Ileal and total tract digestibility of aminoacids in pigs fed cassava leaves and sweet potato vines in their diets  both in silage and in dry form

Nguyen Thi Hoa Ly, Le Duc Ngoan and A C Beynen^*

Hue University of Agriculture and Forestry, Hue, Vietnam
Lydohoai@dng.vnn.vn
* Faculty of Veterinary Medicine, Utrecht University, The Netherlands

Abstract

Ileal and total tract digestibility in fattening pigs fed ensiled cassava root (ECR) based diets with inclusion of protein of cassava leaves and sweet potato vines in both silage and dry form was studied in a 6 x 6 Latin Square design. In four of the experimental diets dried cassava leaves (DCL), ensiled cassava leaves (ECL), dried sweet potato vines (DSPV) and ensiled sweet potato vines (ESPV) were included as the sole protein source. In the other two diets, protein was supplied from a 50: 50 (DM basis) mixture of cassava leaves and sweet potato vines in dried (Mix – D) or ensiled form (Mix –E). The six diets were formulated to contain 120 g CP / kg DM and 13 MJ ME / kg DM. Soybean oil was added to the experiment diets to equalize the calculated content of metabolizable energy. Daily feed allowance was 3 kg 100kg ^–1 of body weight for each individual animal. The daily intakes of DM and dietary components were significantly different among diets (P<0.001). Due to differences in DM intake, the daily intakes of OM, CP, CF and NDF were significantly different among diets (P<0.001). There were no differences (P>0.05) in the coefficients of ileal apparent digestibility (CIAD) of OM among diets but there were significant differences among diets (P<0.05) in the CIAD of DM, CP and NDF. Similarly, there were significant differences in the coefficients of total tract apparent digestibility (CTTAD) of DM, OM, CP, CF and NDF among diets (P<0.05). The CIAD of DM, OM and CP in the ECL, ESPV and Mix –E diets tended to be higher than in the DCL, DSPV and the Mix- D diets. The CTTAD of DM, OM, CP, CF and NDF were higher in the diets, which contained protein from ECL, ESPV and Mix-E than from DCL, DSPV and Mix – D. The CIAD and CTTAD of CP and NDF in the diets which contained protein from the mixture of cassava leaves and sweet potato vine tended to be   higher (P< 0.05) than in the diets that contained protein from cassava leaves or sweet potato vine as the sole protein source in both silage and dry form. There were significantly different coefficients of ileal apparent digestibility (CIAD) of most amino acids (AA) among diets (P<0.05), except for methionine, alanine, glutamic acid and serine. The CIAD of AA in the ECL, ESPV and Mix –E diets tended to be higher than in the DCL, DSPV and the Mix- D diets.  There were no significant differences in CIAD of AA between dried and ensiled cassava leaves. The CIAD of histidine, isoleucine, leucine, lysine, threonine, tyrosine, alanine and aspartic acid in the diets which contained protein from the mixture of cassava leaves and sweet potato vines were higher than in the diets that contained protein from ECL and DCL (P< 0.05). It was concluded that cassava leaves and sweet potato vine have the potential to improve dietary protein and amino acid supply in diets for pigs.

Key words: Amino acids, forages, HCN, mixtures

Introduction

 Vietnam is an agricultural country with a population of 82 million. More than 75% of the population are living in the rural areas and depend on agriculture. Livestock production plays an important role in agricultural production and contributes about 23.0% of total annual agricultural products. The annual growth rate of the pig herd in Vietnam from 2001-2004 was 5.12%, with a total of 27.4 million pigs produced in 2005 (Do Kim Tureen 2005). Farmers in the rural areas of Vietnam account for over 75 % of the total pig production in the country. The main feeds for pigs are rice bran, maize, cassava, sweet potato and some other vegetables. Cassava, maize and rice bran are rich in energy but low in protein and essential amino acids. Commercial feeds, fishmeal, soybean meal and groundnut cake are too expensive to be used by resource–poor farmers.

