The Effects of Planting Date and Tillage Practice on Growth and Yield of Maize and Soybean in Rotation with Winter Onion

Abstract

Introducing summer staple crops to diversify conventional summer paddy rice (Oryza sativa L.) and onion (Allium cepa L.) rotation is important for sustainable agriculture. Herein, we evaluate the effects of planting date (early June to late July) and tillage practice (deep cultivation, DC; conventional tillage, CT) on two maize and soybean cultivars over 4 years (2018–2021) in converted paddy soil in Korea. Due to the growing degree-day differences, the yields of June and July planted crops were 7050 and 5554 total digestible nutrient kg ha⁻¹, respectively, for silage corn, and 7410 and 6473 marketable fresh ear kg ha⁻¹, respectively, for waxy corn. Delaying soybean planting from June to July significantly reduced crops’ yield and interfered with field preparation for the following winter cash crops. The June and July planting yields were 2672 and 2090 kg ha⁻¹, respectively, for large-grain soybean, and 2416 and 1861 kg ha⁻¹, respectively, for small-grain soybean. Deep tillage had no effect on summer crops yield. Additionally, the summer crop growing practice had no residual effect on onion yield. Our study recommends mid to late June planting for growers who wish to introduce maize and soybean in rotation with winter onion in the region.

1. Introduction

Cropping system diversification is important for sustainable agriculture. Onion (Allium cepa L.) is a major winter cash crops in South Korea grown in rotation with paddy rice (Oryza sativa L.) [1,2,3]. Farmers have adopted plastic mulching for onion seedling transplanting during the winter growing season and transplanted early maturing rice cultivars [2]. However, onion harvest followed by rice transplanting in mid-June does not allow for a sufficiently long grain filling period, thereby lowering grain quality [4,5]. Intensive fertilization during onion cultivation oversupplies soil nutrition for rice production, resulting in a low-quality rice production [2,5,6].

In Korea, maize is primarily used for silage and fresh consumption [7] and has a relatively short growing period; hence, it is a practical candidate for a winter cash crops–summer upland crops rotation [8]. Meanwhile, soybean is used for various purposes such as production of tofu and soy sauce and sprout, among others [9]. The optimal and marginal planting date period must be established to improve winter cash crops–summer upland food crops rotation [10].

For instance, upon completion of onion harvest, farmers require 1–2 weeks of field preparation period for the following summer crops. During the summer monsoon season (mid to late July in Korea), it is difficult to operate machinery for seed planting on upland fields, as the soil becomes muddy. Additionally, heavy rainfall during the early developmental stage of summer upland crops remains challenging, although data on the effect of heavy rainfall at an early growing stage on summer crops remain limited. Therefore, it is critical to determine the optimal planting date and planting window. In Korea, onion seedlings are typically transplanted in October and harvested late May to early June of the following year. Considering crops rotation, summer maize and soybean are planted after winter onion harvest. To maximize land use efficiency for optimal yield, onion harvest must not overlap with maize and soybean planting. As winter onion is an important cash crops, summer maize and soybean cultivation season should be completed prior to onion seedling transplant. However, delayed planting date decreases the nutrient value and yields of silage corn [11,12]. For instance, delayed planting offers a high temperature during vegetative growth, with a relatively short vegetative period, thereby lowering plant canopy height and reducing leaf number [13,14]. Delayed planting also reduces the growing degree-days (GDD) of maize during reproductive period [15]. Hence, different environmental conditions associated with the different planting dates affect the overall maize yield [16]. Hu and Wiatrak [17] indicated that reduced vegetative and reproductive stages associated with a delayed planting date contribute to yield loss in soybean, which is sensitive to the photoperiod; the authors also reported that appropriate planting date is essential for soybean canopy development and a high grain yield. However, the effect on grain yield may vary due to the influence of various environmental factors [18,19]. Regardless of the soybean maturity group, appropriate planting date is essential for a high yield, by extending the duration of plant development stages and avoiding cooler temperature during seed filling [19].

Unlike rice, maize does not develop aerenchyma, which alleviates waterlogging stress [20,21]. Further, one difficulty of soybean production is its susceptibility to logging water stress, which commonly occurs in a converted paddy field [22,23]. Waterlogging stress during the vegetative stage leads to 17–43% reduction in soybean yields, with 50–56% yield reduction upon waterlogging stress during the reproductive stage [24,25]. Hence, proper management practices are needed for a converted field (lowland paddy to upland field), such as deep cultivation, to improve vertical infiltration. During deep tillage, the topsoil and subsoil are mixed [26]. Therein, plant root can access subsoil resources by reducing soil mechanical strength and enhance water infiltration, as compared to a conventional shallow tillage [27]. In addition, continuous shallow tillage (conventional or rotary tillage) increases subsoil soil bulk density and limits water and nutrient accessibility [28,29,30]. Implementation of deep tillage in upland soil reduces penetration resistance, following increased water infiltration [31]. However, the effects of tillage practice are site specific and remain to be assessed in maize and soybean with onion rotation in Korea.

Therefore, the objectives of the current study are to (1) evaluate the effect of planting date on summer upland crops (maize and soybean) growth and yield in rotation with winter onion; (2) evaluate the environmental conditions associated with the planting date; (3) investigate the effect of different tillage practices on the yield of summer and following winter crops.

2. Materials and Methods

2.1. Research Sites and Field Management

The field research was conducted in the Gyeongsang National University (GNU) Research Farm in Sacheon, Korea (35.11° N 128.12° E), for 3 years (2018–2020). Additional on-farm trial performed in 2021, on a farm located 85 km NE from the GNU Research Farm, Goryeong, Korea (35.82° N 128.41° E; hereafter referred to as the “farmer’s field”).

Historically, the GNU Research Farm has been used for sole lowland rice production and converted to upland for the current study. The famer’s field trial site has been used for rice–onion rotation for a decade. The weather data during the 4 years of research were obtained from nearby Korea Meteorological Agency stations (available at https://data.kma.go.kr, in Korean, accessed on 18 July 2022). GDD is reported in

Table 1. Growing degree-days (GDD) during summer growing season for each planting date.

Year	Planting Date (m/d)	GDD (°C)
		5WAP	7WAP	9WAP	12WAP		At Harvest (WAP †)
							Kwangpyeongok	Ilmichal	Daewon	Haepum
2018	6/10	556	850	1198	1414		1775 (15)	1533 (12)	1865 (20)	1865 (19)
	6/25	651	998	1215	1487		1660 (15)	1584 (13)	1666 (18)	1666 (17)
	7/10	781	997	1270	1438		1449 (16)	1443 (13)	1449 (16)	1435 (16)
	7/25	715	987	1155	1166		1169 (15)	1166 (14)	1166 (14)	1148 (14)
2019		4WAP	6WAP	8WAP	11WAP		Kwangpyeongok	Ilmichal	Daewon	Haepum
	6/15	473	731	1040	1263		1530 (13)	1363 (11)	1847 (19)	1777 (17)
	6/30	523	831	1055	1298		1538 (14)	1419 (12)	1645 (17)	1606 (16)
	7/15	597	820	1063	1334		1334 (13)	1303 (12)	1410 (15)	1404 (15)
2020		4WAP	6WAP	8WAP	10WAP	12WAP	Kwangpyeongok	Ilmichal	Daewon	Haepum
	6/15	497	684	907	1137	1337	1384 (13)	1265 (11)	1631 (18)	1631 (17)
	6/30	462	685	915	1115	1286	1317 (12)	1286 (12)	1409 (16)	1409 (15)
	7/15	473	703	903	1074	1161	1197 (13)	1197 (12)	1197 (13)	1197 (13)
2021		4WAP	6WAP	8WAP	10WAP	12WAP	Kwangpyeongok		Daewon
	6/15	558	861	1093	1293	1467	1467 (12)		1835 (18)
	6/30	657	889	1089	1263	1443	1442 (12)		1631 (17)
	7/15	649	848	1023	1202	1391	1391 (13)		-

† Weeks after planting.

. Soil samples were collected at a 30-cm depth using a soil auger (diameter, 2.54 cm) before (spring) and after (fall) summer crops planting. Soil samples were collected from the field in a “W” pattern and then homogenized, while soil surface bulk density was randomly sampled. Environmental conditions data are provided in the Supplementary File (Supplementary Figure S1 and Tables S1 and S2). To improve horizontal drainage, a 40-cm deep ditch was created at the experiment field border using a small excavator (SV08, Yanmar, Osaka, Japan) on GNU Research Farm. Seeds were planted either side, on the ridge, with 70-cm inter-row spacing. Intra-row spacing was 30 cm for maize and 10 cm for soybean (Figure 1). Maize seeds were sown at one seed per hill (48,000 seed ha⁻¹), while soybean seeds at two seeds per hill (286,000 seed ha⁻¹) using hand-pushed disk planter (TP-10RA, Agritechno Yazaki Korea, Cheongju, Korea). Split application of fertilizer was implemented at 100–150–150 kg ha⁻¹ at seed bed preparation, and 100–0–0 kg ha⁻¹ at V5 for maize. A small amount of starter fertilizer was applied prior to soybean planting (30–30–30 kg ha⁻¹). Two different maize cultivars were selected for the 3-year experiment on GNU Research Farm namely ‘Kwangpyeongok’ (silage corn) and ‘Ilmichal’ (specialty fresh waxy corn for humans’ consumption), both of which are widely used in Korea. Two different soybean cultivars were selected namely ‘Daewon’ (large-grain, a commonly used soy sauce in Korea) and ‘Haepum’ (small-grain, a commonly used soy sprout in Korea). After the 3-year GNU Research Farm field trial, silage corn Kwangpyeongok and large-grain soybean Daewon were selected for on-farm trial in 2021, due to better overall performance and stable yield compared with the other cultivars, respectively. Herbicide alachlor (1.5 kg a.i. ha⁻¹) was applied immediately following planting for weed control. Other management practices were also conducted following the guidelines of agricultural science technology standards for investigation of research of the Korea Rural Development Administration (RDA, 2003).

