Ⅰ. INTRODUCTION
The sorghum × sudangrass hybrid (Sorghum bicolor (L.) Moench) is a forage crop widely cultivated during summer in Korea. It grows rapidly and is a warm-season crop with strong drought resistance and high dry matter productivity. For these reasons, it is widely used as roughage for livestock in the form of grazing forage, hay, and silage. Recently, as forage production in paddy fields has been emphasized as part of rice production adjustment policies, the use of paddy fields for forage production has emerged as an alternative. However, forage production in paddy fields can be limited by wet injury caused by poor drainage. Compared with corn, sorghum × sudangrass hybrids show relatively greater tolerance to such conditions and are therefore recommended for paddy-field cultivation (Lee, 2019).
However, sorghum species contain a compound called dhurrin, which can be degraded to release prussic acid, namely hydrogen cyanide (HCN), and may cause harm to livestock. In general, sorghum is known to have the highest dhurrin content, while sorghum × sudangrass hybrids contain an intermediate level. This dhurrin content is high when the growth is young, and decreases to a safe level when the plant height is more than 120 cm (Kim, 1990).
In addition, sorghum × sudangrass hybrids may contain high levels of nitrate nitrogen, which can also cause various symptoms in livestock. In general, nitrate from ingested forage enters the rumen and may induce symptoms such as salivation, muscle tremors, abortion, and sudden death (Undersander, 2026).
The contents of various harmful compounds in forage crops can be reduced through appropriate cultivation and forage processing practices. The most effective methods include regulating nitrogen fertilizer application, selecting the proper utilization stage, applying suitable forage processing methods, and adjusting cutting height (Choi et al., 2017).
In Korea, various studies have been conducted on sorghum × sudangrass hybrids, but most have focused on productivity and cultivar selection (Choi et al., 2017;Kim et al., 2012;Jeon et al., 2012). However, few studies have investigated the exact concentrations of these harmful substances or their improvement through processing methods.
Therefore, this study was conducted to investigate the forage value and concentrations of harmful substances at different plant height at harvest of the cultivar ‘Turbo Gold’, which is registered in Korea as an imported adaptation-certified cultivar. In addition, changes in these concentrations during silage preparation were also examined.
Ⅱ. MATERIALS AND METHODS
1. Cultivation of Sorghum × Sudangrass Hybrid
The sorghum × sudangrass hybrid used in this study was cultivated in an experimental field at Seoul National University Pyeongchang Campus, located in Pyeongchang, Korea (N 35°19′58″, E 128°35′01″). The crop was sown on May 27, 2024. The experimental field had previously been used for corn cultivation and was left fallow before this experiment.
The sorghum × sudangrass hybrid cultivar used in this study was ‘Turbo Gold’. Seeds were drilled at a seeding rate of 50 kg/ha with a row spacing of 50 cm. On the sowing date, May 27, 2024, phosphorus and potassium fertilizers were uniformly applied over the field at a rate of 120 kg/ha, while nitrogen fertilizer was applied at a rate of 200 kg/ha, with half of the total amount applied at sowing.
2. Harvesting of Sorghum × Sudangrass Hybrid
To investigate the chemical characteristics of the sorghum × sudangrass hybrid according to plant height, four harvest stages were established, and the experiment was conducted in a randomized complete block design with three replicates for each harvest stage. The crop was harvested when it reached target plant heights of 50 cm on July 3, 100 cm on July 10, 150 cm on July 22, and 200 cm on July 30. The harvested samples were separated into whole plant, leaf, and stem fractions, and the dry matter content of each plant part was determined.
Samples for observing changes in silage quality were harvested on August 22. A portion of the harvested material was used for silage preparation and dry matter analysis. For dry matter determination, weighed samples were dried in a forced-air oven at 65°C for 72 h, and dry matter content was then calculated.
For silage preparation, approximately 400 g of sample was placed into plastic bags, vacuum-sealed, and stored indoors in a shaded area. The silage bags were opened after 0, 1, 2, 3, 5, 10, 15, 30, 45, and 60 days of ensiling, and silage quality was analyzed.
3. Analysis of Feed Value
Samples for feed value analysis of each plant part, including leaf, stem, and whole plant, were collected on the day of harvest. The samples were dried in a forced-air oven at 65°C for more than 72 h, first ground using an electric mixer, and then ground again through a 20-mesh mill. The ground samples were placed in double-capped plastic sample containers and stored away from direct sunlight until analysis.
