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3 May 2026

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Effects of Replacing Soybean Meal with Cashew Nut Roasting Residue on Feed Intake, Nutrient Digestibility, Rumen Fermentation, Blood Metabolites, and Economic Efficiency in Lactating Goats

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and
1
Department of Agricultural and Fishery Science, Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani 94000, Thailand
2
Department of Science, Faculty of Science and Technology, Prince of Songkla University, Pattani Campus, Pattani 94000, Thailand
3
Department of Agricultural Science and Technology, Faculty of Innovative Agriculture, Fisheries and Food, Prince of Songkla University, Surat Thani Campus, Surat Thani 84000, Thailand
*
Author to whom correspondence should be addressed.
Ruminants2026, 6(2), 32;https://doi.org/10.3390/ruminants6020032
This article belongs to the Special Issue Nutrients and Feed Additives in Sheep and Goats
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Simple Summary

Feed costs represent one of the greatest challenges in dairy goat production, particularly the cost of protein-rich ingredients such as soybean meal. In many regions, soybean meal is expensive and often imported, increasing production costs for farmers. At the same time, the processing of cashew nuts generates large amounts of roasting residue that are usually discarded, even though this by-product still contains valuable nutrients. This study investigated whether cashew nut roasting residue could replace part of soybean meal in the diets of lactating goats without negatively affecting feed intake, rumen function, or measured blood indicators. Goats were fed diets containing increasing levels of this by-product, and feed intake, digestion, rumen conditions, blood indicators, and economic outcomes were evaluated. The results showed that goats maintained normal feed intake and stable physiological conditions when part of the soybean meal was replaced. Although protein digestion was slightly lower at the highest inclusion level, no changes were observed in the measured blood indicators. Importantly, the use of cashew nut roasting residue reduced feeding costs. These findings suggest that this agricultural by-product can be used as a practical and low-cost feed resource, helping farmers reduce production costs while promoting more sustainable livestock production systems.

Abstract

This study evaluated the effects of replacing soybean meal with cashew nut roasting residue (CNRR) on feed intake, nutrient digestibility, rumen fermentation, blood metabolites, and economic efficiency in lactating crossbred Saanen goats. Fifteen does were assigned to a completely randomized design with three treatments: 0% CNRR (T1), 10% replacement of soybean meal protein (T2), and 20% replacement (T3). Goats were fed pangola hay ad libitum and concentrate according to milk yield (1:2). Total dry matter, organic matter, crude protein, and fiber intake were not affected by dietary treatments (p > 0.05). However, crude protein digestibility decreased from 68.83% in the control group to 52.72% and 51.21% in the CNRR treatments (p < 0.05). Rumen pH and total volatile fatty acids remained within normal physiological ranges, suggesting stable rumen fermentation. Blood parameters, including packed cell volume, glucose, and blood urea nitrogen, were not influenced by CNRR supplementation (p > 0.05). Economic analysis showed reduced feed cost with increasing CNRR inclusion. In conclusion, CNRR can replace soybean meal protein at rates of up to 20% without affecting feed intake, ruminal pH, or measured blood metabolites; however, crude protein digestibility was reduced, indicating that dietary formulation may require adjustment.