Cassava (Manihot esculenta Crantz) and Sweet potato (Ipomoea batatas L.) are the second and third most important food crops after rice in terms of total production. Because of their high crude protein (CP) content, cassava leaves and sweet potato leaves have been used as a protein supplement for feeding pigs. Earlier studies have indicated the vegetative plant parts may have nutritional properties that should allow them to be used in diets for pigs. Bui Huy Nhu Phuc (2000) showed that cassava leaves, leucaena leaves and groundnut foliage could be used to improve the dietary protein and amino acid supply to growing pigs under tropical conditions. Many studies have concluded that cassava leaves and sweet potato leaves can be considered as potentially valuable protein sources for pigs, and can replace fish meal, soybean meal and groundnut cake in growing pig diets (Nguyen Thi Hoa Ly et al 2005; Bui Huy Nhu Phuc 2000; Peter et al 2001). The nutritive value as well as the ileal and total tract digestibility of ensiled and dried cassava leaves and sweet potato leaves for growing pigs have recently been reported (Phuc and Lindberg 2001; Hoang Huong Giang et al 2004; Le Van An 2004; Chhay Ty, Preston TR and Ly J 2003). However, no data could be found on mixtures of cassava leaves and sweet potato vines in both silage and dry form for fattening pigs.

This study was conducted to determine the ileal and total tract digestibility in fattening pigs of nutrients, in particular crude protein and amino acids, in cassava leaves and sweet potato vines. Also, the influence of preservation method on the digestibility was evaluated by feeding the cassava leaves and sweet potato vines in ensiled and dried form to fattening pigs.

Materials and methods

The experiment was carried out at the Hue University research farm from May to July 2005.

Animals

Six castrated F1 crossbred  (Mong Cai x Large White) fattening pigs of about 5 months of age, with an average body weight of 60.2 ± 1 kg were used. The pigs were vaccinated against pasteurellosis and hog cholera, and surgically fitted with post-valve T-caecum (PVTC) cannulas to allow collection of ileal digesta.  Pigs were kept individually in 2m² pens and had free access to water from nipple drinkers.

Diets and feeding

Ensiling

The fresh leaves of cassava were collected at the time of root harvest or were collected at 90 days after planting, with subsequent harvests at 60 day intervals, and spread out on the floor some hours for wilting. The leaves were separated from the stems and petioles, chopped into small pieces (2 - 3 cm), mixed with salt (0.5 %) and then ensiled with rice bran at 10 % weight of the wilted cassava leaves. The cassava leaf silage was kept in sealed airtight plastic bags with a capacity of 30 kg, and was stored for 2 months prior to feeding.

Sweet potato vines were harvested at 60 days after planting, with subsequent harvests at 20 day intervals. The vines were chopped into small pieces 2- 3 cm long and spread out on the floor overnight for wilting to reduce the moisture content. Rice bran was used as additive at 10 % of the wilted weight of the leaves and also common salt (NaCl) was added at 0.5 % of the wilted weight of the leaves. The silage also was kept in sealed airtight plastic bags with a capacity of 30 kg, and was stored for 21 days prior to feeding.

Drying

Fresh cassava leaves and sweet potato vines were collected and spread out on concrete outdoors in the sun for 2-3 days. The dried leaves were collected and milled through a 1mm screen, and stored in a dry place.

Experimental design

The 6 experimental diets were fed according to a 6 x 6 Latin Square and the experiment lasted for a total of 72 days. Each of the six experimental periods was 12 days, comprising 5 days of adaptation to each diet followed by 4 days of collection of faeces, one day of collection of ileal digesta, one day of rest and finally a second day of collection of ileal digesta.

  Six different diets were used in the experiment:

·        DCL: Cassava leaves, sun-dried

• ECL: Cassava leaves, ensiled

• DSPV: Sweet potato vines, sun-dried

• ESPV: Sweet potato vines, ensiled 

• MIX-D: Mixture (DCL + DSPV) consisting of 50 % cassava leaves, sun-dried (DCL) and 50% sweet potato vines, sun-dried (DSPV) in dry basis

• MIX-E: Mixture (ECL + ESPV) consisting of 50 % ensiled cassava leaves (ECL) and 50% ensiled sweet potato vines (ESPV) in dry basis

In four of the experimental diets, dried cassava leaves (DCL), ensiled cassava leaves

(ECL), dried sweet potato vines (DSPV) and ensiled sweet potato vines (ESPV) were included as the sole protein source. In the other two diets, protein was supplied from a 50: 50 (DM basis) mixture of cassava leaves and sweet potato vines in dried or ensiled form. The six diets were based on ensiled cassava roots as the energy source (Table 1).

