Soil mulch in control of soil temperature and incidence of weeds in the production of crisphead lettuce

Deisy Lúcia Cardoso ORCID: https://orcid.org/0000-0003-1982-3413 Universidade Estadual de Mato Grosso do Sul, Brasil E-mail: deisycardoso@uems.br Denise Renata Pedrinho ORCID: https://orcid.org/0000-0002-8522-6126 Universidade Anhanguera, Brasil E-mail: denise.pedrinho@uniderp.com.br Giselle Feliciani Barbosa ORCID: https://orcid.org/0000-0001-5456-1039 Universidade Estadual de Mato Grosso do Sul, Brasil E-mail: giselle.barbosa@uems.br Wolff Camargo Marques Filho ORCID: https://orcid.org/0000-0001-6407-1375 Instituto Federal de Educação, Ciência e Tecnologia Goianos, Brasil E-mail: wolff.filho@ifgoiano.edu.br José Antônio Maior Bono ORCID: https://orcid.org/0000-0002-9573-9494 Universidade Anhanguera, Brasil E-mail: bono@anhanguera.com José Francisco Assis Rolim ORCID: https://orcid.org/0000-0002-8306-176X Universidade Anhanguera, Brasil E-mail: assisrolimp@gmail.com


Introduction
The lettuce (Lactuca sativa L.) is a leafy vegetable of mild climate that belongs to the family Asteracea. It is one of the most important vegetables grown in the world and in Brazil, certainly one of the most popular and consumed, being an important source of minerals, mainly calcium and vitamins (Filgueira, 2013).
It is estimated that the area planted of vegetables in Brazil is 5,1 million hectares, having as production the value of R$ 36 billion, and the volume, more than 53 million of tons (Anuário, 2019).
Brazil has an area of approximately 39,000 hectares for intensive production of lettuce, through cultivation in small areas and by small producers with family farming (up to 10 hectares) concentrated close to major centers and contributing to the generation of employment and income (Heredia Zárate et al., 2010;Vilela & Luengo, 2017) In Brazil, the lettuce marketed can be classified into six groups according to the type of leaf lettuce: butterhead lettuce; crisphead lettuce (American); loose-leaf lettuce; green-leaf lettuce; mimosa lettuce and romaine lettuce (Filgueira, 2013). Among the types of lettuce grown today, the type iceberb has been highlighted, aiming at fulfilling mainly the fast foods chains.
The temperature and photoperiod are the main factors that interfere with the cultivation of lettuce, directly affecting their development. High temperatures affect the texture of the lettuce leaves, making them more fibrous, cause the burning of edges of the outer leaves, affecting the proper formation of head (Filgueira, 2013). High temperatures also stimulate the tasseling, which is intensified to the extent that the same increases (Luz et al., 2009). Development, v. 9, n. 11, e1869119729, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9729 4 To mitigate the effect of high temperatures on cultivations, in recent decades, new cultivation techniques have been used, such as the use of soil cover. The mulching induces less loss of water through evaporation and reduction of the oscillations of temperature, soil, it is used commonly for the lettuce cultivation, providing a microclimate that is more favorable to the development of the cultivation, so it has the capacity to express better their genetic potential (Kosterna et al., 2014).
The advantages presented by this system are: Improvement of the soil structure, increase of the infiltration and soil water retention, improvement on the root system of the invasive plants control, keeping the soil temperature milder (Gilley et al., 1990;Derpsch et al., 1991;Marouelli et al., 2008).
However, the high temperatures are still one of the main problems faced by producers of lettuce in the State of Mato Grosso do Sul. High temperatures during most of the year are marked characteristics of the climate of the region and interfere and affect the production of the cultivation. Considering that use of stubble straw can soften the soil temperature, the objective of this work was to evaluate the effects of the use of plants mulching on the incidence of weeds and soil temperature in the cultivation of crisphead lettuce, cultivar "Lucy Brown", in Campo Grande -MS during the spring, as well as the effects of this system of cultivation on the productivity and the quality of the lettuce.