1. Introduction

Dairy goat production plays an important role in tropical livestock systems by providing milk for household consumption and income for smallholder farmers. In Thailand, the expansion of dairy goat farming has increased the demand for affordable and reliable feed resources. Feed cost is one of the major expenses in ruminant production, especially protein sources such as soybean meal (SBM), which significantly increase production costs [1]. In addition, the heavy reliance on imported SBM exposes farmers to price fluctuations, highlighting the need for locally available and lower-cost protein alternatives.
Cashew (Anacardium occidentale L.) is an important tropical crop mainly processed for kernel production. The processing of cashew nuts generates considerable amounts of agro-industrial by-products, such as cashew nut roasting residue (CNRR), which may serve as alternative feed resources due to their residual nutrient content. CNRR is an agro-industrial by-product obtained from the cashew nut roasting process. It consists of broken and damaged kernels, partially roasted or cracked whole nuts, testa, and residual oil remaining after roasting. This material is usually mixed together during processing and is often discarded or underutilized despite its nutritional potential. Recently, interest in using unconventional feed resources such as CNRR as a strategy to reduce feed costs and improve feed availability has increased [2]. Cashew by-products, particularly kernel-derived fractions, are known to contain moderate crude protein levels (18–27%) and high ether extract contents (36–51%) due to the presence of residual kernel material, and they can provide useful energy and fiber for ruminant diets [3]. In addition, the use of agricultural by-products as animal feed can help reduce waste and support more sustainable livestock production systems [4]. These characteristics suggest that CNRR could be a practical and sustainable feed ingredient, especially in areas with intensive cashew processing.
Previous studies suggest that cashew-derived by-products have potential for use in animal diets, although their effects on performance may vary depending on the inclusion level and processing conditions. An in vitro study suggested that cashew nut meal could partially replace soybean meal without adverse effects on rumen fermentation characteristics or nutrient digestibility [5]. Similar findings have been reported in poultry, where cashew nut meal was included in quail diets and improved antioxidant status in broiler chickens [6,7]. These results indicate that cashew by-products have potential for use across different animal species. However, further studies are needed to evaluate their long-term effects and to develop suitable processing methods for wider application [3]. In addition, some studies have reported that cashew-derived by-products may improve feed utilization and economic returns under certain production conditions. For example, Okolo et al. [8] evaluated digestibility and bio-economic responses in goats. Although some studies have evaluated the economic responses of cashew-derived by-products, these outcomes are context-dependent and influenced by local conditions. However, information on the use of cashew nut roasting residue (CNRR) specifically in lactating dairy goats remains limited, particularly regarding rumen fermentation, metabolic responses, and economic efficiency.
Diets with high energy or fat contents may negatively affect rumen function when fed at excessive levels, including changes in rumen microbial activity and nutrient utilization [9,10]. Although the effects of dietary fat depend on its source and physicochemical properties [11], the specific effects of CNRR, which contains residual oil and kernel fractions, have not yet been fully investigated. Potential impacts on rumen fermentation, physiological responses, and feeding cost need to be carefully evaluated.
Blood metabolites such as packed cell volume (PCV), glucose, and blood urea nitrogen (BUN) are relevant indicators for evaluating physiological and metabolic responses of animals to dietary treatments. CNRR contains residual protein and fat, which may influence nutrient digestion and metabolism. Changes in protein utilization are reflected in BUN, an indicator of nitrogen metabolism in ruminants [12]. Similarly, dietary energy supply can affect blood glucose concentrations, reflecting energy balance [13]. In addition, PCV is commonly used to assess physiological status in animals [14]. Therefore, these blood metabolites were included to evaluate metabolic responses to CNRR inclusion.
To our knowledge, no previous studies have examined the economic impact of replacing SBM with CNRR in diets of lactating dairy goats. This represents an important knowledge gap, particularly for smallholder farmers who are highly sensitive to feed costs. Therefore, the objective of this study was to evaluate the effects of replacing soybean meal with CNRR in concentrate diets on feed intake, nutrient digestibility, rumen fermentation characteristics, blood metabolites, and economic efficiency in lactating dairy goats. Understanding these responses will help determine whether CNRR can serve as a sustainable ingredient to improve profitability and resilience in tropical dairy goat production systems.

