Dead Cover and Agronomic Characteristics of Cowpea

Dead cover, or mulch, consisting of plant residues, plays an important role for the success of diverse agricultural crops, working as an insulating layer protecting the soil from daytime temperature variations and maintaining the soil moist and rich in organic matter. Cowpea is a source of proteins, carbohydrates, vitamins and minerals. Its importance in the North, Northeast and Midwest regions of the country is associated with economic and social aspects, since it is an important food for low-income populations, supplying their nutritional needs. This study was carried out under greenhouse conditions in Manaus, state of Amazonas, with the purpose of assessing the effect of different dead covers on the agronomic characteristics of cowpea cultivars. It consisted of a completely randomized design in a 4 × 4 factorial arrangement. The treatments comprised four cowpea cultivars (BRS Caldeirão, BRS Tumucumaque, BRS Guariba and BRS Tracuateua) and three species of cover plants (Brachiaria decumbens, Brachiaria ruziziensis and Mucuna pruriens) and one control treatment, without soil cover, in a total of 16 treatments, with four replications and two plants per experimental unit. Analysis of variance was applied to the data, and the means were compared by the Scott-Knott’s test at 5% probability level. The following characteristics were examined: number of pods per plant, pod length, number of seeds per pod, weight of shoot dry matter, and grain yield. Mulching provided better results for all characteristics assessed in the four cultivars when compared to the control. BRS Caldeirão is the recommended cultivar for the state of Amazonas and the other regions with similar edaphoclimatic characteristics (high air temperature, rainfall, air humidity, and low-fertility tropical soils) because it exhibited the greatest number of pods per plant, number of seeds per pod, shoot dry matter, and the highest average grain yield (Freire Filho et al., 2011; Souza et al., 2016).


Introduction
Cowpea (Vigna unguiculata) is a legume that integrates the staple diets of populations in developing countries, rich in proteins, minerals, and fibers (Frota et al., 2008). In addition to providing an important food supply, it generates jobs and income to farmers (Freire Filho et al., 2011).
In Brazil, cowpea is largely cultivated in the North and Northeast regions of Brazil by small farmers (Locatelli et al., 2014), but the production of this legume has grown in the Midwest in recent years due to its high adaptability to tropical conditions, low production costs, and the intense genetic improvement efforts that have been applied to the culture (Freire Filho et al., 2011). association with the nitrogen-fixing bacteria, enriching this nutrient in the tissues and providing it to the crop (Costa et al., 2015).
Grass plants are an alternative because they have a high C/N ratio and, therefore, a low decomposition rate, protect the soil for longer periods, conserve soil moisture and nutrients cycling, with less variation in the soil temperature (Torres, Cunha, Pereira, & Vieira, 2014). Given the above, this work aimed to evaluate the effect of different mulches on agronomic characteristics of cowpea cultivars.
Each experimental unit consisted of a polyethylene pot measuring 23 cm in diameter and 17 cm in height, filled with 5.104 dm 3 of substrate. Each pot contained two cowpea plants, spaced 10 cm apart inside each pot, and 50 cm distant from each pot.
Correction of soil acidity and fertilization were carried out based on the soil analysis and recommendations for the culture. Dolomitic limestone was used for liming (90% Relative Total Neutralization Power). After 90 days allowed for the liming reaction, cover fertilization was performed at the rate of 70 kg ha -1 of P 2 O 5 and 60 kg ha -1 of K 2 O, of simple superphosphate and potassium chloride sources, respectively (Novais & Alvarez, 2002;Sousa, de Castro, Damasceno, & de Matos, 2019).
Sowing was conducted ten days after fertilization, at a depth of 2.0 cm from the surface, with four seeds per pot. After 15 days, thinning was done, and the two most vigorous plants, with good phytosanitary aspects, were maintained. After thinning the seedlings, a 4-cm thick layer of each plant cover species, according to the treatments described above, was spread on the soil surface. The plant material used as mulch in the experiment was specifically cultivated for this purpose in an area of 30-m 2 . The plants were dried during 20 days in natural environment. Using scissors, the material was cut into pieces to be accommodated in the pots.
