Elastic sowing dates with low seeding rate for grain yield maintenance in mechanized large-scale double-cropped rice production

Elastic sowing dates (ESDs) are correlated with rice grain yield. ESD is the easiest factor for farmers to manipulate in mechanized large-scale farming. In this study, field experiments were conducted over a 2-year period to determine the effects of different sowing dates on growth duration, effective accumulated temperature, and yield attributes in two early- and late-season machine-transplanted rice cultivars. In early rice (ER), a delay in the sowing date led to decreased grain yield and shorter growth duration. In late rice (LR), delayed sowing led to significantly lower grain yield and prolonged growth duration. In LR, significantly positive correlations were detected between effective accumulated temperature in the post-heading stage and both filling ratio and yield. Reproductive redundancy increased markedly in LR, by 7.72% over a 5-day interval. We determined that the ESDs for LR were 10 days later than the control, and that of ER was recommend early sowing rather than late sowing. These findings suggest a new strategy to meet the demands of mechanized large-scale rice farming: the development of thermal sensitive high-yield long-duration ER cultivars and high-yield short-duration LR cultivars.

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).
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.   www.nature.com/scientificreports www.nature.com/scientificreports/ stress at anthesis in machine-transplanted LR 7 . The number of days with daily mean temperature <22 °C, which is the critical low temperature for anthesis in rice 13 , 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 mechanization 14 . 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 temperatures 15,16 , 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 both 17 . Growth duration of rice cultivars must also be considered 18 ; 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.  www.nature.com/scientificreports www.nature.com/scientificreports/ Rice planting in a randomized block design was established with different sowing dates and three replicates in a 25-m 2 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.

Materials and Methods
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 m 2 for early season (ER, row spacing × planting spacing = 25 cm × 14 cm), and 36.4 hills per m 2 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 P 2 O 5 ha −1 and 120.0 kg K 2 O ha −1 for ES and 82.5 kg P 2 O 5 ha −1 and 132.0 kg K 2 O 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.  www.nature.com/scientificreports www.nature.com/scientificreports/ 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-m 2 area, and the effective number of panicles per m 2 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 m 2 . 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).