2. Materials and Methods

2.1. Experimental Design

The study was conducted using a completely randomized design (CRD) comprising three dietary treatments with five replicates per treatment, using one goat per replicate. The experiment was conducted at the research farm of Prince of Songkla University, Pattani Campus, Pattani, Thailand. Goats were individually housed in well-ventilated pens equipped with feeding and watering facilities. Clean drinking water and mineral blocks were provided ad libitum. Animals were managed under standard husbandry conditions throughout the experimental period. Fifteen crossbred lactating goats (75% Saanen), averaging 39.86 ± 5.41 kg body weight, with an average 90.50 ± 10.00 days in milk and initial milk yield of 573.20 ± 60.74 g/day, were used in this study, with no significant differences among treatments at baseline (p > 0.05). Prior to the experiment, all goats underwent a 30-day adaptation period during which they were fed pangola hay (Digitaria eriantha) as the roughage source together with a 16% crude protein concentrate. During this adaptation period, animals were also dewormed to eliminate internal and external parasites. The experimental diets were formulated to evaluate the effects of replacing soybean meal with cashew nut roasting residue (CNRR) at three inclusion levels. The control diet (T1) contained soybean meal as the main protein source, without CNRR. The second diet (T2) included 10% CNRR on a concentrate dry matter basis, partially replacing soybean meal, whereas the third diet (T3) contained 20% CNRR, replacing a greater proportion of soybean meal protein. All goats received concentrate at a ratio of 1:2 (milk yield:concentrate), while pangola hay was provided ad libitum. The ingredient compositions and chemical characteristics of the concentrate diets used in each treatment are presented in Table 1.
Table 1. Ingredient proportions (as-fed basis), and nutritional composition (dry matter basis) of the concentrate diets for lactating dairy goats.

2.2. Animals and Management

A total of 15 lactating does were randomly allocated to three treatments (five animals per treatment) with similar stages of lactation and milk yield. All goats were randomly allocated to dietary treatments. Before the trial, each animal was treated with ivermectin (IDECTIN®, Thainaoka Pharmaceutical Co., Ltd., Samut Sakhon, Thailand) at 1 mL/50 kg BW administered subcutaneously to control gastrointestinal and external parasites, and niclosamide (Yomesan®, Bayer AG, Leverkusen, Germany) at a dosage of two tablets per animal to eliminate tapeworms. Goats were housed individually throughout the 3-month experimental period.
All experimental procedures were conducted in accordance with institutional guidelines for animal care and use and were approved by the relevant ethics committee (Approval No.: 67A115000284; Ref. AG088/2024).

2.3. Feeding Management

All goats received pangola hay ad libitum as the roughage source and the experimental concentrates according to their assigned treatments. Concentrate was offered based on milk yield at a ratio of 1:2 (milk yield:concentrate). Feed was supplied twice daily at 07:00 and 16:00 h. Feed offered and refusals were recorded daily to determine daily feed intake.

2.4. Chemical Analysis of Experimental Diets

Samples of pangola hay and concentrate mixtures were collected every three days and ground using a Wiley mill with a 1 mm screen. Samples were analyzed for dry matter (DM), crude protein (CP), ether extract (EE), and ash according to AOAC [15]. Cell wall constituents including neutral detergent fiber (NDF), acid detergent fiber (ADF), and lignin were analyzed following Goering and Van Soest [16]. Organic matter (OM) was calculated as:
OM (%) = DM (%) − ash (%)
Total digestible nutrients (TDN) were estimated from proximate analysis using standard equations based on digestible crude protein, crude fiber, nitrogen-free extract, and ether extract fractions, following established procedures.
The chemical composition of pangola hay, CNRR, and the concentrate diets is presented in Table 2 and Table 3.
Table 2. Chemical composition of pangola hay and cashew nut roasting residue (CNRR) on a dry matter basis (% DM).
Table 3. Chemical composition of concentrate diets containing different levels of cashew nut roasting residue (CNRR) as a protein source (%, DM basis).

2.5. Measurement of Feed Intake

Daily intake of concentrate and roughage was calculated for each animal based on the difference between feed offered and refusals. Feed intake was expressed relative to metabolic body weight (BW0·75) to standardize intake among animals of different sizes. This approach is based on the principle that metabolic rate scales with body weight to the power of 0.75 [8] and is widely used in small ruminants to evaluate nutrient intake and feeding efficiency [13,17].

