Chemical characteristics, degradation kinetics and gas production of arboreal species for ruminants1 Características químicas, cinética de degradação e produção de gases de espécies arbóreas para ruminantes

The breeding systems of the small ruminants are extensive in the great majority, in which the animals are created in areas of native pasture named Caatinga, being necessary the knowledge on the chemical composition and its characteristics of feed degradation is crucial to increase nutrient use efficiency. Thereat, the objective of this study was to evaluate the chemical composition, the minerals, the carbohydrates and nitrogenous compounds fractionation, the degradability, in vitro dry matter digestibility and the in vitro gas production of arboreal species, those are consumed by goats and sheeps. The species evaluated were Cnidoscolus phyllacanthus (Muell. Arg.) Pax. et K. Hoffman, Mimosa tenuiflora (Willd.) Poiret, Myracrodruon urundeuva Fr. All., Poincianella bracteosa (Tul.) L. P. Queiroz, Spondias tuberosa Arr. Cam. and Ziziphus joazeiro Mart. It was found that Mimosa tenuiflora has a higher crude protein quantity (174.9 g.kg-1 DM). Spondias tuberosa and Myracrodruon urundeuva had the highest values for total carbohydrates compared to the other species analyzed. As for the fractionation of nitrogenous compounds, the highest proportion of fraction A was found for Cnidoscolus phyllacanthus (375.5 g.kg-1 CP). Among the species studied, Cnidoscolus phyllacanthus is highlighted with the highest ruminal degradation (61.63%) and dry matter digestibility (627.1 g.kg-1 DM), reflecting higher gas production (206.02 mL.g-1 DM). Cnidoscolus phyllacanthus, Poincianella bracteosa and Myracrodruon urundeuva showed a greater availability of nutrients in the rumen which is fundamental to increase the amount of ruminal microbial protein which reaches the small intestine for use by the animal.


INTRODUCTION
Brazilian Northeast has the largest national herd of goats and sheep (IBGE, 2017), specifically in Semiarid region. The breeding systems of these animals are extensive in the great majority, in which the animals are created in areas of native pasture named Caatinga. This biome presents a great diversity of forage species and participates in about 70 % in the feeding of these animals, so that the dry period is prolonged, the contribution tends to increase due to the availability of dry leaves (NUNES et al., 2016).
The diet of small ruminants in the Brazilian Semiarid region is often based on the grazing of Caatinga native plants, which are the only sources of protein and energy for those animals. Given its botanical diversity, management practices should be adopted to ensure the preservation of the vegetation, which, according to Souza et al. (2013) makes the knowledge of quantitative and qualitative parameters necessary.
The knowledge on the chemical composition and its characteristics of feed degradation is crucial to increase nutrient use efficiency. As feed quality depends on its constituents, the nutritional evaluation of forages produced in Caatinga with a potential for feeding small ruminants is essential to increase the efficiency of the livestock in that region. The use of values presented in foreign tables of feed composition for diets formulation is common in the Brazilian Northeast region.
However, not all characteristics of feed available in the region are known. Studies on Caatinga arboreal species discussed the potential of these plants regarding its composition, especially crude protein content. However, it is important to note that such nutrients are complexed with the plant cell walls and secondary compounds, making them unavailable to animals (SANTANA et al., 2011). Thus, this study aimed to evaluate the chemical characteristics, minerals, carbohydrates and nitrogenous compounds fractionation, degradability and in vitro digestibility of dry matter and the in vitro gas production of the browse species from Brazilian Semi-arid region.
