Efficient weed management in wheat

Herbicide-resistant weeds are a recurring problem, which with each harvest threatens even more the productivity and sustainability of wheat crops. Thinking about the production system as a whole, with the integration of chemical, physical and cultural methods, as well as crop rotation and action mechanisms are fundamental measures for efficient management

24.03.2022 | 14:16 (UTC -3)

Herbicide-resistant weeds are a recurring problem, which with each harvest threatens even more the productivity and sustainability of wheat crops. Turnip, ryegrass and horseweed are among the most limiting winter cereals. Thinking about the production system as a whole, with the integration of chemical, physical and cultural methods, as well as crop rotation and action mechanisms are fundamental measures for efficient management.

Annually the Farmers face new challenges in managing weeds in cereals winter. These are unwanted species that vegetate simultaneously with cultures. They are among the main factors that cause losses in productivity agriculture and its management/control is essential to avoid production losses. You Herbicides are the preferred control tool for growers. However, the repeated use of herbicides caused the emergence of species resistant. Resistance reduces efficiency and makes control costly with use of herbicides. Among the main weeds in winter cereals that are resistant to herbicides are turnip (Raphanus sativus e R. raphanistrum), ryegrass (Annual ryegrass) and horseweed (Conyza spp.).

Turnip is a type of ground cover of autumn/winter widely used in agricultural systems in southern Brazil, as it establishes quickly, cycles nutrients and improves physical characteristics of the soil, in addition to suppressing the growth of other plants weeds (Balbinot Jr et al., 2007). However, despite these desirable characteristics, turnip is the main dicotyledonous weed present in winter crops, as it competes and reduces final productivity of cultures (Bianchi et al., 2011).

Inhibitor herbicides of the enzyme acetolactate synthase (ALS) were for many years the main tool for turnip management, due to high efficiency, selectivity to winter crops and low cost (Vargas & Roman, 2005). However, the use continuation of the same mechanism of action selected resistant plants (Walsh et al., 2007). The first records of the occurrence of resistant turnip biotypes to ALS enzyme inhibitors occurred in Brazil in 2001, with cross-resistance to metsulfuron-methyl, chlorimuron-ethyl, imazethapyr, nicosulfuron and cloransulam-methyl (Heap, 2017).

Another important winter weed is ryegrass, which although it can be used as a forage species and in the supply of straw for the system of direct planting, when it occurs in winter cereals it compromises the quality and quantity of product harvested (Roman et al., 2004; Vargas et al., 2005). In addition Furthermore, the emergence of resistant biotypes and the difficulty of control are factors that contribute to the worsening of the problem. The occurrence of ryegrass resistant to glyphosate (inhibitor of the enzyme 5-enolpyruvyl shikimate 3-phosphate EPSPS synthase) in Brazil, registered in 2003, directed the control of ryegrass by herbicides that inhibit acetyl coenzyme A carboxylase (ACCase) and ALS.

However, continued use of ACCase and ALS inhibitor herbicides selected ryegrass plants resistant to ALS inhibitors in 2010. To make matters worse, cases of multiple resistance of ryegrass to ACCase and EPSPS inhibitor herbicides in 2011; ALS and ACCase in 2016; and EPSPS and ALS in 2017 (Heap, 2017).

In 2017, infestation with glyphosate-resistant ryegrass in Brazil is more than four million hectares, with most areas in Rio Grande do Sul (Figure 1). Already, the areas infested with ryegrass resistant to ALS and ACCase inhibitors in the state gaucho were greater than 1200 hectares and 1000 hectares, respectively (Figures 2 and 3).

Figure 1 – Region, in red, infested with glyphosate-resistant ryegrass (Lolium multiflorum) in Rio Grande do Sul. Embrapa Trigo, 2015.
Figure 1 – Region, in red, infested with glyphosate-resistant ryegrass (Lolium multiflorum) in Rio Grande do Sul. Embrapa Trigo, 2015.
Figure 2– Region, in blue, infested with ryegrass (Lolium multiflorum) resistant to ALS inhibitors in Rio Grande do Sul. Embrapa Trigo, 2015.
Figure 2– Region, in blue, infested with ryegrass (Lolium multiflorum) resistant to ALS inhibitors in Rio Grande do Sul. Embrapa Trigo, 2015.
Figure 3 – Region, in green, infested with ryegrass (Lolium multiflorum) resistant to ACCase inhibitors in Rio Grande do Sul. Embrapa Trigo, 2015.
Figure 3 – Region, in green, infested with ryegrass (Lolium multiflorum) resistant to ACCase inhibitors in Rio Grande do Sul. Embrapa Trigo, 2015.