Material and Methods
The study was carried out between May and November of 2014 in the experimental area at the Anhanguera Uniderp University in Campo Grande -MS (20°26'16.62"S, 54°32'16.07"W). The climate of the region, according to the Köppen (1948) classification system, is Aw, humid tropical climate with the rainy season in the summer and dry in winter.
At 91 days after seeding (DAS), evaluations were conducted on the fresh mass of the coverages, using a square frame of iron of 50cm/side placed in the center of each experimental plot, where it was cut the fresh mass in the area bounded by the square, weighed on precision scale, and, after drying in oven to 60 -70°C until constant mass, it was measured the dry mass; and the values were extrapolated to kg.ha -1 . Research, Society and Development, v. 9, n. 11, e1869119729, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9729 5 The evaluation of weeds infestation was performed immediately before the chemical desiccation of the experimental area, which was performed at 92 DAS, with application of glyphosate in the dose of 1.44kg of a.i..ha -1 . It was used the method of visual evaluation according to methodology of Research Method in Weed Science (1997): 100 (full control), 90 -99 (excellent control), 80 -89 (acceptable control), 50 -79 (not acceptable control), 0 -49 (insufficient control to no effect).
The seedlings of lettuce cultivar "Lucy Brown", were produced in a greenhouse, transplanting was done manually 30 DAS, on the desiccated plants, with a spacing of 0.30 m between rows and between plants. The corrective and coverage fertilizations were performed based on the soil analysis of the area and following the recommendations of Ribeiro et al. (1999). The water supply was performed daily by means of a system of sprinkler irrigation.
It was evaluated the soil temperature by means of daily measurements, before the beginning of the irrigation, always at 14 hours to a depth of five centimeters below the surface of the soil. This is an experimental research qualitative and quantitative, with visual assessments of weed infestation and collection and statistical analysis of the data measured in lettuce plants (Pereira et al., 2018).
The experimental design adopted was in randomized blocks with four treatments The data were interpreted statistically through analysis of variance, and the averages were compared among themselves through Tukey test at 5% probability; using the statistical program Genes (Cruz, 2013). The statistical model adopted was the following: Research, Society and Development, v. 9, n. 11, e1869119729, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9729 6 Y ijk = μ + t i + b j + e ij Where: μ = general average of treatments ti = fixed effect of the i-th treatment (i = 1, 2, 3, 4) bj = end of the j-th block (j = 1, 2, 3, 4); eij = experimental error associated with the observation Yij.

Results and Discussion
The materials, millet and oats, used as mulching on the lettuce cultivation were equally effective and with significantly different results from the lack of coverage on maintenance for the soil temperature (Table 1). On average, these covers kept the soil with a temperature gradient of approximately 5°C below the control (without coverage). The use of mulching in other vegetables show similar results. According to Resende et al. (2005), in the culture of carrots the use of mulching has reduced the gradient temperature gradient at 3.5°C.
In the garlic cultivation, the mulching provides greater insulating effect on the soil surface and contributes to milder temperatures (Costa et al, 1997). The temperature range between the control and treatment coverage more efficient in reducing the gradient was 7, Penninsetum glaucum was the most efficient coverage in reducing soil temperature.
In Table 2 are the weed species identified in the experiment, as well as their respective densities. The highest densities of weeds were the species of Gamochaeta coarctata and Cenchrus echinatus. Development, v. 9, n. 11, e1869119729, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9729    The coverage provided by B. ruziziensis was not efficient for the control of Gamochaeta coarctata, the other coverages were effective in suppressing the weed. Trezzi;Vidal (2004), emphasize the ability to weed suppression through millet cultivation. The authors found average reductions of 91, 96 and 59% of the infestation of Brachiaria plantaginea, Sida rhombifolia and Bidens pilosa, respectively, in areas covered millet stubble. The literature reports that the straw in decomposition can release allelochemicals, which, in turn, can be associated to reduce the emergence of plants seeds (Trezzi;Vidal, 2004;Souza et al., 2006;Carvalho et al, 2016). Jacobi;, claim that the allelopathic action of some oat genotypes is attributed to its ability to exude scopoletin. Scopoletin. is a secondary product from the class of coumarins and has an inhibitory effect on root growth of plants. Hagemann (2010) reports allelopathic potential of oats on the germination and the weeds development.
The allelopathic effect of mulching is so much more intense as the greater the amount of dry matter produced, thus ensuring a sufficient concentration of toxins to lethal action on the seeds or weeds seedlings (Almeida;Rodrigues, 2018). In Figure 1, it is possible to see the data of fresh and dry mass produced by mulches. It is observed that the coverage oats provide the greatest amount of fresh matter, whereas at B. ruzizienses the largest dry mass. In organic cultivation, under direct seeding, it is recommended that the pre-cultivation of species that have large amount of straw, allowing the soil cover, reducing then the undesirable or invasive plants (Vaz de Melo et al., 2007). B. ruzizienses was the covering with the best mass production, the largest amount of dry mass provided a greater reduction in the temperature gradient in the soil, however, it was not efficient for the suppression of weeds.

Soil coverage Notes of visual evaluation
For most of the characteristics related to productivity, showed no difference between treatments, i.e., there was no significant effect of different coverage on the productivity of the lettuce, a fact that can be attributed to the high degree of adaptability to high temperature conditions of this cultivar (Sala;Costa, 2012). There were significant differences only for two variables: dry mass of the commercial head and the fresh mass of the root of lettuce (Table 4).
The mass of commercial head, however, is an important feature for the producer, because the remuneration, in this case, occurs according to the mass of the head. It is also important, as it affects yield during processing.

Conclusions
The use of soil mulch is a beneficial practice for lettuce cultivation, improving the soil temperature and reducing the incidence of invasive plants. The soil coverage with P. glaucum and B. ruziziensis stood out in reducing the soil temperature gradient, while with P. glaucum and A. strigosa reduced the incidence of weeds.
The coverage with P. glaucum has also provided an increase in the fresh mass of the commercial head, causing an improvement in product quality.
It is suggested, as a continuation of the studies, new experimental research with other leafy vegetable plants, and other cover crops in order to meet other advantageous possibilities for cropping systems.