2.2. Tillage Treatments and Planting Dates

During 2018–2020, the experimental design was a randomized complete block design with split plot arrangement; the main plot was tillage practice, while the sub-plot was planting date. Tillage practice treatments were conventional tillage (CT, rotovating with tractor-mounted rotary tiller) and deep tillage (DT, plowing up to 30 cm depth, then rotovating with tractor-mounted rotary tiller). For the 2021 farmer’s field trial, deep tillage was not used as no physiological, while agronomical differences associated with tillage type were observed during the 2018–2020 GNU Research Farm field test (see Section 3.3 for details). The planting dates tested during the first year (2018) were 10 June, 25 June, 10 July, and 25 July (hereafter 6/10, 6/25, 7/10, and 7/25) and later adjusted to 15 June, 30 June, and 15 July (hereafter 6/15, 6/30, and 7/15) based on the first-year crops performance. The size of each plot was 5.6 × 6 m, with three replications. The experimental design was randomized complete block design with three blocks and a single factor (planting date only) with CT only in the 2021 experiment. The size of each plot was 7 × 13 m, with three replications.

2.3. Canopy Height and Leaf Area Index (LAI) Characteristics

Data were collected at approximately 2-week intervals from 4–5 weeks after planting (WAP) to 12WAP. Canopy height was the average undisturbed height of plants determined based on 10 measurements per plot. Maize and soybean LAI measurements were obtained from the two center rows to minimize the border effect. The LAI of each plot was recorded using a LI2200-C canopy analyzer (Li-Cor BioSciences, Lincoln, NE, USA) with a 90° view angle cap and six under-canopy readings in a “W” pattern.

2.4. Yield and Yield Components

The biomass yield of silage corn was determined by harvesting an area of 2.8 × 4 m from the center two rows of each plot. The collected corn shoots were manually divided into stalk and ear components. The plant mass was then dried at 70 °C until constant weight was achieved to determine DM yield. Total digestible nutrients (TDN) were calculated using the equation TDN = (DM yield of maize stalk × 0.582) + (DM yield of maize ear × 0.85) [32], where 0.582 and 0.85 are constants [32,33]. All ears of waxy corn planted in the two adjacent rows were collected and counted in a non-destructive manner to determine the average ear length, ratio of kernel set length to ear length (RKSLEL), number of ears, and marketable fresh ear yield (for ears ≥ 10 cm; 2003 RDA). For soybean, 10 plants from each plot were assessed to determine the number of pods per plant and seeds per plant. Soybean grain yields were determined by harvesting an area of 2.8 × 4 m from the center two rows of each plot adjusting for 14% moisture content. Three replicate measurements of 100-seed weights were obtained for each plot.

2.5. Onion Yield and Yield Components

To compare the residual effect of tillage practice and planting date on the following winter crops in the rotation system, onion agronomic characteristics were determined for 2 years (2018–2019 and 2019–2020) on the GNU Research Farm. Fresh marketable bulb yield and average bulb weight were determined for plants in the central 2.8 × 4 m area of the plots.

2.6. Statistical Analysis

PROC UNIVEARIATE function of SAS 9.4 software (SAS Institute Inc., Cary, NC, USA) was used to assess the normality of data distribution. Plant canopy height, LAI, yield, and yield components were analyzed using PROC MIXED model. Tillage treatment (2018–2020), planting date, and interaction between tillage and planting date were treated as fixed effects. WAP was considered repeated measure (fixed). The block was treated as a random effect. Cultivar was not included in the statistical analysis since the purpose of usage was different, and neither was the year since soybean and maize are annual crops, which were randomized each year. Least significant difference (LSD) test (significance threshold of p = 0.05) was performed for mean separation.

3. Results

3.1. Effect of Planting Date and Tillage Practice on Maize Canopy Height and LAI

We analyzed the tillage × planting date effects on the canopy height of two maize cultivars (Kwangpyeongok, 2018–2021; Ilmichal, 2018–2020) using repeated measures, over 4 years and observed no significant difference with tillage practice (

Table 2. Source of variation and probability level (p-value) of plant canopy height and LAI for two maize cultivars (Kwangpyeongok and Ilmichal) over 4 years.

Year	Source of Variation	Plant Canopy Height		LAI
		Kwangpyeongok	Ilmichal	Kwangpyeongok	Ilmichal
2018	Tillage (T)	0.1612 †	0.5792	0.4118	0.6223
	Planting date (P)	0.0002	0.0005	0.0443	0.0025
	T × P	0.6149	0.4209	0.4781	0.7257
	WAP ‡ (W)	<0.0001	<0.0001	0.0015	0.0003
	T × W	0.7547	0.8250	0.7412	0.5652
	P × W	0.0031	0.0226	0.0793	0.0045
	T × P × W	0.8759	0.8700	0.8648	0.4869
2019	T	0.6699	0.7140	0.8743	0.9870
	P	0.0026	0.0704	0.9957	0.4531
	T × P	0.3883	0.9842	0.9244	0.7142
	W	<0.0001	<0.0001	0.0106	0.0448
	T × W	0.5184	0.9690	0.9675	0.8472
	P × W	0.0003	0.0950	0.0786	0.3081
	T × P × W	0.9982	0.9987	0.9369	0.9706
2020	T	0.2939	0.2539	0.8234	0.7451
	P	0.0037	0.0016	0.0119	0.0117
	T × P	0.4380	0.3259	0.3284	0.2086
	W	<0.0001	<0.0001	0.0020	0.0007
	T × W	0.7946	0.9344	0.5150	0.2701
	P × W	0.0026	0.0003	0.0079	0.0933
	T × P × W	0.8876	0.9919	0.3857	0.9510
2021	P	0.6271	-	0.8288	-
	W	<0.0001	-	<0.0001	-
	P × W	<0.0001	-	0.0051	-

† Significance evaluated at 0.05 probability level; ‡ Weeks after planting.

). The effect of planting date and tillage on plant canopy height is listed in

Table 3. Plant canopy height of two maize cultivars (Kwangpyeongok and Ilmichal), as affected by the planting date (6/10, 6/25, 7/10, and 7/25 in 2018; 6/15, 6/30, and 7/15 in 2019–2021) and tillage practice (deep tillage, DT; conventional tillage, CT).

Year	Maize Cultivar	Treatment	Canopy Height (cm)
			5WAP †	7WAP	9WAP	12WAP
2018	Kwangpyeongok	6/10	85.2 a ‡ C §	148.8 abB	209.8 aA	215.1 aA
		6/25	79.1 aC	160.1 aB	178.2 bA	190.7 bA
		7/10	94.4 aC	135.0 bB	209.2 aA	210.4 aA
		7/25	40.5 bC	114.8 cB	177.0 bA	171.0 cA
		CT	80.1 ns	141.5 ns	197.1 ns	198.2 ns
		DT	69.5	137.9	189.9	195.3
	Ilmichal	6/10	102.0 aB	168.3 aA	176.8 aA	177.6 aA
		6/25	74.0 bB	153.4 abA	148.2 bA	141.2 bcA
		7/10	110.5 aB	137.2 bcA	150.8 bA	152.0 bA
		7/25	41.2 cB	119.7 cA	139.8 bA	129.6 cA
		CT	80.9 ns	148.4 ns	155.3 ns	150.7 ns
		DT	82.9	140.9	152.5	149.5
2019	Kwangpyeongok		4WAP	6WAP	8WAP	11WAP
		6/15	54.7 cC	120.1 bB	198.7 bA	202.2 aA
		6/30	69.9 bC	131.5 bB	212.8 aA	200.7 aA
		7/15	95.0 aC	166.5 aB	197.0 bA	-
		CT	73.1 ns	141.2 ns	205.5 ns	200.1 ns
		DT	73.3	137.5	200.2	202.8
	Ilmichal	6/15	49.5	108.4	170.1	169.3
		6/30	61.6	129.4	184.9	177.8
		7/15	78.0	126.0	162.4	-
		Avg.	63.0	121.3	172.5	173.6
		CT	64.8 ns	122.9 ns	172.5 ns	173.3 ns
		DT	61.3	119.5	172.5	173.9
2020	Kwangpyeongok		4WAP	6WAP	8WAP	10WAP	12WAP
		6/15	80.5 aD	136.2 aC	204.7 aA	201.2 aA	175.7 aB
		6/30	55.5 bD	97.1 bC	164.7 cB	185.3 bA	179.3 aB
		7/15	47.5 bC	91.4 bB	183.6 bA	186.1 abA	183.4 aA
		CT	62.3 ns	109.3 ns	186.8 ns	195.8 ns	179.3 ns
		DT	60.0	107.1	181.9	186.0	179.6
	Ilmichal	6/15	76.7 aC	135.8 aB	181.9 aA	179.2 aA	146.0 aB
		6/30	46.9 bC	93.0 bB	156.0 bA	165.4 bA	158.7 aA
		7/15	49.4 bC	90.2 bB	156.5 bA	157.1 bA	158.0 aA
		CT	59.7 ns	108.8 ns	167.2 ns	167.8 ns	154.6 ns
		DT	55.6	103.9	162.4	166.6	153.8
2021	Kwangpyeongok		4WAP	6WAP	8WAP	10WAP	12WAP
		6/15	89.8 bD	198.4 bC	258.3 aB	272.0 aA	262.9 aB
		6/30	124.4 aC	211.1 aB	253.4 abA	247.1 cA	255.0 aA
		7/15	130.1 aD	188.5 cC	247.5 bB	258.4 bA	257.8 aA

