Full Paper

Journal of Agricultural Meteorology

Effect of early sowing on growth and yield of determinate and indeterminate
soybean(Glycine max (L.) Merr.)cultivars in a cool region of northern Japan（

Agricultural Meteorology Group, Agro-Environmental Research Division, Tohoku Agricultural Research Center,
National Agriculture and Food Research Organization, 4 Akahira, Shimokuriyagawa,Morioka, Iwate 020-0198, Japan（

Etsushi KUMAGAI †Abstract
To increase Japanese soybean yield, both practical and genetic improvements are needed. Early sowing improved
yields in the United States, where indeterminate cultivars may produce better yields than determinate cultivars,
particularly with early sowing. However, the mechanism for the yield response is not fully understood. Furthermore,whether these results are valid for cool regions in northern Japan is unknown. This study tested the effects of earlysowing on yield and its attributes in three years using a Japanese determinate cultivar (Ryuhou) and three indeterminate
cultivars (Karikei-881 and Karikei-879, recombinant inbred lines from determinate × indeterminate crosses; UA4910,
a high-yield American cultivar). The effects of early sowing varied among the years. Early sowing significantly
increased pod number and seed yield by increasing cumulative intercepted solar radiation (thus, growth during early
reproductive stages) in years with high precipitation during mid-season (July and August). High precipitation
potentially inhibited canopy development from vegetative to early reproductive stages more strongly in normally sownplants than in early sown plants. In contrast, no significant effect of early sowing was observed in a year with low
precipitation during mid-season, when low precipitation during vegetative stage promoted canopy development in bothsowing dates. Therefore, early sowing could escape from excess water stress and ameliorate canopy development andthereby seed yield in this region. There was no significant sowing date × cultivar interaction for seed yield in any year;
thus, the indeterminate cultivars did not benefit more from early sowing in northern Japan. UA4910 had greater seedyield than Ryuhou owing to its longer growth duration and greater aboveground biomass and pod number; Karikei-881,
Karikei-879, and Ryuhou showed similar seed yields. Thus, indeterminate cultivars did not necessarily have higher
yields in northern Japan. These results suggest that early sowing could be effective for increasing soybean yield in
northern Japan.
Key words: Cumulative intercepted radiation, Seed yield, Soil moisture, Sowing date, Stem determinacy

1. Introduction
Soybean (Glycine max [ L.] Merr.) is a major source of plant
protein and oil, and a major contributor to the world’ s foodsupply. In the United States, soybean productivity has been
increasing for many years. The mean soybean yield increased-1-1from 2.0 t ha to 3.0 t ha in the past 30 years (USDA-NASS,2016). On the other hand, the yield in Japan is much lower andhas changed little over time; the 30-year average (1982-2012)-1
soybean yield is only 1.65 t ha , and has increased by less than-1
0.3 t ha since 1982 (MAFF, 2013). Thus, a strategy to boost
Japanese soybean yield is needed based on both agronomic
practices and genetics.
Optimization of sowing dates is the most important and least
expensive agronomic practice that affects soybean yield(Robinson et al., 2009). Some researchers have suggested thatearlier sowing dates contributed to recent soybean yield gains in
the United States. For example, sowing soybeans in late Apriland early May is currently recommended in the Midwestern
Received; April 24, 2017
Accepted; September 13, 2017†
Corresponding Author: ekumagai@affrc.go.jp
DOI: 10.2480/agrmet.D-17-00009

United States to achieve the maximum seed yield (Bastidas etal., 2008; De Bruin and Pedersen, 2008; Robinson et al., 2009).
Sowing before the optimal date is restricted mainly by cool soil
temperatures (Hu and Wiatrak, 2012). Cool and wet soils during
early sowing may delay germination and seedling emergence(Andales et al., 2000) and may reduce canopy development,
thereby decreasing seed yield (Kane et al., 1997). Moreover, theeffects of sowing date on soybean yield depend on meteorological conditions and location, especially in terms of the amount andpattern of precipitation (Egli and Cornelius, 2009). Because thebeneficial effects of early sowing could be negated undersuboptimal soil water conditions (Heatherly and Elmore, 1983;
Kane et al., 1997; Robinson et al., 2009), efforts to determine theoptimal sowing date should account for regional variations in theamount and pattern of precipitation.
In Tohoku, a cool region of northern Japan (from 36 to 43°′
N)at about the same latitude as the Midwestern United States (from36 to 49°′N), the currently recommended sowing date forsoybeans is late May to early June. In this region, rice transplantmonoculture is the dominant crop system, and the optimaltransplanting time for paddy rice is mid-May to late May. If the
optimal soybean sowing date is earlier than the current
recommendation, soybean sowing and rice transplanting would
overlap. Therefore, early sowing of soybeans would be difficult− 1 −Journal of Agricultural Meteorology
for farmers. In addition, the spring is cooler in this region than in
the Midwestern United States: May and June temperatures are 2
to 5°
C lower in Tohoku (Supplemental Table S1). Furthermore,
there is a risk that earlier-sown soybean may be exposed to
extremely low temperatures and late-spring frosts, which canalso inhibit germination and emergence and early canopy
development. The complexity of these climatic factors has
prevented researchers from verifying the effect of early sowing
on soybean yield, and it remains unclear how much yield could
be improved by earlier sowing.
Waterlogging is one of the major problems in the productionof soybean in converted paddy fields, which sometimes havepoor drainage and a high-water table in Japan (Bajgain et al.,2015). This circumstance is common in East Asia (Kokubun,2013). The rainy season lasts from June to July in northern Japan,
so waterlogging occurs during the early vegetative stage. Excess
soil water during the vegetative stage inhibits root growth, andsoybean with poor root systems suffers from water deficits in thesummer (Hirasawa et al., 1998). Since soybean production innorthern Japan depends on the amount and pattern of
precipitation, which vary between years and locations, the effectof early sowing on seed yield will also depend on theseprecipitation characteristics.
In addition to climatic factors, genetic factors affect soybean
growth and yield. Stem determinacy is an important trait that
affects seed yield. Soybean cultivars can be classified into
indeterminate, semi-determinate, or determinate, depending on
how strongly their growth responds to environmental cues. Stem
growth and leaf formation and expansion in indeterminate
accessions continue for a long period after flowering. In
determinate accessions, growth of the stem node ends when
flowering begins or soon afterward. Most Japanese cultivars are
determinate, but indeterminate cultivars are the dominant form in
the Midwestern United States. Several American researchers
have reported the effects of stem growth habit on soybean seedyield; some reported a higher seed yield for indeterminate
cultivars (Parvez et al., 1989; Robinson and Wilcox, 1998),
whereas others have found a higher yield for determinate
cultivars (Weaver et al., 1991). The yield components of thesegrowth types also respond differently to sowing date. Beaver andJohnson (1981) reported that seed yield of indeterminate
cultivars in Illinois increased steadily with an increasing shift
from early-June sowing to early-May sowing, whereas the yieldof determinate cultivars did not. Wilcox and Frankenberger(1987) compared the two types using three to five sowing dates
from early May to late June in Indiana. They reported that
determinate cultivars had the highest yield when sown in late
May or early June, whereas indeterminate cultivars produced
their maximum yield when sown from early to late May.
Robinson and Wilcox (1998) also reported that the magnitude of
the yield increase caused by shifting from July to May sowing
was greater in indeterminate lines than determinate lines.
Although a superior yield increase by indeterminate cultivars in
response to early sowing was reported in the United States, to myknowledge, there has been no attempt to clarify the underlying
mechanisms that determine these responses. Furthermore, there
have been no studies that compared determinate and indetermi-

