Care with reduced soil preparation in sugarcane

At the same time as it presents numerous advantages from a conservation and cost reduction point of view, reduced soil preparation, through desiccation and subsoiling of sugarcane growing areas, has the disadvantage of favoring intense weed infestation. The use of pre-emergent herbicides is an important tool to help manage this problem. However, the producer needs to be aware of the correct choice of product and the most appropriate time for application.

Soil preparation is a fundamentally important step in the sugarcane production process. Traditionally, it includes a sequence of mechanized operations whose purpose is to guarantee adequate conditions for the implementation and development of the crop in its new cultivation cycle.

However, this is a set of extremely costly activities for producers, which constitute an important portion of the crop production cost. Currently, conventional soil preparation is the modality most commonly adopted by sugarcane farmers. 

Conventional preparation is understood as the sequence of operations that begins with the chemical desiccation of the ratoons, followed by sequential harrowing (heavy or intermediate), plowing the area and finally, leveling harrowing. These stages require high-power tractors and consequently high fuel consumption. As a consequence, soil disturbance leaves it unstructured and exposed to the effects of rain and consequently susceptible to the effects of erosion.

With the aim of reducing costs and focusing on more rational and conservationist soil management, the adoption of reduced soil preparation has been observed in recent years. In the method, the sequence of mechanized soil disturbance operations is replaced solely by subsoiling after the area has dried out. As a result, the absorption of rainwater is favored and cultural remains on the soil surface are maintained, factors that discourage the occurrence of erosion. 

In sugarcane fields subjected to reduced soil preparation, intense weed infestation occurs, particularly in the period of time between the eradication of the previous ratoon and the implementation of the new sugarcane field. Due to the absence of harrows in this preparation system, mechanical control of invaders is compromised. As a result, it is common for additional glyphosate applications to those carried out to eradicate ratoon to be required for the management of invasive plants in the sugarcane field. 

If weeds are not controlled, they may complete their cycle and reinfest the sugarcane field with seeds, increasing the seed bank in the soil and further aggravating the problem for the next cultivation cycle. The use of pre-emergent herbicides following glyphosate represents an alternative to seed bank management. Management applications should preferably be carried out before the weeds complete their life cycle, thus avoiding the so-called seed rain in the area.

However, to guarantee the dynamics of residual herbicides in the soil, it is essential to consider the size of the weed plants. The species cannot be too developed for the herbicide to reach the soil surface and begin its dynamics. If the size of the weeds is very high, a situation can be created that completely covers the application target (weeds), but prevents the herbicide from reaching the soil and impairs the pre-emergence action. The benefits brought by the use of pre-emergent herbicides will not be fully realized in a situation like this.

Once care is taken with the cycle of weeds, their size and coverage of the application target, attention must be paid to the physicochemical characteristics of the pre-emergent herbicides to be applied, particularly their solubility. The time of year and the time elapsed between application and planting of sugar cane must also be taken into account.

As most cultivation areas are prepared during the rainy season of the year, the use of high solubility herbicides can result in loss of residual effect, due to the movement of the herbicide in the soil profile caused by excess rainfall. The herbicide then ends up being positioned below the first few centimeters of soil, allowing seeds close to the surfaces to germinate and develop. This movement can also be harmful to the crop, causing phytotoxicity, as it tends to leave the herbicide in direct contact with the newly sprouted sugarcane plants after planting. Therefore, in humid seasons, herbicides with low solubility are more suitable, both in relation to the most effective residual period and in relation to crop safety.

Another physical-chemical characteristic that must be paid attention to is photodecomposition. As there is no possibility of mechanical incorporation of the herbicide into the soil through the use of a leveling harrow, the pre-emergent herbicides that best fit this type of use are those that do not undergo decomposition by sunlight.

Paying attention to the points mentioned, the sugarcane grower can then choose the products and the most appropriate time to carry out the management applications, always adapting the product and dose to the weed infestation present in the area and the time elapsed between application and planting.

Fábio César Rodrigues do Amaral, Carlos Alberto Mathias Azania, IAC

Article published in issue 204 of Cultivar Grandes Culturas.