† Weeks after planting; ‡ Within each WAP, values followed by different lowercase letters are significantly different among treatments at p ≤ 0.05; ns, not significant. § Within each planting, values followed by different uppercase letters are significantly different among treatments at p ≤ 0.05; ns, not significant.

.

For Kwangpyeongok (silage) corn, in 2018, planting date × WAP interaction (p = 0.0031) was apparent. The canopy height of the crops from the 7/25 planting was consistently low, the lowest among other plantings throughout the growing season. Crops from different planting dates maintained high canopy height from 9 to 12WAP. In 2019, planting date × WAP interaction (p = 0.0003) was detected. In the early growing stage, the canopy height of crops from the 7/15 planting was the greatest, but as the growing season progressed, the difference between crops from different plantings decreased. In 2020, planting date × WAP interaction (p = 0.0026) was observed. Crops from the 6/15 planting was the tallest at 8 to 10WAP, while the canopy height decreased at 12WAP. For silage corn from the farmer’s field in 2021, planting date × WAP interaction (p < 0.0001) was also observed. The canopy of crops from the 6/15 planting was smaller than that of crops from other plantings at 4WAP, however from 8WAP, it was consistently the highest among the plantings.

For Ilmichal (waxy) corn, in 2018, planting date × WAP interaction (p = 0.0226) was apparent. The canopy height of the 6/10 planting was consistently the greatest among crops from other plantings throughout the growing season. By contrast, only the main effect of the repeated measure (p < 0.0001) was observed in 2019. Similar to 2018, planting date × WAP interaction (p = 0.0003) was observed for 2020. The canopy height of crops from the 6/15 planting was greatest at 8–10WAP, and decreased at 12WAP. The canopy height of crops from the 6/30 and 7/15 plantings was stable from 8 to 12WAP.

We also analyzed the tillage × planting date effects on LAI of the two maize cultivars (Kwangpyeongok, 2018–2021; Ilmichal, 2018–2020) using repeated measures, over 4 years (Table 2). No significant difference with tillage practice was observed for silage corn (Table 2). The effect of planting date and tillage on LAI is shown in

Table 4. LAI of two maize cultivars (Kwangpyeongok and Ilmichal), as affected by the planting date (6/10, 6/25, 7/10, and 7/25 in 2018; 6/15, 6/30, and 7/15 in 2019–2021) and tillage practice (deep tillage, DT; conventional tillage, CT).

Year	Maize Cultivar	Treatment	LAI
			5WAP †	7WAP	9WAP
2018	Kwangpyeongok	6/10	0.40 ns	0.91 b	2.75 ns
		6/25	0.32	1.58 b	2.44
		7/10	0.97	2.52 a	N/A ¶
		7/25	0.95	N/A ¶	2.19
		CT	0.65 ns	1.66 ns	2.23 ns
		DT	0.67	1.68	2.69
	Ilmichal	6/10	0.49 b ‡ C §	1.15 bB	3.25 aA
		6/25	0.31 bC	1.36 bB	1.98 bA
		7/10	1.09 aB	2.61 aA	N/A
		7/25	077 abB	N/A	2.08 bA
		CT	0.71 ns	1.70 ns	2.31 ns
		DT	0.62	1.70	2.57
2019			4WAP	6WAP	8WAP
	Kwangpyeongok	6/15	0.84	2.35	2.71
		6/30	1.92	2.04	2.25
		7/15	1.58	2.06	2.38
		Avg.	1.45	2.15	2.45
		CT	1.49 ns	2.13 ns	2.42 ns
		DT	1.40	2.16	2.47
	Ilmichal	6/15	1.09	2.34	2.66
		6/30	1.75	1.83	1.98
		7/15	1.56	2.08	2.43
		Avg.	1.47	2.08	2.36
		CT	1.57 ns	2.09 ns	2.25 ns
		DT	1.36	2.07	2.46
2020			4WAP	6WAP	8WAP	10WAP	12WAP
	Kwangpyeongok	6/15	1.39 aC	2.85 aA	3.05 aA	2.24 aB	1.79 aBC
		6/30	1.07 aB	1.74 bA	1.70 bA	1.79 aA	2.17 aA
		7/15	0.98 aB	1.50 bAB	1.59 bA	2.03 aA	1.85 aA
		CT	1.10 ns	2.09 ns	2.07 ns	2.21 ns	1.85 ns
		DT	1.20	1.98	2.16	1.84	2.01
	Ilmichal	6/15	1.33 ns	2.84 a	2.95 a	2.09 ns	N/A
		6/30	1.02	1.83 b	2.04 b	1.76	2.10 ns
		7/15	1.33	1.71 b	1.80 b	1.77	2.04
		CT	1.12 ns	1.92 ns	2.32 ns	2.00 ns	2.07 ns
		DT	1.33	2.33	2.22	1.74	2.07
2021			4WAP	6WAP	8WAP	10WAP	12WAP
	Kwangpyeongok	6/15	1.78 aC	2.66 aB	2.86 aB	3.25 aA	2.75 aB
		6/30	2.04 aB	2.37 aB	2.77 aA	2.86 aA	2.90 aA
		7/15	1.86 aD	2.65 aB	3.18 aA	3.01 aAB	2.26 bC

† Weeks after planting; ‡ Within each WAP, values followed by different lowercase letters are significantly different among treatments according to LSD test at p ≤ 0.05; ns, not significant. § Within each planting treatment, values followed by different uppercase letters are significantly different among treatments according to LSD test at p ≤ 0.05; ns, not significant. ¶ Not applicable: LAI could not be measured because of continuous rainfall events or plots were harvested (12WAP only).

.

For the Kwangpyeongok maize in 2018, a significant main effect of planting date on LAI was noted (p = 0.0443), although this was not significantly different for tillage practice (p = 0. 4118). LAI of crops from the 7/10 planting was the highest at 7WAP, however no difference among crops from the different plantings at 5 and 9WAP was apparent. In 2019, only the main effect of repeated measure (p = 0.0106) was detected, although not significantly different with planting date (p = 0.9957) or tillage practice (p = 0.8743). In 2020, planting date × WAP interaction (p = 0.0079) was observed due to a robust LAI increase of the crops from the 6/15 planting between 6 and 8WAP, followed by a decrease between 10 and 12WAP. LAI of crops from the 6/30 and 7/15 plantings was relatively stable from 6 to 12WAP. In 2021, a significant planting date × WAP interaction (p = 0.0051) was observed. Crops from the 6/15 planting reached a maximum LAI at 10WAP, which then decreased; however, LAI of crops from the 6/30 planting remained high from 8 to 12WAP. Finally, LAI of crops from the 7/15 planting reached a maximum at 8WAP and then significantly decreased.

During the 3 years of field research, no significant difference by tillage practice was apparent for the waxy corn Ilmichal (Table 2). In 2018, planting date × WAP interaction (p = 0.0045) was detected due to a relatively delayed LAI development of crops from the 6/10 and 6/25 plantings early in the season (5WAP), with the highest overall LAI of crops from the 6/10 planting late in the season (9WAP). Conversely, in 2019, no significant difference with planting date (p = 0.4531) or tillage practice (p = 0.9870) was detected, while in 2020, a significant main effect of planting date (p = 0.0117) was apparent, but with no statistical difference by tillage practice (p = 0.7451).

For instance, during the mid-season (6–8WAP), LAI of crops from the 6/15 planting was higher than that of other plantings, suggesting that the LAI seasonal development of Ilmichal was responding less to planting date difference than silage corn.

