“Soils can retain herbicides and make them unavailable to the weed seed bank (the underground stock of weed seeds)…”
Kassio Ferreira Mendes is a researcher and professor of traceability in agricultural and environmental quality at the Center for Nuclear Energy in Agriculture at the University of São Paulo, an agronomist from the State University of Mato Grosso, a M.Sc. in plant production at UFV and a Ph.D. in nuclear energy in agriculture at USP.
Mendes has experience in integrated weed management, environmental behavior and pesticide traceability.
AgriBrasilis – What are the interactions between herbicides and the environment?
Kassio Mendes – Herbicides can interact in different ways with the environment. It is the retention, transport and transformation processes that govern the behavior and fate of herbicides in the environment. Therefore, it is necessary to know the physical and chemical properties of herbicides and of the soils, in addition to environmental conditions.
AgriBrasilis – What is the effect of soil conditions on the dynamics and efficiency of these products?
Kassio Mendes – Soils can retain herbicides and make them unavailable to the weed seed bank (the underground stock of weed seeds), therefore, it is necessary to carry out a physical-chemical analysis of the soil before recommending a residual herbicide (applied pre-emergence).
The levels of organic matter and clay are mainly responsible for the retention of herbicides. Thus, in clayey soils and with more organic matter, the recommended doses of herbicides are higher when compared to sandy soils. The pH only interferes with acidic residual herbicides, which may become more ionizable (negatively charged) after liming and suffer from the transport process via leaching. This causes inefficiency in controlling weeds in the arable layer of the soils.
AgriBrasilis – How important is traceability for herbicides?
Kassio Mendes – The traceability of herbicides in the soil is important to avoid problems of contamination and carryover (negative residual effect) in crops in rotation and succession with the crop that received the herbicide application. Injuries caused by herbicides can reduce crop yields.
Another important point is the traceability within the cultivated plant, in which systemic herbicides are absorbed and translocated within the plant, and residues can reach the grains or seeds. With that being said, the maximum residue limit (MRL) in crops must be strictly respected in products for export, import and internal consumption in the country.
AgriBrasilis – Which active ingredients are most persistent in the environment and why?
Kassio Mendes – Herbicides with a high degradation half-life persist longer in soils, as they are moderately sorbed to soil colloids and have a slower biological degradation. Herbicides applied to sugarcane, such as indaziflam, tebuthiuron and sulfentrazone, are very persistent in the soil.
AgriBrasilis – Taking this into account, what management is recommended for the application of these products?
Kassio Mendes – When applying a herbicide, it is necessary to know the physical-chemical properties of the product and of the crops being cultivated in the production system, to avoid problems of injuries and environmental impact. Another fundamental point is that weed control methods must be integrated into agriculture, not just chemical control with the use of herbicides.
AgriBrasilis – How does the time of application affect the performance of herbicides?
Kassio Mendes – Herbicides can be applied in dry and wet seasons, depending on their solubility in water and retention in the soil. During the rainy season, it is recommended to apply herbicides with low water solubility and high retention, as done in sugarcane farming. But it is extremely important to know that herbicides only have residual action in the soil when there is moisture, so a rainfall of 10-20 mm is necessary to activate the herbicide and its bioavailability in the soil solution.
AgriBrasilis – Why is horseweed (Conyza sumatrensis) considered the most difficult weed to control?
Kassio Mendes – It produces a high number (up to 200 thousand) of seeds per plant, which germinate in a staggered manner in the soil, increasing the longevity of the seed bank/underground stock. In addition to this, there are already biotypes of horseweed in Brazil with multiple resistance to herbicides (up to 5 mechanisms of action). Therefore, pre-emergence control is necessary, using residual herbicides and the integration of physical and cultural controls, as this weed is positive photoblastic [Seeds that are stimulated to germinate by light are described as positively photoblastic] and needs light to emerge.
AgriBrasilis – What are the main non-chemical control methods?
Kassio Mendes – Integrated weed management consists of 6 methods: preventive and cultural management, and physical, chemical, biological and mechanical controls. So, there are 5 non-chemical controls, but the most used are mechanical, cultural and physical. There is no bioherbicide registered in Brazil for the biological control of weeds.