Heavy soil drying during mid-to-late grain filling stage of the main crop to reduce yield loss of the ratoon crop in a mechanized rice ratooning system

Chang Zheng, Yuechao Wang, Shen Yuan, Sen Xiao, Yating Sun, Jianliang Huang, Shaobing Peng

National Key Laboratory of Crop Genetic Improvement, Hubei Hongshan Laboratory, MARA Key Laboratory of Crop Ecophysiology and Farming System in the Middle Reaches of the Yangtze River, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan 430070, Hubei, China

Keywords:Crushing damage Mechanical harvesting Ratoon rice Soil drying Yield loss

ABSTRACT Yield loss(YLoss)in the ratoon crop due to crushing damage to left stubble from mechanical harvesting of the main crop is a constraint for wide adoption of mechanized rice ratooning technology.Soil drying before the harvest of the main crop has been proposed to overcome this problem.The objective of this study was to determine the effect of soil drying during the mid-to-late grain filling stage of the main crop on grain yield of the ratoon crop in a mechanized rice ratooning system.Field experiments were conducted to compare YLoss between light(LD)and heavy(HD)soil drying treatments in Hubei province,central China in 2017 and 2018.YLoss was calculated as the percentage of yield reduction in the ratoon crop with the main crop harvested mechanically, relative to the grain yield of the ratoon crop with the main crop harvested manually.In comparison with LD, soil hardness was increased by 42.8%-84.7% in HD at the 5-20 cm soil depth at maturity of the main crop.Soil hardness at 5 and 10 cm depths reached respectively 4.05 and 7.07 kg cm-2 in HD.Soil drying treatment did not significantly affect the grain yield of the main crop.Under mechanical harvesting of the main crop,HD increased the grain yield of the ratoon crop by 9.4%relative to LD.Consequently,YLoss was only 3.4%in HD,in contrast to 16.3%in LD.The differences in grain yield and YLoss between the two soil drying treatments were explained mainly by panicles m-2,which was increased significantly by HD in the track zone of the ratoon crop compared with LD.These results suggest that heavy soil drying practice during the mid-to-late grain filling stage of the main crop is effective for reducing YLoss of the ratoon crop in a mechanized rice ratooning system.

1.Introduction

Ratooning is a rice production practice for harvesting a second(ratoon) crop, which develops from tillers regrowing from stubble remaining after the first (main) crop harvest [1].The ratoon crop reaches harvest time much more quickly than the main crop and incurs much lower production cost,owing to savings in labor,seed,water, pesticides, and seedbed preparation [2].The grain yield in the ratoon crop can be as high as 60% of that in the main crop[3].Moreover, ratoon rice affords a higher net economic return and lower global warming potential than double-season rice [4].Accordingly, ratoon rice is considered an economic, green, and resource-efficient cropping system and has been adopted in many rice-growing regions of the world [5].

In central China, rice ratooning has been a common practice since the middle of the last century [6].Among the provinces in central China, Hubei had a relatively large area of ratoon rice,reported to be 73,000 ha, in the early 1990s [7].However, the planting area fell to only about 6600 ha by 2010 [5].A key reason for this decline was high labor cost for manual harvesting of the main crop (HMAN) [4].This conventional ratoon rice practice is no longer suitable for the current socioeconomic situation in China,with a reduced labor force and increased labor wages.Mechanical harvesting of the main crop(HMEC)is thus an inevitable option for the sustainable development of ratoon rice, and mechanized rice ratooning technology has been developed by Huazhong Agricultural University in Wuhan, Hubei province [8].This technology has been quickly adopted by farmers in Hubei,where the planting area of ratoon rice reached 213,300 ha in 2020[8],and it has been extended to neighboring provinces such as Hunan, Jiangxi, and Anhui in recent years [9-11].

However, there is often a yield loss (YLoss) in the ratoon crop caused by crushing damage to stubble during HMEC, limiting the further extension of mechanized rice ratooning technology [12].The harvesters used for ratoon rice production in China are usually tracked combines, whose tracks cover 40%-50% of the field area[13].This high coverage could result in a large YLossin the ratoon crop.Wang et al.[10] reported that HMECreduced the grain yield of the ratoon crop by 36.1%, 23.7%,and 40.9% when the main crop was cut at stubble heights of 20, 40 and 60 cm, respectively.In 15 cultivars in Sichuan province, YLossranged from 12.5% to 40.1%with a mean of 24.0% [14].A similar YLossof 21.3% was observed in an on-farm demonstration in Hubei province [15].The challenge, therefore, is to reduce YLossby developing specialized harvest machines or crop-management strategies.

