Basic concepts in the management of residual herbicides in cherry orchards (Part I)

Basic concepts in the management of residual herbicides in cherry orchards (Part I)

Agr. Eng. Jorge Lundstedt, UPL Chile.Agr. Eng. Juan Guillermo Márquez, UPL Chile.Agr. Eng. M.Sc. Carlos Tapia. Avium SpA Technical Director.

One of the most important jobs with impact upon orchard health, resource efficiency like water and nutrients, and cultural work; is weed control and the efficient as well as effective use of herbicides according to their need.

In practical cases, at the initial stage of cherry orchards, weed control could have greater importance even than plant formation management and other tasks. In general, this is due to soils being intervened with physical preparation work, nutritional plans and control of more efficient irrigation, fundamental factors so that weeds also take advantage of their development related to these programs.

While there is general management of the efficient use of herbicides in the season, without a doubt the use of narrowed down programs that allow more control time, seems to be the best way to go. 

The utilization of a program based upon residual herbicides applied in the winter months becomes a fundamental part of the weed control program, often along with the complement of particular objectives in each case.

General Classification of herbs

There are many ways to classify herbicides, including the way they are used, their chemical properties and their action mode.

Considering the part of the plant that is sensitive to the action of the herbicide and according to the physical place where mass entry of the product takes place, these are classified as root-absorbed herbicides and foliar herbicides.

Depending upon the behavior of the herbicides that penetrate through the foliage of the plant, these may be classified as contact, those that enter in a very localized fashion and practically do not move internally. On the other hand, systemic products are those that are absorbed and move extensively inside the plant, either via the symplast (by the floem) and/or apoplast (xylem).

Foliar- acting herbicides, in general, do not act through the soil, also called residual, and they are only active upon the plants present at the time of application. Considering its persistence in the soil, they are called residual herbicides those products that remain active in the soil for a prolonged time and, therefore, they are effective in weed control that has not yet germinated and/or emerged.

RESIDUAL HERBICIDES

All residual herbicides are degraded in the soil. However, the degradation of the herbicides takes time, with adequate moisture levels and soil temperature, to establish a basic population of microorganisms that degrade the molecules of the herbicides.

Some herbicides decompose rapidly, avoiding the cause of some type of problem in the crops that will be established in the next season (CarryOver). Others take longer to decompose, and as a result of that, they persist in the following seasons of the year they were applied. These residues may damage sensitive crops that will be established in the following seasons. The dose used plays an important role, since its degradation state depends upon this. A herbicide with a half-life of 30 days, means that if we use 1.0 kg/ha, 30 days after its application theoretically there will be 0.5 kg still in the soil.

CONSIDERATIONS FOR THE APPLICATION

Product mobility in the soil. Herbicides used as active or residual soil, are generally not very mobile in the soil. This is why they must be positioned in a suitable layer thickness through irrigation or tilling. The low solubility of these products in water is what makes them not very mobile and their selectivity, in some cases, depends on them not coming into contact with the roots or rootlets of our crops (positional selectivity).

Soil moisture. For the proper operation of a residual herbicide, it is essential that the soil have moisture. All active/residual soil herbicides require moisture between the field capacity and saturation; otherwise, its effectiveness decreases or is very reduced. If you cannot assure irrigation after the application, it is of great importance to apply it with certain moisture in the soil. In areas with little precipitation, it is preferable to apply post-emergent herbicides rather than pre-emergent ones. Technified irrigation allows localizing moisture, but in the case of herbicides, the use of micro sprinklers is ideal.

Residues from the previous crop on the surface or in weed emergence. The presence of plant or weed remains of a larger size may make a physical barrier, preventing the residual herbicide to reach the soil, with which, if sprinklers are available irrigation will be done to “wash” these remains and to make sure that the largest amount possible of the herbicide reaches its destination. 

Incorporation of herbicide into the soil. Herbicides that are applied to the soil and are not incorporated mechanically are called pre-emergent herbicides, referring to their application before the emergence of the crop and/or weeds. In these applications, the herbicide forms a true film over the surface of the soil and when the seedlings pass over this area, this is when the herbicide comes in and causes the death of the weeds. Removing or breaking this herbicide “film” will leave spaces without the product, which is why it is not recommendable to walk or operate machinery there during the first couple of weeks after application. A strong rain or incorporation irrigation are a good way to avoid this waiting period.

When the application of the herbicide is after the crop and weed emergence these are called post-emergent treatments. Some typical pre-emergent herbicides can also be applied when the weeds have recently emerged, which is called early post-emergence. Post-emergence itself is a practice directed to control weeds in the seedling state that are more or less developed, which have escaped a pre-sowing or pre-emergent application. These herbicides tend to be foliar action, may it be by contact or systemic, however there are some that in addition to this foliar effect also have a residual action, which is the case of oxyfluorfen and many other ureas, sulfonylureas and triazines.

This is why it is possible to use tank mixing of two or more herbicide products formulated independently and, in some cases, other pesticides. The benefits of the tank mixing are the time saved and use of less machinery. A classic example is the use of a total action foliar herbicide such as glyphosate or paraquat and to take advantage of this application in order to “incorporate” a residual that will begin controlling weeds once they are in the soil. With weed coverage above 50% of the soil, it is recommended to do two separate applications, but beneath this limit the herbicides may be mixed, taking into account that the use of an adjuvant is key to the arrival of products to the foilage and soil.

It is good to remember that you must follow all the instructions on the labels with regards to the tank mixing and if mixes «not included on the label» are considered, they should be evaluated for effectivity and safety for the cultivation before its routine use.

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