Digestibility of dried and ensiled sweet potato roots and vines and their effect on the performance and economic efficiency of F1 crossbred fattening pigs

Hoang Huong Giang, Le Viet Ly and Brian Ogle*

National Institute of Animal Husbandry, Ha Noi, Viet Nam
gianghoang97@yahoo.com
*Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences
Uppsala, Sweden

Abstract

Two experiments were carried out to evaluate the digestibility of sweet potato vine (SPV) and root (SPR) silage and meal, and to determine the effects on performance of including a mixture of SPV and SPR in both ensiled and dried form in diets for F1 crossbred fattening pigs (Mongcai sow x Yorkshire boar). The digestibility trial (on-station) was done according to a 3*3 Latin Square design with 3 treatments on 3 castrated F1 crossbred fattening pigs with an initial mean live weight of 35 kg. Treatment SP0 was a basal diet without sweet potato, and was replaced by sweet potato meal (SPM) or sweet potato silage (SPS) at a level of 50% (DM basis) to give treatments SPM50 and SPS50. The feeding trial (on-farm) was conducted with a total of 40 F1 crossbred pigs (20 females and 20 males) of 15 kg average initial live weight at 4 farms as replicates. At each farm, 10 pigs were kept in 5 pens (1 male and 1 female in each pen), with each pen representing a treatment. The pigs were fed diets consisting of mixtures of the basal diet (SP0) and the SPM or SPS (40% or 60% SPM, and 40% or 60% SPS) to give treatments SP0, SPM40, SPM60, SPS40 and SPS60, respectively. The SPM and SPS in both experiments were mixtures of 50% SPV and 50% SPR (DM basis).

The digestibility of DM, CP, CF and NDF in both SPM50 and SPS50 diets was about 90, 77, 92 and 89%, respectively, of the values recorded for treatment SP0. There were no differences due to method of processing the sweet potatoes. Pigs on the control diet (SP0) performed better than on all other treatments as regards average daily gain and feed conversion ratio.  Diets SPM40 and SPM60 resulted in higher average daily gain and better feed conversion ratio compared to diets SPM60 and SPS60. There were no differences between  SPM40 and SPS40. The back fat thickness of the pigs ranged from 1.99 to 2.28 cm and was closely correlated with final live weight and rate of live weight gain.  The labour cost for processing was higher for SPM than for SPS.

Key words:  Conversion, digestibility, feed intake, growth, pigs, sweet potato meal, sweet potato silage.

Introduction

Animal feed costs are the largest component of total production costs, and so reducing feed expenses is very important, for example by using local feed resources. Pig feeds are no exception and, furthermore, pigs require high protein levels in their diets, which makes the feed cost even higher compared to ruminants. Sweet potato is an ideal livestock feed because the roots are a source of energy and the leaves provide protein, while both can be used in fresh and dried form or fermented into silage (Woolf 1992). Sweet potato production is closely linked to pig production in Vietnam (Bottema 1992). In the context of the sweet potato-pig system, pigs have several important functions for rural households: they generate much needed cash income through market sales; they provide manure for maintaining and improving soil fertility; and they convert the generally undesirable and low-value sweet potato into highly desired foods or marketable commodities (Peters 1998). Sweet potato is a tropical and sub-tropical plant that can be harvested twice per year in the summer and spring-winter seasons in North Vietnam. In summer, sweet potato roots (SPR) and sweet potato vines (SPV) are usually cut into small pieces by hand and dried in the sun. This practice is the farmer's way of preserving excess sweet potato, but the dried vines have not been used much as feed. Koh (1976) (cited by Peters 1998) reported that the dried chips constitute a more valuable feed, due to their lower cost and the higher daily weight gains achieved than for fresh roots. Another study showed that it is possible to use sweet potato chips in heavy pig production as a substitute for maize meal at levels of up to 40% (Manfredini et al 1993). In the wet season, SPR and SPV can be preserved as silage for later use as pig feed. Nguyen Thi Tinh et al (2000) concluded that SPV ensiled with chicken manure can be fed to pigs after 21 days of ensiling, and the pigs had higher weight gains compared to pigs fed fresh SPV.