Several research groups have attempted to design specialized harvest machines with reduced track coverage, but none has become commercially available [13,16].The alternative is to develop crop-management strategies that can reduce crushing damage to the stubble during the harvest of the main crop using conventional combines.In ratoon rice production, the common practice is to drain the paddy field about two weeks before the harvest of the main crop for soil drying, so that the combines can move easily in the field for the harvest of the main crop.However,limited information is available on the effectiveness of soil drying during the grain filling of the main crop for reducing crushing damage to the stubble of the main crop and consequently increasing grain yield of the ratoon crop.

The objectives of this study were(1)to determine the effects of soil drying treatments during the mid-to-late grain filling stage of the main crop on grain yield of the main and ratoon crops,and(2)to identify yield-associated traits responsible for yield differences between soil drying treatments.

2.Materials and methods

Field experiments were conducted in 2017 and 2018 at Jiupu village (30°14′N, 115°25′E), Chidong town, Qichun county, Hubei province, China.The topsoil (0-20 cm layer) of the experimental field had a clay loam texture with the following properties:pH 4.6, 31.5 g kg-1organic matter, 2.3 g kg-1total N, 10.4 mg kg-1Olsen P, and 169.0 mg kg-1available K.

Experiments were arranged in a split-split-plot design with three replications in 2017 and four replications in 2018.The main plots were two soil drying treatments of the main crop,designated as light drying (LD) and heavy drying (HD).The LD plots were drained from 23 to 24 days after heading to maturity of the main crop (with soil drying duration of 14 days in 2017 and 9 days in 2018), whereas the drainage was advanced by 4-5 days in HD(with soil drying duration of 19 days in 2017 and 13 days in 2018).The subplots consisted of two harvest methods of the main crop:HMANand HMEC(Fig.S1).The sub-subplots comprised theindicahybrid cultivar Liangyou 6326 (LY6326) and theindica/japonicahybrid cultivar Yongyou 4949 (YY4949).Both cultivars have been widely grown for ratoon rice in central China.Subsubplots measuring 6 × 8 m and plots were separated by a 40-cm-wide alley with plastic film inserted into the soil to form a barrier.

Seedlings were raised in seedbeds with sowing date of March 20 in 2017 and 2018.Transplanting was done on April 23, 2017 and April 24, 2018 at two seedlings per hill.The row spacing was 30.0 cm and the plant-to-plant distance within rows was 13.3 cm.During HMEC, the harvest machine went straight along rows in plots and turned around in the trackways (400 cm wide)located outside the plots.The harvest machine was a combine(WORLD-DR4LZ-4.0E, WORLD GROUP, Danyang, Jiangsu, China)commonly used in the study region, with a cutting width of 200 cm, a track width of 45 cm, and a track-to-track distance of 70 cm (measured between the insides of two tracks).Seven rows were cut by the combine in one run, but one of the seven rows was cut again in the next run to avoid missing panicles(Fig.S1b).This pattern of harvesting operation is commonly practiced by local rice farmers.Thus,six rows were considered as a harvest unit, consisting of two rows in the track zone and four in the track-free zone.The height of stubble left after the harvest of the main crop was 45 cm in both HMANand HMEC.

In the main crop,plants received 200 kg ha-1total N,which was split-applied:40%basally(1 day before transplanting),30%at early tillering (7 days after transplanting), and 30% at panicle initiation.Basal N was applied and incorporated into the soil together with P(40 kg ha-1),K(60 kg ha-1),and Zn(5 kg ha-1).For the ratoon crop,75 kg N ha-1and 60 kg K ha-1were applied as bud-promoting fertilizer 10-12 days after heading of the main crop,and 75 kg N ha-1was applied as tiller-promoting fertilizer 3 days after harvest of the main crop.All N,P,K,and Zn fertilizers were applied in the form of urea,calcium superphosphate,potassium chloride,and zinc sulfate heptahydrate, respectively.The field was held at 3-5 cm water depth from transplanting to starting of soil drying treatments in the main crop,except that the water was drained at maximum tillering to reduce unproductive tillers.After harvest of the main crop, irrigation was immediately applied to a water depth of only 1-2 cm to prevent submergence of regenerated buds.When the buds grew to more than 10 cm,a water depth of 3-5 cm was established by irrigation and maintained until heading of the ratoon crop.Alternate wetting and drying irrigation was practiced from heading to maturity of the ratoon crop.Diseases and insects were intensively controlled with chemicals, and weeds were controlled with a combination of herbicide and hand weeding.

