In vitro estimates of nitrogen digestibility for pigs and water-soluble nitrogen are correlated in tropical forage feeds

J Ly and T R Preston

University of Tropical Agriculture Foundation
Royal University of Agriculture
Chamcar Daung, PO Box 2423 Phnom Penh 3, Cambodia
E-mail: julioly@uta.edu.kh

Abstract

In vitro digestibility (pig pepsin/pancreatin) and water-soluble nitrogen (N) (by water extraction during three 30 minute cycles in a commercial washing machine) were determined in samples from seventeen tropical forage feeds available in Indochina (N range: 0.90 to 5.33 % on dry basis). 

Aquatic macrophytes revealed a high in vitro N digestibility (average of 67.6%) and a high water-soluble value of N (average of  51.5%). In vitro N digestibility of foliage of several trees and shrubs was variable, with a minimum for N. fruticans (1.0%) and a maximum for H. rosasinensis (67.8%). The corresponding water-soluble N values for these two types of forages were 0.8 and 51.8%, respectively. Crop residues exhibited variable values for in vitro N digestibility, with a minimum for rice straw (O. Sativa) of  0.2%) and a maximum for cassava (M. Esculenta) leaves of 57.8%. In all cases the values for in vitro N digestibility (range from  0.2 to 69.0%) were higher (P=0.001) than the water-soluble N values (range from 0.1 to 52.0%).  A close, linear relationship (R^{2 =} 0.98; P=0.001) was established between the in vitro N digestibility (Y in %) and the water-soluble N values (X in %), the equation being: Y = 0.693 + 1.283X (SExy ±0.051). 

It is suggested that in vitro digestibility (estimated with pig pepsin / pancreatin)  and water-soluble N (measured by repeated water extraction during three 30 minute cycles in a commercial washing machine) are closely related.  It is concluded that the water-soluble N measurement is an adequate first approximation on which to predict the true digestibility of protein in tropical forages for  monogastric animals.

Key words: in vitro digestibility, water-soluble,  nitrogen, tropical forages, aquatic plants, shrubs and tree leaves, pigs

Introduction

Washing value procedures have been used as a means of predicting the readily available nutrients in feeds either for ruminants  (Huntington and Givens 1997) or  monogastric animals (Hyslop et al 1999). The water-soluble dry matter is essentially equivalent to the independent term in the equation proposed  by McDonald (1981) for estimating protein digestibility for ruminants using the in sacco method described by Ørskov et al (1980). On the other hand, it has been found that in vitro digestibility of N (pig pepsin / pancreatin) is closely associated to in vivo, ileal digestibility of N in several feeds for pigs (Dierick et al 1985), including a variety of feeds available in tropical latitudes (Ly and Domínguez 1997).

The use of in vivo, ileal protein and amino acid digestibility has been recommended as an appropiate instrument for the determination of N requirements in certain animal species, such as pigs (Boisen and Fernández 1995). However, the determination of in vivo, ileal digestibility of nutrients is expensive and time consuming and not suitable in many situations due to the dependence on surgical skills and advanced laboratory conditions (see Dierick 1991). In this connection, the search for practical methods closely related to well known digestive characteristics of animals, could be of great interest.

The aim of the present study was to determine the possible association between two assessments of the N characteristics of several tropical forage feeds, many of which have potential as  protein supplements for pigs. As is well known, many of the truly tropical sources of energy for pigs are devoid of fibre and N (see Pérez 1997). Thus the use of forages as a source of protein seems to be very convenient in these types of diets.

Materials and methods

Samples from seventeen different tropical forages were collected in Vietnam, either at the Ecological Farm of the University of Tropical Agriculture situated on the campus of the University of Agriculture and Forestry in Thu Duc, Ho Chi Minh city, or from the local market in Thu Duc district, Ho Chi Minh City. Samples were grouped in three major categories:

• Aquatic macrophytes: azolla (Azolla spp), water spinach (Ipomoea aquatica) and lemna (Lemna spp), grown on water  fertilized with effluent from biodigestors charged with pig manure at the Ecological Farm.
• Leaves from trees and shrubs: Acacia auriculiformis, Acacia mangium, cashew (Anacardium occidentalis), jackfruit (Artocarpus heterophyllus), gliricidia (Gliricidia sepium),  hibiscus (Hibiscus rosasinensis),  leucaena (Leucaena leucocephala), nypa palm (Nypa fruticans) and  trichanthera (Trichanthera gigantea) [all available at the Ecological Farm, except the nypa palm collected from plantations close to Ho Chi Minh city]
• Crop residues: cowpea vines (Vigna sinensis),  banana leaves (Musa paradisiaca) and  cassava leaves (Manihot esculenta) [all available at the Ecological Farm]; and  groundnut vines (Arachis hypogea) and rice straw (Oryza sativa) obtained from the local market in Thu Duc, HNo Chi Minh city.

