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Abstract The effect of slow-release fertilizer application on reducing the fertilization times of ratooning rice was explored by carrying out plot experiments using different slow-release fertilizer formulas. The results showed that compared with conventional fertilization of farmers （TCK）， there were no significant differences in plant heights of the first rice crop and ratoon rice crop and the number of tillers was slightly increased， by applying different ratios and types of slow-release fertilizers， indicating that reducing fertilization times did not affect rice growth. Compared with the TCK， the yields of treatments T1， T2， T3 and T4 of slow-release fertilizers increased to a certain extent. The yield of the first rice crop increased by 14.7%， 16.7%， 23.5% and 17.5%， respectively， the yields of the ratoon rice crop increased by 6.3%， 5.8%， 7.4% and 8.1%， respectively， and the yields of the two rice crops increased between 10.8% and 16.1%， indicating that reasonable combined application of slow-release nitrogen and fast-acting nitrogen fertilizer could meet the nutrient requirements of ratooning rice at different growth stages， and the application of slow-release fertilizers could reduce the fertilization times of ratooning rice. Based on the combination of rice yield and slow-release fertilizer cost in the first rice crop and ratoon rice crop， the T3 formula was preferred， that is， urea-formaldehyde slow-release nitrogen fertilizer was used as the base and tillering fertilizer， and polyurethane-coated slow-release nitrogen fertilizer was used as the panicle-sprouting fertilizer.
Key words Ratooning rice; Slow and controlled release fertilizer; Dry matter quality; Yield
Ratooning rice refers to the rice planted with the axillary buds of the first rice crop after the harvesting of the first rice crop[1]， which has the characteristics of seed and labor saving and high economic benefits[2-3]. The development of ratooning rice in areas where there is enough heat for planting one-season rice but insufficient heat for planting double-season rice and in double-season rice areas where one-season mid-season rice is planted is one of the effective measures to increase the multiple cropping index and increase rice yield and economic income per unit area of rice field[4].
In recent years， the planting area of ratooning rice in China has been continuously improved. For example， Hubei Province is an advantageous area for the development of ratooning rice in China. The area of ratooning rice in the province has increased from 29 800 hm2 in 2013 to 153 300 hm2 in 2017. The application prospect is very good[5-7]. At present， there are more in-depth studies on ratooning rices nutrient demand law， fertilization period[8]， nitrogen fertilizer operation[9-11]， etc. However， most studies ignore the problem of fertilization demand in the process of planting ratoon rice. Since the whole rice field growth period of ratooning rice is about 170 d， farmers generally applied fertilizers for 5 times， namely the base fertilizer， tillering fertilizer， panicle fertilizer， bud-promoting fertilizer and Qi Miao fertilizer. In the current situation of rural labor shortage and high cost of artificial machinery， the profitable space for ratooning rice has been greatly compressed.
In this study， through the use of different types （polyurethane-coated， urea-formaldehyde） of slow-release fertilizers， the impact of slow-release fertilizers on the growth and yield of ratooning rice was clarified， so as to provide theoretical basis and technical support for the reduction of fertilization times and amount during the whole growth period of ratooning rice and the large-scale application of such fertilizers to the production of ratooning rice， which is of great significance for the realization of fertilizer-reducing， efficiency-improving and labor-saving simplified sustainable production of ratooning rice.
Materials and Methods
Experimental materials
The experiment was carried out in 2018 at the Ratooning Rice Experimental Base in Zhefu Village， Maoshi Town， Jianli County， Hubei Province. The test soil is paddy soil， and the top soil （0-20 cm） contains organic matter 32.46 g/kg， total nitrogen 2.1 g/kg， alkali-hydrolyzale nitrogen 94.08 mg/kg， rapidly available phosphorus 15.74 mg/kg， and rapidly available potassium 203.65 mg/kg， and has a pH value of 6.3.
The rice variety to be tested was Liangyou 6326， the main local variety， which was planted by transplanting.