Climate data including daily minimum/maximum temperatures,rainfall,and radiation were obtained from a weather station located near the experimental site.Flag leaf SPAD value was measured every 5 days from heading to 30 days after heading of the main crop in 2017.Ten flag leaves were selected from each plot for SPAD measurements.Soil water potential was monitored with tensiometers during soil drying treatments.Two tensiometers were installed to 15 cm depth in each main plot,and readings were recorded at 12:00 AM.Soil hardness was measured at 5,10,15,and 20 cm depths with a hand-held digital soil penetrometer (TYD-2,TOP Instrument Co., Ltd., Hangzhou, Zhejiang, China) in each plot at maturity of the main crop.

After harvest of the main crop in 2018,10 plants in track zone of each plot were selected and tagged for recording the number of regenerated buds.At heading of the ratoon crop,18 hills were sampled in each plot.After recording the number of regenerated tillers,the plant samples were separated into stubble(45 cm),stems(rest of culm and sheath), leaves, and panicles.The green leaf area was measured using a leaf area meter (LI-3100C, LI-COR, Lincoln, NE,USA) to determine leaf area index.The dry weight of each organ was measured after oven-drying at 80 °C to constant weight.The dry weight of regenerated tillers was the sum of dry weights of stems, leaves, and panicles.

Yield attributes were measured at maturity for both the main and ratoon crops.In the main crop, plants were sampled from a 0.48-m2area (4 × 3 = 12 hills) in each plot.In the ratoon crop,plants were sampled from a 0.72-m2area(6×3=18 hills)in each plot of HMANand HMEC.In HMEC, an additional 18-hill sample was taken from one row in the track zone.Yield components, aboveground total dry weight, and harvest index were determined according to Wang et al.[3].

Grain yield was measured in the center of each plot at maturity for both the main and ratoon crops.Plants were sampled from a 5.12-m2harvest area (8 × 16 = 128 hills) in the main crop, and from a 6.00-m2harvest area (10 × 15 = 150 hills) in HMANof the ratoon crop.For HMECof the ratoon crop,samples were taken from a 6.24-m2harvest area (6 × 26 = 156 hills) and the plants of the track and track-free zones were harvested separately to measure their respective grain yield.All grain yield was adjusted to the standard moisture content of 14%.The combined grain yield of HMECwas calculated based on the grain yield of the track and track-free zones.YLosswas calculated as the percentage of yield reduction in the ratoon crop in HMECrelative to the grain yield in HMAN.

Statistical data analysis was performed with Statistix 9.0 (Analytical Software, Tallahassee, FL, USA).Analyses of variance were fitted and treatments means were compared based on the least significant difference (LSD) test at 0.05 probability level.

3.Results

Minimum and maximum temperature displayed increasing trends during the main crop but decreasing trends during the ratoon crop (Fig.S2).Weather conditions were similar between the two years for the main crop, whereas daily mean temperature and daily solar radiation of the ratoon crop increased by 1.7°C and 3.8 MJ m-2day-1, respectively and total rainfall decreased by nearly 40%in 2018 compared with 2017.The ratoon crop matured in 82-93 days,representing 59%-69%of the growth duration of the main crop (Table S1).The growth duration of the main crop was longer in 2017 than in 2018,but the reverse was true in the ratoon crop.At maturity of the main crop,soil hardness showed a consistent and significant difference between the two soil drying treatments at all soil depths (Table 1).In both years, HD showed 42.8%-84.7%higher soil hardness than LD at 5-20 cm depth.Averaged across the two years, soil hardness at soil depths of 5 and 10 cm reached respectively 4.05 and 7.07 kg cm-2in HD.Soil hardness was generally higher in 2017 than in 2018,especially at 5 cm depth.

