Sweet corn refers to a kind of corn suitable for being eaten raw, which tastes juicy, crisp, tender and sweet with very thin skin. It can be eaten raw like fruit, so it is called ‘sweet corn’ (Wu et al., 2014; Ren et al., 2016; Huang et al., 2018; Lu et al., 2019, Shanghai Vegetables, (2): 32). Sweet corn has good flavor and high nutrition (Liu et al., 2010, Guangdong Agricultural Sciences, 37(12): 90-94). And its grains contain rich sugar, fat, protein, crude fiber and 8 kinds of amino acids needed by the human body. Besides, the grains also contain a variety of vitamins and nutrients such as Ca, Mg, Fe, and Zn (Cui et al., 1996; Gao et al., 2020). Therefore, sweet corn has been more and more popular in recent years, and the cultivation area has been expanding.

There are more people and less land in Zhejiang, which is known as ‘70% are mountains, 10% are water and 20% are fields’ (Zheng, 2010), so the cultivated land resources are in short supply. Farmers have low income from growing grain, but they need to ensure food security, over-rely on pesticides and fertilizers, and protect the ecological environment, which is a contradiction (Luo, 2010; Zhong, 2018). In recent years, in order to pursue the development of green and efficient agriculture, Zhejiang Province has actively carried out reforms and innovations in the cultivation system. Rotation between upland sweet corn and paddy late rice has been demonstrated and promoted in Shengzhou, Jiande, Dongyang, Wenzhou, Jiaxing and other places, with more than 3,000 hm² area. In 2019, farmers who planted sweet corn in Shengzhou had a good harvest, with ten-thousand yuan of income per mu (Zhou and Guo, 2019, Zhejiang Agricultural Sciences, 60(11): 2013-2014). After the sweet corn were harvested, the full amount of straw was returned to the field to plant rice, significantly improving the soil structure, reducing the occurrence of rice pests and diseases and the application of chemical fertilizers and pesticides, increasing the yield and quality of rice and corn, and achieving the goal of half-a-ton of grain and ten-thousand yuan of income per mu, which shows significant economic, ecological and social benefits. The green and efficient cultivation technology of rotation between upland sweet corn and paddy late rice was listed as the main technology in 2019 by Zhejiang Agriculture and Rural Affairs Department. The rotation as an agricultural innovation system was promoted to the province. In 2020, 7 hm² sweet corn was planted on Shengzhou Guoyonghua family farm, which drove the development of 80 hm² sweet corn planed by the surrounding professional farmers.

1 Application Effect

The situation and outcome analysis of upland sweet corn-paddy late rice planting on Shengzhou Guoyonghua family farm from 2018 to 2019 (Table 1). In 2018, the sowing period of sweet corn was from January 1^st to 30^th, using electric stove wire to heat. The emergence period was from mid-January to early February. And they were transplanted into plastic greenhouses and medium arched sheds from late January to late February. The first batch was picked on May 10^th. The average yield was 12 750 kg/hm² and the output value reached 150 000 yuan/hm². The output value of late rice was 24 570 yuan/hm², and the annual output value reached 174 570 yuan/hm². Due to the influence of temperature, the sowing period of sweet corn in 2019 was from late January to the end of February. The emergence period was from early February to early March. And the sowing and transplanting period was from February 15^th to March 28^th. The first batch was harvested on May 18^th. The average yield was 13 230 kg/hm² and the output value reached 154 500 yuan/hm². The output value of late rice was 6 300 yuan/hm², and the annual output value reached 160 800 yuan /hm².

Table 1 The outcome and cropping system of rotation between upland sweet corn and paddy late rice  Note: Data from Shengzhou Guoyonghua family farm

2 Green and High-Value Cultivation Techniques for Sweet Corn

2.1 Variety selection

Sweet corn varieties with good quality, moderate plant height and short growth period, which can be eaten raw, were selected in this study such as Xuetian 7401 (Zheshenyu 2018003), Jinyin 208 (Hushenyu 2015009, Zhejiang Introduction [2017] No. 001) and so on (Figure 1).

