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INTRODUCTION
Cuba needs to import more than 400,000 tons of rice per year, so a strong investment program is carried out with the purpose of substituting imports and guaranteeing, before 2030, a national production of at least 85% of the 700,000 tons of rice the country consumes annually (Reyes, 2019).
However, the average yields obtained in the last 25 years do not exceed 3.75 t/ha and the traditional production system predominates in most areas where rice is grown (Miranda, 2020), which requires a high degree of mechanization (specialized cultivation), conditioned by the different sowing technologies that are used and the land extensions that are destined for its exploitation. There is a novel experience in the introduction of mechanized rice transplantation technology with a self-propelled transplanter to guarantee the production of seeds of new rice cultivars. This technology does not prevail yet, although it has a series of advantages, such as: cost reduction (better weed control, mainly red rice; and reduction in the amount of seed/ha). In addition, it generates better health of the rice plants, due to the low sowing density, better root development, which allows better absorption of nutrients and greater resistance to overturning. It also increases the vigor of plant stems, as there is less competition for nutrients, water and light. The technology of sowing by transplantation allows the control of contaminating rice, since the cultivation has a certain advantage over weedy rice, at the time of transplantation, in addition to the management of the water layer, which allows obtaining a culture free of contaminating rice (Miranda et al., 2019; Miranda, 2020; Domínguez et al., 2021b).
In Cuba, producers associated or not in cooperative farms, outside the lands of state companies, use manual transplantation as the fundamental way for sowing, where the producer, supported by his family, faces this great task that involves physical effort and direct exposure to the polluting medium of the rice plantation (Hernández et al., 2016). In the world, the mechanized transplantation of rice is being imposed, which requires certain requirements to develop the process efficiently and among them is the production of the seedlings in trays, which implies a system of equipment that grinds and sifts the soil, fill the trays, fertilize, water and sow the pre-germinated seed (Domínguez et al., 2021a). One hectare of soil for transplantation requires 400 trays of 0.30 x 0.60 cm (ERP-60, 2000).
MATERIALS AND METHODS
The experimental investigations were carried out in the experimental areas of Los Palacios Base Scientific and Technological Unit in Pinar del Río Province and had the objective of evaluating the quality of rice planting by mechanized transplantation with the use of the DAEDONG ERP-60 transplanter model. Figure 1 and Table 1 show some of its technical characteristics (Miranda et al., 2021; ERP-60, 2000).
TABLE 1 Technical characteristics of the ERP 60 transplanter machine
| Total length, mm | 3,100 |
| Overall width, mm | 2,095 |
| Total height, mm | 1,880 |
| Clearance, mm | 405 |
| Weight, kg | 662 |
| Model | FD620D |
| Kind | Two-cylinder, water-cooled gasoline engine |
| Power / Revolutions (max) (kW/rpm) | 11.4 / 3,600 (14.7) |
| Displacement (cc) | 617 |
General Methodology for the Elaboration of the Seedbeds
As established by Minh (2012); Guerra et al. (2013); Hernández et al. (2016), the fundamental variants that affect the quality of seedlings in trays are: substrate composition, percentage of seed germination, seed selection, cultural care and plant vigor.
The methodology used for the preparation of the tray seedling consists of the following steps:
Sifting the soil and other components of the substrate
Mixng all the components of the substrate relationship
Chemical analysis of the substrate relationship
Depositing the substrate up to two centimeters high in the tray
Moistening the substrate at the rate of two liters of water per tray
130 g of seed are deposited per tray, at an average rate of 2.4 seed / cm2
The remaining space of the tray is covered with substrate and it is smoothed
The substrate is moistened again until it drains through the lower holes
The trays used have the following characteristics: length of 60 cm, width of 30 cm, depth of three centimeters, diameter of the holes of 0.3 cm; number of holes, 105 per tray.
To fill the trays, a semiautomatic seeder designed for "row by row" sowing is used, which allows using small quantities of seeds (naked or coated) with any type of tray. The change of the seeding bars and/or the nozzles is easy and fast; it allows the use of different types of trays with different varieties of seeds, Figure 2.
FIGURE 2 Tray seeding machine, 1- Support frame, 2- Movable tray support, 3- Seedling head, 4- Suction jar for seeds.
The rice tray seeder machine is pneumatically operated (it is connected to a compressor), it is manipulated by an operator in charge of assembling and disassembling the trays on the mobile support (2) and keeping the seed assortment in the head tank (3). The sowing is carried out in a synchronized way of four operations, the gauges make 20 traces on the moving tray, while the jars (4) suck the same amount of seed in the head deposit and release them into the conduits that deposit them on the trays of the tray. With this equipment a productivity of 30 trays sown per hour was achieved.