The aim of the present research was to determine the optimum level of SPR and SPV in both silage and dried form in diets for F1 crossbred fattening pigs, as well as evaluating the economic efficiency of using these products in different seasons under small-farm conditions in North Vietnam.

Experiment 1.  Evaluation of the digestibility of mixtures of sweet potato vines and roots in both silage and dry form by F1 crossbred fattening pigs

Materials and methods

The experiment was carried out at the National Institute of Animal Husbandry, Hanoi, Vietnam, from April to June 2002

Experimental animals and design

Three castrated F1 crossbred (Mongcai sow x Yorkshire boar) fattening pigs with mean body weight of 35 kg were used according to a 3*3 Latin Square design with 3 treatments. The pigs were placed in individual metabolism cages containing steel feeders and automatic nipple drinkers. The experiment consisted of three 12-day periods. In each period, the pigs were fed ad-libitum in the adaptation period (7 days) to determine feed intake, and restricted to 80% of this intake in the collection period (5 days).

Experimental diets

Mixtures of 50% SPR and 50% SPV (DM basis) in both silage (SPS) and meal (SPM) form were used. SPS was offered to the pigs after 3 weeks of ensiling. SPM was prepared as follows: after harvesting, the vines were immediately chopped into small pieces (1-2 cm), and the tubers were chopped into small, thin pieces (3-4 mm) and dried until reaching a moisture content of 13-14%. After drying the SPR and SPV were milled and used as ingredients in diets for the pigs.

Three different diets were used in the experiment. Treatment SP0 was a basal diet, and consisted of maize meal, rice bran, soybean meal, cassava meal and fish meal. The analysed chemical composition is shown in Table 1. The nutrient content of the basal diet was formulated according to the recommendations of NIAH (2001) for F1 crossbred pigs. Treatment SPM50 consisted of 50% of the basal diet and 50% of SPM (DM basis), while treatment SPS50 consisted of 50% of the basal diet and 50% of SPS (DM basis) (Table 2).

Measurements

During the collection periods, weights of feed offered, feed refused, faeces and urine were recorded daily. The feed was offered at 08.00h and 15.00h, and refusals were weighed before every feeding. The faeces and urine were collected twice per day at 09.00 h and 16.00 h (one hour after feeding). Samples of 10% of faeces and 10% of urine were taken from each pig in each collection. The samples of faeces were put into sealed plastic bags and the samples of urine were put into capped plastic boxes and stored in a freezer at -18^oC until analysis. Urine was kept at a pH below 4 to preserve the ammonia by using 50 ml 5% H₂SO₄ for every collection. After each 5-day collection period, the faeces and urine samples from each pig were mixed together and representative samples were taken for analysis.

Samples of feed and faeces were analysed for DM, CP, NDF, and ash to determine digestibility, and urine samples were analysed for nitrogen to determine nitrogen retention. Based on digestibility of the basal diet (SPO = B), the digestibility of the mixed diet (SP50 = C), the digestibility of SP100  (A) was calculated as follows:

C = 0.5A + 0.5B
or
C = 0.5(A + B)
Therefore:
A = C/0.5 - B

Chemical analyses

Samples were analysed for DM, N, NDF, CF and ash using standard procedures (AOAC 1990).

Statistical analysis

The data were analysed by ANOVA using the General Linear Model procedure in the MINITAB 12.21 software (1998). Tukey pair-wise comparisons were used to determine the differences between treatments with confidence level of 95.0%.

Results

Experimental diets

After substituting 50% of the basal diet by SPM or SPS, to give diets SPM50 and SPS50, these were found to be lower metabolisable energy (ME), crude protein (CP), lysine, methionine and ash, and higher in crude fiber (CF) and NDF (Tables 1 and 2).

