RESEARCH WORK

 

 

 

Nutritional quality and fractionation of carbohydrates and protein in the forage components of an intensive silvopastoral system

 

 

 

Xiomara Gaviria1, 2, J. Rivera1and R. Barahona2*

1Centro para la Investigación en Sistemas Sostenibles de Producción Agropecuaria-CIPAV. Carrera 25 # 6-62, Cali, Valle Del Cauca, Colombia
2Departamento de Producción Animal, Facultad de Ciencias Agrarias, Universidad Nacional de Colombia, Sede Medellín
*Corresponding author: rbarahonar@unal.edu.co

 

 

 


ABSTRACT

The objective of this study was to evaluate the nutritional quality of the forage components of a SPSi based on Leucaena leucocephala associated to improved pastures, as well as its biomass production. The forage production was determined at several moments of the year and the nutritional quality was evaluated through the Cornell model. The soluble protein proportion (fraction A) was similar between the grasses and L. leucocephala, and represented as minimum 34 % of the total protein. The proportion of protein B2 (intermediate degradation) of the legume was higher than that of the grasses (53,7 vs. 30,2 %, respectively). Protein B3 of the diet (slow degradation) was around 22 % of the total protein, and more than 71 % of it can be considered degradable in rumen. L. leucocephala showed a higher concentration of soluble carbohydrates (16,7 %) and lower quantity of fraction B2 (14,94 %) than the grasses. Concerning the biomass availability, a production of 19,26 t DM/ha year-1 was reached. It is concluded that in SPSis a high quantity of quality forage is produced throughout the year, and that this offer is sufficient to cover the requirements of ruminants.

Key words: Cynodon plectostachyus, Leucaena leucocephala, Megathyrsus maximus, silvopastoral systems.


 

 

INTRODUCTION

Tropical livestock production faces great challenges, especially in the dry season, during which the forage availability and quality drastically decrease (Cuartas et al., 2014). The incorporation of legumes (for example, Leucaena leucocephala) in the grasslands has brought about great environmental as well as productive benefits, and constitutes a choice to avoid seasonality in production, by increasing the availability of high-quality forage throughout the year (Cuartas et al., 2014).

Legumes contribute to nutrient recycling in the grassland, as well as to the protection of the water sources, the increase and conservation of biodiversity and the improvement of the diet of ruminants (Tarazona et al., 2013). Their inclusion in the systems also favors the occurrence of changes in the nutritional indicators, with a better utilization of the companion grasses, due to the increase of the protein content and the digestibility of the diet, and to the reduction of the levels of neutral detergent fiber NDF (Cuartas et al., 2013; Rivera et al., 2013).

Likewise, intensive silvopastoral systems (SPSis) have positively contributed in the purpose of achieving sustainable livestock production (Murgueitio et al., 2014). These systems are an intensive modality of environment-friendly production, and show high density (more than 10 000 per hectare) of forage shrubs such as L. leucocephala (leucaena), associated with improved high biomass-producing grasses and trees introduced in models of intensive rotational grazing (Tarazona et al., 2013).

A priority in the research about SPSis should be their characterization in different areas, in order to improve their utilization. The use of the Cornell Net Carbohydrate and Protein System (CNCPS) developed by Cornell University has been very useful for this purpose; because, by taking into consideration the different protein and energy fractions of forages, it allows to know the ruminal performance of feedstuffs and identify alternatives to maximize their use and utilization (Fox et al., 2004).

The objective of the research was to evaluate the nutritional value and the fractionation of carbohydrates and protein in the forage component of a SPSi.

 

MATERIALS AND METHODS

Location. The study was conducted at the Agricultural Station Cotové (property of the National University of Colombia, Campus Medellín), which is located in the Santafé de Antioquia municipality, 74 km away from Medellín, at 6º 33' 32'' North latitude and 77º 04' 51'' West longitude and at a height of 540 m.a.s.l. The mean annual temperature of this site is 27 ºC and the rainfall, 1 100 mm per year; and it is located below the life zone of tropical dry forest T-df (Holdridge, 1978).

