2025 年 94 巻 3 号 p. 313-322
Production and consumption of broccoli in Japan are gradually increasing; however, the country is highly dependent on imports to meet the growing demand. Large head production systems with ‘Grandome’ can decrease the reliance on imports, but their adaptability across broader areas remains unclear and it is also necessary to consider other cultivars according to regional and cultivation conditions. Therefore, in this study, ‘Grandome’ yield was evaluated from 2019 to 2021 at six autumn to spring production sites (Hyogo, Hiroshima, Kumamoto, Kanagawa, Kagawa, and Ehime). In addition to the yield evaluation of ‘Grandome’, cultivar selection tests suitable for large head production were conducted at the former three sites using 25 cultivars in Experiments 1 to 3, while the yield evaluation exclusively for ‘Grandome’ was conducted under different fertilization conditions and planting densities at the latter three sites in Exp. 4, comparing floret yield (small pieces of broccoli separated from the head). Experiments 1 to 3 revealed that ‘SK9-099’ produced high yield during the relatively warm November harvest, whereas in the subsequent December–January harvest, ‘Grandome’ and ‘Clear’ produced high yields. In years that were colder than usual during the aforementioned period and for cropping types with a harvest period from February to March through a severe cold season, late-maturing cultivars, such as ‘Tomoe’ and ‘Konbanwa’, produced high yields due to their strong low-temperature enlargement capacity. Among cropping types with a harvest period from April to May, ‘Grandome’ produced a high yield. Moreover, these cultivars could be grown in converted paddy fields. In Exp. 4, high fertilization rates and wide planting promoted head enlargement. Wide planting increased the yields under simultaneous harvesting. However, under selective harvesting, where enlarged heads were individually harvested, head size was comparable between the standard and wide planting. Therefore, wide planting had no yield-increasing effect. The optimal planting density and harvest size for maximizing yield remain unclear. Nevertheless, for the first time, this study showed that the effect of planting density on yield varied based on the harvesting method. This study achieved a floret yield of 20,000 kg per ha across all six locations in nine cultivation trials in Japan, highlighting the broad adaptability of large head production systems, particularly in the autumn to spring production areas in Japan.
Broccoli, known for its vibrant color and rich vitamin and mineral contents, has become particularly popular in recent years. The government aims to elevate it to “designated vegetable” status by 2026 in Japan. From 2012 to 2022, its cultivation areas in Japan increased from 13,600 to 17,200 ha, and the quantity of domestic broccoli shipments increased from 123,000 to 157,000 t (Ministry of Agriculture, Forestry, and Fisheries (MAFF), 2024; https://www.maff.go.jp/index.html, October 23, 2024). With the increase in domestic production, the quantity of imports decreased slightly from 86,000 t in 2012 to 75,000 t in 2022. However, a notable point is the composition of fresh and frozen products. The quantity of fresh product imports dropped from 50,000 t to a level that cannot be statistically confirmed, whereas that of frozen products more than doubled, increasing from 36,000 to 75,000 t (Agriculture and Livestock Industries Corporation (ALIC), 2024; https://vegetan.alic.go.jp/index.html, October 23, 2024). Frozen products, which are preprocessed (cut or boiled) and can be stored for long periods, are highly convenient as they can be used simply by thawing for the necessary period. Moreover, the price of imported frozen products is nearly half that of domestically produced fresh products, making them more affordable (Takahashi et al., 2021). These advantages render frozen products highly valuable for processing, resulting in the increase in imports of frozen products.
In other countries, it is common for the harvest standard for broccoli to have a head diameter of 15 cm or more (Jett et al., 1995; Schellenberg et al., 2009); however, in Japan, the central standard for fresh retail markets is a diameter of 12 cm. Several yield improvement techniques reported in Japan have been designed based on this standard (Kodera, 1988; Nakano et al., 2017; Sato, 2015; Takahashi et al., 2018, 2019). Broccoli for processing is utilized in floret form, by which small pieces of broccoli are separated from a head. Therefore, without being bound by the common domestic harvest standard of 12 cm, we attempted to increase the yield by maximizing head size (Takahashi et al., 2021). A comparison of 10 domestic cultivars revealed that ‘Grandome’ maintained its quality even with a head diameter of 20 cm. The head yield exceeded 30,000 kg per ha, and the floret yield exceeded 20,000 kg per ha. In recent years, the average head yield in Japan has been approximately 10,000 kg per ha (10,500, 10,200, and 10,100 kg per ha in 2020, 2021, and 2022, respectively) (MAFF, 2024; https://www.maff.go.jp/index.html, October 23, 2024). As the proportion of florets used as edible parts in processing is approximately 50–70% of the head weight, average floret yield is estimated as approximately 5,000–7,000 kg per ha. Therefore, the results obtained from a previous study suggest yields approximately three times the domestic average, demonstrating the significant potential of the large head production system. By testing the adaptability of ‘Grandome’ across broader areas, this large head production system can be widely adopted in Japan.
