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Pre-emergent herbicides in wheat: strategy for ryegrass control

It is estimated that the presence of one ryegrass plant per square meter can reduce wheat productivity by 0.4%, and maximum losses can exceed 80%
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Ryegrass is an annual grass with C3 metabolism. Its reproduction through seeds, which can occur, emerge and establish itself in different environmental conditions. This species grows competitively, which gives it with wheat. Furthermore, in southern Brazil, ryegrass is cultivated as a forage or ground cover crop before wheat sowing, which favors its occurrence at high densities.

Impact of ryegrass interference on wheat
Control at inadequate stages (large plants) and high plant density are factors that make it difficult to manage ryegrass during desiccation. After the defined wheat and the seed bank of the flows of individual origin of the azem, making difficult the increasing control as the productivity in function of the interferences in the culture. It is estimated that one ryegrass plant per square meter can reduce wheat productivity by 0.4%, and maximum losses can exceed 80% (Photos 1 and 2).


How ryegrass has been managed


In the last decades, the control of desiccated ryegrass has been carried out through the application of post-emergent herbicides, especially glyphosate. However, its widespread application selected resistant populations, the first case being reported in Rio Grande do Sul, in 2003. Currently, according to Embrapa data, resistant ryegrass occupies about 85% of the agricultural area of RS. To overcome this resistance, the tank mixture of ACCase inhibitor herbicides (graminicides) with glyphosate has been used. In many cases, sequential applications with glufosinate ammonium or diquat are necessary to obtain maximum efficiency in controlling desiccation.


After the establishment of wheat, the herbicide options to control ryegrass are: clodinafop, iodosulfuron-methyl and pyroxsulam. However, resistance to these herbicides has been reported and, as a consequence, a reduction in ryegrass control (Photos 1 and 2).


Pre-emergent herbicides in ryegrass control


The application of herbicides to control ryegrass in pre-emergence of wheat was low in RS until the 2021 crop. However, new registrations of products for this type of application should favor the increase in the treated area. Herbicides such as S-metolachlor, trifluralin and pyroxasulfone are registered and show excellent pre-emergence ryegrass control.


In addition, these herbicides have control over other weed species, such as horseweed, which can vary from 40% to 70%. Thus, the action of the pre-emergent herbicides in question favors the management of a broad spectrum of plants in wheat, which could benefit crops that succeed wheat, such as soybeans (Photo 3).


Attention points


The action of pre-emergent herbicides occurs during the germination process of ryegrass seeds, when they are more vulnerable. It is important to highlight the importance of the presence of moisture in the soil to improve the absorption of herbicides.
In addition, the recommendation of doses and the type of application must consider the type of soil (clay or sandy), organic matter in the soil, presence of straw, forecast of torrential rain after application, among other factors.


Wheat sowing must be carried out cleanly (without the presence of weeds) and with maximum efficiency, so that the seeds are not deposited on the surface of the soil. This will reduce the direct contact of wheat seedlings with the herbicides and minimize the risk of phytotoxicity on the crop.

Developed by Agência Jung
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