nate cultivars in cool regions of northern Japan.
Several researchers recently suggested that genetic improvement has contributed to the rapid soybean yield gain in the
United States (Koester et al., 2014; Rincker et al., 2014;
Rowntree et al., 2014; Specht et al., 2014). More recently,
Kawasaki et al. (2016) reported that American cultivars had
better seed yield than Japanese cultivars in a warm region ofJapan. Matsuo et al. (2016) also compared the yields with both
early (late May) and normal (late June) sowing between
American and Japanese cultivars in a warm region ofsouthwestern Japan, and found a greater yield increase with earlysowing in the American cultivars.
Based on this review of the literature, I hypothesized that in
the cool climate of northern Japan, early sowing would increasesoybean yield, and that indeterminate soybean cultivars would
have higher seed yield and a greater yield increase thandeterminate cultivars with early sowing. To test these hypotheses, I compared the changes in yield and yield components
between determinate and indeterminate cultivars with earlysowing (mid-May) and normal sowing (late May) in threeconsecutive years. Although the yield responses to early planting
were well documented in previous studies, those studies
provided limited information about the plant growth parameters
responsible for these responses. The present experiments wereconducted to quantify the increases in crop growth and yield,
cumulative intercepted radiation, and radiation-use efficiency indeterminate and indeterminate cultivars sown earlier than normal,
which the literature suggested would increase soybean yield.

2. Materials and Methods
The experiments were conducted at the NARO Tohoku
Agricultural Research Center in Morioka, Iwate Prefecture,
Japan (39°44′
N, 141°7′E), in 2013, 2014, and 2015. Different
converted paddy fields were used in each of the three years.
Supplemental Table S2 summarizes the soil characteristics andcultivation conditions. The soil in each field was an Andosol.
-2Each field received 200 g m of fused phosphate fertilizer, and-2
100 g m of magnesium lime approximately 1 month before-2
sowing in each year. In addition, the fields received 3 g m of N,-2-2
12. 5 g m of P (P2 O5 equivalent), and 5 g m of K (K2 O
equivalent) in the form of a compound fertilizer 1 day before
sowing in each year. Fertilizers were applied and incorporated toa depth of approximately 15 cm using rotary tillers.
In 2013, I grew two cultivars: a determinate Japanese cultivar(Ryuhou) and an indeterminate line (Karikei-881). In 2014,
I grew four cultivars: Ryuhou and three indeterminate cultivars(Karikei-881 and Karikei-879 from Japan, and UA4910 fromthe United States). In 2015, I grew three cultivars: Ryuhou,
Karikei-879, and UA4910. Ryuhou is a leading cultivar frommaturity group (MG) IV in the Tohoku region (Kumagai andSameshima, 2014). Karikei-881 and Karikei-879 are recombinant inbred lines derived from crosses between the determinate
Japanese cultivar Ohsuzu and the indeterminate American
cultivar Athow (Kato et al., 2015). Karikei-881 and Karikei-879
are late- and early-maturing lines, respectively, and the latter’s
growth duration is similar to that of Ryuhou. UA4910 is anindeterminate American cultivar from MG IV, with a high yield− 2 −E. Kumagai : Effect of early sowing on growth and yield of soybean in a cool region of northern Japan(Chen et al., 2011). It had greater seed yield than commonlygrown Japanese cultivars in southwestern Japan (Matsuo et al.,2016).
These cultivars were grown to maturity (September-October).
Seeds were treated with a combined insecticide and fungicide(CruiserMaxx, Syngenta Co., Tokyo, Japan) at the dosage
recommended by the manufacturer and then sown by hand at adensity of three seeds per hill in mid-May (the early-sowing [ES]treatment) and late May (the normal-sowing [NS] treatment),
with a difference of 14 to 15 days between the two dates(Supplemental Table S2). The sowing density was 9. 5 hills-2(plants) m , at 0.15 m between hills × 0.70 m between rows,-2in 2013 and 2015, and 8.9 hills m , at 0.15 m between hills ×
0.75 m between rows, in 2014. This corresponds to therecommended local planting densities, which range from 7 to 15-2hills m . Plants were thinned to one per hill after establishment.
A pre-emergence herbicide (Ecotop, Maruwa Biochemical Co.,Tokyo, Japan), which contained 1. 0% Dimethenamide-p and1.4% linuron, was applied just after sowing, at the dosage
recommended by the manufacturer, for weed control.
Plants were not irrigated, and relied entirely on precipitation
for their water. A month after sowing, tilling was performed
between the rows to a depth of 15 cm, and ridging was performed
to control weeds and lodging. The pesticides and insecticides
were reapplied to maximize the yield and seed quality. In eachyear, the experiment desigin was a split-plot arrangement;
the main plot was sowing date (NS and ES) and the subplot wascultivar, with three replicates. The subplot consisted of 7 rows ×
4 m long in 2013, 7 rows × 3 m long in 2014, and 6 rows ×
3.3 m long in 2015.
For both NS and ES, I monitored the phenology of 10 plants inthe center row of each plot at intervals of 2 or 3 days andrecorded the dates of each growth stage based on the staging
system of Fehr and Caviness (1977): VE, emergence; R1, thebeginning of flowering; R2, full flowering; R5, the beginning ofseed filling; and R7, the beginning of maturity.
I determined dry weight of aboveground parts per m2 bymeans of periodic sampling two or three times during thegrowing stage of each year. Aboveground parts of center threerows (0.63 m2 and 0.68 m2 in 2013 and 2014, respectively) or
center four rows (0.84 m2 in 2015) were harvested. The dryweight of aboveground part was measured after oven-drying at
80°
C for 3 days. Sampling dates (day of year, DOY) are listed in
Supplemental Table S3. In 2013, the sampling dates were 4 July(late vegetative stage), 24 July (0 to 20 days after R1; thisnumber depended on the sowing date and cultivar), and 22
August (0 to 15 days after R5; this number depended on thesowing date and cultivar). In 2014, sampling was performed at
R1 and 30 days after R1. In 2015, sampling was performed at R2
and the beginning of seed filling (R5).
During vegetative stage (from VE to 4 July, R1 and R2 in
2013, 2014 and 2015, respectively), the crop growth rate per unit
-2
-1
area (CGR; g m day ) was calculated as the aboveground dryweights on the first sampling date, divided by the number of days
from VE to the first sampling. Because plants were not sampled
at VE, the aboveground dry weights at VE were assumed to be
zero. During reproductive stage (from 4 July to 22 August, from