Methodology to Determine the Composition of the Substrate
According to studies of Hernández et al. (2016), in order to determine and adapt the components of the substrate for a carpet rice seedbed under conditions of the southern plain of Pinar del Rio, four variants were decided to prepare the substrate, taking into account the recommendations of the consulted bibliography in Philippine Rice Research Institute (2009) [10] , the substrates tested were:
Sifted Dry Soil (ST).
Sifted Dry Soil + Sifted Organic Matter (ST + MOT).
Sifted Dry Soil + Sifted Organic Matter + Ground Cane Dry Fiber (ST + MOT + FCSM).
Sifted Dry Soil + Sifted Organic Matter + Ground Cane Dry Fiber + Carbonized Rice Husk (ST + MOT + FCSM + CAC).
The substrates, according to composition, remained at rest after mixing for: 40, 30, 20, 10, 0 days. For each substrate with its corresponding rest days, 4 trays (30 cm x 60 cm) were mounted, according to Table 2.
TABLE 2 Location of the different substrates and resting time.
| 40 days | ST | ST + MOT | ST + MOT + FCSM | ST + MOT + FCSM + CAC |
| 30 days | ST + MOT + FCSM + CAC | ST + MOT + FCSM | ST + MOT | ST |
| 20 days | ST + MOT + FCSM | ST | ST + MOT + FCSM + CAC | ST + MOT |
| 10 days | ST + MOT | ST + MOT + FCSM + CAC | ST | ST + MOT + FCSM |
| No rest | ST | ST + MOT + FCSM | ST + MOT + FCSM + CAC | ST + MOT |
Methodology and Standards for Seed Selection
Selection of the seed. This is done by the seed selection method by specific gravity, for which they were immersed in a saline solution with a concentration of 1.13 g/cm3, taking only the seeds submerged in the bottom of the container (Minh, 2012).
Determination of germination. The purpose of germination tests is to determine the maximum germination potential of a batch of seeds, to estimate its value for sowing in cultivation land and to provide results that allow comparing the different seed lots (Minh, 2012; NRAG/CTNR, 2012).
The actual seed mass (Mr) per tray varies depending on the percentage of germination of it (Expression 1).
Methodology to Analyze the Vigor of Plants
To measure the height and thickness of the seedlings, the method of the Standard for Rice according Graeguiles (2000) was used, supported by a tape measure and a caliper with accuracy ± 1 mm and 0.05 mm, respectively.
Methodology to Determine the Quality of the Transplantation Process
The following variables are to be evaluated in the experiment, assuming that the water sheet and the ground are parallel lines (r || p) and the plant (s) is perpendicular (Figure 3) (Menéndez et al., 2012a; 2012b).
Inclination of the Plants when Transplanted
The angle of inclination of the seedling (∢BDL) after transplantation was calculated using the Pythagorean theorem and trigonometric identities (Figure 3).
Where,
1) The ∢ABC was calculated by the expression (2) determined by the inverse of sin∢ABC;
2) The ∢ABC and ∢BDL are equal because they correspond between parallels (r║p) and secant (s). The depth of transplantation (EF) perpendicular to the surface (p) was calculated by the following expressions.
Where:
1) The ∢EDF = ∢BDL for being opposite by the vertex;
2) Segment BD is determined by expression (3);
3) Sum of expression segments (4);
4) Clearing expression (4);
5) EF transplantation depth was calculated by expression (6).
All measurements were carried out with 10 repetitions per plot randomly, with a tape measure with an accuracy of ± 1 mm.
Number of Seedlings per Transplanting Organ. Five plant counts were carried out at random in the three transplanting organs in each experimental plot.
Distance between Plants per Row. It was determined with a tape measure with an accuracy of ± 1mm, the distance between the base of the stems of the consecutive plant in a row, with at least 10 random repetitions and traversing the test plots along their diagonals. Subsequently, the mean value of such measurements was calculated.
Transplantation Effectiveness (Et). In order to know the effectiveness of the transplanter in the process of transplanting the rice crop, a count of the drives of the transplant organs was performed in a work pass, later the niches with transplanted seedlings were counted in the pass carried out and the per percent effectiveness was determined by expression (7).
Where:
Tp |
- Number of niches with transplanted seedlings, unit; |
Ca |
- Number of drives of transplanting organs, unit. |
Seedling Survival (Sp). Seedling survival one month after semi-mechanized transplantation was determined:
Number of Offsprings. The number of offsprings in 15 plants, taken at random in each experimental plot, which will be identified once the plants germinated from two months, and the evaluations were carried out every 15 days throughout the crop cycle.
