Improving small-scale pig production in northern Viet Nam

D. Peters

The author is Rural Development Specialist at the International Potato Center, PO Box 929, Bogor 16309, West Java, Indonesia.
Tel.: +62 251 317 951; fax: +62 251 316 264;
e-mail: [email protected]

AM�LIORER L'�LEVAGE ARTISANAL DE PORCS DANS LE NORD DU VIET NAM

Un essai d'alimentation des porcs a �t� conduit en ferme dans la province de Thanh Hoa, dans le nord du Viet Nam, dans le but d'am�liorer le syst�me actuel � base de patates douces, en prescrivant une ration alimentaire journali�re �quilibr�e, en rempla�ant les racines de patate douce par des lamelles d�shydrat�es de patate douce, et en ajoutant des quantit�s plus importantes de compl�ments prot�iques. La formulation et la pr�paration de la nourriture �taient bas�es sur les r�sultats d'une enqu�te sur les pratiques actuelles, de mani�re � tenir compte de la situation locale. L'essai en ferme a �t� r�alis� aupr�s de six m�nages sous la supervision du Service national de vulgarisation. Les r�sultats obtenus ont montr� que la ration journali�re �quilibr�e et les lamelles de patate douce d�shydrat�es favorisaient la croissance des porcs, et que ces derni�res comportaient en outre un co�t inf�rieur par kilogramme de prise pond�rale. En revanche, l'augmentation des compl�ments prot�iques entra�nait des frais suppl�mentaires importants, faisant ainsi chuter la rentabilit�. Ces r�sultats font actuellement l'objet d'un essai ult�rieur de validation.

MEJORA DE LA PRODUCCI�N PORCINA EN PEQUE�A ESCALA EN VIET NAM SEPTENTRIONAL

En la provincia de Thanh Hoa, de Viet Nam septentrional, se realiz� un ensayo de alimentaci�n de cerdos en la explotaci�n para buscar la manera de mejorar el sistema actual de administraci�n de batatas, estableciendo una raci�n diaria equilibrada de piensos en la que se sustituyeron las batatas naturales por otras secas en trocitos, con la adici�n de una cantidad mayor de suplemento proteico. La formulaci�n del pienso y los tratamientos se basaron en los resultados de un estudio de las pr�cticas utilizadas, a fin de tener en cuenta la situaci�n local. El ensayo en las explotaciones se realiz� en seis hogares, con la supervisi�n del servicio de extensi�n del Gobierno vietnamita. Los resultados del ensayo indican que la raci�n diaria equilibrada y las batatas secas en trocitos mejoraron el crecimiento de los cerdos y que los trocitos secos favorecieron un mejor crecimiento con un costo menor por kilogramo de aumento de peso. Por otra parte, la mayor cantidad de suplemento proteico aument� considerablemente los costos adicionales, debido a lo cual la rentabilidad fue baja. Estos resultados se est�n comprobando en un ensayo complementario de validaci�n.

INTRODUCTION

With rising income, food consumption trends in Asia have shifted to increased meat intake. Meat production is often constrained by the shortage of feed or costly feed imports, and alternatives to large-scale and feed-based livestock production are needed. Sweet potatoes are an ideal livestock feed because the roots provide a source of energy and the leaves provide protein, while both can be used in fresh and dried form or fermented into silage (Woolfe, 1992). Scott (1991) estimates that 65 percent of sweet potato output in China goes into animal feed, mainly for pigs. Sweet potato production is also substantially linked to pig production in Viet Nam (Bottema, 1992). Sweet potato-pig systems also play an important role in the rural economies of many other parts of Asia, for example in the Philippines, India, the Republic of Korea, some eastern islands of Indonesia (Bali and Irian Jaya) and Papua New Guinea. Similar approaches have also been adopted in Latin America and Africa, but to a lesser extent (Scott, 1991).

1
Periodic daily weight gain of the four treatments
Prise pond�rale journali�re p�riodique, pour chacun des quatre traitements
Aumento diario peri�dico de peso en los cuatro tratamientos

On-farm weighing of pigs inside the pen
Pes�e des porcs � l'exploitation
Pesada de cerdos en la celda dentro de la explotaci�n

