Effects of fermented soybean meal and feed restriction on the performance and egg quality of laying hens

Nguyen Tuyet Giang^1,2, Le Thi Thuy Hang^1,2, Le Thi Thuy Loan^1,2, Le Thanh Binh³, Pham Ba Toan³, Truong Minh Quan³, Nguyen Hoang Nhat³, Nguyen Thi Ngoc Linh⁴, Nguyen Thi Thuy Trang⁵ and Do Vo Anh Khoa⁵

¹ An Giang University, An Giang, Vietnam
² Vietnam National University Ho Chi Minh City, Vietnam
³ Thai Nguyen University of Agriculture & Forestry
⁴ Can Tho University
⁵ Vietnam National University of Forestry at Dongnai
dvakhoa@gmail.com

Abstract

This study investigated the combined effects of fermented soybean meal (FSBM) supplementation and feed restriction on the performance and egg quality of Isa Brown laying hens. A total of 120 Isa Brown hens were randomly assigned to four treatments: CON-100 (basal diet, 100% intake), FSBM-100, FSBM-95, and FSBM-90. All were formulated to meet the nutrient requirements of layers. As a result, a feed restriction up to 5% (FSBM-95) maintained egg production and improved feed conversion ratio, while a 10% restriction (FSBM-90) negatively affected performance.Hens receiving FSBM produced eggs with thicker shell, while internal egg quality was unchanged. Although higher feed intake tended to increase egg production and egg weight, the combination of FSBM inclusion and moderate feed restriction (5%) yielded the greatest overall efficiency, reflected in an improved laying rate, feed conversion, egg weight, and potential feed cost savings compared with the control.

Keywords: Isa Brown chicken, feed conversion ratio, laying rate, restricted feeding, soybean meal, yeast fermentation

Introduction

Feed cost, accounting for approximately 70% of total production expenses in livestock and poultry farming, has continued to rise, prompting an intensified search for cost-effective and sustainable alternatives to conventional feed sources (El-Sabrout et al 2022). Soybean meal (SBM), a by-product of the oil extraction process, is one of the most widely used protein sources in animal feeds because of its high protein content and well-balanced amino acid profile. However, the presence of anti-nutritional compounds such as trypsin inhibitors, lectins, phytic acid, and oligosaccharides limits its nutrient digestibility and availability, particularly in non-ruminant species (Mukherjee et al 2016). In addition, similar to other oilseed cakes, SBM contains residual fibers and complex carbohydrates that reduce nutrient utilization efficiency unless properly processed (Sarkar et al 2021). Among potential approaches, microbial fermentation has attracted considerable attention as an eco-friendly technology that enhances the nutritional value and reduces anti-nutritional factors of plant-based ingredients (Mukherjee et al 2016; Adebo et al 2022).

Fermentation of soybean meal (FSBM) using microorganisms such as Saccharomyces cerevisiae, Bacillus subtilis, Lactobacillus sp., effectively degrades anti-nutritional compounds (e.g., trypsin inhibitors, glycinin and β-conglycinin) while improves amino acid profile, protein solubility, and overall digestibility (Inthapanya et al 2020; Nguyen et al 2021; Nguyen et al 2023; Preston 2023). Furthermore, fermentation enriches the feed nutrients with bioactive peptides and beneficial metabolites that may modulate the gut microbiota, thereby improving feed conversion efficiency (Bilal et al 2022; Fathima et al 2023). Recent studies have confirmed that incorporating FSBM into layer diets can enhance nutrient utilization, intestinal health, immune response, as well as improve egg-quality traits such as yolk color and shell strength (Lu et al 2023; Obianwuna et al 2024).

While fermented feeds effectively improve the nutrient availability and digestibility, restricted feeding represents a distinct management strategy aimed at regulating energy intake and optimizing production efficiency. In broiler chickens, quantitative feed restriction during the growing phase has been shown to reduce excessive fat deposition, improve feed conversion, and modulate blood metabolites without compromising growth performance and carcass traits (Attia et al 2017; Rubel and Beg 2018; Gobane et al 2021).

In laying hens, excessive body weight resulting from feed intake exceeding the breed standards can adversely affect health, leading to excessive abdominal fat deposition, elevated body temperature, obesity, lameness, and skeletal disorders all of which contribute to higher mortality. Overfeeding may also cause the production of oversized or double-yolk eggs, which are less marketable. Moderate restriction levels, therefore, have been applied to control body weight, delay sexual maturity, and enhance feed efficiency, particularly under tropical production conditions (Anene et al 2023). Moreira et al (2012) further demonstrated that a feed restriction up to 5% maintained egg production while improving yolk pigmentation and feed efficiency. Similarly, Molapo et al (2020) reported that temporary restriction during the pre-lay phase did not adversely affect subsequent egg production in Koekoek hens, suggesting the presence of compensatory adaptation mechanisms. Sonkamble et al (2020) also found that imposing a 20-30% feed restriction during the rearing period delayed sexual maturity but maintained egg production and quality in White Leghorn layers. Collectively, these findings indicate that strategic well-managed feed restriction strategies can sustain nutrient utilization and improve economic efficiency in layer chickens.