Discussion
Characterization of the Research Conditions
The experimental investigations were developed with six rice cultivars INCA-LP5, ROANA LP-15, GINES LP-18, GUILLEMAR LP-19 and JOSE LP-20 by Colectivo de autores (2019) in the research areas of the Los Palacios Scientific Technological Unit (UCTB), of the National Institute of Agricultural Sciences (INCA), Los Palacios Municipality, Pinar del Río Province, during the 2019/2020 rice campaign.
Quality Parameters of the Seedlings Required by the ERP-60 Transplanter for Rice Cultivation
Analysis of Seed Gemination and Seedling Evolution
In studies carried out by Minh (2012); Guerra et al. (2013); Hernández et al. (2016), the best quality of the rice seedling is achieved when they are developed in the substrate made up of four parts of sieved soil, four parts of sieved organic matter, one part of dry cane fiber ground and a part of charred rice husk. With the aim of performing the investigations close to the real conditions of farms, these studies were mounted on the basis of the two main variants of substrates that can be obtained without difficulties on the farms themselves (Hernández et al., 2016).
In the experimental tests, the results obtained in the two treatments carried out were analyzed (Table 3), for this, the size and thickness of the seedling was measured as a function of the substrate relationship, as well as the population per tray, at 19 days of germination of the seed, achieving in a shorter period of time that the seedlings reach the necessary height recommended 15… 20 cm, maintaining the quality required by the manufacturer for the transplanter (ERP-60, 2000).
TABLE 3 Result of the mean values of the measurements in seedbeds
| Substratum | Rest time (days) | Result of mean values | ||
|---|---|---|---|---|
| Population per tray (un) | Seedling height (mm) | Diameter of the plants 24 mm from the base (mm) | ||
| 100% sifted soil | 30 | 342.71 | 14,782 | 2.22 |
| 50% sifted soil and 50% sifted organic matter | 30 | 577,547 | 15,175 | 2.37 |
Analysis of at the Time of Being Transplanted
Comparisons were made among the means (Figure 4) of treatments to know statistically how the relationships of substrates used influence each of the variables analyzed in the research. The analysis of the population per tray showed that the two treatments differ statistically, being the treatment with 50% of organic matter and sifted soil the one that presents the largest population with a coefficient of variation of 2,372, although the two treatments are within the necessary range for mechanized transplantation.
Quality Evaluation of Mechanized Transplantation
The production of seedbeds with the required quality is of vital importance, due to their direct dependence on the requirements of transplantation, to achieve a process of seedling sowing that allows a vigorous development in the environment in a satisfactory way (Minh, 2012; Guerra et al., 2013; Hernández et al., 2016).
Table 4 shows the statistical analysis carried out from the samples taken from the height of the water sheets and the thickness of the mud layer by plots at the time of rice transplantation, where the mean values, standard error, maximum and minimum coefficient of variation were determined.
Analysis of the Functioning of the Transplant Organs Carried Out at the Time of Transplantation
After the transplant activity, a count of the niches planted by the machine was made in two variants: 12 rows with the use of trays B1 and 12 rows with the use of trays B2, depending on the number of times the transplant organs were activated, according to the regulation of the machine, compared with those that had to be transplanted and verified a month later (Figure 5), to perform an analysis of the quality of the transplant and to measure technically the work of the transplanter.
As shown in Figure 5, it can be seen that there are no significant differences in terms of the counting of niches in different grooves when the trays B1 were used, compared to the operation of the working organs according to the machine regulation. With the use of tray B2, there are significant differences in four rows that put the quality of the transplant at risk, which shows that this result is a direct dependence of the population reached in the trays and not of the operation of the machine.
After performing the mechanized transplant, the number of niches (1 ... 3 seedlings) transplanted per square meter was counted at random by treatments carried out using trays B1 and B2. Significant differences were presented only in the areas planted with trays B2 (Figure 6), corroborating the result of the analysis (previous) of the row count. One of the indicators that is most required for mechanized transplantation is to achieve that the sedlings reach in 18 or 20 days of germination ), heights that fluctuate between 15 and 20 cm (Washio, 2004), being the height of 15 cm the most suitable for the process of sowing with transplanting machines, since if the seedling exceeds these dimensions, it causes interruptions, once the transplant organ deposits it on the ground (Menéndez et al., 2012a; 2012b; Minh, 2012).
CONCLUSIONS
In the tray seedbed technology at the time of transplantation, interaction was found between the factors under study, when the component elements of the substrate were mixed and left at rest, the plants found the appropriate conditions for growth, in the four-element substrate (ST + MOT + FCSM + CAC), with 30 or more days at rest. This makes possible to achieve seedlings of 15.37 cm high and 2.19 mm thick, 19 days after the seed germinated, complying with the requirements for transplantation with the ERP-60 machine.
