SYSTEMATIC BIBLIOGRAPHICAL REVIEW OF SCIENTIFIC PUBLICATIONS ON THE USE OF MILLET AND SORGHUM AS VEGETABLE COVER

The use of alternative systems seeking unsustainability in agricultural production in Brazil, has developed strongly with the use of No-Tillage System (NTS), therefore remodeling the production system. For the production efficiency, it is necessary to use suitable crops as plant cover, depending on the region where the no-tillage system will be set, in which sorghum and millet crops were very positive. Thus, the objective of the work is to perform a systematic literature review on papers approaching the subject with the use of sorghum and millet as a vegetative cover for subsequent no-tillage. It is possible to verify a concentration of the studies after the 2000 decade, in the Southeastern and Central-Western regions of Brazil.


INTRODUCTION
Agriculture is one of the oldest food production activities performed by the man. It is associated with the development of the first civilizations. Furthermore, its activities have been technologically intensified over the years due to the global food demand caused by the population increase over the years (BUAINAIN, et al., 2016).
However, over the years, the relationship between the cropped area for food production and population growth has been reduced, which makes it necessary to use precise measures for global food service, since estimates indicate that, to provide enough food to meet the world's demand until 2050, food production needs to be increased by around 60% to 110% (RAY et al., 2013).
Due to the increase in population, land use exploitation was intensified through the intensive soil management practices, the adoption of monoculture in addition to the large use of pesticides to ensure greater production of food (FELEMA, et al., 2013 ). Thus, the use of agrochemicals and new technologies is expanding, which aim to provide better soil management, assuring the quality and productivity of crops combined with the saving of natural resources such as soil and water for irrigation (PUTTI et al., 2014;KLAR, et al., 2015;PUTTI et al., 2017;OLIVEIRA, et al., 2018).
The development of modern agriculture began in the mid-1950s with the Green Revolution, which has been spreading on a larger scale since 1970, with the need for food production after the war, with the insertion of technological innovations, use of varieties of high-yield seeds with the addition of genetically modified plants, management practices and application of pesticides in the production SANTOS;JOHN, 2018).
However, changes in attitudes regarding the use of those techniques and the use of systems that can be more sustainable have intensified over the years, guaranteeing na efficient and nutritious production and in a way that does not harm the environment intensely, as well as studies based on the economy of natural resources, for example the use of water for irrigation (VIAIS NETO et al., 2019a;GERHARDT, 2016;PEREIRA, et al., 2017).
Allowing the insertion and diffusion of Conservation Agriculture (CA), which developed because of those obstacles, it is sought a production that will guarantee the safety and preservation of the environment and the soil, minimally changing the structure and biodiversity, with gains in food productivity, in addition to generating savings in the use of natural resources (VIAS NETO et al., 2019b;PUTTI et al., 2017;BARBITO;NYARUWATA, 2015).
Thus, the No-Tillage Planting System (NTS) is found, which consists of a cultivation technique, in which, one of the requirements is that the planting is carried out without the conventional use of preparation (plowing and harrowing), that is, the soil is not disturbed, and the plant cover of the crop remains and straw from the previous crop is maintained, in addition to resulting in a system with a lower degree of impact on the environment and stimulating the restoration of biodiversity in the soil. In that system, less water is used for irrigation, as there is a lower incidence of evapotranspiration FAVARATO et al., 2016;MILAGRES et al., 2018).
However, the success of a cover crop is associated with its high biomass and/or dry matter yield, directly related to the level of C/ N (carbon / nitrogen) concentration, in addition to the edaphoclimatic factors typical of each region ( SILVA FILHO et al., 2018).
Thus, millet and sorghum crops are considered excellent cover crops, adapted for the Central-West region, as in addition to the high C/N concentration, they have a deep root system for greater water uptake in dry periods, typical of the Caatinga (Brazilian dry forest) (GOES, 2016;SEBEN JUNIOR et al., 2016). Thus, the objective of this work was to carry out a systematic review of the literature, about the works related to the No-Tillage System using the vegetable coverings of millet and sorghum.

MATERIAL AND METHODS
This work used the methodology proposed by Conforto et al., the RBS Roadmap as a search script for the Systematic Bibliographic Review (2011), which consists of a sequence of three phases: Input, in which the information to be processed to search for scientific articles is defined, as well as other bibliographies indicated by the specialists; Processing, which will make the whole process of searching for input information, which in turn will cause the Output, which will allow for the synthesis of results, reports, among others (LEVY; ELLIS, 2006;CONFORTO et al., 2011).
List of the steps performed in the RBS Roadmap construction process ( Table 1).
The details of the information contained in each stage of the RBS implementation will be described throughout the work, as we can see below (Table 2), the information related to the first stage of the process.
During the realization of the first SBR step, the research Problem was defined, as well as the Objective of the work. It is also stressed for the inclusion of bibliographies recommended by experts, as defined in the "Primary Sources" in addition to the search strings in the database selected for the research together with the criteria adopted for the selection of scientific papers.
Information processing defined above, is described according to the input of different strings, as it is observed in Table 3.

