Effect of partially replacing concentrate mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf meal on the feed intake, digestibility, body weight gain, and carcass characteristics of Arsi-Bale sheep fed a grass hay-based diet

This study evaluated the effects of replacing a concentrate mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf (PJL) meal on the feed intake, digestibility, nitrogen balance, body weight gain, and carcass characteristics of Arsi-Bale sheep fed a grass hay-based diet. A total of 30 male sheep (initial body weight of 20.7 ± 1.6kg) were blocked into six groups and allocated to diets in which the concentrate mix was substituted with PJL meal at 0% (PJL0), 5% (PJL5), 10% (PJL10), 15% (PJL15), and 20% (PJL20). The experiment consisted of 84-day feeding and 10-day digestibility and nitrogen balance trials, which ended with carcass evaluation. Data were analyzed using general linear model (GLM) followed by polynomial contrast analysis using SAS (ver. 9.4). The intake of dry matter (DM), organic matter (OM), crude protein (CP), metabolizable energy (ME), neutral detergent fiber (NDF), and acid detergent fiber (ADF) decreased linearly and quadratically (p < 0.05) when the level of substitution with PJL meal increased. The least intake was observed when the PJL replacement of the concentrate mix exceeded 15%. The apparent digestibility of DM, OM, and CP was significantly (p < 0.05) higher in the PJL5 than in the PJL20 group. The daily nitrogen intake, urinary nitrogen loss, and nitrogen retention of the sheep decreased quadratically (p < 0.05) as the proportion of PJL meal increased. The daily nitrogen intake and retention were significantly lower (p < 0.05) in PJL20 compared with others. The urine nitrogen excretion was higher (p < 0.05) in PJL5 than that in PJL20. The average daily weight gain (ADG) of the sheep decreased linearly and quadratically, whereas the feed conversion efficiency (FCE), hot carcass weight, and slaughter body weight-based carcass dressing percentage dropped linearly (p < 0.001) when the substitution level increased. Substituting the concentrate mix with up to 15% PJL meal did not negatively affect the feed intake, apparent digestibility, or production performance of Arsi-Bale sheep. However, higher inclusion levels (20%) resulted in a reduced feed intake and weight gain and also showed a tendency for reduced digestibility. Further studies should assess the long-term effects and economic viability of treated PJL meal as an alternative feed ingredient in ruminants.

1 Introduction

The shortage of high-quality forage is a major constraint for livestock production in the majority of tropical and sub-tropical regions, including Ethiopia. Natural pastures and crop residues are the main feed sources for ruminant livestock (Begna and Masho, 2024; Dejene et al., 2024). On the contrary, these feedstuffs are low in crude protein (CP) and high in fiber concentration, resulting in poor digestibility and low metabolizable energy (ME) value (Feyisa et al., 2021). In particular, the nutrient concentrations in natural pasture and crop residues are much below the requirements of rumen microbes for their optimum functions to support sheep productivity. Supplementation of natural pasture hay and crop residue-based diets with concentrate feeds could improve the productivity and feed use efficiency of ruminants. However, the utilization of concentrate supplements is limited to the urban and peri-urban production systems, whereas smallholder farmers and pastoralists have limited access to concentrate supplements due to their high cost and unreliable supply (Mengistu et al., 2017; Dejene et al., 2024). On the other hand, proper utilization of locally available nutrient-rich forages such as browse species has been seen as a potential method to overcoming the gaps of feed supplement scarcity for smallholder livestock producers. Browse tree and shrub species, including Prosopis species, have good nutrient compositions and can offer alternative sources of proteins for ruminant supplementation (Belete et al., 2024).

Prosopis species are evergreen leguminous tree/shrubs widely grown in arid and semi-arid regions and have multiple uses including serving as sources of wood, food, and feed and providing environmental services (Abdulahi et al., 2017; Hernández-Ruiz et al., 2022). Prosopis species can survive under extreme conditions, especially in a prolonged dry season, making their biomass available for use during the non-forage growing season. Among the diverse Prosopis species, Prosopis juliflora was introduced to Ethiopia in the late 1970s for the rehabilitation of degraded soil. Since then, it has encroached large areas of land, including productive rangelands, croplands, forest areas, and wildlife reserves (Abdulahi et al., 2017), causing significant ecological and economic harms. In Ethiopia, P. juliflora is an invasive alien species, and national management strategy has been developed to control its further expansion (MoLF (Ministry of livestock and fisheries), 2017). The utilization of P. juliflora for livestock feed is among the top proposed management strategies (Abdulahi et al., 2017; Ravhuhali et al., 2021; Syomiti, 2015) due to the good nutrient composition of the foliage. Belete et al. (2024) reported the nutritive value of P. juliflora foliage studied in Ethiopia to be within the range of 7.8%–26.1% CP, 23.8%–47.8% neutral detergent fiber (NDF), 33.2%–59.6% in vitro organic matter digestibility (IVOMD), and 5.7–8.3 MJ ME/kg dry matter (DM). It can provide an alternative source of protein for all livestock during the dry season, when the availability of herbaceous forage is limited (Heuzé et al., 2015; Ravhuhali et al., 2021). Earlier studies discussed the contribution of P. juliflora to livestock feed supply with a recommended inclusion rate of 20%–40% in the total diet (Ruiz-Nieto et al., 2020). The same authors further reported that the nutritional contribution of P. juliflora mainly depends on the utilization of the fruits/pods. The pods are safe for animal feed due to their relatively low anti-nutritional properties over the leaves (King’ori et al., 2011; Sawal et al., 2004), which can be managed by limiting their inclusion level in the diets.

