How the herbicide glyphosate interacts with inoculants in soybeans

Genetically modified soybeans (Glycinemax (L.) Merril) Roundup Ready (RR), resistant to glyphosate, is one of the most economically important biotechnological products developed in the last 20 years. This variety received a gene from another organism (bacteria) that gives it resistance to the herbicide glyphosate, used in Round-Up Ready, to combat weeds. As it is resistant to this element, the entire plantation can be sprayed with glyphosate, with only invasive (undesirable) plants being eliminated.

Soybean cultivation occupies a prominent position in the Brazilian agricultural scenario, a fact demonstrated by the growth in the cultivation area, according to data from Conab (2019). The total planted area in 2018/19 is expected to grow 2,1%, from 61,72 million hectares in 2017/18 to 63,03 million hectares in 2018/19. In Brazil, 96,5% of the entire area planted with oilseeds is transgenic and very little of the area currently under cultivation is conventional.

Soy has traditionally been produced in systems that include the use of herbicides, and this is exactly where transgenic soy offers an advantage to the producer, especially due to the ease of management it provides, enabling the application of glyphosate across the entire area, with good control efficiency. of weeds. Reducing the application of other pesticides also influences the use of the product, in addition to saving on machinery for spraying these products, resulting in lower fuel consumption.

The expansion of the no-till system and recent biotechnological advances that have resulted in glyphosate-tolerant plants have led to an increase in sales and use of the herbicide glyphosate in agroecosystems, increasing the presence of this molecule in the environment, especially in the soil. Concomitantly with the increased use of these products, greater tolerance to herbicides has been developed in weed species, leading to greater and more frequent use of herbicides than recommended by manufacturers.

Biological Nitrogen Fixation and glyphosate

To produce one ton of soybeans, around 80kg of nitrogen is needed. This nutrient is the most required by culture and can be obtained free of charge in nature, through some bacteria of the genus Bradyrhizobium (rhizobia). They are capable of capturing nitrogen (N2) present in the air and transforming it into a form that can be assimilated by plants. This resource is known as biological nitrogen fixation (BNF) and is considered the most important biological process on the planet. It is a true “biological factory”, capable of meeting the needs of various legumes such as soybeans, eliminating the need for chemical nitrogen fertilization. This is a process that makes the production of this crop viable in the tropics and is also of great importance in low-carbon agriculture. The use of inoculants is the most environmentally and economically efficient way to supply nitrogen to soybeans. Studies confirm that annual soybean reinoculation provides an average increase in grain yield of 8,4% in relation to areas that are not inoculated annually.

Biological nitrogen fixation (BNF) can be impaired by glyphosate, through effects both directly on the rhizobia and indirectly on the host plant, which can reduce its effectiveness.

In an experiment with pots conducted in the open air, in an experimental area at the Universidade Estadual Paulista (Unesp) at the Faculty of Engineering of Ilha Solteira, São Paulo, the effects of doses of the herbicide glyphosate on Bradyrhizobium strains and biological nitrogen fixation in BMX Potência RR soybean plants. A commercial liquid inoculant was used. The treatments consisted of the absence and presence of bacteria from strains of Bradyrhizobium elkanii (Semia 5019) and Bradyrhizobium japonicum (Semia 5079), inoculated via seed and four doses of the herbicide glyphosate via foliar (0; 1, 2 and 4 L/ha of the commercial product) and applied at the V3 phenological stage, simulating a normal period of application of this herbicide in commercial crops. Other variables were analyzed at the end of the R2 stage, when plants were collected from pots by a basal cut of the stem. The height of the plant, the number of nodes on the main trunk, the number of branches, the number of leaves and the dry mass of the aerial part obtained after drying in an oven were evaluated.

As a result of this study, an initial yellowing of glyphosate-resistant (RR) soybean plants was observed due to the accumulation of the first phytotoxic metabolite of glyphosate, aminomethylphosphonic acid (Ampa), and this chlorotic effect was more pronounced in the first days after application. of herbicide in the absence of inoculation. In the inoculated condition, the effect did not occur (Figure 1A). This effect was also observed in other studies, which stated that despite the change in color, due to the glyphosate effect, there was no change in the leaf chlorophyll index.

Biological nitrogen fixation via inoculation led to an increase in the values ​​observed for plant height, number of leaves and total dry mass of plants, obtained after drying in an oven at 65°C for 72 hours, when compared to non-inoculated plants. Only the number of nodes per plant and the number of branches did not present significant results (Table 1). This is also due to the greater accumulation of nitrogen in the aerial part, necessary for numerous plant growth and development processes, in addition to being a structural part of molecules such as chlorophyll, which act directly in the production of photoassimilates that are used in the plant's metabolic activities. . These variables are intrinsic factors to the vegetal and spatial distribution of plants, since some soybean cultivars have a compensatory effect, adapting to different environments and systems.

For leaf nitrogen content, the inoculated plants presented higher values ​​due to the efficiency of biological nitrogen fixation (BNF), which results in the production of ureides (allantoin and allantoic acid) in the nodules (Table 1). These are sent to the aerial part, where they are dissociated into other forms of nitrogen, contributing to the good physiological and biochemical functioning of the plant.

Regarding the application of glyphosate, no significant effect was observed on any agronomic parameter studied, indicating that, for cultivars that have the glyphosate resistance gene (RR), there was no damage due to applications during the crop cycle. Another factor that influences the absence of negative effects of the herbicide on the FBN efficiency of symbiotic bacteria is that most of the applied glyphosate that reaches the soil is strongly adsorbed on colloids, contributing to their inactivation and unavailability to plants.

It was found that the inoculated RR soybean does not change the leaf chlorophyll index due to the application of glyphosate and, regardless of the inoculation, it has the ability to recover from the application of the product, without harming its development.

Glyphosate is one of the most used herbicides in agriculture, and several authors consider it to be one of the least toxic herbicides for nodulation and subsequent FBN. Some studies indicate that soil microorganisms have the ability to adapt to the application of this herbicide, obtaining, over time, low sensitivity to its presence and growing adequately for symbiosis even at higher product concentrations.

Nadia M. Poloni,
João W. Bossolani,
Edson Lazarini,
João VT Bettiol and
Matheus M. Negrisoli,
unesp
João A. Fischer Filho,
Imesb

Cultivating Great Crops May 2020

With each new edition, Cultivar Grandes Culturas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on the plant health of Major Crops, the Magazine helps farmers in the search for management solutions that increase their profitability. In the May edition you can also see: 

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