Total carbohydrates (TC) were calculated according to Sniffen et al. (1992), where TC = 100 -(CP + EE + Ash), and fractionated in A+B1, B2 and C, being the nonfibrous carbohydrates (NFC), corresponding to the fractions A+B1, by the difference between TC and NDFap. The fraction C represented by the indigestible NDF was obtained according to Valente et al. (2011). The fraction B2, was obtained by the difference between NDFap and the C fraction. For the fractioning of nitrogenous compounds, the non-protein nitrogen (fraction A), the neutral detergent insoluble nitrogen (NDIN) and the acid detergent insoluble nitrogen (ADIN) were estimated according to the methodology described by Licitra, Hernandez and Van Soest (1996). The fraction B1+B2 was obtained by means of the equation: B1+B2 = 100 -(A + B3 + C), while fraction B3 was obtained by the difference between NDIN and ADIN and fraction C considered as ADIN.
Chemical characteristics, degradation kinetics and gas production of arboreal species for ruminants The in vitro dry matter degradability was determined according to the first stage of the Tilley and Terry (1963) method. Samples (milled in sieve of 2 mm) were incubated at 0, 3,6,9,12,18,24,36 and 48 hours at 39 °C. To estimate the parameters a, b and c, the model proposed by Ørskov and McDonald (1979) was used and processed by the PROC NLIN procedure of the SAS ® statistical software, once the parameters a, b, and c were calculated, they were applied to the equation (ØRSKOV; McDONALD, 1979). The analysis of in vitro dry matter digestibility (IVDMD) was conducted according to Tilley and Terry (1963), with the modifications proposed by Holden (1999).
The in vitro gas production using technique a pressure transducer was used as proposed by Theodorou et al. (1994). The cumulative gas production was estimated by measuring the pressure of the gases produced during the fermentation process using a pressure transducer (LOGGER AG100 -Agricer) and graduated syringes for gas volume at 2, 4,6,8,10,12,15,18,21,24,30,36,42 and 48 hours after incubation. Using pressure and volume data, an equation was generated, (y = 5.1612x -0.3017), R 2 = 0.9873, where the pressure (psi) and the gas volume (mL) were related by the PROC REG of SAS ® from 937 observations. In the equation developed at the Laboratory of Gas Production (-8°90'77" S, -36°49'49" W, altitude of 844 meters), it was observed that one psi = 4.859 mL of gases. From the equation, data (in psi) reported for volume of gas produced during incubation were used. To determine the parameters, the bicompartmental logistic model was used Schofield, Pitt and Pell (1994) with the aid of the PROC NLMIXED of SAS ® . All incubations were used 1.0g of sample in each jar of glass (160 mL), and the ruminal fluid was taken from goats fed a balanced diet with Pennisetum purpureum and concentrate (80:20).
The treatments were arranged in a completely randomized design with ten replications (Y ij = µ + t i +e ij ; Y ij -observation j, submitted to treatment i; µ -general constant; t i -effect of treatment I; e ij -randon error associated with each observation). The data obtained from chemical analyses, fractionation of carbohydrates, nitrogenous compounds and IVDMD were submitted to analysis of variance using the PROC GLM procedure, and the means were compared by Tukey test at 0.05 significance level using the SAS ® statistical software.

RESULTS AND DISCUSSION
The highest DM values found were 528.5 g.kg -1 of natural matter (NM) for Poincianella bracteosa, followed by Mimosa tenuiflora and Ziziphus joazeiro, and the lowest value was found for Spondias tuberosa (297.4 g.kg -1 NM) and Cnidoscolus phyllacanthus (270.1 g.kg -1 NM) ( Table  1). The highest DM values obtained for P. bracteosa, Z. joazeiro and M. tenuiflora can be explained by the different morphological characteristics of these forages. The highest concentration of ash was observed for Z. joazeiro (83.7 g.kg -1 DM) and the lowest concentration was observed for P. bracteosa (45.4 g.kg -1 DM). M. tenuiflora showed the highest ether extract (EE) content among species (79.4 g.kg -1 DM) and Z. joazeiro presented the lowest quantity of EE (10.4 g.kg -1 DM).