Figure 1  Sweet corn varieties Note: A: Xuetian 7401; B: Jinyin 208

2.2 Timely sowing and cultivating strong seedlings

Generally, seedlings are grown in plugs or nutrient bowls in early and middle February. If large and small arched sheds are used for cultivation, seedlings can be grown in advance. Then strengthen seedbed management, pay attention to freezing damage or high temperature scalding seedlings, and keep the seedbed moist before emergence to ensure that the seedlings are healthy. After emergence, control watering to prevent overgrowth. Seedlings can be transplanted at 22~25 days with 3 leaves and 1 heart, and the film can be removed 3~5 days before transplanting (Figure 2).

Figure 2  Sweet corn seedlings

2.3 Spreading sufficient base fertilizer and planting closely

Before soil preparation, spread 15 000~30 000 kg/hm² farmhouse organic fertilizer or 3 000~6 000 kg/hm² commercial bio-organic fertilizer and 600 kg/hm² ternary compound fertilizer (N:P:K is 16:16:16, the same below). Deeply plow and cover them with film, and use degradable film if it permits. Two rows can be planted in each patch, and the density of transplanting is 45 000~50 000 plants/hm². After transplanting, water should be poured.

2.4 Field management and pest control

After the seedlings survive, 75 kg/hm² urea can be dissolved in water to fertilize the seedlings in the 4~5 leaf stage. And 300 kg/hm² urea can be flushed with water to fertilizer the ears in the flaring stage (8~10 leaves). Depending on the occurrence of pests and diseases, insecticides (chlorantraniliprole, emamectin benzoate, tebufenozide, lufenuron, indoxacarb, ethyl spinosad and chlorfenapyr as the main ingredients can be selected) and fungicides (azoxystrobin, difenoconazole, pyraclostrobin, and propiconazole as the main ingredients can be selected) can be mixed and sprayed at one time to control Pyrausta nubilalis and prevent Helminthosporium maydis, Rhizoctonia solani, Puccinia polysora Underw. and so on, especially Spodoptera frugiperda. Sweet corn is prone to tillering and all tillers should be removed. Generally, one ear is reserved for one plant, and the ears should be thinned in time when spinning.

2.5 Timely harvest

Sweet corn should be harvested in batches 20~25 days after spinning according to the appearance. After the ear filaments turn dark brown, the grains are fully swollen and plump, the color gets bright, and the grains become milky when squeezed, they should be harvested and placed in a cool and ventilated place. And they ought to be on the market as soon as possible to ensure the quality and taste.

3 Green and High-Value Cultivation Techniques for Late Rice

3.1 Selecting good varieties

It is advisable to choose indica-japonica or japonica rice varieties with large panicle type, medium tillering ability, strong lodging resistance, and excellent rice quality such as Yongyou 1540, Yongyou 4550, Yongyou 7850, Zhejing 96, Jiayou Zhongke 10 and so on.

3.2 Sufficient base fertilizer and soil preparation

After the sweet corn are harvested, the full amount of straw should be returned to the field, reducing the application of chemical fertilizers. 300~375 kg/hm² ammonium bicarbonate and 225 kg/hm² ternary compound fertilizer can be spread. Then plow, rake and flatten the field to make it to be leveled. Each patch is 3~4 m wide with operation ditch. Finally, the ‘cross’ shaped ditch and surrounding ditch should also be opened in the field.

3.3 Germination and sowing

It can be planted by direct seeding, throwing seedling or machine transplanting. In order to ensure rice safety and ear filling, the central Zhejiang region should be planted before June 28. Before sowing, it is better to sun the seeds for 1~2 d to improve the germination rate, and then select the seeds with muddy or salt water to remove impurities and chaff. The average seed consumption of hybrid rice is 12~20 kg/hm², and that of conventional rice is 60~90 kg/hm² in this field. The seeds are soaked with 2 000 times 25% phenamacril (Liangdi) or 1 500 times prochloraz (Shibaike) for 36~48 h. And then clean them with water to promote germination. When sowing, germinated rice can be mixed with 15 g 35% carbosulfan and 20 g 10% imidacloprid. After mixing, sow immediately to prevent insects and birds. After sowing, collapse the grain with shovel or broom. If possible, cover with the vegetable husks or wheat awns.