However, as indicated by the 134 broccoli cultivars on the latest list in Japan (Maruo, 2023), there are numerous cultivars circulating domestically and seed companies continuously release new ones. Stansell and Björkman (2020) reported a positive correlation between harvest index and release year, while there have been fewer changes in the aboveground biomass of broccoli over time. A similar trend was observed among domestic cultivars. For example, the proportion of apical head for ‘Ryokurei’ released in 1980, was approximately 17%, while that of ‘Madoka’, released in 2013, has reached 31% (Takahashi and Sasaki, 2019). Additionally, owing to regional differences in climate and soil conditions in Japan, the most suitable cultivars may vary depending on the cropping type and production area. Therefore, it is essential to verify the yield potential with a large number of cultivars across different regions. Since it is well known that planting density has opposing effects on the productivity per plant and the number of harvested products (Francescangeli et al., 2006; Jett et al., 1995), it is also necessary to consider planting density as well.
Therefore, in this study, we conducted the tests of adaptability of ‘Grandome’ from 2019 to 2021 at six autumn to spring production sites (Hyogo, Hiroshima, Kumamoto, Kanagawa, Kagawa, and Ehime). In addition to the evaluation of ‘Grandome’, cultivar selection tests suitable for large head production were also conducted at three former sites using 25 cultivars, while the yield evaluation exclusively for ‘Grandome’ was conducted under different fertilization conditions and planting densities at the latter three sites.
The soil properties of each field are listed in Table 1. According to the statistical data in 2018, when we were planning this research, the broccoli cultivation area in the Shikoku region increased by 8% compared to the previous year, while Japan’s total acreage saw a 3% increase (MAFF, 2024; https://www.maff.go.jp/index.html, October 23, 2024). The relatively warm and dry Seto Inland Sea region appeared to be suitable for broccoli production, and it was expected that the cultivation area would continue to increase in this region. Therefore, Hyogo, Hiroshima, Kagawa, and Ehime were selected as test sites. Additionally, Kumamoto, which showed the largest increase in Kyushu region (13% increase in cultivation area in 2018), and Kanagawa, which is relatively warm and appeared suitable for cultivation during the cool season in the Kanto region, were also selected as test sites. The average temperature, precipitation, and sunshine hours during the cultivation period (from planting to harvest) in each field were obtained from the Agro-Meteorological Grid Square Data, NARO (Ohno et al., 2016; https://amu.rd.naro.go.jp/wiki_open/doku.php?id=start2) (Table 2). Fertilization and planting densities were designed based on conventional cultivation methods at each site, and daily management practices were aligned with local standard methods. After harvesting, only heads that were sufficiently dark green without yellowing, tightly packed beads, and free from lesions were selected as being of marketable quality and were trimmed to a length of 15–17 cm, leaving the upper two to three leaves intact, while removing the remaining leaves. After measuring the head diameter and fresh weight (FW), the heads were cut into florets of approximately 5 cm in length and FW was measured. Based on the FW of the head and florets and the planting density, the head and floret yields were calculated on a per ha basis. Statistical analyses were performed in R (R Core Team, 2015; http://www.R-project.org).
Soil conditions at each experimental field.
Weather conditions at each experimental field and period.
