Assessment of crude protein solubility in Psophocarpus scandens compared to Ipomoea aquatica and Sesbania grandiflora as potential protein sources for livestock

Nguyen Van Thu¹, Nguyen Thi Kim Dong¹ and T R Preston²

¹ Tay Do University, Vietnam
nvthu@tdu.edu.vn
² Centro para la Investigación en Sistemas Sostenibles de Producción Agropecuaria (CIPAV), Carrera 25 No 6-62 Cali, Colombia

Abstract

This study evaluated the chemical composition and protein solubility of three tropical forage species including Psophocarpus scandens (PS), Sesbania grandiflora (SG), and Ipomoea aquatica (IA) to determine their potential in ruminant feeding systems. The experiment was arranged in a completely randomized design (CRD) with three treatments (forage species) and four replications. Each forage was assessed in two forms: fresh and dried. The results showed that significant differences (p<0.01) were observed among the forage species for all nutritional parameters. Sesbania grandiflora had the highest dry matter content (194 g/kg), while Ipomoea aquatica had the lowest (108 g/kg) but exhibited the highest crude protein content (281 g/kg DM). Psophocarpus scandens presented the highest fiber values (NDF, ADF, CF: 430, 358, and 186 g/kg DM, respectively), suggesting lower digestibility. Ash content was highest in Ipomoea aquatica (117 g/kg DM), reflecting its higher mineral content. Protein solubility was significantly influenced by forage type and form (p<0.01). In the fresh state, Ipomoea aquatica showed the highest solubility (724 g CP/kg DM), while Psophocarpus scandens had the lowest (387 g CP/kg DM), a trend also seen in dried samples. The low solubility in Psophocarpus scandens suggests a higher proportion of rumen-undegradable protein, which could enhance nitrogen efficiency and reduce methane emissions. Despite its lower crude protein content (207 g/kg DM), Psophocarpus scandens shows promise as a functional component in sustainable ruminant diets.

Keywords: protein forages, nutrient composition, ruminants, soluble protein, environment

Introduction

Psophocarpus scandens, a wild relative of the winged bean (Psophocarpus tetragonolobus), is a leguminous vine native to tropical Africa and Southeast Asia. Traditionally used in human diets, it has gained attention in recent decades as a protein-rich forage option for livestock due to its high crude protein content, adaptability, and palatability (Devendra and Sevilla, 1978 and Gross, 1983). In the Mekong delta of Vietnam Psophocarpus scanden (18.8% CP) grows well and is popularly used as a protein supplemented source for ruminants and rabbits to reduce the feed cost (Nguyen Van Thu et al 2021). The research conducted in Vietnam highlights the potential of Psophocarpus scandens as a valuable forage and protein supplement in animal diets. These findings suggest that Psophocarpus scandens can be an effective and sustainable alternative to conventional feed sources in livestock production systems.​ While Ipomoea aquatica and Sesbania grandiflora foliates with high protein content are also used as the forage crude protein sources for both monogastic animals and ruminants in Vietnam.

Effective ruminant production must prioritize the reduction of methane emissions, as these emissions significantly contribute to global warming. Mitigating methane release not only supports environmental sustainability but also enhances the economic efficiency of livestock systems by reducing nutrient losses from feeds. According to Preston and Leng (2021), optimizing protein utilization in ruminants involves identifying feed sources with low protein solubility—an important characteristic that allows a portion of dietary protein to bypass rumen fermentation. This rumen escape contributes to better protein use efficiency and reduced methane generation. Therefore, refining the classification of protein sources, as suggested by Preston and Leng (2021), represents a key strategy in improving ruminant productivity while simultaneously lowering methane emissions. This experiment aimed to assess the protein solubility of Psophocarpus scandens in comparison to Ipomoea aquatica and Sesbania grandiflora foliate, with the goal of determining its potential as a protein supplement for future research and practical use in livestock feeding systems.

Material and methods

Location

This experiment was carried out from May to Aug 2024 at the laboratory od Department of Animal Sciences of Faculty of Agriculture, Can tho University

Design and treatments

The experiment was conducted using a completely randomized design (CRD) with three forage treatments and four replications per treatment. The three forages evaluated were:

• Psophocarpus scandens
• Ipomoea aquatica
• Sesbania grandiflora

Each forage was prepared and tested in two forms:

1. Fresh foliage, ground in a blender with 1M NaCl solution
2. Dried foliage, oven-dried at 65°C and ground to pass through a 1 mm sieve

The procedures for sample preparation and protein solubility assessment followed the methods described by Nguyen Thi Thu Hong and Nguyen Ngoc Trang (2022). This design allowed for the comparison of protein solubility characteristics across different forage species and processing methods.