On the other hand, P. juliflora leaves have higher CP (21.6%–26.1%) than the pods (14.9%–17.8%), moderate fiber content (Ali et al., 2012; Melesse et al., 2019; Srinivas and Chaturvedi, 2019), and are available year-round; however, they remain underutilized as feed sources. The lower contribution of P. juliflora leaves to the feed supply has been reported to be associated with their strong unpalatable properties (Ali et al., 2012; Sawal et al., 2004). Furthermore, due to the low gas production potential of the leaves, with subsequent low organic matter (OM) digestibility and ME values, Melesse et al. (2024) reported P. juliflora leaves as a less favorable protein supplement in ruminants. Plants from Prosopis species generally contain diverse types of plant secondary metabolites (PSMs) with anti-nutritional effects. Among the many PSMs, tannins, alkaloids, saponins, flavonoids, and phenolic compounds have been screened in P. juliflora (Hernández-Ruiz et al., 2022; Ibrahim et al., 2013). The concentrations of PSMs significantly vary among the different Prosopis plant parts, with the leaf being the richest source of the majority of PSMs (Srinivas and Chaturvedi, 2019). This may make the leaves less favorable than the pods or fruits for animal feed use. Moreover, tannins and alkaloids are the two main anti-nutritional factors in P. juliflora leaves that require special attention when fed to livestock (Hernández-Ruiz et al., 2022; Ruiz-Nieto et al., 2020). However, Melesse et al. (2024) give more emphasis on the non-tannin phenolic compounds in inducing anti-nutritional effects in P. juliflora leaves. Tannins are the reason for the low palatability, whereas beyond affecting the palatability of the feed, alkaloids can intoxicate the animals if present at high doses in the diet (Heuzé et al., 2015). An in vitro assay by Srinivas and Chaturvedi (2019) showed that P. juliflora leaves can impair feed fermentation and could be extremely toxic to rumen microbes even if subjected to sun and oven drying. Reduced DM intake (DMI) and body weight gain were reported in sheep supplemented with untreated P. juliflora leaf (PJL)–pod mixture (Ali et al., 2012). Similarly, Abedelnoor et al. (2009) reported that the inclusion of PJL silage in concentrate supplements at increasing levels (5%, 10%, and 15%) led to a linear decrease in the feed intake and weight gain performance of sheep and goats.

Overall, P. juliflora is an evergreen browse species whose leaf biomass is available year-round, even during the extended dry season and drought periods. The biomass productivity of P. juliflora in Ethiopia has been estimated at 2.2–2.6 t/ha DM and 0.49–0.54 t/ha CP from the leaves (Bayssa et al., 2016a). The leaf biomass of P. juliflora remains underutilized or unutilized due to palatability issues. The utilization of the leaves is very limited as ruminants only occasionally forage on dry fallen leaves mainly during the dry season or droughts, when no other forage sources are available (Heuzé et al., 2015). On the other hand, it has invaded large areas of the rangelands of Ethiopia, causing shrinkage and serious degradation of the grazing lands. Therefore, the proper utilization of this invasive species is required not only to solve the feed shortage problem but also to contribute to the effort of reducing its further spread (Ravhuhali et al., 2021). Despite the achievements made in the utilization of P. juliflora pods and fruits for ruminant feeding, there is a lack of effort on the use of the leaf biomass as animal feed in Ethiopia. In order to use the leaf biomass for animal feed purposes, appropriate postharvest treatments are required to mitigate or reduce the negative effects of the reported anti-nutritional factors (Hernández-Ruiz et al., 2022; William and Jafri, 2015; Zhong et al., 2022). The current study hypothesized that the application of combined treatments targeting the deactivation of tannins and alkaloids could improve the palatability and overcome the toxic effects of P. juliflora leaves for inclusion into ruminant diets. Among the tannin deactivation treatments, polyethylene glycol (PEG) has been tested on P. juliflora leaves in vitro and has shown positive effects on the nutritive values (Bayssa et al., 2016b). A study reported that feed block containing Prosopis cineraria leaves and PEG was safely fed to goats (Bhatta et al., 2005). Despite progress in the use of PEG for tannin deactivation, only a few studies have addressed the mitigation of the anti-nutritional effects caused by other metabolites, such as alkaloids, in P. juliflora leaves. The application of heat treatments such as heating (cooking) and soaking in water has been reported to reduce the toxic effects of alkaloids in lupine seed used for human consumption (Das et al., 2022; Haddad et al., 2006). Furthermore, boiling followed by soaking for optimum duration significantly reduced the phytochemical concentrations of alkaloids, tannins, flavonoids, and saponins in plant materials (Ugwuanyi et al., 2020). Hence, the application of heating with soaking in water can be adapted to remove the water-soluble and heat-unstable alkaloids from P. juliflora foliage before feeding to animals. The objective of the present experiment was to determine whether hot water- and PEG-treated PJL meal can partially substitute the concentrate mix in sheep fed a grass hay basal diet.

2 Materials and methods

2.1 Study site and ethical approval

The experiment was conducted at the Small Ruminant Research Unit of the School of Animal and Range Sciences, Hawassa University, Ethiopia. The site is located at 7°4′ N latitude and 38°3′ E longitude, at an altitude of 1,700 m above sea level. The area has average minimum and maximum temperatures of 13.1°C and 27.5°C, respectively, and receives a bimodal annual rainfall ranging from 965 to 1,110 mm. Before the commencement of the experiment, all experiment protocols were reviewed and approved by the Institutional Review Board/Research Ethics Committee (IRB/REC, protocol no. YA001) of Hawassa University, College of Agriculture.

2.2 Study forage collection and processing

The presence of diverse PSMs with potential anti-nutritional effects is the main challenge. Alkaloids are a major anti-nutritional factor in P. juliflora, with the leaf containing 3.6%–8.5% total alkaloids (Ibrahim et al., 2013; Sirmah, 2019). Our pretreatment analysis of the leaf sample showed 3.87% ± 0.23% total alkaloid content. Another important fact is that the alkaloids of PJL are extremely diverse in type. Ahmad et al. (1989) identified the juliflorine, julifloricine, and julifloridine alkaloid types from the leaf extract of P. juliflora. These are classes of alkaloids belonging to the piperidine and indolizidine alkaloid groups, which are known for their strong anti-nutritional effects. The negative effects of anti-nutritional compounds on the nutritional quality of Prosopis can be reduced by heating treatment (Zhong et al., 2022). Accordingly, the combined application of boiling and soaking effectively removed toxins, such as pyrrolizidine alkaloids, from the edible plant parts (Takenaka et al., 2022). These methods have the potential for adoption in PJL treatment to reduce the heat-unstable and water-soluble anti-nutritional metabolites to safe levels for animal feeding. PJL was collected from a communal rangeland in the Awash Fentale District of Afar regional state, Ethiopia. The leaf biomass was harvested during the main dry season of 2024 and transported to a small ruminant research site of Hawassa University. Subsequently, it was sun dried on a plastic sheet and crushed using a mortar and pestle. Physical treatment (a combination of soaking and boiling) was applied to reduce the alkaloid content of the dried and chopped leaves before feeding the leaves to the sheep (Makiko et al., 2022; Ugwuanyi et al., 2020). The boiling and soaking treatments were slightly modified from previous settings considering the nature of the leaf biomass and the intended uses of the study plant. The dried leaves were first submerged in boiling water (97.2 ± 3.1°C) for 9.5 ± 1.6 min. The boiled leaves were then transferred into a bigger container and soaked in water for approximately 22–24 h at a weight-to-volume ratio of 1:50 (w/v). After soaking, the water was completely drained and the leaves dried in the sun. The treated leaf meal was stored until mixture with other ingredients and fed to the animals. The hot water- and soaking-treated leaf meal was sampled and analyzed for total alkaloid content, with the value reduced to 1.75% ± 0.45% (Table 1), a 54.9% reduction from that of untreated intact leaf of P. juliflora.

