Nutritional evaluation of tropical leaves for pigs; 
Desmanthus (Desmanthus virgatus)

J Ly and Pok Samkol

University of Tropical Agriculture Foundation, 
Royal University of Agriculture
Chamcar Daung, Dangkor District,
PO Box 2423, Phnom Penh 3, Cambodia
jlyca@yahoo.com; samkol@uta.edu.kh

Abstract

Eight Mong Cai castrate male pigs of 17.0 kg mean initial live weight were used according to a balanced change over design to study the nutritive value of desmanthus (Desmanthus virgatus) leaf meal obtained from a plantation cut at 60 days of regrowth. The basal diet (N, 2.82%) consisted of sugar palm (Borassus flabellifer) syrup and fresh water dry fish from which 17.7% was replaced by meal made from sun-dried desmanthus leaves (N, 4.41, NDF 61.0% of dry matter). 

Organic matter and N digestibility of the diet significantly decreased as determined either by the direct or the indirect (acid insoluble ash) method when desmanthus was introduced in the diet. DM digestibility estimated by the direct procedure could be predicted by the indirect method (r = 0.73; P<0.001). There were no significant differences between both methods for organic matter, NDF and N digestibility. 

Organic matter, NDF and N digestibility of desmanthus determined by difference revealed a low nutritive value of this type of leaf meal (37.3; 30.3 and 2.0% respectively). In vitro pepsin/pancreatin digestibility of N from desmanthus leaves meal was rather low (22.2%) in accordance with in vivo digestibility results. N retention as percentage of intake was lowest (P<0.001) in pigs fed desmanthus (58.2 and 45.3% for control  and desmanthus diets, respectively) but there was no treatment effect when N retention was related to the amount of digested N (67.6 and 64.3% respectively). 

The use of desmanthus leaf meal in diets for pigs could be justified if methods to increase its nutritive value could be developed. 

Key words: Pigs, digestibility, leaf meal, Desmanthus virgatus

 
Introduction

The leguminous shrub Desmanthus virgatus has been proposed as an alternative fodder tree the leaves of which could be used as animal feed (Evans et al 1992; Battad 1993). However, very little is known about the nutritive value of desmanthus leaves for pigs, in comparison with Leucaena (Leucaena leucocephala), for which there are results from a number of studies (Ly et al 1997, 1998; Argenti and Espinoza 1998; Bui Huy Nhu Phuc and Lindberg 2000). 

Desmanthus is a shrub belonging to the Mimosaceae family, originating in Central and South America, which has been introduced in many other tropical regions, including South East Asia (Allen and Allen 1981). The high crude protein content of desmanthus leaves (24-30% in dry matter) and its high yield (23 to 35 tonnes DM/ha,  according to Gutteridge (1994)) are factors that suggest the possible introduction of desmanthus leaves in pig diets. Gutteridge (1994) has suggested that since desmanthus does not contain mimosine, its foliage could be given to monogastric animal species with no harmful effects. 

The study of the nutritive value of potential foliage feeds for pigs could be improved if knowledge concerning digestibility could be readily assessed. The indirect method for estimation of total digestibility, based on use of acid insoluble ash as an inert marker (Van Keulen and Young 1977), offers such a possibility. The method has been used in experiments carried out with pigs in different feeding conditions (Ly et al 1997; Ly 2000). 

The aim of the present study is to report some results on the nutritive value of desmanthus leaves for pigs with a comparison of the direct and indirect method for estimation of digestibility.
 

Materials and Methods 

Eight Mong Cai castrate male pigs of 17.0 kg mean initial live weight were used according to a balanced two-period changeover design (Gill and Magee 1976; Gill 1978) to study the nutritive value of meal of desmanthus (Desmanthus virgatus) leaves obtained from a plantation cut at 60 days of regrowth. The plants were grown in the Chamcar Daung Campus of the Royal University of Agriculture. After harvesting, the leaves, secondary and tertiary stems included, were separated from the primary stems and then sun-dried before being ground in a hammer mill to obtain the leaf meal (N 4.41; organic matter 93.7; NDF 61.0% in dry basis respectively). Samples of desmanthus obtained from 10 terminal branches of 1 m length indicated  that the proportion of leaves:primary stems by weight was 1.06 (± 0.20):1.0 (fresh basis). The average number of leaves per tertiary stem was 70.5 ±  5.1, whereas the fresh weight of leaves plus secondary and tertiary stems was 343 ± 36 mg. The DM content of leaves and secondary plus tertiary stems was 35.4%. The DM concentration in primary stems was 39.3%.

 The basal diet consisted of sugar palm (Borassus flabellifer) syrup and fresh water dried fish, while in the experimental diet, 17.7% of the DM of the basal diet was replaced by desmanthus leaf meal. 