Experimental design
There were six treatments in this experiment： ① CK： no fertilization， ② TCK： conventional fertilization of farmers， ③ T1： base and tillering fertilizer using urea-formaldehyde slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using urea-formaldehyde slow-release fertilizer 25-5-15， UF-UF， ④ T2： base and tillering fertilizer using polyurethane slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using polyurethane slow-release fertilizer 25-5-15， PP， ⑤ T3： base and tillering fertilizer using urea-formaldehyde slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using polyurethane slow-release fertilizer 25-5-15， UF-P， and ⑥ T4： base and tillering fertilizer using polyurethane slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using urea-formaldehyde slow-release fertilizer 25-5-15， P-UF. Three replicates were set for each treatment in the experiment， and there were a total of 18 plots， each of which had an area of 30 m2. Randomized block arrangement was adopted. Fertilizer application： During the whole growth period of the main crop and the ratoon crop， the TCK treatment was applied with ordinary compound fertilizer （N-P2O5-K2O=15-15-15） 600 kg/hm2 as the base fertilizer， and topdressed with urea 150 kg/hm2 at tillering stage， urea 150 kg/hm2 and potassium chloride 75 kg/hm2 as the panicle fertilizer， and urea 150 kg/hm2 and potassium chloride 75 kg/hm2 as the bud-promoting fertilizer 15 d after full heading of the main crop; and treatments T1， T2， T3 and T4 combined the base fertilizer and tillering fertilizer into one， and the panicle fertilizer and bud-promoting fertilizer into one. Treatment T1 was applied with urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=24-6-12， containing 10% urea-formaldehyde） 600 kg/hm2 as the base and tillering fertilizer， and urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=25-5-15， containing 50% urea-formaldehyde） 600 kg/hm2 as the panicle-sprouting fertilizer; treatment T2 was applied with polyurethane-coated slow-release fertilizer （N-P2O5-K2O=24-6-12， 20% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as the base and tillering fertilizer， and polyurethane-coated slow-release fertilizer （N-P2O5-K2O=25-5-15， 40% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as the panicle-sprouting fertilizer; treatment T3 was applied with urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=24-6-12， containing 10% urea-formaldehyde） 600 kg/hm2 as the base and tillering fertilizer， and polyurethane-coated slow-release fertilizer （N-P2O5-K2O = 25-5-15， 40% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as panicle-sprouting fertilizer; treatment T4 was applied with polyurethane-coated slow-release fertilizer （N-P2O5-K2O=24-6-12， 20% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as the base and tillering fertilizer， and urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=25-5-15， containing 50% urea-formaldehyde） 600 kg/hm2 as panicle-sprouting fertilizer; and except for the treatment without fertilization， all the treatments were applied with urea 225 kg/hm2 as Qi Miao fertilizer after the first rice crop was harvested.
In order to prevent water and fertilizers from exchanging with adjacent plots， land ridges were formed in each plot and wrapped with agricultural film to 30 cm below the field surface. The rice in each plot was manually transplanted. The seeds were sown on March 20 and transplanting was performed on April 27 according to 2-3 seedlings per hole and row and plant spacing of 20 cm×20 cm. The base fertilizer （or base and tillering fertilizer） was applied on April 26; the tillering fertilizer was applied on May 5; the panicle fertilizer （or panicle-sprouting fertilizer） was applied on June 10; the bud-promoting fertilizer was applied on July 15; and the Qi Miao fertilizer was applied on August 3. The field management method was the same as that of the local farmer. Determination items and methods
Determination of plant height and tiller number： Fifteen consecutive representative hills were selected in each plot and marked. The tiller number was determined at the tillering stage， full heading stage， filling stage and maturing stage of the first rice crop， and the full heading stage and maturing stage of the ratoon rice， respectively.
Dry matter accumulation： Three representative hills were selected at the tillering stage， full heading stage， filling stage， and maturing stage of the first rice crop， and at the full heading stage and maturing stage of the ratoon rice crop， respectively. The aboveground part of the selected hills was cut at the level of the ground， dried at 105 ℃ for 30 min， and then dried at 65 ℃ to constant weight， and the weight was then measured.
Yield determination： During the full ripe stage， plants in 1 m2 were collected from each plot and threshed， and the seeds were mixed. Part of the seeds was weighed to obtain a fresh weight， and then filled in a net bag and hung to air dry to a constant weight. The moisture content and the actual yield were calculated. At the full ripe stage， 5 consecutive representative hills were selected to measure the effective panicle number， number of grains per panicle， 1 000-grain weight， and seed setting rate.
Statistical analysis method
The test data was analyzed using Microsoft Excel 2010 software， and statistical analysis was performed using SPSS 20.
Results and Analysis
Effects of different fertilizer treatments on plant height of ratooning rice
As shown in Fig. 1， compared with the CK， after applying fertilizer， the plant height of each treatment in different periods increased significantly， and reached its maximum value at the maturing stage of the first rice crop. Compared with the TCK， there were no significant differences in plant height between the slow-release fertilizer treatments， and reducing fertilization times did not affect plant growth.
