Progress and Challenges of Rice Ratooning Technology in China

The following excerpt is taken from Peng, Shaobing, Chang Zheng, and Xing Yu. "Progress and Challenges of Rice Ratooning Technology in China." Crop and Environment, vol. 2, 2023, pp. 5–11. For brevity, certain parts have been omitted. Please refer to the full text here: https://www.sciencedirect.com/science/article/pii/S2773126X23000060

Introduction

Rice ratooning, a practice to harvest a second crop from the stubble of the main crop, is an ancient technology because farmers have practiced ratooning for thousand years (Guo, 1993; Vergara et al., 1988). Ratooning was practiced by farmers more as an afterthought or when their rice crops were damaged severely by flooding (Bai et al., 2008; Lampe, 1988). The adoption of ratoon rice was very limited in the past due to low and unstable grain yield in the ratoon season, although it has many advantages such as saving labor, seeds, fertilizers, water, pesticides, nursery bed, and time compared with double-season rice (Peng, 2014; Santos et al., 2003; Yuan et al., 2019).

At the beginning of this century, there was a rapid decline in ratoon rice production in China. The following were the reasons for the decline in ratoon rice area from 1990s to early 2010s. Firstly, the grain yield of the ratoon crop was still very low and unstable (Xiong et al., 2000; Xu et al., 2015). Secondly, the grain yield of hybrid rice was further improved, especially due to the development of super hybrid rice. Farmers could achieve satisfactory yield by planting middle-season rice with super hybrid rice varieties (Wang et al., 2021). Thirdly, there was a surplus of rice production in China during that time and it became difficult for farmers to sell rice grain. As a result, the price of rice was not very attractive to further increase rice production (Luo, 2016). Finally, labor shortage and rapid increase in labor costs prevented farmers to continue the practice of traditional ratoon rice production with the main crop harvested manually (Fei et al., 2013).

To reverse the decline of ratoon rice area, scientists at the Crop Physiology and Production Center (CPPC) of Huazhong Agricultural University in collaboration with researchers and extension workers in Honghu and Qichun Counties of Hubei Province have developed mechanized rice ratooning technology with the main crop harvested mechanically since the early 2010s (Wang and Peng, 2018; Xiao, 2018; Zheng et al., 2022a, 2022b). Studies have been conducted on the physiology of ratoon rice yield potential and improved crop management with focus on minimizing the yield loss in the ratoon season from mechanical harvesting of the main crop. With proper varieties and management practices, the grain yield of mechanized ratoon rice has increased significantly, and farmers could produce as much as 9–10 tons per hectare in the main season and 5–6 tons per hectare in the ratoon season. Consequently, mechanized ratoon rice quickly replaced traditional ratoon rice practice, and the planting area has expanded rapidly in China since 2013. Current total area of ratoon rice has reached slightly over 1 million hectares in China (Yu et al., 2022b).

Key practices of mechanized ratooning technology

Because traditional ratoon rice practice with the main crop harvested manually is no longer suitable for the current social and economic situation due to high labor inputs, it is important to develop rice ratooning technology with the main crop harvested mechanically. In the mechanized ratoon rice, however, rolling damage to stubbles will occur during the mechanical harvesting of the main crop (Xiao, 2018; Zhou et al.,2022). As a result, axillary buds are partially destroyed and regeneration of tillers is slow, which cause low and unstable yield of the ratoon crop (Chen et al., 2021; Zheng et al., 2022a). Ordinary combine harvesters could cause rolling damage by as much as 50% of ground area, which could result in the yield loss of as much as 45% in the ratoon season (Fu et al., 2022; Li et al., 2019; Zhang et al., 2015). In addition to designing specialized harvesters with reduced rolling damage, key practices for achieving high yield in mechanized rice ratooning system include variety selection, optimization of planting date, proper water and fertilizer management, and optimal straw cutting height.

