Chemical Composition and in vitro Nutrient Digestibility of Sulla aculeolata subsp. aculeolata and Sulla aculeolata subsp. mauritanica from Semi-arid Rangelands of the Mediterranean Area
Anass Elyemlahi
1
(
Research Team of Biotechnology and Biomolecular Engineeing, FSTT, Abdelmalek Essadi University Tetouan, Morocco
)
Abdelhay Arakrak
2
(
Research Team of Biotechnology and Biomolecular Engineeing, FSTT, Abdelmalek Essadi University Tetouan, Morocco
)
Amin Laglaoul
3
(
Research Team of Biotechnology and Biomolecular Engineeing, FSTT, Abdelmalek Essadi University Tetouan, Morocco
)
Mohammed Ayadi
4
(
Animal Production Research Unit, National Institute of Agricultural Research, 78 Av. Sidi Mohamed ben Abdellah, Tangier, Morocco
)
Mohammed Bakkali
5
(
Research Team of Biotechnology and Biomolecular Engineeing, FSTT, Abdelmalek Essadi University Tetouan, Morocco
)
Keywords:
Abstract :
Chemical Composition and in vitro Nutrient Digestibility of Sulla aculeolata subsp. aculeolata and Sulla aculeolata subsp. mauritanica from Semi-arid Rangelands of the Mediterranean Area
Anass EL Yemlahi A*, Abdelhay ArakrakB, Amin LaglaouiC, Mohammed AyadiD, Mohammed BakkaliE
A PhD., Biotechnology and Biomolecular Engineeing Research Team, FSTT, Abdelmalek Essadi University Tetouan, Morocco, * (Corresponding Author), E-mail: elyemlahi@hotmail.fr
B Full Prof., Biotechnology and Biomolecular Engineeing Research Team, FSTT, Abdelmalek Essadi University Tetouan, Morocco
C Full Prof., Biotechnology and Biomolecular Engineeing Research Team, FSTT, Abdelmalek Essadi University Tetouan, Morocco
D PhD Senior Chief Engineer., Animal production research unit, National Institute of Agricultural Research, 78 Av. Sidi Mohamed ben Abdellah, Tangier, Morocco
E Full Prof., Biotechnology and Biomolecular Engineeing Research Team, FSTT, Abdelmalek Essadi University Tetouan, Morocco
Abstract. This study focuses on the evaluation of the chemical composition and in vitro nutrient digestibility of two forage legumes namely, Sulla aculeolata subsp. aculeolata and Sulla aculeolata subsp. mauritanica. from the semi-arid rangelands of the Mediterranean area. For the first time, a study has been conducted to determine the nutritive value of these two pasture forages. Plants were harvested at the flowering stage from two distinct regions of Morocco, namely Beni Chiker and Saïdia. Samples were wilted in the field, oven-dried, and assayed for chemical composition, in vitro organic matter and crude protein digestibility (IVOMD and IVCPD). The results showed that both subspecies were a good source of crude protein up to 13 %DM at the early flowering stage and registered significant differences regarding their fiber contents, where S. aculeolata subsp. aculeolata appear very fibrous (ADF 39.09 %DM and ADL 15.66 %DM) resulting in a decrease of IVOMD (49.04 %OM) and IVCPD (45.54 %DM) as compared to S. aculeolata subsp. mauritanica., which recorded lower values of ADF (24.30 %DM) and ADL (8.62 %DM) and the higher values of IVOMD (74.76 %OM) and IVCPD (56.23 %DM). The use of Sulla aculeolata spp., particularly Sulla aculeolata subsp. mauritanica as a forage crop is suitable to enhance pasture productivity and to ensure animal nutrition of small ruminants in Mediterranean pastures.
key words: Sulla aculeolata spp., Chemical composition, In vitro digestibility, Natural pastures
Introduction
Legumes with 20.000 species are one of the widest families of flowering plants with worldwide distribution (Lewis et al., 2005). Within this family, the genus Hedysarum spp., tribe Hedysareae is one of the most important temperate forage legumes in the Mediterranean basin. The genus was reported to harbor several annual or perennial herbaceous species, distinguishable by their native distribution, morphology, and genetic diversity (Boussaïd et al., 1995; Ben Fadhel et al., 1997; 2006). However, only a few of them have been identified and evaluated for their potential value as ruminant fodder.