R1 to 30days after R1, from R2 to R5 in 2013, 2014 and 2015,
respectively), in 2013, the CGR was determined from the slopeof the linear regression of aboveground dry weight against DOY
from three sampling dates in two cultivars. In 2014 and 2015,
CGR was calculated as the difference between the aboveground
dry weights on the two sampling dates, divided by the number ofdays between the two measurements.
The fraction of intercepted solar radiation (F) was estimatedby measuring solar radiation below the canopy (Io) and incident
radiation above the canopy (I) using integrated solarimeter films(color acetate films; Opto Leaf Y1-W; Taisei Chemical
Industries, Tokyo, Japan). In 2013, the intercepted solar radiation
was calculated as follows (Shiraiwa et al., 2011):
F = 1 - (Io / I)

(1)

Io and I were estimated by installing several 1-m-long bars
holding seven squares of film (each 2 cm × 1.5 cm at 15-cm
intervals), between the rows, below and above the canopy. The
color acetate films faded as they absorbed solar radiation, and thedegree of fading of each film was determined using a portable
photometer (D-Meter RYO-470, Taisei Chemical Industries,
Ltd., Tokyo, Japan). This degree was assumed to be negatively
and nonlinearly correlated with the amount of solar radiation
following an empirical equation derived by the manufacturer.
The measurements were collected for 5 to 7 days, and Io and I
were subsequently determined. These measurements wereconducted repeatedly between the first and the last sampling
dates.
In 2014 and 2015, I estimated the intercepted solar radiationby using digital imaging techniques (GACS1; Kimura
Ouyou-Kougei Co. Ltd., Saitama, Japan). The fractional canopy
cover (Cf) was determined by using digital images taken above
the canopy at 1-week intervals from the early vegetative to late
reproductive stages. In each year, total daily solar radiation (MJ-2-1
m day ) was recorded at a weather station near the study field,
and daily incident radiation above the canopy (DIR) was
computed as the product of daily solar radiation and Cf.
Cumulative intercepted radiation (CumIR) from VE to the last
sampling was computed by summing the DIR values. The mean
radiation-use efficiency (RUE) for biomass production wasdetermined from two or three samples as the slope of the linear
regression between the dry weight of the aboveground parts and
CumIR (Monteith, 1994). Although I used two different methods
to estimate intercepted solar radiation, a previous study ofsoybean showed a significant correlation in the estimated values
between the methods (Shiraiwa et al., 2011).
In 2014 and 2015, I sampled soil between the plants to a depthof 30 cm in each plot using a hand soil sampler (30 cm in lengthand 5 cm in inner diameter; HS-30S; Fujiwara Scientific, Co.,
Ltd., Tokyo, Japan) at the final sampling dates to determinate theroot length density (RLD; cm cm-3). I divided the samples into
depths of 0 to 15 cm and 15 to 30 cm, and then carefully removed
the soil from the root samples with tap water. I then completely
removed the root nodules before measuring RLD with a
WinRHIZO system (Regent Instruments, Montreal, PQ,
Canada).
At maturity, I manually harvested the aboveground parts from− 3 −

Journal of Agricultural Meteorology
3.36 m2 of each plot in 2013, 3.38 m2 in 2014, and 4.20 m2 in
2015. After completely air-drying the plants, I removed the few
remaining leaves and petioles from the aboveground parts, andthen weighed the remaining parts. Aboveground biomass
equaled the sum of the weights of the stems, pod shells, andseeds. Filled pods and the number of mature seeds were counted
and the mature seeds were weighed. Seed yield per m2 wasadjusted to a 15% moisture content, and the 100-seed weight wasdetermined as (the seed yield / the number of seeds) × 100.
In addition to solar radiation, daily means of temperature andprecipitation during the growing season were recorded at aweather station near the field. The soil volumetric moisture
content (SMC, m m-3) was monitored with soil moisture sensors(EC5; Decagon Devices Inc., Pullman, WA, USA) at 7.5 cm
below the soil surface (the midpoint of the plow layer) of the NS
and ES plots. SMC was measured and recorded every 30 min
using a datalogger (CR10X; Campbell Scientific Inc., Logan,
UT, USA).
To test for significant differences between the two sowing
dates and between the cultivars in each year, and for significant
date × cultivar interactions, I used two-way ANOVA performedby the General Linear Model procedure of SPSS 22. 0 for
Windows (IBM, Tokyo, Japan). Sowing date, cultivar, and theirinteraction were each considered as fixed effects, and replication(block) was considered as a random effect. When the ANOVA
produced a significant result, Fisher’s LSD test for significant
differences between means was performed. For Cf, I used
Student’s t-test to detect a significant difference between the twosowing dates in each cultivar on a given date (DOY) in 2014 and2015. I performed a curvilinear regression for the relationshipbetween mean precipitation and mean Cf during the vegetative
stage.