3.2. Effect of Planting Date and Tillage Practice on Soybean Canopy Height and LAI

We analyzed tillage × planting date effects on the canopy height of two soybean cultivars (Daewon, 2018–2021; Haepum 2018–2020) using repeated measures over 4 years and observed no difference with tillage practice for Daewon (large-grain) soybean (

Table 5. Source of variation and probability level of plant canopy height and LAI of two soybean cultivars (Daewon and Haepum) over 4 years.

Year	Effect	Plant Canopy Height		LAI
		Daewon	Haepum	Daewon	Haepum
2018	Tillage (T)	0.2784 †	0.1173	0.3294	0.5962
	Planting date (P)	<0.0001	0.0015	0.0009	0.0044
	T × P	0.2540	0.1921	0.5960	0.4303
	WAP ‡ (W)	<0.0001	<0.0001	0.0003	0.0003
	T × W	0.7802	0.8296	0.8847	0.7824
	P × W	<0.0001	<0.0001	0.0004	0.0017
	T × P × W	0.9903	0.9810	0.9909	0.9508
2019	T	0.0630	0.2933	0.1934	0.2019
	P	0.0653	0.5111	0.1577	0.0732
	T × P	0.0414	0.7614	0.6952	0.4990
	W	<0.0001	<0.0001	0.0003	<0.0001
	T × W	0.8288	0.9701	0.7219	0.7678
	P × W	<0.0001	<0.0001	0.2227	0.0188
	T × P × W	0.6199	0.9635	0.8487	0.8274
2020	T	0.2541	0.1854	0.7200	0.4609
	P	0.0002	0.0033	0.0146	0.1218
	T × P	0.4623	0.8580	0.4849	0.9016
	W	<0.0001	<0.0001	0.0049	0.0014
	T × W	0.7956	0.8119	0.7142	0.8053
	P × W	0.0002	0.0232	0.0002	0.0106
	T × P × W	0.9588	0.9588	0.9316	0.4255
2021	P	<0.0001		<0.0001
	W	<0.0001		<0.0001
	P × W	<0.0001		<0.0001

† Significance evaluated at 0.05 probability level; ‡ Weeks after planting.

). The effect of planting date and tillage on plant canopy height is shown in

Table 6. Plant canopy height of two soybean cultivars (Daewon and Haepum), as affected by the planting date (6/10, 6/25, 7/10, and 7/25 in 2018; 6/15, 6/30, and 7/15 in 2019–2021) and tillage practice (deep tillage, DT; conventional tillage, CT).

Year	Soybean Cultivar	Treatment	Canopy Height (cm)
			5WAP †	7WAP	9WAP	12WAP
2018	Daewon	6/10	26.3 c ‡ C §	39.8 cB	80.6 aA	80.6 aA
		6/25	26.3 cC	70.8 aB	81.3 aA	77.1 abA
		7/10	45.9 aB	67.3 aA	69.8 bA	71.7 bA
		7/25	35.0 bB	52.2 bA	53.0 cA	49.1 cA
		CT	32.6 ns	56.1 ns	70.7 ns	69.5 ns
		DT	34.2	59.0	71.6	69.8
	Haepum	6/10	17.0 bC	29.7 bB	75.1 aA	78.3 aA
		6/25	16.1 bC	58.2 aB	72.5 aA	71.9 abA
		7/10	33.4 aC	57.6 aB	70.8 aA	70.5 bA
		7/25	30.6 aB	52.7 aA	54.2 bA	50.7 cA
		CT	23.6 ns	47.5 ns	66.7 ns	66.3 ns
		DT	25.0	51.6	69.5	69.4
2019			4WAP	6WAP	8WAP	11WAP
	Daewon	6/15	33.8 bD	73.4 bC	82.1 bB	94.0 aA
		6/30	43.0 aC	66.8 cB	87.6 aA	85.3 bA
		7/15	42.5 aC	79.2 aA	78.4 bA	68.1 cB
		CT	41.0 ns	74.9 ns	84.8 ns	85.0 ns
		DT	38.5	71.4	80.5	79.9
	Haepum	6/15	25.5 bD	57.5 bC	74.5 bB	90.8 aA
		6/30	32.1 aC	50.8 cB	80.8 aA	78.8 bA
		7/15	32.5 aC	72.1 aAB	74.7 bA	68.1 cB
		CT	30.5 ns	61.1 ns	77.1 ns	80.2 ns
		DT	29.5	59.2	76.2	78.3
2020			4WAP	6WAP	8WAP	10WAP	12WAP
	Daewon	6/15	41.3 aC	70.1 aB	96.9 aA	98.6 aA	100.8 aA
		6/30	37.3 aD	62.3 bC	74.4 bB	83.8 bA	74.9 bB
		7/15	36.4 aC	53.8 cB	67.8 bA	67.0 cA	63.8 cA
		CT	39.7 ns	61.6 ns	81.5 ns	84.0 ns	81.2 ns
		DT	37.0	62.5	77.9	82.3	78.5
	Haepum	6/15	31.3 aD	52.2 bC	80.5 aB	86.2 aAB	92.3 aA
		6/30	27.4 aD	46.7 aC	63.4 bB	74.4 bA	69.1 bAB
		7/15	28.3 aC	42.9 bB	68.0 bA	64.9 cA	68.3 bA
		CT	29.6 ns	49.1 ns	72.9 ns	75.3 ns	79.2 ns
		DT	28.4	45.4	68.3	75.0	73.9
2021			4WAP	6WAP	8WAP	10WAP	12WAP
	Daewon	6/15	45.6 bC	88.2 bB	102.7 aA	105.5 aA	106.1 aA
		6/30	65.0 aB	99.8 aA	102.6 aA	102.5 aA	98.4 bA
		7/15	50.0 bC	77.2 cA	77.8 bA	74.3 bAB	71.8 cB

† Weeks after planting; ‡ Within each WAP, values followed by different lowercase letters are significantly different among treatments according to LSD test at p ≤ 0.05; ns, not significant. § Within each planting treatment, values followed by different uppercase letters are significantly different among treatments according to LSD test at p ≤ 0.05; ns, not significant.

.

For the Daewon soybean, in 2018, a significant planting date × WAP interaction (p < 0.0001) was observed. During early season (5–7WAP), the canopy height of crops from the 7/10 and 7/25 plantings was relatively greater than that of crops from other plantings; however, as the season progressed, the canopy height of crops from the 6/10 and 6/25 plantings exceeded that of crops from other plantings. Meanwhile, in 2019, planting date × WAP interaction (p < 0.0001) was detected, although no significant difference with tillage practice (p = 0.0630) was detected. The canopy height of crops from the 6/15 planting gradually increased, reaching a maximum at 11WAP, while the canopy height of crops from the 6/30 planting remained relatively large from 8 to 11WAP. The canopy height of crops from the 7/15 planting was the greatest from 6 to 8WAP, and then decreased. In 2020, planting date × WAP interaction (p = 0.0002) was observed. The canopy height of crops from the 6/15 and 7/15 plantings showed a similar trend, i.e., it was high from 8 to 12WAP, while that of crops from the 6/30 planting peaked at 10WAP, then decreased. The canopy height of crops from the 6/15 planting was the greatest at 12WAP, and the value was the highest among the three plantings. For the farmer’s field, in 2021, planting date × WAP interaction (p < 0.0001) was observed. The canopy height of crops from the 6/15 planting was the greatest from 8 to 12WAP, and was the greatest from 6 to 12WAP for the 6/30 planting. The canopy height of crops from the 7/15 planting was relatively large from 6 to 10WAP, then decreased at 12WAP. The canopy height of crops from the 7/15 planting was the smallest from 8 to 10WAP, which was the lowest among the three plantings, while the canopy height of crops from the 6/15 planting at 12WAP was greater than that of crops from the 6/30 planting at that time point.

For Haepum soybean, no significant difference with tillage practice was observed during over the 3-year period (Table 5). In 2018, a significant planting date × WAP interaction (p < 0.0001) was detected. Similar to Daewon soybean in 2018, the canopy height of crops from the 7/10 and 7/25 plantings was greater than that of crops from other plantings early in the growing season (5 to 7WAP); additionally, the canopy height of crops from the 6/10 and 6/25 plantings was greater than that of crops from other plantings late in the growing season (9 to 12WAP). In 2019, planting date × WAP interaction (p < 0.0001) was apparent. The canopy height of crops from the 6/15 planting was the greatest among all the plantings at 11WAP. In 2020, planting date × WAP interaction (p = 0.0232) was detected. The canopy height of crops from the 6/15 planting was the greatest from 10 to 12WAP, while that of crops from the 6/30 planting peaked at 10WAP, then decreased. Additionally, the canopy height of crops from the 7/15 planting was consistently low throughout the growing season.

Results of the effect of planting date and tillage practice on LAI of the two soybean cultivars are shown in

Table 7. LAI of two soybean cultivars (Daewon and Haepum), as affected by the planting date (6/10, 6/25, 7/10, and 7/25 in 2018; 6/15, 6/30, and 7/15 in 2019–2021) and tillage practice (deep tillage, DT; conventional tillage, CT).

