The use of herbicides in blueberry production—especially pre-emergent ones—continues to raise questions, caution, and, in many cases, myths within the industry. To delve into the technical foundations behind these perceptions, we spoke with agronomist Claudio Alister, MSc, PhD, a specialist in the field, who addresses everything from the processes that determine herbicide efficacy to the specific challenges across different production systems.
These will be among the topics Dr. Alister—General Director and co-founder of SIDAL—will cover during his participation in the 1st International Course on Blueberry Production Specialization, to be held on May 26–27 at the Monticello Conference Center, an event that will bring together specialists and key stakeholders from the industry.
More than mechanisms: processes that define weed control
Regarding the mode of action of herbicides, Dr. Alister makes a key distinction: rather than ‘mechanisms,’ it is more appropriate to speak of processes. “The term mechanism is associated with the metabolic pathway where the herbicide acts. But when we talk about weed control, the processes are much broader and include factors such as climate, the developmental stage, and the physiology of the weed,” he explains.
In this context, he clearly distinguishes between pre- and post-emergent herbicides. In the former, the soil plays a central role, influencing both the efficacy and selectivity of the product. In contrast, for post-emergent herbicides, the critical factors are application conditions and herbicide formulation, which determine the plant’s capacity for absorption.
Why is there concern about pre-emergent herbicides?
In the blueberry industry, there is a perception of high risk associated with the use of pre-emergent herbicides. According to Dr. Alister, this perception has historical roots, but is not always technically grounded.
“It has been associated with a lack of information on the actual selectivity of herbicides in this crop, considering that blueberry is a relatively recent species in terms of expansion, unlike more traditional fruit crops such as apples or grapevines,” he explains.
In addition, significant changes in the industry—such as new varieties with lower chilling requirements, different production systems (soil, pots, tunnels), and differences in earliness—directly influence tolerance to pre-emergent herbicides.
Selectivity: a complex equation
One of the critical aspects in the use of pre-emergent herbicides is their selectivity, especially in a crop like blueberry, which has a shallow root system and high sensitivity to chemical stress.
In general terms, the specialist points out that the key factors are the crop’s inherent tolerance to the active ingredient, the crop growth habit (with or without winter dormancy), herbicide–soil or substrate interactions (adsorption, desorption, persistence), soil or substrate moisture retention, and the variety.
Sandy soils: greater mobility, higher risk
Under conditions such as those found on the Peruvian coast, where sandy soils with low organic matter predominate, herbicide behavior changes significantly.
“These soils tend to have lower retention capacity and pose higher risks of groundwater contamination,” he explains.
However, he notes that not all herbicides behave the same way. Those with high lipophilicity (logKow greater than 4) may exhibit more favorable adsorption even under these conditions.
The Chilean contrast: high adsorption in volcanic soils
In southern Chile, the situation is almost the opposite. Volcanic soils, rich in organic matter and with high adsorption capacity, tend to strongly retain herbicides.
One point highlighted by the agronomist is that, over more than two decades of pesticide research—with an emphasis on herbicides—and their relationship with the physicochemical properties of soils, “we have found that the adsorption of many soil-active herbicides used in fruit crops and other crops shows adsorption coefficients 50% higher than the average values reported in the literature and international databases.”
The challenge in pots: total precision
In container-based production systems, the scenario shifts again. ‘The herbicide’s availability in solution will be determined by the type of substrate used, and this opens up a wide range of possibilities,’ he explains.
In these cases, calibration and application technique become especially critical, as errors can easily result in underdosing (weed management failures) or overdosing (phytotoxicity).
The problem of the lack of specific information
Today, artificial intelligence (AI) tools are available to analyze the physicochemical properties of herbicides and support decision-making. However, Dr. Alister warns about their limitations.
“We have seen cases where an advisor, following AI recommendations, has encountered serious issues, both in terms of selectivity and weed management, leading to significant problems,” he notes.
The main reason is the lack of specific scientific information, particularly regarding the doses required to ensure effective concentrations in the soil solution over defined periods.
“This lack of specific technical-scientific information is not only an issue in our South American context, but also in more developed countries,” he adds.
Errors, myths, and poor decisions
When asked about the most common mistakes in herbicide application, the specialist is clear: “If we focus on “management errors,” the main issue is usually calibration and application technique.”
More than operational mistakes, many problems stem from decisions made without a sufficient technical basis, which later contribute to the spread of myths within the industry.
For example, he explains that herbicide incorporation into the soil should be carried out with light mechanical operations (≤5 cm), requires rainfall or irrigation above 5 mm, and is more effective with sprinkler or microsprinkler systems rather than drip irrigation.
He also questions widespread practices such as adjusting doses solely based on soil texture, noting that this is only valid if the herbicide’s adsorption mechanism is understood.
As an example, he mentions certain herbicides from the ALS family, which bind to the soil through interaction with the silt–clay complex—a process that depends on the availability of bi- and trivalent cations such as iron, as this element facilitates the herbicide–soil bond. “If iron is not available, even if there is sufficient clay, its retention in the soil will be low,” he explains.
Leaching: interaction between molecule and management
In sandy soils, the risk of leaching does not depend solely on the herbicide, but also on post-application water management, which must allow sufficient time for interaction between the herbicide and the soil.
“High water loads in soils with macropores will produce “facilitated flow,” which does not provide enough time for chemical and/or physical bonds to form between the herbicide and soil particles,” the expert explains.
Combining strategies to address resistance
In the current context—marked by a reduction in registered herbicide molecules in markets such as the European Union, and the limited introduction of herbicides with new modes of action—weed management programs require integrated approaches.
“The reduced availability of new molecules makes it necessary to combine both strategies,” the specialist states.
The integrated use of pre- and post-emergent herbicides helps reduce selection pressure, prevent the dominance of resistant ecotypes, and improve the management of difficult weeds.
Finally, Dr. Alister outlines the key technical criteria for evaluating the incorporation of pre-emergent herbicides into blueberry weed management programs:
- Crop tolerance to the herbicide
- Weed emergence history and dynamics
- Herbicide–soil interaction
- Water management and moisture conditions
