The Role of Agronomic Factors in Salibu Rice Cultivation

3.1. Availability of Soil Water

SR allows rice harvests to be carried out 3.5–4 times a year with yields equivalent to the parent rice [7]. The increasing harvest index depends on the water availability conditions. According to Negalur et al. [2], SR has a short growing duration, suitable in rainfed areas with residual soil moisture. Rainfed rice fields can be planted once or twice in harvests, making planning difficult depending on the unpredictable rainwater. Harvesting can be done 3-4 times a year on semi-technical rice fields and fields involving technical irrigation. The cropping system patterns of TS (1^st stage), SR (2^nd stage), and SR (3^rd stage) are more profitable than TS (1^st stage) and TS (2^nd stage) in one year.

The selection of rice varieties is better determined by water conditions in the field [25]. Increasing land productivity is a determining factor for rainfed rice farming [26]. Rice breeders regard ratooning as an important practice for sustainable rice production in tropical agricultural systems to maximize profits [27]. Rainfed land has important benefits in increasing food production. TS is better than the seed-direct system as it takes advantage of residual soil moisture [16]. Rice cultivation in rainfed lowland can increase from one to two harvests using the TS (1^st stage) and SR (2^nd stage) in a year.

The soil moisture content influences the main crop harvest [28]. The inundation height of 5 cm from the soil surface results in taller plants and more tillers than the saturated condition treatment with a 0.5 cm water layer [29]. Saturated conditions up to 1 cm in height can be applied in rice cultivation by farmers, which will not affect rice production and soil characteristics [30]. Ratoon rice yields show that inundation (full flooding) provides the highest yield than inundation intervals of 2, 4, and 6 days [31].

In contrast to the findings presented by Elkheir et al. [32], each soil type essentially conserves water due to which yields show better rice production under aerobic conditions. According to Bleoussi et al. [33], the protein content of each rice variety increases with increasing drought intensity. The groundwater deficit has significant implications for grain quality. Momolu [34] stated that grain yields for all rice cultivars decrease with the formation of tillers due to the extended stress period.

The rice field ecosystem is divided into lowlands, highlands, and water. Rice ratooning is more suitable for lowland rice fields [35]. The SR cultivation can be done on rainfed land, although it can be done in two stages, TS (1^st stage) and SR (2^nd stage), due to the limited availability of soil water. Barnaby et al. [36] stated that rice plants have a variety of physiological and metabolic strategies in producing tolerance to water stress. It is necessary for rice varieties selection that is adaptable to deficit irrigation production systems or less water. The model of SR cultivation pattern is presented in Table 3.

Table 3 shows that soil water availability for one year will determine the number of SR cultivation stages. In semi-technical irrigated land, it can be increased to 3 harvests per year with the patterns, TS (1^st stage), SR (2^nd stage), and SR (3^rd stage). In comparison, on technical irrigated land, it can be increased to 4 rice harvests a year with the patterns, the TS (1^st stage), SR (2^nd stage), SR (3^rd stage), and SR (4^th stage). The SR can be cultivated more than four times from one planting of parent rice.

3.2. Time and Height of Stem Cuttings

The SR cultivation is better implemented in a land that is not always inundated [37]. During two weeks before and after harvest, soil water content should be maintained according to field capacity. Initial shoots growth of SR is better if the field capacity conditions are maintained rather than inundated. Rice varieties’ ability to produce the number of shoots or tillers after cutting the stem of main rice depends on genetic and environmental characteristics, including soil water content availability. Shiraki et al. [38] stated that soil moisture conditions two weeks before and after the harvest significantly affect grain yield. Ratoon plants in dry soil moisture conditions can increase yields by 69% than humid or flood conditions. According to Oda et al. [39], late irrigation is recommended in ratoon rice management practices.

Cutting of the stem at physiological maturity provides the best ratoon yield [40]. Rice yields are proportional to the number of ratoon tillers produced. The late stem cuttings have the most effect on the ratoon tillers number [39]. The shorter stem cutting causes the ratoon growth to be longer at the vegetative stage of growth, and delays the maturity stage of the grain [41]. Cutting height significantly influences plant height, the number of grains per panicle, and filled grains. Fitri et al. [7] showed that the optimal results for the number of productive tillers, the number of seeds per panicle, and rice productivity are obtained when the remaining stems are cut at the height of 3-5 cm above the soil surface, and carried out at the age of 7-8 days after harvest (DAH).

The main crop’s first harvest is done at 5 cm of cutting height [42]. Carbohydrates are needed to maintain metabolic activity during the initial growth stages. The remaining stems provide the energy requirements for new tillers’ growth after cutting, and then the tillers immediately become autotrophic [8]. The highest yield of ratoon rice (Ciherang variety) is achieved when stems are cut at harvest with a height of 3 cm above the soil surface with a grain yield of 3.54 tons ha^-1 [43]. The effect of cutting height on rice yields depends on the photosynthetic conditions and the number of internodes remaining in the main stem as a place for ratoon shoots to appear [44].

There are differences in the dynamics of shoot growth at the stem nodes. The morphology of vegetative organs shows that hormones regulate shoot growth on the main stem nodes. There are several proteins involved in brassinosteroid biosynthesis. Brassinosteroid signaling plays a role in the germination of axillary buds of ratoon rice [45]. The ability of rice crops to produce ratoon is strongly influenced by the carbohydrate and phytohormone content left in the stem meristem tissue after harvest [46]. If the stems’ cutting is done 2-3 cm from the soil surface, the new germin shoots could be reproduced to form the next new rice tiller. The difference in the emergence of new shoots on the parent stem can be seen in Fig. (1).