The experiments were conducted at the Awaji Agriculture Technology Institute (34°31′ N, 134°80′ E, at 40 m altitude) in Minami-Awaji city, Hyogo prefecture, on experimental upland fields converted from paddy fields. In Exp. 1-1, the eight cultivars shown in Table 3 were sown in 128 cell trays on August 15, 2019, and transplanted into the field on September 18. The field was fertilized with N:P2O5:K2O = 400:210:230 kg per ha. The planting arrangement consisted of two rows with a row width of 135 cm and plant spacing of 45 cm (planting density: 32,920 plants per ha). These settings involved wider spacing and higher fertilization conditions than conventional practices to achieve larger heads. Heads were individually harvested after growing them to a diameter of approximately 18 cm. Ten plants were used for each cultivar with three independent replicates. In Exp. 1-2, the 19 cultivars shown in Table 3 were sown in 128 cell trays on August 24, 2020, and transplanted into the field on September 18. The field was fertilized with N:P2O5:K2O = 350:140:210 kg per ha. Plant spacing was 45 cm (32,920 plants per ha). Other settings, harvest standards, and numbers of replicates were the same as those in Exp. 1-1.
List of cultivars used in Experiments 1, 2, and 3.
The experiment was conducted at the Japan Agricultural Cooperatives (JA) West Japan Agricultural Research and Development Center (34°42′ N, 132°85′ E, at 236 m altitude) in Higashi-Hiroshima city, Hiroshima prefecture, on experimental upland fields. The six cultivars shown in Table 3 were sown in 128 cell trays on August 20, 2019, and transplanted into the field on September 19. The field was fertilized with N:P2O5:K2O = 300:200:300 kg per ha. The planting arrangement consisted of two rows with a width of 180 cm, and three different plant spacings were set: 30 cm (37,040 plants per ha), 40 cm (27,780 plants per ha), and 50 cm (22,220 plants per ha). The heads were enlarged to the maximum size while maintaining marketable quality, and were harvested simultaneously for each cultivar. Specifically, ‘Ohayo’, ‘Arakusa #53’, and ‘12SKE5’ were harvested on February 7, 2020, ‘Grandome’ on February 26, and ‘Winter dome’ and ‘Konbanwa’ on March 12. Three plants were used for each cultivar with four independent replicates.
Exp. 3) Trials in KumamotoThe experiments were conducted at the Kumamoto prefectural Agricultural Research Center (32°56′ N, 130°66′ E, at 3 m altitude) in Yatsushiro city, Kumamoto prefecture, on experimental upland fields converted from paddy fields. In Exp. 3-1, a cropping type with a harvest period in autumn, the four cultivars shown in Table 3 were sown in 128 cell trays on August 14, 2020, and transplanted into the field on September 10. In Exp. 3-2, a cropping type with a harvest period in winter, the four cultivars shown in Table 3 were sown on August 28, 2020, and transplanted on September 24. In Exp. 3-3, a cropping type with a harvest period in spring, and the five cultivars shown in Table 3, were sown on December 28, 2020, and transplanted on February 9, 2021. The field was fertilized with N:P2O5:K2O = 400:400:330 kg per ha in Exp. 3-1 and 3-2, and 360:220:340 kg per ha in Exp. 3-3. The planting arrangement consisted of two rows with a width of 140 cm and plant spacing of 50 cm (28,570 plants per ha). The heads were enlarged to the maximum size while maintaining marketable quality, and were harvested individually. Three plants were used for each cultivar with three independent replicates.
Examination of the cultivation conditions and adaptability of the large head production system of ‘Grandome’ (Exp. 4)The experiments were conducted in Kanagawa (Exp. 4-1) and Kagawa (Exp. 4-2), and Ehime (Expt. 4-3). The ‘Grandome’ cultivar was used for all experiments. Four plots were established by combining two fertilization levels (standard and high) and two planting densities (standard and wide), except for the high fertilization level with standard planting density plots in Exp. 4-3. Wide planting plots were set at approximately 80% of the planting density of the respective standard plots. The cultivation details for each experimental plot were as follows:
Exp. 4-1) Trials in KanagawaThe experiment was conducted at ZEN-NOH Agricultural Research and Development Center (35°35′ N, 139°36′ E, at 7 m altitude) in Hiratsuka city, Kanagawa prefecture, on experimental vegetable fields. Seeds were sown in 128 cell trays on August 5, 2019, and transplanted into the field on August 29. The standard fertilization was N:P2O5:K2O = 200:200:200 kg per ha, and the high fertilization was 400:400:400 kg per ha. The planting arrangement consisted of one row with a row width of 60 cm and plant spacing of 40 cm (41,670 plants per ha) for standard planting and 50 cm (33,330 plants per ha) for wide planting. The heads were enlarged to the maximum size while maintaining marketable quality, and were harvested individually. Five plants from each plot were used for measurements, with three independent replicates.