Crude protein content was analyzed according to the Dumas method (1826). Neutral detergent fiber (NDF) and acid detergent fiber (ADF) contents were analyzed according to the method of Goering and Van Soest (1970). Total digestible nutrient (TDN) content was estimated from ADF content according to Holland et al. (1990) using the following equation:
Relative feed value (RFV) was calculated by estimating digestible dry matter (DDM) from ADF content and dry matter intake (DMI) from NDF content using the following equations:
In vitro dry matter digestibility (IVDMD) was determined using the method of Tilley and Terry (1963), as modified by Moore (1970). Rumen fluid used in the experiment was collected from Hanwoo cattle that had been fed roughage ad libitum, before morning feeding.
Nitrate content in the samples was measured using a LAQUAtwin ion-selective electrode (ISE) pocket meter (model NO3-11; Horiba, Kyoto, Japan). Before sample measurement, two-point calibration was performed and repeated regularly throughout the analysis. Nitrate standard solutions of 150 mg NO3− L−1 and 2000 mg NO3− L−1, corresponding to 2.4 and 32 mmol L−1, respectively, prepared by the manufacturer, were used for calibration (Peña-Fleitas et al., 2022).
4. Silage Analysis
Approximately 200 g of sample from each treatment was collected and stored in a freezer at −20°C until analysis of silage characteristics. To measure silage pH, 10 g of silage was mixed with 100 mL of distilled water and stored in a refrigerator for 24 h with occasional shaking. The extract was then squeezed and filtered through four layers of gauze, and the pH of the filtrate was measured using a pH meter (HI 9024; HANNA Instruments Inc., UK).
For the analysis of WSC was analyzed by the anthrone method (Yemm and Willis, 1954) and ammonia nitrogen (NH3-N) content determination by a modification of the phenol-hypochlorite reaction method (Broderick et al., 1980).
5. Statistical Analysis
Statistical analysis was performed using the SAS package program (version 9.4, 2014). Analysis of variance was conducted, and treatment means were compared using the least significant difference (LSD) test.
Ⅲ. Results and Discussion
1. Changes in Feed Value According to Cutting Height and Plant Part
Changes in feed value according to plant part are shown in Table 1. The crude protein (CP) content of the whole plant harvested at 200 cm was 7.35%, and the content was higher in the leaves (9.53%) than in the stems (4.81%). In addition, the CP content in the 50 cm harvest group (11.57%) was significantly higher than that in the 150 cm harvest group (8.81%) (p<0.05). Choi et al. (2017) reported that the average CP content of sudangrass cultivars harvested in the Cheonan region in mid- to late July ranged from 8.9 to 10.0%, which was higher than the value observed in the present study (7.35%).
However, regional differences were also observed; in the Gimje region, CP content ranged from 7.1 to 8.2%, which was lower than that in Cheonan and similar to the results of the present study (Choi et al., 2017).
The ADF and NDF contents, which indicate the fiber content of the plant, increased as plant height increased, but no clear differences were observed among plant parts. In particular, NDF content was lowest in the stems, whereas ADF content was lowest in the leaves; however, no significant differences were observed among treatments (p>0.05). The digestibility of the sorghum × sudangrass hybrid according to plant height did not differ significantly among plant parts, but clear differences were observed according to plant height at harvest. Digestibility decreased as plant height increased.
TDN content tended to decrease with increasing plant height. In the 200 cm harvest group, TDN content was approximately 5% higher in the leaves than in the stems, whereas no clear differences among plant parts were observed in the other harvest groups. The RFV values, calculated based on ADF and NDF contents, averaged around 90. RFV tended to be higher when plant height was shorter, but differences among plant parts were not large.
Meanwhile, Alatürk (2024), in a study on harvest plant height and harvest timing of sudangrass cultivars, reported that delayed harvesting increased dry matter yield but decreased crude protein content by 54%. They also reported that crude protein content decreased by 65% as plant height at harvest increased from 30 to 150 cm.
Dry matter content tended to increase as plant height increased, with a particularly marked increase observed in the stem fraction (Table 2). The 50 cm harvest group did not differ significantly from the 100 cm harvest group, but it showed significantly lower dry matter content than the 150 and 200 cm harvest groups (p<0.05). This suggests that fiber accumulation may have increased during this period.
The WSC content, which affects silage fermentation, increased significantly as plant height increased, while no differences were observed among plant parts. In addition, there was no significant difference between the 50 and 100 cm harvest groups, nor between the 150 and 200 cm harvest groups.