All foliages were sun-dried, then ground (2mm screen) for analysis. The dry matter content of the fresh and the sun-dried material was estimated by microwave radiation until constant weight (Undersander et al 1993). Nitrogen was determined by the macro-Kjeldahl technique (AOAC 1990). The determination of the water soluble N in the samples was conducted according to previous studies developed for estimation of water-soluble dry matter (Ly and Preston 1997), following by the measurement of N content in the undissolved foliage residues contained in the washed nylon bags. Commercial, pure wood cellulose (solkafloc), ground by the same procedure as for the forage samples, was employed as reference material for the water-soluble N measurements. The formula for the calculation of the water-soluble N was:

Water-soluble N (%)  = 100*(N_t – N_uw )/ N_t)

Where N_t and N_uw are total and non-water-soluble N, expressed in mg, respectively.

The in vitro pig pepsin/pancreatin digestibility was conducted in quadruplicate according to the procedure of Dierick et al (1985) using casein (analytical grade) as the reference protein.

Data were subjected to the paired ‘t’ test and regression analysis using standard procedures as outlined by Steel and Torrie (1980).

Results and discussion

The determined dry matter and N content of the forages and N solubility characteristics are set out in Table 1. The corresponding water-soluble value of DM in solkafloc samples was very near zero (0.5 ± 0.3 %), which suggested that no particular material was escaping through the nylon bag used in the washing procedure. This type of cellulose revealed zero N content, as expected. On the other hand, in vitro digestibility of casein as estimated by the sequential incubation procedure with pepsin, then with pancreatin, was 97.2 ± 1.5 % (n = 4).

The aquatic floating macrophytes (lemna, azolla, water spinach) had a high in vitro N digestibility (average of 67.6%) together with a high water-soluble N value (average of 51.5%). The high in vitro digestibility of protein from lemna obtained in the present study is in accordance with previous reports on either  in vitro or in vivo ileal digestibility of N from lemna per se (Domínguez et al 1996) or in vivo apparent digestibility of N in diets of sugar cane juice and lemna (Rodríguez and Preston 1996) and cereal grains and lemna (Gutiérrez et al 2001). To the knowledge of the authors, there are no previous reports of  in vitro N digestibility of water spinach.

In vitro N digestibility of the foliage from the selection of trees and shrubs foliage showed a wide range with a minimum for nypa palm leaves (1.0%) and a maximum for hibiscus leaves (67.8%). The corresponding water-soluble N values for these two extreme types of forages were 0.8 and 51.8%, respectively. In this connection, water-soluble DM values from a similar selection of  crop residues and tree and shrub leaves, such as those examined in the present study, have been found to vary considerably (Keir et al 1997; Ly et al 1997a,b). The crop residues, as expected, also showed a wide range of values for in vitro N digestibility, with a minimum for rice straw (0.2%) and a maximum for cassava leaves (57.8%).

Due to the wide variation in in vitro digestibility and washing characteristics of N in the samples examined, a general, consistent trend (P=0.001) was observed for the seventeen samples, in all cases the values for in vitro N digestibility being higher than those for the water-soluble N (Figure 1).  This suggests that the procedure of enzymatic attack of the foliage protein was more effective than the mechanical action of repeated wash cycles in the washing machine.

It is obvious that the results of the present study give no indication as to the nature of the N compounds determined by the two methods. However, since a close (P<0.001) linear relationship (R² = 0.98; SE_yx ±3.53) was established between the in vitro digestibility and water-soluble N value (Figure 2), it could be assumed that a similarity between both type of compounds does exist de facto.

Conclusions

According to the data reported here, in vitro (pig pepsin/pancreatin) digestibility of protein is closely related with the water-soluble N value in a wide range of tropical forages.  It is therefore proposed that the water-soluble N measurement is an adequate first approximation on which to predict the true digestibility of protein in tropical forages for  monogastric animals.

Acknowledgements

The financial support for this study was partially supplied by the Swedish Agency for Research Cooperation with Developing Countries (SAREC), courtesy of Dr Brian Ogle,  and by FAO, Rome (Manuel Sánchez, certifying officer from the Animal Production and Health Division). Dr. Julio Ly is a visiting scientist from the Swine Research Institute, Havana, Cuba.

References

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