The highest concentrations of neutral detergent fiber corrected for ash and protein (NDFap) were 520.8 and 485.9 g.kg -1 DM for S. tuberosa and Z. joazeiro, respectively, and the lowest concentration was 361.3 g.kg -1 DM for M. urundeuva. For ADF, there was variation from 416.7 g.kg -1 DM for S. tuberosa to 231.2 g.kg -1 DM for M. tenuiflora (Table 1). The cellulose (CEL) concentration was the highest in P. bracteosa (307.5 g.kg -1 DM) followed by C. phyllacanthus (227.3 g.kg -1 DM) and S. tuberosa (210.9 g.kg -1 DM), and the lowest concentration was observed for M. tenuiflora (117.0 g.kg -1 D M ) . T h e highest concentration of LDA was observed for S. tuberosa (205.8 g.kg -1 DM) and lowest concentration for P. bracteosa (85.5 g.kg -1 DM) (Table 1).
For non-fibrous carbohydrates (NFC), M. urundeuva showed the highest concentration in its composition, 451.7 g.kg -1 DM, followed by P. bracteosa (368.9) and S. tuberosa (314.3). The content of fibrous components presented in this study are similar to the values found by Neves et al. (2014) and Nunes et al. (2016). Part of the energy used by the ruminal microflora is obtained from the fermentation of dietary carbohydrates, which are converted into short-chain fatty acids (SCFA), glucose precursors and fatty acids (OWENS; BASSALAN, 2016).
The levels of minerals present in the evaluated species presented a difference, except for potassium (K) ( Table 2). The highest amount of phosphorus (P) was observed in P. bracteosa, with 1.51 g.kg -1 DM and the lowest in S. tuberosa with 0.83 g.kg -1 DM. Calcium (Ca) found in Z. joazeiro was 22.18 g.kg -1 DM and 2.35 g.kg -1 in M. tenuiflora. The presence of sulfur (S) was higher in Z. joazeiro (2.22 g.kg -1 DM). The iron content (Fe) observed in the species ranged from 78.5 to 142.05 mg.kg -1 DM, with a higher concentration found in C. phyllacanthus and lower in M. urundeuva.
The quantity of total carbohydrates (TC) was observed in the composition of S. tuberosa (835.1 g.kg -1 DM), followed by M. urundeuva (813.0 g.kg -1 DM) and the lowest concentration was observed for M. tenuiflora (690.2) ( Table 3). The highest concentration of TC presented by S. tuberosa does not mean more energy, approximately 55% of the TC are not digestible (fraction C), in contrast, 70% of the TC of M. urundeuva can be degraded. Regarding the fractionation of carbohydrates (Table 3), the highest value for the fraction A+B1, corresponding to carbohydrates with a fast degradation rate, was obtained for M. urundeuva (555.6 g.kg -1 TC) followed by P. bracteosa (474.6). The lowest values were observed for S. tuberosa (376.6) and Z. joazeiro (356.6). C. phyllacanhus, Z. joazeiro, M. urundeuva and P. bracteosa showed a higher fraction B2 of TC (166.9; 152.4; 149.5 and 132.6 g.kg -1 TC, respectively) ( Table 3).
Species with a higher proportion of the fraction B2, i.e., potentially digestible fibers, better support the growth of microorganisms that use fibrous carbohydrates. Moreover, as the fiber undergoes a slow degradation, the use of protein sources with a slow degradation in the rumen is also appropriate because it allows a better synchronization between carbohydrates and proteins for a maximum efficiency of microbial protein synthesis and promotes a decrease in energy and nitrogen losses resulting from ruminal fermentation (LICITRA; HERNANDEZ; Van SOEST, 1996). S. tuberosa participated the most in the fraction C of TC, confirmed by the larger amount of lignin found in that forage. Pereira et al. (2010) performed the fractioning of the TC of Z. joazeiro, in vegetative phenological stage, and found 309.1 g.kg -1 TC (A+B1), 284.5 g.kg -1 TC (B2) and 406.3 g.kg -1 TC (C), values close to those observed in this study.