Effects of different fertilizer treatments on tillering of ratooning rice
It could be seen from Fig. 2 that compared with no fertilization， the number of tillers of each treatment increased significantly after fertilizer application in the main and ratoon rice crops. According to the investigation on tillering of the main rice crop， compared with the TCK， the number of tillers at the tillering stage was not significantly different between the slow-release fertilizer treatments T1， T2， T3， and T4， but at the full heading stage， it was significantly higher in treatments T1 and T3 than in treatments TCK， T2 and T4. Treatment T1 had the highest effective tiller number at the maturing stage， followed by treatment T3， and these two treatments were both higher than the TCK. For the ratoon rice crop， the number of tillers showed an order of T3>T4>TCK≈T2>T1>CK; and at the maturing stage， except for no fertilization， other treatments exhibited no significant differences. It might be because the demand for slow-release nitrogen is not high in the early growth stage of the first rice crop， that is， the tillering stage. During this period， the urea-formaldehyde compound fertilizer with a relatively low slow-release nitrogen content should be selected. In the middle and late stages of the growth of ratooning rice， the application of fertilizers should not only provide nutrients for filling of the first rice crop， but also provide continuous nutrients for the growth of ratoon plants. It is better to choose a certain proportion of polyurethane-coated slow-release fertilizer during this period. Weiping JIAO et al. Effects of Different Slow-release Fertilizers on Growth and Yield of Ratooning Rice
Effects of different fertilizer treatments on dry matter accumulation of ratooning rice
It could be seen from Fig. 3 that the application of fertilizers significantly increased the dry matter accumulation of the first rice crop and the ratoon rice crop. From the tillering stage to the filling stage， dry matter accumulated rapidly in the first rice crop. The higher dry matter accumulation in the first rice crop of treatments T1 and T3 indicated that the application of urea-formaldehyde slow-release fertilizer in the first rice crop as the base and tillering fertilizer was more conducive to plant growth. When the panicle-sprouting fertilizer was applied with the polyurethane coating type， the slow and continuous release of nitrogen enabled the ratoon rice to maintain a higher dry matter accumulation in the later period.
Effects of different fertilizer treatments on yield components of ratooning rice
It could be seen from Table 1 that the number of grains per panicle and the number of panicles per square meter were significantly improved in the first rice crop and ratoon rice crop after applying fertilizers.
For the first rice crop， the slow-release fertilizer treatments T1， T3 and T4 were significantly different from the TCK in the number of grains per panicle， and the values of treatments T1， T2， T3 and T4 increased by 14.8%， 10.4%， 22.2% and 15.0% compared with the TCK， respectively. Among the slow-release fertilizer treatments， the number of grains per panicle of T3 increased by 6.5%， 10.7% and 6.3% compared with T1， T2， and T4， respectively. There were certain differences in the seed setting rate among the treatments. The seed setting rate was highest in the no fertilization treatment， and the lowest in the T4 treatment. Compared with the TCK， the number of panicles per square meter increased after applying slow-release fertilizer， showing an increase of 8.1%-34.8%.
For the ratoon rice， the slow-release fertilizer treatments T1， T2， T3 and T4 showed no significant differences in the number of grains per panicle， seed setting rate， and number of panicles per square meter from the TCK. It indicated that the nutrient requirements of the ratoon rice could be satisfied by combining the panicle fertilizer and the bud-promoting fertilizer into one through the application of slow-release nitrogen fertilizers at the time for the application of the panicle fertilizer in the first rice crop and then applying the seedling-promoting fertilizer in the ratoon crop， and the yield level of the ratoon rice could be ensured thereby. Effects of different fertilizer treatments on yield and benefit of ratooning rice
The yield of ratooning rice is composed of the first crop yield and the ratoon crop yield. To achieve a high yield in the two rice crops， we must coordinate the relationship between the yields of the first rice crop and the ratoon rice crop while pursuing a high yield of single crop. Compared with the CK without fertilization， the yield increased by 37.4% in the first rice crop， and 141.7% in the ratoon rice crop， and 71.6% in the total production of the two crops （Table 2）， indicating that under fertilization conditions， the number of tillers increased， the number of grains per panicle increased and eventually the rice yield increased. Therefore， fertilization is a prerequisite for high yield of mid-season hybrid rice.