After harvest, the mean values of each experimental unit were analyzed to determine the following agronomic characteristics: a) Number of pods per plant (NPP)-total number of pods with at least one seed, divided by the number of plants; b) Pod length (PL)-measured with metric tape; c) Number of seeds per pod (NSP)-ratio of the total number of seeds to the total number of pods; d) Shoot dry matter (SDM)-obtained by drying leaves, stems and petiole in a forced-air circulation oven at 65 °C to constant weight; weighing on an analytical scale; e) Grain yield (GY)-grain weighing (13% of moisture), and transformation of data to kg ha -1 (Camara, Mota, Nicolau, Pinto, & Silva, 2018). Tumucumaque = Tracuateua (Table 1). For shoot dry matter, cultivars BRS Caldeirão, BRS Tumucumaque and BRS Guariba exhibited the highest averages, not differing from one another. But BRS Tracuateua was the cultivar with the lowest production of dry matter. Ranking for average grain yield was: BRS Caldeirão = BRS Guariba > BRS Tumucumaque = BRS Tracuateua (Table 2).  Note. Means followed by the same letters in the column do not differ by the Scott-Knott test (p < 0.05).

Discussion
All cowpea species studied were classified as early-cycle cultivars, which is an advantageous aspect for producers because it allows faster financial returns due to the shorter time in field, in addition to the shorter time of exposure to possible adverse weather, reducing risks of crop losses (Freire Filho et al.,2011;Osipitan, 2017).
The absence of attacks by pests or diseases may be related to the mulch ability to prevent soil pathogens from spreading on the plant' shoots by water splashed during the irrigation process (Pereira & Pinheiro, 2012).
There was no occurrence of weeds during the cowpea productive cycle. Mulch has the ability to gradually hinder increase of the weed seed bank by preventing the incidence of sunlight required for germination, or functioning as a physical barrier to the emergence of seedlings, minimizing their regrowth, thus reducing control costs and representing a viable alternative (Maia Júnior et al., 2019).
Of all cultivars studied, the cv. BRS Caldeirão indicated the best agronomic performance. It exhibited the greatest number of pods per plant, number of seeds per pod, shoot dry matter, and the highest average grain yield. It was followed by cv. BRS Guariba, which was not different from cv. BRS Caldeirão in number of seeds per pod, average grain yield and shoot dry matter, and exhibited the second greater number of pods per plant and the second longer pod length (Tables 1 and 2).
The number of pods per plant is a production component that is directly related to productivity, being an effective parameter to define the cowpea cultivar to be chosen by farmers (Silva & Neves, 2011;Bezerra, Neves, Alcântara Neto, & Silva Júnior, 2014). Similar results were obtained by Locatelli et al. (2014) when they studied the productivity of cultivars BRS Guariba, BRS Nova Era and BRS Pajeú cultivated in no-tillage system with mulch cover (straw) of Brachiaria ruziziensis, where the NPP was 9.5, 12.84 and 11.08, respectively.
With respect to pod length (Table 1), all cultivars exhibited lower values than those found by Silva and Neves (2011), who recorded average lengths of 19.7 cm and 20 cm in rainfed and irrigated cultivation, respectively. But they were higher than the ones found by Bertini, Teófilo, and Dias (2009), who recorded 11.9 cm as the longest pod length. Large pods favor manual harvesting, usually performed in small crop areas, predominantly by family farmers. Due to this feature, cv. BRS Tumucumaque would be recommended for cultivation in Amazonas, because it exhibited the longest pod length and, together with BRS Guariba, the second largest number of pods per plant, but the smallest number of seeds per pod, and, along with BRS Tracuateua, the lowest average grain yield, thus not justifying the choice based on pod length only (Tables 1 and 2).