Columns means with different superscripts differ significantly (**0.01 and *0.05 of probability). 1 g.kg -1 NM; 2 g.kg -1 DM; 3 g.kg -1 CP   The crude protein (CP) concentration was observed in larger amounts in M. tenuiflora (174.9 g.kg -1 DM), Z. joazeiro (150.6 g.kg -1 DM), P. bracteosa (145.0 g.kg -1 DM) and C. phyllacanthus (143.2 g.kg -1 DM), followed by Myracrodruon urundeuva (111.1 g.kg -1 DM) and S. tuberosa (91.7 g.kg -1 DM) (Table 4). Neves et al. (2014) and Nunes et al. (2016), also in vegetative phenological stage, observed values of CP above 110 g.kg -1 DM, which confirms the potential use of these forage species as protein sources for ruminant feed, particularly in tropical regions. It should be noted that the formulation of diets considering only the feed CP content without the knowledge of its availability may hinder the efficiency of synthesis of rumen microorganisms. It is important to note that the CP contents presented, despite being considered a quality parameter for forages, had significant portions attached to the fiber, i.e., unavailable for micro-organisms. Similarly, the presence of tannins and lignin tend to increase neutral detergent insoluble protein (NDIP) and acid detergent insoluble protein (ADIP) (LICITRA; HERNANDEZ; Van SOEST, 1996;NUNES et al., 2016). The highest proportion of neutral detergent insoluble protein (NIDP) was observed for the forages M. tenuiflora, S. tuberosa, M. urundeuva and Z. joazeiro, with 836.7, 812.1, 773.9 and 685.4 g.kg -1 CP, respectively, while the lowest concentration was observed for C. phyllacanthus (190.9 g.kg -1 CP). The highest ADIP content was obtained for S. tuberosa (581.3 g.kg -1 CP), i.e., approximately 58 % of the protein present in its composition was unavailable.
For the fractioning of nitrogenous compounds, higher proportions of fraction A (NPN) were observed for C. phyllacanthus (375.5 g.kg -1 CP). The fraction B1+B2 had higher concentrations in P. bracteosa (481.5) and C. phyllacanthus (433.6) ( Table 4). For the fraction B3, the highest amount was found for M. tenuiflora and M. urundeuva (434.5 and 376.4 g.kg -1 CP). For the fraction C, there was a higher concentration in S. tuberosa, with 581.3 g.kg -1 CP in its composition, and a lower concentration in C. phyllacanthus (153.2 g.kg -1 CP) ( Table 4).
The highest percentage of fraction A (nitrogenous compounds) indicates the possibility of using this forage as a source of nitrogen readily available for rumen microorganisms. The high amount of N in the fraction A of C. phyllacanthus is noteworthy when associated with the availability of carbohydrates in the fraction A+B1. This is a fraction that presents a fast ruminal degradation, characterizing a synchrony of nutrient degradation rates in the rumen when using these forages, as recommended by the Cornell Net Carbohydrate and Protein System (CNCPS). Thus, when a forage has a high protein content and most of this protein is in fast degradation fractions, it is necessary to supply a carbohydrate source with a high ruminal degradation rate so that the microbial synthesis in the rumen is efficient. The fraction B3 is associated with cell walls with a slow degradation rate in the rumen (LICITRA; HERNANDEZ; Van SOEST, 1996). Thus, it has a larger passage, being a potential source of amino acids in the small intestine to be absorbed by the animal.
The degradation parameters a, b and c correspond to the percentage of compounds which can be soluble (a), compounds that are insoluble but can be degraded by microorganisms according to incubation time (b), and c corresponds to the degradation rate of insoluble compounds according to incubation time (t). The degradation potential (Dp) ranged from 27.12% (M. tenuiflora) to 61.63% (C. phyllacanthus) ( Table 5).