Nitrate nitrogen content, which may limit the utilization of sorghum × sudangrass hybrids, generally tended to decrease in the leaves as plant height increased, whereas it increased in the stems. In the whole plant, nitrate nitrogen content differed significantly among plant heights.
Elgersma et al. (2001) reported that nitrate nitrogen content in different parts of sorghum × sudangrass hybrids was highest in the lower stem, decreased toward the upper stem, and was very low in the leaves. They also reported that, at nitrogen application rates below 200 kg N/ha, nitrate nitrogen levels were low enough not to cause livestock damage. Similarly, in the present study, nitrate nitrogen was detected at an overall safe level.
2. Changes in Feed Value According to Fermentation Period
Changes in the feed value of sudangrass silage during the fermentation period are shown in Table 3. Although dry matter content continued to fluctuate over time, it remained higher than that of the fresh material during the fermentation process. However, after 60 days of fermentation, it tended to be lower than the initial value, although no significant difference was observed.
In an Italian ryegrass silage study, Li et al. (2024) also reported that dry matter content was lowest on day 5 of fermentation and then tended to increase slightly thereafter. Similarly, Li et al. (2023) reported in a rye silage study that dry matter content decreased until the first 5 days of fermentation and then tended to increase slightly.
Crude protein content showed the highest value on day 5 of fermentation and then gradually decreased, continuing to decline thereafter. The ADF and NDF contents generally tended to increase as fermentation progressed, but no significant differences were observed. TDN content, calculated from ADF, tended to gradually decrease with fermentation, but this change was not statistically significant.
Meanwhile, Li et al. (2023) reported that, in rye silage, dry matter content tended to slightly decrease compared with the fresh material as fermentation progressed. They suggested that this may have been due to the loss of organic matter in the form of water and carbon dioxide under anaerobic conditions. They also reported that crude protein content decreased during the early stage of fermentation but tended to increase slightly as fermentation progressed. The decrease during the early stage was attributed to increased protein degradation caused by the activity of plant enzymes and pre-existing microorganisms.
3. Changes in Silage Quality According to Fermentation Period
Regarding changes in fermentation characteristics according to the fermentation period, pH gradually decreased over time and, in particular, rapidly dropped below 4.0 within the first 5 days of fermentation (Fig. 1). Since pH reflects the activity level of lactic acid bacteria, the WSC content, which serves as a fermentation substrate for lactic acid bacteria, showed a similar trend to pH, gradually decreasing and then maintaining a stable level after about 15 days of fermentation.
Ammonia nitrogen content tended to gradually increase as fermentation progressed, with most of the increase occurring within the first 5 days of fermentation. In general, ammonia nitrogen content is an indicator of the degree of protein degradation and is also closely related to crude protein content.
4. Correlations Among Silage Quality Parameters
Regarding the correlations among quality-related parameters, crude protein content showed a weak correlation with digestibility, while ADF content showed strong negative correlations with pH, WSC, and ammonia nitrogen (Table 4). The pH was positively correlated with WSC but negatively correlated with ammonia nitrogen.
WSC content showed a negative correlation with ammonia nitrogen and a positive correlation with nitrate nitrogen. Nitrate content was closely correlated with pH, WSC, and ammonia nitrogen. In particular, nitrate content showed a negative correlation with ammonia nitrogen, indicating that nitrate accumulation decreased when fermentation proceeded well.
Hou et al. (2025), in a study on corn silage, reported correlations between silage quality and feed value, showing that pH was negatively correlated with lactic acid and crude protein contents, while lactic acid content was positively correlated with dry matter content.
Ⅳ. CONCLUSIONS
This study was conducted to observe changes in chemical composition according to the growth stage of a sorghum × sudangrass hybrid and silage quality during fermentation. The cultivated samples were harvested and analyzed when plant height reached 50 cm (July 3), 100 cm (July 10), 150 cm (July 22), and 200 cm (July 30). Harvested silage samples were taken after 0, 1, 2, 3, 5, 10, 15, 30, 45, and 60 days to evaluate changes in silage quality. The crude protein content was 7.35% in the 200 cm harvest group, and it was higher in the leaves (9.53%) than in the stems (4.81%). Nitrate nitrogen content generally tended to decrease in the leaves as plant height increased, whereas it increased in the stems. Although the dry matter content of the silage fluctuated during fermentation, it remained relatively high throughout the fermentation process. When the crop is harvested at a generally appropriate stage, nitrate-nitrogen in the sorghum × sudangrass hybrid does not reach a level harmful to livestock. In addition, nitrate-nitrogen did not reach a harmful level during the silage fermentation process.
