have adequate time for the plant to establish a root system and store reserves to overwinter.  Planting the latter half of September is riskier as the average first frost date is mid-October.  And with the price of establishing an alfalfa field, quite risky agronomically and economically.  Now to today’s topic.

Last week’s column unfortunately only appeared online and was omitted from the print edition.  It dealt with crop hybrids and varieties and breeding for them to be able to tolerate or even resist diseases and insects.  If you are interested, you can find the column at the Tribune website.  This is a positive for crops but what about pesticide resistance for weeds and diseases and insects?  Today, let’s focus on pesticide resistance in weeds, diseases, and insects, how it happens. 

  • All pesticides have a “mode of action”, or in English, how it kills the pest.  There are a variety of the modes of action.  For example, herbicides may attack a part of photosynthesis, cell respiration, cell division, or disrupt cell walls.  An insecticide may attack the nervous system, hormonal functions, and so on.  These modes of actions are classified (grouped) so you can identify it.  The chemical name or brand name will not indicate the mode of action.  In fact, the same chemistry may be sold under a variety of brand names.
  • For simplicity, we will stick with herbicides here, but this applies to insecticides and fungicides.  Also remember that weeds are flowering plants, reproducing sexually with genetic mixing.  A chemistry is determined that is effective in killing a weed but not the crop.  The crop is able to metabolize the chemical, break it down, but the weed can’t.  For example, you have a pigweed infestation and spray it with glyphosate (Roundup ®) and only glyphosate.  These plants are very similar genetically but due to sexual reproduction there are small differences.  One pigweed plant can produce well over 100,000 seeds.  Out of that 100,000, 99,900 or so can’t metabolize the chemistry and die.  Some are genetically different enough they are damaged but still manage to produce some viable seed.  A very few are so genetically different, they can metabolize the chemical and are unaffected.  Those few plants can produce thousands of viable seeds and most are likely resistant to glyphosate.  Next year you are on the same field and notice an area of pigweed and they seem unfazed by glyphosate.  They reproduce and each year the area grows.  What has happened?  By only using one mode of action, a producer has essentially bred a weed species, shifted its genetics, to tolerate and metabolize what was designed to kill it. This doesn’t happen overnight and whether an herbicide, insecticide, or fungicide, the process is essentially the same.

Next week: preventing and eliminating resistance.



Part II
Published September 29, 2019

First the drought monitor remains basically unchanged.  Harvest is slowly underway with corn and some soybeans.  Grain sorghum is all over the place with maturity and the sorghum planted close to on time looks promising.  Wheat and rye and being planted and some has emerged.  The warm weather has helped move things along, however, next week’s forecast predicts a major cool down by the middle of this week.  Last week we discussed herbicide resistance in weeds and how it develops.  This week – can we do much about it and can we prevent it with newer herbicide chemistries?

  • To answer the second question first, yes.  As will be discussed below, using these techniques will allow new chemistries to be effective and remain so.  The key for new products is to rotate modes of action, what kills the weed.
  • Simple and to the point – start with a clean, weed-free seedbed and do everything possible to produce a vigorous actively growing crop that achieves full canopy as quickly as possible. 
  • While expensive, new products are coming on the market with new modes of action.  This doesn’t mean totally abandon products like glyphosate but using them in combination and rotation with other products.  Some of the chemistries must be used in combination with varieties and hybrids specially developed for the herbicide and others not.
  • Even with the expense, move beyond post emergence only herbicide programs and use a pre/post combination.  And while taking rotational considerations into account, consider more soil applied residual herbicide.
  • As much as is possible, weed control, especially of seedling weeds, must be an obsession. And even though no-till and conservation tillage acreage has suffered with herbicide resistance, move back to reducing tillage and where practical eliminate it.  If weed pressure can be controlled for several years under this system, weed pressure starts to diminish.  However, this can be difficult to accomplish with growing conditions.
  • Use as diverse a crop rotation as possible, especially if broadleaves and grass crops can be used.  In some areas, using a broadleaf crop may not be practical.
  • Cover crops that can provide good ground coverage and then be terminated and left on the soil surface can help decrease weed pressure.  This need not be an expensive cover crop blend.  It can be something as simple as planting spring outs at several bushels per acre in the fall and letting them die out over the winter and then plant into the residue.
  • Finally, while not a great option – a fallow period with super aggressive weed control (chemical, mechanical, or a combination) for those severely infested fields.
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