Agr. Eng. Jorge Lundstedt, UPL Chile.
Agricultural Engineer Juan Guillermo Marquez, UPL Chile.
Agr. Eng. M.Sc. Carlos Tapia. Technical Director Avium SpA.
One of the most important tasks that impacts the health of the garden, the efficiency of resources such as water and nutrients, and cultural tasks is weed control and the efficient and effective use of herbicides according to their need.
In practical cases, in cherry orchards in their initial stage, weed control could have a greater role than even plant training management and other tasks. This is generally due to the fact that the soils are intervened with physical preparation work, nutritional plans and irrigation control in a more efficient way, fundamental factors so that weeds also take advantage of their development based on these programs.
Although there are general methods for efficient use of herbicides during the season, the use of more limited programs that allow for more control time undoubtedly seems to be the most appropriate path.
The use of a program based on residual herbicides applied in the winter months becomes a fundamental part of the weed control program, often accompanied by supplements for specific objectives in each case.
GENERAL CLASSIFICATION OF HERBICIDES
There are several ways to classify herbicides, including how they are used, their chemical properties, and their mode of action.
Considering the part of the plant sensitive to the action of the herbicide and according to the physical location where the massive entry of the product occurs, these are classified into herbicides absorbed by the root and foliar herbicides.
According to the behavior of herbicides that penetrate the plant's foliage, they can be classified as contact herbicides, which penetrate in a very localized manner and practically do not move internally. On the other hand, systemic herbicides are those products that are absorbed and move extensively within the plant, either via the symplast (through the phloem) and/or apoplast (xylem).
Herbicides that act on the leaves, in general, do not have a soil action, also called residual action, and are only active on the plants present at the time of application. Considering their persistence in the soil, products that remain active in the soil for a long time are called residual herbicides and are therefore effective for the control of weeds that have not yet germinated and/or emerged.
RESIDUAL HERBICIDES
All residual herbicides are degraded in the soil. However, herbicide degradation requires time so that, with adequate soil moisture and temperature levels, a basic population of microorganisms that degrade the herbicide molecules is established.
Some herbicides break down quickly, avoiding causing any problems for crops established the following season (CarryOver). Others take longer to break down, and as a result, persist in the seasons following the year in which they were applied. These residues can damage sensitive crops established in subsequent seasons. The dose used plays a key role, as the rate of degradation will depend on it. A herbicide with a half-life of 30 days means that if 1.0 kg/ha is used, theoretically 0.5 kg will be available in the soil 30 days after application.
CONSIDERATIONS FOR APPLICATION
Product mobility in soil. Herbicides used as active or residual soil are generally very poorly mobile in the soil. Therefore, they must be placed in an adequate layer thickness by means of irrigation or tillage. The low solubility of these products in water is what makes them poorly 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 a residual herbicide to work properly, soil moisture is essential. All soil-active/residual herbicides require moisture between field capacity and saturation, otherwise their effectiveness decreases or is greatly reduced. If irrigation after application cannot be ensured, it is a priority to apply the herbicide with some moisture in the soil. In areas with little rainfall, post-emergence herbicides are preferable to pre-emergence herbicides. Technological irrigation allows for localizing moisture, but in the case of herbicides, the use of micro-sprinklers is ideal.
Soil organic matter/clay content.All soil-active herbicides, once in the soil, are retained (adsorbed) by the colloids of the soil surface: clay and organic matter (carbon). The higher the content of organic matter and/or clay in the soil, the more strongly the herbicide will be adsorbed or retained by it and, therefore, there will be less herbicide available to act on weeds. In these heavy soils with a high organic matter content, higher doses will be used. If the soil is also dry, the adsorption force will be even greater, since it is through water that the herbicides are “detached” from the colloids (desorption). Another reason to make this type of application in wet soil. The doses of soil-active herbicides must be adjusted imperatively, according to the content of organic matter and/or clay in the soil of each plot.
Residues from the previous crop on the surface or emerged weeds. The presence of plant remains or larger weeds can act as a physical barrier preventing the residual herbicide from reaching the soil. Therefore, if spraying is available, irrigation will be used to “wash” these remains and ensure that as much of the herbicide as possible reaches its destination.
Incorporation of herbicide into the soil. Herbicides that are applied to the soil and not incorporated mechanically are called pre-emergence herbicides, meaning that they are applied before the crop and/or weeds emerge. In these applications, the herbicide forms a real film on the soil surface and when the seedlings pass through this area, the herbicide enters and causes the death of the weeds. Removing or breaking this herbicide “film” will leave spaces without the product, so it is not advisable to walk or drive with machinery during the first weeks after application. Heavy rain or incorporation irrigation prevents this period.
When the herbicide is applied after the emergence of the crop and weeds, they are called post-emergence treatments. Some typical pre-emergence herbicides can also be applied to newly emerged weeds, in what is called early post-emergence. Post-emergence itself is a practice aimed at controlling weeds in a more or less developed seedling stage, which have escaped a pre-sowing or pre-emergence application. These herbicides are usually foliar-acting, either contact or systemic, however there are some that in addition to this foliar effect also have residual action, as is the case with oxyfluorfen and many ureas, sulfonylureas and triazines.
For this reason, it is possible to use tank mixtures of two or more independently formulated herbicide products and, in some cases, other pesticides. The benefits of tank mixtures are time savings and less use of machinery. A classic example is the use of a total action foliar herbicide such as glyphosate or paraquat and taking advantage of this application to “incorporate” a residual that will begin to control weeds once in the soil. With weed coverages above 50% of the soil, it is recommended to make two separate applications, but below this limit the herbicides can be mixed, taking into account that the use of a good adjuvant is key for the products to reach the foliage and the soil.
It is worth remembering that all label directions regarding pond mixes must be followed, and if “off-label” mixes are being considered, they must be evaluated for effectiveness and safety for the culture prior to routine use.
