Evaluation of ten tropical legume forages for their potential as pig feed supplement

S N Heinritz**, S Hoedtke*, S D Martens, M Peters and A Zeyner*

Center for International Tropical Agriculture (CIAT), Multipurpose Tropical Forages, Cali, Colombia
s.martens@cgiar.org
* University of Rostock, Faculty for Agricultural and Environmental Sciences, Chair for Nutrition Physiology and Animal Nutrition, Rostock, Germany
** University of Hohenheim, Landesanstalt für Landwirtschaftliche Chemie, Stuttgart, Germany

Abstract

Herbage of Cratylia argentea, Desmodium velutinum, Fleminigia macrophylla, Leucaena diversifolia, Canavalia brasiliensis, Centrosema brasilianum, Clitoria ternatea, Lablab purpureus, Stylosanthes guianensis and Vigna unguiculata  from the CIAT (International Center for Tropical Agriculture) gene bank were assessed for their nutritional value and in-vitro digestibility for pigs in order to predict their potential as alternative protein supplement in a tropical smallholder context.

Crude protein (CP) contents ranged from 137 to 257 g kg^-1 dry matter (DM) (mean 191 g kg^-1 DM), although a considerable proportion of it, 27 % on average, was bound to neutral detergent fiber (NDF). Interesting levels of lysine were found in Cratylia argentea (14 g kg^-1 DM) and Leucaena diversifolia (13 g kg^-1 DM), whereby the latter was also high in tannic acid concentration (49 g kg^-1 DM) thus limiting the amino acid digestibility. Vigna unguiculata presented highest in-vitro enzymatic degradability (521 g kg^-1 DM), which even increased in a 40:60 mixture with maize. Lowest degradation was obtained with Flemingia macrophylla (248 g kg^-1 DM), while the median of the forages approached 390 g kg^-1 DM. It is concluded, that Vigna unguiculata herbage meal has the highest potential to be successfully included in pig diets, while Cratylia argentea meal should equally be assessed in vivo.

Keywords: amino acids, fiber, in-vitro digestibility, tannins

Introduction

Fluctuating prices for imported feed concentrates, variable availability of high-quality protein supplements and environmental implications are the reasons for searching for locally available solutions for efficient pig production in the tropics. Tropical legume forages adapted to various ecological niches are being evaluated as one option for small and medium size pig producers.

The aim of the study was to determine the nutritional value of selected tropical forage legumes in order to estimate their potential as protein supplement for pig feeding. 

Material and methods

Plant material

In 2009 and 2010 the following forage legumes were harvested at pre-flowering or early flowering stage from different sites between 3°30’N to 2°06’N latitude in the Valle del Cauca and Cauca departments of Colombia: four shrub legumes Cratylia argentea CIAT 18516/18668 (8 weeks regrowth), Desmodium velutinum CIAT 23985 (8-9 weeks regrowth), Fleminigia macrophylla CIAT 21087 (8-9 weeks regrowth) and Leucaena diversifolia CIAT K787, and six herbaceous legumes Canavalia brasiliensis CIAT 17009 (12.5 weeks), Centrosema brasilianum CIAT 5234 (8 weeks regrowth), Clitoria ternatea CIAT 20692 (12 weeks), Lablab purpureus CIAT 22759 (8 weeks), Stylosanthes guianensis CIAT 11995 (8 weeks regrowth) and Vigna unguiculata CIAT 4555 (7.5 weeks). Around 15 kg fresh matter (FM) of each species were chopped for ensiling (Heinritz et al, forthcoming) and a representative sample was taken for chemical analysis. From the shrub legumes, the lignified stems were removed before chopping and leaves with only green, fine stems were included in the chemical analysis.

Chemical analysis

The samples were lyophilized and ground to 1 mm mesh size. Dry matter (DM) was determined at 105 °C for ≥ 3 h and the ash by burning the sample in a muffle furnace for 5 h at 600 °C. Neutral and acid detergent fiber (NDF, ADF) were analyzed by the method of van Soest et al 1991). Nitrogen (N) was determined by microkjeldahl according to Temminghoff (2010) (Skalar, Sampler SA 1000, Chemistry Unit SA 4000, Matrix Photometer 6250/6260, Software: FlowAccess, ver. 1.04.8). Crude protein (CP) was expressed as N x 6.25. Nitrogen bound to fiber (N-NDF) was determined by nitrogen analysis of the NDF residue (Licitra et al 1996). Amino acids (AA) were quantitatively separated in the filtrates by high performance liquid chromatography as described by Hackl et al (2010). Tannins were determined according to Makkar (Makkar et al 1993; Makkar 2003), distinguishing tannic acid from total phenolics by using polyvinyl polypyrrolidone (PVPP) as binding tannin-phenolics. 

In-vitro digestibility for pigs

For measuring the in-vitro digestibility, enzymatic digestion as occurring in stomach and small intestine, as well as the fermentation taking place in the large intestine were determined, to cover the digestion process completely.