In the context of sweet potato-pig systems, pigs serve several important functions for rural households: they generate much needed cash income through market sales; they provide manure for maintaining and improving soil fertility; they convert the generally undesirable and low-value sweet potatoes into highly desired foods or marketable commodities; and, in the case of Irian Jaya, pigs are the basis of social and economic exchange and play a pivotal role in the local culture (Heider, 1979). This feeding practice is used by almost every household in the sweet potato producing regions for the same purposes.
Despite the importance of this system for the rural household economy, slow pig growth has kept profitability low. Hence, improvements in this area would have a widespread impact on augmenting household income through increasing individual pig growth and/or supporting more pigs on the same amount of feed. One potential avenue for improving the system is by adding a protein supplement to the traditional low nutrient-density diet. Thorn (1993) found that adding small amounts of protein supplement, such as locally available fishmeal, greatly improved the efficiency and profitability of smallholder pig production in the Solomon Islands.
Another potential avenue is to improve sweet potato utilization methods. Where the harvest coincides with the dry season, sweet potatoes are commonly cut into chips by hand and dried in the sun. This chip-and-dry practice is the farmers' way of preserving excess sweet potato. Moreover, previous research on the effects of sweet potato as pig feed has shown that the dried chips constitute a more efficient feed, i.e. lower cost and higher daily weight gain, than fresh roots (Koh, Yeh and Yen, 1976). A long cooking time is required for most sweet potato varieties in order to break down the starch to enable subsequent absorption of the nutrients by the pigs. Dried chips require only brief cooking since the starch is already partially released by cutting and solar drying. Further research has also confirmed that "sweet potato chips properly supplemented with protein are considered an adequate energy source for growing finishing pigs" (Wu, 1980, p. 1261). Additional studies have examined the effect of sweet potato chips as pig feed ( Wu, Huang and Chen, 1985; Wang, Yeh and Lee, 1984) and explored various methods of processing sweet potato chips in order to improve their nutritional value (Yeh et al., 1976-77). However, these methods require sophisticated technologies that are not applicable to the pig farmers in developing countries.
Following an initial survey in various sweet potato growing areas of the coastal plains and mountainous regions in northern and central Viet Nam, a sweet potato-pig feed trial was conducted in six households of two villages in Tinh Gia district of Thanh Hoa Province in northern Viet Nam. This article reports on: the findings of this survey, which were instrumental in designing a trial appropriate to the local situation; the goals, objectives, hypotheses and treatments of the trial; participatory methods for conducting the trial; results of the trial; and planning and design of a follow-up validation trial.

Farm pig pens that are too dark
Porcheries trop sombres
Las celdas de los cerdos en la explotaci�n son demasiado oscuras

Pig pen construction that makes weighing difficult
Construction d'une porcherie o� la pes�e sera difficile
Construcci�n de celdas para los cerdos que dificulta la pesada

Front view of a typical pig pen
Vue de face d'une porcherie
Vista frontal de una celda t�pica de cerdos en la explotaci�n

PRELIMINARY SURVEY

A preliminary survey was conducted during April 1997 in two districts of Thanh Hoa Province (northern Viet Nam) - Tinh Gia and Hoang Hoa. Forty households were interviewed and the results were analysed to determine the appropriate locations for trials to increase the efficiency of raising fattening pigs. The findings indicated that Tinh Gia district was the appropriate location for the following reasons (Table 1).

1
The sweet potato-pig system in two districts of Thanh Hoa Province, Viet Nam
Le syst�me patate douce/porc dans les deux districts de la province de Thanh Hoa, Viet Nam
Sistema de administraci�n de batatas a los cerdos en dos distritos de la provincia de Thanh Hoa, Viet Nam

¹ At time of survey, April 1997.
² Estimated growth rate of the pigs currently kept in the pens.

Tinh Gia is best oriented to finishing-pig production, from 9 to 87 kg, compared with 13 to 64 kg in Hoang Hoa district. Tinh Gia's interest in pig production coincides with the trial's aim of improving the growth rate and profitability of finishing pigs.
Even though Tinh Gia households feed their pigs large quantities of sweet potato roots and vines, 40 percent higher than in Hoang Hoa, their growth rate remains only comparable or even lower, indicating a possible excessive ration of sweet potatoes.
Tinh Gia district is less than 2 km from the ocean and the households located on the coast are engaged in fishing. Unmarketable small fish and shrimps are commonly used as protein supplement for pigs but the feeding is sporadic. Samples of both these and other protein supplements were collected locally and analysed in the laboratory in Hanoi for their nutritional content. The crude protein content of salted small fish, according to the laboratory analysis, is shown in Table 2.

2
Laboratory analysis of nutritive values of various protein supplements
Analyse en laboratoire des valeurs nutritives des divers compl�ments prot�iques
An�lisis de laboratorio del valor nutritivo de diversos suplementos proteicos

In addition to small fish and shrimp, fish sauce residue is often fed to pigs but only as an appetite enhancer because it is believed to have little nutritional value. However, some people argue that uncooked fish sauce residue does have its merit in improving growth, in addition to enhancing appetite. Because of its widespread use and its speculative effects, it was considered that the usefulness of fish sauce residue was worth investigation.

Pigs are currently fed both fresh sweet potato roots and dried chips. The major sweet potato harvest season in Tinh Gia is in May and June, which is during the dry season when it is possible to process chips as pig feed.

TRIAL DESIGN

The results of the trial conducted by Koh, Yeh and Yen (1976) in China showed that pigs gained 60 kg (26 to 90 kg) in 83 to 118 days (daily weight gain ranged from 542 to 778 g), depending on the diet. Preliminary survey results indicated that, although the pigs were given large quantities of sweet potatoes, the growth rate (9 kg per month = 300 g of daily weight gain) had not reached its optimal potential.

The goal of the International Potato Center (CIP) is poverty alleviation through livelihood improvements associated with its mandated commodities and, hence, its research goal is to determine how to improve pig-raising techniques by using locally available feed, specifically sweet potato, to reach optimal growth with minimum input through a balanced diet. In order to achieve this goal, the following tasks were set:

• Conduct a pig feed trial for the purpose of comparing the effects of different diets on pig growth.
• Calculate total feed cost for each diet and analyse the cost per kilogram of weight increase in order to identify the most economically efficient diet for raising finishing pigs.