· Variety selection

There is a large variation in the grain yield of the ratoon crop across rice varieties, which is highly related to their ratooning ability (Chen et al., 2018; Negalur et al., 2017). Ratooning ability is defined as the ratio of panicles per square meter in the ratoon season to that in the main season (Wang et al., 2019). Ratooning ability can reach as much as 2.0 or more because each stubble has several node positions with potential to produce regenerated tillers in the ratoon season, especially under high straw cutting height (He et al., 2019; Vergara et al., 1988; Yu et al., 2022a). Ratooning ability of 1.5 is more common with most varieties in the ratoon rice production (Wang et al., 2019; Yang et al., 2021). Furthermore, the ability of varieties to tolerant rolling damage from the harvesting machine is particularly important for reducing yield loss in the ratoon season from mechanical harvesting of the main crop. Large variation in the tolerance to rolling damage was also observed among rice varieties in our recent study (Guo Y., unpublished data).

Although mega varieties for ratoon rice production are varied across provinces, their common features are middle-season varieties, hybrids especially two-line hybrids, high ratooning ability, tolerant to rolling damage, medium duration of 130–140 days, and relatively good grain quality (Wang et al., 2021; Zhang et al., 2021). Nevertheless, three hybrid rice varieties, Liangyou 6326, Fengliangyouxiang 1, and Yongyou 4949, have relatively stable and superior yield performance in both main and ratoon seasons across several provinces (Lin et al., 2022; Wu et al., 2020; Zhang et al., 2021). In general, hybrid rice produces higher grain yield in both main and ratoon seasons than inbred varieties, which could be due to its higher carbohydrate content in the stubble and more vigorous root system at the harvesting of the main crop (Chen et al.,2018; Ren et al., 2006; Xu et al., 2015).

Recently, early-season rice varieties with short growth duration of 110–120 days were tested for ratoon rice in Anhui and Hunan Provinces. There are two purposes for this attempt. One is to avoid high temperature in late July and early August because its main crop is harvested in middle or late July, in which the grain quality is better than the main crop which is harvested from early to middle August. Another is to leave sufficient thermal energy for the ratoon crop so that low cutting height could be practiced during the harvesting of the main crop. With the low cutting height, the negative effects of rolling damage on the grain yield and milling quality of the ratoon crop could be minimized. However, its yield performance in both seasons should be rigorously compared with middle season varieties across a wide range of environmental conditions.

· Optimization of planting date

Currently, ratoon rice is recommended to grow in the region where thermal energy is more than that required for planting single-season rice but not enough for planting double-season rice (Dong et al., 2017; Xu et al., 2021). Therefore, thermal energy is generally limited in ratoon rice production, and it is important to optimize planting date by emphasizing early planting. Preparation of seedbed in plastic greenhouse or dome is done as early as February. Seeds are usually sown in seedling trays or seedbed on March 15–20 and transplanting is done when seedlings are about one-month old. Some progressive farmers even advance seeding to early March. In central China, the harvesting of the main crop is usually done between August 5 and 15, and in general, early harvesting of the main crop provides an assurance for the success of the ratoon crop. The harvesting date and growth duration of the ratoon crop depend on temperature in September and October. If temperature is high in these two months, harvesting will be delayed and growth duration of the ratoon crop will be prolonged and, consequently, the grain yield of the ratoon crop will be high. The prolonged growth duration is because sufficient thermal energy allows the regenerated tillers from the lower nodes in the rolling zone to reach full maturity.

· Proper water management

Heavy soil drying at two weeks after heading during the main season is recommended to achieve proper soil moisture content and soil hardness at the harvesting of the main crop in order to minimizing rolling damage on stubbles by the harvesting machine (Zheng et al., 2022a). Compared with light soil drying treatment, heavy soil drying treatment at mid-tillering and mid-grain filling increased soil hardness at 5 cm soil depth from 2.52 to 4.23 kg /cm² at the maturity of the main crop. Better tiller regeneration and plant growth in the rolling row was observed in heavy soil drying treatment than in light soil drying treatment. The heavy soil drying did not affect the grain yield of the main crop but increased grain yield of the ratoon crop by 9.4%. These results suggest that heavy soil drying during the main season is a very important practice to minimize the yield loss in the ratoon season from mechanical harvesting of the main crop.