Compared with the TCK， the yields of the first rice crop and the ratoon rice crop of the slow-release fertilizer formulas increased to a certain extent， and the yields of the first rice crop in treatments T1， T2， T3 and T4 increased by 14.7%， 16.7%， 23.5% and 17.5%， respectively， and the yields of ratoon rice increased by 6.3%， 5.8%， 7.4% and 8.1%， respectively. The increase in rice yield of the two crops ranged from 10.8% to 16.1% （Table 2）， indicating that reasonable combination of slow-release nitrogen and fast-acting nitrogen fertilizer could increase rice yield.
The rice yields of various treatments were basically at the same level， and the field management was basically the same except for fertilizer application. Therefore， only the cost differences caused by fertilizer application between different treatments were analyzed. The various treatments applied with slow-release fertilizers reduced the fertilization times by 2 times compared with the TCK， thus reducing the labor cost of fertilization， but the cost of slow-release nitrogen fertilizers was higher than that of ordinary nitrogen fertilizers， resulting in high fertilizer cost. Therefore， based on the combination of rice yield and fertilizer input in the two rice crops， the effects of increasing yield and income ranked as the T3>T4>T1≈T2. Formula T3 was preferable， that is， urea-formaldehyde slow-release nitrogen fertilizer was applied as the base and tillering fertilizer， and polyurethane-coated slow-release fertilizer was applied as the panicle-sprouting fertilizer.
Conclusions and Discussion
The slow-release nitrogen fertilizers have the advantage of long fertilizer efficiency compared with ordinary urea[12]， and they are widely used because of its advantages of improving fertilizer use efficiency， reducing pollution and saving labor force with one-time application[13]. In this study， the application of slow-release fertilizers reduced the fertilization times of ratooning rice. The differences in plant height between the TCK and other treatments were not significant， and the numbers of tillers in the first rice crop and the ratoon rice crop slightly increased， indicating that reducing the fertilization times did not affect plant growth. Dry matter is the final product of crop photosynthesis and is the main factor that determines rice yield. With the progress of rice growth， the accumulation of dry matter in plants increases， and rice yield is positively correlated with the net dry matter accumulation of plants in the middle and late stages[14] . In this study， treatments T1 and T3 had higher dry matter accumulation during the grain filling and maturation periods in the first rice crop and the ratoon rice crop， indicating that applying urea-formaldehyde slow-release fertilizer in the growth period of the first rice crop and applying urea-formaldehyde or coated slow-release fertilizer in the later period would not cause the phenomenon of premature aging due to inadequate nutrients in the later period.
In order to obtain a high yield in the two rice crops， the number of effective panicles should be ensured while maximizing large panicles in the first rice crop， while for the ratoon rice crop， the main target is to compensate for the lack of the number of grains per panicle by increasing effective panicles[15-16]. The research by Jiang et al.[17] showed that for both the first rice crop and the ratoon rice crop of ratooning rice， the number of grains per panicle and the number of effective panicles had the closest correlation with yield， contributing 97% to the yield of ratooning rice. In this study， the differences in the yield components of the first rice crop were mainly manifested in the number of grains per panicle. The number of grains per panicle in T3 increased by 6.5%， 10.7%， and 6.3% compared with treatments T1， T2 and T4， respectively; the number of panicles per square meter in the ratoon rice crop was not significantly different between the TCK and the different slow-release fertilizer treatments， indicating the nutrient requirements of the ratoon rice could be satisfied by combining the panicle fertilizer and the bud-promoting fertilizer into one through the application of slow-release nitrogen fertilizers in the first rice crop and then applying the seedling-promoting fertilizer in the ratoon rice crop， and the yield level of the ratoon rice could be ensured thereby.
According to the characteristics of slow-release fertilizers， the application of slow-release fertilizers can reduce the fertilization times of rice and achieve the purpose of saving costs and increasing efficiency[18]. In this study， compared with the TCK， the slow-release fertilizer treatments increased both the yields of the first rice crop and the ratoon rice crop， with an increase of 14.7%-23.5% in the first rice crop and an increase of 5.8%-8.1% in the ratoon rice crop. It shows that slow-release fertilizers release nutrients slowly， prolong the time of fertilizer supply， and have the effect of early application and later use. There is no premature aging due to inadequate nutrients in the later period， which has a certain role in promoting rice growth and increasing yield. The price of slow-release fertilizers is higher， and the combination of quick-acting and slow-release fertilizer is reasonable to reduce the cost of fertilizer application[19]. To ensure good early growth， stable growth in the middle and late stages and more effective panicles of the main rice crop， fast sprouting， high panicle formation rate and enough effective panicle of the ratoon rice crop， and the goal of coordinated high yield in the two crops， we optimized the ratio of quick-acting nitrogen fertilizer and slow-release nitrogen fertilizer， and the T3 formula was preferred， followed by the T1 formula.
