INTRODUCTION

Most of Korea’s edible corn is made up of waxy corn and was grown in about 15,000 ha in 2019 (MIFAFF, 2019). Generally, the cultivation of waxy corn is conducted in the form of sowing in April and harvesting in mid-July and late July, so the harvest is concentrated in the summer, causing frequent price falls due to large quantity (Kim et al., 2014).

Many studies have been conducted on the timing of sowing to prevent such harvesting concentration. Park et al. (1987) showed that waxy corn exhibited a tendency to decrease in yield due to faster transition from vegetative growth to reproductive growth when the seeding period was late. In addition, the yield of waxy corn was reduced in terms of sowing on May 27^th compared to April 15^th when it was grown in Hongcheon, Gangwon-do (Yoon et al., 1999) although the critical sowing period may be delayed due to recent temperature increases. Global warming is a risk factor for deepening weather disasters, including pests; on the other hand, it is also a window of opportunity due to the increased period of crop cultivation. Because the period of growth of vegetable waxy corn is relatively short, it is possible to establish the double cropping system, which is expected to increase the utilization rate of land and to improve income.

However, the lower yield and marketability of vegetable waxy corn have been pointed out as problems due to the lack of technology for the cultivation of a double cropping system (Jung et al., 2012). The present method of cultivation in the double cropping system of the newly developed and distributed waxy corn has diversified its harvest time compared with the past method. Recent studies have shown that the timing of vegetable waxy corn supply has been extended from the early part of July to the early and middle part of October by mens of advancing the sowing time of the first cropping and delaying the sowing time of the second cropping (Jung et al., 2012; Kim et al., 2000).

In addition, the use of slow-releasing fertilizer once when growing double cropping waxy corn can reduce labor and environmental pollution due to low nitrogen loss.

In previous studies, the use of slow-releasing fertilizers increased the plants’ dry weight by increasing the use of nitrogen over more effective fertilizers (Choi et al., 2002).

In the study of the crop application of slow-releasing fertilizer, Yoo et al. (1998) and Chi et al. (2013) reported that the rate of nitrogen utilization was the highest at 50 days after the nitrogen treatment and that nitrogen was slowly released based on the latex-coated fertilizer.

Therefore, this study was conducted to investigate the effects of the slow-releasing fertilizer and seeding date on waxy corn propagated through double cropping without tillage. It also compared the ear yields according to the sowing date with “Mibaek 2” as a test variety in order to investigate the effect of the slow-releasing fertilizers.

MATERIALS AND METHODS

Study field characteristics

This experiment was conducted from 2018 to 2019 at the Chungcheongbuk-do Agricultural Research and Extension Services, and the test waxy corn hybrid was called “Mibaek 2”, a crop mostly cultivated as an early maturing type in the central region of Korea. “Mibaek 2” was sown for first cropping on March 25^th, April 5^th, and April 15^th and for second cropping on July 5^th, July 15^th, and July 25^th six times overall, at 10-day intervals. The slow-releasing fertilizer employed in this study was Latex Coating Urea (LCU), which was used as a full-scale fertilizer once before vinyl mulching.

Fertilizer application and crop cultivation

The level of distribution was different depending on the soil testing-based recommendation (STBR), which was N-P₂O₅-K₂O = 16-3-3 kg/10a. The fertilizer level of the control was N- P₂O₅-K₂O = 30-3-6 kg/10a, with nitrogen separated twice in the first cropping and second cropping. 50% addition to soil testing- based, recommended fertilization (150% STBR) increased the fertilizer level by 1.5 times, while 100% addition to soil testing- based, recommended fertilization (200% STBR) increased the fertilizer level by 2 times. With the aim of producing 6,600 plants per 10a, the sowing was set to 60 cm in width and 25 cm in length, while planting density was maintained uniformly by thinning out and with only one head left in the growing season. The placement of the test tools was tested through a three- repeated split-plot arrangement method, with the seeding dates as a main plot and fertilizer levels as a sub plot. Plant height was investigated in accordance with the Agricultural Science and Technology Research and Analysis Standards of the Rural Development Administration about a week before the harvest. The characteristics of ears were averaged about 26 days after silking by way of harvesting the waxy corn ears, removing the leaves, and investigating 10 pieces of ear weight, ear length, seed set ear length, and ear diameter.

Soil and statistical analyses

Soil samples were taken by a hand spiral auger at growing season. Collected soil samples were air-dried at room temperature for chemical quantification. Soil pH and EC were determined from the 1:5 of soil/water suspension using a pH/conductivity meter (Horiba, F-54). Available phosphorus was analyzed by Lancaster method employing continuous flow spectrometer (Bran + Luebbe Analytics, Autoanalyzer 3). The 1N-NH₄OAC (adjusted pH 7.0) was utilized to extract exchangeable cations (K, Ca, Na and Mg) and quantified by inductively coupled plasma spectroscopy (PerkinElmer, Optima 3300DV). Experimental data were statistically analyzed through the analysis of variance (ANOVA) with the Raleigh program (SAS Inc. ver. 9.2, USA).

RESULTS AND DISCUSSION

Soil nutrient content

Prior to the test, the pH of the soil was slightly acidic at 6.5, and the organic content was 23g/kg, showing the distribution of the average organic content in the central region (Table 1). After the test, the soil showed an increase in pH value and soil salinity compared to the period prior to the test.

Growth of first cropping

There were no differences in the tasseling dates and silking dates according to the fertilizer levels but the days from planting to emergence was shortened as the sowing period was delayed (Table 2). Thus, the sowing on April 15^th shortened the days to silking by 7 days compared to the sowing on April 5^th and by 12 days compared to sowing on March 25^th.

