Cowpea (Unguiculata vine L.) is recognized for being an excellent source of carbohydrates (50% to 60%) and proteins (17% to 28%), playing an important role in combating malnutrition in underdeveloped countries. Consumption and production occur predominantly in underdeveloped countries located in Latin America and Africa. In Brazil, cowpea is commonly cultivated on small properties located in the north and northeast regions. Recently, the crop began to be incorporated into the productive arrangement of the Brazilian center-west, where cowpea is cultivated on large properties. In common, the regions mentioned are characterized by low rainfall during the growing season.
Cowpea has an indeterminate growth habit, so reproductive growth occurs alongside vegetative growth, resulting in uneven flowering and, consequently, grain maturation. Close to harvest time, the same plant may have green leaves and grains with water content above the ideal. In an attempt to standardize maturation and carry out mechanized harvesting on large properties, some producers use herbicides with desiccant action on plants.
There are few products registered in the Agrofit System of the Ministry of Agriculture, Livestock and Supply (MAPA) for the practice of pre-harvest bean desiccation. The products that are registered are Flumioxazine and Ammonium Glufosinate, which must be used, when necessary, always with the guidance of an Agricultural Engineer.
In practice, some producers use products that are not registered for pre-harvest desiccation. Among the most used are paraquat and glyphosate, the latter of which, in some reports from producers, is associated with other herbicides, such as carfentrazone.
Strategy for developing the study
The mechanism of action of different desiccant herbicides is already well elucidated when the application is carried out in the vegetative stage of plants, as their indication for use generally occurs at this stage. However, there is a growing demand in agriculture for pre-harvest applications for desiccation, and reports on the behavior of vegetables and, more specifically, grains under these conditions are still scarce.
To assess the consequences of using desiccants, it is not enough to obtain samples directly from storage units or the shelf, as, in this way, it is not possible to know the origin of the product. Therefore, the study aimed to reproduce as faithfully as possible the cases reported by some producers in the Brazilian center-west and evaluate the technological quality of newly harvested grains, as well as the residual levels of herbicides in the grains. The tests were conducted with paraquat (PAR), glyphosate (GLI) and glyphosate + carfentrazone (GLI/CAR).
The study was developed in Mato Grosso, in the municipality of Primavera do Leste, with support from producers and businesspeople in the region. Four field plots of 25 ha each were used, three for the evaluation of herbicides and one was kept without application (SAP). In each 25 ha plot, three areas of 10mx20m were considered as replicates for sample analysis.
In the non-desiccated area, the plants were uprooted and kept on the ground until they reached the desired water content. All operations, whether or not involving desiccation, were carried out 70 days after sowing.
Technological quality of dried cowpea
Cowpea (Vigna unguiculata) of the Bico de Ouro variety has post-harvest behavior similar to carioca beans (Phaseolus), that is, the seed coat, which is the outermost layer of the grain, tends to redden during storage. The change in color after harvest is associated by producers, retailers and consumers with a poor quality product, which has a low market value.
To estimate this change in color, the a* value was used, which indicates changes in the color of the grains on a colorimetric axis that varies from green (-) to red (+). The higher this value in the colorimetric test, the redder the beans.
It is believed that the reason why the darkening occurred was different for each of the treatments. However, in common, the oxidation of phenolic compounds is identified as the main responsible for the change in the color of the seed coat. These phenolic compounds have a protective and attractive function for vegetables and are also known as antioxidant compounds for those who consume them.
Just like color, cooking time is another extremely important parameter for evaluating the quality of cowpea. In general, regardless of the herbicide used in pre-harvest, desiccation resulted in shorter cooking time when compared to the area without application. Bean cooking time is determined by the Mattson cooker test, for treatment comparison.
In the area where herbicides were not applied, cowpea took longer to reach the ideal harvest point. It is likely that the grain, which is a living organism, protected itself from adverse conditions, such as excess solar radiation. The most common and well-known mechanism for protection is associated with the lignification process of the grain's cotyledon. This mechanism may be one of those responsible for the increase in cooking time.
The physiological consequences of the use of herbicides are not restricted to changes in color and cooking time. To analyze further and be able to infer the extent to which the herbicide acts on the grains, a liquid chromatography analysis coupled to non-targeted mass spectrometry of the different treatments was also carried out to observe the metabolic responses. With this analysis, several compounds were quantified and their variations were determined through a multivariate analysis of principal components (PCA). Groups were formed for each of the treatments, confirming distinct patterns of metabolic response of cowpea grains to different herbicides.
Catechin-3-glucoside and epicatechin, which are phenolic compounds, were present at higher levels in glyphosate-treated grains. Citric acid was responsible for discriminating the treatment with glyphosate + carfentrazone. Quercetin and gluconic acid were responsible for the discrimination of treatment without application. In the paraquat treatment, both quercetin and gluconic acid remained at intermediate levels when compared to the other treatments.