As for digestibility, the highest value was 627.1 g.kg -1 DM (C. phyllacanthus) and the lowest value was 340.5 g.kg -1 DM (M. tenuiflora) ( Table 5). Pereira et al. (2012), working with hay the C. phyllacanthus found 636.6 g.kg -1 DM digestibility. The degradation curves of Columns means with different superscripts differ significantly (**0.01 and *0.05 of probability). 1 g.kg -1 DM; 2 g.kg -1 CP. Standard error median (SEM) Chemical characteristics, degradation kinetics and gas production of arboreal species for ruminants the species evaluated showed distinct behaviors ( Figure  1), reflecting the peculiarities of the composition of each species.
The low percentages of in vitro dry matter degradability (IVDDM) of M. tenuiflora and S. tuberosa are related to higher levels of LDA and fraction C of TC, although all plants were in the same phenological state, but presented early maturation of tissues., which influences nutritional value more than any other factor. As the plant matures, it tends to decrease the production of potentially digestible components, such as soluble carbohydrates ED = effective degradability for passage rate equal to 0.02, 0.05 and 0.08 per hour; PD = potential degradability represented by the sum of a and b. Columns means with different superscripts differ significantly significantly (**0.01 and *0.05 of probability). -Unvalued. Standard error median (SEM)  and proteins, and increase the production of cell wall constituents. As a result, a decrease in IVDDM and dry matter intake by animals is expected. Habib et al. (2016), evaluated plants of the same genus (Ziziphus) and found dry matter digestibility values close to this one.
The total volume of gases observed (V t1 ) was close to that found by the bicompartmental model (V t2 ), proving the model adjustment. The lowest volume of gases produced by the fermentation of TC of M. tenuiflora, Z. joazeiro and S. tuberosa may have been caused because 54, 49 and 55% of the TC, respectively, were present in the fraction C, which is not degradable by ruminal microorganisms. During the early events of ruminal degradation, soluble nutrients are responsible for the greatest volume of gases produced, i.e., the A+B1 fraction is responsible for the initial volume (V f1 ) of gases produced from its degradation, which promotes a greater gas production specific rate (k 1 ).
The greatest gas volumes produced by the fermentation of NFC were verified for P. bracteosa and S. tuberosa. The lowest digestion rate (k 2 ) estimated for the FC of M. urundeuva leads us to infer that other factors may be related to k 2 , such as intermediaries of lignin synthesis and/or the presence of tannins. Although the presence of secondary compounds of Figure 2 -Volume of gases produced during in vitro incubation of arboreal species V t1 = total volume (mL.g -1 DM); V t2 = total volume (mL.g -1 DM) found using the model; V f1 = volume (mL) of gas produced by the degradation of the fraction A+B1 of the Cornell System (NFC); k 1 = specific rate (h) of production of gases by the degradation of the fraction A+B1 (NFC); V f2 = volume (mL) of gases produced by the degradation of the fraction B2 of the Cornell System (FC); k 2 = specific rate (h) of production of gases by the degradation of the fraction B2 (FC); λ = latency (h). Columns means with different superscripts differ significantly (**0.01 and *0.05 of probability). -Unvalued. Standard error median (SEM) the evaluated species is still not well characterized, it is known that significant levels of tannins can affect fermentation, ruminal microbiota and suppress methanogenesis (BHATTA et al., 2012;DURMIC et al., 2017;PAL et al., 2015).
Chemical characteristics, degradation kinetics and gas production of arboreal species for ruminants CONCLUSIONS 1. Cnidoscolus phyllacanthus had the highest predominance of the fractions A+B1 (NFC) of carbohydrates and the fraction A (NPN) of nitrogenous compounds, with a synchrony in the availability of these fractions; 2. Cnidoscolus phyllacanthus, Myracrodruon urundeuva and Poincianella bracteosa showed the highest gas production; 3. In face of all values presented, all the evaluated species present satisfactory nutritional value, especially in the case of animal production in a Semiarid environment.