Year	Soybean Cultivar	Planting Date	LAI
			5WAP †	7WAP	9WAP
2018	Daewon	6/10	0.41 b ‡ B §	1.29 bB	4.76 bA
		6/25	0.55 bC	4.49 aB	5.98 aA
		7/10	2.20 aB	4.94 aA	N/A
		7/25	2.08 aB	N/A ¶	3.35 cA
		CT	1.15 ns ¶	3.48 ns	4.58 ns
		DT	1.47	3.66	4.81
	Haepum	6/10	0.25 bB	1.17 bB	4.85 abA
		6/25	0.36 bC	4.60 aB	5.88 aA
		7/10	1.68 aB	4.31 aA	N/A
		7/25	1.98 aB	N/A	3.91 bA
		CT	0.94 ns	3.54 ns	4.69 ns
		DT	1.20	3.18	5.08
2019			4WAP	6WAP	8WAP	11WAP
	Daewon	6/15	1.94 ab	4.65 ns	5.48 ns	4.92 ns
		6/30	1.59 b	4.37	4.27	5.05
		7/15	3.04 a	4.25	5.42	-
		Avg.	2.19	4.42	5.06	4.99
		CT	2.19 ns	4.74 ns	5.35 ns	5.26 ns
		DT	2.19	4.10	4.76	4.70
	Haepum	6/15	1.34 bD	3.40 aC	5.82 aA	4.84 aB
		6/30	1.41 bB	4.13 aA	4.18 bA	4.53 aA
		7/15	2.89 aC	3.95 aB	5.29 aA	-
		CT	2.06 ns	3.87 ns	5.26 ns	5.00 ns
		DT	1.70	3.78	4.94	4.37
2020			4WAP	6WAP	8WAP	10WAP	12WAP
	Daewon	6/15	2.53 aB	4.71 aA	5.23 aA	2.97 aB	2.74 bB
		6/30	2.04 aC	3.48 bAB	2.73 bBC	3.05 aAB	3.77 aA
		7/15	1.84 aC	2.03 cC	2.31 bBC	3.12 aAB	3.52 abA
		CT	1.89 ns	3.45 ns	3.44 ns	3.14 ns	3.27 ns
		DT	2.38	3.37	3.40	2.96	3.41
	Haepum	6/15	1.64 aC	3.43 aAB	4.30 aA	2.94 bB	3.71 aAB
		6/30	1.79 aB	2.53 abAB	2.46 bB	2.67 bAB	3.55 aA
		7/15	1.94 aB	1.94 bB	2.49 bB	4.14 aA	3.94 aA
		CT	1.66 ns	2.53 ns	3.22 ns	3.07 ns	3.60 ns
		DT	1.92	2.74	2.94	3.43	3.88
2021			4WAP	6WAP	8WAP	10WAP	12WAP
	Daewon	6/15	3.32 bC	4.18 bB	7.59 aA	7.60 aA	7.11 aA
		6/30	4.54 aD	6.27 aB	6.78 bA	5.74 bC	5.81 bBC
		7/15	2.80 cC	4.30 bA	4.38 cA	3.77 cB	3.49 cB

† Weeks after planting; ‡ Within each WAP, values followed by different lowercase letters are significantly different among treatments according to LSD test at p ≤ 0.05; ns, not significant. § Within each planting treatment, values followed by different uppercase letters are significantly different among treatments according to LSD test at p ≤ 0.05; ns, not significant. ¶ Not applicable: LAI could not be measured because of continuous rainfall events.

.

During the 3-year field research, no significant difference with tillage practice was observed for LAI of Daewon soybean (Table 5). In 2018, a significant planting date × WAP interaction (p = 0.0004) was detected. At 5WAP, LAI of crops from the 7/10 and 7/25 plantings was relatively greater than that of soybean planted earlier. However, at 9WAP, crops from the 6/25 planting had the highest LAI among crops from all plantings, followed by that from the 6/10 planting. Meanwhile, in 2019, only the main effect of WAP (p = 0.0003) was observed, and so was the planting date × WAP interaction (p = 0.0002) in 2020. LAI of crops from the 6/15 planting was relatively high from 6 to 8WAP, then decreased, while that of crops from both, the 6/30 and 7/15 plantings was high at 12WAP. When comparing the maximum LAI values of crops from the different plantings, that from the 6/15 planting was the highest, followed by that of the 6/30 planting and 7/15 planting. In 2021, planting date × WAP interaction (p < 0.0001) was detected. LAI of crops from the 6/15 planting was consistently high from 8 to 12WAP, while that of crops from the 6/30 planting was high at 8WAP, then decreased. LAI of crops from the 7/15 planting was consistently low throughout the growing season.

For Haepum soybean, no significant difference in LAI with tillage practice was detected (Table 5). For 2018 soybean, a significant main effect of planting date × WAP interaction (p = 0.0044) was observed; the trend was similar to that of Daewon soybean in 2018. Delayed planting date (7/10 and 7/25) resulted in LAI being relatively greater than that of crops from early plantings at 5WAP, while LAI of crops from the 6/10 and 6/25 plantings was greater than that of crops from other plantings at 9WAP. In 2019, planting date × WAP interaction (p = 0.0188) was observed. LAI of crops from the 6/15 and 6/30 plantings was low at 4WAP, followed by a rapid increase at 6WAP and remained high for the remainder of the growing season. In 2020, planting date × WAP interaction (p = 0.0106) was detected. LAI of crops from the 6/15 planting was relatively high at 8WAP, while that of crops from the 6/30 planting was high at 12WAP. Unlike LAI of crops from the 6/15 and 6/30 plantings, that of crops from the 7/15 planting continuously increased throughout the growing season.

3.3. Effect of Planting Date and Tillage on Yield and Yield Components of Maize and Soybean

We evaluated the flowering (tasseling) and harvest dates with respect to the planting date during the 4 years (

Table 8. Flowering (tasseling) and harvest dates of two maize cultivars (Kwangpyeongok and Ilmichal) and two soybean cultivars (Daewon and Haepum) from 2018 to 2021.

Year	Planting Date	Kwangpyeongok		Ilmichal
		Tasseling Date (No. Days) †	Harvest Date (No. Days)	Tasseling Date (No. Days)	Harvest Date (No. Days)
2018	6/10	14 Aug. (65)	27 Sept. (109)	7 Aug. (58)	6 Sept. (88)
	6/25	30 Aug. (66)	10 Oct. (107)	27 Aug. (63)	28 Sept. (95)
	7/10	10 Sept. (62)	01 Nov. (114)	5 Sept. (57)	12 Oct. (94)
	7/25	5 Oct. (72)	7 Nov. (105)	26 Sept. (63)	5 Nov. (103)
2019	6/15	11 Aug. (57)	18 Sept. (95)	2 Aug. (58)	6 Sept. (83)
	6/30	29 Aug. (60)	8 Oct. (100)	9 Aug. (60)	27 Sept. (89)
	7/15	10 Sept. (57)	14 Oct. (91)	8 Sept. (55)	8 Oct. (85)
2020	6/15	23 Aug. (69)	14 Sept. (91)	88 Aug. (54)	4 Sept. (81)
	6/30	4 Sept. (66)	28 Sept. (90)	23 Aug. (54)	25 Sept. (87)
	7/15	14 Sept. (61)	14 Oct. (91)	4 Sept. (51)	12 Oct. (89)
2021	6/15	10 Aug. (56)	13 Sept. (90)	-	-
	6/30	17 Aug. (48)	27 Sept. (89)	-	-
	7/15	8 Sept. (55)	15 Oct. (92)	-	-
		Daewon		Haepum
		Flowering date (no. days)	Harvest date (no. days)	Flowering date (no. days)	Harvest date (no. days)
2018	6/10	27 Jul. (47)	30 Oct. (142)	13 Aug. (64)	24 Oct. (136)
	6/25	6 Aug. (42)	30 Oct. (127)	23 Aug. (59)	24 Oct. (121)
	7/10	21 Aug. (40)	31 Oct. (113)	21 Aug. (42)	31 Oct. (113)
	7/25	26 Aug. (32)	6 Nov. (104)	3 Sept. (40)	6 Nov. (104)
2019	6/15	31 Jul. (46)	28 Oct. (135)	9 Aug. (55)	14 Oct. (121)
	6/30	9 Aug. (40)	1 Nov. (124)	16 Aug. (47)	21 Oct. (113)
	7/15	16 Aug. (32)	1 Nov. (109)	24 Aug. (40)	28 Oct. (105)
2020	6/15	1 Aug. (47)	20 Oct. (127)	8 Aug. (54)	13 Oct. (120)
	6/30	8 Aug. (39)	20 Oct. (112)	17 Aug. (48)	13 Oct. (105)
	7/15	17 Aug. (33)	20 Oct. (97)	23 Aug. (39)	20 Oct. (97)
2021	6/15	4 Aug. (50)	20 Oct. (127)	-	-
	6/30	17 Aug. (48)	27 Oct. (119)	-	-
	7/15	26 Aug. (42)	N/A ‡	-	-

† Days to flowering (tasseling) or harvest; ‡ Not applicable.