3. Results
3.1 Weather conditions and SMC
Table 1 summarizes the weather conditions during the three
growing seasons. The 6-month (May to October) mean
temperature in the three years was 0.6 to 0.9°
C higher than the
30-year mean. The large difference was observed in June 2013
and 2014 (1.8 and 2.4°
C, respectively, above the mean). In 2015,

Fig. 1. Changes in the soil volumetric moisture content (SMC)
from July to September during the growing season in the three
years of the study. DOY, day of year.

mean temperatures in May and July were 2.5 and 2.1°
C higherthan the 30-year mean. The 6-month mean daily solar radiationin 2013 was similar to the 30-year mean, but those in 2014 and-2 -1
2015 were 1.7 and 2.0 MJ m d higher than the mean. The
6-month mean precipitation in 2014 was close to the 30-year
mean (143.8 vs. 139.4 mm, respectively), but 2013 was wetter
than normal (by 70. 4 mm). 2013 and 2014 had higher-thannormal mid-season (July to August) precipitation, whereas 2015
had lower-than-normal one. In 2015, monthly total precipitationin July and August was 87.9 and 36.4 mm lower than the 30-year
mean, respectively.
Soil moisture contents (SMC) in 2013 and 2014 were highand stable throughout the growing season, whereas that in 2015
decreased to low levels during July and mid-August (Fig. 1).
3.2 Growth duration, aboveground biomass, seed yield, pod
number and 100-seed weight
In all three years, early sowing significantly extended days
from VE to R1 and R1 to R7 (by 3 to 5 days in both parameters)
when averaged across the cultivars (P < 0.001; Table 2). These
parameters differed significantly among the cultivars: Days from
VE to R1 were longest in Karikei-881, followed by Ryuhou,
UA4910 and Karikei-879. Days from R1 to R7 of indeterminate

Table 1. Monthly mean air temperature, solar radiation, and total precipitation during the 2013, 2014, and 2015 growing seasons, andcomparison with the 30-year mean values.

Temperature(°
C)

Solar radiation-2 -1(MJ m d )Precipitation
(mm)

Year
2013
2014
2015
30-year mean
2013
2014
2015
30-year mean
2013
2014
2015
30-year mean

May
13.1
14.9
15.9
13.4
17.2
20.4
21.3
17.4
49.5
69.0
87.0
101.4

June
19.6
20.2
18.5
17.8
20.4
18.6
19.2
17.1
37.5
71.5
123.5
108.1
− 4 −

July
21.8
22.7
23.4
21.3
12.0
18.4
17.8
15.1
503.5
261.5
108.5
196.4

Aug.
23.5
22.7
22.7
22.9
16.6
12.2
15.6
15.1
202.5
236.0
145.5
181.9

Sept.
19.1
17.1
17.9
18.2
12.2
15.3
13.2
11.9
210.0
95.0
104.5
155.9

Oct.
13.3
11.1
10.8
11.5
8.3
11.6
11.1
9.9
255.5
130.0
124.0
92.5

Mean
18.4
18.1
18.2
17.5
14.5
16.1
16.4
14.4
209.8
143.8
115.5
139.4

E. Kumagai : Effect of early sowing on growth and yield of soybean in a cool region of northern Japan
Table 2. Days from emergence (VE) to beginning flowering (R1) and from R1 to beginning maturity (R7), aboveground biomass, seedyield and yield components in the soybean cultivars with normal and early sowing dates (NS and ES, respectively) in 2013, 2014, and
2015.
Days
Days
Aboveground
Seed
Pod
100-seedfrom VE from R1 biomass
yield
numberweightYear
Sowing date
Cultivar
to R1
to R7
-2
-2
-2
(days)(days)(g m )(g m )
(m )
(g)
2013
NS
52
69
781
356
775
33.3
ES
57
73
899
414
862
33.6
Ryuhou
52
63
755
392
809
33.2
Karikei-881
56
78
925
378
828
33.7
ANOVA
Sowing date (S) ***
***
*
*
*
ns
Cultivar (C)
***
***
***
ns
ns
ns
S×C
**
ns
ns
ns
ns
ns
2014
NS
44
78
730
398
827
26.8
ES
48
82
876
470
908
28.5
Ryuhou
47 b
71 d
665 c
394 b
687 c
34.7 aKarikei-881
52 a
83 b
862 ab391 b
780 bc31.4 bKarikei-879
41 c
77 c
760 bc435 b
843 ab
25.4 c
UA4910
44 d
89 a
928 a
517 a
1158 a19.0 d
ANOVA
S
***
***
**
**
*
***
C
***
***
**
**
***
***
S×C
***
***
ns
ns
ns
ns
2015
NS
49
71
689
398
790
27.2
ES
52
76
710
401
813
26.8
Ryuhou
54 a
66 c
649 b
387 b
668 a
34.9 aKarikei-879
48 b
73 b
645 b
371 b
743 b
26.5 b
UA4910
50 c
83 a
804 a
439 a
993 b
19.6 c
ANOVA
S
***
***
ns
ns
ns
ns
C
**
***
***
*
***
***
S×C
*
***
†
ns
ns
ns
Values of a parameter followed by the same letter did not differ significantly between cultivars (ANOVA followed by Fisher’s LSD test,
P < 0.05). ANOVA results: ***P < 0.001; **P < 0.01; *P < 0.05; † P < 0.10; ns, not significant.