Table 1

Table 1. Chemical composition [percent dry matter (DM). unless specified] of the experimental feeds.

2.3 Experimental animals, diets, and feeding

A total of 30 male Arsi-Bale sheep with an average body weight of 20.7 ± 1.6kg (mean ± SD) were purchased from Kofele livestock market. Upon arrival at the research site, the sheep were ear-tagged and managed for 2 months for acclimatization to the environment. During this period, they were treated with ivermectin to control both internal and external parasites. At 3 weeks post-ivermectin injection, they were further administered Tetragozash-900. All sheep were housed in individual pens equipped with separate feeding and watering troughs. The sheep were offered the grass hay free choice for ad libitum intake at approximately 15% refusal rate and supplemented with 300 g/head per day of a concentrate mixture before transition to the experimental diets. The concentrate mixture used as the control diet comprised wheat bran (47%), maize grain (20%), noug seed cake (30%), molasses (2%), and table salt (1%). The treated PJL meal was used for the partial replacement of the concentrate mix in the treatment diets. Natural grass hay purchased from the Sululta area of Central Ethiopia was used as the basal feed. Before feeding, the hay was chopped with a machete to a size of approximately 5–10 cm to minimize wastage. During the study period, feeding was done based on the estimated nutrient requirements of local sheep for maintenance and weight gain according to Kearl (1982). Both the basal and experimental diets were divided into two equal portions and provided to the animals at 0800 and 1500 hours daily. Clean drinking water was available to the sheep with free access during the experiment duration.

2.4 Experimental design and treatments

The experiment was laid out in a randomized complete block design (RCBD), with the sheep blocked into six groups based on their initial body weight. Thereafter, animals from each block were randomly assigned into five experimental/treatment diets. The treatment diets consisted of partial replacement of the concentrate mix with PJL meal at 0% (PJL0), 5% (PJL5), 10% (PJL10), 15% (PJL15), and 20% (PJL20) (Table 2). The replacement levels were determined based on the results of a preliminary test conducted prior to the commencement of the actual experiment. For the treatment diets containing PJL meal, the tannin binding agent polyethylene glycol (PEG-6000) was added to mitigate the expected negative effects of tannins following the protocol previously used in sheep fed tannin-rich browse species (Yisehak et al., 2014). The PEG solution was prepared by dissolving PEG in tap water at daily PEG doses of 0.6, 1.2, 1.8, and 2.4 g per sheep for PJL5, PJL10, PJL15, and PJL20, respectively. The solution was sprayed on the daily feed supplement and thoroughly mixed before feeding.

Table 2

Table 2. Arrangement of the experimental diets/treatments.

2.5 Nutrient intake and weight gain measurement

Following a 14-day adaptation period, the feed intake and weight gain performance were recorded for 84 days. The quantity of feed offered (grass hay and supplements) and the corresponding refusals of each animal were recorded daily. Feed samples were collected fortnightly, while refusals were sampled daily for individual sheep and bulked over time. The intakes of grass hay, supplement, total DM, OM, CP, NDF, acid detergent fiber (ADF), and acid detergent lignin (ADL) were calculated as the differences between the daily feed offer and refusal and the nutrient content of the feed offered and the refusal. ME intake (MEI) was estimated from the digestible organic matter intake (DOMI) in grams per day (AFRC, 1993) using Equation 1.

$\begin{array}{ll}\mathit{\text{ME intake }}(\mathit{\text{MJ per head per day}})=0.0157*\mathit{\text{DOMI}}& (1)\end{array}$

The body weight of the sheep was recorded at the start of the experiment and then after every 2 weeks during the feeding trial. The total weight gain of the sheep during the feeding trial was calculated by subtracting the initial weight from the final body weight. The average daily gain (ADG) was determined by dividing the total weight gain (in grams) by the total number of feeding days (84 days). The feed conversion efficiency (FCE) was measured as the proportion of ADG to the daily DMI (Balehegn et al., 2014).

2.6 Digestibility and nitrogen balance trials

At the end of the growth study, four sheep from each treatment were randomly selected and transferred into metabolic cages for evaluation of digestibility and nitrogen retention. The sheep were allowed a 3-day acclimatization period to metabolic cages, fecal bags, and urine collection facilities, followed by a 7-day collection period. Fecal output was collected over a 24-h period for an individual animal, weighed, and then recorded. Approximately 20% of a daily fecal excretion was sampled after thorough mixing and then taken to the laboratory. Half of the fecal sample was dried at 105°C overnight for DM determination, while the remaining half was stored in a deep freezer at −20°C for determination of the chemical composition. The daily urine excretion of the experimental animals was collected into bottles containing 100 ml H₂SO₄ (10%) to prevent ammonia nitrogen loss. Approximately 10% of the daily urine collection was sampled and kept in a deep freezer at −20°C, following the same procedure as for the storage of fecal samples. Both fecal and urine samples collected daily were bulked over a 7-day period for each sheep. During this trial, the feed intake and refusal were also recorded. In addition, feed and refusal samples collected for chemical composition analysis. After the 7-day collection period, all fecal and urine samples were thoroughly mixed and sub-sampled for chemical analysis.

