Digestibility and nitrogen balance studies in pigs fed diets with ensiled taro (Colocasia esculenta) leaves as replacement for fish meal

Pheng Buntha, Khieu Borin, T R Preston^* and B Ogle^**

^*Center for Livestock and Agriculture Development

buntha@celagrid.org

^**Finca Ecológica, TOSOLY, Socorro Colombia
trpreston@mekarn.org

^***Swedish University of Agricultural Sciences, Department of Animal Nutrition & Management,
.O. Box 7024,750 07,Uppsala, Sweden

Brian.ogle@huv.slu.se

Abstract

The four treatments applied to 4 growing pigs (15 kg live weight) in a 4*4 Latin square arrangement were four levels of ensiled leaves of taro (Colocasia esculenta) equivalent to 25, 50, 75 and 100% substitution of the protein from fish meal in a basal diet of sugar palm syrup. The pigs were housed in metabolism cages for consecutive periods of 12 days, with collection of faeces and urine during the last 5 days of each period.

There were positive curvilinear trends in DM and crude protein intake and in N retention in response to replacement of fish meal by ensiled taro leaves with the optimum proportion being at about 70-75% replacement. Coefficients of DM apparent digestibility were high on all diets but those for crude protein showed a negative curvilinear response with declining values beyond 25% substitution of the fish meal protein by that from taro leaf silage. The limiting factor to utilization of the taro leaf silage appears to be the lower apparent digestibility of the protein in the taro leaves.

Key words: Colocasia esculenta, digestibility, N balance, sugar palm syrup, Taro leaf silage

Introduction

Taro (Colocasia esculenta), also known as “Old Cocoyam”, is a wetland crop in many tropical and subtropical areas of the world, cultivated mainly for root (corm) production. Before the corm can be eaten, the traditional practice is to cook it so as to break down the needle-like calcium oxalate crystals which are present in all parts of the plant. According to Miller (1929), these crystals are “extremely irritating to the throat and mouth lining, causing a burning and stinging sensation”. Many other plants contain substantial quantities of oxalate (Standal 1983). Proposed functions of the oxalate content in plants are said to include: protection from insects and foraging animals through toxicity and/or unpalatability, osmo-regulation, and regulation of calcium levels in plant organs and tissues (Libert and Franceschi 1987).

In a survey in Takeo and Pursat provinces, 51% of the farmers that were interviewed reported feeding the petiole of Taro (Paper I) to pigs. As for the corm, the traditional practice is to boil the petioles before feeding. Very few farmers reported feeding the leaves. In the mountain areas of North Vietnam, there is a tradition of using both the corm and the leaves of Giant taro (Alocasia macrorrhiza) as feed for pigs (Tiep et al 2006). As in the case of taro, it was reported (Tiep et al 2006) that the traditional way is to boil the corm and the leaves prior to feeding, as this reduced the concentration of calcium oxalate present. These authors showed that ensiling the leaves with rice bran and molasses reduced the calcium oxalate content by 78%, and that the silage could be included in the diet of growing pigs at the 10% level without affecting growth rate. In Colombia, Rodriguez et al (2006) showed that the leaves of the New  Cocoyam (Xanthosoma sagittifolium) could be fed fresh to young pigs at levels replacing 50% of the soybean meal (approximately 45% of the diet DM) and that growth rates (500 g/day) were similar to those on the control diet without the leaves. The leaves were chopped before offering them to the pigs which may have reduced the oxalate content as it was observed that the irritating effect from handling the leaves was reduced after chopping them (Preston TR  personal communication).

There appears to be little recent research on the feeding value of taro leaves for pigs. For this reason the present study was carried out to provide preliminary information on digestibility and nitrogen balance in pigs fed ensiled taro replacing fish meal, using a fibre-free basal diet (sugar palm syrup), as according to Preston (2006) this is likely to facilitate the use of fibrous protein sources,  such as leaves from vegetative sources.

Materials and methods

Location and Climate

The experiment was conducted in the ecological farm of the Center for Livestock and Agriculture Development (CelAgrid), located in Rolous village, Rolous commune, Kandal Stoeung district, Kandal province, about 26 km from Phnom Penh City, Cambodia. During the trial (from 1 September to 21 November, 2006), the average ambient temperature was 25.9 ± 0.96 ­C in the morning at 06:00h, 31.7 ± 1.59 C in the middle of day,  and 28.3  ± 1.90 C in the afternoon at 18.00h.