2.6. In Vivo Digestibility

Digestibility was determined using the indicator method with acid-insoluble ash (AIA) as an internal marker [18]. After the 30-day adaptation period, fecal samples were collected twice daily (07:00 and 17:00 h) during the final five consecutive days of the experimental period. The total duration of the experiment was three months. Fecal samples were composited daily and analyzed for AIA and nutrient contents. Digestibility coefficients were calculated as described by Schneider and Flatt [19]:
DM digestibility (%) = 100 − [100 × (%AIA in feed/%AIA in feces)]
Nutrient digestibility (%) = 100 − [100 × (%AIA in feed × %nutrient in feces)/(%AIA in feces × %nutrient in feed)]

2.7. Determination of Rumen Fermentation Characteristics

Rumen fluid was collected on the final day of the experiment at 0 h (before feeding) and 4 h after feeding using a stomach tube connected to a vacuum pump. Approximately 100 mL of rumen fluid was collected from each animal. The pH and temperature were measured immediately using a pH electrode and digital thermometer. A 60 mL subsample was acidified with 1 mL of 1 M sulfuric acid per 10 mL of rumen fluid, centrifuged at 3000 rpm (352× g) for 15 min, and the supernatant was stored at −20 °C for analysis of total volatile fatty acids (TVFA) and individual VFA (acetic, propionic, and butyric acids). TVFA and individual VFA were determined using gas chromatography (GC) following standard procedures, with a capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness; fused silica) equipped with a flame ionization detector (FID).

2.8. Determination of Blood Metabolites

Blood samples were collected from the jugular vein at 0 h and 4 h post-feeding on the final experimental day. Samples were analyzed for packed cell volume (PCV) by microhematocrit centrifugation and glucose concentration using standard biochemical methods with an automated biochemical analyzer, following the manufacturer’s instructions. Blood urea nitrogen (BUN) concentrations were determined according to the method of Crocker [20]. All metabolic indicators were used to assess potential physiological effects of dietary treatments.

2.9. Measurement of Milk Yield and Economic Efficiency

Milk yield was recorded daily for each animal throughout the experimental period. Economic efficiency was evaluated based on feed cost and milk income. Feed cost was calculated from daily feed intake and ingredient prices, while milk income was estimated based on the market price of fresh milk. Feed cost calculations were based on daily feed intake expressed on an as-fed basis, whereas intake values reported in Table 4 are presented on a dry matter basis.
Table 4. Dry matter intake (DM basis) of lactating goats (n = 5) fed pangola hay and concentrate diets containing different levels of cashew nut roasting residue (CNRR) replacing soybean meal.

2.10. Statistical Analysis

All data were analyzed using ANOVA in a completely randomized design using the GLM procedure of SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). Treatment means were compared using Duncan’s Multiple Range Test (DMRT). Statistical significance was declared at p < 0.05. Orthogonal polynomial contrasts were used to evaluate linear and quadratic effects of increasing CNRR levels.

3. Results

3.1. Chemical Composition of Feed Ingredients and Concentrate Diets

The chemical composition of pangola hay, CNRR, and concentrate diets is shown in Table 2 and Table 3. Pangola hay contained 3.45% CP and high levels of fiber (NDF 78.44%; ADF 44.61%). In contrast, CNRR contained higher CP (21.88%) and EE (50.17%), resulting in a high gross energy value (6835.27 cal/g).
Increasing the level of CNRR in the concentrate diets slightly increased dry matter content (86.63–88.60%). Crude protein content remained similar among treatments (17.34–17.88%), whereas ether extract increased markedly from 2.38% in the control diet to 11.41% in the 20% CNRR diet, leading to higher dietary energy density (3862.15–4292.50 cal/g). Fiber fractions (CF, NDF, and ADF) showed moderate increases with higher CNRR inclusion.

3.2. Feed Intake

Dry matter intake of pangola hay, concentrate, and the total diet is shown in Table 4. Pangola hay intake was not different among treatments (p > 0.05), ranging from 259.67 to 449.39 g/goat/day. Concentrate intake was also not affected by the inclusion of CNRR (p > 0.05), with values between 749.24 and 794.78 g/goat/day. As a result, total dry matter intake ranged from 1054.45 to 1215.27 g/goat/day and did not differ among dietary treatments (p > 0.05).
When expressed relative to body weight, total dry matter intake ranged from 2.67% to 3.26% of body weight, while intake expressed per metabolic body weight ranged from 66.95 to 80.44 g/kg BW0·75. Replacing soybean meal with CNRR did not affect feed intake in lactating goats.

