The uncontrolled growth of the weeds causes the cultivations to fight against them to obtain the sunlight and the necessary nutrients, for this reason most of the time significant losses are generated. Since herbicides do not have the ability to differentiate between cultivation and weeds, conventional agricultural systems can only apply to selective herbicide cultivations. This type of herbicide does not affect the crop, however, it does not have a complete effectiveness to destroy all types of weeds. If farmers generate herbicide resistant crops, then it is possible to implement non-selective herbicides to destroy all weeds, quickly and efficiently with a single application. The result would be less spraying, less field traffic and lower operating costs.
Usefulness of non-selective herbicides
The herbicides of broad spectrum, commonly known as non-selective, have great effectiveness in destroying an extensive list of weeds. However, applying non-selective herbicides could destroy the crop. Therefore, non-selective herbicides are most useful when applied before seedling emergence or in unexpected cases in orchards, vineyards and nurseries.
Selectivity of herbicides
The reason why herbicides have a great value because they can be implemented just before planting or emergence of cultivations, they can even be used at the top after the emergence of the cultivation, without getting too many injuries. A large number of herbicides marked for this type of application, can currently selectively eliminate an extensive amount of weeds, without intervening with the crop. To achieve selectivity, two methods can be applied: placement selectivity and real selectivity.
Achieving a selectivity by avoiding or decreasing the contact between the herbicide and the cultivation is known as location selectivity. The most common case is to direct an herbicide (such as: glyphosate) to a weed without exposing it to the crop plant. This type of selectivity is as effective as any other, as long as the excess herbicide is prevented from leaching into the root zone where it can absorb it and is not removed from the weeds. Another method of placement selectivity is when an herbicide that does not leach easily is used on the outer layer of soil to achieve shallow weed control, but it does not leach the root areas of a more established cultivation such as established alfalfa or fruit trees.
The real selectivity is the one that completely tolerates the result of some morphological, physiological or biochemical means. The herbicide can be used on the crop foliage or directly on the soil where the crop is growing without risking any injury. Certainly the actual selectivity may be the best of its kind, however, it is not perfect. This is thanks to aspects such as the stage of cuticle thickness, cultivation development, air temperature and humidity, location of growth point, virulence of leaf surface, size of droplets spraying and the surface tension of the spray droplets which may affect the activity of the herbicide. When the conditions are the most optimal for the herbicidal activity, even a real selectivity may not prevent the cultivation from suffering any injury.
Among the morphological differences we can find the aspects of the plant such as the wax or villi of the leaf surface, the size and orientation of the leaf, the location of the point of growth and the depth of the root.
Almost always, the more waxy or more hairy the leaf surface, the more difficult is the foliar penetration of an applied herbicide. The more the growth point is protected (as in the case of grasses), the decreases the chance that the foliar herbicides will reach their point of growth. As long as the cultivation is deep rooted, it will be harder to get a soil weed herbicide at the roots of the cultivation, and less likely to be harvested for the crop.
Physiological differences may contain various processes that hinder herbicide activity and / or breakdown. Depending on the situation, herbicides may:
- Transport by different routes using the plasma motto.
- Move in a different way inside the plant.
- Combine with some element inside the cell wall.
- Integrate into a cellular cytoplasm
- Piped into sinks where the herbicide will lose its full effect.
All of these aspects could boost tolerance, but any aspect would rarely develop tolerance on its own.
The metabolic aspects have the genetic insensitivity given to an irritated area of herbicidal action that prevents the action of the herbicide. In the case of Roundup Ready soybeans, this generates an excess of the enzyme that glyphosate (Roundup) generally inhibits, so Roundup Ready soybeans do not suffer alterations, however, normal amounts of the herbicide are absorbed by the plant of the cultivation. Maize plants metabolize and transform atrazine into an innocuous metabolite with a rapidity that prevents the herbicide from inhibiting photosynthesis, offering crops tolerance as long as the metabolic system has not been saturated by excess pesticide or a combination of these.