2.7 Carcass yield evaluation

At the end of the digestibility trial, three sheep per treatment were randomly selected and slaughtered to evaluate the carcass parameters. The selected sheep were fasted overnight prior to slaughter and their slaughter body weight recorded. During the slaughter process, the hot carcass and non-carcass (offal) components were weighed. The non-carcass components were separated into edible and non-edible offal and weighed immediately after slaughter. The weight of the digestive tract was determined by calculating the difference between the full and empty digestive tract weights. The weight of the gut content was subtracted from the slaughter weight to obtain the empty body weight (Balehegn et al., 2014). The rib eye area (REA) was measured between the 12th and the 13th rib, and the cross-sectional area was traced on transparent plastic paper (Tadesse et al., 2024), with the REA measured using a planimeter from the paper. The average reading of the right and left sides was taken as the value of the REA. The carcass dressing percentage was calculated by expressing the hot carcass weight as a percentage of the slaughter body weight and the empty body weight using Equation 2:

$\begin{array}{ll}\text{Dressing percentage }=\left[\frac{(\text{Hot carcass weight in kg})}{\text{Slaughter or empty body weight in kg}}\right]*100& (2)\end{array}$

2.8 Chemical composition of the feed, fecal, and urine samples

Chemical analyses of all samples were conducted at the Animal Nutrition Laboratory of the School of Animal and Range Sciences, Hawassa University. Samples of feed, refusal, and feces were dried in a forced-air oven at 60°C for 48 h and ground using a laboratory Wiley mill to pass through a 1-mm sieve. The DM, ash, and CP contents were analyzed following the procedures of AOAC (Association of Official Analytical Chemists) (1990). The nitrogen (N) contents of the feed, fecal, and urine samples were determined using the Kjeldahl method AOAC (Association of Official Analytical Chemists), 1990), while the CP content was calculated as CP = N × 6.25. Fecal N analysis was performed on wet feces. The fiber content was analyzed using ANKOM technology. The NDF content was determined according to Van Soest et al., 1991), whereas the ADF and ADL contents were analyzed using the methods of Van Soest and Robertson (1985).

The total alkaloid content of P. juliflora leaves was determined gravimetrically using a slight modification of the procedure described by Harborne (1973). Briefly, 5 g of ground leaf meal was first subjected to extraction with 200 ml of concentrated ethanol (96%) overnight at room temperature. The mixture was then filtered and concentrated by evaporating the solvent in a water bath at low temperature (<50°C). Acid extraction was performed on the filtrate from ethanol using 200 ml of diluted (2%) HCl at room temperature for 4 h. Any undissolved solids or residues from the acid extraction were filtered. Finally, the concentrated ammonium hydroxide solution (NH₄OH) was added dropwise to precipitate the alkaloid. Ammonium hydroxide solution was added until the pH of the filtrate reaches approximately 9–10. The mixture was kept overnight to allow complete precipitation of the alkaloid, and the precipitated alkaloid mixture was filtered using a pre-weighed filter paper. At the end, the precipitated extract was oven-dried at 60°C until a constant weight was achieved. The alkaloid content was calculated from the residual weight using Equation 3:

$\begin{array}{ll}\text{Alkaloid content }(\%)=\frac{\text{Precipitated alkaloid weight in gram}}{\text{Plant sample weight in gram }(\text{DM})}*100& (3)\end{array}$

Condensed tannins (CTs) were assayed using the butanol–HCl procedure and expressed as leucocyanidin equivalent (% DM) (Makkar, 2003). Equation 4 used for the calculation of CT concentration. The dilution factor was equal to 1 if 10 ml of 70% acetone was used, and 0.5 ml per volume of the extract was taken for analysis.

$\begin{array}{ll}\text{CT}=\frac{(\text{Absorbance at }550\text{nm}*78.26*\text{dilution factor})}{\text{\% DM}}& (4)\end{array}$

2.9 Statistical analysis

All collected data were analyzed using the general linear model (GLM) procedure of SAS (ver. 9.4). Significant differences were declared at p < 0.05. Tukey’s honestly significant difference (HSD) test was used for treatment mean comparisons. For parameters with significant treatment effects, orthogonal polynomial contrast analysis including linear and quadratic effects was performed to determine the response trend. The cubic term was excluded from the model after preliminary evaluation, which showed that it was not significant for the majority of the measured parameters. In addition, prediction equations were generated for selected variables [i.e., DMI, CP intake (CPI), NDF intake (NDFI), MEI, and ADG] using polynomial regression procedures, with the results presented in graphics. In the regression analysis, p < 0.05 and p ≤ 0.1 were considered as tendency of effects and were included into the model. The data were analyzed according to the statistical model shown in Equation 5 below;

$\begin{array}{ll}{\text{Y}}_{\text{ij}}=\mu +{\text{T}}_{\text{i}}+{\text{B}}_{\text{j}}+{\text{e}}_{\text{ij}}& (5)\end{array}$

where Y_ij is the response variable, µ is the overall mean, T_i is the i^th treatment effect (i = 1–5), B_j is the j^th block effect (j = 1–6), and e_ij is a random error.

3 Results

3.1 Nutrient composition of the experimental feeds

The chemical composition of the experimental feeds is presented in Table 1. The grass hay, PJL meal, and concentrate mix had comparable total ash contents. The concentrate mix had a CP content of 20.4%, while that of the PJL meal was 26.5%. In contrast, natural grass hay had a CP (7.1%) lower than that in the concentrate mix and PJL meal (Table 1). Grass hay had the highest NDF and ADF concentrations, followed by the PJL meal, while the concentrate mix had the lowest values. Analysis of the PSMs showed that the treated PJL meal contained approximately 1.75% total crude alkaloids and 0.16% soluble condensed tannins.

3.2 Feed intake

The feed intake of sheep fed diets containing increasing levels of PJL meal is shown in Table 3. The results revealed a significant (p < 0.05) linear and quadratic effects on the intakes of grass hay, supplement, and total DM, with a decreasing trend when the concentrate mix was substituted with increasing levels of PJL meal. Similarly, the intakes of OM, CP, ME, NDF, and ADF were linearly and quadratically reduced (p < 0.05) when the PJL levels in the diets increased. The linear and quadratic responses of DM, NDF CP, and ME intake Y (in grams/day) to increasing levels of concentrate mix substitution with PJL meal X (in percent) are presented in Figures 1A–D, respectively. As indicated in Table 3, the sheep allocated to the high-PJL diet (PJL20) had significantly (p < 0.05) lower intakes of DM, OM, CP, ADF, ADL, and ME than those on the other PJL-containing treatment diets. There were no significant differences (p > 0.05) in these variables among sheep allotted to the PJL0–PJL15 diets. Sheep on the PJL20 diet exhibited similar (p > 0.05) DMI as percentage body weight and NDFI compared with those fed the PJL15 diet. Overall, while the feed intake declined as the proportion of PJL meal in the supplement increased, the reduction was not significant (p > 0.05) up to a 15% substitution of the concentrate mix with PJL meal.