The six diets were formulated to contain 120 g CP/kg DM and 13 MJ ME / kg DM. Soybean oil was added to the experimental diets to equalize the calculated content of metabolizable energy. All diets were supplemented with a standard mixture of vitamins, minerals and trace elements according to requirements given by NRC (1998), added at 5 g kg ^–1. Chromium oxide was used as a digesta flow marker and was added at 5 g kg ^–1 (Table 2).

The feeding level during the collection period was set slightly below the maximum level consumed during the adaptation period. Daily feed allowance was 3 kg 100 kg ^–1 of body weight for each individual animal. The pigs were fed two times per day at 6:00 and 18:00h, with the daily allowance equally divided between the two meals. Food refusals and spillage were recorded, and were used to correct the food intake data.

Digesta and faeces collection and calculations

For the determination of ileal digestibility, 12 digesta samples from each pig were taken during the 2 days of collection in each experimental period. On each day of collection, samples were taken after every 2 h during the 12 h period between the morning and afternoon feeding, giving six samples per day of collection. At each sample collection, digesta were quantitatively collected for 1 h in containers through soft plastic tubing connected to the ileal cannula. The digesta were frequently removed from the tube and container and transferred to a bigger container where they were kept on ice during sampling. The samples were then kept frozen at – 18^oC.

For total tract digestibility, faeces were collected four times per day and kept in a freezer at – 18^oC. Finally, prior to chemical analysis, individual samples of ileal digesta and faeces were thawed and pooled within pig and period. Sub-samples were taken and dried at 60⁰C prior to chemical analysis. The digestibility of the diets at each sampling site was calculated using the indicator technique (Sauer et al 2000) according to the equation:

                 CAD_D  = 1 – (  DC_F/ DC_{D X} I_D/I_F)

Where is CAD_D the coefficient of apparent digestibility of dietary component in the assay diet; DC_F the dietary component concentration in ileal digesta or faeces (%); DCD the dietary component concentration in the assay diet (%); I_D the indicator concentration in the assay diet (%); I_F the indicator concentration in ileal digesta or faeces (%).

Chemical analysis

Food, digesta and faecal samples were dried at 60 ^oC for 24 h and milled through a 1 mm screen before analysis. Nitrogen contents of faeces and digesta was determined on fresh samples, while the other analyses of feed, faeces and digesta were determined on air-dry samples. The chemical composition was determined according to standard methods (AOAC 1990). Chromium oxide in feed, faeces and ileal digesta was determined according to Fenton and Fenton (1979). Amino acids were analyzed according to Spackman et al. (1958) on an ion-exchange column using an HPLC.

Statistical analysis

Analysis of variance was performed according to a 6 x 6 square arrangement, using the General Linear Model of Minitab Statistical Software Version 13 (2000). Tukey pair-wise comparisons were used to determine differences between treatment means at P<0.05.

Results

Intake

The chemical composition of the ingredients is presented in Table 1 and of the experimental diets in Table 2. The CP content of fresh ensiled and dried cassava leaves was higher than in fresh, ensiled and dried sweet potato vines (Table 3). However the CP content of dried cassava leaves (DCL), dried sweet potato vines (DSPV) as well as Mix –D was higher than in ensiled cassava leaves (ECL), ensiled sweet potato vines (ESPV) and Mix – E. There was no difference in CF content between dried and ensiled cassava leaves or dried and ensiled sweet potato vines as well as Mix - D and Mix - E. The NDF content was lower in ECL and DCL than in DSPV and ESPV (Table 1).

The content of amino acids differed between fresh, dried and ensiled cassava leaves. Further, the contents of lysine, histidine, isoleucine, lysine, methionine, phenylalanine and threonine in the ensiled cassava leaves and ensiled sweet potato vines were lower than in fresh or sun-dried cassava leaves and sweet potato vines (Table 3). 

 The effect of ensiling and drying on EAA of cassava leaves and sweet potato vines is shown in Figure1 and Figure 2.

All experimental diets had the same levels of CP and ME. However CF and NDF were lower in the DCL, ECL and Mix – D diets. There were differences in the HCN content among diets, with the highest content of HCN in the ECL diet.

The daily intakes of DM and dietary components were significantly different among diets (P<0.001). Due to differences in DM intake (Table 4) the daily intakes of OM, CP, CF and NDF were significantly different among diets (P<0.001). The lowest daily intake of DM, OM and CP was recorded for the Mix- E diet, while the highest value was found in the DCL diet (P<0.05).

