Ⅰ. INTRODUCTION
The decline in domestic pasture area has weakened the forage production base in Korea, while dependence on imported forage has continued to increase, highlighting the growing importance of establishing a sustainable forage production system (Kim et al., 2000). Cultivating winter forage crops in paddy fields after rice harvest, rather than leaving these fields fallow, has been recognized as an effective strategy to improve land-use efficiency and enhance national forage self-sufficiency. Under these conditions, winter cereal crops such as Italian ryegrass (IRG), oat, and rye have been widely utilized due to their high dry matter (DM) yield and desirable forage quality characteristics (Song et al., 2021).
Hairy vetch (HV), a leguminous species with strong nitrogen-fixing capacity, is known to enhance productivity and nutritive value through complementary resource use when intercropped with cereal forage crops (Poffenbarger et al., 2015). In Korea, rye–HV mixtures have shown increased crude protein (CP) content and improved forage quality depending on seeding ratio and sowing method (Kim et al., 2002). Similarly, field trials in Osaka, Japan demonstrated that HV–oat intercropping enhanced nitrogen uptake and DM yield compared with oat monoculture (Tarui et al., 2013). In addition, various studies have evaluated the forage yield and feed value of winter crops such as IRG, oat, rye, and barley, some of which have also investigated the potential benefits of incorporating HV into mixed cropping systems (Song et al., 2021). Research on rapeseed–hairy vetch mixtures further reported increased DM and CP yields compared with monoculture, suggesting improved nitrogen uptake efficiency and complementary resource use (Ji et al., 2025). Moreover, studies conducted in Europe and Africa have reported that winter-grown HV exhibits high biomass productivity and favorable nutritive value, supporting its potential as a valuable forage crop (Lehutjo et al., 2025).
However, most previous studies have been limited to upland conditions or specific crop combinations, and research comparing the productivity and forage quality of cereal–HV mixtures under paddy-based double-cropping systems remains scarce. Due to the repeated flooding and drainage inherent to rice cultivation, paddy soils differ from upland soils in their physical and chemical properties, which may influence interspecies competition, nitrogen dynamics, and the nutritive composition of forage in intercropping systems. Therefore, empirical evaluation of cereal–HV intercropping under paddy field conditions is required to determine its agronomic feasibility and potential benefits.
This study was conducted in a paddy field located in the southern region of Korea to compare the productivity (DM yield) and forage quality (CP, fiber composition) of IRG, oat, and rye grown either as monocultures or in mixtures with HV following rice harvest. The findings of this study provide meaningful insights into the practical contribution of HV intercropping within rice–forage double-cropping systems and may serve as a foundation for improving forage self-sufficiency and establishing more sustainable agricultural production systems.
Ⅱ. MATERIALS AND METHODS
1. Experimental design
The field experiment was conducted in a paddy field located in Naju, southern Korea, from 13 October 2024 to 7 May 2025. The study followed a rice-winter forage double-cropping system, in which winter forage crops were sown immediately after rice harvest. Four forage species were used: Italian ryegrass (IRG, cv. Early-bird), rye (cv. Blue land), oat (cv. Donghan), and hairy vetch (HV, cv. Pebb-2). The experiment consisted of six treatments, representing monoculture and intercropping systems. IRG, rye, and oat were cultivated individually as sole crops, and each of these three cereals was also intercropped with HV. Accordingly, the treatments included IRG, rye, oat, IRG+HV, rye+HV, and oat+HV. All treatments were arranged in a randomized complete block design with three biological replications. The physicochemical properties of the soil at the experimental site are presented in Table 1.
2. Plant growth and harvest
Each plot measured 2 m × 5 m. Winter forages were sown on 13 October. The seeding rates for the monoculture treatments were 50 kg ha-1 for IRG and rye, and 150 kg ha-1x for oat. HV was sown at 50 kg ha-1 when intercropped with each cereal species. A compound fertilizer was applied at a rate of 200 kg ha-1 nitrogen, 150 kg ha-1 phosphorus, and 150 kg ha-1 potassium. Fertilizer was split-applied, with 50% applied before seeding and the remaining 50% applied at the 7–8 leaf stage. All forage crops were harvested on 7 May 2025.
3. Chemical analysis
A subsample of approximately 400 g was collected from each plot at harvest. Samples were dried at 60°C for 48 h in a dry oven to determine DM content. The dried samples were then ground to pass through a 1.0-mm sieve, and the powdered samples were stored at 4°C in a dark, dry room until analysis. Total nitrogen concentration was determined using the Kjeldahl digestion method (Bremner, 1996). CP content was calculated by multiplying total N by 6.25, assuming that protein contains 16% N. Neutral detergent fiber (NDF) and acid detergent fiber (ADF) were analyzed following the procedures described by Van Soest et al. (1991). Digestible DM (DDM) and DM intake (DMI) were estimated using Eqs. (1) and (2), respectively. Total digestible nutrients (TDN) and relative feed value (RFV) were calculated according to Holland et al. (1990) using Eqs. (3) and (4).
4. Statistical analysis
All data were analyzed using SAS software (SAS Institute Inc., Cary, NC, USA). Analysis of variance (ANOVA) was performed to evaluate treatment effects, and mean separation was conducted using Duncan’s multiple range test at a significance level of p<0.05. Unless otherwise specified, statistical differences among treatments were determined based on comparisons of treatment means.