In this study, an experiment was conducted to evaluate the combined effects of dietary inclusion of FSBM and varying levels of feed restriction on the production performance and egg quality of Isa Brown chickens. The objective was to determine whether the nutritional improvements derived from FSBM could compensate for reduced feed intake and maintain optimal productivity under limited-feeding conditions.

Materials and methods

The experiment was conducted at a layer farm located in Tri Ton district, An Giang province, Viet Nam. A total of 120 Isa Brown laying hens, 22 weeks of age, were randomly assigned to four dietary treatments in a completely randomized design (CRD). The dietary treatments were a basal diet fed at the standard intake level (CON-100), a fermented soybean meal (FSBM) diet fed at the standard intake level (FSBM-100), an FSBM diet fed at 95% of the standard intake level (FSBM-95), and an FSBM diet fed at 90% of the standard intake level (FSBM-90). The standard feed intake (112 g bird/day) was determined according to the recommendations of Hendrix-Genetics (2018). Each treatment consisted of ten replicates, with three birds housed in individual cages (50 × 50 × 40 cm) equipped with an individual feeder and a nipple drinker.

The FSBM used in this study was prepared following the procedure described by Le et al (2024). Commercial soybean meal was moistened to achieve 50% relative humidity and inoculated with Saccharomyces cerevisiae (Instant Success Dry Yeast, Lesaffre, France) at 2% (v/w; ≈ 10⁸ cfu/mL). The mixture was fermented anaerobically for 72 h at ambient temperature, then oven-dried at 55 °C to a moisture content below 10%, and ground into powder using a 3-mm sieve.

The experimental diets (control and FSBM based ones) were formulated to meet nutritional requirements recommended in the Isa Brown Management Guide (Hendrix-Genetics, 2018). In the FSBM based treatments, FSBM was included at 3%, partially replacing conventional soybean meal (Table 1). Feed was provided according to the assigned levels at 100%, 95% and 90% of the standard intake level. Fresh drinking water was provided ad libitum through nipple drinkers. A lighting program of 16 hours of light and 8 hours of darkness was maintained throughout the experimental period.

The body weight of the birds was recorded at 22 and 34 weeks of age, while eggs were collected twice daily to determine laying rate and total egg production. Feed conversion ratio (FCR) was calculated as the ratio of feed intake to egg mass. The performance parameters included daily feed intake (g/bird/day), laying rate (%), egg weight (g/bird/day), feed conversion ratio (FCR), and body weight change (%). Egg quality traits were evaluated every three weeks using 10 eggs per treatment. The parameters assessed included egg weight, shape index, yolk index, shell thickness, the proportions of yolk and albumen and Haugh unit.

Data obtained from the experiment were statistically analyzed using Analysis of Variance (ANOVA) under the general linear model (GLM) procedure in Minitab 16.0 (Minitab Inc., State College, PA, USA). The statistical significance was declared at p<0.05.

Results and discussion

Laying performance

The effects of dietary treatments on the production performance of Isa Brown hens are shown in Table 2. Feed restriction significantly influenced laying rate, feed intake, and feed conversion ratio (p<0.05), whereas body weight change was not affected. (p>0.05).

In the present study, the inclusion of fermented soybean meal (FSBM) improved the production performance of Isa Brown hens under both standard and restricted feeding conditions. Hens fed the FSBM-100 diet achieved the highest laying rate compared to the control group (CON-100), confirming the beneficial effect of dietary fermentation on nutrient utilization and productivity. Interestingly, birds receiving the FSBM-95 treatment, despite a 5% reduction in feed allowance, maintained egg production comparable to that of the unrestricted groups, suggesting that FSBM inclusion effectively compensated for the reduced feed consumption. However, a further restriction to 90% of the standard intake (FSBM-90) resulted in a significant decline in laying rate, indicating that nutrient intake supply at this level was insufficient to sustain optimal production.