). For Kwangpyeongok maize, in 2018, tasseling of crops from the 7/25 planting was delayed compared to that of crops from other plantings due to relatively low temperature during the late vegetative stage in September (Supplementary Figure S1). Crops from the 6/10 to 7/10 plantings were at the R5 stage at harvesting, however crops from the 7/25 planting remained at the R3–4 stage at harvesting which led to yield loss. Crops from the 7/25 planting was harvested to clear the field in preparation for the following onion cultivation. We made similar observations in 2020, for corn from the 7/15 planting. Thus, when the planting date was delayed to 7/15 or later, the silage corn would most likely be harvested prior to reaching the R5 stage. The waxy corn Ilmichal likewise requires warm temperature during the reproductive period; for instance, the harvest of crops from the 2018 7/25 planting was delayed due to low temperature during the reproductive stage in September and October. However, unlike the silage corn, the waxy corn has a relatively short growing period, hence its harvesting period was not affected by the planting date (6/15 to 7/15) during the 3-year observation.

Both Kwangpyeongok and Ilmichal maize from the 7/25 planting could not be easily rotated with winter onion. Consequently, this planting date was omitted from the following-year experiment.

During the 4-year experiment, the flowering time of both Daewon and Haepum soybean cultivars was relatively long when planted on 6/15, and the following harvest time was also extended (Table 8). This might have resulted in a longer grain filling period.

We compared total biomass, ear biomass, stover biomass, and TDN yield for silage corn Kwangpyeongok over 4 years (

Table 9. Statistical analysis of the yield and yield components of two maize cultivars (Kwangpyeongok and Ilmichal).

Year	Cultivar	Parameter	Planting Date (P)	Tillage (T)	P × T
2018	Kwangpyeongok	Total biomass	<0.0001 †	0.4889	0.7866
		Ear	<0.0001	0.4602	0.9378
		Stover	0.0122	0.7497	0.5028
		TDN	<0.0001	0.4774	0.8315
	Ilmichal	Ear length	<0.0001	0.1962	0.8867
		RKSLEL	0.0052	0.6392	0.8811
		Number of ears	0.3009	0.3589	0.6564
		Marketable fresh ear yield	0.0038	0.1986	0.5435
2019	Kwangpyeongok	Total biomass	<0.0001	0.6465	0.3284
		Ear	<0.0001	0.4750	0.7735
		Stover	<0.0001	0.1488	0.1198
		TDN	<0.0001	0.8080	0.3966
	Ilmichal	Ear length	<0.0001	0.3019	0.8542
		RKSLEL	0.0002	0.1836	0.3921
		Number of ears	0.0002	0.3787	0.5093
		Marketable fresh ear yield	<0.0001	0.5413	0.5431
2020	Kwangpyeongok	Total biomass	0.0001	0.9957	0.2126
		Ear	<0.0001	0.9966	0.4114
		Stover	0.0239	0.9887	0.1356
		TDN	<0.0001	0.9971	0.2393
	Ilmichal	Ear length	0.4588	0.8076	0.8863
		RKSLEL	0.0012	0.4887	0.7136
		Number of ears	0.0269	0.8331	0.8977
		Marketable fresh ear yield	0.0340	0.8590	0.7883
2021	Kwangpyeongok	Total biomass	0.0069
		Ear	0.3285
		Stover	0.0002
		TDN	0.0159

† Significance evaluated at 0.05 probability level.

). A significant effect of planting date on the total biomass, ear biomass, stover biomass, and TDN yield was observed only in 2018. Crops from the 7/25 planting showed the lowest ear, stover biomass, and TDN yield among the plantings (

Table 10. Yield and yield components of two maize cultivars (Kwangpyeongok and Ilmichal), as affected by the planting date and tillage practice (deep tillage, DT; conventional tillage, CT).

Year	Cultivar	Treatment	Total Biomass (kg ha−1 D.W.)	Ear (kg ha−1 D.W.)	Stover (kg ha−1 D.W.)	TDN (kg ha−1)
2018	Kwangpyeongok	6/10	10,740 a†	5811 a	4929 a	7808 a
		6/25	8912 b	4200 b	4712 ab	6312 b
		7/10	11,216 a	6009 a	5207 a	8138 a
		7/25	5786 c	1655 c	4132 b	3811 c
		DT	9407 ns	4724 ns	4683 ns	6741 ns
		CT	10,246	5287	4959	7380
			Ear length (cm)	RKSLEL (%)	Number of ears (ea ha−1)	Marketable fresh ear yield (kg ha−1 F.W.)
	Ilmichal	6/10	20.1 a	97.1 a	43,556 ns	7832 a
		6/25	19.3 b	91.8 b	46,222	8374 a
		7/10	19.0 b	96.6 a	45,037	8997 a
		7/25	18.0 c	92.4 b	41,185	6135 b
		DT	19.8 ns	93.9 ns	44,741 ns	8129 ns
		CT	19.6	95.0	45,037	8077
2019			Total biomass (kg ha−1 D.W.)	Ear (kg ha−1 D.W.)	Stover (kg ha−1 D.W.)	TDN (kg ha−1)
	Kwangpyeongok	6/15	12,317 a	4776 a	7541 a	8449 a
		6/30	8626 b	3566 b	5060 b	5976 b
		7/15	7652 b	2344 c	5308 b	5082 c
		DT	10,642 ns	4091 ns	6550 ns	7290 ns
		CT	10,301	4251	6050	7135
			Ear length (cm)	RKSLEL (%)	Number of ears (ea ha−1)	Marketable fresh ear yield (kg ha−1 F.W.)
	Ilmichal	6/15	19.8 a	95.8 a	49,482 a	8720 a
		6/30	18.7 b	92.0 b	44,444 a	8140 a
		7/15	15.2 c	88.4 c	30,222 b	5244 b
		DT	19.1 ns	94.2 ns	46,519 ns	8340 ns
		CT	19.3	93.6	47,407	8519
2020			Total biomass (kg ha−1 D.W.)	Ear (kg ha−1 D.W.)	Stover (kg ha−1 D.W.)	TDN (kg ha−1)
	Kwangpyeongok	6/15	9327 a	4823 a	4504 a	6721 a
		6/30	5681 b	1995 b	3686 b	3841 b
		7/15	5207 b	1800 b	3407 b	3513 b
		DT	7527 ns	3431 ns	4096 ns	5300 ns
		CT	7481	3387	4094	5261
			Ear length (cm)	RKSLEL (%)	Number of ears (ea ha−1)	Marketable fresh ear yield (kg ha−1 F.W.)
	Ilmichal	6/15	17.8 ns	94.5 a	41,905 a	6847 a
		6/30	17.7	84.4 b	30,476 b	4546 b
		7/15	18.2	95.8 a	33,968 ab	5514 ab
		DT	17.7 ns	88.6 ns	35,556 ns	5511 ns
		CT	17.8	90.3	36,825	5881
2021			Total biomass (kg ha−1 D.W.)	Ear (kg ha−1 D.W.)	Stover (kg ha−1 D.W.)	TDN (kg ha−1)
	Kwangpyeongok	6/15	13,156 a	5605 ns	7551 a	9159 a
		6/30	11,383 b	5620	5764 b	8131 ab
		7/15	10,095 b	5033	5062 c	7224 b

† Values followed by different lowercase letters are significantly different among treatments within a same cultivar according to LSD test at p ≤ 0.05; ns, not significant.

). Biomass and TDN yield of crops from the 6/10 planting and 7/10 planting was relatively higher than that of crops from other plantings. Additionally, a significant main effect of planting date only on the total biomass, ear biomass, stover biomass, and TDN yields were observed in 2019 (Table 9). The total biomass, ear biomass, stover biomass, and TDN yield were the highest for crops from the 6/15 planting; TDN was 41% to 66% greater than that of crops from the 6/30 and 7/15 plantings, respectively. Similar to 2019 maize, a significant main effect of planting date only on total biomass, ear biomass, stover biomass, and TDN yields were observed in 2020 (Table 9). The total biomass, ear biomass, stover biomass, and TDN yield were the highest for crops from the 6/15 planting, while TDN yields were 2-fold higher than that of crops from other plantings. Specifically, it was 74% greater than that of crops from the 6/30 planting. Similar to observations from the GNU Research Farm study, a significant main effect of planting date on total biomass, stover biomass, and TDN yield, but not on ear biomass, was observed in 2021. That year, the total biomass, stover biomass, and TDN yield were the highest for crops from the 6/15 planting, for which the TDN yields were 13–26% higher than that of crops from the 6/30 and 7/15 plantings, respectively. Overall, the ear yield in 2021 was higher than that in the previous year.