Water management immediately after the harvesting of the main crop is also crucial for achieving high grain yield in the ratoon season (Negalur et al., 2017). As mentioned above, the main crop is usually harvested in early August when temperature is very high and heat stress could have a detrimental effect on the initiation and survival of axillary buds, and the growth of regenerated tillers. Under this condition, water management should focus on reducing canopy air temperature and increasing transportational cooling effect of plant tissues.

· Unique fertilizer management

The fertilizer management strategy for the main crop is largely similar to that of middle-season rice, except that nitrogen (N) application rate is higher during vegetative stage when air temperature is relatively low, which is the same strategy used for the early-season rice (Zhou et al., 2018). For the ratoon crop, however, fertilizer management practice is very unique. It is recommended to apply bud-promoting N fertilizer two weeks before the harvesting of the main crop, and tiller-promoting N fertilizer within 3 days after the harvesting of the main crop (De Datta and Bernasor, 1988; Lin et al., 2015). The rate for each application is usually 50–75 kg N per hectare. Some farmers apply 75 kg K per hectare along with the application of bud-promoting N fertilizer (Liu, 2016; Sun, 2019).

In one of our field experiments conducted in Qichun County, the application of bud-promoting N at 75 kg per hectare increased the grain yield of the ratoon crop by 15.4%, tiller-promoting N at the same rate increased the grain yield of the ratoon crop by 36.9%, and the application of both increased the grain yield by 39.5% (Sun, 2019). Bud-promoting N increased panicle number, while tiller-promoting N increased panicle size. Although there were more studies that emphasized the necessity of tiller-promoting N application, the importance of bud-promoting versus tiller-promoting N fertilizer for yield increase of the ratoon crop has been a controversy (Turner and McIlrath, 1988; Wang et al., 2019). Yuan et al. (1996) concluded that bud-promoting N should be emphasized when panicle number limited the grain yield of the ratoon crop, whereas tiller-promoting N should be emphasized when panicle size and grain filling percentage limited the grain yield of the ratoon crop.

Farmers are often reluctant to apply bud-promoting fertilizer because crop canopy is fully closed at that crop stage, and it is difficult for farmers to enter the field for the application. In addition, water resource could be unavailable during that time and N topdressing cannot be done without standing water in the field. Slow-release N fertilizer for ratoon rice was formulated by CPPC in collaboration with Sinofert Holding Limited to avoid the application of bud-promoting N fertilizer (Yang et al., 2022). The number of N applications for the entire ratoon rice (main plus ratoon seasons) could be reduced from 5 to 2–3 times by using the slow-release N fertilizer without large reduction in annual grain yield. However, its current high cost prevents from large-scale commercialization and future effort is on the cost cutting of this special N fertilizer for ratoon rice.

· Proper straw cutting height

Straw cutting height at the harvesting of the main crop determines how many nodes are left on the stubble. Except the first node from the top, every node has an axillary bud which can potentially produce a regenerated tiller. At high cutting height, regenerated tillers are usually produced in the upper node positions, which have shorter growth duration than those from the lower node positions (Ichii, 1988; Ling et al., 1989). Therefore, adjusting straw cutting height during the harvesting of the main crop is necessary to avoid cold damage during the panicle development and flowering of the ratoon crop (i.e. high cutting height for the late harvesting of the main crop). In central China where thermal energy is insufficient, high straw cutting height of 40–45 cm is usually practiced. Under this cutting height, regenerated tillers are usually produced in 2nd and 3rd nodes from top (Dong et al., 2017; Yu et al., 2022a).

