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Crop Responses to Herbicides
by Bob Hartzler

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June 11, 2000 -  There has been a higher number of herbicide injury problems this year, and unlike most years the problems are being caused by a diverse number of products.  The typical questions that arise are: 1) 'Why is the damage so severe?' and 2) 'How can I avoid similar problems in the future?'.  In this article I will address some of the underlying factors that may have led to these problems.   Keep in mind that in many situations a combination of factors ultimately results in the problem and it is impossible to place the blame on a single factor.

Background:  A person must first understand the fate of a herbicide applied to a crop plant to understand why crop injury occurs.  As soon as the herbicide is intercepted by the crop it begins to move from the plant surface into the cell cytoplasm.  To accomplish this the herbicide must move across the cuticle, cell wall and then the cell membrane.  This process is driven by diffusion, a physical process in which a chemical moves from an area with a high concentration (leaf surface or soil solution) to an area with a low concentration (cytoplasm).  Once inside the cell the herbicide faces one of two fates: 1) it reaches the target site where it can bind and disrupt some physiological process, or 2) it is intercepted by a degradation system that detoxifies the herbicide before it reaches the target site.  Under ideal conditions, the majority of herbicide is metabolized before it reaches the target site in the crop and no adverse response occurs.  Unfortunately, several factors influence these two processes and result in crop injury.  The following factors may be responsible for many of the injury situations observed this year.

Environment:  Any condition that stresses the crop (extremes in temperature or moisture) will reduce the crop’s tolerance to a herbicide.  The reason for increased susceptibility in stressed crops is largely due to how the rates of herbicide absorption and metabolism respond to stress.  Since herbicide absorption is a physical process (diffusion), environmental factors that stress the plant are unlikely to cause large reductions in herbicide movement from the leaf surface into the plant cell.  Long-distance symplastic transport of herbicides is affected more by stress than absorption or movement within leaves, but this type of transport is relatively unimportant in the activity of herbicides on annual plants (corn, soybeans, most weeds). 

While herbicide absorption and movement to the target site is affected only slightly by environmental stress, the ability of the plant to metabolize the herbicide may be greatly reduced.   First, plants under stress may have less energy reserves available to drive the enzymatic systems that metabolize herbicides.  Secondly, and perhaps more important, the ability of the plant to detoxify the herbicide may be impaired when it is under stress.  Photosynthesis creates numerous highly reactive compounds that are damaging to to the plant, and the production of these compounds increases when the plant is under stress.  Several defense mechanisms in the plant limit the damage from these phytotoxic compounds.  The same enzymatic systems that protect the cell from the toxic chemicals produced by photosynthesis also detoxify many herbicides.  Since a plant has a limited capacity to metabolize toxic compounds, the detoxification system of the plant may be overwhelmed by the combination of stress-induced compounds and herbicides.  This allows the herbicide to reach the target site at concentrations capable of causing crop injury. 

Under stress conditions the rate of herbicide metabolism is affected much more than the rate of herbicide absorption, therefore allowing the herbicide to reach the site of action at relatively high concentrations.  This is why opposite ends of the environmental spectrum (wet vs dry; hot vs cold) can cause a similar response in the crop to a herbicide.

Delayed rain and soil-applied herbicides:  Many areas of the state received little or no rainfall during the first four to five weeks after the crop was put into the ground.  When rain finally came the crop may have reached the V2 to V3 stage and was beginning to show signs of water deficiency.  Under these conditions a plant will absorb a large pulse of water when the first significant rain occurs. This pulse of water moving into the plant is much greater than would occur during normal transpiration of a plant not having been under water stress.  Herbicides present in the soil solution are carried passively into the plant with water absorbed by roots.  Thus, the crop would absorb an abnormally large amount of herbicide following the rain, and in some cases this may have overloaded the detoxification systems.

Herbicide combinations:  A growing trend is to use a multitude of herbicides in order to control all weeds in a single pass across the field.  While these combinations usually do not cause problems, each additional product added to the tank inherently increases the potential for crop injury.  Even under ideal conditions some herbicide reaches the target site and inhibits the activity of that enzymatic system.  A healthy crop is normally able to tolerate this low level of disruption; however, if several processes are affected the end result can be a synergistic response resulting in significant injury. 

Why does a combination of herbicides have greater risk of injury than a single product?  The widespread use of adding 0.5 to 1.0 lb of atrazine to assorted postemergence treatments in corn is an example of how disruption of multiple processes can increase phytotoxicity.  The low rate of atrazine normally would kill very few weeds if applied by itself.  However, the slight disruption of photosynthesis caused by the atrazine is sufficient to make weeds more susceptible to the primary herbicide in the tank mix.  This same thing can happen to the crop when multiple products are applied in an attempt to achieve one-pass control.

Postemergence herbicide combinations may also result in undesirable consequences if the products have different requirements for spray additives.  The best example of this would be the addition of dicamba to various ALS-inhibitors (Accent, Basis Gold, Northstar, Permit, etc.).  ALS herbicides require the addition of spray additives to improve herbicide absorption, whereas dicamba formulations (Banvel, Clarity) do not require these materials.  The inclusion of spray additives with dicamba increases the rate of absorption, therefore increasing the likelihood of the herbicide reaching the target site.

Herbicide carryover:  Many areas of the state are experiencing greater problems with herbicide carryover this spring than typically encountered.  This is largely due to the dry period during 1999 that followed the postemergence application season.  Although in most situations the carryover injury appears to be minor, the low level of crop injury may predispose the crop to other herbicide problems as described earlier.

Miscellaneous factors:   Many additional factors can increase crop stress and therefore reduce herbicide tolerance.  Examples of stress that have placed crops under stress this spring are flea beetles, bean leaf beetle, Rhizoctonia, Phytophthora and Stewart's wilt.  As with the environment, any factor that reduces the vigor of the crop will reduce the ability of the crop to tolerate the herbicide.

Management considerations:  The simplest means of reducing herbicide injury is to avoid applications to crops under stress.  However, to accomplish this might result in only 5 to 10% of the corn and soybean acres being treated in any given year.  Thus, compromises must be made.   However, the risk of injury can be managed by using common sense and monitoring the crop prior to application.  Learn all you can about the different herbicide options and their margins of crop safety.  The following tactics can be used to reduce the potential for crop injury when crops are under stress:

While there may be inconveniences to these tactics (increased trips, increased costs, need for specialized equipment), the reduced risk of crop injury often outweighs the disadvantages.  Due to adverse weather, there will always be situations when a grower has no option but to make an application to a crop with a fairly high level of risk.  However, many of the problems this spring were created either by attempting to design a herbicide treatment that will control all weeds in a single trip or by ignoring the condition of the crop at the time of application.  These situations can be avoided through use of good management.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
http://www.weeds.iastate.edu
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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.