To sum up， the ratooning rice formula fertilizer developed based on slow-release fertilizer formulas， that is， the application of urea-formaldehyde slow-release nitrogen fertilizer as the base and tillering fertilizer and the application of polyurethane-coated slow-release nitrogen fertilizer as the panicle fertilizer， could simplify the fertilization technique of ratooning rice. The improved fertilization technique reduced 5 times of fertilization to 3 times while increasing rice yield. It is conducive to saving costs and increasing efficiency， and can be applied and promoted in actual production.
References
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Key words Ratooning rice; Slow and controlled release fertilizer; Dry matter quality; Yield
Ratooning rice refers to the rice planted with the axillary buds of the first rice crop after the harvesting of the first rice crop[1]， which has the characteristics of seed and labor saving and high economic benefits[2-3]. The development of ratooning rice in areas where there is enough heat for planting one-season rice but insufficient heat for planting double-season rice and in double-season rice areas where one-season mid-season rice is planted is one of the effective measures to increase the multiple cropping index and increase rice yield and economic income per unit area of rice field[4].
In recent years， the planting area of ratooning rice in China has been continuously improved. For example， Hubei Province is an advantageous area for the development of ratooning rice in China. The area of ratooning rice in the province has increased from 29 800 hm2 in 2013 to 153 300 hm2 in 2017. The application prospect is very good[5-7]. At present， there are more in-depth studies on ratooning rices nutrient demand law， fertilization period[8]， nitrogen fertilizer operation[9-11]， etc. However， most studies ignore the problem of fertilization demand in the process of planting ratoon rice. Since the whole rice field growth period of ratooning rice is about 170 d， farmers generally applied fertilizers for 5 times， namely the base fertilizer， tillering fertilizer， panicle fertilizer， bud-promoting fertilizer and Qi Miao fertilizer. In the current situation of rural labor shortage and high cost of artificial machinery， the profitable space for ratooning rice has been greatly compressed.
In this study， through the use of different types （polyurethane-coated， urea-formaldehyde） of slow-release fertilizers， the impact of slow-release fertilizers on the growth and yield of ratooning rice was clarified， so as to provide theoretical basis and technical support for the reduction of fertilization times and amount during the whole growth period of ratooning rice and the large-scale application of such fertilizers to the production of ratooning rice， which is of great significance for the realization of fertilizer-reducing， efficiency-improving and labor-saving simplified sustainable production of ratooning rice.
Materials and Methods
Experimental materials
The experiment was carried out in 2018 at the Ratooning Rice Experimental Base in Zhefu Village， Maoshi Town， Jianli County， Hubei Province. The test soil is paddy soil， and the top soil （0-20 cm） contains organic matter 32.46 g/kg， total nitrogen 2.1 g/kg， alkali-hydrolyzale nitrogen 94.08 mg/kg， rapidly available phosphorus 15.74 mg/kg， and rapidly available potassium 203.65 mg/kg， and has a pH value of 6.3.
The rice variety to be tested was Liangyou 6326， the main local variety， which was planted by transplanting.
Experimental design
There were six treatments in this experiment： ① CK： no fertilization， ② TCK： conventional fertilization of farmers， ③ T1： base and tillering fertilizer using urea-formaldehyde slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using urea-formaldehyde slow-release fertilizer 25-5-15， UF-UF， ④ T2： base and tillering fertilizer using polyurethane slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using polyurethane slow-release fertilizer 25-5-15， PP， ⑤ T3： base and tillering fertilizer using urea-formaldehyde slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using polyurethane slow-release fertilizer 25-5-15， UF-P， and ⑥ T4： base and tillering fertilizer using polyurethane slow-release fertilizer 24-6-12， panicle-sprouting fertilizer using urea-formaldehyde slow-release fertilizer 25-5-15， P-UF. Three replicates were set for each treatment in the experiment， and there were a total of 18 plots， each of which had an area of 30 m2. Randomized block arrangement was adopted. Fertilizer application： During the whole growth period of the main crop and the ratoon crop， the TCK treatment was applied with ordinary compound fertilizer （N-P2O5-K2O=15-15-15） 