We observed a significant main effect of planting date only on ear length, RKSLEL, and marketable fresh ear yield of Ilmichal corn in 2018 (Table 9). Additionally, the earlier the planting date, the longer the ear (Table 10). RKSLEL of waxy corn planted on 6/10 and 7/10 was greater than that of waxy corn planted on 6/25 and 7/25 (average of 96.8% vs. 92.1%, accordingly). In 2018, the number of ears was not affected by planting date or tillage practice, while the marketable fresh ear yield of crops from the 7/25 planting was 27% lower than the average value for crops from other plantings. Hence, in 2018, the yield of silage corn and waxy corn was generally stable for crops from plantings before 7/10. For Ilmichal maize in 2019, a significant main effect on ear length, RKSLEL, number of ears, and marketable fresh ear yield of planting date only was observed. Crops from the 6/15 and 6/30 plantings maintained yield, but decreased for 7/15. No effect of tillage practice was apparent. Additionally, for waxy corn, a significant main effect of planting date only on RKSLEL, number of ears, and marketable fresh ear yields were detected in 2020. Unlike in the previous year, the marketable fresh ear yield, number of ears, and RKSLEL of crops from the 6/30 planting were relatively low. The fresh marketable ear yield and number of ears of crops from the 6/15 planting were 50% and 38% greater than those of 6/30.

We observed a significant effect of planting date, but no effect of tillage practice, for Daewon soybean in 2018 (

Table 11. Statistical analysis (p-value) of yield and yield components of two soybean cultivars (Daewon and Haepum).

Year	Cultivar	Parameter	Planting Date (P)	Tillage (T)	P × T
2018	Daewon	Number of pods	0.0002 †	0.7381	0.9574
		Number of seeds	0.0053	0.8413	0.4984
		100–seeds weight	<0.0001	0.9587	0.4625
		Yield	0.0015	0.2526	0.2305
	Haepum	Number of pods	0.0263	0.1616	0.5051
		Number of seeds	0.0109	0.4429	0.6125
		100–seeds weight	0.9077	0.3208	0.9448
		Yield	0.0002	0.9802	0.5855
2019	Daewon	Number of pods	0.0461	0.7710	0.9544
		Number of seeds	0.2361	0.9470	0.7692
		100–seeds weight	0.0287	0.0717	0.3118
		Yield	0.0012	0.9052	0.0227
	Haepum	Number of pods	0.0154	0.9369	0.6713
		Number of seeds	0.7219	0.7924	0.5875
		100–seeds weight	0.0279	0.5014	0.4561
		Yield	0.0003	0.6024	0.2190
2020	Daewon	Number of pods	0.0678	0.5088	0.5152
		Number of seeds	0.0571	0.8779	0.6150
		100–seeds weight	0.0015	0.1076	0.8224
		Yield	0.0653	0.6065	0.6332
	Haepum	Number of pods	0.0002	0.3405	0.4149
		Number of seeds	0.0025	0.8570	0.6333
		100–seeds weight	0.0711	0.7526	0.5592
		Yield	0.0053	0.2541	0.6975
2021	Daewon	Number of pods	0.1220
		Number of seeds	0.0260
		100–seeds weight	0.5060
		Yield	0.0500

† Significance evaluated at 0.05 probability level.

). Crops from the 7/25 planting showed the lowest yield and yield components, with no difference among crops from the other three plantings (

Table 12. Yield and yield components of two soybean cultivars (Daewon and Haepum), as influenced by the planting date and tillage practice (deep tillage, DT; conventional tillage, CT).

Year	Cultivar	Treatment	Number of Pods (ea)	Number of Seeds (ea)	100–Seeds Weight (g)	Yield (kg ha−1)
2018	Daewon	6/10	68.6 a†	79.4 a	32.8 a	2567 a
		6/25	59.5 a	88.4 a	32.7 a	2752 a
		7/10	58.7 a	78.9 a	32.1 a	2807 a
		7/25	38.4 b	53.3 b	26.2 b	1446 b
		DT	65.5 ns	82.1 ns	32.7 ns	2833 ns
		CT	62.6	85.7	32.8	2486
	Haepum	6/10	104.2 a	156.4 a	11.6 ns	2124 a
		6/25	93.3 a	188.5 a	11.4	2525 a
		7/10	93.3 a	173.5 a	11.4	2460 a
		7/25	69.1 b	108.5 b	11.3	1345 b
		DT	102.7 ns	176.0 ns	11.6 ns	2250 ns
		CT	94.8	169.0	11.4	2399
2019	Daewon	6/15	68.6 a	79.4 ns	27.7 ab	2945 a
		6/30	50.5 ab	64.1	28.4 a	2653 b
		7/15	41.2 b	62.1	26.3 b	2180 b
		DT	60.5 ns	69.6 ns	27.2 ns	2600 ns
		CT	58.5	73.9	28.9	2585
	Haepum	6/15	114.6 a	166.5 ns	11.4 a	2531 a
		6/30	84.3 b	152.5	10.2 b	2182 b
		7/15	98.8 ab	157.2	10.2 b	2146 b
		DT	99.9 ns	160.4 ns	10.5 ns	2301 ns
		CT	98.9	158.5	11.1	2272
2020	Daewon	6/15	31.4 ab	37.3 ab	26.8 a	2690 ab
		6/30	31.9 a	49.1 a	23.8 b	3048 a
		7/15	24.9 b	35.3 b	21.4 c	1925 b
		DT	31.7 ns	44.4 ns	24.3 ns	2828 ns
		CT	31.6	42.1	26.2	2910
	Haepum	6/15	69.4 a	108.7 a	10.3 ab	2574 a
		6/30	53.9 b	94.5 a	10.7 a	2557 a
		7/15	42.4 c	61.0 b	9.3 b	1491 b
		DT	64.4 ns	102.0 ns	10.6 ns	2753 ns
		CT	58.9	101.2	10.4	2378
2021	Daewon	6/15	68.9 ns	109.8 a	27.8 ns	2837 a
		6/30	56.6	65.3 b	28.3	1882 b
		7/15	N/A ‡	N/A	N/A	N/A

† Values followed by different lowercase letters are significantly different among treatments within a same cultivar according to LSD test at p ≤ 0.05; ns, not significant. ‡ Not applicable.

). In addition, the late planting date hindered the following onion transplanting practice. In 2019, a significant effect of plating date on the number of pods, 100–seeds weight, and yields were detected. Crops from the 6/15 planting had the highest number of pods and yield, while that from the 6/30 planting had the highest 100–seeds weight. No significant difference in the yield and yield component was observed for the tillage practice. Similar to 2019, 100–seeds weight of crops from the 6/15 planting was greater than that of crops from other plantings in 2020. In 2021, the crops maturity from the 7/15 planting was severely delayed, therefore, we could not proceed with the harvest before the onion transplanting season, thus the data were removed from analysis. The number of seeds and grain yield significantly differed by planting date. For instance, the seed number and grain yield of crops from the 6/15 planting were greater that those of 6/30.

For Haepum soybean, in 2018, a significant effect of planting date on the number of pods, number of seeds, and grain yields were detected, with no statistical effect of tillage practice. Similar to 2018 Daewon soybean, Haepum soybean from the 6/15 planting had the greatest number of pods and seeds along with grain yield. In 2019, the number of Haepum pods and 100-seeds weight were statistically different by planting date. Crops from the 6/15 planting had the highest pod number and 100-seeds weight. Finally, in 2020, the number of pods and seeds and grain yield were significantly different with the planting date. The number of pods was greatest for crops from the 6/15 planting, while the number of seeds and grain yield were smallest for crops from the 7/15 planting.

3.4. Residual Effect of Rotation Crops Planting Date and Tillage Practice on Onion Yield and Yield Component

Two years of testing different planting dates and tillage practice for the growth of summer crops showed no residual effect on onion yield and quality (

Table 13. Yield components of onion, as affected by the planting date and tillage practice on summer crops (deep tillage, DT; conventional tillage, CT).

Year	Treatment		Fresh Marketable Bulb Yield (Mg ha−1)	Avg. Bulb Weight (g)	Year	Treatment		Fresh Marketable Bulb Yield (Mg ha−1)	Avg. Bulb Weight (g)
2018–2019	6/10		86.2 ns	216.6 ns	2019–2020	6/15		140.5 ns	227.7 ns
	6/25		84.5	216.8		6/30		131.8	210.5
	7/10		86	229		7/15		131.3	220.6
	7/25		85.3	197.1
	DT		86.1 ns	214.6 ns		DT		138.3 ns	221.1 ns
	CT		84.9	215.1		CT		130.8	218
	p-value	Planting date	0.9661	0.974		p-value	Planting date	0.8069	0.7754
		Tillage	0.6666	0.953			Tillage	0.1656	0.7162
		P × T	0.8618	0.9435			P × T	0.1272	0.276

ns, not significant.

). For instance, the average fresh marketable bulb yields were 85.5 Mg ha⁻¹ and 134.6 Mg ha⁻¹ in the 2018–2019 and 2019–2020 growing seasons, with the average bulb weight of 214.9 g and 219.6 g, respectively. Before the farmer’s field research was implemented, fresh marketable onion yields were 86.4 Mg ha⁻¹.

4. Discussion

In the current study, we assessed the effect of planting date (early June to late July) and tillage practice (DC and CT) on two maize (silage and waxy corn) and soybean cultivars (large and small grain) grown for 4 years in a field study with a converted paddy soil condition in Korea. This study identified practical strategies for summer upland crops cultivation season with winter cash crops rotation.