On the other hand, studies on certain Hedysarum species such as Sulla coronaria L and Sulla flexuosa L. have shown high quantities of green matter up to 50.000 Kg/ha (Douglas and Foote, 1985; Chouaki et al., 2006), and adequate protein content by fixing atmospheric nitrogen (Kishinevsky et al., 2003; Issolah et al., 2014; Elyemlahi et al., 2019a). In addition, the species exhibits a high digestibility using either enzymatic (Elyemlahi et al., 2019b) or rumen fermentation methods (Errassi et al., 2018) while low to moderate contents of anti-nutritional substances such as total free phenolics and condensed tannins have been reported (Errassi et al., 2018; Elyemlahi et al., 2019b). Such studies indicate a good forage value, and high animal performance (Burke et al., 2002; Bonanno et al., 2011; Kadi et al., 2011). Hence, they have been established as a forage crop and used for hay, silage, and green feed in several countries (Casella et al., 1984; Mitchell et al., 1999; Trifi-Farah et al., 2002).
In Morocco, this genus is represented by nine species (Fennane et al., 2007) including the species reported in this study, which is Sulla aculeolata syn. Hedysarum aculeolatum (Amirahmadi et al., 2014). The plant is a diploid species similar to Sulla flexuosa L. with white or rosy flowers, showing an inter-population morphological polymorphism as a function of pedoclimatic variations (Kheffache and Combes, 1992). Furthermore, the species is characterized by the occurrence of two subspecies, namely Sulla aculeolata subsp. mauritanica and Sulla aculeolata subsp. aculeolata growing spontaneously under Mediterranean climate (Ionesco and Stéfanesco, 1967; Abdelguerfi-Berrekia et al., 1991). In this framework, this research aimed to investigate the chemical composition and digestibility of these two forage legumes, i.e., Sulla aculeolata subsp. mauritanica and Sulla aculeolata subsp. aculeolata growing in natural pastures located in the North of Morocco. Plants showing high potential agronomic value will be domesticated as a source of forage for ruminant diets in Mediterranean areas.
Materials and Methods
Plants samples and analysis
The whole plant (leaves and stems) at the early flowering stage from two subspecies of Sulla aculeolata species (Sulla aculeolata subsp. aculeolata and Sulla aculeolata subsp. Mauritanica) was collected from two distinct regions (Beni Chiker and Saïdia) located in the North of Morocco (Fig. 1 and Table 1). Soil chemical and physical characteristics have been shown in Table 1. Aboveground plant biomass from three square plots (1m2) established at each field were oven-dried separately at 70°C to constant weight (AOAC 1997: method 930.15) for chemical and gas production analysis, and at 50°C for phenolics compounds essay (Makkar, 2000). Finally, the samples were milled through a 1mm sieve using a mill (POLYMIX® PX-MFC 90 D).
Chemical composition
Dried samples were analyzed for ash (AOAC 1997: method 942.05), crude protein (AOAC 1997: method 955.04), and ether extract (AOAC 1997: method 920.39). Neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) were quantified using sodium sulfate and thermo-stable α-amylase according to the sequential procedure of Van Soest et al. (1991), and expressed inclusive of residual ash. Other chemical components were calculated as follows:
Crude fiber (CF) content was determined by the weende method as described by AOAC (1997: method 978.10), using the ANKOM 200 fiber analyzer (Ankom Technology Co.). In addition, plant samples (200 mg DM), pre-dried at 50°C and sieved through a 0.5 mm mesh screen, were extracted in 10 ml of aqueous acetone (7:3, v/v) in an ultrasonic water bath for 20 min at room temperature, then subjected to centrifugation for 10 min at 3000g at 4°C. The supernatant was collected and the pellet was subject to the same extraction as described earlier. The supernatants were combined and assayed for total extractable phenols (TEPH) using the Folin-Ciocalteu reagent (2N, Sigma, St Louis, MO) and an aqueous solution of sodium carbonate (Na2CO3, 20%), and for total extractable tannins (TET) using polyvinylpolypyrrolidone (PVPP, Sigma, St Louis, MO) to separate tannins from other phenols according to the methods outline by Makkar (2000). Furthermore, the extracts were essayed for Extractable condensed tannin (ECT) using butanol-HCI reagent (95:5 v/v) and 2% ferric ammonium sulfate (in 2N HCl) following the method of Porter et al. (1986), and expressed as leucocyanidin equivalent using Makkar (2000) formula:
Where:
A550nm: is the absorbance at 550nm,
DF: is the dilution factor and
DM: is the dry matter of the sample
Mineral concentrations such as calcium (Ca), phosphorus (P), magnesium (Mg), potassium (K), sodium (Na), sulfur (S), and chlorine (Cl) were determined using the wavelength dispersion X-ray fluorescence (WDXRF) method employing a ceramic X-ray tube with Rhodium anode and Beryllium window (75µm), at National Centre for Scientific and Technical Research (CNRST) in Rabat, Morocco. This analysis was made by grinding and compressing the samples into a pellet (32 mm diameter) under high pressure. Detection and quantification of the elemental composition were performed using eight analyzer crystals, and three detectors (scintillation, gas flow and sealed Xenon).
