INTRODUCTION

Sorghum species are strongly resistant to high temperatures and drought during the summer, making them widely used as high-quality forage across various regions of Korea. In particular, the Sorghum×Sudangrass hybrid is extensively cultivated alongside corn for use as green forage and silage (Kim et al., 2002). Generally, the hybrid is known to have high sugar content in stems and leaves (Kim et al., 2002) and low lignin levels, making it highly suitable for silage production due to its excellent feed value (Li et al., 2013). In Korea, improvements in cultivar selection, cultivation techniques, and management practices for Sorghum×Sudangrass hybrids (Seo and Kim, 1983;Han et al., 1986;Kim et al., 2002;Yoon et al., 2008) have led to high productivity, and it has been widely adopted by livestock farms as a representative summer forage crop alongside corn. However, Korea has recently experienced the effects of climate change more rapidly and severely than the global average (IPCC, 2014), which has significantly affected the growing conditions of forage crops. Therefore, the development and implementation of new cultivation technologies in response to climate change are urgently needed (Choi et al., 2018). According to the Korea Meteorological Administration's RCP 8.5 scenario, summer crops such as corn may face high-temperature damage for more than 10 days annually, raising concerns about heat stress during the growing season (Seo et al., 2016). Under these conditions, vulnerability assessments of crops are effectively used to analyze the impacts of climate change and extreme weather events on crop productivity (Farhangfar et al., 2015). Internationally, such assessments are used to identify critical climate factors and develop various strategies to mitigate yield loss (Hatfield et al., 2018). In Korea as well, it is essential to actively conduct vulnerability assessments of forage crops in response to intensifying climate change to better prepare for and reduce crop-specific climate-related damages (Chung et al., 2019).

Therefore, this study aims to conduct a pilot assessment of the climate impact vulnerability of the Sorghum× Sudangrass hybrid (SX-17), identify key climatic factors affecting the production of forage sorghum, and analyze the potential yield reduction due to climate change. The findings are intended to serve as baseline data for developing strategies to mitigate production losses and adapt to future climate conditions.

MATERIALS AND METHODS

This study was conducted to evaluate the productivity and climate impact vulnerability of the Sorghum×Sudangrass hybrid in central Korea. Experimental plots were established in Daesowon-myeon, Chungju-si, Chungcheongbuk-do, and Ungok-myeon, Cheongyang-gun, Chungcheongnam-do, where cultivation experiments were carried out over a two-year period from 2017 to 2018. The cultivar used in the experiment was SX-17, a heading-type hybrid (Seo et al., 2021). The sowing dates were April 24, 2017, and April 23, 2018, for the Chungju site, and April 22, 2017, and April 26, 2018, for the Cheongyang site. Each plot measured 12m² (3m×4m), with row planting at 50cm intervals. Three replicated plots were prepared at each location for statistical validity. Fertilizer was applied at a rate of 250-150-150kg/ha (N-P-K). Of the nitrogen fertilizer, 40% was applied as a basal dose, followed by 30% during the growing stage, and the remaining 30% after the first harvest. For productivity assessment considering silage usage, two central rows were harvested after heading to measure fresh yield. Harvest dates were August 18, 2017, and August 20, 2018, in Chungju, and August 17, 2017, and August 21, 2018, in Cheongyang. After harvest, subsamples were dried in a 65°C oven for 72 hours to measure moisture content and calculate the final dry matter yield. For the climate impact vulnerability assessment of the Sorghum× Sudangrass hybrid, studies related to its cultivation in Korea over the past 30 years (1990–2018) were reviewed to estimate average yields at heading. Additional data were obtained from local governments and agricultural technology centers in Chungju and Cheongyang. Based on this information, climate factors with potential negative effects on productivity were selected, and a pilot vulnerability assessment was conducted for the period from 1999 to 2018, for which complete data were available in both regions. The vulnerability assessment was conducted using the formula and detailed items defined by the Rural Development Administration. As shown in Table 1, different weights were applied to exposure levels depending on the duration of extreme conditions. Sensitivity was set at 0.5, and adaptive capacity was assumed to be minimal due to a lack of expert personnel and irrigation systems, while all other conditions were assumed to be optimal. Vulnerability was calculated using the following formula:

Vulnerability = Exposure Level + Sensitivity Level − Adaptive Capacity (MAFR et al., 2017).

Meteorological data, including temperature, precipitation, and sunshine hours, were obtained from Weather.go.kr (www.weather.go.kr). Using 20 years of temperature and daily precipitation data from 1999 to 2018, and the criteria presented in Table 1, a pilot climate impact vulnerability assessment was carried out for the two regions. For statistical analysis of the results, ANOVA was conducted using the General Linear Model (GLM) procedure of the SAS software (ver. 9.3, SAS Institute, Cary, NC, USA). Duncan’s multiple range test (Duncan, 1955) was used to compare treatment means, and significance was determined at the p<0.05 level.