600 kg/hm2 as the base fertilizer， and topdressed with urea 150 kg/hm2 at tillering stage， urea 150 kg/hm2 and potassium chloride 75 kg/hm2 as the panicle fertilizer， and urea 150 kg/hm2 and potassium chloride 75 kg/hm2 as the bud-promoting fertilizer 15 d after full heading of the main crop; and treatments T1， T2， T3 and T4 combined the base fertilizer and tillering fertilizer into one， and the panicle fertilizer and bud-promoting fertilizer into one. Treatment T1 was applied with urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=24-6-12， containing 10% urea-formaldehyde） 600 kg/hm2 as the base and tillering fertilizer， and urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=25-5-15， containing 50% urea-formaldehyde） 600 kg/hm2 as the panicle-sprouting fertilizer; treatment T2 was applied with polyurethane-coated slow-release fertilizer （N-P2O5-K2O=24-6-12， 20% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as the base and tillering fertilizer， and polyurethane-coated slow-release fertilizer （N-P2O5-K2O=25-5-15， 40% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as the panicle-sprouting fertilizer; treatment T3 was applied with urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=24-6-12， containing 10% urea-formaldehyde） 600 kg/hm2 as the base and tillering fertilizer， and polyurethane-coated slow-release fertilizer （N-P2O5-K2O = 25-5-15， 40% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as panicle-sprouting fertilizer; treatment T4 was applied with polyurethane-coated slow-release fertilizer （N-P2O5-K2O=24-6-12， 20% polyurethane-coated slow-release nitrogen in total nitrogen） 600 kg/hm2 as the base and tillering fertilizer， and urea-formaldehyde slow-release fertilizer （N-P2O5-K2O=25-5-15， containing 50% urea-formaldehyde） 600 kg/hm2 as panicle-sprouting fertilizer; and except for the treatment without fertilization， all the treatments were applied with urea 225 kg/hm2 as Qi Miao fertilizer after the first rice crop was harvested.
In order to prevent water and fertilizers from exchanging with adjacent plots， land ridges were formed in each plot and wrapped with agricultural film to 30 cm below the field surface. The rice in each plot was manually transplanted. The seeds were sown on March 20 and transplanting was performed on April 27 according to 2-3 seedlings per hole and row and plant spacing of 20 cm×20 cm. The base fertilizer （or base and tillering fertilizer） was applied on April 26; the tillering fertilizer was applied on May 5; the panicle fertilizer （or panicle-sprouting fertilizer） was applied on June 10; the bud-promoting fertilizer was applied on July 15; and the Qi Miao fertilizer was applied on August 3. The field management method was the same as that of the local farmer. Determination items and methods
Determination of plant height and tiller number： Fifteen consecutive representative hills were selected in each plot and marked. The tiller number was determined at the tillering stage， full heading stage， filling stage and maturing stage of the first rice crop， and the full heading stage and maturing stage of the ratoon rice， respectively.
Dry matter accumulation： Three representative hills were selected at the tillering stage， full heading stage， filling stage， and maturing stage of the first rice crop， and at the full heading stage and maturing stage of the ratoon rice crop， respectively. The aboveground part of the selected hills was cut at the level of the ground， dried at 105 ℃ for 30 min， and then dried at 65 ℃ to constant weight， and the weight was then measured.
Yield determination： During the full ripe stage， plants in 1 m2 were collected from each plot and threshed， and the seeds were mixed. Part of the seeds was weighed to obtain a fresh weight， and then filled in a net bag and hung to air dry to a constant weight. The moisture content and the actual yield were calculated. At the full ripe stage， 5 consecutive representative hills were selected to measure the effective panicle number， number of grains per panicle， 1 000-grain weight， and seed setting rate.
Statistical analysis method
The test data was analyzed using Microsoft Excel 2010 software， and statistical analysis was performed using SPSS 20.
Results and Analysis
Effects of different fertilizer treatments on plant height of ratooning rice
As shown in Fig. 1， compared with the CK， after applying fertilizer， the plant height of each treatment in different periods increased significantly， and reached its maximum value at the maturing stage of the first rice crop. Compared with the TCK， there were no significant differences in plant height between the slow-release fertilizer treatments， and reducing fertilization times did not affect plant growth.