In vitro gas production
Samples were collected fresh and then dried at 60°C, crushed, and sieved at 1 mm. To assess the degradation kinetics and degradability of the various by-products, the in vitro method of Menke et al. (1979) improved by Menke and Steingass (1988) was used, which consists in incubating 300 mg of the sample, supplemented by goat’s rumen juice and a buffer solution (with proportions of 1/3 and 2/3) in a graduated glass syringe placed in a water bath at 39°C. At the end of each incubation, the syringe contents are recovered, filtered, and oven-dried at 60°C for 48 h to estimate in vitro dry matter digestibility (IVDMD). The recovered residues were incinerated at 550°C for 12h to determine in vitro organic matter digestibility (IVOMD):
, 
Where:
DMi and DMf: represent the dry matter incubated and recovered after drying samples at 60°C, respectively.
OMi and OMf: are the organic matter contents incubated and recovered in the crucibles after incineration at 550°C.
The volumes of gas produced at different fermentation times (2, 4, 8, 12, 24, 48, 62, and 72 hours) were used to determine the parameters of the food degradation kinetics and predict its potential rumen degradation using Ørskov and McDonald (1979) model. This model was chosen because its constants have been the object of several studies, and great results have been recorded when applying this equation to various forages species (Kamalak et al., 2004; Akinfemi et al., 2009; Bezabih et al., 2014):
Where:
a is the production of gas from the potentially degradable soluble fraction (ml);
b is the production of gas from the potentially degradable insoluble fraction (ml);
c is the production rate of gas from the insoluble fraction (b) expressed in (h-1);
a+b is the gas production potential (ml).
Metabolizable energy (ME) was evaluated using the Menke et al. (1979) equation:
Where:
GP24 (mL/0.2 g DM) is the gas volume at 24 h of incubation and
CP (g/kg DM) is the crude protein and
CF is the crude fat (g/kg DM) in the sample.
The production of microbial biomass in the rumen in mg (PBM) in the below formula is calculated using the volume of gas produced in ml/g MS (Vgaz), IVADMO and the stoichiometric factor (FS) which varies from 2.20 to 2.34 mg/ml for acetic and propionic fermentation, respectively (Blümmel, 2000).
Where:
GV72: is the gas volume at 72 h of incubation,
IVADOM is the actual degraded organic matter (mg) that is obtained as the difference between organic matter incubated and recovered after incineration by the formula:
The partition factor (PF), which measures the efficiency of microbial production, is calculated by the formula of Blümmel et al. (1997) and Blümmel (2000) as follows:
Protein in vitro degradability
In vitro crude protein degradability (IVCPD) was evaluated based on enzymatic hydrolysis following the method described by Aufrère et al. (1989). Dried samples (0.5 g, milled with 0.5 mm sieve) were incubated at 40°C for 1 h in 50 ml of phospho-borate buffer (pH 8.0) containing (0.02 mg.ml-1) protease enzyme solution extracted from Streptomyces griseus (Type XIV, ≥3.5 units/mg, Sigma, St Louis, MO). The mixture was then centrifuged at 845×g at 4°C for 5 min and finally filtered. Protein digestibility was estimated as the proportion of nitrogen disappearing in the supernatant using the Kjeldahl method:
Where,
DN: is degradable nitrogen in the supernatant after digestion with a protease enzyme solution,
TN: is total nitrogen in the sample.