Apparent digestibility and nitrogen retention

Apparent digestibility of all nutrients was reduced when SPM or SPS replaced 50% of the DM of the control diet (SPO) (Table 3). There were no differences between SPS50 and SPM50 except for OM which was higher for SPS50. Differences for N retention were similar to those for digestibility. Nitrogen retention was much higher for treatment SP0 (14.9 g/day) compared to SPM50 (9.9 g/day) and SPS50 (7.5 g/day) (P<0.05), but was not significantly different between treatment SPM50 and SPS50 (P>0.05).

The calculated (by difference) values for digestibility of SPM and SPS (Table 4) indicated a very low crude protein digestibility, which was only about half of the crude protein digestibility of the basal diet. There were no differences between SPM and SPS.

Discussion

In the present study, the basal diet (SP0) was formulated according to the recommendations of NIAH (2001) for requirements of cross-bred F1 fattening pigs. Replacing the basal diet with sweet potato silage and sweet potato meal at 50% of the dry matter to give treatments SPS50 and SPM50 resulted in changes in the chemical composition of SPS50 and SPM50. The fibre content was much higher than in the basal diet, which made the digestibility of nutrients of diets SPS50 and SPM50 much lower than that of SP0. The fibre fraction of a food has the greatest influence on its digestibility, and both the amount and chemical composition of the fibre are important (McDonald et al 1995). The inclusion of sweet potato foliage thus lowered the digestibility of all nutrients due to the increases in fibre content of the diets (Dominguez 1992; Close 1993). Bardon and Frioramonti (1983) and Bach Knudesen and Hansen (1991) (cited by Dominguez and Ly 1997) pointed out that the bulking characteristics of digesta and faeces are usually enhanced by dietary fibre in the pig, which probably resulted in higher rate of feed passage through the gastrointestinal tract that would have reduced nutrient digestibilities of the SPM50 and SPS50 diets. Fernandez and Jorgensen (1986) (cited by 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) also found that the total digestibility of DM, CF and NDF decreased with increases of SPV meal (100 and 200 g SPV meal/kg feed) in the diets. However, the digestibilities of crude fibre and NDF in SPS50 and SPM50 were 92 to 95% and about 89% of the basal diet, respectively, and thus were not very different, while the digestibility of crude protein in SPS50 and SPM50 was about 77% of that of the basal diet. These results are in agreement with Dominguez (1992), who summarized the results from previous studies on the digestibility of the nutrients in sweet potatoes for pigs, and concluded that the digestibility of energy and fibre in sweet potato diets was high, though the nitrogen digestibility was somewhat lower because of the poor digestibility of sweet potato protein, even when cooked. Tor-Abbidye et al (1990) showed that there was no significant difference in DM and GE digestibility, but a significant decrease of CP digestibility, when replacing maize by sweet potato by-product meal in pig diets.

Nitrogen retention depends on the amount of digested nitrogen as well as the protein quality of the diet, so the presence of sweet potato in the diets SPM50 and SPS50 would have made nitrogen retention much lower than in the basal diet due to the lower nitrogen digestibility of SPM and SPS, and due to the fact that 50% of the N in sweet potato is non-protein nitrogen (Hutchinson 1943 and Purcell 1978 cited by Tor-Abbidye et al 1990).

The low value for the digestibility (47%) of the crude protein in SPM and SPS mixtures, calculated by difference, was probably due to the low digestibility of the crude protein in SP vines, as these were the major CP source (81% of the total) in the SPM and SPS mixtures. The low CP digestibility of SPV can be explained by the findings of previous studies. For example Brown and Chavalimu (1985) found that a substantial increase in the amount of unavailable nitrogenous substances in a diet that contained SPV meal could have been a direct effect of drying the sweet potato foliage; Dominguez and Ly (1997) reported that in vivo total CP digestibility of SPV meal was rather low (54%) and 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 pigs (Len 2001). In addition, the presence of trypsin inhibitors and proteinase inhibitors in SPR also makes proteins unavailable (Zhanga et al 2001), and the poorer nitrogen digestibility was probably due in part to antitryptic factors, which although reduced, were not entirely eliminated by the ensiling method of conservation (Lin et al 1988). In general, crude protein in sweet potato is digested poorly by growing pigs (Evan 1939 cited by Tor-Abbidye et al 1990). The higher organic matter digestibility for SPS compared with SPM is in agreement with a study by Bui Nhu Huy Phuc et al (1997) that compared the digestibility of sun-dried cassava leaf with ensiled cassava leaf in growing pigs, and concluded that the ensiled leaf was digested better than the sun-dried leaf. However, a later study by Bui Nhu Huy Phuc (2000) compared the digestibility of crude protein and organic matter between ensiled and sun-dried SPV in rats and found no difference between the two processing methods.
 