cultivars were longer than that of Ryuhou. ANOVA showed nosignificant sowing date × cultivar interaction for abovegroundbiomass, seed yield, pod number or 100-seed weight in all three
years (Table 2). The effect of early sowing on abovegroundbiomass, seed yield and pod number differed among the years.
When averaged across the cultivars in 2013 and 2014, earlysowing significantly increased aboveground biomass (by 15%
and 20%, respectively), seed yield (by 16% and 18%,
respectively) and pod number (by 11% and 10%, respectively).
In 2015, in contrast, sowing date had no significant effect onthese parameters. ANOVA showed a significant cultivar effecton aboveground biomass in 2013: Karikei-881 had 23% higher
aboveground biomass than Ryuhou. The 2014 and 2015
experiments both showed significant cultivar effects onaboveground biomass, seed yield and pod number: When
averaged between the two sowing dates in 2014, abovegroundbiomass was greatest in UA4910, followed by Karikei-881,
Karikei-879, and Ryuhou. Moreover, UA4910 showed significant 31% greater seed yield and 68% greater pod number than
Ryuhou. In 2015, aboveground biomass, seed yield and pod
number in UA4910 were significantly higher than that in
Ryuhou. Averaged across the cultivars, the 100-seed weight was

6% greater in ES than in NS in 2014; there was no significant
difference between sowing dates in the other years. In 2014 and2015, there were significant cultivar effects on the 100-seedweight: that was greatest in Ryuhou, followed by Karikei-879
and UA4910.
3.3 Cf, CGR, RUE, and CumIR
Cf was significantly higher in ES than in NS for most of thegrowing period (DOY 170 to 220) in all cultivars in 2014
(Supplemental Fig. S1). In 2015, Cf in ES plants were also
higher until around DOY 200 in all three cultivars but there wasno difference between two sowing dates during the most of
reproductive stage after R1. This reason was that NS plants
showed more rapid increase of Cf during early stage in 2015 thanin 2014.
Tables 3 and 4 summarizes the growth and light-use
parameters. As shown in Table 3, in 2013 and 2014, early sowing
significantly increased CGR during vegetative stage (by 50%
and 31%, respectively) when averaged across the cultivars. Earlysowing also increased significantly CumIR and mean Cf during
early stage by 31% and 14%, respectively in 2014. In 2015, incontrast, early sowing did not increase these parameters, and− 5 −Journal of Agricultural Meteorology
Table 3. Crop growth rate (CGR), radiation-use efficiency for dry matter production (RUE), cumulative intercepted irradiation (CumIR)and mean fraction of canopy cover (mean Cf) during vegetative stage of the soybean cultivars with normal and early sowing dates (NSand ES, respectively) in 2013, 2014, and 2015.

RUE
CumIR
Mean Cf
CGR
-2 -1
-1
-2
(g MJ )
(MJ m )(%)
(g m d )
NS
0.91
2013a
ES
1.37
Ryuhou1.18
Karikei-881
1.11
ANOVA
Sowing date (S)*Cultivar (C)
ns
S×C
ns
NS
1.56
0.63
114
14
2014b
ES
2.05
0.63
150
16
Ryuhou2.03 b
0.62 b155 b
17 b
Karikei-881
2.96 a0.77 a201 a
21 a
Karikei-879
1.21 c0.58 b91 c
11 c
UA49101.01 c0.55 b
81 c
10 d
ANOVA
S
*
ns
*
*
C
***
**
***
***
S×C
ns
ns
***
***
NS
2.55
0.64
195
21
2015c
ES
2.00
0.52
208
19
Ryuhou3.01 a
0.65
255 a
25 a
Karikei-879
2.14 b
0.57
184 b
19 b
UA49101.66 c
0.52
165 c
17 c
ANOVA
S
†
ns
†
*
C
**
ns
***
***
S×C
ns
ns
ns
ns
a
CGR, RUE and Mean Cf were determined on two sampling dates (VE and day of year [DOY] 187). CumIR was determined from VE to
DOY 187.
b
CGR, RUE and Mean Cf were determined on two sampling dates (VE and R1). CumIR was determined from VE to R1.
c
CGR, RUE and Mean Cf were determined on two sampling dates (VE and R2). CumIR was determined from VE to R2.
Values followed by the same letter did not differ significantly between cultivars (ANOVA followed by Fisher's LSD test, P < 0.05).
ANOVA results: ***P < 0.001; **P < 0.01; *P < 0.05; † P < 0.10; ns, not significant. -, not determined.
Sowing date

Cultivar

these values were higher in 2015 than in 2014. Significant effects
of sowing date on RUE were not observed in 2014 and 2015.
Significant effects of cultivar on CGR, CumIR and mean Cf wasobserved in 2014 and 2015.
The 2013 and 2014 experiments both showed that earlysowing significantly increased CGR (by 23% in 2013 and 18%
in 2014), CumIR (by 8% in 2013 and 26% in 2014) and mean Cf(by 10% in 2013 and 13 % in 2014) during reproductive stage
when averaged across the cultivars (Table 4). In 2015, there
were no significant effects of sowing date on CGR and CumIR.
Mean Cf was significantly higher in NS plants than in ES plants.
The effect of sowing date on RUE differed between the years: ES
plants showed greater RUE than NS plants in 2013, but there wasno significant difference between ES and NS in 2014 or 2015. A
significant effect of cultivar on CGR and RUE was observed in2013 and 2014: However, there were no significant differencesin CGR or RUE between the cultivars in 2015. CumIR differedsignificantly among the cultivars in 2014 and 2015.
When data were combined across all cultivars grown under
two sowing dates in 2014 and 2015, a relationship between mean
precipitation and mean Cf during the vegetative stage wascurvilinear with a significant regression coefficient (R2= 0.351,
P < 0.05, Fig.2).

f

Year

Fig. 2. A curvilinear regression for the relationship between themean precipitation from emergence to the first sampling and themean Cf during the stage for all cultivars combined between
2014 and 2015. All data were combined. *P < 0.05.