There were no significant effects of soil drying treatment on yield and yield-associated traits of the main crop (Table S2).On average, YY4949 produced 16.6% and 5.6% higher yield than LY6326 in 2017 and 2018, respectively.All yield-associated traits but panicles m-2and grain weight were responsible for the yield superiority of YY4949.Owing to simultaneous decreases in grain filling percentage, grain weight, total dry weight, and harvest index, grain yield was significantly lower in 2018 than in 2017,especially for YY4949.

Soil drying treatment showed significant effects on grain yield of the ratoon crop in both HMANand HMEC(Table 2).Averagedacross years and cultivars, HD increased grain yield by 9.4% compared with LD in HMEC.In HMAN, HD reduced grain yield by 5.4%compared with LD.As a consequence, YLosswas 3.4% and 16.3%in HD and LD, respectively, and the effect of soil drying treatment on YLosswas significant.In both years, YY4949 produced higher yield than LY6326 in the ratoon crop especially in HMEC.Overall,the grain yield of the ratoon crop was higher in 2018 than in 2017 in both HMANand HMEC.

Table 1 Soil hardness at the depths of 5, 10, 15, and 20 cm of soil drying treatments at maturity of the main crop in 2017 and 2018.

Table 2 Yield of the ratoon crop with the main crop harvested manually and mechanically, and yield loss(YLoss)in the ratoon crop from mechanical harvesting of the main crop for two rice cultivars grown under soil drying treatments in 2017 and 2018.

The reduction in YLossby HD was due mainly to the increased panicles m-2compared with LD,which resulted in higher spikelets m-2in HD than in LD (Table 3).Heavy soil drying increased panicles m-2by 16.0% in HMECbut did not significantly affect panicles m-2in HMANcompared with LD.HD increased spikelets m-2by 14.1% in HMECbut reduced spikelets m-2by 6.6% in HMANcompared with LD.No significant differences in spikelets panicle-1,grain filling percentage, or grain weight were found between the two soil drying treatments for both HMANand HMEC(Tables 3 and S3).In both years,higher spikelets m-2and grain filling percentage contributed to higher grain yield of YY4949 compared with LY6326.The difference in spikelets m-2between the cultivars was explained by spikelets panicle-1instead of panicles m-2.Higher grain yield in 2018 than in 2017 was attributed mainly to higher grain filling percentage and grain weight.

Total dry weight rather than harvest index explained the reduction in YLossby HD compared with LD (Table 3).Heavy soil drying increased total dry weight by 12.2% in HMECbut reduced total dry weight by 6.2% in HMANcompared with LD.Soil drying treatment showed very little effect on harvest index,although the difference was significant in HMAN.Both higher total dry weight and harvest index contributed to higher grain yield of YY4949 over LY6326.Higher harvest index was responsible for the higher grain yield in 2018 than in 2017.

There were significant effects of soil drying treatment on grain yield of the ratoon crop in the track zone of HMEC(Table 4),whereas the effects of soil drying treatment on grain yield was insignificant in the track-free zone of HMEC(7.74 t ha-1in LD vs.7.71 t ha-1in HD, data not shown).Compared with LD, HD increased the grain yield of the track zone by 36.2% in 2017 and 126.9%in 2018.Overall,there were similar grain yields in the track zone in the two cultivars.Averaged across years and cultivars, the grain yield of the track zone was significantly higher in 2017 than in 2018.

Higher yield of the track zone in HD over LD was due mainly to higher spikelets m-2,total dry weight,and harvest index(Table 4).The increase of spikelets m-2in HD was attributed solely to more panicles m-2, because spikelets panicle-1was significantly lower in HD than in LD.Averaged across years and cultivars,the panicles m-2of HD was increased by 119.2%compared with LD in the track zone.The effects of soil drying treatment on grain filling percentage and grain weight was insignificant.Averaged across years and cultivars, HD increased total dry weight by 67.1% and harvest index by 12.8% compared with LD in the track zone.