RESULTS

During the experimental period, the productivity of the Sorghum×Sudangrass hybrid (SX-17) is presented in Table 2. In 2017, the fresh matter yield was 44,757kg/ha in Chungju and 53,436kg/ha in Cheongyang, indicating favorable productivity under the average climatic conditions of that year. In contrast, due to higher temperature and lower precipitation in 2018 than in 2017, yields declined sharply to 16,436kg/ha in Chungju and 17,116kg/ha in Cheongyang, representing a significant reduction compared to the previous year. The dry matter yield in 2018 also decreased markedly compared to 2017 and was significantly lower than the national average.

Analysis of meteorological data for both regions showed that the normal average maximum temperatures for Chungju (1991–2020) were 29.7°C in July and 30.3°C in August, whereas in 2018 they reached 32.8°C and 33.4°C, respectively-approximately 3.1°C higher than normal. Similarly, in Cheongyang, the normal maximum temperatures for July and August were 29.3°C and 30.2°C, but in 2018 they increased to 31.5°C and 32.4°C, respectively, indicating a significant heatwave. Notably, the average temperatures in July and August of 2018 were significantly higher than those in 2017, whereas temperatures in May and June-the early growth periodtended to be lower in 2018, suggesting that late-season weather conditions adversely affected forage productivity. Furthermore, although precipitation in May and June of 2018 was higher than in 2017 in both regions, rainfall during July, which is the most vigorous growth period, was less than half of that recorded in 2017 (p<0.05). This substantial decrease in rainfall, combined with high temperatures, likely resulted in drought stress, which in turn contributed significantly to the decline in productivity. Using temperature data (Tables 3 and 4), precipitation data (Table 5) from the Korea Meteorological Administration, and the climate impact vulnerability criteria outlined in Table 1, a trial vulnerability assessment was conducted for Chungju and Cheongyang. The results are illustrated in Figures 1 and 2. Both regions showed a gradual increase in the climate impact vulnerability index after 2010. In particular, the index reached 0.7 in 2018 in both regions, the highest recorded value during the study period. This suggests that extreme heat and drought had a serious negative impact on crop growth and productivity.

DISCUSSION

Based on the criteria established in this study, a pilot assessment of the climate impact vulnerability of the Sorghum×Sudangrass hybrid was conducted for the period from 1999 to 2018. The results revealed a clear trend of increasing vulnerability over time. In particular, both Chungju and Cheongyang experienced a sharp decline in yield in 2018, during which the climate impact vulnerability index reached its highest level. While various factors influence crop growth, meteorological variables such as temperature and precipitation are among the most critical. When conditions deviate from optimal ranges, crops become susceptible to damage from low temperatures, heat stress, drought, or excessive moisture (Shim et al., 2013). Therefore, the extreme heat and drought during the summer of 2018 are considered the primary causes of the observed yield reduction. In both regions, temperature levels in 2018 were significantly higher than in 2017, while precipitation during the peak growth months (June and July) was reduced to nearly half of that in the previous year. This indicates that the combination of heat and drought stress severely affected the productivity of the Sorghum×Sudangrass hybrid (Chung et al., 2019). The findings of this study are consistent with previous research, which has identified heatwaves and abnormal climate events as major limiting factors for forage crop productivity (Kim et al., 2002;Chung et al., 2019).

The increasingly unstable weather patterns observed in recent years are also reflected in the climate impact vulnerability trends over the past two decades (Figures 1 and 2). Both regions maintained an index below 0.4 until 2011, after which a continuous upward trend was observed, with 2018 showing the highest vulnerability due to intense heat and drought. Kucharik and Serbin (2008) reported that temperature and precipitation are the most evident climatic factors influencing corn productivity. Similarly, the present study indicates that elevated maximum temperatures and drought contributed to increased vulnerability. Along with corn, crops such as Sorghum×Sudangrass hybrid, soybean, and wheat are major food and forage crops not only in Korea but also worldwide. According to GCM climate change scenarios, climate-induced changes in weather patterns are expected to have a significant impact on crops during their growing seasons, potentially reducing yields (Alexandrov and Hoogenboom, 2000). Furthermore, Kucharik and Serbin (2008) warned that additional summer temperature increases due to climate change may lead to reduced yields in corn and soybean, with compounded effects when high temperatures and drought coincide. They also emphasized the close relationship between rising temperatures, declining precipitation, and crop yield reduction.

Collectively, the results indicate that extreme heat and drought driven by climate change will have a significantly negative impact on future crop yields. Although C4 crops such as the Sorghum×Sudangrass hybrid are generally more tolerant of high temperatures and drought than other crops, persistent or intensified extreme weather events, like those observed in 2018, could lead to further reductions in productivity (Chung et al., 2019). The uncertain climatic conditions resulting from climate change will continue to pose a significant risk to crop production. The results of climate impact vulnerability assessments can serve as important baseline data not only for researchers but also for policymakers and local governments in forecasting climate-related production losses and developing countermeasures (Chung et al., 2019). For more accurate assessments of summer forage crops such as the Sorghum×Sudangrass hybrid, future studies should incorporate a wider range of climatic variables, including soil moisture and humidity, as well as cultivation practices. Although this study analyzed the impact of climate vulnerability on forage crop productivity based on short-term research, further investigations are needed to prevent future yield reductions caused by extreme weather events. For more accurate climate impact assessments, long-term studies and detailed analyses based on their results will be essential.