06 March 2023
The dispersion of plant protection products and its impacts is examined by weather station specialists.
THE weather conditions during a plant protection treatment are fundamental. Spraying in unfavourable weather conditions increases the risk of dispersion of phytosanitary products and thus reduces the effectiveness of the treatment and exposes the environment to contamination.
Plant protection products can be dispersed at several levels. They can be carried by the air or by water and are dispersed in several environments: the atmosphere, the soil and the aquatic environment. The transfer of pesticides into the atmosphere is the main source of dispersion of active ingredients in the environment. It is difficult to estimate precisely the losses for a single pass, but they can be very significant (up to 50%) depending on particular on the weather conditions and the spraying equipment. A distinction is made between phenomena that occur during application and those that occur afterwards.
Drift is the first mechanism involved in spraying. It corresponds to all the active ingredient transported by the wind outside the treated area during application. The amount of drift is very strongly correlated to the wind speed. It is generally quantified by counting the number of drops of active ingredient outside the treated area.
In order to limit drift, the French government has introduced a decree prohibiting spraying with wind speeds above 19 km/h. To avoid drift, spraying should be carried out when the winds are as light as possible. It is also forbidden to spray in certain areas considered sensitive.
Drift is strongly related to wind, but not only. It also depends on application factors such as boom height or forward speed, as well as on the product formulation (particle size, density, etc).
During a plant protection treatment, the majority of the active ingredient applied comes into contact with the plant cover or the soil surface. The active substance must then be assimilated by the plants, but a non-negligible proportion of it may be volatilized into the atmosphere. Volatilisation is the "passage of a compound from its solid, liquid, aqueous solution or adsorbed phase on the soil to the gas phase". The compound is then transported into the atmosphere. This mechanism represents from a few tenths to a few tens of percent of losses of active matter.
Volatilisation depends on :
- weather conditions (hygrometry, temperature)
- application techniques
- the type of crop
- cultivation practices
- the characteristics of the active ingredient
The second vector for the transfer of plant protection products in the environment is water. The risk associated with transport phenomena involving water is high, but more easily controlled. Indeed, losses can be high under certain conditions, but by monitoring and forecasting weather data, it is possible to reduce the risk.
The primary mechanism of active ingredient transfer related to water is leaching. The objective of a treatment is to ensure that as much of the product as possible is in contact with the canopy. In case of rain during or after application, the products can be thrown on the soil surface, which corresponds to leaching. The challenge is greater for contact products which must remain on the surface of the plants for a long time to protect them. Rain can remove this protective layer, requiring a new treatment.
It corresponds to the transport of compounds by surface water outside the plot, often to a watercourse. The importance of this phenomenon is very much linked to rainfall, but also to the characteristics of the environment, particularly the topography. On land that is prone to runoff, this can be problematic. Indeed, if the rainy episode occurs quickly after a treatment and is heavy rain, the concentration of pesticides in the run-off water can be high. The resulting environmental contamination can then be significant.
The last mechanism of transfer of phytosanitary products by water is leaching. This mechanism corresponds to the entrainment of the active ingredient, solubilised in water, to the depths. Seepage water can carry the active ingredient to the water table, which is then contaminated. The risk associated with this phenomenon is very much linked to the nature of the product, in particular its persistence (shelf life). Regular heavy rainfall increases this phenomenon.
Agronomic and financial impacts
A dispersion of phytosanitary products means that the intervention was not carried out under optimal conditions and therefore that its effectiveness was limited. This has a direct impact on the crops, which may have been poorly protected. They may then be at risk from diseases and pests. This represents a financial risk for the farmer on two levels.
If the crops are poorly protected and therefore affected and destroyed by diseases or pests. This can lead to a reduction in yields and crop quality.
if the treatments have to be carried out again following a bad application. In an economic context of inflation and increasing prices of plant protection products, it is more than necessary to optimise and reduce the number of applications.
The economic stakes involved in crop protection treatments are high and an improvement in the efficiency of interventions can make it possible to reduce the number of applications. The provision of ultra-local weather data and the combination with DSTs help farmers in their crop protection strategy. During treatment, the maximum amount of active ingredient must reach its target and be assimilated to optimise the efficiency of the passage and limit the associated risk.
Plant protection products, particularly through leaching, infiltrate the soil, which can have serious consequences for the soil fauna. This has direct repercussions on the structure of the soil and therefore its fertility. Indeed, it is the living organisms present in the soil that allow its aeration and the formation of organic matter.
These products can also infiltrate watercourses, as previously mentioned. This has a direct impact on aquatic fauna and flora (fish, algae, crustaceans) and can cause their death. This is known as aquatic toxicity.
Through their dispersion in the air, the products can also be toxic for various organisms, notably bees and other crop protection agents. Impacts on human health can occur in different ways. First of all, there is the risk during spraying. This is why spraying is regulated in terms of the wind speed in order to limit its dispersion outside the plot and the distance to houses to avoid any impact on the neighbourhood.
There is also a risk after spraying. There may be pesticide residues directly on foodstuffs which may represent a toxicity risk, and a risk to the quality of water which may be contaminated.
Thus, there are many mechanisms by which pesticides are transferred outside the field. Taking into account the weather conditions for the planning of treatments is essential to improve their effectiveness and reduce the environmental risk.