Cutting height of the main crop is one of the most important practices that determines the grain yield of the ratoon crop because it has substantial effect on axillary bud initiation and the growth of regenerated tillers (Harrell et al., 2009). However, there is a controversy on whether high or low cutting height results in high grain yield of the ratoon crop. Yazdpour et al. (2012) reported that the high cutting height of 40–50 cm significantly increased the grain yield of the ratoon crop due to increased panicles per square meter compared with the cutting height of 10–20 cm. On the other hand, Jones (1993) found that increasing cutting height had little effect on panicles per square meter, but significantly decreased spikelets per panicle, which caused yield reduction in the ratoon crop. The controversy might be a result of different types of variety and climatic conditions, especially the availability of thermal energy during growth period of the ratoon season (Xiong et al., 2000; Yi et al., 2009).

Challenges for further upscaling

There are still several challenges for further scaling up of this technology. The first and most important one is the yield loss of the ratoon crop from rolling damage of machine harvester. The magnitude of this yield loss depends on variety's ability to tolerate rolling damage, the way how farmers operate the harvester, soil water management, and fertilization practices. However, the most effective solution is to design a special harvester for ratoon rice. Various prototypes of special harvesters have been developed by several research institutions such Huazhong Agricultural University, Jiangsu University, and South China Agricultural University (Fu et al., 2020; Huang et al., 2020; Yang et al., 2019). Those machines could reduce the percentage of rolling area from 40-50% to 26.7% and reduce the yield loss in the ratoon crop from rolling damage substantially, but their mobility, working efficiency, and durability are not satisfactory for commercialization. The efforts for improving those special harvesters for ratoon rice have to be continued.

Another challenge is the milling and eating quality of the main crop, which is better than the early-season rice but worse than middle-season and late-season rice. Although varieties with relatively good grain quality are recommended for rice ratooning, high temperature during the grain filling period increases chalkiness, which reduces head rice yield (Yang et al., 2022). Furthermore, bud-promoting N fertilizer increases grain protein content and consequently reduces palatability of the main crop. Research is underway to screen varieties with relatively stable milling and eating quality under high temperature. Longjingyou 1212 appears to be a variety with those traits (Yang C. unpublished data). Although the ratoon crop has superior eating quality, its milling quality is not satisfactory in mechanized ratoon rice. The problem of milling quality lies in poor head rice yield, which is caused by the late maturity of regenerated tillers in rolling zones (Zheng et al., 2022b). Intersubspecific hybrid varieties such as Yongyou 4949 could partially solve this problem because they have round grain shape.

Overall, crop management practices of the ratoon rice are more complicated compared with single- or double-season rice. This is because the performance of second crop is affected by the first crop. Furthermore, key procedures have to be completed within a short period of time. Climatic condition has profound impact on the grain yield, especially in the ratoon season. Therefore, it is not easy to achieve stable and high grain yield in ratoon rice compared with ordinary rice crops. To overcome these constraints, simplified crop management practices have to be developed, and more training and technology dissemination are needed.

Almost all rice varieties currently used for ratoon rice production in China were screened from existing varieties. Most of those varieties were released many years ago and their resistances to diseases, insects, and lodging are not as good as newly developed varieties. Blast and sheath blight are most serious diseases, while brown plant hopper and stem borer are most devastating insects for ratoon rice. Lodging is another major problem affecting the regeneration of tillers and the grain yield of the ratoon crop. As the planting area of mechanized ratoon rice increases, rice breeders have started to show interests in developing varieties that is targeted to mechanized rice ratooning system.

Conclusions

Rapid development of mechanized rice ratooning technology in China strongly suggests that this technology is the most effective in increasing total rice production with reduced labor requirements and reduced agronomic inputs. Breeding that is targeted to mechanized rice ratooning system offers great opportunities for further improvement in both grain yield and quality. There is a large potential to further expand ratoon rice in China, and there will be increased research interests from various disciplines when total ratoon rice planting area is further expanded. The future research work should focus on the improvement of specialized harvester, grain quality, and resistance to pests and lodging.

Related Links:

China Releases the Guidelines for Promoting Ratoon Rice Development (2025–2030) 

http://www.wfpchinacoe.net/2025-09/08/content_118066651.shtml