Exp. 4-2) Trials in KagawaThe experiment was conducted at Kagawa Prefectural Agricultural Experiment Station (34°23′ N, 133°93′ E, at 53 m altitude) in Ayagawa town, Kagawa prefecture, on experimental upland fields converted from paddy fields. Seeds were sown in 200 cell trays on August 13, 2019, and transplanted into the field on September 17. The standard fertilization was N:P2O5:K2O = 400:220:270 kg per ha, and the high fertilization was 600:310:390 kg per ha. The planting arrangement consisted of two rows with a row width of 150 cm and plant spacing of 31 cm (43,010 plants per ha) for standard planting and 40 cm (33,330 plants per ha) for wide planting. The heads were enlarged to the maximum size while maintaining marketable quality, and were harvested individually. Ten plants were used for the measurements from each plot, with three independent replicates.
Exp. 4-3) Trials in EhimeThe experiment was conducted at an upland field in JA Shuso (33°91′ N, 133°04′ E, at 100 m altitude) in Saijo city, Ehime prefecture. Seeds were sown in 200 cell trays on September 2, 2019, and transplanted into the field on September 27. The standard fertilization was N:P2O5:K2O = 380:450:410 kg per ha, and the high fertilization was 560:680:610 kg per ha. The planting arrangement consisted of two rows with a row width of 135 cm and plant spacing of 40 cm (37,040 plants per ha) for standard planting, and 50 cm (29,630 plants per ha) for wide planting. The heads were enlarged to their maximum size while maintaining marketable quality and were harvested simultaneously on February 5, 2020. Ten plants were used for the measurements from each plot, with three independent replicates.
In Exp. 1-1, which began in 2019, the harvest period for each cultivar was from early December to mid-February (Table 4). The top three high-yielding cultivars were the medium to late-maturing cultivars, ‘Clear’, ‘Grandome’, and ‘ATS-206’ (Fig. 1), with floret yields of 24,250, 20,040, and 17,850 kg per ha, respectively.
Harvest data from the Hyogo field for 2019 cropping (Exp. 1-1).
Broccoli heads of moderate (15–16 cm) and large (> 20 cm) sizes in the ‘Clear’, ‘Grandome’, and ‘ATS-206’ cultivars in Exp. 1-1.
In Exp. 1-2, which began in 2020, in addition to the three high-yielding cultivars from Exp. 1-1, 16 new cultivars were studied. The top three high-yielding cultivars were the medium to late-maturing cultivars, ‘Tomoe’, ‘Mutsumi’, and ‘Konbanwa’, with floret yields of 22,590, 21,850, and 20,560 kg per ha, respectively (Table 5). Floret yields of ‘Clear’, ‘ATS-206’, and ‘Grandome’, which exhibited high yields in the previous year, were 17,710, 18,510, and 18,310 kg per ha, respectively, in this period. Although the yields of ‘Clear’ and ‘Grandome’ decreased, they were still relatively high at approximately 18,000 kg per ha.
Harvest data from the Hyogo field for 2020 cropping (Exp. 1-2).
It was observed that the wider the planting spacing, the larger the head diameter, and FW of the heads and florets (Table 6). However, no effect on head and floret yield of planting spacing was detected due to the offsetting effect of reduced harvest numbers with wider spacing.
Harvest data from the Hiroshima field (Exp. 2).
Regardless of planting density, floret yields of the three cultivars harvested on February 7 were all relatively low (< 18,000 kg per ha), whereas those of the three cultivars harvested on February 26 and March 12 were all high (> 21,000 kg per ha). The top three high-yielding cultivars were ‘Konbanwa’ (40 cm spacing), ‘Grandome’ (30 cm spacing), and ‘Winter dome’ (40 cm spacing) with yields of 28,730, 24,260 kg and 23,080 per ha, respectively.