Table 3

Table 3. Effect of substituting concentrate mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf (PJL) meal on the dry matter and nutrient intake (in grams/day, unless specified) of sheep fed a grass hay basal diet.

Figure 1

Figure 1. Polynomial regression relationships between the concentrate mix substitution levels with Prosopis juliflora leaf (PJL) meal (in percent) and intake of dry matter (DM, in grams per day) (A), neutral detergent fiber (NDF, in grams per day) (B), crude protein (CP, in grams per day) (C), and metabolizable energy (ME, in megajoules per day) (D) of sheep fed a grass hay-based diet.

3.3 Apparent digestibility of feed

The apparent digestibility of DM and nutrients in sheep fed diets containing increasing levels of PJL meal is presented in Table 4. A significant (p < 0.05) difference in the digestibility of DM, OM, and CP was observed between sheep fed the PJL5 and PJL20 diets, with the former showing the highest values. The digestibility of DM, OM, and CP in the sheep supplemented with the PJL0, PJL10, and PJL15 diets was not significantly (p > 0.05) different from that of the PJL5 and PJL20 groups, which had the highest and lowest values, respectively (Table 4). The results also showed a significant quadratic reduction (p < 0.05) in the digestibility of DM, OM, CP, and NDF when PJL replacement of the concentrate mix increased (Table 4). As shown in Table 4, sheep on the PJL5 diet had significantly (p < 0.05) higher NDF digestibility than those on the PJL15 and PJL20 diets. On the other hand, the NDF digestibility in sheep fed the PJL0 and PJL10 diets was comparable and not significantly (p < 0.05) different from the values recorded in the PJL5 and PJL15 groups. The apparent digestibility of ADF in the PJL20 group was significantly (p < 0.05) lower than that in all other treatments. Similarly, the ADL digestibility in the PJL20 group was lower than that in PJL0, PJL5, and PJL10 (Table 4).

Table 4

Table 4. Effect of substituting the concentrate feed mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf (PJL) meal on the apparent digestibility of dry matter and nutrients of sheep fed a grass hay basal diet.

3.4 Nitrogen balance

The nitrogen utilization of sheep fed experimental diets is given in Table 5. The results demonstrated significant (p < 0.05) linear and quadratic effects of increasing levels of concentrate mix substitution with PJL meal on the daily nitrogen intake and nitrogen retention of the sheep, which decreased in value at higher replacement levels. The results demonstrated that the daily total nitrogen intake and retention were significantly affected (p < 0.05) by the treatment diets, with the sheep fed the PJL20 diet showing lower N intake and N retained than all the other treatment groups. Sheep allocated to the PJL5, PJL10, and PJL15 diets had similar (p > 0.05) daily total nitrogen intake and retention to the group fed the PJL meal-free diet (PJL0). Significant linear (p < 0.01) and quadratic (p < 0.05) effects were also observed on the daily urinary nitrogen excretion (in grams) in sheep allocated to diets with varying levels of PJL meal. The urine nitrogen excretion in sheep was influenced (p < 0.05) by the experimental diets. The loss of urinary nitrogen was higher in the PJL5 group than in the PJL20 group (Table 5).

Table 5

Table 5. Effect of replacing the concentrate feed mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf (PJL) meal on the nitrogen utilization of sheep fed a grass hay basal diet.

3.5 Growth performance and feed conversion efficiency

The body weight change, ADG, and FCE of the sheep are presented in Table 6. The final body weight, body weight change, ADG, and FCE of the sheep were quadratically influenced (p < 0.001) and responded with a declining trend as the proportion of PJL meal in the treatment diets increased (Table 6). However, only the sheep fed the PJL20 diet had lower final body weight, body weight gain, ADG, and FCE compared with all the other treatment groups. However, the sheep in the PJL5, PJL10, and PJL15 treatments had similar (p > 0.05) final body weight, total weight change, ADG, and FCE to sheep that received PJL0. Furthermore, the regression prediction equation comprising both the linear and quadratic responses for the ADG of sheep Y (in grams/day) to increasing levels of concentrate mix replacement with PJL meal X (%) are shown in Figure 2. A numerical decreasing trend was observed with increasing levels of PJL meal inclusion in the diet, as seen in Figure 3 for body weight gain.

Table 6

Table 6. Effect of substituting the concentrate feed mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf (PJL) meal on the body weight change and feed conversion efficiency of sheep fed a grass hay basal diet.

Figure 2

Figure 2. Polynomial regression relationships between the concentrate mix substitution level with Prosopis juliflora leaf (PJL) meal (in percent) and the daily body weight gain (ADG, in grams per day) of sheep fed a grass hay-based diet.

Figure 3

Figure 3. Body weight change trends of sheep fed experimental diets during the experimental period.

3.6 Carcass parameters

The carcass parameters of sheep fed the different treatment diets are presented in Table 7. A significant linear drop (p < 0.01) was observed for the slaughter body weight, empty body weight, hot carcass weight, REA, and slaughter weight-based carcass dressing percentage of the sheep following an increase of PJL proportion in the diets (Table 7). Sheep slaughter from the PJL0, PJL5, PJL10, and PJL15 diet groups had similar (p > 0.05) slaughter body weight, empty body weight, and hot carcass weight. Greater (p < 0.05) slaughter body weight and hot carcass weight were observed in the sheep fed the PJL0 diet than in those fed the PJL20 diet. The dressing percentage calculated as a proportion of empty body weight was not affected (p > 0.05) by the treatment diets. However, when expressed as a percentage of slaughter body weight, the dressing percentage was significantly lower (p < 0.05) in the PJL20 group compared with the PJL0 and PJL5 groups. The REA was greater (p < 0.05) in the control (PJL0 group) compared with all the treatments containing PJL in the diet. Overall, a decreasing trend was observed in all the carcass parameters with increasing level of PJL in the diet.

Table 7

Table 7. Effect of substituting the concentrate mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf (PJL) meal on the carcass parameters of sheep fed a grass hay basal diet.