Experiment 2. Effect of dried and ensiled sweet potato vines and roots on production performance and economic efficiency of F1 crossbred fattening pigs.

Materials and methods

The experiment was carried out in Cat Que village, Hoai Duc district, Ha Tay province, from June to November 2002.

Treatments, animals and design

There were five treatments comprising combinations (DM basis) of the basal diet (Table 5) and sweet potato meal (SPM), or sweet potato silage (SPS), prepared according to the procedures described in Experiment 1:

• SP0, the basal diet
• SPM40: basal diet 60% and SPM 40%
• SPM60: basal diet 40% and SPM 60%
• SPS40: basal diet 60% and SPS 40%
• SPS60: basal diet 40% and SPS 60%

Forty crossbred fattening pigs (F1: Mongcai local sow x Yorkshire boar) of 15 kg average initial weight were allocated to 4 households, with 10 pigs in each. Each household was a replicate. Animals in each household were kept in 5 pens (1 male and 1 female pig in each pen), with each pen as a treatment. All pigs were castrated and vaccinated against Hog Cholera, Paratyphoid and Erysipelas. The pigs were adapted to the experimental diets for 2 weeks then the data were recorded.

Measurements

During the experimental period the pigs were fed semi ad-libitum twice per day, at 8.00h and 16.00h, which is the normal practice of the farmers in the village. The semi ad-libitum system was as follows: the amount of feed remaining about 4 hours after every feeding was recorded, then in the next meal, the amount of feed offered would be adjusted by this amount, so that the feed refused after around 4 hours would be consumed before the next feeding. Feed offered was weighed and recorded daily for calculation of feed intake. The pigs were weighed in the morning before feeding at the beginning and at the end of each period to determine live weight gain and feed conversion ratio. Back fat thickness of all pigs was measured at the end of the study using a "lean meter" based on the principle of ultra sound. Feed and labour costs were also calculated to compare economic efficiency between treatments.

Statistical analysis

The data were analysed by ANOVA using the General Linear Model procedure in MINITAB 12.21 program (1998). Tukey pairwise comparisons were used to determine the differences between treatments with confidence level 95.0%.
 

Results

Ingredient and chemical composition of the diets

The sweet potato meal and silage (50% SPV and 50% SPR DM basis) were higher in fibre and lower in CP, lysine and methionine, than the basal diet (SP0) (Tables 5 and 6).

Feed and nutrient intakes

Daily DM intake was the highest in treatment SP0 and tended to decrease as sweet potato levels increased (Table 7). There were no differences due to method of processing the sweet potato (SPM and SPS).

Pig performance

Pigs fed the basal diet (SP0) had the highest final weight and ADG and the best FCR (Table 8). There were indications that, at the highest level of inclusion, diets with sweet potato meal were superior to those with sweet potato silage.  The back fat thickness ranged from 1.99 to 2.28 cm and was closely correlated with final live weight (r=0.95) and live weight gain (r=0.96).

Economical analysis

Feed cost, diet cost and economic analysis for the experimental treatments are shown in Tables 9, 10 and 11.

Feed cost per unit increase in live weight was less on the diets containing sweet potato meal or silage than on the control diet (Table 11). The cost for processing SPM, including labour and cost of grinding materials, was higher than for SPS (150 VND vs. 100 VND/kg DM).
 