Exp. 3) Trials in KumamotoThe harvest period was mid-November in Exp. 3-1, from late January to early February in Exp. 3-2, and from late April to mid-May in Exp. 3-3 (Table 7). The cultivars with particularly high floret yields were ‘SK9-099’ in Exp. 3-1 (24,290 kg per ha; Fig. 2), ‘Grandome’ and ‘Konbanwa’ in Exp. 3-2 (24,790 and 23,300 kg per ha, respectively), and ‘Grandome’ in Exp. 3-3 (23,080 kg per ha).
Harvest data from the Kumamoto field (Experiments 3-1, 3-2, and 3-3).
Large heads of broccoli from the ‘SK9-099’ cultivar in Exp. 3-1 and ‘Konbanwa’ cultivar in Exp. 3-2.
The harvest period was from mid to late December in Exp. 4-1 (Kanagawa), mid to late January in Exp. 4-2 (Kagawa), and early February in Exp. 4-3 (Ehime) (Table 8). Although the floret yields of ‘Grandome’ varied depending on the trial site and cultivation method, the highest yields at each site were 22,550 kg per ha in the high-fertilization standard planting plot in Exp. 4-1, 24,390 kg per ha in the high fertilization standard planting plot in Exp. 4-2, and 26,290 kg per ha in the high fertilization wide planting plot in Exp. 4-3. In Exp. 4-1, two typhoons occurred on September 9 and October 12, causing damage, including lodging, and the rainfall during the growing period (August 29 to December 27) was 741 mm, approximately 30% greater than the average of 572 mm (Table 2). Despite the reduction in yield due to head rot disease caused by heavy rainfall, the yield was comparable to that at other trial sites. Rainfall levels during the cultivation period in Experiments 4-2 and 4-3 were 278 and 320 mm, respectively, which were less than half those observed in Exp. 4-1. Notably, no significant disease outbreaks were observed. Two-way analysis of variance of the results of Exp. 4-1 and 4-2, which exhibited the same experimental design, revealed that fertilization had no significant effect on the head diameter, but that fertilization and planting space significantly affected other parameters, with no interaction between the two. In other words, high fertilization and wide planting increased the productivity per plant; however, no interaction occurred between the two factors. Wide planting reduced the yield per unit area due to the decrease in the number of heads harvested.
Harvest data from the Kanagawa (Exp. 4-1), Kagawa (Exp. 4-2), and Ehime (Exp. 4-3) fields.
In addition to testing the regional adaptability of ‘Grandome’, we also tested other cultivars in Experiments 1, 2, and 3 to see if they were suitable for large head production. Not only yield, but also broccoli’s appearance, including bead size, uniformity, and coloration of the heads (Farnham and Björkman, 2011; Ordiales et al., 2017), are also important. In Hyogo (Exp. 1-1), some early cultivars with longer growing periods showed bead expansion and a decline in quality, leading to some heads being harvested before reaching a large size. In contrast, the medium to late-maturing cultivars ‘Grandome’, ‘Clear’, and ‘ATS-206’ did not show any bead expansion with longer growing periods. However, ‘Clear’ and ‘ATS-206’ exhibited slightly lighter green heads, raising concerns about a decline in quality. In the following year (Exp. 1-2), the temperature was lower than that of the previous year (Exp. 1-1) (Fig. S1). Consequently, the late-maturing cultivars ‘Tomoe’, ‘Mutsumi’, and ‘Konbanwa’, which are more cold-tolerant, achieved high yields. However, although ‘Mutsumi’ showed no issues with floret quality, it did exhibit “leafy head” (a physiological disorder where leaves emerge from the head), potentially lowering its grade.
Hiroshima (Exp. 2) is located at almost the same latitude as Hyogo (Exp. 1). However, the trial site in Hyogo was at a low altitude (40 m) and surrounded by the sea, whereas the trial site in Hiroshima was relatively high (236 m) and somewhat inland. Therefore, compared to Hyogo and Kumamoto, the trial site in Hiroshima was colder, with average daily minimum temperatures reaching −3°C in winter (Fig. S2). The late-maturing cultivar ‘Konbanwa’, which has high enlargement capacity at low temperatures, performed well. In cabbage, larger heads improve the yield efficiency and are preferred for processing use (Saka et al., 2011; Watanabe et al., 2012). Similarly, in broccoli, larger heads increase the proportion of heads and improved the harvest index (Takahashi et al., 2021). ‘Konbanwa’ had greater potential to increase yields in the 40 cm spacing compared to the 30 cm spacing, suggesting that it is more efficient to harvest larger heads even if the number of plants is reduced.
