Introduction

Double rice cropping is a typical rice production system in China, accounting for >20% of the total rice production area, and is considered an efficient system for the improvement of multiple-crop indices and total rice production1. Yang et al.2 reported that rice production in China increased by 4.0% with the concomitant favorable adoption of a multiple-cropping system. Therefore, rice production plays a pivotal role in ensuring national food security3. Machine transplanting is a labor-saving alternative rice cultivation technology4. As efficient agriculture and land transfer systems have been popularized in recent years, mechanized large-scale farming technology has developed rapidly for rice production in China5,6, leading to increases in the time required for farming operations (e.g., land preparation and seedling transplantation)7; therefore, reasonable seedling transplantation practices in terms of sowing date and seedling age are needed. Important non-monetary factors affecting potential rice yield include transplanting time (sowing date if seedling age is constant) and seedling age at transplanting8. Sowing date is correlated to rice grain yield, and is the easiest factor for producers to manipulate9,10. However, little information is available on the role of elastic sowing dates (ESDs) in mechanized large-scale farming because seedling age is generally limited to 20 days for early rice (ER) and 15 days for late rice (LR).

Under small-scale manual transplanting practices, ESDs are March 25–30 for ER and June 25–28 for LR in China. Due to increases in the length of farming operations in mechanized large-scale farming systems, delayed sowing dates have been adopted by most farmers; however, delayed sowing reduces the effectiveness of accumulated temperature due to decreasing daily temperatures in LR fields6, resulting in poor rice growth and development, and ultimately grain yield loss. Furthermore, ESDs for ER necessarily affect those for LR. Therefore, ESDs for ER and LR should be improved. In this study, we investigated ESDs for ER and LR to maintain yield in mechanized large-scale farming systems. We conducted field experiments over a 2-year study period to determine the effects of ESDs on growth duration, effective accumulated temperature, and yield attributes of ER and LR cultivars under machine-transplanted conditions.

Results

Growth duration vary with different sowing dates

Growth duration was shorter for ER with delayed sowing (Table 1). A 15-day delay in sowing led to a 3-day reduction in growth duration compared with the control. In LR, growth duration was prolonged as sowing date was delayed; the growth duration following a 20-day delay in sowing was more than 8 days longer than that of the control, especially in the reproductive stage. Total effective accumulated temperature exhibited the opposite trend. Growth duration decreased and significantly increased as effective accumulated temperature increased in ER and LR, respectively (Table 2). Effective accumulated temperature decreased by an average of 52.2 °C in LR, with a 5-day interval.

Table 1 Growth duration in machine-transplanted double-cropped rice grown under different sowing periods in 2015 and 2016.
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Table 2 Effective accumulated temperature in machine-transplanted double-cropped rice grown under different sowing periods in 2015 and 2016.
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Reproductive redundancy and nutrition redundancy under different sowing dates

No significant difference in aboveground biomass was observed in ER or LR among different sowing dates (Table 3); aboveground biomass was 1467–1526 g m−2 in LR. Variation in crop growth rate (CGR) exhibited a similar trend. Harvest index (HI) declined in both ER and LR as sowing date was postponed (Table 3), and decreased dramatically in LR, by 3.8% with a 5-day interval. Reproductive redundancy increased by 2.68 and 7.72% in ER and LR, respectively, with a 5-day interval, following sowing date delay (Fig. 1b). Nutrition redundancy increased by 2.72 and 1.57% in ER and LR, respectively, with a 5-day interval following sowing date delay (Fig. 1a).

Table 3 Aboveground biomass, crop growth rate (CGR), and harvest index (HI) in machine-transplanted double-cropped rice under different sowing periods in 2015 and 2016.
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Figure 1
figure 1

Variation in nutritional and reproductive redundancy (%) in machine-transplanted double-cropped rice with delayed sowing in 2015 and 2016.

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Yield difference under different sowing dates

Grain yield in ER and LR decreased as sowing date delay increased (Table 4). In LR, grain yields differed significantly among different sowing dates (P < 0.05). Grain yield was 6.82–7.36 t/ha in ER and 7.40–7.98 t/ha in LR with a sowing date delay of 0–10 days. In LR, filling rate decreased significantly (by 8.85%, with a 5-day interval) as sowing date delay increased. We detected significant positive correlations between effective accumulated temperature in the post-heading stage and yield (P < 0.01) and filling ratio (P < 0.01) in LR (Table 5). No significant differences were observed in panicles m−2, spikelet panicle−1, or grain weight among treatments in ER or LR. Therefore, sufficient effective accumulated temperature (low daily temperature occurred at anthesis with delay ESD, Fig. 1a,b) at anthesis was the key to determining ESD in LR to maintain high grain yield.

