Timing of harvesting reverses the effect of cutting twice with ratoon rice

Introduction

The cultivation of rice using ratooning has several advantages over conventional cultivation, including lower costs, lower labor intensity, and lower greenhouse gas emissions (Sacks, 2013; Safruddin, 2017). A new practice for ratooning rice crops using the lower nodes of rice stubble, known as the SALIBU system, is gradually spreading throughout the tropical regions (Fitri et al., 2019; Yamaoka et al., 2017). The practice involves harvesting most of the above-ground portion of the plant but leaves the growing shoots intact. The SALIBU method implements 10 additional management practices to conventional management of rice cultivation in Indonesia. (1) early harvesting, (2) pre-fertilization (at seven days before harvesting), (3) cutting twice (rice was harvested 25 cm above the ground, then cut again at 3–5 cm above ground), (4) late irrigation (irrigation was started on day 14 after harvesting), (5) hand weeding, (6) dividing (dividing hills into two or three tillers and replanting to fill the space), (7) pushing (pushing the rice plants into the soil if the root came up on the soil surface), (8) removing excess plants (to keep original plant density), (9) draining from day 29 to 43 after harvesting, and (10) draining for the above (5) to (9) treatments. However, this new methodology has yet to be well studied. Oda et al. (2019) evaluated the Cohens’ d effect size (Cohen, 1992) of each practice in the technology package and clarified that the process of cutting twice has a large effect on the number of ratoons. Interestingly, the effect can be reversed depending on the management conditions. The cutting twice has a large positive effect when poor management conditions are used but a medium negative effect when standard management conditions of direct seeding triple-cropping rice in the Mekong Delta are employed. The poor management conditions consisted of not using fertilizer, continuous-flooding water management, and harvesting 10 days late. Obviously, there is an interaction between the management conditions and the practice of cutting twice; however, which of the three conditions was responsible for the interaction has not been clarified. The best harvesting time for good ratooning is when the culms were still greenish because of the growth of ratoon tillers depends on the carbohydrate reserves of the stubble (Fukai, 1988). Hence, the ratoons decrease with the delay in harvesting. The amount of total available carbohydrates in stubble increases with cutting (Fukai, 1988). This means that cutting twice will have the advantage of leaving carbohydrates in longer stems for a period of time. However, after cutting the main crop, there are more dead buds in the nodes closer to the ground, and lower cutting results in the loss of many hills and reduces tillering (Fukai, 1988). This will be the reason why ratoons decrease with the cutting twice. Due to the balance of the above mechanisms, the effect of cutting twice increase or decrease will be reversed. In contrast, the application of N fertilizer to the main crop results in a steady increase in ratoons (Fukai, 1988).

We assumed that the cause of the reverse effect was the timing of harvesting. Therefore, we examined the relationship between delayed harvesting and the number of ratoons under conditions with and without fertilizer. Consideration of water conditions was omitted to avoid complexity.

Methods

A relationship between delayed harvesting and the number of ratoons was analyzed. We harvested rice plants grown under conditions of both with and without fertilizer. Rice was harvested on the day when 75% of the seeds on the main stems turned yellow (days 0), and after on days 4, 8, and 14. The water management approach we adopted was alternative wet and dry (AWD). Note, SALIBU practices are not used in this experiment except cutting twice. Early harvesting (one of the SALIBU practices) is a conventional practice in the Mekong delta triple cropping.

Results

Effect of fertilizer

The application of fertilizer to the main crop did not reverse the number of ratoons. There was a consistently greater number of ratoons with fertilizer than without fertilizer (Figure 1a).

Figure 1. Ratoons and Living leaves.

•: fertilized, Δ: unfertilized. Bars represent 95% CI (n = 4). Living leaves at the harvesting. Ratoons on day 45 after harvesting. Rice was harvested on the day when 75% of the seeds on the main stem turned yellow (the standard SALIBU method; days 0), and after on days 4, 8, and 14. Rice was harvested at 25 cm aboveground first, then cut again at approximately 5-cm aboveground on day 7 after each harvesting.

Discussion and conclusion

Oda et al. (2019) show cutting twice can have a large effect on the number of ratoons, and the effect can be reversed by different management conditions, such as adding fertilizer, delaying harvesting, or changing the type of water management used. We performed a pot experiment to investigate the effects that the level of fertilizer and the delay in harvesting time had on the number of ratoons.

Our results showed that the application of fertilizer to the main crop does not reverse the effect of cutting twice on the number of ratoons. The application of N fertilizer to the main crop results in a steady increase in ratoons and the result is consistent with previous studies (Fukai, 1988). In contrast, the timing of harvesting can reverse the effect of cutting twice on the number of ratoons. Despite the growth of ratoon depends on the carbohydrate storage of the stem, the number of ratoons continuously increased with the decrease of the number of living leaves. Especially, the second cutting conducted on the rice plant harvesting on days 0 resulted in no ratoons. Harvesting increases the number of dead buds on nodes close to the ground (Fukai, 1988). When the harvesting is early, the growth of ratoons is fast, so that growing ratoons are removed and few viable buds remain after the second cutting. However, when the harvesting is late, the growth is slow, so that viable buds remain after the second cutting. On the other hand, cutting twice has the advantage of leaving carbohydrates on the long stem for a period. This increases the number of ratoons for later harvesting.

Harvesting earlier than normal (at the physiological maturing stage) is emphasized in the SALIBU technology package; however, our results showed that the cutting twice decreased ratoons when harvesting at the recommended timing, and that increased ratoons when harvesting late timing. This is a significant finding for the SALIBU practice. The practice of cutting twice originated partially the custom of conventional harvesting of which cutting height is high. That should be distinguished from using lower nodes of stubble. In other words, cutting twice is recognized as an essential practice in SALIBU (Fitri et al., 2019; Yamaoka et al., 2017) but cutting twice is not essential for some varieties when we can cut rice plant stems near the soil surface at harvesting. Previous work also shows that cutting twice reduces the number of ratoons (Shiraki et al., 2020), but if varieties with high ratoon ability are adopted, twice cropping may suppress the growth of weak ratoons from higher nodes and result in higher yields. Cutting twice has a significant effect on ratoon number, but further study is needed because the effect of cutting twice varies among varieties and the timing needs to be adjusted.

Data availability