Ⅰ. INTRODUCTION
Alfalfa (Medicago sativa L.) is a perennial forage legume valued for its high productivity and nutritive value, particularly its high crude protein (CP) content. Alfalfa supply in South Korea relies largely on imports. However, climate-related production instability in major exporting countries may threaten the reliability of this supply, increasing interest in domestic alfalfa production. Expanding domestic alfalfa production in South Korea requires suitable land and production systems adapted to local conditions. Paddy fields may provide an alternative land resource; however, their poor drainage can limit alfalfa production, particularly during periods of heavy summer rainfall (Zhang et al., 2019).
Excess soil moisture can reduce oxygen availability in the root zone, impairing root function and plant growth. Waterlogging has been shown to reduce alfalfa growth and photosynthetic performance (Zhang et al., 2019). Under flooding conditions, root activity and plant survival may also vary with water temperature, flooding depth, and duration (Deng et al., 2025). These risks may lead to unstable alfalfa production in poorly drained paddy fields. An appropriate cropping system is therefore needed to maintain forage production when alfalfa growth is reduced or the stand is damaged under wet summer conditions. Establishing a suitable summer forage crop after alfalfa may help fill resulting forage-supply gaps. Crop selection should account for establishment conditions and the time available before subsequent autumn field operations.
Waterlogging damage to alfalfa has been reported under the summer monsoon conditions of South Korea (Kim, 2023;Jeong et al., 2024). The summer monsoon season, known as Changma, generally occurs between late June and late July. According to the 1991–2020 climate normals provided by the Korea Meteorological Administration, the monsoon season in southern South Korea lasts an average of 31.4 days, with an average precipitation of 341.1 mm (KMA, 2026). In the cropping system considered in this study, this period coincides with the time when growers must decide whether to continue alfalfa production after the spring harvest or introduce a summer forage crop. The periods before and after the monsoon season therefore represent important decision points for determining whether and when to establish a subsequent crop based on alfalfa growth and field conditions.
Establishing a subsequent crop before or after the monsoon season presents different challenges in terms of the available growing period and establishment conditions. Early establishment after alfalfa harvest provides a relatively long growing period before optimal sowing dates, but seedlings may be exposed to excessive moisture during emergence and early growth. Conversely, introducing a replacement crop after assessing alfalfa damage, or delaying field operations because of rainfall, may shorten the period between sowing and harvest. Crop selection should therefore consider both establishment ability and dry matter production within the available growing period.
Emergency forage crops are introduced to compensate for forage shortages caused by reduced production or failure of the primary crop. In South Korea, barnyard millet and teffgrass have been evaluated as potential summer emergency forage crops. Park et al. (2022) reported that the time required to reach harvest and dry matter yield (DMY) differed among summer emergency forage crops and sowing dates. These findings indicate that emergency forage selection should consider not only yield potential but also the available growing period and crop-specific growth characteristics.
In cropping systems involving alfalfa, summer forage production must also accommodate harvesting and field preparation for autumn alfalfa sowing. It is therefore necessary to evaluate the emergence, growth, and DMY of summer forage crops that can follow alfalfa in poorly drained paddy fields, including their potential under delayed sowing conditions.
This study compared the growth characteristics and forage productivity of early- and late-maturing barnyard millet, silage maize, a sorghum–sudangrass hybrid, and teffgrass grown after alfalfa in paddy fields in two years. The objective was to provide basic information for selecting subsequent and emergency forage crops after cultivating alfalfa in the paddy field.
Ⅱ. MATERIALS AND METHODS
1. Experimental site and design
The experiment was conducted in a paddy field in Jeongeup, Jeonbuk province, South Korea, in 2024 and 2025. The cultivars represented forage crops differing in maturity and origin. ‘Shirohie’ was the early-maturing barnyard millet entry, whereas ‘Jeju native’ was a late-maturing local accession collected from Jeju Island. Both entries have previously been evaluated for forage production in Korea. ‘32P75’ was a medium maturity forage maize developed by Pioneer Hi-Bred International. ‘Jumbo’ was a non heading sorghum–sudangrass hybrid previously evaluated as a forage cultivar in Korea. ‘Tiffany’ was a teff cultivar (Eragrostis tef), a warm season annual grass native to Ethiopia that has also been evaluated for forage production in Korea.