− 6 −E. Kumagai : Effect of early sowing on growth and yield of soybean in a cool region of northern Japan
Table 4. Crop growth rate (CGR), radiation-use efficiency for dry matter production (RUE), cumulative intercepted irradiation (CumIR)and mean fraction of canopy cover (mean Cf) during reproductive stage of the soybean cultivars with normal and early sowing dates(NS and ES, respectively) in 2013, 2014, and 2015.
RUE
CumIR
Mean Cf
CGR
Year
Sowing date
Cultivar(g MJ-1)(MJ m-2)(%)(g m-2 d-1)
2013a
NS
10.1
1.13
436
61
ES
12.4
1.28
470
67
Ryuhou
12.0
1.28
446
65
Karikei-881
10.5
1.12
460
64
ANOVA
Sowing date (S)
**
*
***
*
Cultivar (C)
*
*
ns
ns
S×C
ns
ns
***
**
2014b
NS
9.6
0.85
336
79
ES
11.3
0.79
423
89
Ryuhou10.9 b0.83 b396 b
88 b
Karikei-881
12.0 a0.92 a405 a
96 a
Karikei-879
10.0 b0.80 b382 b
78 c
UA49108.7 c
0.73 b
335 c
72 d
ANOVA
S
*
ns
***
*
C
**
*
***
***
S×C
*
ns
***
***
2015c
NS
19.5
1.15
479
95
ES
18.9
1.17
494
93
Ryuhou
21.9
1.23
368 b
95 a
Karikei-879
18.6
1.16
542 a
95 a
UA4910
17.1
1.10
564 a
93 b
ANOVA
S
ns
ns
ns
*
C
†
ns
***
*
S×C
ns
†
ns
ns
a

CGR, RUE and Mean Cf (= F) were determined on three sampling dates (day of year [DOY] 187, 207, and 235). CumIR wasdetermined from DOY 187 to DOY 235.
b
CGR, RUE and Mean Cf were determined on two sampling dates (R1 and R1 + 30 days). CumIR was determined from R1 to R1 + 30
days.
c
CGR, RUE and Mean Cf were determined on two sampling dates (R2 and R5). CumIR was determined from R2 to R5.
Values followed by the same letter did not differ significantly between cultivars (ANOVA followed by Fisher's LSD test, P < 0.05).
ANOVA results: ***P < 0.001; **P < 0.01; *P < 0.05; † P < 0.10; ns, not significant.

3.4 RLD
Table 5 summarizes the RLD results in 2014 and 2015. Earlysowing significantly increased RLD at a depth of 0 to 15 cm in2014 but not in 2015. RLD at a depth of 15 to 30 cm, averaged
across the cultivars, was significantly higher in NS than in ES in
2015, but there was no difference between the sowing dates in
2014. The 2014 experiment showed marginally significant andsignificant effects of cultivar on RLD at depths of 0 to 15 cm and
15 to 30 cm, respectively. In 2014, Karikei-881 tended to
produce greater RLD than the other cultivars in both depth
ranges and for both sowing dates. In 2015, there was no
significant difference in RLD between the cultivars in the upper
soil depth and only a marginally significant effect on RLD in thelower soil depth. RLD of UA4910 and Karikei-879 tended to behigher than that in Ryuhou. RLD of the NS plants at both soil
depths was higher in 2015 than in 2014, presumably because of
the lower SMC in 2015.

4. Discussion
The objective of this study was to test the hypotheses that earlysowing would increase soybean yield and that the indeterminate

cultivars would show greater yield and a greater yield response to
early sowing in the cool climate of northern Japan. The first
hypothesis was partially validated, but the second hypothesis was
rejected.
4.1 Early sowing ameliorated seed yield by increasing Cf,
CumIR and CGR from vegetative to reproductive stage
in years with high precipitation, but not in a year withlow precipitation
Some researchers previously reported that the beneficial
effects of early sowing on soybean yield were affected by
precipitation (Heatherly and Elmore, 1983, Kane et al., 1997,
Robinson et al., 2009). The present study provides the first dataon the causes of the yield gain due to early sowing anddifferences in yield responses between years in terms of CumIRand RUE and root growth. The effect of early sowing on yield
varied substantially among the three years: When averaged
across the cultivars, early sowing significantly increased
seed yield in 2013 and 2014 with high precipitation during
mid-season (July and August) but not in 2015 with low
precipitation (Table 2). This discrepancy resulted from the− 7 −Journal of Agricultural Meteorology
Table 5. Root length density (RLD) at depths of 0 to 15 cm and 15 to 30 cm for the soybean cultivars withnormal and early sowing dates (NS and ES, respectively) in 2014 and 2015.

Year

Sowing date-3

Cultivar

RLD (cm cm )0-15 cm
15-30 cm
2014
NS
3.89
2.01
ES
4.55
2.29
Ryuhou4.44
1.77 bKarikei-881
4.88
3.02 aKarikei-879
3.81
2.12 b
UA49103.76
1.68 bANOVA
Sowing date (S)*ns
Cultivar (C)
†
*
S×C
ns
ns
2015
NS
4.72
3.18
ES
5.12
1.99
Ryuhou4.82
1.89
Karikei-879
5.00
2.64
UA4910
4.93
3.22
ANOVA
S
ns
*
C
ns
†
S×C
ns
ns
Values followed by the same letter did not differ significantly between cultivars (ANOVA followed by Fisher's
LSD test, P < 0.05). ANOVA results: *P < 0.05; † P < 0.10; ns, not significant.