The CF and NDF intakes were higher in the DSPV and ESPV diets compared to ECL and DCL (P<0.001). The DM, CP, CF and NDF intakes were not different between ESPV and Mix –E diets (P>0.05).

Ileal and total tract apparent digestibility of nutrients

There were no differences (P>0.05) in the coefficients of ileal apparent digestibility (CIAD) of OM among diets (Table 5) while there were significant differences between diets (P<0.05) in the CIAD of DM, CP and NDF. Similarly, there were significant differences in the coefficients of total tract apparent digestibility (CTTAD) of DM, OM, CP, CF and NDF among diets (P<0.05, Table 5).

The CIAD of DM, OM and CP in the ECL, ESPV and Mix-E diets tended to be higher than in the DCL, DSPV and the Mix- D diets. The CTTAD of DM, OM, CP, CF and NDF were higher in the diets which contained protein from ECL, ESPV and Mix-E than from DCL, DSPV and Mix – D.  The data in Table 5 also show that the CIAD and CTTAD of CP and NDF in the diets that contained the mixture of protein from cassava leaves and sweet potato vines tended to be   higher  than  in the diets that contained protein from cassava leaves or sweet potato vines as the sole protein source in both silage and dry form.  The CIAD and CTTAD of CP and NDF  were highest  in the ESPV and Mix-E diets and lowest in the DCL diet  (P< 0.05, Figure 3 ). However, there were no differences in the CIAD and CTTAD of CP and NDF between Mix-D and Mix-E diets (P>0.05). 

There were significant differences in the coefficients of ileal apparent digestibility (CIAD) of most amino acids (P<0.05, Table 6), except for methionine, glutamic acid, glycine and serine. Similarly for CP, the coefficients of ileal apparent digestibility of AA in the ECL, ESPV and Mix-E diets tended to be higher than in the DCL, DSPV and the Mix-D diets. However, there were no differences in the CIAD of AA between the dried and ensiled cassava leaf diets (P>0.05).The CIAD for the Mix-D and Mix-E diet  also tended to be higher than in the  dried cassava leaves (DCL) or dried sweet potato vines (DSPV) diets, as well as the ensiled cassava leaves (ECL) and ensiled sweet potato vines (ESPV) diet (Figure 4).  The CIAD of histidine, isoleucine, leucine, lysine, threonine, tyrosine, alanine and aspartic acid in the diets which contained the mixture of protein from cassava leaves and sweet potato vines were  higher  than  in the diets that contained protein from  ensiled cassava leaves and dried cassava leaves  (P< 0.05).The highest  CIAD for lysine was in the Mix-E diet (P<0.05, Table 6). 

Discussion

The HCN content of the ECL (80mg/kg DM) and DCL (59.8 mg/kg DM) diets was higher than in the other treatments, but there were no indications of cyanide toxicity on any of the diets in the present study. Du Thanh Hang and Preston (2005) reported that cassava leaves were readily consumed, providing 38 % of the dietary DM and over 70 % of the dietary protein in pigs from 27 to 31 g/kg live weight with no effect on performance.  Chhay Ty et al (2003) did not find any symptoms of intoxication in pigs when using ensiled cassava leaves, intakes of which averaged 45.6 g DM/ kg live weight. Recently Chahay Ty and Preston (2005) reported that fresh cassava leaves could be fed at 41 % of the diet DM with no apparent signs of toxicity. A possible reason that has been suggested for this lack of effect is that the enzymes responsible for the release of HCN from the cyanogenic glucosides are inactivated at the low pH in the pig stomach.

The CP and CF contents of the cassava leaves and sweet potato vines used in the present study were in good agreement with early reports in the literature (Eggum 1970; Rawindran 1990; Phuc 2000; Wanapat 2001; Peter et al 2001; Woolfe 1992; NIAH 2001). The contents of CP in ensiled cassava leaves (ECL) and ensiled sweet potato vines (ESPV) were lower than in the dried material, in agreement with previous studies on cassava leaves (Phuc and Lindberg 2001).  

The sun drying of cassava leaves and sweet potato vines did not affect the total CP, although ensiling resulted in a reduction of the CP contents. The CP content in dried and ensiled sweet potato vines was lower than in cassava leaves. Similarly, the content of most amino acids in sweet potato vines was lower than in cassava leaves, due to the lower CP and higher CF content in sweet potato vines.