Silvopastoral system. The silvopastoral system had 5,6 ha and included L. leucocephala (Lam.) de Wit shrubs, sown at a density higher than 10 000 trees ha-1 and associated to the pastures Megathyrsus maximus (Jacq.) B. K. Simon & S. W. L. Jacobs and Cynodon plectostachyus (K. Schum.) Pilger. The forages were collected in the summer (June), and five subsamples of each species were taken.

The area was grazed with 20 commercial Zebu animals, which had an initial weight of 300 kg and a final weight of 420 kg. Rotational grazing was performed, in 1 200-m2 strips, divided by electrical fence, with one day of occupation and resting times of 45 days.

Evaluation of the forage production. The forage production was measured at four moments (every two months, approximately), during an experimental period of eight months (a summer and a winter). The quantification of the grass offer was made according to the double sampling method, described by Haydock and Shaw (1975); while in order to quantify the biomass of leucaena such method was modified. For such purpose five strata were defined in one linear meter, which represented the different growth levels of the shrub, from which the material that the animals could consume (leaves and fresh stems) was harvested and weighed. Taking this scale into consideration 50 frames were used, in the same way as for grasses, and the total forage availability of each strip was estimated.

Evaluated forages. The most representative forages of the SPSi were evaluated: C. plectostachyus, M. maximus and L. leucocephala, which had an average age of 45 days of regrowth. To facilitate the evaluation and discussion of the results, the nutrient offer from the grasses was grouped and this allowed to contrast it with the contribution of the legume.

Chemical composition of the forages. The analyses related to the forage quality were made in the laboratory of chemical and bromatological analysis of the National University, Campus Medellín. The following was analyzed in the samples: dry matter (DM), through the method ISO 6496, in forced-air oven at 105 ºC until reaching constant weight (ISO, 1999); crude protein (CP), by the Kjeldahl method, according to NTC 4657 (ICONTEC, 1999); NDF and insoluble fiber in acid detergent (ADF), according to the sequential technique described by Van Soest et al. (1991); lignin (Van Soest, 1963) and ethereal extract (EE) through Soxhlet extraction, by immersion, with the method NTC 668 (ICONTEC, 1973). The ash content was determined by direct incineration in muffle furnace at 500 ºC, according to AOAC 942.05 (AOAC, 2005); while calcium and phosphorus, by AA and UVVIS spectrophotometry, based on NTC 5151 (ICONTEC, 2003) and 4981 (ICONTEC, 2001), respectively. Finally, the crude energy was determined by calorimetry, following method ISO 9831 (ISO, 1998).

Determination of the protein and carbohydrate fractions. To analyze the protein fractionation, the scheme proposed by the CNCPS system (Sniffen et al., 1992), modified by Licitra et al. (1996), was used. The total nitrogen (TN) was determined by the Kjeldahl method (NTC-4567; ICONTEC, 1999). The non-protein nitrogen (NPN), easily soluble protein nitrogen (ESPN), hardly soluble protein nitrogen (HSPN) and insoluble protein nitrogen (IPN) were analyzed from the modified method proposed by Pichard and Van Soest (1977). For such purpose, 0,5 g of sample were weighed in a 100-mL beaker, and 5 mL of 10 % tert-Butyl alcohol (as moisturizing agent) and 5 mL of buffer were added. The resulting sample was kept at room temperature during one hour, with agitation every 10 minutes, to be filtered then on Whatman paper no. 54. The residue was filtered with 50 mL of buffer and 250 mL of distilled water, and the residual N was determined which is in correspondence with the insoluble N through the Kjeldahl method. In this process a borate-phosphate solution was used as buffer, composed by 12,2 g of NaH2PO4 H2O/L and 8,91 g of NaB4O7 10 H2O/L, and it was verified that the pH of the solution was 6,8.