3.3. Apparent Nutrient Digestibility

Apparent nutrient digestibility coefficients of lactating goats fed diets containing different levels of CNRR are shown in Table 5. Dry matter digestibility decreased with increasing levels of CNRR, showing a significant linear effect (p = 0.03), although the overall treatment effect was not statistically significant (p = 0.07). Dry matter digestibility values were 50.40%, 45.21%, and 40.38% for T1, T2, and T3, respectively.
Table 5. Apparent nutrient digestibility coefficients of lactating goats (n = 5) fed pangola hay and concentrate diets containing different levels of cashew nut roasting residue (CNRR) replacing soybean meal.
Crude protein digestibility was significantly affected by dietary treatment (p < 0.01). Goats in the control group (T1) had higher crude protein digestibility (68.83%) compared with those fed diets containing CNRR, with values of 52.72% and 51.21% for T2 and T3, respectively. In contrast, ether extract digestibility increased significantly with the inclusion of CNRR (p < 0.01), with higher values observed in T2 (80.08%) and T3 (76.50%) than in the control diet (70.94%).
The digestibility of organic matter, crude fiber, neutral detergent fiber, acid detergent fiber, and lignocellulose did not differ among treatments (p > 0.05). These results indicate that replacing soybean meal with CNRR up to 20% did not affect fiber digestibility in lactating goats.

3.4. Rumen Fermentation Characteristics

Rumen fermentation parameters of lactating goats fed diets containing different levels of CNRR are shown in Table 6. No overall treatment effect was detected for mean rumen pH (p = 0.21), with values ranging from 6.19 to 6.60, remaining within the normal physiological range for goats. However, significant linear effects were observed for rumen pH at 0 h before feeding (p = 0.01) and 4 h after feeding (p = 0.03), indicating treatment-related variation across sampling times.
Table 6. Rumen pH and volatile fatty acid (VFA) profiles of lactating goats (n = 5) fed diets containing different levels of cashew nut roasting residue (CNRR) replacing soybean meal.
Total volatile fatty acids (TVFA) and ruminal ammonia nitrogen (NH3-N) concentrations are also presented in Table 6. Mean TVFA concentration was not significantly affected by dietary treatment (p = 0.69), although a tendency toward a linear response was observed (p = 0.06). Mean NH3-N concentration was not significantly different among treatments (p = 0.41); however, it decreased linearly with increasing CNRR inclusion (p = 0.02), with values of 23.75, 20.92, and 18.62 mg/dL for 0, 10, and 20% CNRR, respectively. The NH3-N values remained within the normal physiological range, indicating that ruminal nitrogen availability was adequate for microbial protein synthesis across treatments.
Acetic acid proportion was significantly affected by dietary treatment (p < 0.05). Goats fed the control diet (T1) showed higher acetic acid proportions both before and after feeding compared with those receiving CNRR. Mean acetic acid concentrations were 66.51%, 61.36%, and 59.55% for T1, T2, and T3, respectively. A significant linear decrease was also observed with increasing CNRR inclusion (p < 0.01).
Mean propionic acid proportion was not significantly affected by dietary treatment (p = 0.16); however, propionic acid increased linearly with increasing CNRR inclusion (p < 0.01). Mean propionic acid values ranged from 19.56% to 28.55%. At 4 h post-feeding, propionic acid proportion was significantly affected by treatment (p < 0.01).
Butyric acid proportion was not affected by dietary treatment (p > 0.05), with mean values between 11.87% and 13.92%.
Mean acetate:propionate ratio was not significantly affected by dietary treatment (p = 0.20). Mean acetate to propionate ratios ranged from 2.22 to 3.58, remaining within the normal physiological range. However, the mean acetate:propionate ratio decreased linearly with increasing CNRR inclusion (p < 0.01).