Ⅲ. RESULTS AND DISCUSSION
In the comparison between monoculture and HV intercropping for IRG, oat, and rye, a clear difference in DM yield was observed only in rye (Fig. 1A). The DM yield of rye monoculture was significantly higher than that of rye+HV, indicating that HV intercropping did not enhance the biomass accumulation of rye. In contrast, IRG and oat showed a slight increase under intercropping, although these differences were not statistically significant, suggesting that the DM-enhancing effect of intercropping was limited in these two species. This pattern is consistent with previous findings showing that cereal-legume intercropping does not necessarily increase biomass production. Caballero et al. (1995) reported that although CP yield increased in oat-vetch mixtures, DM yield responses were highly variable, and Bacchi et al. (2021) similarly noted that hairy vetch intercropping does not consistently improve forage biomass.
A similar response was observed for TDN yield. Rye monoculture produced significantly higher TDN than rye+HV, indicating that intercropping did not improve the energy yield of rye (Fig. 1B). No significant differences were observed between monoculture and intercropping in IRG or oat. Comparable biomass levels between cereal rye monocultures and rye-hairy vetch mixtures have been reported, with cereal rye generally dominant in mixtures and HV monocultures producing less biomass than rye monocultures (Poffenbarger et al., 2015).
In contrast, intercropping had a pronounced effect on CP yield in IRG and oat (Fig. 1C). Both IRG+HV and oat+HV showed significantly higher CP yields than their respective monocultures, indicating that nitrogen fixation and protein accumulation by HV directly contributed to protein productivity in these two grasses. These results agree with earlier reports that grass-legume intercropping enhances CP production (Sleugh et al., 2000; Lithourgidis et al., 2006). In rye, CP yield tended to be higher under intercropping, but the difference was not statistically significant, likely because rye’s rapid early growth and dominance in mixtures was associated with lower relative development of HV and constrained its contribution to protein production in the combined biomass (Poffenbarger et al., 2015).
For forage quality, intercropping consistently improved key quality parameters across all three species (Fig. 2). NDF concentration decreased significantly in IRG, oat, and rye under intercropping, demonstrating that HV reduced the proportion of structural carbohydrates (Fig. 2A). This trend aligns with the global meta-analysis by Liu et al. (2023), which reported consistent reductions in NDF and ADF in cereal-legume mixtures. ADF also declined significantly in oat and rye (Fig. 2B), consistent with the findings of Bacchi et al. (2021), who showed that intercropping delays or reduces cell-wall accumulation in forage crops.
DMI increased significantly in all species under intercropping, and DDM increased significantly in oat+HV and rye+HV (Fig. 2C, D). These improvements reflect the direct influence of reduced fiber fractions on digestibility and intake potential. Zhang et al. (2015) similarly reported that cereal-legume intercropping enhances ruminal degradability and animal intake responses. The strong improvement in DMI observed in oat+HV aligns with Kchaou et al. (2023), who noted that oat-legume mixtures often show synergistic enhancements in forage quality.
Nutritionally, CP concentration significantly increased under intercropping in IRG+HV and oat+HV, further indicating the contribution of HV to nitrogen enrichment (Fig. 2E). In rye, CP concentration showed an increasing trend but did not differ significantly, and cereal species with rye often limiting the nitrogen contribution of companion legumes in mixture systems (Poffenbarger et al., 2015). TDN (%) also increased significantly in oat+HV and rye+HV, indicating improved energy availability under intercropping (Fig. 2F). RFV, the integrated forage-quality index, increased significantly in all three species under intercropping (Fig. 2G). This reflects the combined effects of reduced fiber, improved digestibility, and enhanced intake potential. The most pronounced RFV improvement occurred in oat+HV, consistent with earlier observations that oat-legume mixtures generate strong quality advantages (Bacchi et al., 2021; Liu et al., 2023).
It should be noted that crop-specific recommended seeding rates were applied for each species in this study, reflecting standard agronomic practices under field conditions. Therefore, the observed differences in productivity among species are interpreted primarily in terms of intercropping effects rather than differences in seeding rates. Overall, the effects of intercropping in this study differed clearly between yield and forage quality. Yield responses varied by species, with rye monoculture consistently outperforming its mixture in DM and TDN, while significant increases in CP yield were observed only in intercropped IRG and oat. By contrast, forage quality showed consistent and pronounced improvements across all species under intercropping, including reduced fiber concentrations, enhanced digestibility and intake potential, and increased RFV. These findings indicate that, under a paddy double-cropping system, intercropping is more effective as a strategy for improving forage quality than for increasing yield. Accordingly, decisions regarding the adoption of intercropping should consider the primary production objective, whether the goal is to maximize DM and TDN productivity or to enhance CP content and overall forage quality, and species combinations should be selected accordingly.
Ⅳ. CONCLUSIONS
This study evaluated the effects of intercropping HV with IRG, oat, and rye under a paddy-based double-cropping system. The results showed that intercropping had limited influence on DM and TDN yields, with significant reductions observed only in rye mixtures, while no yield benefits were detected in IRG or oat. In contrast, CP yield increased significantly in IRG+HV and oat+HV, indicating the clear nitrogen contribution of the legume component. For forage quality, intercropping consistently improved key indicators across all species by lowering NDF and ADF and enhancing DMI, DDM, TDN, and RFV. Overall, intercropping was more effective as a strategy for improving forage quality than for increasing yield under paddy field conditions. Thus, its adoption should depend on production goals, with monoculture being preferable for maximizing DM and energy yield, while HV intercropping is better suited for systems that prioritize CP enhancement and overall forage quality.