For the fractionation of carbohydrates the procedure proposed by the CNCPS was used, which classifies their fractions from their degradation rate. Fraction A is constituted by rapidly degradable and soluble carbohydrates (including sugars); B1 are intermediate degradation carbohydrates (including pectins and starches); carbohydrates B2 have slow degradation and fraction C represents the non-degradable part (lignin and fractions bound to the cell wall). The total carbohydrates (TC) were calculated from the formula: 100 (CP + EE + ash). The structural carbohydrates (SC) were estimated by the difference of the NDF and the insoluble protein in NDF; while the non-fibrous or non-structural carbohydrates (NSC), by the difference between the total carbohydrates and the structural carbohydrates (Sniffen et al., 1992).

 

RESULTS AND DISCUSSION

The grasses showed a higher protein percentage (table 1) than the ones described for tropical grasses, possibly due to their association with leucaena shrubs. According to Barahona and Sánchez (2005), this type of interaction allows to improve the pasture quality, with which their protein content and also the total forage offer increase, compared with the results obtained in monocrop systems. This facilitates to offer a diet more adjusted to the nutritional requirements of the animals and increase the livestock efficiency (Cuartas et al., 2013).

The chemical composition shown by leucaena (high protein contents and low fiber contents) is typical of this legume, which coincides with the reports by García et al. (2009) and Gaviria et al. (2012) for the forage of this species: CP values between 19 and 28 % and NDF, between 35 and 51 %; this confirms its favorable properties to be included as protein source for feeding grazing cattle. The results are also in agreement with those reported by Sandoval et al. (2005), who obtained 26,7 % of ADF and 39,5 % of NDF for L. leucocephala.

Figure 1 shows the grass-legume ratio and the forage production (kg ha-1 every 45 days) at different moments of the experimental period; the latter was not constant and its variation was mainly due to the changes in the rainfall regime. The highest biomass offer appeared in April (2,9 t) and the lowest, in June (1,9 t), which means that the biomass availability was acceptable; but, in turn, it indicates that to maintain an adequate productivity of the system, the stocking rate should be adjusted according to the forage availability during the grazing time.

When making a projection for one year, a total production of 19,26 t DM ha-1 can be reached, with predominance of the grass in the system. In each grazing cycle (every 45 days), the average production of the grasses was 2 100 kg DM ha-1 and that of the legume, 308 kg DM ha-1, for a total of 2 408 kg DM ha-1. This is highly contrasting with regards to the biomass production in traditional systems in Colombia, where degraded pastures produce as average 7,0 t DM ha-1 year-1 (Cajas et al., 2011a), and the improved pastures can vary from 7,6 t DM ha-1 year-1 under inadequate management conditions (Córdoba et al., 2010), to 19,2 t DM ha-1 year-1 with an intensive use of resources (fertilizers and machinery) to renovate degraded pastures (Cajas et al., 2011b; Naranjo et al., 2012). In this sense, it should be emphasize that in a SPSi it is not necessary to use chemical fertilizers. The forage production in the SPSi of this research was higher than the one reported for a SPSi in the Cesar department: 15,62 t DM ha-1 year-1 (Gaviria et al., 2012).

Table 2 shows the content and offer of the protein fractions of the forages. The soluble protein is rapidly degraded in the rumen (Krishnamoorthy et al., 1982) and comprises fraction A (non-protein nitrogen), which turns into ammonia in the rumen, and B1 (true protein of fast degradability). Fraction B also includes other two fractions of true protein, which have different degradation rates: B2, of intermediate degradability, and B3, of slow degradability, also called escape protein. Fraction C refers to non-degradable protein, as it is bound to ADF.

In this study, the concentration of soluble protein was similar between the grasses and the legume, and represented as minimum 34 % of the total protein. On the other hand, the concentration of protein B2 was higher in the legumes than in the grasses; while the concentration of fractions B3 and C was higher in the grasses. Fraction C of the protein is associated to the lignin contents, which increase with the forage maturity. This fraction includes proteins associated to lignin, tannin-protein complexes, as well as Maillard products which are highly resistant to the hydrolysis by microbial enzymes (Krishnamoorthy et al., 1982), which causes that it cannot be digested in the rumen.