It was thought that this was due to the delay in sowing period, which led to the arrival of the silking date of waxy corn in the range of 1,400-1,420°C, and that these results were consistent with the findings of Kim et al. (2014) who reported that silking dates from the seeding dates of the “Ilmichal” varieties were about 70 to 78 days.

Culm height and ear length had no statistical significance, but the kernel set length was the highest at 123 cm in the sowing date on April 5^th (Table 3). In addition, the seed set ear length was 17.8 cm long in the sowing date on April 5^th and April 15^th, indicating that the later the sowing period is, the more marketable it becomes.

Nielsen (2007) showed that the grain line number of corn had a strong genetic effect, while the ear length and the seed set ear length had a great influence on the cultivation environment.

This test also showed that the kernel set length and seed set ear length varied depending on the sowing time.

Culm height, kernel set length, ear length, and seed set ear length tended to elongate in 100% addition to soil testing-based recommended fertilization, but there was no statistical significance. Park et al. (2015) reported that as nitrogen content increased, chlorophyll content increased until the late stages of corn production, resulting in high light utilization, whereas_;Lim et al. (2014) showed that there was no significant difference in culm height, kernel set length, and ear length in the treatment zones of 18 kg and 36 kg of nitrogen per 10a.

However, despite the increased volume of the fertilizer, the fact that it did not show much difference in growth was attributed to the ingredients of the slow-release fertilizer being slowly eluted and having no significant impact on the first cropping production.

The weight of marketable ear was the heaviest in the sowing date on April 5^th, although ear weight and number of marketable ears per 10a showed no statistical significance (Table 4).

The number of marketable ears will be the most important factor in income, as early harvests can get high prices while corn sales are most common in 30 single-sack units.

Therefore, it was thought that the sowing time for the first cropping propagated through double cropping system in the central region of the country could be advanced by March 25^th.

Early April is reported to be the economic sowing limit of the super sweet corn hybrid in southern Korea (Yang et al., 2007) while it is mid-May in central and northern Korea (Sim, et al., 2019).

Therefore, given that sowing early at an appropriate time increases the number of marketable ears compared to late sowing, the sowing time could be advanced with global warming if conditions are not to advance sowing to an extreme degree.

Moreover, due to the increase in the fertilizer levels, there was no statistical significance in the ear weight and the number of marketable ears, but the weight of marketable ear was the highest when there is in the 100% addition to soil testing-based, recommended fertilization.

Growth of second cropping

There was no difference in days in terms of tasseling and silking according to the fertilizer levels but the days from planting to emergence was shortened as the sowing period was delayed (Table 5).

In this test, the longer the sowing date was delayed, the faster the silking date was. This is consistent with the report of Jung et al. (2012) which stated that the sooner the seeding period was, the faster it was by 33 days for the “Ilmichal” variety.

The possible reason for this decrease in silking days may be the presence of differences in photoperiod and temperature among sowing dates.

As the sowing date was delayed, the culm height, kernel set length, ear length, and seed set ear length were shortened, with the decline being the largest in the sowing date on July 25^th, the latest sowing date (Table 6).

These results are due to the shorter day effect of late sowing during the growth period of the waxy corn and supported by Jung et al. (2012) who reported that growth temperature was decreased during the ripening stage of second cropping.

In particular, the large decrease in the ear length and seed set ear length in the sowing date on July 25^th was thought to have a significant impact on the decline in the marketability of waxy corn.

The culm length and the kernel set length tended to increase as the fertilizer levels increased, but there was no statistical significance, so it was thought that a one-time, slow-releasing fertilizer application could result in a reduction in labor force without any difference in growth compared with the conventional distribution. The characteristics of culm length and ears differed greatly in the first cropping and the second cropping, showing 69-83% of the previous cropping in culm length, 80-90% in ear length, and 67-88% in seed set ear length.

In the second cropping, growth occurred under higher temperatures, with associated reductions in the duration of growing cycles (Jung et al., 2012; Kim et al., 2014).

Also, as the sowing date was delayed, the diameter of the ears was reduced, and the weight of marketable ears and the number of marketable ears decreased. At the sowing date of July 25^th, especially, the decline was significant (Table 7).

Statistical analysis of the data revealed that marketable ear yield of waxy corn was significantly affected by sowing date and fertilizer. Waxy corn plated on July 5^th produced more yild (1,059 kg/10a), while the July 25^th sowing produced the lowest marketable ear yield (506 kg/10a).

In particular, since the income of waxy corn is determined by the number of marketable ears (Lee & Kim, 1986), it was thought that the sowing of the second cropping in the cultivation of double cropping without tillage in the central region of the country should be completed by July 15^th to secure the yield . The diameter of the ear and the average ear weight were not statistically significant, depending on the difference in the levels of the fertilizers.

However, the weight of marketable ears and the number of marketable ears were high in the 100% addition to soil testing- based recommended fertilization. It was also thought that it would be appropriate to add 100% to the soil testing-based recommendation for slow-releasing fertilizer. Lim et al. (2014) reported that as the ratio of nitrogen increases, photosynthesis also increases, showing a defined correlation with the content of follicle nitrogen. In this test, the weight of marketable ears and the number of marketable ears differed significantly between each treatment due to an increase of up to 32kg of nitrogen per 10a.

Therefore, it was thought that 100% addition would not only reduce the labor force, but also increase the number of marketable ears, which would be advantageous for farming households.

However, since the effects of the fertilization are thought to vary depending on weather and soil conditions, it was thought that additional fertilizer spraying in the double cropping of waxy corn should be determined by the examination of the growth of the waxy corn.