For cropping systems that use semi-mechanized or mechanized harvesting, smaller pods are more suitable for handling. Because of the pods' light weight, they tend not to touch the ground, which reduces the likelihood of occurrence of losses caused by rot pods and seeds. BRS Caldeirão showed to be an ideal cultivar for semi-mechanized or mechanized harvesting, exhibiting the best performance for all parameters assessed, including a smaller pod length (Silva & Neves, 2011;Freire Filho et al., 2011).
Regarding the number of seeds per pod (Table 2), cv. BRS Guaribaand BRS Caldeirão exhibited the highest values, 10.31 and 9.84, respectively, higher than the other species, with no significant difference between them. The average number of seeds per pod was similar to the one found by Silva and Neves (2011). Locatelli et al. (2014) assessed the effect of use of no-tillage system and irrigation on straw mulch of Brachiaria ruzizienses with regard to the physiological characteristics of cowpea and obtained 17.6 g of shoot dry matter for cv. BRS Guariba, a lower value than the one found in this study. Lacerda et al. (2009) obtained a maximum value of 27 g of cowpea' shoot dry matter in a study conducted with 326 mm of water.
Shoot dry matter is considered a productivity indicator. Plants that undergo water stress close their stomata, reducing the assimilation of CO 2 and photoassimilates and, consequently, the plants'shoot dry matter, growth and yields may be reduced (Oliveira, Fernandes, & Rodrigues, 2005). Mulch provides better water retention, and the cowpea plant can respond to different stimuli from the environment, optimizing photoassimilate partitioning and increasing the accumulation of dry matter throughout the cycle, as a physiological response to ensure grain yields (Silva, Maracaja, Medeiros, F. Oliveira, & M. Oliveira, 2009).
The interaction between the cowpea cultivars and the plant species used as dead cover was not significant. However, application of mulch favored cowpea yields compared to the control (without mulch). A similar result was found in a study with cowpea where mulching provided a greater amount of shoot dry matter and greater pod number and length (Maia Júnior et al., 2019).
In a study with soybean cultivation in succession to maize intercropped with different forage plants,there was no significant difference for the parameters assessed, which was justified by the similar process of decay and mineralization of the forage mulch (Garcia, Andreotti, Teixeira Filho, Lopes, & Buzetti, 2014). The same may have occurred in the present study. It should be noted that in greenhouse cultivations, there is little influence from external environments, resulting in a slow decomposition process. So, because it is a short-cycle culture, mulch decay and mineralization may not have occurred in a timely manner to influence the crop yield more significantly.
According to Bastos (2017), hot temperatures during flowering may cause a significant reduction of grain yields caused by flower abortions. However, although the state of Amazonas has high temperatures, the results achieved in this study were higher than the average Brazilian yield, which is of 480 kg ha -1 , as recorded by CONAB (2017), thus justifying the practice of mulching. According to Mota, Libardi, Brito, Assis Júnior, and Amaro Filho (2010), mulch on the soil surface reduces evaporation and increases water storage, especially in the initial and vegetative phases of the culture.
Thus, the presence of mulch on the soil surface provided better grain yields, probably because of the greater availability of water in soil and reduced soil temperature, and also because of the incorporation of nutrients generated from the cover plants decomposition and mineralization.

Conclusion
BRS Caldeirão is the most recommended cultivar for the state of Amazonas and other regions with similar soil and climate characteristics (high air temperature, rainfall, air humidity, and low-fertility tropical soils). Of the cultivars assessed, BRS Caldeirão exhibited a greater number of pods per plant, number of seeds per pod, amount of shoot dry mater and higher average grain yield.
There is no significant relationship between the cowpea cultivars BRS Caldeirão, BRS Tumucumaque, BRS Guariba, BRS Tracuateua and the use of mulch formed by weed species Brachiaria decumbens, Brachiaria ruziziensis and Mucuna pruriens.
Regardless of the species used in the present study, dead soil cover provides better results in the number of pods per plant, pod length, number of seeds per pod, shoot dry matter, and grain yield.