Manage weeds in rice cultivation and preventing resistance problems requires a complex solution, with the adoption of a set of integrated actions and not just the use of isolated tools. When choosing the herbicide, it is necessary to consider the species weeds in the area, the presence of resistant plants, the time in which intends to carry out the applications, the physical-chemical characteristics of the soil, the type of soil preparation, the product’s offer on the market, its cost and efficiency about the target species.
In irrigated rice crops in Rio Grande do Sul, the main weeds are barnyard grass (Echinochloa spp.), weedy rice (Oryza spp.), angiquinho (Aeschynomene spp.) and sedges (Cyperus spp.). A importance of these invaders is mainly due to their wide distribution and high infestation.
Weedy rice and barnyard grass deserve to be highlighted. The first by belong to the same species as cultivated rice, which makes control difficult selective chemical. Rice grass, in turn, is highly competitive, being that controlling 99% of the infestation may still not be enough to prevent impacts on crop productivity.
Furthermore, the emergence of weed species resistant to main herbicides have worsened this situation, sparking the search for control alternatives.
In the Southern Region of Brazil, weed resistance to Herbicides represent an additional problem in the management of rice crops. A introduction of herbicide-resistant rice cultivars, such as the Clearfield (CL), allowed the control of many of these species. However, the use intensive and outside of technical recommendations led to partial loss of its efficiency (endurance!).
In the current scenario, the Minimum cultivation is the main system used, with more than 95% of areas applying herbicides (excluding the agroecological rice niche), almost entirety under the CL system. Rice weeds resistant to herbicides in Brazil are listed in Table 1.
Rice must be kept free of competition with weeds between the 10th day and the 45th day after emergency (DAE). The later the control is carried out, the lower the productivity. However, in the current scenario, leaving plants to be controlled weeds on the 8th or 9th DAE can be a “trap”. Resistant weed plants herbicides are difficult to control and the ideal is to start farming cleanly, with these species properly controlled.
Therefore, before thinking about chemical control it is necessary awareness that the adoption of some crop management practices (including prior to establishment) can result in a reduction in infestation and improve herbicide efficiency (20% ‑ 30%).
Plant management weeds in rice begins with the use of quality seeds, sown in area with lower infestation rates, in a cultivation system that facilitates this management.
Direct planting and minimum cultivation present, among others, benefits, the reduction in the use of herbicides, as the use of pastures in Winter contributes to reducing germination/development of some plants weeds (Figure 1). For example, ryegrass cover, associated with drainage and animal production, reduces the emergence of some weeds (Figure 2). Additionally, minimum tillage allows for early soil preparation in the fall or spring, stimulating the seed bank and allowing its control.
Research shows that 80% of barnyard grass emergence occurs in the first three weeks after preparing the soil, therefore control it in this period is essential to reduce its occurrence in the area.
The pre-germinated system, in particular, allows the application of herbicides directly into the irrigation water (benzedura), in addition to applications by the usual methods. In “blessing”, there is the possibility of application of herbicides in any weather condition. This particularity may constitute an advantage in the management of weeds in certain situations.
Although there are a number of difficulties in crop rotation in Lowlands, associated with drainage problems and soil compaction, soybeans, corn and sorghum are good alternatives for managing weeds, mainly in areas with herbicide-resistant plants (Figure 3). O fallow in the summer, if used, must be combined with successive soil preparation to stimulate the emergence of weeds and allow their control.
Due to some technical and economic advantages, rice began to be cultivated in rotation with soybeans, in the system known as “ping pong” (one year rice, one year soybeans), enabling herbicide rotation. It is important to highlight that regardless of the culture being implemented, the rotation of action mechanisms of herbicides is essential, and pre-emergents are good alternatives. In soybeans, for example, good results have been obtained in the control of grasses with a application in pre-emergence and two in post-emergence.
When choosing the herbicide, the weed species must be considered. in the area; presence of resistant plants; the time in which you intend to do the applications; the physical-chemical characteristics of the soil; the type of preparation ground; the offer of the product on the market; the cost of the pesticide and its efficiency about the target species.
It is always recommended to apply the recommended dose and not use herbicides not registered for the crop, weed, or application modality. Furthermore, herbicides with different characteristics must be associated. Pre-emergent herbicides help reduce initial infestation by purpose of reducing the population to be eliminated post-emergency (Figure 4).
The use of a full-action herbicide, such as glyphosate, at the “point needle”, allows you to eliminate invasive plants that have already emerged from the crop, including those resistant to ALS-inhibiting herbicides (Figure 4).
Some measures preventive measures indicated to prevent the emergence of resistant weeds to herbicides are:
Once a resistance problem is found, carry out sowing, cultural treatments and harvesting of the problem area lastly, cleaning the equipment soon after to avoid spreading the problem to other areas.
Preventing the emergence of resistant weeds is a task difficult. However, it is almost impossible to eliminate them. Remember this!
Recommendations general
Pre-emergent herbicides are good tools to start clean farming. Furthermore, water management is essential for inhibiting weed plants. Therefore, nitrogen fertilization and water input must occur maximum until the stage of 4 developed leaves of the crop, right after application of the usual herbicides at this stage.
Post-harvest soil preparation (mainly knife roller) also helps to standardize the terrain and eliminate weeds. However, This operation can incorporate recently weed seeds into the soil. scattered. Therefore, this practice should be discussed between technical assistance and producer to evaluate the pros and cons of this tool.
In summary, a set of actions is required to manage sustainable use of weeds in rice, in order to control them efficiently and with the lowest cost in the medium/long term. Therefore, crop planning must be carried out in the long term, always considering crop rotation associated with other management tools. The sustainability of the rural producer.
Article published in issue 226 of Cultivar Grandes Culturas, March, 2018.
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