The experiment was arranged in a randomized complete block design (RCBD) with three replicates. Each plot measured 2 × 3 m (6 m2). Sowing dates were classified relative to the monsoon period reported by the Korea Meteorological Administration (KMA) for the southern region, where the experimental site was located. The monsoon period was from late June to late July. Sowing dates falling before and after these periods were classified as pre-monsoon in 2024 and post-monsoon in 2025, respectively.
2. Sowing and crop management
This study evaluated the growth and forage productivity of five summer forage crops as candidates for cultivation following alfalfa.
In 2024, early- and late-maturing barnyard millet, forage maize, and the sorghum–sudangrass hybrid were sown on June 26, whereas teffgrass was sown on August 1. In 2024, teffgrass was sown on August 1 because it was included as a candidate late-sown emergency forage crop following alfalfa. The other forage crops were sown on June 26. In 2025, all summer forage crops were sown on July 25. Because sowing dates differed between years, crop growth and productivity were compared among treatments within the conditions of each annual trial.
Seeding rates were 10 kg/ha for barnyard millet, 20 kg/ha for silage maize, 30 kg/ha for the sorghum–sudangrass hybrid, and 10 kg/ha for teffgrass. Barnyard millet and the sorghum–sudangrass hybrid were sown in rows spaced 22 cm apart, and teffgrass was sown in rows spaced 12 cm apart. Forage maize was sown at a spacing of 75 cm between rows and 18 cm between planting positions within rows.
Fertilizer application rates, expressed on an N–P2O5–K2O nutrient basis, were 120–200–70 kg/ha for barnyard millet and teffgrass, 100–150–150 kg/ha for the sorghum–sudangrass hybrid, and 200–150–150 kg/ha for forage maize. Nitrogen was applied in two equal portions, with 50% applied at establishment and the remaining 50% as topdressing.
In 2024, both barnyard millet cultivars were harvested on August 14, and the remaining crops were harvested on September 24. In 2025, all summer forage crops were harvested on September 23.
3. Measurements of growth characteristics and forage production
Growth measurements included emergence score, lodging score, and plant height. Yield measurements included dry matter (DM) content, fresh matter yield (FMY), and DMY. Emergence and lodging were evaluated following RDA (2012). Emergence was scored on a scale of 1–9, with scores of 1, 5, and 9 representing emergence rates of ≥80%, 60–79%, and ≤59%, respectively. Lodging was scored on a scale of 1–9, with scores of 1, 5, and 9 representing lodging percentages of ≤20%, 41–60%, and ≥81%, respectively.
Plant height was measured from the soil surface to the top of the plant using five representative plants per plot, and the mean value was calculated. FMY was determined by harvesting each plot at approximately 7 cm above the soil surface, weighing the harvested biomass, and converting the yield to a per-hectare basis.
A representative 300 g sample was collected from the harvested material in each plot. Samples were weighed fresh and dried at 65℃ for 72 h to determine DM content. DMY was calculated by multiplying the FMY of each plot by its corresponding DM fraction, after which treatment means were calculated.
4. Weather data collection
Daily mean temperature and precipitation data were obtained from the Jeongeup Automated Synoptic Observing System (ASOS) station through the Open MET Data Portal of the KMA. These data were used to calculate monthly mean temperature and precipitation during the experimental period in each year. The normal climate data (1991–2020) for the same station were used for comparison. Data aggregation periods were specified to account for differences in sowing and harvest dates among summer forage crops.
5. Statistical analysis
Statistical analyses were performed using R version 4.4.2 (R Core Team, 2024). The treatment factor comprised five levels as early-maturing barnyard millet, late-maturing barnyard millet, forage maize, the sorghum–sudangrass hybrid, and teffgrass.