3.7 Edible and non-edible offal

The effect of the experimental diets on the edible and non-edible offal components of sheep is presented in Table 8. Except for heart weight, all other studied edible offal components of the sheep were unaffected (p < 0.05) by the treatment diets. The heart weight of sheep decreased linearly (p < 0.05), being lower at higher levels of PJL meal in the diet. On the other hand, sheep fed the PJL0, PJL5, and PJL15 diets had significantly heavier heart weight compared with those fed the PJL20 diet. All of the evaluated non-edible components of the sheep were not affected (p > 0.05) by the experimental diets.

Table 8

Table 8. Effect of substituting the concentrate feed mix with hot water- and polyethylene glycol-treated Prosopis juliflora leaf (PJL) meal on the edible and non-edible offal components of sheep fed a grass hay basal diet.

4 Discussion

4.1 Nutritional value

The present study examined PJL meal subjected to hot water soaking as a partial replacer of concentrate mix in the sheep diet. The PJL meal had a CP content comparable to the previously reported 26.1% for untreated P. juliflora leaves by Melesse et al. (2019). In contrast, lower CP values of 19.5% and 21.6% were reported for P. juliflora leaves by Ali et al. (2012); Bayssa et al. (2016a), respectively. The observed variations in the CP values of PJL meal from some of the previous reports could be due to differences in the plant age and maturity at harvesting, the collection season, the soil type, and the postharvest processing applied to the leaves. The grass hay used in the current study was comparable to that reported by Feyisa et al. (2021); Belete et al. (2024). However, grass hay with CP contents (3.2%–6.0%) lower than the current findings have been reported in several previous animal trials (Bekele et al., 2022; Hintsa et al., 2015; Tadesse et al., 2022). On the other hand, the grass hay by Yinnesu and Nurfeta (2012) and Yasin and Animut (2014) was lower than the current result. In general, the CP content of the grass hay in the present study was adequate for optimum microbial activity, which otherwise would affect the voluntary feed intake (Van Soest, 1982). Regarding the fiber content, the NDF and ADF values in the present study were higher than those reported by Ali et al. (2012); Melesse et al. (2019), who found NDF values ranging from 27.1% to 33.1% and ADF values between 18.2% and 24.5%. The fiber contents recorded in the present study, however, were slightly lower than those previously reported for natural pasture grass hay, which ranged from 68.2% to 78.3% for NDF and from 36.8% to 46.9% for ADF (Belete et al., 2024; Feyisa et al., 2021; Tadesse et al., 2022; Yinnesu and Nurfeta, 2012).

P. juliflora is known to contain diverse types of cytotoxic alkaloids in its different parts, including the leaf and pod. Therefore, foliage of Prosopis have anti-nutritional effects of causing toxicity to livestock particularly when consumed in high doses and reducing the feed palatability and intake (Heuzé et al., 2015; Ruiz-Nieto et al., 2020). The quantified total alkaloid content in the PJL meal used in the present study (1.75%) was lower than the reported value of 3.6% for dried leaves of P. juliflora (Ibrahim et al., 2013). A comparable concentration of 1.35% was also reported for oven-dried leaves in another study (Srinivas and Chaturvedi, 2019). The CT content (0.156%) was similar to the 0.1% soluble CT reported by Melesse et al. (2019), but lower than the 1.0% CT recorded in the leaves analyzed by Sisay et al. (2018). The quantified alkaloid and tannin in the PJL in the present study showed that the applied treatments, which were based on boiling in hot water followed by soaking in water, did not completely deactivate or remove the PSMs of the P. juliflora leaves. Apart from the concentration or dose, factors such as the types and chemical characteristics of the phytochemicals could play an important role in determining their anti-nutritional properties.

4.2 Feed intake

In the present study, a significant quadratic reduction (p < 0.05) in the intake of the measured variables—DM, OM, CP, and ME—was observed with an increase in the PJL mean in the experimental diets. From the analysis of variance, the intakes of grass hay, supplements, total DM, OM, CP, and ME were significantly (p < 0.05) lower in the treatment diet with a high PJL meal (PJL20) than in the other PJL meal-containing diets. The reductions in the intake of DM, OM, CP, and ME in the high-PJL meal group compared with the PJL meal-free diet (PJL0) were 22.7%, 21.3%, 15.3%, and 28.8%, respectively. The fiber (NDF and ADF) intake of the sheep responded in a similar trend to that of the other nutrient intake variables. In agreement with the present study, Abedelnoor et al. (2009) reported a linear decrease in DMI in sheep and goats when P. juliflora leaf silage was included at 0%, 5%, 10%, and 15% in the concentrate mix used for supplementation. Their findings indicated that the reduction in feed intake is more pronounced in goats than in sheep, with DMI decreasing by 50.2% in goats and 25.9% in sheep when fed a high proportion of PJL silage. Ali et al. (2012) studied the effects of supplementing sheep fed Rhodes grass with 300 g/head of a 1:1 mixture of P. juliflora pod–leaf, comparing it to supplementation with either sole P. juliflora pods or a commercial concentrate mix. They observed a significant reduction in the supplement diet and total DM intake in sheep that received the pod–leaf mixture. According to these previous reports, PJL possesses strong unpalatable characteristics. Moreover, a reduced consumption of feed ingredients mixed with the leaf of P. juliflora has been reported (Abedelnoor et al., 2009; Ali et al., 2012). Tadesse et al. (2022) substituted the concentrate mix with Leucaena leucocephala leaves at 0%, 5%, 10%, 15%, and 20% in a sheep feeding experiment. They reported a lower DMI than that in the current study, with the exception of the sheep fed the PJL20 diet, for sheep fed L. leucocephala-containing diets, which could be due to differences in the body weight of the sheep used for the experiment. The sheep allocated to the PJL0–PJL15 diets showed comparable DMI to sheep given a concentrate mix partially (0%, 5%, 10%, 15%, and 20%) substituted with Cajanus cajan leaf meal, with reported DMI of 834–873 g/head (Tadesse et al., 2024). The DMI as percent of body weight of the sheep fed the PJL20 diet was comparable to the previously reported DMI of 3.2%–3.4% of body weight for sheep supplemented with varying levels of P. juliflora pods (Yasin and Animut, 2014).