Table 4 Grain yield in machine-transplanted double-cropped rice grown under different sowing periods in 2015 and 2016.
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Table 5 Correlations between effective accumulated temperature and other indices.
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Discussions

Ensuring elastic sowing date in mechanized large-scale double-cropped rice production

The ER and LR cultivars used in this study are temperature- and light-sensitive, respectively. In a study of two field stations in China during 1981–2000, Tao et al.11 reported that growth duration was significantly shorter and longer in ER and LR, respectively, as temperature increased. Our data also indicated indirectly that ER growth duration decreased as effective accumulated temperatures increased. In contrast, Zhang et al.12 demonstrated that planting short-duration cultivars at increasing temperatures exacerbated undesired phenological changes, with the growth duration of the late rice cultivar ‘HY518’ 7–14 days shorter than that of ‘TY390’. In the current study, the reproductive stage of rice sown after a 20-day delay was dramatically prolonged, to 8 days longer than that of the control. Therefore, priority should be given to the ESD of late rice; in long-duration cultivars, reasonable ESD should be set at no later than July 10, especially as the ESD of ER is dependent on that of LR.

Annual rice grain yield maintenance was dependent on late rice grain yield

In late rice (LR), delayed sowing led to significantly lower grain yield and prolonged growth duration. Similar results have been reported previously, with delayed transplanting reducing spikelet filling and grain yield due to low temperature stress at anthesis in machine-transplanted LR7. The number of days with daily mean temperature <22 °C, which is the critical low temperature for anthesis in rice13, increased as sowing date was postponed, at 2–10 days in 2015 and 0–7 days in 2016 during the first 10 days after heading. These variation were directly affect the filling rate at the repining stage. Our data indicated that filling rate decreased significantly (by 8.85%, with a 5-day interval) as sowing date delay increased.

In additional, total dry matter of late rice among different sowing dates was averagely 1467~1526 g m−2, if HI was normal (about 0.5), theoretical yield was 7.3~7.6 t ha−1, our results also suggested that dry matter production was not affected as sowing date was delayed. However, reproductive redundancy increased by 7.72% in LR, with a 5-day interval, following sowing date delay. Possible reason was that the inability to transfer aboveground biomass in the vegetative organs to grain yield in LR due to less effective accumulated temperature or low-temperature stress at anthesis was intensified as sowing date was delayed, although there was no significant correlation between effective accumulated temperature of growth duration after transplanting and aboveground biomass (P > 0.05) or HI (P > 0.05). Double cropped rice production in China is undergoing an unprecedented period of transition to large-scale mechanization14. Precondition of high annual rice yield under the double cropped rice production was high early rice yield and high late rice yield, therefore, reasonable allocation of limited thermo-unit conditions annually was the keys for rice growth and development and even yield. Adjusting sowing date (seedling age was fixed) was for improving allocation of limited thermo-unit conditions under the double cropped rice production. In this study, in view of low temperature stress under late rice season, and to promote high grain yield in mechanized large-scale farming, we examined ESD for LR, and determined that it should be no later than July 10 (a 10-day delay compared with the control), and that ESD for ER should be 10 days earlier than the control, rather than later. Even if reasonable ESD is adopted, the potential threat of meteorological factors, especially high and low temperatures15,16, remain a threat to sustainably increasing grain yield. The increase in reproductive redundancy observed in this study indicates that affecting yield remains a risk in this approach. Therefore, our results suggest that greater effort should be made to develop high-yield multi-resistant rice cultivars to meet the development of mechanized large-scale rice farming, whether via conventional breeding, biotechnology, or both17. Growth duration of rice cultivars must also be considered18; our data demonstrate that thermal sensitive long-duration rice cultivars are appropriate for the early season, and day-neutral and thermal sensitive short-duration cultivars for the late season (e.g. 6,12).