Discussion

Feed and nutrient intake

The reduced DM intake on the treatments with sweet potato meal or silage could have been due to the bulkiness as well as the high fibre content of these feeds. It can be calculated that if the pigs on the SPS treatments had consumed the same amount of DM as the pigs on the basal diet (1.19 kg DM/day in the 15-50 kg period and 2.16 kg DM/day in the 50-80 kg period), they would have consumed actual (as fed) amounts of 4.67 and 8.47 kg of the SPS60 diet in the first and the second periods, respectively, which are very large amounts. The high water-holding capacity of fibre in SPV meal could at least partially account for the increase in the bulking characteristics of the digesta (Metz 1985, cited by Dominguez and Ly 1997).   These results are in agreement with those of  Kyriazakis and Emmans (1995),who reported that the voluntary feed intake of pigs is largely governed by the bulkiness of the diet. Ly (2002) reported that dry matter intake decreased in treatments containing cassava foliage silage due to its bulkiness. Dominguez (1990) showed that the use of fresh SP foliage for pigs, substituting 25 and 50% of soybean meal as a protein source in sweet potato roots-soybean diets, also decreased the intake of dry matter.

Cheeke et al (1980) and Rosales et al (1993) ( Bui Nhu Huy Phuc 2000) reported that other major problems which preclude the use of some plants as protein sources for monogastrics are their limited palatability and high levels of fibre, which may limit the feed intake and availability of nutrients.  For example, Marrero (1975) (cited by Dominguez 1992) found that feed intake decreased when raw sweet potato replaced cereals in the diet for pigs. Garcia et al (1999) evaluated effects of inclusion of four levels of dehydrated sweet potato foliage and roots on the performance of growing pigs and concluded that the feed consumption declined as dehydrated sweet potato foliage increased from 15% to 45%  in the diets. Bui Nhu Huy Phuc (2000) pointed out that the feed intake of rats was lower when the diets included SPV silage.

Pig performance

The poorer responses in ADG and FCR when sweeet potato meal or silage was included in the diet can be related to the lower digestibility of nutrients as shown clearly in Experiment 1. Fuller and Chamberlain (1982) (cited by Dominguez 1992) pointed out that the amino acid contents of SPR and SPV were somewhat deficient in total sulphur amino acids and lysine compared to the ideal protein. In addition, replacing the basal diet with SPM or SPS was partly at the expense of  maize and, according to Wu (1980, cited by Yeh 1982),  the digestible and net energy of sweet potato root chips are only 91 and 79%, respectively of those of maize. Manfredini et al (1993) concluded that the use of sweet potato root chips in heavy pig production, as a substitute for maize meal up to the 40% level, decreased daily gain and feed efficiency. Tor-Agbidye et al (1990) concluded that pigs fed sweet potato by-product meal and soybean meal grew more slowly and had poorer feed efficiency than those fed a maize-soybean meal diet, and suggested that this probably was due to the low digestible energy, methionine and threonine contents of the experimental diets.

Dominguez (1990) summarized previous studies with sweet potato root chips and concluded that half the maize could be substituted (about 40% sweet potato root chips in the diet) without affecting pig growth rate and feed conversion. There is support for this recommendation on the basis of the similarity in performance of pigs fed the basal diet, and the diets with 40% of SPM or 40% SPS during Period 2 (50 to 70 kg live weight).  However, this result is confounded with the possible effects of adaptation to the high fibre levels, as a consequence of exposure to the SP diets in Period 1.

The data for back fat thickness mainly reflected differences in rate of live weight gain and final live weight, criteria that resulted from the levels of  sweet potato meal or silage in the diet.

Economical analysis

Improvement of the economic efficiency, as well as making use of locally available feed resources for fattening pigs, were the main objectives of the present study. Although the sweet potato diets gave lower growth rate and poorer FCR compared to the basal diet, the costs of feed per kg weight gain were lower, especially for treatments SPM40 and SPS40. Processing SPM was more expensive than SPS, because after drying SPV and SPR had to be ground to make SPM. Peters et al (2001) also concluded that SPS had three advantages over SPM: improved pig growth rate, reduced cost per per kg of weight gain, and saving in labor and fuel for cooking.
 