Statistical analysis
All tests were performed in triplicate and expressed as mean ± standard deviation. Analysis of variance (ANOVA) was performed with species as the main factor using (Proc GLM) procedure of the Statistical Analysis System (SAS, 2002).
Degradation characteristic parameters (a, b and c) were estimated using nonlinear regression models (Proc NLIN) of SAS.
Results
Primary results showed that both subspecies of Sulla aculeolata grow predominantly in natural pastures near Beni Chiker and Saïdia region located in the North region of Morocco (Fig. 1), in fine-calcareous sandy soil with pH value up to 8.82, poor in phosphorus (<10 ppm), nitrogen (<0.15%) and potassium (<150 ppm) except from Saïdia site, i.e., Sulla aculeolata subsp. Aculeolata which revealed a very high potassium soil concentration (Table 1).
Figure 1. Location of sampling sites.
Table 1. Soil characteristics of sampling sites
Site | Beni Chiker | Saïdia |
Species | Sulla aculeolata subsp. mauritanica | Sulla aculeolata subsp. aculeolata |
Slope | 30°(NE) | 0° |
pH (water) | 8.75 | 8.82 |
N, % | 0.11 | 0.08 |
P2O5, ppm | 5.13 | 6.21 |
K2O, ppm | 111 | 250 |
OM, % | 2.00 | 0.60 |
CaCO3, % | 33.6 | 21.0 |
Clay, % | 10.2 | 5.13 |
Fine silt, % | 10.2 | 20.5 |
Coarse silt, % | 4.14 | 17.3 |
Fine sand, % | 27.4 | 35.2 |
Coarse sand, % | 14.4 | 0.82 |
The chemical composition of two subspecies of Sulla aculeolata is shown in Table 2. As expected, small differences emerged among samples as a result of subspecies effects regarding the crude protein (CP) content (Table 2). On average, 13 %DM of CP was recorded in both subspecies. On the other hand, significant differences (P<0.05) among the subspecies were observed in their fiber content. Comparatively, a very high dietary fiber content was recorded in Sulla aculeolata subsp. aculeolata (NDF 48.78 %DM) compared with Sulla aculeolata subsp. mauritanica (NDF 30.50 %DM), which also showed the lowest ADF and ADL contents (Table 2).
Table 2. The chemical composition (% DM) of Sulla aculeolata spp.
Chemical composition | Abbrev. | S. aculeolata subsp.aculeolata | S.aculeolata subsp.mauritanica | SEM | P-value |
Dry matter | DM | 18.2±0.48 | 36.3±1.13 | 4.43 | 0.00 |
Total ash | Ash | 13.1±1.64 | 25.6±1.76 | 3.13 | 0.00 |
Organic matter | OM | 87.8±0.12 | 85.8±0.96 | 0.65 | 0.10 |
Crude protein | CP | 14.0±1.42 | 13.8±1.59 | 0.57 | 0.89 |
Crude fiber | CF | 40.6±1.53 | 26.6±0.15 | 3.47 | 0.00 |
Ether extract | EE | 2.10±0.16 | 2.08±0.18 | 0.06 | 0.89 |
Neutral detergent fiber | NDF | 48.8±1.97 | 30.5±0.09 | 4.52 | 0.00 |
Acid detergent fiber | ADF | 39.1±2.49 | 24.3±0.62 | 3.71 | 0.00 |
Acid detergent lignin | ADL | 15.7±0.16 | 8.62±0.75 | 2.04 | 0.00 |
Hemicellulose | HEM | 9.68±0.66 | 6.20±0.52 | 0.89 | 0.01 |
Cellulose | CEL | 24.8±1.80 | 8.77±0.62 | 3.98 | 0.00 |
Non-fiber carbohydrate | NFC | 24.9±1.31 | 28.0±1.50 | 1.09 | 0.21 |
Total extractable phenols | TEPH1 | 21.7±0.77 | 6.92±0.89 | 3.32 | 0.00 |
Total extractable tannins | TET1 | 12.8±0.82 | 3.84±0.21 | 2.02 | 0.00 |
Extractible condensed tannin | ECT2 | 0.09±0.01 | 0.10±0.01 | 0.00 | 0.32 |
SEM: standard error of the mean
1 expressed as eq-g tanic acid/100 g DM.