3.4 Seedling management

From sowing to turning green, the soil of direct seeding rice should keep moist. Before 3 leaves, the soil should be dry mainly with moist management and needs ventilation and oxygenation to promote root growth. After 3 leaves, shallow water layer should be established to promote tillering. At the time of two leaves and one heart, weaning fertilizer, that is 100~150 kg/hm² urea, should be spread. And chemical weeding should also be carried out according to the occurrence of weeds at this time. In the 3~4 leaf stage, make artificial seedlings and fill the gaps in time.

3.5 Field management

Keep shallow water to protect seedlings and promote tillering and drain paddy field lightly for many times at the right time. Before 6 leaves, shallow water management is the main way to promote early tillering and rapid growth. When the number of seedlings in the field reaches 80% of the expected number, drain the field (if the seedlings are enough early, the field should also be drain early). The method of draining paddy field for many times should be adopted (Zou, 2012). Drain the field from lightly to heavily, and the degree of draining requires that the edge of the field should be split, and the foot should not fall in the field (Shao et al., 2008). In 5~6 leaf stage (tillering), 90~150 kg/hm² compound fertilizer should be spread. In the round stalk jointing stage (the third leaf grows), 75~120 kg/hm² urea can be spread on the seedlings, and 120~150 kg/hm² potash fertilizer can be spread on the ears. In the beginning and full ear stage, disease control and pest control should be carried out, foliar fertilizer such as ‘Penshibao’ should be spread once or twice. Pay attention to controlling the Chilo suppressalis, Cnaphalocrocis medinalis, Nilaparvata lugens, Rhizoctonia solani and Ustilaginoidea oryzae. Farmers can also refer to local pest information to control timely. After full ears, intermittent irrigation ought to be implemented to keep the soil moist and keep the leaf and root healthy. Stop irrigation one week before harvest, and don’t be too early.

3.6 Timely harvest

When more than 95% of the rice grains are yellow and ripe, mechanical harvesting can be carried out. Don’t harvest too early to affect the seed setting rate, thousand-grain weight and rice quality.

4 Prospects for the Popularization and Application of Upland Rotation Technology

4.1 The significant effect on cutting costs and increasing income

Upland rotation is one of the main farming systems in the Yangtze River basin of China, and it plays an important role in continuously increasing the crop yield and guaranteeing the food supply. Yuan (1981) researched that upland rotation can increase crop yield by 5%~8%, even over 20%. Upland rotation can promote the development of rice roots, which helps to absorb the nitrogen, phosphorus and potassium in soil. After upland rotation, rice plant height, grain number, thousand-grain weight, and straw biomass are significantly improved, compared with traditional farming systems (Linh et al., 2015). In 2019, the use of chemical fertilizers on the sweet corn planting on Shengzhou Guoyonghua family farm reduced by 16.5% and the use of pesticides reduced by 10.7%. The labor for spraying and weeding reduced by 15 workers/hm². After rice rotation, the obstacles to continuous cropping of corn in the next season are basically eliminated, which can save labor cost by 2 250 yuan and chemical fertilizer and pesticide cost by 1 032.75 yuan per hectare.

4.2 Helping to cultivate healthy soil

Healthy soil is the foundation for cultivating high-yield and high-quality crops. In pursuit of high yields, farmers rely heavily on quick-acting chemical fertilizers, causing soil compaction, acidification, poor soil, and low fertilizer utilization, which are not conducive to crop growth and environmental safety. Upland rotation is a combination of land using and land cultivation, which is an effective agricultural measure to increase yield and improve quality of crops. A large number of experiments have shown that upland rotation can improve soil physical and chemical characteristics, increase soil aggregate structure, prevent soil acidification and salinization, and also have certain effects on soil bulk density and soil aggregates (Liu and Jiang, 2019). Besides, upland rotation can adjust the content of nitrogen, phosphorus, potassium and trace elements in soil, increase soil nutrients, and maintain the continuous yield increase (Fan et al., 2008). Upland rotation can also improve the soil aeration, increase the number of ammonifying bacteria, nitrifying bacteria and cellulose decomposing bacteria in soil, improve the microbial activity and community structure, promote the biochemical reaction of the soil, make the low fertilizer fungal soil change into high fertilizer bacterial soil, and improve the soil enzyme activity, which can promote crop growth and increase crop yield (Xiong et al., 2008).