Search Strings
"sorghum OR Sorgo", "millet OR milheto", "vegetal cover OR cobertura vegetal", "vegetable cover", "cover plants OR plantas de coberturas", "notillage OR plantio direto", "direct planting". During the processing phase, the search was performed using keywords related to the strings topic. Firstly, a search was made for scientific works containing strings ("Sorgo" OR "Sorghum"), covering works in Portuguese and English, obtaining a total of 1,351 scientific papers in the SICELO database.
Likewise, it was made for strings ("milheto" OR "millet"), with the equivalent of 537 works.
The objective is to search for topics related to the use of millet and sorghum as a vegetation cover, so the search proceeded through publications referring to the use of millet and sorghum, simultaneously, defined as search strings: (("sorgo" OR "Sorghum") AND ("millet" OR "milheto")), resulting in 120 publications in SCIELO.
As a result, the strings defined in the previous paragraph were combined with publications related to vegetable coverings, characterized as search strings: AND (("Vegetable cover" OR "vegetable cover" OR "cover plants" OR "plantas de coberturas" OR "cobertura vegetal")), resulting in a total of only 13 publications. Due to the small number of works to be researched, a new search was carried out in the database, covering publications that deal with topics on no-tillage, resulting in the search string: (("Sorghum" OR "Sorgo") AND ("Millet" OR "milheto")) AND (("Vegetal Cover" OR "vegetable cover" OR "cover plants" OR "plantas de coberturas" OR "cobertura vegetal" OR "crop rotation" OR "rotação de cultura") OR ("no-tillage" OR "direct planting" OR "plantio direto")), aiming to cover the largest number of publications related to the research objective, with a total of 61 works, distributed over the years as illustrated in Figure 1.
Following the previous step, search filters are started, in which, due to the small number of papers published over the years, all publications resulting from the search were taken into account, totaling 61 documents, to start the filters search results as shown in Table 4.
According to Table 4, after the filter in the SCIELO database, to return only scientific articles, there were 61 articles; all of which are published in the field of agricultural sciences, with 60 articles published in Brazilian journals.
After the return of the 60 articles, with the filtering available through the database, the reading of the titles and abstract begins, according to the pre-established exclusion criteria in accordance with the objective of the study. Table 5 shows the volume of publications after performing the post-reading filters.
Over the process of reading the titles and abstracts of the 60 papers selected by the search filters, a total of 22 articles were excluded according to pre-defined exclusion criteria, out of which five dealt with topics related to animal nutrition; 10 that approached topics related to the control of weeds and herbicides, three on the crop-livestock system and four papers that had an emphasis on the irrigation system. "Millet" OR "milheto" 537 (("Sorgo" OR "sorghum") AND ("millet" OR "milheto")) 120 (("sorgo" OR "sorghum") AND ("millet" OR "milheto")) AND (("Vegetal cover" OR "vegetable cover" OR "cover plants" OR "plantas de coberturas" OR "cobertura vegetal"))
Once 38 articles were selected using the exclusion criteria, the introduction and conclusion were read in order to confirm the previous criteria. Hence, 21 scientific papers were removed from the selection of 38 articles, according to the criteria, which was 1 due to the focus on animal nutrition; 2 to weed control; 4 to the crop-livestock system; 3 with emphasis on the irrigation system, in addition to nine papers on the physical characteristics of the soil and three referring to agronomic analysis of productivity.
Thus, there were 16 scientific papers left for the complete reading process, which was characterized in the final number of papers related to the analysis of the chemical attributes of the soil using millet and sorghum as a vegetation cover, which will be made the analysis of results about the publications on the purpose of the work. Table 6: List of the papers selected for the analysis.

Selected papers
Thus, the selection of the 16 articles concludes the information processing phase, then, the third stage of the work starts with the analysis of the results.

RESULTS AND DISCUSSION
Following the final selection of the 16 articles, the analysis shown in the third phase of the SBR Roadmap process was carried out. Figure 2 shows the overview of the publications over time.
The year 2002 is the starting point of the publications as the no-tillage system is a recent technique used in agriculture, which has had an increase in the study on the topic over the years with two publications in 2004, 2005, 2009, 2010 and 2012, three publications in 2008, and no papers in 2007 and 2011. Thus, the regions where the studies on the use of sorghum and millet as vegetation cover were carried out showed a greater incidence in the Southeast region, with 11 publications and five publications in the Central-West region as an experimental area, showing satisfactory results in relation to dry matter yield and nutrient accumulation, as it can be seen in the graph below.
Of the 69% representation of the Southeast region, three experiments were carried out in the State of Minas Gerais, and eight studies in the State of São Paulo; nevertheless, in the Central-West region, three were in the State of Mato Grosso do Sul and two in Goiás, the ones representing 31% of the region where the experiment was carried out.  Regarding the published journals, their analyses was made regarding the Qualis of the journals in the area of agricultural sciences, Table 7.
It can be observed the volume of publications in journals above B1, with nine articles submitted and six in journals classified with A2 Qualis and only 1 without Qualis, which characterizes publications with scientific impact value.
In general, the articles selected for analysis are classified in Table 8, below with information from the published journal, Qualis and number of citations of the paper in scientific papers.  Thus, publications on the use of millet and sorghum as vegetable coverings for later no-tillage crops are configured.

CONCLUSIONS
• Based on publications in specialized journals referring to the use of the No-Tillage Planting System using plant coverings, millet and sorghum crops, the highest concentration of papers is observed between 2002 and 2012, since it is considered a recent activity in agriculture, with its experimental study in the Brazilian southeastern and central-western regions, obtaining publications in journals with scientific value, with A2 and B1 Qualis, in addition to the high number of citations of the papers, proving to be scientifically important for the area of agricultural sciences.