4.Discussion

The higher grain yield of the main crop in 2017 than in 2018 resulted from longer growth duration (Table S1).The lower grain yield of the ratoon crop in 2017 than in 2018 was due to lower temperature and solar radiation (Fig.S2).Significant and consistent differences in YLosswere observed between the two soil drying treatments.The difference in YLossbetween HD and LD was caused mainly by higher grain yield of the ratoon crop in HD than in LD in HMEC.This was because HD increased the grain yield of the ratoon crop by 9.4% relative to LD in HMEC, whereas HD decreased the grain yield of the ratoon crop by only 5.4% relative to LD in HMAN.Among yield attributes,the increased panicles m-2, spikelets m-2,and total dry weight contributed to the reduction of YLossin HD.These results indicate that the negative effects of HMECon grain yield of the ratoon crop can be alleviated with heavy soil drying practice during the grain filling stage of the main crop.

Soil drying treatment affected the grain yield and YLossof the ratoon crop mainly by changing the grain yield of the track zone.The difference between the two soil drying treatments in grain yield of the track zone was explained primarily by panicles m-2.The key to increasing panicle number of the ratoon crop is to increase the number of regenerated buds [5].In our study, the number of regenerated buds in the track zone in HD were significantly higher than those in LD(Fig.S3),resulting in more regenerated tillers in HD at heading of the ratoon crop (Table S4).These findings suggest that heavy soil drying reduced crushing damage to regenerated buds and increased tiller production of the ratoon crop in the track zone.This is also supported by the facts that leaf area index and dry weight of regenerated tillers at heading of the ratoon crop in the track zone were significantly higher in HD than in LD (Table S4).

Increasing soil hardness by soil drying is critical to improving mechanical operation during harvest in rice production.To guarantee that harvest machine does not sink easily when working in the fields,the soil hardness at 3-5 cm depth has been recommended to be kept above 4 kg cm-2[17].However,increasing soil hardness by soil drying will inevitably affect soil water status,possibly impairing rice crop growth.Soil water potential is an indicator of soil water availability for plant growth and development regardless of soil texture[18,19].Rice yield is generally reduced when the soil water potential is lower than -30 kPa [19,20].Zhao et al.[21]reported that rice yield reduction from water stress varied from 2.0%to 26.4%,depending on the cultivars,when a soil water potential of-50 kPa was maintained for at least 40 days from heading to maturity.In our study,soil drying during the mid-to-late grain filling stage of the main crop significantly reduced soil water potential (Fig.S4).In HD, soil water potentials between -30 kPa and-50 kPa were reached 25 and 28 days after heading in 2017 and 2018, respectively, although these levels lasted for only 3-4 days.Because the reduction of soil water potential occurred during the late stage of grain filling, which is generally insensitive to soil water deficit [19], HD did not significantly affect the grain yield of the main crop.This inference could be confirmed by the SPAD value of flag leaves during the grain filling period of the main crop,which was not decreased in HD compared with LD (Fig.S5).

To ensure that mechanical harvesting can be performed efficiently, rice farmers usually choose to start drainage 10-15 days after heading to increase soil hardness, although there is a threat of yield reduction in comparison with drainage at 25-30 days after heading [17].For increasing the grain yield of the ratoon crop in mechanized rice ratooning system, we recommend starting drainage 18-20 days after heading of the main crop.The target soil hardness values at maturity of the main crop following soil drying are 4 and 7 kg cm-2at the respective depths of 5 and 10 cm.Although heavy soil drying is effective for reducing crushing damage to residual stubble and increasing the grain yield of the ratoon crop along the track, there is still severe damage to stubble where the combine turns around and where it unloads grain.Developing specialized harvest machines with reduced track coverage remains a research challenge.

CRediT authorship contribution statement

Chang Zheng:Investigation, Data curation, Writing - original draft.Yuechao Wang:Investigation, Data curation.Shen Yuan:Methodology, Data curation.Sen Xiao:Investigation.Yating Sun:Investigation.Jianliang Huang:Methodology,Supervision.Shaobing Peng:Conceptualization, Funding acquisition, Supervision,Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgments

This work was supported by the Major International(Regional)Joint Research Project of National Natural Science Foundation of China (32061143038), the China Agriculture Research System(CARS-01-20),and the Fundamental Research Funds for the Central Universities (2662020ZKPY015).

Appendix A.Supplementary data

Supplementary data for this article can be found online at https://doi.org/10.1016/j.cj.2021.06.003.

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