In higher cutting (Fig. 1a), new lateral shoots emerge from the upper stem node. Taller stem cuttings produce more new shoots, and soon, new leaves form. Shoots have a short vegetative growth so the leaves number is small and the leaf area size is narrow, and they immediately move into the reproductive phase. In lower stem cutting (Fig. 1b), new shoots grow from the basal stem. The shorter stem cuttings cause new shoots to grow in the soil surface that immediately form new roots, producing new tillers. Rice tillers have a more extended vegetative growth phase and produce more leaves extending to a broader area. The leaf area affects the photosynthesis process, producing carbohydrates.

The ratoon tillers are related to the level of stem carbohydrates at harvest time [28]. Higher stem cuttings cause the plant to grow and flower quickly because the shoots take advantage of the remaining carbohydrate reserves in the stem of the parent plant. In contrast to shorter stem cuttings, new shoots are formed to produce new roots. Furthermore, new shoots arise from the next new tillers so the vegetative life is longer. Mareza et al. [44] indicated that the lower stem cuttings cause the tiller number per clump to be lower and more productive. The number and leaves area per tiller^- is more wider, but the leaf area per clump is lower. Also, stem cutting results in longer flowering age and harvesting, higher dry weight of the tillers, and the same seed carbohydrate content as the parent plant, but plant dry weight per clump is lower.

3.3. Dose and Time of Fertilization

The regrowth of tillers depends on the carbohydrate reserves in the plant stems or roots remaining after pruning. The carbohydrates are needed to maintain metabolic activity during the initial stages of regrowth. The energy is required to supply new tillers that immediately become autotrophic [47]. Furthermore, after the shoots grow, it takes a sufficient supply of nutrients from the environment to support normal growth like the previous parent rice. Nutrients in the soil during the harvest of the main crop greatly affect the performance of the shoots.

The growth and yields of SR are primarily influenced by the balanced nutrients [8]. Better yields of ratooned crops are possible by increasing fertilization, mainly the supply of nitrogen [46]. SR age is shorter than the parent rice, so a balanced fertilization system is needed to strengthen its vegetative growth. Nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) are the primary nutrients for increasing and sustaining SR productivity.

The urea fertilizer application can increase the N element in the soil. The N availability can cause plant leaves to look greener, the number of tillers to be more, accelerate the growth of shoots, roots, photosynthesis, and stimulate rice plants’ growth. According to Ambarita et al. [48], the excessive N application can be inhibited by the absorption of other elements, which may lead to decreased growth and yield and increased amount of empty grain. Mamun et al. [49] showed that using an additional 25% nitrogen of the recommended dose for ratoon crops after harvesting the main crop can yield economic yields for hybrid rice.

In a study, fertilizer application was done in 3 stages. The first fertilization was applied at 10 DAH (20% of urea dose), the second at 21 DAH (40% of urea dose), and the third at 35 DAH (40% of urea dose). The dose of 375 kg ha^-1 urea application significantly affected the increase in the number of tillers (32.06 tillers clump^-1), the number of panicles (18.86 units clump^-1), and grain dry weight (0.63 kg m^-2) in SR cultivation of Ciherang variety [50]. The dose of 50 kg ha^-1 urea and 50 kg ha^-1 NPK Phonska fertilizer did not significantly affect rice production between the three zones of high, medium, and lowland on the Salibu system [7]. NPK Phonska is an inorganic fertilizer with the name NPK compound fertilizer consisting of several macronutrients, such as nitrogen (N), phosphorus (P), potassium (K), and sulfur (S). The content of NPK Phonska 15-15-15 is N (15%), P₂O₅ (15%), K (15%), and S (10%). Rice farmers widely use this fertilizer because it can increase the yield and grain quality.

An increase in nitrogen dose within a certain range is followed by an increase in leaf area index, plant height, number of tillers, net photosynthesis rate, transpiration rate, and grain yield. In addition, the effect of increasing nitrogen dose on grain yield index and nitrogen contribution rate are parabolic [51]. The dosage of nitrogen fertilizer has a different effect on the number of tillers per clump, the percentage of filled grains per panicle, and the weight of 1000 grains [52]. The vegetative age of SR is shorter than the parent plant. A delay in fertilization can lead to fewer tillers and decreased grain yields. Rice plants must absorb sufficient N, P, and K during the growth stage to obtain optimal growth characteristics and yields [53]. Fertilization of SR is carried out according to site-specific recommendations. According to Kristamtini et al. [4], the dose of fertilizer application should comply with recommendations from paddy soil test equipment and be given in two times. The first fertilization should be done at 40% of the recommended dose at 15-20 days after stem cuttings (DASC). The second fertilization should be as much as 60% of the recommended dose at 30-35 DASC. Suparwoto and Waluyo [54] reported, in their study, the first fertilization at 14 DASC by applying 150 kg ha^-1 urea fertilizer and the second at 40 DASC with 25 kg ha^-1 SP-36 and 25 kg ha^-1 KCl fertilizers.

Based on the results of previous studies, a summary of the dose of fertilizer and time of fertilization in the cultivation of SR can be made. For more details, this summary is shown in Table 4.

Table 4 shows that the dose of fertilizer and the time of fertilization vary. Therefore, it is necessary to research on the type and dose of fertilizer and the right time of application. Even though there is no recommendation for site-specific fertilization, SR cultivation is still profitable compared to TS from the farming aspect.

The SR has been found to be more profitable than TS. The farmers save the costs of the production process in the second planting season. Production activities, such as plowing, harrowing, nursery, seedlings removal, planting, and purchasing seeds, are not carried out. According to Surdianto et al. [1], the comparison of costs between SR and TS is shown in Table 5.

Table 5 shows that the cost difference between the SR and TS is Rp. 1,874,667 ha^-1 for each growing season. It means that the SR cultivation system can increase farmers’ income and production costs can be eliminated.