Fraction B3 is degraded in the rumen only in 10-25 %, for which a large part of this protein passes to the intestine, where it is digested due to the enzymatic action and it turns into the highest-efficiency fraction in ruminants (Sniffen et al., 1992). The content of B3 protein in the diet represented around 22 % of the total protein; more than 71 % of the total protein is degradable in rumen, for which, under the conditions of SPSis, there is good protein contribution to cover the requirements of the animals (Chamorro et al., 2002).

The protein fractions of L. leucocephala are similar to the ones reported by Rey et al. (2005), who obtained 42,99 % of fraction B2 and 10,96 % of fraction B3. However, they differ from those reported by Chamorro et al. (2002).

The fractionation scheme of structural and non-structural or soluble carbohydrates of the CNCPS is based on the one proposed by Goering and Van Soest (1970). In general, tropical grasses, unlike legumes, show a high content of structural carbohydrates and low contents of soluble carbohydrates (Juárez et al., 1999). Table 3 shows the content and offer of the carbohydrate fractions of both evaluated forages.

In terms of DM percentage, leucaena showed higher concentrations of soluble carbohydrates and lower concentrations of fraction B2 (which corresponds to the available fiber) with regards to grasses. Fraction C was similar in both forages, because they have equal lignin content. Fractions B2 and C represent structural carbohydrates, although the contents of B2 are available carbohydrates for the rumen flora (Chamorro et al., 2002).

Singh et al. (2012) reported 272 g kg-1 DM for the soluble fraction, higher value than the one found in this research, but fraction C was similar (181 g kg-1 DM). Regarding grasses, the values were close to the ones reported by these authors.

On the other hand, it is important to know the quantity of protein and carbohydrates which are rapidly degraded in the rumen, because the fermentation of these nutrients covers the main requirements for the growth of rumen microorganisms, by providing ammoniacal nitrogen, aminoacids, carbon skeletons and energy in the form of ATP for the synthesis of microbial protein (Sniffen et al., 1992).

Aldrich et al. (1993), when using diets that contained 36 % of non-structural carbohydrates (80 % fermentable in rumen) and 17,5 % of crude protein (66 % degradable in rumen), reported that a maximum contribution of microbial nitrogen to the duodenum occurs and, also, that a decrease in the non-structural carbohydrates or in the fermentable protein reduced the microbial protein synthesized in the rumen.

In each grazing cycle (i.e. every 45 days) the CP offer was 329 kg ha-1 as average with a stocking rate of 3,57 animals ha-1, which means 2,0 kg of available CP per animal per day (table 4). Some in vivo experiments conducted in this system showed that the average DM intakes were equivalent to 2,46 % of the live weight of the animals, with proportions of 75,3 % for the grass and 24,7 % for leucaena (Gaviria et al., 2015). Thus, a 400-kg animal would consume 1,26 kg day-1 CP, which is adequate to cover its maintenance needs and allow a weight gain of 1 kg day-1. In Colombia, the weight gain of the animals that graze in a SPSi varies between 650 and 800 g day-1 (Cuartas et al., 2014). On the other hand, Barahona and Sánchez (2005) state that the NDF content of the diet can limit productivity. In this sense, such content was 59,4 %, value that can be considered low if it is compared with that of tropical grasses, but is still limiting for the animal productivity.

It is concluded that SPSis offer a higher quantity of better-quality forage with regards to traditional systems, and that although there are variations in the biomass availability throughout the year, the forage offer is sufficient to cover the requirements of ruminants.

 

ACKNOWLEDGEMENTS

The authors thank the work team of CIPAV and UNAL and the Ministry of Agriculture and Rural Development, for funding the projects «Comparative analysis of beef production in steers produced in an Intensive Silvopastoral System and Confinement» and «Studies for the increase of silvopastoral productivity and environmental services in the sustainable Colombian cattle production project», respectively.

 

 

 

Received: September 29, 2014
Accepted: November 17, 2014