Conclusions

Elastic sowing dates (ESD) with low seeding rate was conductive to grain yield maintenance in mechanized large-scale double-cropped rice production. ESD for LR should be no later than July 10 (a 10-day delay compared with the control), and ESD for ER should be earlier sowing rather than later.

Materials and Methods

Experiments designs

Field experiments were conducted in Yongan, Hunan Province, China (28°09′N, 113°37′E, 43 m a.s.l.) in the early and late seasons in 2015 and 2016. Maximum and minimum temperatures between March 25 and November 22 were 26.8 °C and 19.3 °C in 2015, and 26.4 °C and 19.3 °C in 2016, respectively (Fig. 2). High temperatures (daily maximum temperature ≥35 °C) occurred more frequently in 2016 than in 2015. Solar radiation between March 25 and November 22 were 13.9MJ/m2/d in 2015, and 13.3MJ/m2/d in 2016, respectively (Fig. 3). The soil was clayey with a pH of averagely 6.30, organic matter content of averagely 18.4 g kg−1, and total nitrogen (N) content of averagely 1.09 g kg−1. We performed a soil test using samples collected from the upper 20 cm. The ER cultivars used in this study were Zhongzao 39 (ZZ39, inbred) in 2015 and Lingliangyou268 (LLY268, hybrid) in 2016. The LR cultivars were Hyou 518 (HY518, hybrid) in 2015 and Taiyou390 (TY390, hybrid) in 2016. These four cultivars were selected because they are widely grown by rice farmers in the study region.

Figure 2
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Daily mean temperature (Mean T), maximum temperature (Max T), minimum temperature (Min T) and solar radiation during the rice growing season from seeding to maturity of double rice in Yongan County, Hunan Province, China in 2015–2016.

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Figure 3
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Solar radiation during the rice growing season from seeding to maturity of double rice in Yongan County, Hunan Province, China in 2015–2016.

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Rice planting in a randomized block design was established with different sowing dates and three replicates in a 25-m2 plot. We planted 20-day-old ER seedlings on four sowing dates between March 25 (control) and April 9, at 5-day intervals. We planted 15-day-old LR seedlings on five sowing dates between July 1 (control) and July 21, at 5-day intervals.

Seeding was performed on paper using a single-seed printing seeder (HDBZJ-580-A, Hande Co. Ltd.), and sowing was performed using seedling trays (length × width × height = 58 cm × 25 cm × 2 cm) at a rate of 15 g per tray. According to local recommended density of ER and LR, the transplanting density was 28.5 hills per m2 for early season (ER, row spacing × planting spacing = 25 cm × 14 cm), and 36.4 hills per m2 for late season (LR, 25 cm × 11 cm).

One seedling was transplanted per hill. N content of the soil was 150.0 kg N ha−1 for ES and 165.0 kg N ha−1 for LS, with 70% of total N at basal dressing and 30% of total N at panicle initiation. Phosphorus (P) and potassium (K) rates were 75.0 kg P2O5 ha−1 and 120.0 kg K2O ha−1 for ES and 82.5 kg P2O5 ha−1 and 132.0 kg K2O ha−1 for LS. P was applied initially; K application was split equally between initial application after transplantation and at panicle initiation. The water management strategy was flooding, followed by midseason drainage, re-flooding, moist intermittent irrigation, and drainage. Weeds, insects, and diseases were intensively controlled with chemicals.

Growth period, tillers, dry matter and yield attribute sampling

Dates of sowing and the full-heading and mature stages were recorded accurately. Excluding the three border plants, 10 hills were labeled in each plot to count tillers at fixed intervals from 5 to 40 days after transplanting; 10 hills were sampled and aboveground biomass was determined in the flowering stage. In the mature stage, yield components were determined for 12 hills, including spikelet panicle−1, filling ratio, and grain weight. Finally, grain yield was determined in a selected 5-m2 area, and the effective number of panicles per m2 was determined for 20 hills.

Nutrition redundancy was calculated from the difference between the maximum number of tillers and the effective number of panicles per m2. Reproductive redundancy was calculated from the barren grain number and total grain number. Effective accumulated temperature at the different stages was calculated as the difference between daily average temperature and biological initial temperature (10 °C, Indica rice).

Statistical analysis

Statistical analyses were performed using analysis of variance (ANOVA) with Statistix 8 software (Analytical Software, Tallahassee, FL, USA). Treatment means and years were compared based on the least significant difference test at a significance level of 0.05.