2 expressed as eq-g leucocyanidin/100 g DM.
Secondary metabolite analysis (Table 2) of the evaluated forage legumes revealed significant variation (P<0.05) of total extractable phenols (TEPH) and tannins (TET), A maximum content was noted for Sulla aculeolata subsp. aculeolata (21.69 and 12.82 %DM respectively), compared to Sulla aculeolata subsp. mauritanica (6.91 and 3.84 %DM). By contrast, no significant difference (P>0.05) has arisen between the studied Sulla subspecies concerning their extractable condensed tannins mean value (0.09 %DM for Sulla aculeolata subsp. aculeolata and 0.010 %DM for Sulla aculeolata subsp. mauritanica).
Regarding the mineral composition (Table 3), the highest mineral value was observed in Sulla aculeolata subsp. mauritanica (25.6 %DM) in comparison with Sulla aculeolata subsp. aculeolata (13.1 %DM). Furthermore, mineral analysis (Table 3) showed a high concentration of Ca (27.5 g/Kg DM) and Na (6.01 g/Kg DM) in Sulla aculeolata subsp. mauritanica compared with Sulla aculeolata subsp. aculeolata (Ca 11.8 g/Kg DM and Na 2.20 g/Kg DM). Whereas a decrease in K contents was recorded in Sulla aculeolata subsp. mauritanica (10.9 g/Kg DM) more than in Sulla aculeolata subsp. aculeolata (18.1 g/Kg DM).
Table 3. Mineral contents (g/kg DM) of Sulla aculeolata spp.
Mineral contents | Sulla aculeolata subsp. aculeolata | Sulla aculeolata subsp. mauritanica | SEM | P-value |
P | 1.46±0.25 | 1.43±0.38 | 0.13 | 0.93 |
Ca | 11.8±0.99 | 27.5±0.94 | 4.55 | 0.00 |
K | 18.1±0.74 | 10.9±0.67 | 2.08 | 0.01 |
Mg | 2.46±0.20 | 2.74±0.84 | 0.26 | 0.53 |
S | 6.28±0.16 | 5.95±0.60 | 0.20 | 0.53 |
Na | 2.20±0.66 | 6.01±1.12 | 1.16 | 0.05 |
Cl | 28.0±1.24 | 22.8±1.04 | 1.59 | 0.04 |
SEM: standard error of the mean.
Similarly, a comparison of gas production parameters indicated some significant differences (P<0.05) between the two subspecies of Sulla aculeolata (Table 4). Generally, gas production from Sulla aculeolata subsp. mauritanica was higher than in Sulla aculeolata subsp. aculeolata at all times of incubation, suggesting the occurrence of highly fermentable cytoplasmic substances such as crude protein and soluble carbohydrates. It was reputed that the fermentability of forages is mainly determined by the plant’s nutrient composition and digestibility (Doane et al., 1997; Boadi and Wittenberg, 2002; Boadi et al., 2004).
Table 4. Gas production (ml/200 mg DM) and estimated parameters of Sulla aculeolata spp. after 72 h of incubation
Incubation time | Sulla aculeolata subsp. aculeolata | Sulla aculeolata subsp. mauritanica | SEM
| P-value
|
2 h | 32.5±1.31 | 31.0±2.27 | 0.75 | 0.40 |
4 h | 39.8±0.12 | 53.8±2.23 | 3.47 | 0.00 |
6 h | 58.2±2.35 | 76.7±2.19 | 5.43 | 0.01 |
8 h | 64.8±1.66 | 90.1±1.67 | 6.23 | 0.00 |
10 h | 91.4±1.66 | 116±2.23 | 6.01 | 0.00 |
12 h | 106±0.00 | 122±2.22 | 4.65 | 0.01 |
24 h | 157±1.66 | 181±2.17 | 5.71 | 0.00 |
48 h | 191±2.35 | 201±1.99 | 2.91 | 0.05 |
72 h | 198±2.35 | 214±1.56 | 4.77 | 0.01 |
Estimated parameters |
|
|
|
|
a, ml/g DM | 1.19±0.23 | 0.59±0.11 | 0.19 | 0.08 |
b, ml/g DM | 202±2.69 | 212±0.64 | 3.06 | 0.03 |
c, %/h | 0.06±0.00 | 0.07±0.00 | 0.00 | 0.00 |
a+b, mL | 203±2.91 | 213±0.75 | 2.92 | 0.04 |
ME, MJ/kg DM | 24.5±0.12 | 27.7±0.03 | 0.70 | 0.00 |
MBM, mg | 368±22.4 | 432±18.4 | 17.8 | 0.04 |
PF, mg/ml | 2.90±0.11 | 3.13±0.08 | 0.07 | 0.09 |
a: the gas production from the soluble fraction; b: the gas production from the insoluble fraction; c: the gas production rate constant; a + b: potential gas production; ME: metabolizable energy; MBP: microbial biomass production; PF: partitioning factor. SEM: standard error of the mean.