difference in CGR, CumIR and Cf from VE to the early
reproductive stage (Tables 3 and 4) that might be associatedwith the differences in precipitation (Table 1) and in SMC(Fig. 1). In general, seedling establishment and early canopy
development were important to achieve high yield of soybean. It
is widely accepted that early leaf expansion and canopy
development are both inhibited strongly by excess water stress(e.g., Bajgain et al., 2015). The present study showed that thecurvilinear relationship existed between mean Cf and mean
precipitation during vegetative stage when combined across two
sowing dates in 2014 and 2015 (Fig. 2): In 2014, NS plants(especially Karikei-879 and UA4910) showed lower Cf underhigher precipitation condition than ES plants. In contrast, both
NS and ES plants showed higher mean Cf under low
precipitation during vegetative stage in 2015. The differences of
Cf between two sowing dates were also observed during early
reproductive stage in the two years. These results raised thepossibility that early sowing would escape from the excess water
stress and ameliorate canopy development and thereby yield inyears with higher-than normal precipitation of this region.
Generally, CGR, CumIR and RUE during reproductive stage
are significantly positively correlated with soybean yield (e.g.,Kawasaki et al., 2016). In the present study, CGR, CumIR and
RUE were measured from the different stages in each year (fromlate vegetative stage to around R5 in 2013, from R1 to 30 days
after R1 in 2014, and from R2 to R5 in 2015, respectively).
Although CGR cannot be accurately compared among the years
in this study because the values were strongly depended on thegrowth stages (e.g., Van Roekel and Purcell, 2014), this value
was higher in 2015 than in 2013 and 2014 (Table 4). The
increases of CGR and CumIR caused by early sowing appear tobe the main contributors to the increased yield in the two years.
On the other hand, low precipitation and SMC promoted morestrongly Cf, CumIR and CGR in NS than in ES in 2015. These
differences of parameters between sowing dates were probably
owing to those in root development as discussed later. Thus,
early sowing did not increase mean Cf, CumIR, CGR and thereby
aboveground biomass and seed yield in 2015 (Tables 2, 4).
It was reported that the longer growth duration was involved
with yield increase (Rowntree et al., 2014; Matsuo et al., 2016).
The present study revealed that early sowing extended theperiods from VE to R1 and R1 to R7 regardless of the cultivars in
the three years (Table 2), but the longer growth duration by earlysowing did not always resulted in yield increase.
Robinson et al. (2009) reported that the difference in yield
between sowing dates was strongly explained by pod number per
unit area. Dry matter production during the early reproductive
stage is essential for determining the pod and seed number insoybean (Egli, 2010; Shiraiwa et al., 2004). The present results
confirm that the increased pod number due to early sowing wasmost likely derived from the increased CGR, which in turnresulted from the increased CumIR from the flowering stage (orlate vegetative stage) to the middle reproductive stages. The
100-seed weight was significantly increased by early sowing in2014 (Table 2). However, the increase of the 100-seed weight
was not consistent throughout the experiments. Several
researchers reported no effect of sowing date on the 100-seedweight (Bastidas et al., 2008; De Bruin and Pedersen, 2008;
Rowntree et al., 2014), which agrees well with the present
results.
Early sowing significantly increased RLD in the upper soil in
2014, when there was excess soil water (Table 5). This agrees
with the result of Turman et al. (1995), who found that soybean
sown in mid-May in Missouri had a larger root system 30 days
after emergence than plants sown in mid-June. In contrast, in2015 with a low SMC, NS plants had significantly higher RLD− 8 −E. Kumagai : Effect of early sowing on growth and yield of soybean in a cool region of northern Japan-3

than ES plants in the deep soil layer (3.18 vs 1.99 cm cm ,respectively), although there was no difference in RLD between
the two dates in the upper soil. Water deficiency stimulates root
growth of soybean (Hoogenboom et al., 1987). In Japan,
soybean grown under conditions with less-than-normal SMCs
before flowering had a larger RLD (Hirasawa et al., 1998). Thisagrees with the present result: the RLD in 2015 was tended to behigher than that in 2014 for both sowing dates and both depths.
ES plants in 2015 gradually restricted their canopy development
due to low SMC and RLD, whereas NS plants could absorb
water from the deeper soil and develop their canopy better.
Although I did not explore possible causes for the difference in
RLD in the deeper soil between the two dates, one possible
explanation was the effect of the timing of water deficiency onroot development. Hoogenboom et al. (1987) observed a large
increase in root growth when water deficits occurred during laterstages of vegetative growth or early stages of reproductive
development (from R1 to R2). Root growth was less affected bydrought after plants had reached the pod development stage (R4)and finally ceased during R5. On the other hand, roots continued
to extend linearly into deep and barrier-free soils from the earlyvegetative stage to R5 (Mayaki et al., 1976; Mitchell andRussell, 1971), but the pattern of root growth could be site- andcultivar-specific. In 2015, roots of NS plants might have beenstimulated by a low SMC during the most developmental stages
and then grow more in the deeper soil. Further investigation ofroot dynamics during the growing season in the cool Tohoku
region of northern Japan will be needed in future studies.
The magnitude of the relative seed yield increase due to earlysowing (16% in 2013 and 18% in 2014, on average, across thecultivars) was similar to previous results obtained in the
Midwestern United States. Bastidas et al. (2008) reported
higher yields from late-April and early-May sowings in both
years of an irrigated experiment in Nebraska, but the increase
was only 8% in the first year and 16% in the second year. De
Bruin and Pedersen (2008) evaluated the response to sowing
date in two years in Iowa, and found that late-April andearly-May sowings produced higher yields (by an average of8%) than late-May plantings.
Although the present study suggests that early sowing is aneffective management strategy to increase soybean yield in thecool Tohoku region of northern Japan, several points must be
considered. First, as discussed above, the beneficial effect ofearly sowing was affected by the amounts and patterns ofprecipitation during growing season. Therefore, the combinationof both early sowing and water management such as irrigationand drainage will be required to increase soybean yield in thisregion. Second, the present results were derived from field datain years with unusually warm weather early in the growing
season (May in all years except 2013 and June in all years). The
inhibition of seedling emergence and therefore of canopy
development that occurs at low temperatures was not observed inany year, even in 2013, when the May weather was slightly
colder than the long-term average. As Horai et al. (2013) noted,
the mean air temperatures in May and June have been increasing
significantly in Iwate Prefecture since the 1920s, but there has
been high fluctuation between years. Further experiments during