3.5. Blood Metabolites

Blood metabolite profiles of lactating goats fed diets containing different levels of CNRR are presented in Table 7. No overall treatment effect was detected for mean packed cell volume (PCV) (p = 0.52); however, a significant linear increase was observed with increasing CNRR inclusion (p = 0.01). Mean PCV values ranged from 25.16% to 26.66%.
Table 7. Blood metabolites of lactating goats (n = 5) fed diets containing different levels of cashew nut roasting residue (CNRR) replacing soybean meal.
Mean blood glucose concentration was not significantly affected by dietary treatment (p = 0.12), although a significant quadratic response was observed (p < 0.01), with the highest value at the 10% CNRR level. Mean glucose values ranged from 61.16 to 69.00 mg/dL and remained within normal reference ranges.
Mean BUN concentration was not significantly different among treatments (p = 0.72), although a tendency toward quadratic response was observed (p = 0.05). Mean BUN values ranged from 18.89 to 21.16 mg/dL, suggesting no evident disturbance in nitrogen metabolism.

3.6. Milk Yield and Economic Efficiency

Milk yield was 511.34, 677.04, and 892.54 g/day for 0, 10, and 20% CNRR inclusion levels, respectively, with no significant differences among treatments (p > 0.05). Economic outcomes of lactating goats fed diets containing different levels of CNRR are presented in Table 8. Feed cost per goat per day was 21.73, 14.60, and 14.52 THB/day for 0, 10, and 20% CNRR inclusion levels, respectively, and numerically decreased with increasing CNRR inclusion; however, the difference was not statistically significant (p = 0.36), and no significant linear effect was detected (p = 0.43). No overall treatment effect was detected for feed cost per kilogram of milk or net profit per kilogram of milk by ANOVA (p > 0.05). However, feed cost per kilogram of milk tended to decrease linearly (p = 0.05), whereas net profit per kilogram of milk increased linearly with increasing CNRR inclusion (p = 0.02).
Table 8. Feed cost and economic return of lactating goats (n = 5) fed diets containing different levels of cashew nut roasting residue (CNRR) replacing soybean meal.