Conclusions

• There was no effect of processing method (ensiling and sun-drying) on the digestibility of nutrients in sweet potato (combined roots and vines) for fattening pigs but the former was less expensive
  
  
• Pig performance (weight gain and feed conversion) was negatively affected by feeding sweet potato meal or silage as partial replacement for maize and soybean/fish meal, but feed costs per unit live weight gain were reduced.
  
  
• Ensiling both sweet potato vines and roots for later inclusion in diets for fattening pigs is the most suitable preservation method in the rainy season, while sun-drying is more suitable for the summer dry season.

Acknowledgements

The authors are very grateful to the Swedish International Development Authority (Sida/SAREC) and the Swedish University of Agricultural Sciences, Department of Animal Nutrition and Management, for their support of this study. They would like to thank the National Institute of Animal Husbandry, Hanoi, Vietnam, for allowing and helping them to carry out the digestibility trial and the Department of Feed Analysis of the National Institute of Animal Husbandry for analysis of samples.Special thanks are due to the farmers in Cat que village, Hoai duc district, Hatay province namely: Nguyen thi Ha, Tran thi Chi, Nguyen thi Hue and Tran van Mau for allowing us to conduct the feeding trial and to colleagues Mrs. Len, Mr. Kien and Mr. Linh for their help. .This paper is based on research submitted by the senior author to the Swedish University of Agricultural Sciences in partial fulfillment of the requirements for the MSc degree in Tropical Livestock Systems.

References

AOAC  1990 Official Methods of Analysis (15^th edition). Association of Official Agricultural Chemists. Washington, DC 1.

Bottema T 1992 Rapid market appraisal, issues and experience with sweet potato in Viet Nam. In G.Scott, S.Wiersema and P.I. Ferguson, editors. Product development for root and tuber crops. Volume 1-Asia. Lima, International Potato Center.

Brown D L and Chavalimu E 1985 Effect of ensiling or drying on five forage species in Western Kenya: Zea mays (maize stover), Pennisetum purpureum (Pakistan napier grass), Pennisetum sp. (Bana grass), Ipomoea batatas (sweet potato vines) and Cajanus cajan (pigeon pea leaves). Animal Feed Science and Technology 13: 1-6.

Bui Nhu Huy Phuc 2000 Tropical forages for growing pigs. Digestion and nutritive value. Doctoral thesis. Swedish University of Agricultural Sciences. Uppsala, 2000.

Bui Nhu Huy Phuc, Ogle R B, Lindberg J E and Preston T R 1997 The nutritive value of sun-died and ensiled cassava leaves for growing pigs. In: National seminar-workshop "Sustainable livestock production on local feed resources". Agricultural publishing house. Ho chi minh city, 1997. pp: 95-102

Close W 1993 Fibrous diets for pigs. Animal Production in Developing Countries (Editors: M Gill, E Owen, G E Pollot and T L J Lawrence) British Society of Animal Production Occasional Publication No. 16, pp 107-117.

Dominguez P L 1990 Ipomoea batatas. Sweet potato. Animal Feed Resources Information System. http://www.fao.org/ag/AGA/AGAP/FRG/afris/Data/542.HTM

Dominguez P L 1992 Feeding of sweet potato in monogastrics. In: Roots, tubers, plantains and bananas in animal feeding (Editors: David Machin and Solveig Nyvold). Animal Production and Health paper No 95, FAO: Rome, 1992. pp. 217-233. http://www.fao.org/ag/aga/agap/frg/AHPP95/95-217.pdf

Dominguez P L and Ly J 1997 An approach to the nutritional value for pigs of sweet potato vines (Ipomoea batatas (l.) lam). Livestock Research for Rural Development. Volume 9 Number 2 http://www.cipav.org.co/lrrd/lrrd9/2/ly92.htm