years with a cooler May and June might have produced different
results. Previous growth chamber experiments by my researchgroup showed that future increasing temperatures and atmospheric CO2 concentrations will increase the seed yield with anormal sowing date (Kumagai et al. 2012, Kumagai andSameshima, 2014; Kumagai et al., 2015). I hypothesize that earlysowing may be more successful in the future and that soybean
production will therefore increase if temperatures continue toincrease in this region owing to global warming.
A final problem is that the early sowing dates tested in thisstudy overlap the currently recommended period for ricetransplanting in Iwate Prefecture (Shimono et al., 2010). Field
experiments by Horai et al. (2013) revealed that transplanting
10 days earlier did not increase the grain yield of the current ricecultivars that are grown widely in this region. To develop an
agricultural system that will increase the production of both riceand soybean in this region, the positive effect of ultra-earlysowing (in early May, which does not overlap the paddy ricetransplanting period) on soybean growth and yield should betested in future studies.
4.2 Indeterminate cultivars did not always surpass determinate Ryuhou in a cool region
The present study revealed that early sowing significantly
increased the seed yield, with no sowing date × cultivar
interaction, in the two years with adequate soil moisture(Table 2), indicating that the magnitude of the yield increase in
response to early sowing did not differ among the cultivars
tested. However, it is possible that using only a single
determinate cultivar (Ryuhou) prevented the present study fromdetecting a difference between determinate and indeterminate
cultivars. Further investigation of the response to early sowing
dates using many cultivars will be needed to select materials withthe strongest response to ES in northern Japan.
A significant effect of cultivar on seed yield was observed in2014 and 2015, but not in 2013 (Table 2). However, theindeterminate cultivars did not always surpass the determinate
cultivar Ryuhou. Kato et al. (2015) compared the yield ofindeterminate and determinate lines among Ohsuzu × Athow
recombinant inbred lines with a normal sowing date at Akita, in
the Tohoku region, and showed that indeterminate lines withearly maturity had significantly higher yield than determinatelines with early maturity. Similarly, the present study showedthat early-maturing Karikei-879 had a growth duration (daysfrom VE to R7) similar to that of Ryuhou when averaged
between the sowing dates in two years, but that the abovegroundbiomass and seed yield of this cultivar were tended to be higher
than those of Ryuhou in 2014 (a year with high precipitation andhigh SMC), but not in 2015, the year with low SMC (Table 2).
On the other hand, late-maturing Karikei-881 had a longer
growth duration and heavier aboveground biomass than Ryuhou,
but a similar pod number and yield, when averaged between thetwo dates in 2013 and 2014 (Table 2). In both 2014 and 2015,
UA4910 had a longer growth duration, and greater pod number
per m2 than Ryuhou, and thereby had a higher seed yield. A
recent yield comparison between Japanese and American
cultivars by Matsuo et al. (2016) showed that this cultivar had a− 9 −Journal of Agricultural Meteorology
higher pod number and seed yield than several Japanese cultivars
in southwestern Japan. Kawasaki et al. (2016) also reported that
the American cultivars showed superior seed yield to Japanesecultivars due to their high biomass production capacity and high
RUE during the seed-filling stage in a warmer region of Japan.
In the present study, CGR and RUE were calculated during the
30 days after R1 in 2014 and from R2 to R5 in 2015, which isearlier than the stages studied by Kawasaki et al. (2016).
Therefore, further analysis of these parameters, with a focus on
the whole growing season, but especially on the late seed-fillingstage, will be needed to elucidate the factors responsible forsuperior seed yield.
An interesting finding was the plasticity of RLD in UA4910 in2014 and 2015: RLD values of this cultivar in both the shallow
and deep soil layers were similar to those of Ryuhou in 2014, theyear with high SMC, whereas there was a tendency towardhigher RLD in the deep soil layer in UA4910 in 2015, the yearwith low SMC. This characteristic probably contributed to thecultivar’s high yield even in 2015 with low SMCs (Table 2).
Karikei-881 had higher RLD in both soil layers than the otherthree cultivars in 2014. There has been limited information
obtained on the effect of stem determinacy on root characteristics
so far. An early report by Allmaras et al. (1975) in southwestern
Minnesota showed that the total root dry weight per unit of shootdry matter in an indeterminate isoline of Harosoy was generally
greater than that in a determinate isoline, but that more root dry
matter was concentrated in the upper 30 cm of the soil.
However, since these differences are presumably site- andcultivar-specific, it will be essential to compare many accessions
in different years and at different locations. Further investigations of the physiological bases of yield differences between
Japanese and American soybean cultivars will be needed to guide
breeding for high-yielding Japanese cultivars in future studies.

5. Conclusions
Three years of field experiments in northern Japan showed thatearly sowing increased soybean yield in years with warm
temperatures and with high precipitation and SMC. The yield
increase was owing to greater pod number and higher CGR and
CumIR due to a greater canopy development during vegetativeand reproductive stage, which might be associated with thedecreased precipitation and SMCs by early sowing. These results
suggest that early sowing is an effective option for reducing therisk of excess water stress in vegetative stage and increasing
yield of soybean in this region. In both the indeterminate anddeterminate cultivars, seed yield was increased by earlier sowing,
but without a cultivar × sowing date interaction, suggesting thatthe indeterminate soybean cultivars did not benefit more than thedeterminate soybean from early sowing in northern Japan.

Supplemental information
Supplemental information for this paper is available at
http://doi.org/10.2480/agrmet.D-17-00009.

Acknowledgments
This study was financially supported by the Research Fund forSocial Requests from the National Agriculture and Food

Research Organization (NARO). I am grateful to Hideyuki
Mochida (Tohoku Agricultural Research Center [TARC],
NARO) for his advice on conducting these experiments, Shin
Kato and Akio Kikuchi (TARC, NARO) and Makita Hajika(Institute of Crop Science, NARO) for providing the seeds, andNaoki Matsuo (Kyushu Okinawa Agricultural Research Center,
NARO) and Maya Matsunami (TARC, NARO) for their advice
on root sampling and analysis. I thank Hisashi Tamura, Eisaku
Kumagai, Fumihiko Saitou, Akihiko Umihata, and Kafumi
Segawa for their technical assistance. I would like to thank theanonymous reviewers for invaluable comments and suggestions.

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