The sheep fed PJL meal-containing diets (PJL5–PJL20) had daily CPI ranging from 83.2 to 99.3 g/head, which are higher than the 68.27 g/head reported for Afar sheep fed diets containing dried P. juliflora pod–leaf mixture (Ali et al., 2012). Furthermore, the CPI of the sheep is in close agreement with that reported for sheep supplemented with 300 g P. juliflora pods by Yasin and Animut (2014). In general, the reduction in feed intake observed in sheep fed the PJL20 diet may be attributed to the increasing levels of PSMs as the proportion of leaf meal in the diets increased. PEG is effective in mitigating the anti-nutritional effects of tannins to improve the intake of tannin-rich forages by ruminants (Yisehak et al., 2014). PEG supplementation to the PJL meal-containing diets could not overcome the negative effects of PSMs in the high-leaf meal diet (PJL20), resulting in a significant reduction in the feed intake of the sheep. A recent study on PJL revealed the presence of high concentrations of non-tannin phenolics, which have potential anti-nutritional effects on the feed fermentation and energy value (Melesse et al., 2019). An increase in the alkaloids following increasing levels of PJL meal in the sheep diet could be one reason for the observed depressed feed intake in the sheep supplemented with a high-PJL meal diet (Hernández-Ruiz et al., 2022). In addition to tannins and alkaloids, literature sources have shown the negative effects of saponins in P. juliflora foliage, which can be attributed to its low feed palatability, and the subsequent reduction of voluntary feed intake (Badri et al., 2017; Ibrahim et al., 2013). In general, Sirmah (2019) explained that the diverse PSMs in P. juliflora leaves might act synergistically to impose strong anti-nutritional properties, including suppression of the feed intake particularly when consumed at high levels.

4.3 Digestibility and nitrogen balance

Despite the linear and quadratic effects (p < 0.05) noted on the apparent digestibility, the suppression of DM, OM, CP, and NDF digestibility became significant (p < 0.05) only when the concentrate substitution level reached a maximum of 20% (PJL20) compared with the PJL5 diet. The apparent digestibility of DM and NDF in the present study was comparable to the reported 63.0%–73.0% and 61.2%–74.4%, respectively, for goats fed a diet where the concentrate supplement was partially substituted with Acacia tortilis leaves treated using calcium hydroxide (Bayssa et al., 2016c). According to the same report, the OM and CP digestibility values of the goats were higher than the current results for sheep fed different PJL meal diets. Similarly, Chaturvedi and Sahoo (2013) reported lower DM, OM, and NDF digestibility, but higher CP digestibility (80.4%–81.3%), than the current results for sheep supplemented with P. juliflora pods at 30% and 40% of the concentrate mix. Significantly (p < 0.05) high apparent digestibility of DM, OM, and CP was observed in PJL5 compared with PJL20. The maximum level of digestibility in the treatment with low leaf meal (PJL5) might be attributed to the absence of adverse effects of the low level of PSMs in PJL. In contrast, the reduced nutrient digestibility in the sheep that received PJL20 could have resulted from the increased intake of PSMs. Studies support that, if a high dose of PSMs is present in the diets of ruminants, this can induce anti-nutritional effects of suppressing the feed digestibility, with subsequent consequences of reducing the voluntary feed intake (Maasdrop et al., 1999, as cited in Yinnesu and Nurfeta, 2012). However, PEG was added to mitigate the suppressive effects of tannins on DM and nutrient digestibility. The observed reduction in nutrient digestibility in the PJL20 group might be further associated with the presence of alkaloids and other non-tannin phenolics. In agreement with the current results, a linear decrease in the apparent digestibility of NDF and ADL was reported with an increasing dose of alkaloids in sheep diets, while the digestibility of DM, OM, and CP remained unaffected (Coufal-Majewski et al., 2017).

Similar to the feed intake performance (Table 3), the lowest daily intake and retention of nitrogen (in grams/day) was exhibited by sheep allotted to the high-PJL meal (PJL20) diet (Table 5). The sheep fed the leaf meal-containing diets (PJL5–PJL20) showed fecal and urine nitrogen excretion higher than the reported values for goats supplemented with diets containing varying proportions of treated A. tortilis leaves (Bayssa et al., 2016c). The sheep in the current study had nitrogen retention lower than those reported for the goats in a similar study, which might be associated with the high fecal and urine nitrogen losses. Moreover, Chaturvedi and Sahoo (2013) reported fecal nitrogen excretion (5.18–5.64 g/day) comparable to the current results when P. juliflora pods replaced 30% and 40% of the concentrate supplement in fattening sheep. The findings of Chaturvedi and Sahoo (2013) further revealed that sheep fed 30%–40% P. juliflora pods in concentrate supplements had daily nitrogen intake (27.65–28.0 g), urine nitrogen excretion (8.27–9.86 g), and nitrogen retention (12.66–14.81 g) much higher than the present results. The variations in the reported nitrogen utilization efficiency of sheep among studies might be attributable to discrepancies in the plant parts studied (pods versus leaves); the characteristics of the experimental animals, mainly species, breed, age, and weight difference; the feed processing and treatment methods applied; the proportion of Prosopis foliage in the daily allowance diet; and the nature of the basal diets used. More importantly, despite the variations in the proportions of PJL meal in the studied diets, the fecal nitrogen excretion of the sheep was not affected (p > 0.05) in the current trial. Tannins have biological activities of binding protein and then preventing excessive microbial degradation of the protein in the rumen (Dey et al., 2008; Piluzza et al., 2013). The suppressive effect of tannins on ruminal protein degradation leads to a low ammonia concentration in the rumen with a concomitant drop in the urine nitrogen concentration. Thus, it can result in a high nitrogen excretion in the form of feces than urine. Based on the current results for nitrogen excretion (both fecal and urine), it appears that the addition of PEG was effective in mitigating the adverse effects of PJL tannins on the nitrogen metabolism in sheep. The positive effects of PEG in improving the feeding values of tropical tanniferous browse species for ruminants have been well studied (Bayssa et al., 2016a; Yisehak et al., 2014). Overall, the significant (p < 0.05) difference in urine nitrogen excretion observed between the sheep fed low (PJL5) and high (PJL20) PJL meal diets might be due to the higher intake and digestibility of protein in the former group (Tables 3, 4).