For the present study, the in vitro dry and organic matter digestibility (IVDMD and IVOMD) is shown in Table 5. The result showed a marked difference between subspecies of Sulla aculeolata (P<0.05). The lowest values were observed in Sulla aculeolata subsp. aculeolata (IVDMD 51.52 %DM and IVOMD 49.04 %OM). While the highest values were registered in Sulla aculeolata subsp. mauritanica (IVDMD 69.53 %DM and IVOMD 74.76 %OM). Likewise, a high value of in vitro crude protein degradability (CPD) up to (56.23 %DM) was registered in Sulla aculeolata subsp. mauritanica in comparison with Sulla aculeolata subsp. aculeolata which recorded the lowest IVCPD value (45.5 %DM) (Table 5).
Table 5. Nutrient digestibility of Sulla aculeolata spp.
Nutrient digestibility | Sulla aculeolata subsp. aculeolata | Sulla aculeolata subsp. mauritanica | SEM | P-value |
IVDMD, %DM | 51.5±1.96 | 69.5±1.01 | 5.24 | 0.01 |
IVOMD, %OM | 49.0±1.26 | 74.8±1.36 | 7.44 | 0.00 |
IVCPD, %DM | 45.5±1.63 | 56.2±1.84 | 3.17 | 0.02 |
IVDMD: in vitro dry matter digestibility;
IVOMD: in vitro organic matter digestibility;
IVCPD: in vitro crude protein degradability.
SEM: standard error of the mean.
Discussion
In the Mediterranean pastures, forage legumes such as Sulla species are important components in many ruminant diets. However, information about their nutritional value as fodder crops remains scant. Therefore, screening of species with high nutritive value is important. Within this framework, two Sulla subspecies, namely Sulla aculeolata subsp. aculeolata and Sulla aculeolata subsp. mauritanica, which grows spontaneously in calcareous sandy soil, were harvested and analyzed. Similar findings were reported by Ionesco and Stéfanesco (1967); Abdelguerfi-Berrekia et al. (1991); Hannachi-Salhi et al. (2002) who indicated the occurrence of Sulla aculeolata spp. in a very limited distribution area in scrub and pasture zones at low altitudes, on sloping sandy-clayey to clayey soils moderately watered, under the sub-humid and semi-arid climates of Morocco and Algeria.
From a nutritive point, results showed some significant variations (P<0.05) among the studied Sulla subspecies which could be mainly due to the genotypic characteristics of each species, but also to agronomic and environmental factors such as harvesting frequency (Farzinmehr et al., 2020), altitude (Mountousis et al., 2006) and soil type (Snyman and Joubert, 1995).
In this respect, results showed a similar value of CP content in both Sulla subspecies, lower than those observed in Trifolium pratense L. and Medicago sativa L. evaluated at different growth stages (Homolka et al., 2012). Nevertheless, they were above a threshold value, i.e., 7% and 10%, necessary for rumen function (Van Soest, 1994) and livestock maintenance (Waghorn and Clark, 2004). Such a result was granted to the capacity of the subspecies to establish nodule-specific symbiosis with rhizobia bacteria (Kishinevsky et al., 2003). However, those bacteria could have an important effect on plant growth and quality (Bennett et al., 2015; Zhang et al., 2016).
