Rice grass management

Grass difficult to manage, rice grass should also be included in the list of concerns in production areas of the main Brazilian oilseed, not only in the rotation soybean-rice, as well as in the main producing regions in highlands, with chances of presenting difficulties in control with the use of glyphosate.

To the grasses, in the current context of weed management, represent a great challenge to be overcome. In rice cultivation, due to the main system used to be basically served by enzyme inhibitor herbicides ALS (such as imidazolinones), which are potentially latiphocidal (control of broadleaf plants) but with an effect on some grasses (leaves narrow), the problem, in terms of the number of species occurring, is greater than in soybeans, where herbicides with graminicidal efficiency that allow for broader and more efficient control. Among the grasses of the rice, rice grass stands out (a complex of plants of the genus Echinochloa).

To the different species of rice grass grow in varied environments, from fields flooded areas from rice paddies to drained highlands. They are rustic plants, wide adaptability to different locations and conditions. However, damage to commercial crops by these species have been reported for the rice, and have shown resistance to herbicides with different mechanisms of action. In recent years, in surveys carried out by the Interdisciplinary Group of Research and Herbology at the Federal University of Santa Maria, rice grass, In addition to occurring throughout the rice production area, it was found as weed in soybean crops in Santa Maria, Tupanciretã, Júlio de Castilhos, Cruz Alta, Augusto Pestana, Cachoeira do Sul, Dom Pedrito, Rosário do Sul, São Sepé, São Borja and São Gabriel.

The studies reveal that a standard population of a given grass species does not occur rice that is well adapted to all highland environments, occurring at least four species: Echinochloa crusgalli there. crusgalli (grass rice, canevão, jaú, rooster comb), Echinochloa crusgalli there. mitis (mitis, white loin), Echinochloa colona (coloninho),  It is a kind of more recent introduction in Rio Grande do Sul Echinochloa crusgalli there. oryzoides (also called tenderloin grass white). Concerned soybean producers have stated that “rice grass the coxilha is rising!!”, in reference to the understanding that it is a flooded lowland plant that is currently also occurring in plateau in soybean crops and in areas not susceptible to flooding. But this It's a mistake! When traveling through soy producing regions of Rio Grande do Sul, from the South Zone with the flooded lowlands to the plateau with the highlands drained, it is possible to find these species of rice grass growing in roadsides in non-flooded environments, which reveals that they are species adapted to drier terrain.

The studies initial phases with plants collected from soybean production sites allowed verify that some of these rice grass biotypes present varying levels susceptibility to the herbicide glyphosate, among other herbicides used. Species resistant to glyphosate have also been identified by researchers from Argentina, Uruguay, Australia and the United States. In areas where collections, management with glyphosate applications was carried out within programs that include graminicidal herbicides, such as clethodim, allowing control grasses in escapes, when the plants were already in advanced stage of development. There are areas where the population level is already worrying It is necessary to use pre-emergent herbicides to control the flow of sequential germinations originating from the considerable amount of disseminations of the soil seed bank. This gives flexibility for control with herbicides post-emergences within the management program.

The grass rice should also be included in the list of concerns about grasses that infest soybean production areas, not only in the soybean-rice rotation, but also in main producing regions in highlands, which may present difficulties in control with glyphosate. To avoid the introduction and naturalization of these species in the soybean production environment, attention is needed to prevent reproduction and dissemination of the weed plant with an increase in the amount of seeds in the soil, which would result in more problems for producers in the main Brazilian oilseed.

In this context, the cleaning of harvesting machines that are used in different places where this weed commonly occurs, the use of seeds of excellent quality both of soybeans and forages sown in winter and adequate management with quarantine of cattle herds rotated within the integrated system, require care to avoid the introduction of these species. These have been the main means of spreading rice grass to highland areas. O The use of post-emergent graminicide herbicides to control species is necessary, especially the herbicide Cletodim, which showed superior efficiency to other graminicides in the control of these rice grass biotypes in trials field and greenhouse carried out.

Figure 1 shows that two groups of rice grass species occur. The first presents those with an annual cycle, characterized by the absence of ligules, rhizomes and hairiness in the stem nodes. The second, presents perennial species, with hairy ligule on the leaf sheaths (less on the flag leaf), hairiness on the stem nodes, stolons and rhizomes.

In between Among the species with an annual cycle, subgroup 1 corresponds to the species Echinochloa colona, whose biotypes were collected in 16 rice production areas. Subgroup 2 brings together the species Echinochloa crusgalli variety mitis (10 areas). Subgroup 3 Echinochloa crusgalli variety crusgalli (12 areas) . Subgroup 4 Echinochloa crusgalli variety oryzoides (1 area). Subgroup 5 Echinochloa crusgalli variety cruspavonis (9 areas).

In addition of these species, in flooded areas, the two cycle species were found evergreen Echinochloa helodes e Echinochloa polystachya.

Recently another species with an annual cycle identified as Echinochloa crusgalli variety caudata.

CHARACTERIZATION OF RESISTANCE TO GLYPHOSATE HERBICIDE

You studies carried out with rice grass plants from the Central Depression of Rio Grande do Sul, collected in soybean areas, show that they were detected coloninho rice grass biotypes (Echinochloa settler) resistant to the herbicide glyphosate. These biotypes present a resistance factor of 4,54, therefore, theoretically they require a dose of at least five times greater than that which causes a loss of 50% of the dry mass of the aerial part in the susceptible biotype.

Nonetheless, despite being considered academically as a low level of resistance, tests carried out in the field, in an area provided by a soybean producer, accompanied by three consecutive harvests, reveal that control at a level considered satisfactory (above 80%) has only been obtained with doses greater than 12 liters per hectare when applied to resistant rice grass with 1-2 tillers. Control was obtained from this resistant biotype (figure 2) with the application of the herbicide Cletodim.

Article published in issue 218 of Cultivar Grandes Culturas, July 2017.