By: Claudio Farias, Market Developer Curicó-Talca Corteva Agriscience; Constanza Lozano, KAM VII Region – Corteva Agriscience.
Reaching the harvest season with a weed-free orchard is what every fruit grower would wish for, however, this is often out of the question. During the season, other tasks such as pest and disease control, fertilization, pruning, thinning, preparation for harvest, etc., are prioritized, and finally the weed control program is reduced to rather reactive or “calendar” applications in the case of residual herbicides.
This is why early planning for weed control, with applications at the right time, is key to optimizing resources, minimizing the number of applications and increasing the efficiency of each one. Why do it? Because it allows the orchard to remain clean until harvest, reducing weed pressure in the medium-long term, at a reasonable cost, with the minimum possible interventions and under an integrated management scheme (different MOA).
Within this planning, the application of residual herbicides (soil-active) is undoubtedly key, but it is important to carry out an efficient and effective application, so this article will address some important considerations for weed control in winter.
Considerations for winter weed control
When we talk about a weed control program, we seek to reduce the pressure on the orchard, which is why it is essential to control those species originating from seeds before they emerge.
These annual weeds germinate mainly in winter and spring, so it is essential to control them during the autumn-winter months. To do this, the use of residual herbicides (soil-active) is recommended, which prevent the emergence of these weeds by acting on their early stages.
Being a pre-emergent application, it is important to consider some aspects such as:
- Seed bank
- Choosing the herbicide
- Dose
- Incorporation
- Application
- Degradation and leaching
Regarding the seed bank, if we were to make a soil profile from the surface to 50-80 cm with a microscope, it could be observed that along it there is a large seed bank at different depths, of different ages, most of them dormant, however in the first 3-5 cm of soil there is the so-called "Active seed bank" (Alister, Kogan, 2018) which are those seeds that will germinate and emerge year after year and therefore are the focus of control with active soil.
The incorporation The active soil use of herbicides is key in the days following their application, for which the presence of a minimum rainfall of 5 mm is essential (ideally, 10 mm of water falling after application). This allows the herbicide to be distributed in the soil profile, allowing it to reach this active seed bank.
Regardless of the method of application (motorcycle, tractor with bar, backpack sprayer, etc.), the important thing is to achieve adequate ground coverage, considering aspects such as: dosage, nozzles, anti-drift handling, application speed, etc.
As for the active soil herbicide, the choice of the product to be used is key when making applications during this period. The decision must consider factors such as: label (use only herbicides labeled for fruit trees), spectrum of action (what weeds it controls), herbicide movement in soil (which moves so much beyond the first few centimeters), adsorption to soil colloids (duration of control), Restrictions on use indicated by the manufacturer (some herbicides have restrictions during the first years after planting, information found on the labels).
To make a good choice it is necessary to understand some concepts related to the physical and chemical characteristics of herbicides such as:
- Half-life (DT50): measures the persistence of a herbicide in the soil. This value indicates the number of days in which the applied dose is halved. For example, if a herbicide has DT50= 40 days and 3 L/ha were applied, after 40 days in the soil there will be 1.5 liters. This value is not directly related to the period of weed control in the soil by the herbicide, since for example, the active ingredient Pendimethalin has a DT50 of 182 days (IUPAC, 2024), and its control period does not exceed 60 days.
- Adsorption Constant (Koc): indicates the retention of the active ingredient in the soil's organic matter and clays. The higher the value, the greater the retention.
- Solubility: represents the amount of a substance capable of dissolving in a solvent. As a practical example for residual herbicides, the higher its value, the more soluble it will be and the less water it will need to activate in the soil.
- GUS Index (Groundwater Ubiquity Score): It is a tool that allows herbicides to be classified according to the risk of leaching into groundwater. It integrates the half-life and KOC of an active substance; the lower the value, the lower the risk of leaching.
To finish defining an application of soil-active herbicides, it is important, in addition to the previous concepts, to integrate information on the soils, specifically the values of organic matter, clays and cation exchange capacity that must be met to carry out an application of residual herbicide. These values are the following:: Organic matter 1.5%, CEC 10 meq/100 g of soil and 10% of claysThis information should be available in each orchard and can be obtained through a soil analysis containing all these parameters.
Below is an informative table with the main active soil herbicides available on the market and their DT50, Koc, Kfoc, GUS and Solubility values:
Does applying a residual herbicide increase production costs?
The cost of the weed control program is undoubtedly a key factor when deciding what to do. The cost of soil-active herbicides often seems excessive and conditions the decision to use them. However, the analysis must go beyond the cost per kg/L of an application, and the value of an annual weed control program must be considered, including application costs, number of interventions in the season, as well as the cost of resistance management.
Below is a cost analysis of different “typical” weed control programs. Program A considers the use of 2 applications of residual herbicides in autumn and winter, in addition to 2 perennial weed removals in spring-summer. This program is defined under normal weed pressure in the orchard. Program B considers the use of 1 residual herbicide at the end of winter and 2 perennial weed removals in spring-summer. This program is established for a clean orchard with a strategy of more than one season of using residuals as a weed control strategy. Program C considers only a post-emergence strategy throughout the year, seeking to control a broad spectrum of weeds.
*Tables prepared by the authors, with cost data provided by distribution chain based on list prices of herbicides analyzed.
GRANITE, a new standard for weed control
Corteva Agriscience has enriched its portfolio with a new standard in weed control, Granite® (Oxyfluorfen + Penoxsulam). A pre-emergence herbicide (active soil) with a long residual period on a broad spectrum of broadleaf (dicotyledonous) and narrowleaf (monocotyledonous) species and with a post-emergence effect on weeds in the early stages of development (contact and systemic action through xylem and phloem). Granite® should be applied during the winter break and prior to the active development of roots in the orchard.
Granite®'s maximum potential will be achieved when applied to a soil free of weeds, clods and moisture. However, in orchards with the presence of emerged weeds, it is recommended to mix with another post-emergence herbicide, either systemic and/or contact. Granite® requires to be incorporated into the soil solution either by rain or irrigation in the days following application and, in order to continue actively controlling weeds in the orchard, ideally successive rains or irrigations to maintain a necessary concentration in the soil solution to continue the control effect. It is recommended to apply with machines specially designed for herbicides, with a volume of water between 100 - 300 L / Ha, according to the volume of foliage in the canopy of herbaceous vegetation, ideally with a fan-type nozzle achieving adequate coverage of the weeds.
For more information about the product, you can contact your Corteva Agriscience regional representative or at https://www.corteva.cl.
