Excessive herbicide treatment can cause a great deal of damage to a tolerant plant because deactiva-
tion or degradation systems can become saturated.
B. Herbicide Application
- Herbicides of Preemergence Application
Within this category are the herbicides that are generally applied to the soil before crops are sown and al-
ways before the plants reach the surface.
Because seeds of many species are quite small and germinate at a depth of 0.7 to 1.5 cm, it is im-
portant that herbicides are applied in the upper 2 or 3 cm of the soil.
Once herbicides come into contact with the plant, they are absorbed through roots or apical meris-
tems. Herbicides that are absorbed by roots will be absorbed while the top of the root is in the layer of soil
that contains the herbicide. Upon continuing growth, the root at a greater depth will decline in absorption.
If the tops of the roots leave the contaminated layer soon, the plant will be likely to survive.
Many herbicides, upon application to the soil, are absorbed through apical meristems and can kill
many scions before leaving the soil. Among of these herbicides, some enter the apical meristems as gases
(e.g., thiocarbamates) or as a liquid (e.g., alachlor). There are environmental and physical factors that pro-
mote rapid growth in crop plants and therefore their time in touch with the herbicides is reduced.
Some herbicides have different abilities to move in the plant. Thus, some herbicides are absorbed by
roots and move to the leaves. Others do not have movement capacity; therefore, their symptoms are ex-
pressed only at the absorption sites. Generally, symptoms are more prominent at sites where the mobile
herbicides are concentrated.
- Herbicides of Postemergence Application
These herbicides are applied when the plant has emerged from the soil. Generally, they are absorbed by
leaves. The relationship between the quantity of herbicide and the surface of the leaf, size of the plant,
age, hydric stress, air temperature, relative humidity, etc. are factors that can influence the quantity of ab-
sorbed herbicide; for example, in dry conditions and at high temperatures, absorption is less.
Herbicides also differ in their capacity to move in the plant. Those that are nonmobile (e.g., diphenyl
ether, bipyridine) should cover more of the plant. The mobile herbicides can move from their site of ap-
plication to the site at which they act.
C. Herbicide Resistance
A great many weeds that at one time were susceptible to certain herbicides and were easily treated but at
present they have now developed resistance to them. In fact, currently not less than 53 kinds of weeds are
known to be resistant to at least five different herbicide families [1,2].
Resistance to herbicides is probably developed by selection of the resistant biotypes after extended
exposure of a species of weed to a herbicide (we define biotypes as plants within the same species that
are characterized by having a given feature in common). The resistant plants will survive the herbicide,
produce seeds, and create new resistant weed generations.
Resistance mechanisms vary depending on the herbicide family. The resistant biotypes can have a
slightly different biochemistry in their susceptible reactions, which can determine the resistance. For ex-
ample, although photosynthesis is inhibited in the biotypes susceptible to triazine, a small change in pro-
tein D1 makes resistant biotypes that accomplish perfect photosynthesis in the presence of the herbicide.
The appearance of the resistant biotypes is much easier if the herbicide has only one site of action.
In spite of resistance mechanisms, knowing the mode of action of herbicides would help develop
prevention and extinction programs for resistant biotypes. The use of a single strategy with a herbicide
family would increase the number of resistant biotypes, which would cause serious problems in the near
future.
There is a series of recommendations to prevent and to treat resistance to herbicides:
Practice crop rotation.
Rotate herbicide families.
Mix herbicides with different modes of action.774 DE LA ROSA