Enzymatic hydrolysis

Porcine pepsin (2000 FIP-U g^-1, Roth) and porcine pancreatin (Pancreatin 8 x USP specifications, Sigma n°P-7545) were used for the enzymatic hydrolysis (three test runs in triplicates each) according to the method of Boisen and Fernandez (1997). Additionally to eight forage samples, ground maize and a mixed sample, containing 60% maize and 40% Vigna unguiculata, representing a simplified pig diet, were included in the in-vitro digestibility studies.  

Modified in-vitro gas production technique

The in-vitro gas test followed the protocol of Menke and Steingass (1987), as adapted to pigs by Bindelle et al (2007). Faeces were sampled from donor pigs of 5 months of age which had a bodyweight of 59 kg on average. They were fed with a conventional diet, consisting of 300 g kg^-1 wheat, 215 g kg^-1 extracted soybean meal, 190 g kg^-1 barley, 50 g kg^-1 rye and 5 g kg^-1 maize. Each gas production (GP) test included three replicates per sample, which were run simultaneously. Additionally, a blank, i.e., faeces alone, was measured in each test run. The volume (V) of the produced gas was read from the syringes at the following times: 0, 2, 5, 8, 12, 16, 20, 24, 30, 46, 52, 58 and 72 h. Finally, fermentation products were measured by gas chromatography and lactic acid by HPLC in three replicates. 

Statistical analysis

Statistical differences in digestion parameters were detected by Tukey Pairwise Mean Comparisons (95% confidence interval) and Pearson correlations with Bonferroni probabilities and linear regressions calculated using SYSTAT® 12 (Systat Software, Inc., 2007). 

Results and discussion

Crude protein values of the forage legumes ranged from 137 to 257 g kg^-1 DM with an average of 191 g (Table 1). When subtracting the protein bound to NDF an average of 138 g kg^-1 DM remained, although tannic acid especially in Flemingia and Leucaena (genus name will be used in the following text for convenience) might further reduce protein availability in the animal. With up to 300 g kg^-1 DM ADF, fiber concentrations of Vigna, Cratylia, Lablab and Leucaena were acceptable, while Stylosanthes, Centrosema and Flemingia were at the upper level.

Cratylia, which showed a high content of potentially available crude protein also presented highest concentrations of the essential amino acids lysine and methionine + cystine (Met+Cys) (Table 2) and was thus higher than alfalfa, which is reported with an average of 10.5 g for lysine and 6.1 g kg^-1 DM  for Met+Cys (protein 22-25% DM) (INRA 2004), and had about 54 and 56 % of these AA compared to soybean meal, 46 (26.6 g and 12.7 g kg^-1 DM resp.) (INRA 2004). Leucaena would also be interesting considering the AA pattern if it was possible to reduce the tannin content considerably.

Highest enzymatic degradability within the 8 studied forages was achieved with Vigna, followed by Leucaena, Canavalia, Desmodium and Cratylia (Table 3). This was well correlated with the respective ADF contents (R²=0.77, P=0.005) and less with NDF (R²=0.50, P=0.051). According to the obtained regression equation with ADF (f(x) =-0.758x+642), the theoretic enzymatic degradability of Lablab would be 413 g and of Clitoria 342 g kg^-1 DM, thus ranking in the upper middle and middle digestibility class of the studied tropical forages. Highest gas volume during in-vitro fermentation was also produced by Vigna together with the highest amount of short chain fatty acids. The contrasting legume was Flemingia (Table 3). The mixture with maize increased the calculated digestibility of Vigna by 100 g kg^-1 DM. An explanation could be the mixture of high contents of starch and protein, and low fiber content. The gas production of the mixture was lower than with maize alone and quite similar to the calculated gas production of the combination based on the GP of both materials alone. This might be explained by the increased enzymatic degradation of Vigna in the mixture which left fewer residues to ferment in the simulated large intestine.

Conclusions

• Because of its highest in-vitro digestibility and high protein availability, Vigna unguiculata herbage has the highest nutritive potential to be included in pig diets. At the same time, Cratylia argentea should be assessed as alternative protein supplement for pigs because of its interesting concentration in lysine and intermediate digestibility. When screening tropical forages for their potential in monogastric farm animal nutrition, a low ADF concentration should be favored because of its correlation to enzymatic digestibility.
  
  
• The presented investigation is accompanied by feeding trials with pigs (Sarria et al 2010, first results) and  agronomic studies (Peters, Maass, van der Hoek, unpublished) to assess the suitability of the tropical forages identified for high nutritional value for smallholder pig production systems in South America (Colombia), Central America (Nicaragua and Honduras) and Central and Eastern Africa (DR Congo).

Acknowledgements

We kindly thank for the technical assistance of the team from the Tropical Forages Program, CIAT, and from the Chair for Nutrition Physiology and Animal Nutrition, University of Rostock. The financial support for the project “More chicken and pork in the pot, and money in the pocket: Improving forages for monogastric animals with low-income farmers” (2009-2012) by the Federal Ministry for Economic Cooperation and Development, Germany (BMZ), is gratefully acknowledged.

References

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Received 7 December 2011; Accepted 14 December 2011; Published 4 January 2012

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