The content of amino acids was affected by the ensiling process (reduced content of arginine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tyrosine and valine) and also by sun drying (reduced content of leucine, lysine, phenylalanine). This is in agreement with a study where the contents of lysine, histidine, and methionine  were  found  to be lower in ensiled  than in sun–dried cassava leaves (Phuc and Lindberg 2001).  This  is also in agreement with Le Van An and Lindberg (2003), who reported  that  the content of amino acids in ensiled sweet leaves was lower than in the dried material. The sun drying of cassava leaves and sweet potato vines did not affect the total content of EAA, although ensiling resulted in a reduction of the CP content compared with the fresh material (Table 3). The reduction in the CP content in ECL and ESPV could be due to microbial decomposition to ammonia during the ensiling process. The change in AA content in ECL and ESPV can be explained by microbial activities during the ensiling process. However, the reduced content of AA in DCL and DSPV can be due to the action of endogenous plant enzymes during the initial phase of the process.

The daily intakes of DM were lower (4.5 - 8.2 %) for the diets with ensiled materials than for diets with dried materials. The daily DM feed allowance was similar (3 kg 100 kg –1), but feed intake was affected by the preservation method. The reduction in water content by sun drying, and the subsequent reduction in particle size by milling, increased feed intake.

The negative effects of fiber on diet digestibility and pig growth are well documented, and it is generally accepted that diets containing more than 7-10 % of fiber result in decreased growth rates (Kass et al 1980). In the present study, the CF was from 7.4 -10.2 % in all treatments, levels that were acceptable for F1 fattening pigs.

The CIAD of DM, CP, NDF and most of the AA in the Mix - D and Mix - E diets also tended to be higher than in the other diets. There were no significant differences in CIAD and CTTAD of DM, OM and NDF between ensiled and dried cassava leaf diets. However, the CIAD and CTTAD of DM, OM, CP and NDF for the ECL diet tended to be higher than in the DCL diet. The CIAD and CTTAD of CP in the ECL diet were significantly higher than in the DCL diet (P<0.05). The were no differences in CIAD and CTTAD of dietary components between sweet potato vine diets except for NDF, but all parameters in the ESPV diet tended to be higher than in the DSPV diet. These results are in agreement with previous studies by Phuc et al (1997) and Hoang Huong Giang et al (2004), who concluded that the ensiled leaf was digested better than the sun-dried leaf in growing pigs. This suggests that the preservation method used affected the digestibility of the material. However, a later study by Phuc (2000) compared the digestibility between ensiled and sun-dried SPV in rats and found no difference between the two processing methods. According to Le Van An (2004) there are no major differences in nutritive value between fresh, sun-dried or ensiled sweet potato leaves and there were no differences in CIAD and CTTAD of dietary components between dried and ensiled sweet potato leaves diets.

The data in Table 5 show that the CIAD of the CP in ECL, DCL, ESPV, DSPV, Mix- D and Mix - E diets ranged from 44 to 49 % and CTTAD of CP in these treatments ranged from 52 to 58 %, which is in agreement with Phuc and Lindberg (2000), Chhay et al (2005), Hoang Huong Giang et al (2004) and Dominguez and Ly (1997). Hoang Huong Giang et al (2004) reported that the apparent digestibility of CP of ESPV and DSPV was 47.6 and 47.7 %, respectively. Dominguez and Ly (1997) reported that in vivo total CP digestibility of SPV meal was 54%. However, compared with sweet potato leaves studied in growing pigs, the CIAD of the CP of cassava leaves and SPV showed a lower CIAD and CTTAD of CP than sweet potato leaves. Le Van An (2004) reported that the CIAD of CP in ensiled and dried sweet potato leaves was 74 % and the CTTAD of CP was 77 and 75 % for ensiled sweet potato leaves and dried sweet potato leaves, respectively. This can be explained by a higher in fibre content in cassava leaves and sweet potato vines. The CF in sweet potato leaves (SPL) had a mean value of 11 %, while the CF in SPV was 20.7 % (Woolfe 1992).

The CIAD of the NDF in ECL, DCL, ESPV, DSPV, Mix-D and Mix-E diets were low, ranging from 22 to 38 %, and the CTTAD of CF and NDF in these treatments ranged from 38 to 47 % and 31 to 40 %, respectively which is also in good agreement with Phuc and Lindberg (2000) and Hoang Huong Giang et al (2004). 