Experimental design

The experiment was carried out as a Latin Square 4x4 arrangement with 4 pigs and 4 periods (Table 1), each of 12 days. The first 7 days were for adaptation to the diet, followed by 5 days for collection of faeces and urine.

Animals and housing

Four crossbred (Large White x Local) castrated male pigs, initially averaging 15.0 ± 1.0 kg live weight were used for the experiment. They were housed in metabolism cages (1.6 x 1.4 m floor area), elevated 0.6 m and allowing the separate collection of faeces and urine. The cages were made of wood and were situated in a building with a thatch roof of Imperata grass, and open sides.

Diets and feeding management

Wild taro (Trav Prey) leaves were harvested from a lake receiving waste water from Phnom Penh City. The leaves were chopped into small pieces and ensiled with 5% of sugar palm syrup (79% of DM) and 2% of water (fresh basis) to facilitate the mixing of the viscous syrup. Leaves and syrup (plus the water) were carefully mixed and stored in plastic bags, inside rigid plastic containers. The bags were tightly closed to prevent air contact and stored for 30 days before feeding. Every container thus prepared contained approximately 150 kg of fresh material. The ensiled Taro leaves had a pH of 3.85 after one month and DM content of 18.3%.

Four diets were formulated with protein from ensiled taro leaves replacing fish meal protein at levels of 25, 50, 75 and 100% (Table 2). The pigs were fed ad libitum, with fresh feed supplied 3 times per day during the adaptation period, and given 80% of the mean ad libitum intake in the collection period to avoid residues. All ingredients were mixed together before feeding. Fresh water was available at all times from nipple drinkers. The duration of the experiment was 48 days.

Measurements

Feeds offered and residues, and output of faeces and urine were recorded daily during the last 5 days of each period. The animals were weighed in the morning before feeding at the beginning and the end of each period. Samples of each component (offered) were retained for analysis of DM, Ash, OM and N. A sub-sample (10%) of faeces was put each day in plastic bags in the freezer (-20 °C). Urine was collected in a bucket containing 10 ml of 10% sulphuric acid (H₂SO₄) to keep the pH below 4 so as to prevent escape of ammonia. A sub-sample (10%) was collected each day and kept in the freezer.

Chemical analysis

Feed and faecal samples were dried by microwave radiation to measure the DM content (Undersander et al., 1993). Total N of samples of feed and faeces and urine was determined by the Kjeldahl procedure as outlined in AOAC (1990). The ash content of feed and faeces was determined following the AOAC (1990) recommendations; organic matter was assumed to be the result of subtracting the percentage of ash from 100. The calcium oxalate was determined according to AOAC (1990) procedures.

Statistical analysis

The data were analyzed using the general linear model (GLM) procedure in the ANOVA software of MINITAB (release 13.31). Sources of variation were treatments, periods, animals (pigs) and error.

Results and discussion

The process of ensiling the taro leaves was more effective than sun-drying in reducing the calcium oxalate content (Table 3). The concentration in the ensiled leaves was lower than reported by Tiep et al (2006) (0.7% in DM) for ensiled leaves of Giant taro (Alocacia macrorrhiza). The fish meal used was of extremely poor quality, with a crude protein content of only 35.6% of DM, and an ash content of 56.8%, probably because the meal was contaminated, possibly by, for example, addition of sand.

There were slight differences between the planned and observed levels of substitution of fish meal protein by the protein from taro leaves, with higher than intended levels for the intermediate treatments TS25, TS50, and TS75 (Table 4).  This was because of a small degree of selection by the pigs.

Daily feed intake

There were significant differences between treatments for intake of palm syrup and total DM (Table 5). The relationship between DM intake and dietary protein from ensiled taro leaves was curvilinear (Figure 1) with maximum intake at about the 75% replacement of fish meal, followed by a decline to the 100% level. The trend for intake of palm syrup was similar. The reason for the decrease in intake of the TS100 diet may have been the “bulkiness” of the diet because of the high content of the low DM silage. The DM intake expressed as a function of live weight on the TS100 diet (30.1 g/kg) was lower than reported by Rodriguez et al (2006) (48.5 g DM/kg live weight) for a similar diet fed to pigs of similar live weight, but with sugar cane juice instead of palm syrup and fresh leaves of Xanthosoma sagittifolium instead of ensiled leaves of Colocasia esculenta.