4.4 Weight gain and feed conversion efficiency

The decreasing trend in the final body weight, total weight change, ADG, and FCE observed in the current study with increasing proportions of PJL meal in the diets (Table 6) could be due to the decreased feed intake following increased concentrations of PSMs in the diets. However, a significant (p < 0.05) reduction in weight gain performance was recorded only when the replacement level of concentrate mix with PJL meal reached 20% in the PJL20 group. The observed linear and quadratic responses in the ADG of sheep were significant. The ADG of sheep on a high-PJL meal diet (PJL20) was lower than that of sheep on the PJL0, PJL5, PJL10, and PJL15 diets by 90.2%, 67.9%, 61.9%, and 51.6%, respectively. Consistent with the current results, Abedelnoor et al. (2009) reported a decline in weight gain performance in Sudanese sheep from 186 g in the control group (0% PJL silage) to 128, 71, and 36 g in diets containing 5%, 10%, and 15% PJL silage, respectively. The authors replicated the same feeding regime that had PJL silage for lactating dairy goats and reported weight loss results at 10% and 15% inclusion levels. The weight gain (43.1–54.5 g ADG) of sheep fed the PJL5–PJL15 diets in the present study was higher than the reported 24 g weight of sheep that received a P. juliflora pod–leaf mix, but comparable to the 50.5 g of sheep fed sole P. juliflora pods (Ali et al., 2012).

Despite a daily supplement intake of 242.2 g, the sheep allotted to the PJL20 diet had the lowest weight gain (6.85 g/day), similar to the 6.67 g/day reported by Yasin and Animut (2014) and lower than the 12.7 g/day observed in Afar sheep fed grass hay ad libitum without supplementation (Ali et al., 2012). These discrepancies might be due to variations in the quality of the basal diets, as some studies used Rhodes grass hay with a high CP content (10.5%–12.2%). This may be an indication that the presence of higher PJL in the diet suppressed the nutrient use efficiency of the sheep. Except for those in the PJL20 group, the sheep in all treatment diets demonstrated weight gain performance within the range of 33.8–72.4 g previously recorded by Yasin and Animut (2014) for sheep supplemented with a mix of cottonseed cake and P. juliflora pods at varying ratios. Similarly, Afar goats supplemented with concentrate partially replaced with P. juliflora pods (0%–40%) exhibited ADG ranging from 40 to 70 g (Hintsa et al., 2015). In contrast, Bekele et al. (2022) reported higher ADG and FCE (83.6–115 g and 0.10–0.123, respectively) than the current results in crossbred sheep supplemented with a concentrate mix containing varying amounts of Acacia nilotica leaf meal (0%, 26.5%, 46.4%, and 61.3%). A key observation from the current results is that all treatment groups showed similar growth performance during the first month, but as the feeding duration extended, the high-leaf meal diet (PJL20) resulted in a declining weight gain (Figure 1). The poor weight gain performance of PJL20 sheep could be explained by their low DM, CP, and ME intake performance and the accumulation of possibly non-tannin PSMs in their body over time, with subsequent effects on the nutrient utilization and overall animal performance (Table 3).

4.5 Carcass and non-carcass parameters

The sheep fed the PJL20 diet showed hot carcass, slaughter, and empty body weight lower than those of the control (PJL0) group (Table 7). However, there were no significant (p > 0.05) differences in the hot carcass yield of the sheep fed different PJL meal-containing diets (PJL5–PJL20). The hot carcass weights of the sheep from all treatment diets were slightly higher than those reported for sheep supplemented with a mixture of PJL and pod, but comparable to those supplemented with sole pods in ground form (Ali et al., 2012). The slaughter weight, empty body weight, and hot carcass weight of the sheep in the current study were lower than the reported results for crossbred sheep supplemented with different proportions of A. nilotica leaves (Bekele et al., 2022). However, the dressing percentages (as a proportion of empty body weight) were not significantly (p > 0.05) different among treatment groups. The lower slaughter weight-based dressing percentage (38.1%) in the PJL20 group compared with the PJL0 and PJL5 groups may be attributed to the higher gut content, resulting in a lower hot carcass yield. Sheep supplemented with PJL meal-containing diets had similar (p > 0.05) REAs, which were comparable to the values reported by Yasin and Animut (2014). Except for kidney weight, the analyzed edible and non-edible offal components were not significantly affected (p > 0.05) by the experimental diets.

5 Conclusion

PJL meal has an optimum nutrient profile, particularly in terms of CP content. The examined alkaloid deactivation methods using hot water treatment and overnight soaking did not fully remove it from the leaf, and the assessed PJL meal was not free of anti-nutritional factors. Thus, there was a decreasing trend in the feed intake, growth performance, and carcass yield of the sheep when the levels of concentrate mix substitution with PJL meal increased. A significant drop in the production performance of the sheep was noted when the replacement of concentrate mix with PJL meal exceeded 15% These results indicate that, if proper postharvest treatment methods are developed, PJL meal has the potential to be considered as a partial replacement for the rarely available and expensive concentrate supplements in pastoral areas where P. juliflora is the predominant vegetation and leaves are widely available throughout the year. It can be concluded that replacing up to 15% concentrate mix with treated PJL meal is possible without adverse effects on the performance of sheep. Further studies on the economic performance, the impact of long-term feeding, and cost–benefit analysis of leaf treatment methods that consider additional anti-nutritional substances are required for the development of a practical PJL-based supplementation strategy.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

Ethics statement

The animal study was approved by Institutional Review Board/Research Ethics Committee (IRB/IRC, Protocol No. YA001)) College of Agriculture, Hawassa University. The study was conducted in accordance with the local legislation and institutional requirements. No potentially identifiable images or data are presented in this study.

Author contributions

SSB: Investigation, Writing – original draft, Conceptualization, Data curation, Methodology, Formal analysis. AT: Writing – review & editing, Conceptualization, Funding acquisition, Methodology, Supervision. SRB: Conceptualization, Funding acquisition, Supervision, Writing – review & editing. UD: Conceptualization, Funding acquisition, Supervision, Writing – review & editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The authors declare that the study was supported by DAAD funding from the Federal Ministry for Economic Cooperation and Development (BMZ) of the German Government under the project ‘Climate Change Effects on Food Security’ [CLIFOOD, (Grant No. 57562534)].

Acknowledgments

The authors appreciate the assistance of Mr. Asrat Bizuneh during forage collection from Afar rangeland. We also thank, Mr. Tadesse Bokore for his unreserved help during laboratory analysis of feed, feces and urine samples.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author AT declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

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