Similarly, a high concentration of crude fat content (expressed as ether extract ’’EE’’) was recorded in both subspecies, comparable to those obtained in other Sulla species (Issolah et al., 2014; Elyemlahi et al., 2019b), and generally below the threshold value of 8%DM, above which the metabolism and ruminal digestion could drastically be affected (Wilson and Brigstocke, 1981). It was reported that feed rations based on grazed fodder or grass silage containing a high concentration of crude fat such as those recorded in Sulla flexuosa L. and Sulla coronaria L. may improve zootechnical characteristics of ruminants feed with Sulla (Bonanno et al., 2011; Kadi et al., 2011; Ponte et al., 2022).
On the other hand, the results of measured fibers indicated some significant differences (P<0.05) among the evaluated Sulla subspecies, generally above the recommended minimum value (25 to 28%) of dietary fiber, measured as a percentage of NDF in dry matter yield (NRC, 2001), and below the critical level of 50% (NASEM, 2016). Such a variation is mainly attributed to the genetic factors that control fiber synthesis and distribution in plants, but also to environmental conditions including soil microorganisms (Bennett et al., 2015; Zhang et al., 2016). In fact, the host plants preferentially selected those bacteria as they can promote not only plant growth and health but also nodulation and N availability in sustainable agriculture systems under stress conditions (Benhizia et al., 2004; Muresu et al., 2019). However, they may induce an increase in lignin content as a response by plants to bacteria invasion (Zhang et al., 2016). For the present study, the content of lignin (represented by ADL fiber fraction) was within the range obtained by Homolka et al. (2012), except that observed in Sulla aculeolata subsp. aculeolata.
Low quantities of condensed tannin (CT) were recorded in both subspecies within the range obtained by Stienezen et al. (1996) and Amato et al. (2005) for similar species. In animal nutrition, both lignin and CT are considered as critical factors limiting forage digestibility (Van Soest, 1963; Balogun et al., 1998). Therefore, the reduction of lignin content could have an advantage in enhancing forage digestibility and quality while maintaining a moderate quantity as those observed in Sulla flexuosa L. (Elyemlahi et al., 2019b) as CT protects protein from degradation in the rumen (Waghorn et al., 1994; Bonanno et al., 2011; Patra and Saxena, 2011).
Remarkably, a significant increase in organic matter digestibility (IVOMD) was observed in Sulla aculeolata subsp. mauritanica in comparison with Sulla aculeolata subsp. aculeolata as the lignin content tends to be higher. Similar results were stated by Balogun et al. (1998); Gemeda and Hassen (2015) who indicated a significant correlation between ADL content and digestibility using ruminal degradability methods. By the same token, the in vitro crude protein degradability (IVCPD) tends to be higher in Sulla aculeolata subsp. mauritanica as compared with Sulla aculeolata subsp. aculeolata and Sulla flexuosa L. using Streptomyces griseus enzymatic method to predict protein degradability (Elyemlahi et al., 2019b). However, the values obtained in this study remain slightly lower than those reported by Hoffman et al. (1993) for a group of perennial forage using ruminally degradable protein methods. In fact, the use of non-ruminal proteases for estimating protein degradability might result in limited predictive value, due to the enzyme-substrate specificity. Consequently, the use of enzymatic extracts of ruminal origin appears more reliable for the determination of IVCPD (Kohn and Alien, 1995; Velásquez, 2008; Velásquez and Pichard, 2010).
Conclusion
The present study provides the first characterization of two forage legumes, i.e., Sulla aculeolata subsp. aculeolata and Sulla aculeolata subsp. mauritanica growing spontaneously in natural pastures located in Northern Morocco. Both subspecies of Sulla aculeolata revealed great agronomic potential even at the flowering stage, particularly Sulla aculeolata subsp. mauritanica in terms of average crude protein (13 %DM), crude fiber (26.59 %DM), and IVOMD (up to 74.76 %OM), which indicated a very valuable fodder material to exploit in pastures-farming systems under Mediterranean environments. However, performing in vivo analysis is necessary to confirm such results.
Acknowledgments
The authors would like to thank the National Centre for Scientific and Technical Research (CNRST) for mineral analysis.
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