4.3 The obvious effect on pest and weed control

Upland rotation is an effective measure to improve the ecological environment of farmland, change the field microclimate required for the growth of pathogenic bacteria in the soil, eliminate pathogens and eggs, and reduce diseases, insects and weeds. Upland rotation has more significant effects on controlling corn stalk rot disease (Luo and Kong, 1995), sheath blight (Ye et al., 2015), rice sheath blight (Chen and Wang, 1992), sclerotinia sclerotiorum of rape (Xu et al., 2012), and fusarium and verticillium wilt of cotton (Xu, 2003, Jiangxi Cotton, 25(2): 6-6). If the conventional farming system is adopted, the incidence rate of bacterial wilt, fusarium wilt and root knot nematode will increase by 12%, 16% and 8% respectively, compared with adopting upland rotation. Due to the different types and living habits of weeds in paddy fields and dry fields, upland rotation can significantly reduce the types and density of weeds in the field, and effectively prevent weeds from growing wildly. Wang et al. (2002) found that weed coverage of rice field decreased by 80% and weed coverage of corn field decreased by 24% during three consecutive years of upland rotation.

4.4 The prospects for application

Upland rotation technology is one of the important measures to increase yield, improve quality, increase the economic benefits of the planting industry, and ensure the sustainable development of agriculture, which shows good economic, social and ecological benefits. Upland rotation technology has been widely used in southern regions of China. In 2016, the upland rotation area of vegetable and rice in Zhejiang Province exceeded 58 400 hm², accounting for about 4% of grain planting area and 7% of the vegetable planting area in Zhejiang Province (Hu et al., 2016, Chinese Melons and Vegetables, 29(2): 52-56). In the single-season rice planting area of Zhejiang Province, the rotation between upland sweet corn and paddy late rice was promoted and achieved the goal of half-a-ton of grain and ten-thousand yuan of income per mu, which was listed as the main technology by Zhejiang Agriculture and Rural Affairs Department. The application area of the rotation between upland sweet corn and paddy late rice is about 2×10 ³ hm² in 2019, which is expanding year by year.

Agricultural production is a multi-factor integrated system. Upland rotation technology should not only fully consider the simplification of stubble arrangement of paddy field and dry field, but also the simplification of fertilizer management and operation. In order to pursue high yield, heavy application of chemical fertilizer often leads to overgrowth of the next crop because of excessive nutrient, which affects the yield or quality. During the rotation between upland sweet corn and paddy late rice, farmers should add some nitrogen fertilizer to decompose straw and pay attention to the effects of harmful substances produced in the process of decomposition on rice seedling stage. In the next step, the nitrogen, phosphorus and potassium nutrient cycling of the rotation between upland sweet corn and paddy late rice should be taken as the starting point to conduct in-depth research on the impact of upland rotation on the improvement of soil nutrient utilization, the cultivation of soil fertility and the ecological environment. Besides, the mechanism of crop yield and quality formation should be analyzed from the physiological level to provide theoretical support for the popularization and application of the technology of rotation between upland sweet corn and paddy late rice.

Authors’ contributions

ZFC, who was the executor and the person in charge of this project, wrote the first draft of the manuscript. CRX and ZZF were responsible for the implementation and writing of rice cultivation techniques. THP, HHL and BF carried out and modified the cultivation techniques of sweet corn. WGY, who was the conceiver and the person in charge of this project, proposed the design of the project and directed the writing of the manuscript. All authors read and approved the final manuscript.

Acknowledgements

This study was jointly funded by National Key R&D Program of China (2018YFD0200706), Major Special Project for Breeding New Agricultural (Grain) Varieties in Zhejiang Province (2016C02050-9-1), and Science and Technology Cooperation Program of ‘San Nong Liu Fang’ in Zhejiang Province (2020SNLF010).

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