Garcia C, Gonzalez C, Diaz Ivonne and Vecchionacce H 1999 Features of behavior in pigs fed with root and foliage of Batata (Ipomoea batatas L.). http://bibliovet.veter.ucv.ve/Revistafcv/4003/vo;4003.htm

Kyriazakis I and Emmans G C 1995 The voluntary feed intake of pigs given feeds based on wheat bran, dried citrus pulp and rass meal, in relation to measurements of food bulk. British Journal of Nutrition 73: 191-207

Lin Y H, HuangT C and Huang C 1988 Quality improvement of sweet potato (Ipomoea batatas L. Lam.) roots as feed by ensiling. The British Journal of Nutrition, volume 60, Issue 1, July 1988. pp 173-184.

Ly J 2002 The effect of methionine on digestion indices and N balance of young Mong Cai pigs fed high levels of ensiled cassava leaves. Livestock Research for Rural Development 14 (6) 2002. http://www.cipav.org.co/lrrd/lrrd14/6/ly146.htm

Manfredini M, Badiani A, Nanni N and Chizzolini R 1993 Sweet potato chips in heavy pig production. Livestock Production Science, 35 (1993), pp. 329-340

McDonald P, Edwards RA, Greenhalgh J F D and Morgan C A (Editors) 1995 Animal Nutrition. Fifth edition. Longman Scientific and Technical, Longman Group Limited, Longman house, Burn Mill, Harlow, Essex CM20, 2JE, England.

MINITAB 1998 GLM. In Minitab Reference Manual Release 12. 1998.

NIAH 2001 Composition and nutritive value of animal feeds in Vietnam. National Institute of Animal Husbandry.  Agricultural Publishing House, Hanoi.

Ninh Thi Len 2001 Evaluation of chicken manure and cassava residue as feed for fattening F1 pigs under village conditions in north Vietnam. M.Sc. Thesis, Swedish University of Agricultural Sciences, Department of Animal Nutrition and Management, Uppsala, Sweden.

Nguyen Thi Tinh, Tran Phung Thanh Thuy, Pham Ngoc Thach and Peters D 2000 Ensiling sweet potato vines as feed to fattening pigs. Information of Science and Technologies on Animal production, No 4/2000. (In Vietnamese language). Hanoi Agricultural Publishing House, Hanoi 2001. pp 18-30.

Nguyen Thi Tinh, Tran Phung Thanh Thuy, Pham Ngoc Thach and Peters D 2001 Processing, storing sweet potato root by fermentation and using the fermented feed to fatten pigs. Information of Science and Technologies on Animal production, No 2/2001. (In Vietnamese language). Hanoi Agricultural Publishing House, Hanoi 2001. pp 18-30.

Peters D 1998 Improving small-scale pig production in northern Vietnam. World Animal Review. FAO. No 91, 1998/2. pp. 2-12 http://www.fao.org/ag/AGa/AGAP/FRG/FEEDback/War/W9980T/w9980e02.htm#TopOfPage

Peters D,  Nguyen Thi Tinh, Thai Thi Minh, Phan Huu Ton, Nguyen The Yen and Mai Thach Hoanh  2001 Pig Feed Improvement through Enhanced Use of Sweet Potato Roots and Vines in Northern and Central Vietnam. In: International Potato Center (CIP), Hanoi, Vietnam

Tor-Abbidye Y, Gelaye S, Louis S L and Cooper G E 1990 Performance and carcass traits of growing-finishing swine fed diets containing sweet potato meal or corn. Journal of Animal Science 1990, 68: 1323-1328.

Yeh T P 1982 Utilization of sweet potatoes for Animal feed and industrial. Uses: Potential and Problems. In: Sweet potato. Asian Vegetable Research and Development Center, 1982. pp. 385-392)

Woolf J A 1992 Sweet potato: An untapped food resource. Cambridge University Press, Cambridge, 643 pp.

Zhanga Z, Christopher C, Wheatley B and Corke H 2001 Biochemical changes during storage of sweet potato roots differing in dry matter content. Postharvest Biology and Technology. Volume 24, Issue 3. April 2002. pp. 317-325