 The pigs were housed in metabolism cages previously described by Chiev Phiny and Rodríguez (2001). The diets were given at a rate of 30 g DM/kg body weight per day at approximately 07:30 h each morning and refusals taken at 07:00 h the following day. The ration was offered as a mash, by mixing diluted sugar palm syrup (1:1: by weight with  water, in fresh basis) with the other ingredients. Water was available ad libitum through drinking nipples. A preliminary adjustment period of five days was followed by another five days of collection. Other details related to faeces and urine sampling were described by Ly et al (2001). Nutrient digestibility and N balance coefficients were estimated by the conventional method and thereafter the nutritive value of desmanthus leaf meal was calculated by difference (Crampton and Harris 1969).

Chemical analysis of the diets and faeces were undertaken following the methods of Goering and Van Soest (1970) and Van Soest et al (1991) for NDF and AOAC (1990) for ash and N. The DM content was determined using the microwave method of Undersander et al (1993). Fresh faeces were analyzed for pH according to a procedure described by Ly et al (2001). Acid insoluble ash was analysed by digestion with 2N HCl according to the procedure of Van Keulen and Young (1977).

Ground samples of desmanthus leaves were incubated in vitro according to the method of Dierick et al (1984). Ground, dried samples of fresh water fish and casein were incubated similarly. The analytical procedures applied to the residues after incubation were the same utilized in the in vivo experiment.

The data were subjected to analysis of variance to test the effect of dietary desmanthus leaves using standard procedures described elsewhere (Steel and Torrie 1980). Data were processed using the software package of MINITAB (Ryan et al 1985).

Results and Discussion

 Nutritive evaluation of desmanthus leaf meal

All pigs gained weight during the in vivo trial, and there were no symptom of discomfort in the experimental animals. There were no feed refusals at any time. Organic matter and N digestibility of the diet decreased (P<0.001) when desmanthus meal was introduced in the diet. This same effect was evident (P<0.01) for DM digestibility. N retention as percentage of intake was lower (P<0.001) in pigs fed desmanthus meal, but there was no treatment effect when N retention was related to the amount of digested N.

Organic matter, NDF and N digestibility of desmanthus leaf meal determined by difference revealed a low nutritive value of this type of leaf meal (Table 3), as compared to other tree leaf meals previously studied (Ly et al 2001). 

In vivo digestibility of DM and N in cattle fed desmanthus leaves meal has been reported to be rather low: 47.7 and 44.0%, respectively (see Gohl 1981). Other results related to N digestibility in cattle are somewhat higher (58.0%) according to Kharat et al (1980). It is possible that one of the main constraints of desmanthus foliage for a better use in pigs could be the difficulty encountered for leaf disruption by mechanical procedures, including chewing. This assumption arise from the fact that desmanthus leaflets are extremely small (vide supra) and therefore, the preservation of its integrity would not facilitate the digestion of the cell content in the small intestine of animals.

As in other previous studies (Ly et al 2001), dry fish showed a high in vitro pepsin/pancreatin digestibility values for the assayed indices (Table 4). On the contrary, it was observed that desmanthus leaves had a low in vitro digestibility, in accordance to in vivo results. In this connection, in vitro digestibility data for desmanthus resemble those observed in leucaena leaf meal (Ly et al 1998), but are very low if contrasted with leaf meals from other tropical, non-legume trees such as Morus alba and Trichanthera gigantea (Ly et al 2001).

In vitro pepsin/pancreatin digestibility of N from desmanthus leaf meal was rather low (Table 4) in accordance with the in vivo digestibility results. However, the relatively higher in vitro N digestibility for desmanthus leaf meal as compared to the similar in vivo parameter in pigs could be the consequence of a laboratory grinding process to reduce the particle size of samples, an essential feature of this method as outlined by Dierick et al (1985). 

 Use of acid insoluble ash as inert marker in digestibility studies

 There was a significant effect of the method of estimation on the digestibility of DM (Table 5). On the other hand, DM digestibility estimated by the direct procedure could be predicted by the indirect method (Figure 1).

Figure 1: Relationship between digestibility determined by the 
insoluble ash method and by in vivo total collection

There were no significant differences between the methods for organic matter, NDF and N digestibility. These results suggest the suitability of the Van Keulen and Young (1977) procedure for indirect estimation of nutrient digestibility of non-conventional feeds in pigs.    

Conclusions

• The apparently low nutritive value of desmanthus leaf meal makes it unsuitable as an alternative protein source in pig diets.
• The use of desmanthus leaf  meal in diets for pigs could only be justified if methods to increase its nutritive value could be successfully developed.
• Use of the acid insoluble ash method for indirect determination of digestibility in pigs fed non-conventional diets, as those available in Cambodia, is recommended as a highly viable alternative to the in vivo method.

 
Acknowledgments

This publication is an output from a collaborative project between the Swine Research Institute at Havana, Cuba, and the University of Tropical Agriculture Foundation, Phnom Penh. This project has been partially funded by FAO, Rome (certifying officer, Dr. Manuel Sanchez, AGAP). The technical assistance of Mr. Kim San Sophon during the conduct of the experiment is gratefully acknowledged. 

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Received 23 August 2001

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