4. Discussion

This study evaluated the effects of CNRR inclusion in diets of lactating goats. Overall feed intake was not significantly affected, while several rumen fermentation and blood variables showed dose-dependent linear or quadratic responses. Daily feed cost was numerically reduced with increasing CNRR inclusion.
It should be noted that the inclusion of CNRR involved a partial replacement of both soybean meal and ground corn, resulting in simultaneous adjustments in dietary protein and energy sources. This combined substitution may have influenced nutrient intake and utilization patterns observed in the present study.
Previous studies support the use of cashew by-products as alternative protein sources in ruminant diets. Rashmi et al. [5] reported that CNRR showed no adverse effects on rumen fermentation kinetics and digestibility in vitro. Similarly, Okolo et al. [21] showed that cashew nut products improved feed conversion and reduced feed costs, with optimal inclusion levels around 20%. However, complete replacement with alternative protein sources should be carefully managed, as Kraiprom et al. [22] reported reduced milk yield and nutrient intake under full replacement conditions.
Feed intake was not affected by CNRR inclusion, indicating that diet palatability was maintained. This agrees with earlier studies in goats, sheep, and dairy cows showing that moderate inclusion (20–30%) of cashew by-products does not reduce voluntary feed intake [21,23].
Increasing CNRR inclusion showed a linear decrease in dry matter digestibility and a significant reduction in crude protein digestibility, while fiber digestibility remained unaffected. The ether extract (EE) intake increased with CNRR inclusion, with values of 20.94, 30.88, and 62.30 g/kg DM for the control, 10%, and 20% CNRR diets, respectively. Previous studies have reported that dietary fat levels exceeding approximately 70 g/kg DM can negatively affect rumen microbial activity and fiber digestibility [24]. In the present study, the EE level in the 20% CNRR treatment approached this threshold, which may have reduced rumen microbial population and activity.
High dietary lipid levels may inhibit rumen microbial activity, particularly proteolytic bacteria, and reduce nitrogen utilization efficiency. Lipids may also coat feed particles and limit microbial attachment, thereby reducing the enzymatic degradation of protein. Similar responses have been reported in goats and other ruminants fed lipid-rich diets, where moderate inclusion improved digestibility, but higher levels resulted in reduced protein utilization [5,23,25]. In addition, Costa et al. [25] reported that the inclusion of cashew nut meal altered the rumen environment and microbial activity, which may influence microbial protein synthesis and protein digestibility. These findings suggest that the reduction in crude protein digestibility observed in the present study may be associated with dietary lipid effects rather than a general impairment of rumen function.
Furthermore, this pattern suggests that CNRR inclusion may exert a more selective effect on nitrogen metabolism rather than on overall rumen fermentation. While crude protein digestibility was reduced, the stability of rumen pH, VFA, and blood metabolites indicates that the general rumen environment remained unaffected. This may reflect the specific inhibition of proteolytic microbial activity without disrupting total fermentative capacity.
Rumen fermentation characteristics remained within normal physiological ranges with CNRR inclusion, indicating maintenance of normal ruminal conditions [17,26]. However, significant linear responses in NH3-N concentration, rumen pH at specific sampling times, and acetate:propionate ratio indicated dose-dependent changes in fermentation patterns. The decrease in acetate proportion and the concomitant increase in propionate, particularly at 4 h post-feeding, are consistent with responses observed when lipid-rich feed ingredients are included in ruminant diets, thereby shifting ruminal fermentation toward a more glucogenic pattern without causing a major disruption in rumen conditions [27,28,29]. Measurements of NH3-N and total VFA provide useful indicators of ruminal nitrogen metabolism and overall fermentation activity. Although no overall treatment effect was detected by ANOVA for mean NH3-N concentration, a significant linear decline was observed with increasing CNRR inclusion, suggesting altered ruminal nitrogen dynamics. Ruminal ammonia concentrations remained within the range considered adequate for microbial activity despite the reduction in crude protein digestibility. This response may reflect changes in microbial capture of ammonia, nitrogen recycling, or synchronization between available energy and nitrogen supply under the experimental conditions. Similarly, the unchanged total VFA concentrations suggest that overall fermentative activity was maintained despite shifts in specific VFA proportions following CNRR inclusion.
Blood metabolite values, including packed cell volume, blood glucose, and blood urea nitrogen, remained within normal physiological ranges, and no adverse changes were detected in the measured blood indicators. Although no overall treatment effect was detected by ANOVA for mean blood metabolites, significant linear and quadratic responses were observed for some variables, suggesting adaptive physiological variation rather than metabolic disturbance. The absence of major changes in BUN despite lower crude protein digestibility may indicate effective nitrogen recycling and maintenance of metabolic balance. Similar findings have been reported in goats fed balanced diets with alternative protein sources, where metabolic status was maintained within normal limits [14,30,31]. Although digestible protein intake differed among treatments (126.4, 101.2, and 107.9 g/day for T1, T2, and T3, respectively), BUN concentrations were not significantly different, indicating that nitrogen metabolism remained within a similar physiological range across treatments.
The linear decline in ruminal ammonia nitrogen (NH3-N) concentrations (23.75, 20.92, and 18.62 mg/dL for T1, T2, and T3, respectively) may indicate altered ruminal nitrogen utilization with increasing CNRR inclusion. Lower NH3-N concentrations may reflect changes in ammonia capture by rumen microbes or improved synchronization between available energy and nitrogen supply in the rumen. Nevertheless, NH3-N values remained within the range generally considered adequate for microbial activity (approximately 15–25 mg/dL) [32], which may partly explain the stable BUN concentrations observed among treatments.
A key factor underlying this response is likely the inclusion of CNRR, which is rich in lipid content. The substantially higher digestible ether extract (EE) intake in T2 and T3 (53.3 and 106.1 g/day, respectively), compared with T1 (17.8 g/day), suggests that goats in these treatments received a greater supply of dietary energy. Lipids provide approximately 2.25 times more energy than carbohydrates, and increasing dietary fat is known to improve energy balance and support milk production in lactating ruminants [13,29]. CNRR may increase dietary energy density due to its lipid content, consistent with previous reports on fat supplementation [29].
In addition, the observed decrease in the acetate-to-propionate ratio (A:P) across treatments (from 3.58 in T1 to 2.61 and 2.22 in T2 and T3, respectively) indicates a shift in rumen fermentation toward greater propionate production. Propionate is the primary precursor for gluconeogenesis in ruminants and plays a critical role in lactose synthesis, which directly regulates milk volume [33,34]. The increased availability of propionate in T2 and T3 may have enhanced glucose supply and may have supported lactose synthesis, contributing to the numerical differences in milk yield observed among treatments. Similar shifts in rumen fermentation patterns have been reported when lipid-rich feed ingredients are included in ruminant diets [35,36].
Moreover, dietary lipids—particularly from agro-industrial by-products such as CNRR—can alter rumen fermentation by partially suppressing methanogenesis and modifying microbial activity, thereby improving the efficiency of energy utilization [35,36]. The combined effects of higher energy density and a more glucogenic fermentation pattern may have allowed animals in T2 and T3 to allocate more energy toward lactation rather than maintenance, even under lower intake conditions. In such cases, mobilization of body reserves may also contribute to sustaining milk production. The increased ether extract intake likely contributed to the higher dietary energy availability [29], which may have partially compensated for the reduced intake of digestible dry matter and protein.
Overall, these findings suggest that the inclusion of CNRR may have enhanced dietary energy supply through increased EE intake and promoted a shift in rumen fermentation toward propionate production (lower A:P ratio). These mechanisms may help explain the numerical differences in milk yield observed among treatments, despite reduced digestible protein intake.
One practical advantage of CNRR was the numerical reduction in daily feed cost due to its lower price relative to soybean meal. Although daily feed cost was not statistically different among treatments, feed cost per kilogram of milk tended to decline, whereas net profit per kilogram of milk showed a significant positive linear response with increasing CNRR inclusion. These findings suggest that CNRR may improve economic efficiency when animal productivity is maintained, supporting the use of low-cost, locally available feed resources in sustainable livestock production systems [1].
One methodological limitation of this study is the relatively small sample size (n = 5 per treatment), which may have limited the statistical power to detect differences in feed intake, ruminal parameters, and blood metabolites, particularly in lactating animals with high biological variability. Nevertheless, statistical power analysis indicated that the experimental design had acceptable statistical power (power = 0.83) to detect treatment effects.
In addition, rumen fluid collection using an esophageal tube may be subject to salivary contamination, which could influence pH measurements. This limitation should be considered when interpreting the results.