Measurements and chemical analysis

To assess protein solubility and chemical composition, both fresh and dried forage samples were processed and analyzed as follows:

Protein Solubility in Fresh Foliage

For each treatment (e.g., Psophocarpus scandens, Ipomoea aquatica and Sesbania grandiflora), 10 g of fresh foliage was homogenized in 100 mL of 1M NaCl solution using a laboratory blender. The homogenate was passed through a 1 mm screen, then shaken for 3 hours at room temperature. After shaking, the mixture was filtered through Whatman No. 4 filter paper. The nitrogen content of the resulting filtrate was determined using the Kjeldahl method as described by AOAC (1990), and protein solubility was calculated based on the soluble nitrogen fraction.

Protein solubility in dried foliage:

To prepare dried samples, 100 g of fresh forage was dried at 65°C in a convection oven until reaching constant weight. The dried samples were then ground to pass through a 1 mm sieve. For each dried sample, 3 g was shaken in 100 mL of 1M NaCl solution for 3 hours. The mixture was subsequently filtered through Whatman No. 4 filter paper, and the nitrogen content of the filtrate was analyzed by the Kjeldahl method (AOAC, 1990), following the procedure outlined by Whitelaw and Preston (1963).

Chemical composition

All forage samples were analyzed for basic chemical composition using standard procedures:

• Dry Matter (DM): Determined by oven-drying at 105°C for 24 hours (AOAC, 1990).
• Organic Matter (OM): Estimated by ashing the dried sample at 550°C for 4 hours (AOAC, 1990).
• Crude Protein (CP): Measured using the Kjeldahl method (AOAC, 1990).
• Crude Fiber (CF): Determined by AOAC standard methods (AOAC, 1990).
• Neutral Detergent Fiber (NDF) and Acid Detergent Fiber (ADF): Analyzed according to the procedures of Van Soest et al (1991).

Statistical analysis

The data were analyzed by the general linear model in the Minitab software Release 18.1 (Minitab 2017). Sources of variation were: treatments, repetitions and error.

Results and discussion

The chemical composition of three forages are presented in Table 1.

In Table 1 showed that significant differences (p<0.01) were observed among the forages for all measured parameters, including dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber (NDF), acid detergent fiber (ADF), crude fiber (CF), and ash.

Sesbania grandiflora had the highest DM content (194 g/kg), followed by Psophocarpus scandens (170 g/kg), while Ipomoea aquatica had the lowest (108 g/kg). In contrast, Psophocarpus scandens had the highest OM content (914 g/kg DM), reflecting its relatively lower ash content compared to the other two forages. All forages had high CP content (207–281 g/kg DM), with Ipomoea aquatica containing the most protein and Psophocarpus scandens the least. These findings are consistent with previous reports highlighting the protein-rich nature of Ipomoea aquatica and Sesbania grandiflora as suitable forage sources in tropical regions (Nguyen Van Thu et al 2021; Nguyen Thi Thu Hong and Nguyen Thi Ngoc Trang, 2023). Fiber components (NDF, ADF and CF) were significantly higher in Psophocarpus scandens, indicating a more fibrous structure. This may impact digestibility and nutrient availability in ruminants. Conversely, ash content followed the opposite trend, with Ipomoea aquatica showing the highest mineral content, while Psophocarpus scandens had the lowest.

The protein solubility of the foliates in the experiment was indicated in Table 2.

Figure 1. The solubility of crude protein (g/1000g) of three foliates in fresh and drying at 65oC

Protein solubility values for the fresh and dried forms of the three forages are shown in Table 2 and Figure 1. Solubility was significantly affected (p<0.01) by both forage species and form. In all cases, fresh forages had higher protein solubility than dried forages.

Ipomoea aquatica exhibited the highest protein solubility (724 g CP/kg) in the fresh form and 655 g CP/kg in the dried form reflecting its highly soluble protein content. In contrast, Psophocarpus scandens had the lowest solubility (387 and 284 g CP/kg for fresh and dried forms, respectively), with Sesbania grandiflora displaying intermediate values (406 and 349 g CP/kg). These findings are in agreement with previous studies. Le Thanh Phuong et al. (2025) reported similar solubility values for Psophocarpus scandens, while the results for Ipomoea aquatica align closely with those reported by Nguyen Thi Thu Hong and Nguyen Thi Ngoc Trang (2023). The relatively low protein solubility in Psophocarpus scandens may be advantageous for ruminant nutrition, as it suggests a higher proportion of rumen-undegradable protein, which can improve nitrogen retention and reduce methane production (Preston and Leng, 2021).

Conclusion

Among the forages studied, Psophocarpus scandens exhibited the lowest protein solubility, which indicates a higher proportion of rumen-undegradable protein. This characteristic makes it a valuable forage option for ruminants, as it can enhance nitrogen utilization efficiency and potentially reduce methane emissions. Despite its lower crude protein content compared to the other species, its fibrous nature and low solubility suggest strategic nutritional benefits in ruminant diets.

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