Effects of different fertilizer treatments on tillering of ratooning rice
It could be seen from Fig. 2 that compared with no fertilization， the number of tillers of each treatment increased significantly after fertilizer application in the main and ratoon rice crops. According to the investigation on tillering of the main rice crop， compared with the TCK， the number of tillers at the tillering stage was not significantly different between the slow-release fertilizer treatments T1， T2， T3， and T4， but at the full heading stage， it was significantly higher in treatments T1 and T3 than in treatments TCK， T2 and T4. Treatment T1 had the highest effective tiller number at the maturing stage， followed by treatment T3， and these two treatments were both higher than the TCK. For the ratoon rice crop， the number of tillers showed an order of T3>T4>TCK≈T2>T1>CK; and at the maturing stage， except for no fertilization， other treatments exhibited no significant differences. It might be because the demand for slow-release nitrogen is not high in the early growth stage of the first rice crop， that is， the tillering stage. During this period， the urea-formaldehyde compound fertilizer with a relatively low slow-release nitrogen content should be selected. In the middle and late stages of the growth of ratooning rice， the application of fertilizers should not only provide nutrients for filling of the first rice crop， but also provide continuous nutrients for the growth of ratoon plants. It is better to choose a certain proportion of polyurethane-coated slow-release fertilizer during this period. Weiping JIAO et al. Effects of Different Slow-release Fertilizers on Growth and Yield of Ratooning Rice
Effects of different fertilizer treatments on dry matter accumulation of ratooning rice
It could be seen from Fig. 3 that the application of fertilizers significantly increased the dry matter accumulation of the first rice crop and the ratoon rice crop. From the tillering stage to the filling stage， dry matter accumulated rapidly in the first rice crop. The higher dry matter accumulation in the first rice crop of treatments T1 and T3 indicated that the application of urea-formaldehyde slow-release fertilizer in the first rice crop as the base and tillering fertilizer was more conducive to plant growth. When the panicle-sprouting fertilizer was applied with the polyurethane coating type， the slow and continuous release of nitrogen enabled the ratoon rice to maintain a higher dry matter accumulation in the later period.
Effects of different fertilizer treatments on yield components of ratooning rice
It could be seen from Table 1 that the number of grains per panicle and the number of panicles per square meter were significantly improved in the first rice crop and ratoon rice crop after applying fertilizers.
For the first rice crop， the slow-release fertilizer treatments T1， T3 and T4 were significantly different from the TCK in the number of grains per panicle， and the values of treatments T1， T2， T3 and T4 increased by 14.8%， 10.4%， 22.2% and 15.0% compared with the TCK， respectively. Among the slow-release fertilizer treatments， the number of grains per panicle of T3 increased by 6.5%， 10.7% and 6.3% compared with T1， T2， and T4， respectively. There were certain differences in the seed setting rate among the treatments. The seed setting rate was highest in the no fertilization treatment， and the lowest in the T4 treatment. Compared with the TCK， the number of panicles per square meter increased after applying slow-release fertilizer， showing an increase of 8.1%-34.8%.
For the ratoon rice， the slow-release fertilizer treatments T1， T2， T3 and T4 showed no significant differences in the number of grains per panicle， seed setting rate， and number of panicles per square meter from the TCK. It indicated that the nutrient requirements of the ratoon rice could be satisfied by combining the panicle fertilizer and the bud-promoting fertilizer into one through the application of slow-release nitrogen fertilizers at the time for the application of the panicle fertilizer in the first rice crop and then applying the seedling-promoting fertilizer in the ratoon crop， and the yield level of the ratoon rice could be ensured thereby. Effects of different fertilizer treatments on yield and benefit of ratooning rice
The yield of ratooning rice is composed of the first crop yield and the ratoon crop yield. To achieve a high yield in the two rice crops， we must coordinate the relationship between the yields of the first rice crop and the ratoon rice crop while pursuing a high yield of single crop. Compared with the CK without fertilization， the yield increased by 37.4% in the first rice crop， and 141.7% in the ratoon rice crop， and 71.6% in the total production of the two crops （Table 2）， indicating that under fertilization conditions， the number of tillers increased， the number of grains per panicle increased and eventually the rice yield increased. Therefore， fertilization is a prerequisite for high yield of mid-season hybrid rice.
Compared with the TCK， the yields of the first rice crop and the ratoon rice crop of the slow-release fertilizer formulas increased to a certain extent， and the yields of the first rice crop in treatments T1， T2， T3 and T4 increased by 14.7%， 16.7%， 23.5% and 17.5%， respectively， and the yields of ratoon rice increased by 6.3%， 5.8%， 7.4% and 8.1%， respectively. The increase in rice yield of the two crops ranged from 10.8% to 16.1% （Table 2）， indicating that reasonable combination of slow-release nitrogen and fast-acting nitrogen fertilizer could increase rice yield.