These observations are consistent with previous reports. Moreira et al (2012) and Sonkamble et al (2020) demonstrated that moderate feed restriction (5-10%) improved feed efficiency without compromising egg production in Hisex Brown and White Leghorn hens. Similarly, Lu et al (2023) and Obianwuna et al (2024) reported that dietary inclusion of FSBM enhanced nutrient digestibility, gut health, and protein utilization, thereby improving performance. Collectively, these findings reinforce the synergistic benefit of combining FSBM with mild feed restriction as an effective strategy to optimize laying production and feed efficiency in hens.

The improved FCR observed in FSBM groups could be attributed to enhanced amino acid availability, reduced anti-nutritional factors, and increased beneficial microflora activity resulting from fermentation. Yeast fermentation of soybean meal has been shown to increase the bioavailability of amino acids, decrease anti-nutritional factors such as trypsin inhibitors, and enrich the feed with beneficial metabolites that promote intestinal balance (Lu et al 2023; Obianwuna et al 2024). These effects collectively enhance nutrient absorption and metabolic efficiency, as similarly reported in yeast-fermented sorghum meal, which improved laying rate and feed conversion in hens without compromising egg quality (Nguyen et al 2025).

The moderate feed restriction (5%), on the other hand, may contribute to improved FCR by slowing the digesta passage rate and increasing nutrient retention time, thereby facilitating nutrient utilization. However, excessive restriction (10%) likely reduced intake below maintenance levels, resulting in lower productivity. Overall, the synergistic combination of 3% FSBM inclusion with mild feed restriction (5%) appears optimal for sustaining egg production while improving nutrient conversion efficiency in Isa Brown chickens.

Egg quality

The effects of dietary treatments on egg quality traits are presented in Table 3. There were no significant differences among treatments in internal egg quality traits such as yolk proportion, albumen proportion, yolk index, albumen index, or Haugh unit (p>0.05). However, the variation was noted in external egg characteristics. Hens in the FSBM-95 group produced the heaviest eggs (55.1 g), whereas those in the FSBM-90 group had the lowest egg weight (52.4 g; p<0.05). Eggshell thickness was significantly higher in the FSBM-100 and FSBM-95 groups (0.53 and 0.50 mm, respectively) compared with CON-100 (0.45 mm), but markedly decreased in FSBM-90 (0.37 mm; p<0.05).

The absence of significant differences in external egg traits among treatments indicates that variations in feed intake at or below the breed standard did not markedly affect the egg shape index, an important characteristic related to the egg’s suitability for packaging and its resistance to handling or transport damage (Anene et al 2023).

The improvement in shell thickness observed in hens fed FSBM diets could be attributed to enhanced mineral utilization and gut health benefits associated with fermented soybean meal inclusion, as similarly reported by Lu et al (2023) and Obianwuna et al (2024). In contrast, excessive feed restriction (90% of standard allowance) likely reduced calcium intake below the threshold required for proper shell formation, resulting in thinner eggshells and lower egg weight. The lack of differences in internal egg quality parameters (Haugh unit and yolk index) observed in this study is consistent with the findings of Moreira et al (2012) and Sonkamble et al. (2020), who reported that moderate feed restriction did not impair albumen or yolk quality, indicating that laying hens were able to maintain normal egg composition under a moderate feed restriction regime. The absence of significant differences in most variables among treatments indicates that different feed intake at or below the breed standard did not affect shell thickness or shape index. These parameters are critical indicators of shell strength and handling resistance, as an eggshell thickness of approximately 0.33 mm has been estimated to provide at least a 50% probability of withstanding normal transport and handling without breakage.

The improvement in shell quality observed in FSBM-inclusion groups could be attributed to enhanced nutrient bioavailability and the probiotic effects associated with yeast fermentation. Fermentation increases the digestibility and utilization of key nutrients such as amino acids, calcium, and phosphorus, while simultaneously promoting beneficial gut microbiota that support efficient mineral metabolism and eggshell formation. In contrast, hens in the FSBM-90 group likely experienced nutrient intake below the threshold required for effective shell calcification, resulting in thinner shells.

Overall, these results indicated that including 3% FSBM in the diet combined with a restricted feeding regimen below the breed standard did not adversely affect body weight change and most egg quality parameters. However, at the 90% feeding level, even with 3% FSBM inclusion, laying rate and shell thickness were reduced. Therefore, feeding at 95% of the standard intake with 3% FSBM can be recommended, as it maintains egg production and quality while contributing to feed cost savings.

Conclusions

The findings of this study indicate that feeding Isa Brown hens with a diet containing 3% FSBM at 95% of the standard feed level allowance (FSBM-95) maintained high egg production and improved feed efficiency without compromising egg quality. Therefore, the FSBM-95 treatment can be considered the optimal feeding strategy under the conditions of this study.

Acknowledgments

The authors would like to gratefully acknowledge all individuals who provided technical and logistical support during the study.

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