Canopy height development varies by year and cultivar, however, no tillage practice effect on canopy height was observed during 2018–2020. Chaudhary et al. [34] have reported that deep tillage reduces soil bulk density and enhances water infiltration of deep coarse-textured soil and is more effective than conventional tillage. However, no significant differences in soil bulk density between the different tillage practices (explained in Supplementary S1) was observed. Similar to canopy height, no difference in LAI by tillage practice was observed. Previous studies have reported conflicting results: DT was shown to enhance LAI [35], while LAI of maize was reported to increase with CT [36]. We suspect that no beneficial effect of deep tillage was possibly due to the drainage issue that was not appeared in the research field, with a drainage ditch around the border on the GNU Research Farm (2018–2020) and farmer’s farm (2021). A ditch adjacent to the field can improve the soil physical properties and enhance plant growth and yield [37,38]. Thus, with well drained converted-paddy field condition, additional agronomic input, such as deep tillage, is not essential.

We observed a significant difference with planting date treatment in both maize cultivars. The canopy height of maize from the 6/10 planting was relatively elevated mid-growing season. In 2019, the canopy height of crops from the 7/15 planting was greater in the early season, with a high GDD (0–4WAP; 597). However, the overall GDD up to 11WAP was similar for the 6/15 and 6/30 plantings (Table. 1). This might have affected the overall canopy development of crops from the different plantings to a similar extent. Crops from the 6/30 and 7/15 plantings were exposed to heavy rainfall during early growing season in 2020 (Supplementary Figure S1), with a similar overall GDD. This might have reduced plant growth during the seedling stage, thereby reducing plant canopy height throughout the growing season in 2020. These observations are in line with the notion that heavy rainfall early in the growing season significantly reduces the overall maize growth in Korea [21]. In 2021, crops from the 6/30 and 7/15 plantings exhibited robust canopy height development due to a high GDD early in the growing season; by contrast, crops from the 6/15 planting experienced the lowest GDD and high accumulated rainfall. These results are in line with previous studies showing that delayed planting reduced overall GDD [39]. Our results are also in line with a previous report that low GGD until the V6 stage with heavy rainfall reduces maize plant height [40]. Crops LAI, a key biophysical variable, reflects the crops physiology and ecosystem function [41]. Interestingly, the developmental pattern of maize LAI differed from that of canopy height. In 2018, delayed planting (7/10 and 7/25) was resulted in a highly robust LAI development until 7WAP, however, crops from the 6/10 planting had the highest LAI for both silage corn and waxy corn at 9WAP. The four planting dates resulted in similar GDD for the crops (1155–1270); only crops from the 6/10 planting received adequate amount of accumulated rainfall (475 mm). Crops from the other plantings received 769 mm rainfall on average, which negatively affected plant growth during the mid-growing season. Flesch and Dale [42] have reported that excessive rainfall reduces maize LAI in mid-growing season. Unlike in 2018, the overall GDD until 8WAP of crops from all three plantings was similar in 2019. In 2020, crops from all plantings reached maximum LAI at 6–8WAP, however, LAI of crops from the 6/15 planting began to decrease earlier (from 10WAP) than that of crops from other plantings since at 12WAP; crops from the 6/15 planting received up to 1337 (°C) GDD and entered senescence.

In 2021, early LAI development of crops from the 6/30 and 7/15 plantings was more robust than that of crops from the 6/15 planting due to a high GDD during the early stage of development. Leaf number of maize grown in a warm environment increases during the vegetative stage [11,43]. In the current study, early planting produced a crops with a higher maximum LAI than that of crops from relatively delayed plantings. This observation was in line with that of Williams and Lindquist [44], who showed maximum LAI reduction, from 27% to 44%, with shorter daylength. Similar trend was reported for potato [45]. Therefore, our results suggest that 6/15 to 6/30 is a practical planting window that allows for appropriate canopy height and LAI development in maize.

For soybean, similar to maize, delayed planting date rapidly increased the canopy height early in the growing season, while early planting date resulted in a greater canopy height later in the growing season (Table 6). Delayed planting produces crops with smaller canopy height in late growing season [46,47]. Herein, canopy height increased with a high GDD, although excessive rainfall negatively affected canopy development of crops from the 6/15 and 7/15 plantings (up to 130 mm more cumulative rainfall than that for crops from the 6/30 planting). In addition, the flowering of crops from the 7/15 planting proceeded up to 2 weeks earlier than that of crops from the 6/15 planting, as soybean is sensitive to daylength [48]. Additionally, delayed planting date negatively affects plant height development later in the season [49,50]. Similar to the effect on canopy height development, delayed planting date of soybean significantly enhanced LAI development during the early growing season, while early planting date resulted in crops with greater LAI later in the season (Table 7). LAI, crops growth rate, and leaf expansion rate of early-planted soybean are increased due to a high temperature during the reproductive stage [51]. Similarly, in this study, an increased crops growth rate during early season following delayed planting was associated with warm temperature at the seedling stage. Collectively, both for maize and soybean, delayed planting improved plant growth early in the season due to warm temperature, yet hindered development during the reproductive stage with low late season temperature, thereby reducing biomass and grain yield.

We observed similar time to harvest (less than 10 days difference) of all planting date in two tested maize cultivars. By contrast, for two soybean cultivars, the difference was up to 30 days between planting dates. A sufficient growing period is critical for rotation with winter cash crops. The growing period to maturation of early-planted soybean (mid to late June) was 2–4 weeks longer than that of late-planted soybean which led to sufficient grain filling and a higher yield. For instance, delayed planting shortens the growing period of soybean to maturity and reduces yield [50]. Soybean is sensitive to daylength [48]; hence, early-planted soybean experiences more long days than late-planted soybean, which extends the vegetative growth and flowering to pod stages [44,48]. However, delaying soybean planting reduces the growth period and lowers GDD, which in turn negatively impact the overall yields [44]. Unlike soybean, maize is not sensitive to daylength, hence the similar growth duration regardless of maize planting date. However, since late-season GDD is low, delayed maize planting leads to yield reduction [52].

During the study, neither maize nor soybean yields were affected by tillage practice. However, we observed a significant difference based on the planting date of both crops. Considering maize yield and yield component, the yield of silage corn from the 6/10 planting was the highest in 2018. During 2019–2021, biomass yield of silage corn was reduced as the planting date was delayed. Similarly, previous studies indicated that delayed planting negatively affects maize yield [11,12,53]. Specifically, in the current study, the overall yield difference between crops from the 6/15 to 6/30 plantings was smaller than that from the 6/15 to 7/15 plantings. This observation was similar to a previous report showing that GDD reduction between the early and late planting date is associated with a decreased yield [54]. In the current study, planting date also affected soybean yields.

Similar to the current study, previous research reported that yield and yield components decrease with delayed planting date [51,55,56].

Especially for the upland summer crops and winter cash crops rotation, the 7/15 planting date of soybean hindered the crops from reaching the harvest stage. In 2021, we could not harvest the crops prior to the field being prepared for onion transplanting. Consequently, delayed planting (mid-July) should be avoided in the soybean–winter cash crops rotation system. Early planted soybean experiences a high temperature during the grain filling period (R5–8) [57]. Additionally, a high LAI during the reproductive stage is associated with a high grain yield [58]. In the current study, early-planted soybean developed a high LAI during the reproductive stage. Hence, a high GDD and time for canopy development extended by early planting provide favorable conditions to increase soybean grain yield. We therefore recommend planting maize (both silage and waxy corn) and soybean immediately following winter onion harvest for a good summer crop yield and quality. If this is not possible due to rainfall or delayed winter onion harvest, mid–late June is a practical window for maize and soybean planting in Korea.

Two years of onion yields data acquired in the current study indicated no residual effect of planting date or tillage practice during summer crops. Deep tillage and early planting of onion can increase marketable onion yields [59,60,61]. However, in this study, the interval between treatment implementation and onion cultivation was more than 4 months as the implementation targeted summer maize and soybean. Thus, a 4-month interval could be a sufficient time to compensate for any specific effect of these treatments. The onion yields varied annually: 85.5 Mg ha⁻¹ in 2018–2019 and 134.6 Mg ha⁻¹ in 2019–2020, which was associated with the overall warm winter conditions in the winter of 2019.

5. Conclusions

For both silage and waxy corn, crops yields were maintained despite planting date delayed from mid to late June, as the maize grows rapidly with a high GDD, and the harvest time precedes the actual physiological maturity for silage and fresh consumption purposes. However, delayed planting of maize reduced crops yields due to low temperature during the reproductive stage. Hence, the performance and yields of the two soybean cultivars were sensitive to planting date. Planting delay to July significantly reduced the yield due to low GDD and insufficient period of reproductive stage and conflicted with field preparation for the following onion cultivation. We also observed that deep tillage is not required when adequate horizontal drainage is established. Considering these findings, we recommend mid-late June planting for growers who wish to introduce maize and soybean in rotation with winter onion in the region.