5. Conclusions

Cashew nut roasting residue (CNRR) can be used as an alternative protein and energy source in concentrate diets for lactating dairy goats. Replacing soybean meal with CNRR at rates up to 20% did not significantly affect dry matter intake, rumen pH, or measured blood metabolites. However, crude protein digestibility was reduced at higher CNRR inclusion levels, indicating that dietary formulation may require further adjustment. Although several variables showed no overall treatment effect by ANOVA, orthogonal polynomial contrasts revealed significant linear or quadratic responses, indicating dose-dependent physiological adaptations to increasing CNRR inclusion. From an economic perspective, CNRR numerically reduced feed cost and significantly increased net profit per kilogram of milk in a linear manner. These results indicate that CNRR is a low-cost feed ingredient with potential to support the efficient use of local agro-industrial by-products in tropical dairy goat systems. Future studies should focus on long-term performance, milk quality, fatty acid composition, and strategies to better balance diets when using high-fat by-products such as CNRR.

Author Contributions

Conceptualization, U.P. and T.K.; methodology, H.J. and T.K.; validation, T.K., H.J. and S.J.; formal analysis, T.K. and U.P.; investigation, T.K., H.J. and S.J.; resources, U.P.; data curation, T.K.; writing—original draft preparation, T.K.; writing—review and editing, U.P., T.K., H.J. and S.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by National Science, Research and Innovation Fund (NSRF) and Prince of Songkla University (Ref. No. SAT6701325S).

Institutional Review Board Statement

The experiment was conducted in accordance with ethical guidelines for animal research and was approved under project number 67A115000284 (Ref. AG088/2024), approved on 15 August 2024, with a scientific animal use license number U1-02168-2558.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ethical and confidentiality restrictions.

Conflicts of Interest

The authors declare no conflicts of interest.

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