Plant height, FMY, and DMY were analyzed separately for each year by analysis of variance (ANOVA) using an RCBD model that included treatment and block effects. Treatment means were compared using the least significant difference (LSD) test at the 5% significance level. Emergence and lodging scores were summarized descriptively, and DM content was presented as treatment means.
Ⅲ. RESULTS AND DISCUSSION
1. Weather conditions during the experimental period
Monthly mean temperature and precipitation during the experimental period are presented in Fig. 1. In both 2024 and 2025, monthly mean temperatures from June to September exceeded the normal climate condition. In 2024, the highest monthly mean temperature occurred in August, and temperatures remained above normal climate condition in September. In 2025, the highest monthly mean temperature occurred in July, whereas temperatures in August and September were lower than those in 2024.
The monthly distribution of precipitation differed between years. In 2024, precipitation was above normal in July but below normal climate condition in August. In contrast, precipitation in 2025 was above normal climate condition in June and September but below normal climate condition in July and August.
2. Emergence and growth characteristics
In 2024 trial, both barnyard millet cultivars and teffgrass had emergence scores of 1, indicating favorable emergence, whereas forage maize and the sorghum–sudangrass hybrid had scores of 7, indicating relatively poor emergence (Table 1). In 2025 trial, both barnyard millet cultivars and teffgrass had emergence scores of 1, whereas forage maize and the sorghum–sudangrass hybrid had scores of 3 (Table 1). These results indicate that barnyard millet and teffgrass achieved favorable emergence under the conditions of this study. However, teffgrass in the pre-monsoon trial was actually sown on August 1, later than the other summer forage crops, and its emergence was therefore evaluated under different establishment conditions.
Lodging scores were 1 for both forage maize and the sorghum–sudangrass hybrid, 2 for early-maturing barnyard millet, 3 for teffgrass, and 4 for late-maturing barnyard millet in 2024 trial (Table 1). In 2025 trial, teffgrass had a lodging score of 2, whereas all other summer forage crops had scores of 1 (Table 1).
In 2024 trial, the sorghum–sudangrass hybrid reached a plant height of 267 cm and was significantly taller than early-maturing barnyard millet, forage maize, and teffgrass (p<0.05). Although the sorghum–sudangrass hybrid was numerically taller than late-maturing barnyard millet (266.8 vs. 189.5 cm), the difference was not statistically significant (p>0.05; Table 1). In 2025 trial, the sorghum–sudangrass hybrid was significantly taller than all other crops, with a plant height of 148 cm (p<0.05; Table 1).
The sorghum–sudangrass hybrid achieved a relatively high plant height under both sowing conditions despite its less favorable emergence scores. It also produced the highest DMY under pre-monsoon sowing conditions, indicating that favorable initial emergence did not necessarily correspond to high final DMY. Growth after establishment may have partly compensated for its poorer initial emergence.
3. Forage production in the southern region
The sorghum-sudangrass hybrid had the highest FMY at 77,910 kg/ha, significantly exceeding all other summer forage crops in 2024 trial (p<0.05; Table 1). FMY of early-maturing barnyard millet, late-maturing barnyard millet, and teffgrass were 46,288, 43,566, and 40,517 kg/ha, respectively, with no significant differences among these treatments (p>0.05; Table 1). Forage maize produced the lowest FMY at 16,689 kg/ha (Table 1).
The sorghum–sudangrass hybrid also produced the highest DMY at 11,728 kg/ha in 2024 trial (p<0.05; Fig. 2). DMY of late- and early-maturing barnyard millet were 9,236 and 7,795 kg/ha, respectively. These two treatments did not differ significantly from each other (p>0.05), but both produced significantly higher DMY than teffgrass and forage maize (p<0.05; Fig. 2). DMY of teffgrass and forage maize were 5,875 and 5,312 kg/ha, respectively (Fig. 2).
