A note on the effect of fresh mulberry leaves, fresh sweet potato vine or a mixture of both foliages on intake, digestibility and N retention of growing pigs given a basal diet of broken rice

Chhay Ty, Khieu Borin and Chiv Phiny

Center for Livestock and Agriculture Development (CelAgrid), P.O box 2423, No 05, Street 181, Phnom Penh, Cambodia
chhay-ty@celagrid.org

Abstract

Three crossbred castrate male pigs, weighing on average 24 kg were allotted at random to three diets to study the effect of fresh sweet potato vine and fresh mulberry leaves, given separately or mixed together on intake, digestibility and N retention of growing pigs with a basal diet of broken rice.

The actual intakes of the foliages accounted for 21 to 28% of the total DM intake, .and about 55% of the crude protein intake. There was a strong indication (P=0.066) that intakes of foliage DM were higher when the sweet potato vines were all or part of the foliage supplement. Total intakes of DM and crude protein did not differ among the treatments. Coefficients of apparent digestibility were high for DM and organic matter with no differences among the treatments. The apparent digestibility coefficients of the DM, OM and crude protein were higher for the mulberry diet compared with the diets in which the protein supplement contained sweet potato vines. N retention was high on all diets, equivalent to a live weight gain of the order of 250-300 g/day, with a suggestion of a higher value for the diet with mulberry leaves

It is concluded that the protein in the fresh foliage of mulberry leaves is well utilized by growing pigs fed a basal diet of broken rice. There were no advantages from giving a mixture of sweet potato vines and mulberry leaves compared with either foliage given alone.

Key words: broken rice, digestibility, mulberry leaves, N retention, pigs, sweet potato vine

Introduction

Increasing prices of both fishmeal and soybean meal have focused attention on the use of leaves and vines of tropical food crops as alternative sources of protein in pig diets (Bui Huy Nhu Phuc 2006; Preston 2006).

Sweet potato (Ipomoea batatas L.) is a tropical food crop which originated in Central America, and is now extensively grown in many counties, especially China and in Southeast Asia. It is grown primarily for production of tubers for human food. However, attention has been directed recently on its use as live stock feed, with both the vines and the tubers being used for this purpose, the former as a source of protein and the latter for energy (Woolfe 1992; Le Van An 2004).

Mulberry trees have been domesticated for many centuries for feeding silkworms (Tingzing et al 1988). Several reports have emphasized the potential of the foliage as a feed for ruminants (Benavides 1999; Sánchez 1999; Yao et al 2000); however, very little is known about the nutritive value of the leaves for non-ruminant animals other than some preliminary research done by Trigueros and Villalta (1997) and Ly et al (2001).

The aim of the present experiment was to study the effect of fresh sweet potato vine and fresh mulberry leaves, given separately or mixed together, on intake, digestibility and N retention of growing pigs given a basal diet of broken rice.

Materials and methods

Location and climate

The experiment was carried out at the Center for Livestock and Agriculture Development (CelAgrid), located in Kandal village, Rolous Commune, Kandal Steung district, Kandal province about 25km from Phnom Penh City, Cambodia from 21 December 2005 to 27 January 2006.

Treatments and design

Three treatments were compared as protein sources in diets based on broken rice:

• ML: Mulberry leaves
• SP: Sweet potato vines
• SPML: Mixture between fresh sweet potato vines and fresh mulberry leaves

The experimental design was a 3*3 Latin Square arrangement of the three dietary treatments (Table 1) with three castrated male pigs weighing on average 24 kg.

Each experimental period lasted for 12 days; 7 days for adaptation to the diets followed by 5 days for collection of faeces, urine and feed refusals. The pigs were housed in metabolism cages in an open shed during the whole trial. Details of the cages have been described elsewhere (Chhay Ty et al 2003a,b).

Experimental feeds and feeding

Sweet potato vines and mulberry leaves were harvested daily from the cropping area of CelAgrid. The mulberry leaves were from trees that were being harvested at 3-month intervals. The sweet potato vines (60% stems and 40% leaves) were harvested 45 days after the plants had been established. Broken rice was purchased from a rice mill close to CelAgrid. The composition of the dietary ingredients, based on samples taken during the experiment, is in Table 2.

The broken rice was offered at a level of 20 g DM/kg live weight per day; the foliages were offered ad libitum. On the SPML treatment the offer level of the two foliages was in a ratio of 50:50 (DM basis). The feeds were offered in 3 meals daily at 8:00 am, 12:00 am and 4:00 pm. Broken rice and salt were mixed together and fed first. After the broken rice and salt were consumed, the foliages were offered. Water was permanently supplied through drinking nipples.

Data collection

Feed refusals and faeces were collected every day. Faeces were kept frozen in plastic bags until analysis but feed offered and residues were analysed for DM every day, using a representative sample from every treatment. At the end of each period, representative samples of faeces, feeds and feed refusals were analyzed for DM, N, ash and organic matter. Urine was collected in a plastic bucket to which sulphuric acid was added daily (10ml of 10% concentrated H₂SO₄) to maintain the pH below 4.0. The volume of urine was measured every day and 10% of the total volume stored until the end of each period, when it was analyzed for N. The animals were weighed at the beginning of the trial and then every 12 days.

Chemical analyses

Chemical analyses of the feed ingredients and faeces were undertaken following the methods of AOAC (1990) for ash, and N. The DM content was determined using the microwave method of Undersander et al (1993). The N content of urine was determined by the AOAC (1990) procedures.