Kumamoto (Exp. 3) was the warmest region in this study, with an average temperature of 7.0°C during the coldest months of January and February (Fig. S2). All cultivars achieved good results, with floret yields exceeding 18,000 kg per ha. Notably, ‘Grandome’ achieved the highest yield of 24,790 kg per ha at this site. The field in Kumamoto was originally a semi-wet paddy field and because of the high groundwater level, cultivation was performed on highly raised beds. Therefore, the conventional planting density (28,570 plants per ha) tended to be sparse compared to other trial sites, making ‘Grandome’, which has strong enlargement capacity under sparse planting conditions, highly suitable. Furthermore, because of the warm climate in Kumamoto, there is a cropping type in which planting is done in early February (and harvesting occurs from April to May), as seen in Exp. 3-3. Finishing harvests in January avoids any overlap between planting and harvesting operations in February. In Exp. 3-2, only ‘Grandome’ among the four cultivars completed its harvest period by January, demonstrating the advantage of a shorter cultivation period compared to other late-maturing cultivars.
In Japan, the conversion of paddy fields to upland farming is anticipated (Araki et al., 2000; Ohtake and Aoyagi, 1998), and broccoli is expected to be a crop suitable for this purpose (Nakano et al., 2020). In this study, Experiments 1, 3 and 4-2 were conducted on representative paddy soils. This is the first study to demonstrate that large broccoli heads can be produced in such soils, making it highly significant from the perspective of utilizing paddy fields in Japan.
Cultivation conditions and adaptability of ‘Grandome’In Exp. 4, the regional adaptability and response to fertilization and planting density of ‘Grandome’ were examined. ‘Grandome’ consistently achieved floret yields exceeding 20,000 kg per ha across all regions, demonstrating stable high yields over a wide area. The adaptability of various vegetables, including broccoli, to ensure yield across different regions has been evaluated (Lyon et al., 2020). For example, a study comparing the performances of broccoli cultivars under conventional and organic farming methods between Oregon and Maine in the United States revealed that the ranking of suitable cultivars significantly changed depending on the region (Renaud et al., 2014). Although our study did not involve geographical isolation as extreme as that between the West Coast (Oregon) and the East Coast (Maine) of the United States, the consistent high yield of ‘Grandome’ across multiple regions demonstrated its high practical utility. In Experiments 4-1 and 4-2, the head diameters between standard and wide planting were almost the same; however, the harvest date tended to be later for standard planting (Table 8). This was likely due to selective harvesting, allowing the remaining heads to be enlarged using the space created by the harvested heads. Consequently, heads in standard planting could also be harvested at a similar size to those in wide planting, reducing the advantage of the individual head enlargement effect in wide planting and leading to a lower yield per unit area in wide planting than in standard planting. However, in Exp. 4-3, in which both standard and wide planting plants were harvested simultaneously, head diameter, FW of the head, and FW of the florets significantly increased with wide planting, resulting in increased yield per unit area. The finding that the impact of planting density varies depending on the harvesting method is a novel and practical discovery. However, in Exp. 2, where ‘Grandome’ was also harvested simultaneously, there was no significant difference in yield between planting densities (Table 6). In Exp. 4, wide planting was set at approximately −20% compared to standard planting, while in Exp. 2, the 40 cm spacing was −25% and the 50 cm spacing was −40% compared to the most densely planted 30 cm spacing, resulting in sparser planting than in Exp. 4. It is speculated that excessive sparser planting had a greater effect on the reduction in the number of heads harvested than on head enlargement. Therefore, determining the planting density that maximizes yield requires additional experiments with finer planting density intervals while considering the harvesting method.
We thank Takafumi Kawai, Kaori Otsuka, and Mayuko Yamada of the Hyogo Prefectural Technology Center for Agriculture, Forestry, and Fisheries, Awaji Agricultural Technology Institute, and Shoko Ogawa of the Chusan Agricultural Improvement and Extension Center for growing the plants and supporting the experiments.