The rice yields of various treatments were basically at the same level， and the field management was basically the same except for fertilizer application. Therefore， only the cost differences caused by fertilizer application between different treatments were analyzed. The various treatments applied with slow-release fertilizers reduced the fertilization times by 2 times compared with the TCK， thus reducing the labor cost of fertilization， but the cost of slow-release nitrogen fertilizers was higher than that of ordinary nitrogen fertilizers， resulting in high fertilizer cost. Therefore， based on the combination of rice yield and fertilizer input in the two rice crops， the effects of increasing yield and income ranked as the T3>T4>T1≈T2. Formula T3 was preferable， that is， urea-formaldehyde slow-release nitrogen fertilizer was applied as the base and tillering fertilizer， and polyurethane-coated slow-release fertilizer was applied as the panicle-sprouting fertilizer.
Conclusions and Discussion
The slow-release nitrogen fertilizers have the advantage of long fertilizer efficiency compared with ordinary urea[12]， and they are widely used because of its advantages of improving fertilizer use efficiency， reducing pollution and saving labor force with one-time application[13]. In this study， the application of slow-release fertilizers reduced the fertilization times of ratooning rice. The differences in plant height between the TCK and other treatments were not significant， and the numbers of tillers in the first rice crop and the ratoon rice crop slightly increased， indicating that reducing the fertilization times did not affect plant growth. Dry matter is the final product of crop photosynthesis and is the main factor that determines rice yield. With the progress of rice growth， the accumulation of dry matter in plants increases， and rice yield is positively correlated with the net dry matter accumulation of plants in the middle and late stages[14] . In this study， treatments T1 and T3 had higher dry matter accumulation during the grain filling and maturation periods in the first rice crop and the ratoon rice crop， indicating that applying urea-formaldehyde slow-release fertilizer in the growth period of the first rice crop and applying urea-formaldehyde or coated slow-release fertilizer in the later period would not cause the phenomenon of premature aging due to inadequate nutrients in the later period.
In order to obtain a high yield in the two rice crops， the number of effective panicles should be ensured while maximizing large panicles in the first rice crop， while for the ratoon rice crop， the main target is to compensate for the lack of the number of grains per panicle by increasing effective panicles[15-16]. The research by Jiang et al.[17] showed that for both the first rice crop and the ratoon rice crop of ratooning rice， the number of grains per panicle and the number of effective panicles had the closest correlation with yield， contributing 97% to the yield of ratooning rice. In this study， the differences in the yield components of the first rice crop were mainly manifested in the number of grains per panicle. The number of grains per panicle in T3 increased by 6.5%， 10.7%， and 6.3% compared with treatments T1， T2 and T4， respectively; the number of panicles per square meter in the ratoon rice crop was not significantly different between the TCK and the different slow-release fertilizer treatments， indicating the nutrient requirements of the ratoon rice could be satisfied by combining the panicle fertilizer and the bud-promoting fertilizer into one through the application of slow-release nitrogen fertilizers in the first rice crop and then applying the seedling-promoting fertilizer in the ratoon rice crop， and the yield level of the ratoon rice could be ensured thereby.
According to the characteristics of slow-release fertilizers， the application of slow-release fertilizers can reduce the fertilization times of rice and achieve the purpose of saving costs and increasing efficiency[18]. In this study， compared with the TCK， the slow-release fertilizer treatments increased both the yields of the first rice crop and the ratoon rice crop， with an increase of 14.7%-23.5% in the first rice crop and an increase of 5.8%-8.1% in the ratoon rice crop. It shows that slow-release fertilizers release nutrients slowly， prolong the time of fertilizer supply， and have the effect of early application and later use. There is no premature aging due to inadequate nutrients in the later period， which has a certain role in promoting rice growth and increasing yield. The price of slow-release fertilizers is higher， and the combination of quick-acting and slow-release fertilizer is reasonable to reduce the cost of fertilizer application[19]. To ensure good early growth， stable growth in the middle and late stages and more effective panicles of the main rice crop， fast sprouting， high panicle formation rate and enough effective panicle of the ratoon rice crop， and the goal of coordinated high yield in the two crops， we optimized the ratio of quick-acting nitrogen fertilizer and slow-release nitrogen fertilizer， and the T3 formula was preferred， followed by the T1 formula.
To sum up， the ratooning rice formula fertilizer developed based on slow-release fertilizer formulas， that is， the application of urea-formaldehyde slow-release nitrogen fertilizer as the base and tillering fertilizer and the application of polyurethane-coated slow-release nitrogen fertilizer as the panicle fertilizer， could simplify the fertilization technique of ratooning rice. The improved fertilization technique reduced 5 times of fertilization to 3 times while increasing rice yield. It is conducive to saving costs and increasing efficiency， and can be applied and promoted in actual production.
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