Resistance consists of inherent and inheritable capacity of a given weed biotype survive and reproduce after application of the recommended dose of a certain herbicide, which would normally be lethal to the original population (Heap & Lebaron, 2001; Gazziero et al., in Agostinetto, D.; Vargas, L, 2014). There are two types of resistance, crusade and multiple. Cross resistance occurs when the biotype is resistant to more than one chemical group within the same mechanism of herbicidal action. Multiple resistance occurs when the biotype is resistant to more than one mechanism of action (Agostinetto; Vargas, 2014).

For cases of ALS and ryegrass resistant turnip resistant to EPSPs, ALS and ACCase increasing dose does not improve control significantly. In work carried out by Cechin (2016), it was necessary doses 33 times and 274 times (115 g a.i./ha and 959 g a.i./ha) higher to that recommended for the herbicide iodosulfurom-methyl, to control turnip resistant to ALS, making its unfeasible use. Studies carried out at the Federal University of Pelotas (UFPel) and Embrapa Trigo, to determine cross-resistance between herbicides iodosulfurom-methyl and pyroxsulam, showed that the application of 16 times the commercial dose (56 g a.i./ha) 288 g a.i./ha) did not control resistant plants, which makes application unfeasible of these herbicides on plants resistant to ALS inhibitors. Already in work carried out with biotypes with multiple resistance to ALS inhibitors (iodosulfuron-methyl) and ACCase inhibitors (clethodim) required 16 times and 15 times the commercial dose (56 g a.i./ha and 1820 g a.i./ha), respectively, to obtain control of more than 90% of the resistant biotype.

Ryegrass plants resistant to EPSPs and ALS herbicides
Ryegrass plants resistant to EPSPs and ALS herbicides

Cases of multiple resistance are becoming more frequent in recent years, which poses a threat to agriculture. This fact is worrying as it reduces chemical management alternatives for these populations, along with with the increase in productivity losses in crops due to the presence of weeds in crops. In addition to control difficulties due to the resistance, control costs increase due to the need for the application of herbicides with different mechanisms of action. In this context, the use of different weed control methods (such as physical methods to avoiding the production of seeds from resistant biotypes) is essential to prevent the selection, multiplication and dispersal of resistant plants.

Management of resistant ryegrass and turnip and perspectives for control

Practices like rotation of action mechanisms are important to increase the time of use of the herbicides and slow the evolution of resistance. To minimize the effects of resistance and avoid increasing the selection of tolerant and resistant species and also prolong the efficient use of herbicides to control weeds It is recommended to adopt the measures indicated below.

Preventive measures to minimize the effects of resistance

  • Rotate herbicides with different mechanisms of action;
  • Destroy plants suspected of resistance;
  • Rotate crops;
  • Follow the weed population in order to detect the emergence of resistance at first;
  • Prevent plants resistant or suspected seeds produce seeds and spread throughout the area.

The cases of resistance multiple ryegrass plants have a direct impact on management, requiring use of herbicides with different mechanisms of action from those for which there is already documented resistance. Each case of resistance requires different products and forms of management (Table 1).

Tabela 1
Tabela 1

For ryegrass control pre-sowing the crop (desiccation) in cases of resistance to EPSPs, it is necessary the addition of ACCase inhibitor herbicides or isolated applications of paraquat, paraquat+diuron or ammonium glufosinate. When using paraquat, paraquat+diuron and glufosinate the application stage must be observed, as these herbicides control efficiently young ryegrass plants, preferably at the beginning of the tillering.

In cases of multiple resistance to EPSPs and ALS inhibitors, paraquat, paraquat+diuron and glufosinate can be used in desiccation, or glyphosate associated with ACCAse inhibitor herbicides. In post-emergency the use of iodosulfuron-methyl and pyroxsulam, both inhibitors of ALS. For cases of multiple resistance to EPSPs inhibitors and ACCase, there are also the full-action herbicides, paraquat, paraquat+diuron and glufosinate for use in desiccation. In some cases of ACCase resistance, ryegrass may present resistance to only one chemical group. This way, if the ryegrass is not controlled by aryloxyphenoxypropionates (Fop’s), it may be sensitive to cyclohexanediones (Din’s) and vice versa. However, it is necessary to have prior knowledge of the history of applications in the area to recommend this type of management and observe the interval between application of the herbicide and sowing, to avoid damaging the crop. For post-emergence, clodinafop-propargil should be avoided in areas with suspected ACCase resistance.

When ryegrass presents resistance to ALS and ACCase, in desiccation management The same recommendations are followed for ACCase-resistant ryegrass. However, for post-emergency, control tools are extinguished, and can be ALS or ACCase inhibitors are used, only if there is prior knowledge of which chemical groups resistance occurs, and if the biotype present in the area presents resistance simple or crossed for that mechanism of action.