The CIAD of histidine, isoleucine, leucine, lysine, threonine, tyrosine, alanine and aspartic acid in the diets which contained the mixture of protein from cassava leaves and sweet potato vines were higher than in the diets that contained protein from ensiled cassava leaves and dried cassava leaves (P< 0.05). The CIAD of CP and AA were slightly higher in the diets based on ensiled cassava root supplemented with a mixture of cassava leaves and sweet potato vines than in the diet with only cassava leaves or sweet potato vines. Further, the CIAD of aginine, isoleucine, leucine, threonine, tyrosine and alanine in sweet potato vine were significantly higher in cassava leaves in both silage and dry form. The CIAD of lysine in the Mix-E diet was the highest among the diets (P<0.05). A likely explanation for the increased digestibility of protein and amino acids could be that the mixed materials have an amino acid balance closer to that of the “ideal” protein for pigs. Chhay Ty and Preston (2005) reported that total DM intake, digestibility of DM, OM, CP, CF and N retention in a diet containing a mixture between fresh water spinach and cassava leaves were higher than in a diet with fresh cassava leaves alone. Comparing the two treatments, the digestibility of CP was 72.2 and 60.2 % for the mixture and fresh cassava leaves alone diets, respectively.

When cassava leaves or sweet potato vines were included in the diets, the CIAD and CTTAD of OM and CP were lower compared to some earlier reports, in which the protein was from fish meal, shrimp by product (Ngoan 2000) and soybean (Phuc 2000). This can be explained by the increase in fibre content of the diets and an imbalance of essential amino acids in cassava leaves and sweet potato vines. Fernandez and Jorgensen (1986)(cited by Ninh Thi Len 2001) determined the effect of fibre content in the diet on digestibility and absorption of nutrients in pigs, and concluded that the digestion of CP generally is depressed by around 2 % units per 1 % increase of crude fibre. Dominguez and Ly (1997) found that the total digestibility of DM, CF, NDF and CP decreased with increases of SPV meal (100 and 200 g SPV meal /kg feed) in diets.

Hutchinson (1943) and Purcell (1978) (cited by Tor-Abbidye et al 1990) demonstrated that 50 % of the N in sweet potato is non- protein in origin, and non- protein nitrogen is digested poorly by growing pigs (Ninh Thi Len 2001). In addition, the presence of trypsin inhibitors and proteinase inhibitors in SPV makes proteins unavailable (Zhanga et al 2001) and decreases CP digestibility. The presence of cyanogenic glucosides and tannins (from 30 to 50g/kg DM, according to Ravindran 1993) in cassava leaves have adverse effects on growth and have the capability to lower protein and amino acid digestibility.    

 The CIAD and CTTAD of CP and amino acids of cassava leaves and SPV were low in this study. This also can be explained by the high fibre content of the diets and was in agreement with studies on the inclusion of tropical foliages (Phuc and Lindberg 2000) and other fibre-rich feedstuffs in diets for growing pigs. A likely explanation for the reduced digestibility of protein and amino acids in fibre-rich diets is that amino acids are bound to or encapsulated in the cell wall, and that fibre will stimulate the secretion of endogenous nitrogen.

Conclusions

·        The present data suggests that cassava leaves and sweet potato vines have the potential to improve dietary protein and amino acid supply in diets for pigs.

·        The most suitable strategy for preserving cassava leaves and sweet potato vines can be decided from a consideration of the prevailing climatic conditions.

·        Possibly, the CP and AA content can be slightly lower in the ensiled materials than in dried materials.

·        Also, it should be noted that the content of EAA in cassava leaves and sweet potato vines can be affected by the preservation process, which needs to be accounted for in diet formulation.

·        Using a mixture of cassava leaves and sweet potato vines in diets of fattening pigs could improve the digestibility of protein and amino acids.

Acknowledgements

This study was carried out at the Department of Animal Nutrition and Biochemistry and at the research farm of Hue University of Agriculture and Forestry (HUAF) with financial support from the Swedish International Development Cooperation Agency, Department for Research Co-operation (Sida-SAREC). We gratefully acknowledge the MEKARN Sida/SAREC program for supporting this research. The authors would like to thank Prof. Dr. R. Brian Ogle and Prof. Dr. Thomas R. Preston for their help and valuable advice. We also would like to express our gratitude to Dr. Doan Thi Khang of the National Institute of Animal Husbandry in Hanoi for performing the chemical and amino acid analyses.

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