Apparent digestibility

There were no significant differences among treatments in apparent digestibility of DM and organic matter (Table 6) which were high in all cases. The value for DM of 87.7 on the TS100 diet was slightly higher than was reported by Rodriguez et al (2006) for a diet of sugar cane juice and fresh leaves of Xanthosoma sagittifolium (83.4%). If it is assumed that the sugar palm syrup is 100% digested, and as it contributed 53% of the diet DM in treatment TS100, it can be predicted that the apparent digestibility of the taro leaf silage would be 73.8% for the DM and 86% for the organic matter.

Crude protein apparent digestibility differed among treatments (P<0.05) with a negative curvilinear trend (Figure 4), as the proportion of protein from taro silage was increased, with the lowest value of 66.1% on the TS100 diet. A similar negative trend was reported by Rodriguez et al (2006) with a value of 61.4% for the diet with only cane juice and Xanthosoma leaves. These negative effects can be attributed to the binding of some of the proteins to the fibre which reduces its digestibility (Jørgensen et al 1996; Kass et al 1980).  Shayo and Uden (1999) reported that 64% of the protein in Xanthosoma leaves was in the NDF fraction; however, a much smaller fraction (21%) was reported by Leterme et al (2005) for leaf protein in Xanthosoma.

Nitrogen balance

There were significant differences in all the components of N balance (Table 7; Figure 5). Faecal N tended to increase (P<0.05) and urine N to decrease as the proportion of taro leaf silage increased. Similar trends were reported by Rodriguez et al (2006) for diets of sugar cane juice and Xanthosoma leaves. There were significant effects of N intake on N retention (P<0.001), but not on N retention as % of intake and % of digested N (P>0.05). The responses to increasing intake of dietary taro leaf protein were curvilinear for N retention (adjusted for N intake; Figure 6) and for N retained as % of N intake and as % of digested N (Figure 7).  This response followed closely the trend for DM intake (Figure 1) implying that the best results would be with 75% replacement of the fish meal by the taro leaf silage. The similar values for N retention on TS0 (all fish meal) and TS100 (all taro silage) suggests that the amino balance was not a limiting factor to pig performance. This is supported by the conclusion of Leterme et al (2005) that “tropical tree leaves offer a variable amount (170-240 g kg⁻¹) of proteins that are well balanced in essential amino acids but not well digested by pigs”.

The N retention values in general were lower (range 2.9 to 5.4 g N/day) than those recorded by Rodriguez et al (2006) (range 6.1 to 9.7 g N/day). The main reason for this would seem to be the lower DM intake (25 to 34 g DM/kg LW) compared with the experiment of Rodriguez et al (2006) (49 to 56 g DM/kg LW). The diets were similar in overall composition (mainly sugars as the energy component and leaves of similar plants species (Old Cocoyam versus New Cocoyam)). The difference in response in feed intake could be due to the previous feeding history of the two sources of pigs. These were of similar live weight (15 kg in this study versus 13.4 kg in the experiment of Rodriguez et al (2006)). However, the pigs in the latter study were from sows that had been fed leaves of New Cocoyam during pregnancy and lactation, when the piglets also had access to the leaves. Tien and Preston (2003) concluded that “reductions in growth rate at weaning time when piglets are first offered solid foods, or subsequently due to frequent changes in diet, can be reduced if novel foods are eaten pre-natally by mother or post-natally by mother and piglets together”. 

Conclusions

• There were positive curvilinear trends in DM and crude protein intake and in N retention in response to replacement of fish meal by ensiled taro leaves with the optimum proportion being at about 70-75% replacement.

•  

• Coefficients of DM apparent digestibility were high on all diets but those for crude protein showed a negative curvilinear response with declining values beyond 25% substitution of the fish meal protein by that from taro leaf silage.

•  

• The limiting factor to utilization of the taro leaf silage appears to be the lower apparent digestibility of the protein in the taro leaves and maybe amino acid imbalance of the protein, or a greater proportion of crude protein fermented in the caecum.

Acknowledgements

The present experiment is part of a study on ensiled taro (Colocasia esculenta) leaves in diet of local crossbred pigs in Cambodia, supported by the MEKARN project financed by SIDA-SAREC. The authors express their gratitude to all the personal of the CelAgrid Ecological Farm, for help with the experiment. This paper forms part of the MSc thesis (MEKARN-SLU, Uppsala, May 2007) of the senior author.

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