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How do selective herbicides work?

Vegetables
03.04.2017
Man spraying - Selective Herbicides

By Scott Mathew, Senior Solutions Development Lead @HortApplication

Recently, a couple of growers have mentioned to me that they have found it particularly difficult to manage their crops this summer due to the extreme climatic conditions experienced. The common example provided was where herbicide use appeared to have set the crop back mildly at times compared to normal. This raised a few questions…

What makes a herbicide ‘selective’?

When a herbicide is referred to as being selective it means that the herbicide can be applied to your crop and will control or suppresses the targeted weed species without affecting the growth of your crop.

How is herbicide selectivity achieved?

Herbicide selectivity is generally achieved broadly by either:

  1. Selectivity by placement:
  • By applying the herbicide in your crop in such a way that you minimize any contact between the herbicide applied and your crop. For example, where you might apply a pre-emergent herbicide to soil at, or immediately after planting, but before crops and weeds emerge.  Weeds germinating in the chemical band in the soil will be controlled whereas the crop germinates at a different depth and not in contact with the herbicide which ensures herbicide selectivity or crop safety.
  1. True selectivity:
  • When the herbicide can be applied to the foliage of the crop or to the soil in which the crop is growing without concern of injury or damage to the crop. The crop can achieve this generally by one of three ways:
  1. Morphological which refers to the plant characteristics such as the leaf orientation, waxiness or pubescence (hairiness) etc. For example, a hairy leaf surface makes it more difficult for the herbicide to reach the leaf surface and affect the crop.
  2. Physiological differences refers to the different processes that affect the activity and/or the breakdown of the herbicide. For example, your crop plant may translocate the herbicide at a slower rate than the weeds resulting in the herbicide controlling the weeds and not affecting the crop. Your crop may have the ability to integrate the herbicide with something in the cell cytoplasm, or channel the herbicide into ‘sinks’ where the herbicide will have no effect on your crop, however controls the weeds.
  3. Deactivation or metabolism which is ability of some plants to slow or prevent the activity of the herbicide so the plant is tolerant to a particular product. For example, Sweet Corn plants metabolise and convert atrazine to an innocuous metabolite within the plant so rapidly that the herbicide does not have time to inhibit photosynthesis which provides crop tolerance. Metabolic insensitivity and/or the ability to metabolise the herbicide usually are the best types of crop selectivity.

Why do the selective herbicide sometimes cause a yellowing of my crop?

Commonly herbicide selectivity is achieved through the ability of the crop to metabolise the herbicide into a form that has no herbicidal impact on the crop. Anything that places the crop under stress such as environmental stresses (waterlogging, frost, drought etc.) can affect the crops ability to effectively metabolise the selective herbicide that you have applied. As a result, a herbicide that typically doesn’t cause any crop effects may in fact cause some  symptoms if the crop is under stress and consequently can’t metabolise the herbicide as efficiently as it  would. Fortunately, most times these symptoms are transient and the crop quickly recovers from the herbicide affects resulting in no impact on the final yield.