Given this scenario, the application of herbicides pre-emergence is important to reduce selection pressure in post-emergence (Table 1). However, no herbicides are currently registered in the Ministry of Agriculture, Livestock and Supply (Map) for this modality application, with ryegrass control in wheat. Pyroxasulfone is a new pre-emergent herbicide, in the registration phase with Mapa, which, as it is a mechanism different from those currently used and which will assist in the management of resistant ryegrass. Pinoxadem is also in the registration phase, which presents selectivity for wheat and barley in post-emergence applications, acting as an ACCase inhibitor herbicide but in a new chemical group, that of Phenylpyrazolines (Den’s). These herbicides will help in the management of ALS-resistant ryegrass, and for ACCase when the mechanism of resistance is not metabolization of the herbicide. Another worrying factor is the loss of the paraquat molecule due to legal issues related to the toxicology of product. Without this herbicide, crops with the presence of biotypes with multiple resistance to EPSPs and ACCase, the only molecule available in desiccation will be ammonium glufosinate, which, in addition to the problem of limiting at the application stage, it can favor the selection of ryegrass biotypes resistant, as is already a reality in the United States, due to its use continued and repeated.

Figure 4 - Wheat crop with ryegrass resistant to EPSPs and ACCase
Figure 4 - Wheat crop with ryegrass resistant to EPSPs and ACCase

Available herbicides for use in turnip desiccation are 2,4-D, metsulfuron-methyl, MCPA, dicamba, glyphosate, glufosinate ammonium paraquat and paraquat + diuron. While the first four essentially control turnip and other broad leaves, glyphosate, ammonium glufosinate, paraquat and paraquat + diuron are herbicides with total, control both broadleaves and grasses.

For pre-emergence only metribuzim is registered for turnip control, while flumioxazin is an important tool that can be used in this modality application, in order to control horseweed at the beginning of winter, with the possibility of herbicide contribute to reducing the future turnip population in post-emergency. Herbicides registered for turnip control in post-emergence are metsulfuron-methyl, pyroxsulam, iodosulfuron-methyl, bentazone, metribuzim, 2,4-D, dicamba and MCPA.

Metsulfurom-methyl, pyroxsulam, iodosulfuron-methyl are inhibitors of ALS, while metsulfuron controls only broadleaf, pyroxulam and iodosulfuron-methyl control both grasses and broad leaves. Bentazone and metribuzim, herbicides photosystem II inhibitors, can be applied to turnip management in post-emergency. However, care must be taken when carrying out applications in initial stages of weed growth. 2,4-D auxin herbicides, dicamba and MCPA are efficient in controlling turnip, but caution regarding use of these herbicides resides at the crop stage for application. The use of these products should be restricted only to the tillering stage, as applications precocious affect the beginnings of spikelets causing ear retention, while late applications affect the sporogenesis reducing crop productivity.

The presence of ALS-resistant turnip makes it difficult to manage weed mainly in post-emergence of wheat, as we end up restricting to molecules that are less safe in relation to harm to culture, in this case auxin inhibitors, or initial stages of application in the case of auxin inhibitors. photosystem II. To manage resistant biotypes, the farmer must avoid the use of ALS-inhibiting herbicides and the use of rotation of action, especially in desiccation, where there are greater alternatives, such as also the use of pre-emergent in order to reduce selection pressure. Unfortunately, in practice the choice of herbicide is based solely on cost and product efficiency, where auxinics meet these two requirements, and consequently, they are the main form of post-emergence management of turnip resistant to ALS in winter cereals. If spraying with auxinics continue at this intensity, possibly turnip biotypes will be selected resistant to these herbicides, as is already the case in Australia. Therefore, the Maximum efforts are needed to avoid the loss of this important group of herbicides. Another herbicide that can be a good alternative for controlling dicotyledons in pre-emergence in wheat is saflufenacil (Protox inhibitor). However, this product is limited in terms of application stage in the same way than full-action herbicides.

Faced with the worrying scenario of plant resistance weeds to herbicides, it is essential that the recommendations techniques thinking about the production system as a whole. In plant management weeds, in addition to chemical methods, physical and cultural, with the aim of minimizing the reproduction and replenishment of banks of weed seeds in the soil, as well as their dispersion to other areas. Crop rotation helps to diversify cultural practices and mechanisms of action of applied herbicides, which favors management. Attitudes proactive measures will contribute to the sustainability of agricultural activity in the point of view of weed resistance to herbicides, ensuring maximum crop productivity and profitability for farmers.

Article published in issue 217 of Cultivar Grandes Culturas, June 2017. 

Mosaic Biosciences March 2024