Statistical analyses

The data were subjected to analysis of variance according to the general linear model of the Minitab software (Minitab release 13.31, 2000). Sources of variation were: pigs, periods, treatments and error.
 

Results

During the entire trial, the pigs were in a positive body weight balance and there were no symptoms of ill-health from feeding fresh mulberry leaves or fresh sweet potato vine or the mixture of the two foliages.

The pigs consumed all the broken rice. The actual intakes of the foliages accounted for 21 to 28% of the total DM intake, and about 55% of the crude protein intake (Table 3 and Figures 1 and 2).

There was a strong indication (P=0.066) that intakes of foliage DM were higher when the sweet potato vines were all or part of the foliage supplement. Total intakes of DM and crude protein did not differ among the treatments.

Coefficients of apparent digestibility were high for DM and organic matter with higher values for the ML diet compared with diets containing SP (Table 4).

The apparent digestibility of the crude protein was also higher for the mulberry diet (ML) compared with the diets in which the protein supplement contained sweet potato vines (Figure 3).

N retention was high on all diets (Table 5), equivalent to a live weight gain of the order of 250-300 g/day, assuming that approximately 20% of the live body of the pigs is protein.

There was a suggestion (P=0.19) that daily N retention was higher on the ML treatment, but no differences for retention as percentage of N intake or N digested.

Discussion

There are no apparent reasons for the higher coefficients of apparent digestibility of DM, OM and crude protein in the diet with mulberry as the sole forage compared with the diets containing sweet potato vine. The levels of methionine + cystein as proportion of the lysine are reported to be similar for mulberry and sweet potato leaves (0.56 and 0.55), as are levels of crude fibre (17.2 and 12.8% in DM) and ash (12.6 and 10.9% in DM) (Preston 2006). Both plants were grown in adjacent plots in CelAgrid hence differences in mineral content were also unlikely to have been a factor.

High values for digestibility of DM (85%) and crude protein (84%) in pigs fed diets with 30% dried mulberry leaf meal were reported by Ly et al (2001). Using the "difference method" they estimated the values for the mulberry leaves alone as 84 and 81% for DM and crude protein, respectively. In an experiment with 50% of fresh mulberry leaves in the diet DM, replacing a combination of fish meal and rice bran, values of apparent digestibility were 82 and 71%, for DM and crude protein (Chiv Phiny et al 2003). By comparison, with sweet potato vines at 25% of the diet DM (Hoang Huong Giang et al 2004), the dietary values for apparent digestibility of DM and crude protein were 77 and 67%, respectively; while estimates (using the difference method) for the sweet potato vines per se were 68 and 48% for DM and crude protein. The implications from these studies, that the crude protein in mulberry leaves is more digestible than in sweet potato vines, is supported by the higher apparent crude protein digestibility for the diet with mulberry compared with the diets containing sweet potato vines in the present study.

The use of protein-rich foliages in pig diets as complete replacements for conventional protein supplements such as fish meal and soybean meals is a relatively recent development (Ogle 2006; Preston 2006). Much of the research has been directed to the use of single foliages such as those from cassava (Bui Huy Nhu Phuc 2006), sweet potato (Le Van An 2004), water spinach (Chhay Ty et al 2005a,b) and mulberry leaves (Chiv Phiny et al 2003). Chhay Ty et al (2005b) evaluated the use of cassava leaves and water spinach alone, or as 50:50 mixtures (DM basis), as sole supplements to a basal diet of broken rice. They reported a synergistic effect from the inclusion of water spinach, as pig performance on the mixture of the two foliages was superior to the average of the foliages fed as the sole supplement. In a recent N-balance experiment with young pigs (Chiv Phiny et al 2007) there were reduced losses of N in urine and superior N retention when a 50:50 mixture (DM basis) of mulberry leaves and water spinach was fed compared with water spinach alone. This positive effect from diluting the water spinach with mulberry leaves was observed with contrasting energy sources: cassava root meal plus rice bran compared with sugar palm juice plus broken rice. Chittavong Malavanh and Preston (2006) also reported reduced N losses in urine and higher N retention when sweet potato vines replaced water spinach in the diet of pigs fed broken rice and cassava root meal as energy sources. In the present experiment, the increase in apparent digestibility of crude protein with increasing proportions of mulberry leaves in the diet, replacing sweet potato vines, lends support to the findings of Chiv Phiny et al (2003, 2007) that the protein in mulberry leaves is well utilized by growing pigs.

There were no apparent advantages from providing a mixture of both mulberry leaves and sweet potato vines, compared with either foliage given alone. This is contrary to the synergistic effect of combining water spinach with cassava leaves (Chhay Ty et al 2005b) and the superior value of a mixture of mulberry leaves with water spinach compared with water spinach alone (Chiv Phiny et al 2007).
 

Conclusions

• The protein in the fresh foliage of sweet potato vines and mulberry leaves was well utilized by growing pigs fed a basal diet of broken rice, with higher apparent digestibility for the diet containing mulberry leaves.
  
  
• There were no advantages from giving a mixture of sweet potato vines and mulberry leaves compared with either foliage alone.
  

Acknowledgments

The authors would like to express their gratitude to the MEKARN project financed by the SIDA-SAREC Agency and to the Center for Livestock and Agriculture Development (CelAgrid), for providing resources for conducting this experiment.

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