Studies in Korean paddy fields have also reported high forage productivity among sorghum–sudangrass hybrids, with productivity varying according to hybrid characteristics and drainage conditions (Lee, 2019;Ji et al., 2010). Jeon et al. (2012) compared sorghum–sudangrass hybrids grown in a paddy field and reported that the non-heading cultivar ‘Jumbo’ had the highest mean DMY among the cultivars tested. Ji et al. (2011) also reported that sorghum forage productivity varied with paddy-field drainage conditions and that ‘Jumbo’ showed the smallest absolute reduction in DMY between drainage conditions. In the present study, the sorghum–sudangrass hybrid produced a high DMY in 2024 trial, however, its poor emergence suggests that stand establishment may constrain its suitability as a crop following alfalfa in paddy fields. In comparison, barnyard millet combined favorable emergence with relatively high DMY, suggesting that it may offer a more reliable option when both establishment and forage yield are considered. Also, previous studies have shown that the growth and forage yield of barnyard millet vary with sowing date (Lee et al., 2013). Park et al. (2022) likewise reported high DMY for late-maturing barnyard millet in a domestic summer emergency forage trial. Taken together, these findings support barnyard millet as a potentially preferable crop following alfalfa under the conditions of the 2024 trial.
Under 2025 trial, teffgrass had the highest mean FMY at 16,556 kg/ha, whereas forage maize had the lowest at 2,222 kg/ha. However, FMY did not differ significantly among summer forage crops (p>0.05; Table 1). In contrast, significant differences were observed in DMY (p<0.05). Teffgrass produced 5,567 kg/ha, significantly exceeding both barnyard millet treatments and forage maize (p<0.05). However, its DMY did not differ significantly from that of the sorghum–sudangrass hybrid, which produced 2,879 kg/ha (p>0.05; Fig. 2). DMY of early-maturing barnyard millet, late-maturing barnyard millet, and forage maize were 2,666, 2,292, and 405 kg/ha, respectively, with no significant differences among these treatments (p>0.05; Fig. 2).
The different statistical results for FMY and DMY under post-monsoon sowing conditions highlight the importance of evaluating forage productivity on a DM basis. Teffgrass had the highest mean DM content at 33.6%, which contributed to its high DMY relative to its FMY.
4. Potential use as subsequent and emergency forage crops following alfalfa
Selecting a crop to follow alfalfa requires consideration of yield, initial establishment, the available growing period, and compatibility with autumn sowing. Under 2024 trial, the sorghum–sudangrass hybrid produced the highest DMY, whereas barnyard millet combined favorable emergence with relatively high DMY. Although the sorghum–sudangrass hybrid produced a higher DMY, barnyard millet may be the preferred summer forage option under 2024 trial when both stand establishment and DMY are considered.
Under 2025 trial, teffgrass produced the highest mean DMY, significantly exceeding the DMY of both barnyard millet treatments and forage maize, but not that of the sorghum–sudangrass hybrid. This result suggests that teffgrass has potential as an emergency forage crop under late-sowing conditions.
These findings should be interpreted as crop-specific responses under the particular conditions of the 2024 and 2025 trials. Because the sowing schedules differed between years, the effects of year and sowing date could not be separated; moreover, teffgrass was sown on a different date from the other crops in 2024. Annual system productivity, including alfalfa yield and the yield of the subsequent forage crop, was not evaluated. Further trials that repeat sowing schedules across years and measure both components are needed to assess the productivity of the overall cropping system.
Ⅳ. CONCLUSIONS
This study aimed to identify suitable summer forage crops for use following alfalfa in paddy fields by evaluating the establishment, growth, and forage productivity of five forage crops under the sowing schedules examined in 2024 and 2025. Overall, barnyard millet and the sorghum–sudangrass hybrid showed potential as summer forage crops following alfalfa under pre-monsoon sowing conditions, whereas teffgrass showed potential under post-monsoon sowing conditions. These findings can inform forage production planning in paddy fields by supporting crop selection based on establishment timing, initial establishment performance, and the available growing period.