Residual levels of herbicides in grain
Cowpea dried with glyphosate and glyphosate + carfentrazone showed residual glyphosate content of 13mg/kg and 14mg/kg of dry weight, respectively. The residual levels of carfentrazone were lower than the limit of quantification of the test by liquid chromatography coupled to mass spectrometry. The glyphosate values of both treatments were higher than the maximum limits of 2,0mg/kg and 0,1mg/kg established by Codex Alimentarius (2016) and the European Union (2011), respectively.
The paraquat content was 0,84mg/kg dry weight in cowpea treated with paraquat. As determined in the study with glyphosate, the value of paraquat determined in the present study is higher than the maximum limits of 0,5mg/kg and 0,02mg/kg of grains allowed by the Codex Alimentarius (2016) and the European Union (2011), respectively. .
The effects of exposure to glyphosate on human health have been studied for years and there is still no consensus among researchers about its real harm. The widespread use of paraquat in agriculture is reported to be responsible for the detection of residues in environmental, food and biological samples (AWADALLA, 2012; CHEN et al., 2012).
Problems with the use of desiccants, without following agronomic recommendations, are not new in Brazil. In 2016, for example, wheat producers were warned not to use glyphosate and paraquat, as had happened in the two previous harvests, due to the presence of the aforementioned herbicides in wheat flour, at levels higher than the legal limits. Whenever producers need to use desiccants or other agrochemicals, it is mandatory that the products are registered for this purpose.
Final considerations
It is essential that producers consult Agricultural Engineers when it is necessary to use desiccants. The Engineers will provide guidance on the use of products registered by the Ministry of Agriculture, Livestock and Supply (MAPA), their method of application and the safety interval. In the absence of options for cowpea cultivation, there is an urgency to study and develop protocols to regulate products and application management of desiccants, considering that the use of products such as glyphosate and paraquat, as done by some producers, poses risks to the health of consumers, due to the residual levels observed in this work.
For the consumer, it is emphasized that the practices used as the basis for this study are isolated and that research and inspection bodies and technical assistance professionals are alert to protect the population's health.
Igor da Silva Lindemann1
Galileo Rupollo2
Moacir Cardoso Elias3;
Nathan Levien Vanier4
1Agricultural Engineer, Master, PhD student at the Post-Harvest, Industrialization and Grain Quality Laboratory (Labgrãos) of the Department of Agroindustrial Science and Technology (DCTA), of the “Eliseu Maciel” Faculty of Agronomy (FAEM), Federal University of Pelotas (UFPEL ). Email: igor_lindemann@hotmail.com
2Agricultural Engineer, PhD, Grain Post-Harvest Consultant in Mato Grosso. Email: galileuru@yahoo.com.br
3Agricultural Engineer, PhD, Full Professor at DCTA-FAEM-UFPel. Email: eliasmc@uol.com.br
4Agricultural Engineer, PhD, Adjunct Professor at DCTA-FAEM-UFPel. Email: eliasmc@uol.com.br
6. References
AWADALLA, EA Efficacy of vitamin C against liver and kidney damage induced by paraquat toxicity. Experimental and Toxicologic Pathology, v. 64, p. 431–434, 2012.
CHEN, YW; YANG, YT; HUNG, DZ; SU, CC; CHEN, KL Paraquat induces lung alveolar epithelial cell apoptosis via Nrf-2-regulated mitochondrial dysfunction and ER stress. Archives of Toxicology, v. 86, p. 1547–1558, 2012.
EUROPEAN COMMISSION. Commission Regulation, (EU) No. 520/2011, amending Annexes II and III to Regulation (EC) No 396/2005 of the European Parliament and of the Council as regards maximum residue levels for benalaxyl, boscalid, buprofezin, carbofuran, carbosulfan, cypermethrin, fluopicolide, hexythiazox, indoxacarb, metaflumizone, methoxyfenozide, paraquat, prochloraz, spirodiclofen, prothioconazole and zoxamide in or on certain products, 2011.
FAO/WHO. Codex Alimentarius Commission: Procedure Manual, 39th session. Ed. Rome, Italy. 2016.
LINDEMANN, I.; LANG, G.H.; HOFFMANN, J. F.; ROMBALDI, V.; OLIVEIRA, M.; ELIAS, M. C.; ROMBALDI, C. V.; VANIER, N. Foliar desiccators glyphosate, carfentrazone and paraquat affect the technological and chemical properties of cowpea grains. Journal of Agricultural and Food Chemistry, v. 65, p. 6771-6778, 2017.
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