​2,4-D: Controversial since always, essential as never

2,4-D is a herbicide whose mechanism of action is to mimic natural plant auxins, and was the first organic herbicide synthesized by the chemical industry, in 1941. A few decades after its discovery, during the Vietnam war, it was used the mixture of 2,4-D with 2,4,5-T - known as “Agent Orange" - by the Americans with the intention of defoliating parts of the jungle between Vietnam, Laos and Cambodia in order to expose the supply route that supplied the guerrillas in Vietnam. The idea did not work from a military standpoint, since it was very difficult to defoliate such large areas, especially when guerrillas could easily change route in response to Agent Orange. One of the biggest problems was related to dioxin, TCDD, originating from the manufacturing process of 2 4,5-T, a component banned more than 25 years ago, while 2,4-D continues to be on the market today - wrongly associated with Agent Orange.

It is known that all auxin-mimicking herbicides are capable of causing similar symptoms, characterized by changes in plant growth. These effects can be noticed even when small amounts of these herbicides come into contact with sensitive plants. Hence the other name by which this group of herbicides is known – hormonal herbicides.

Formulated mixtures of picloram+2,4-D are possibly the most used herbicides for the management of pasture weeds, although they are not the only ones (other examples are fluroxypyr, triclopyr, picloram, aminopyralid and clopyralid). Manure from pastures treated with this mixture may have sufficient residue to cause symptoms in sensitive plants in which the manure is used as a fertilizer. This occurs because picloram has a long persistence in the soil (20 to 300 days), and not because of 2,4-D, which has a short persistence (4 to 10 days). Because they present similar symptoms, it is common to associate the effects observed with 2,4-D and not with picloram.

In fact, perhaps the biggest controversy related to the use of auxin mimics and 2,4-D in particular is the potential that the drift of these products can cause in nearby sensitive crops. Although drift is an inherent problem in any application of agricultural products, the fact that these herbicides cause symptoms at very low doses ends up drawing attention. Since current amine formulations are not considered volatile (PV=1,9 x 10^-5 Pa at 25^oC), it is necessary to redouble efforts so that the application technology and application conditions are the most appropriate.

2,4-D and glyphosate are two systemic herbicides that are suitable for controlling already developed weeds. Used alone or together, they constitute the basis of desiccation management in direct grain planting areas in Brazil (the largest in the world), both because they present a broad spectrum of control and because of the excellent cost-benefit ratio when compared to other options on the market. . Mixtures of 2,4-D and glyphosate also make an important contribution to the control of weeds that are tolerant or resistant to glyphosate, such as viola, trapoeraba and horseweed.

With all the folklore that has been created around 2,4-D, it would be natural for there to be a need to reevaluate this herbicide in light of new knowledge and methodologies developed by modern science. Reevaluation processes (toxicological, environmental and agronomic) of 2,4-D took place in the USA, Canada, New Zealand and the European Community, as well as in Brazil. In all reevaluation processes, the results led to the maintenance of 2,4-D in the respective markets. In fact, there is no country in the world that legally prohibits the use of this herbicide.

From a toxicological point of view, which is the one that most directly affects the applicator, there is no evidence that 2,4-D has carcinogenic or teratogenic effects. In relation to the environment, it is degraded by microorganisms, has low leaching into the soil, reaching a maximum depth of 60 cm and has no capacity to contaminate the water table.

Therefore, although it is involved in historical controversies, 2,4-D has an unparalleled record of services provided within Brazilian agriculture.