In the course of these activities the following four hypotheses, drawn from literature, previous research and local conditions, were tested:

H₁ - Replacing sweet potato fresh roots with dried chips promotes better growth.
H₂ - Adding small fish as extra protein to the current diet improves growth.
H₃ - Adding fish sauce residue improves growth.
H₄ - These alternative diets result in a lower cost per kilogram of weight gain.

In addition to fish and sweet potato roots, the preliminary survey showed that farmers normally include sweet potato vine, maize, rice bran and some groundnuts and rice in the pigs' diet since they are all available on the farm. Based on this traditional diet, four treatments with prescribed balanced daily rations were designed to test the hypotheses (Table 3). With Treatment 1 approximating the traditional feeding practice by standardizing and balancing the conventional dietary elements, the four treatments are as follows:

Treatment 1. Fish (traditional ration) + sweet potato (fresh and dried chips);
Treatment 2. Fish (traditional ration) + sweet potato chips;
Treatment 3. Fish (added ration) + sweet potato (fresh and dried chips);
Treatment 4. Fish (traditional ration) + fish sauce residue + sweet potato (fresh and dried chips).

3
Formulation of daily feed ration for the four treatments of the trial
Formulation de la ration alimentaire journali�re, pour chacun des quatre traitements test�s
Formulaci�n de la raci�n diaria de pienso para los cuatro tratamientos del ensayo

¹ During this period, the daily feed was formulated for a ten-day period instead of being calculated according to the weight of the pigs.
² FSR = fish sauce residue.

METHODS

On-farm participatory trial

The trial was conducted between 20 April and 31 August, lasting a total of 133 days. For this trial the method developed by Koh, Yeh and Yen (1976) was adapted to on-farm conditions. The piglets were weighed for three consecutive days, and the average of these three days was taken as the starting weight while the second day of weighing marked the first day of the trial. The same procedure was repeated for the end weight and the last day of the trial. During the trial period, pigs were weighed every two weeks.

Six male pigs were purchased for each treatment, i.e. four treatments with six replications, totalling 24 pigs. They were placed in six households in two villages, and each of the participating households constructed four individual pens for the four treatments. Prior to the trial, a meeting was held with the women participating (raising pigs is traditionally women's responsibility) for consultation on feed formulation, construction of the weighing cage and the weighing schedule. The women were also informed of the purpose of the trial, the meaning of the treatments, the method of feeding and the importance of standardized methods for raising the trial pigs.

Author discussing on-farm pig trials with local farmers
Auteur discutant d'essais sur les porcins avec des �leveurs locaux
El autor examina los ensayos realizados con los cerdos en las explotaciones con los agricultores locales

Pig manure and straw used for fertilizer
Emploi de fumier comme engrais
Esti�rcol de cerdo y paja utilizados como fertilizante

Sweet potato roots and vines used for pig feed
Utilisation de racines et feuilles de patates douces pour l'alimentation des porcs
Batatas y sarmientos utilizados para alimentar a los cerdos

The participants, at their request, were allowed to select their own piglets as long as they adhered to the weight specification of 8 to 12 kg. The piglets were kept in their individual pens for a week to enable them to settle into the new environment before the trial started. During this week, they were vaccinated against diseases and dewormed by local extensionists. Since the women were not accustomed to weighing feed, the local extensionists weighed and distributed the daily rations to the participating households. The participants themselves ground maize, bran and chips into flour and cooked the flour together with chopped vines, sliced roots (except for Treatment 2), grass and fish. For Treatment 4, fish sauce residue was added at the end of the cooking process. The pigs were fed three times a day - between 6 and 6.30 a.m., at 12 noon and between 6.30 and 7 p.m.

Problems encountered

On-farm trials do produce relevant results for farmers, but the logistics are often difficult to control. Some of the problems encountered only increased the complexity of the trial, but others had implications for improvements and led to the adjustments made in the follow-up validation trial discussed at the end of this article.

Pig pen design. Weighing. Most pens were not designed to enable the gate to be opened for pigs to walk out directly into the weighing cage. Some did not even have gates and others had gates which were difficult to open because of the way the pens were angled. The problem was even worse when the pens were low because this made it difficult to move the cage inside the pen for weighing.
Housing. Three problems were observed here: i) the low or flimsy bamboo walls between the pens were not sufficient to keep the pigs out of each other's pens; ii) the front wall was too low to keep the pigs in; and iii) the pigs escaped by squeezing through the gaps between the bamboo bars of the gate.
Health. Some households had pens with poor drainage, hence the pigs were constantly lying in puddles of water. Other pens had low and thick thatched roofs which made it excessively dark inside.

Problems associated with the design of the weighing cage. First, the two gates on either side of the cage were designed for large pigs and the spaces between the bars were so wide that the piglets routinely escaped. Second, the cage weighed 13.5 kg, which made it difficult for the farmers to carry it from house to house.

Participants' pig-raising practices. Despite repeated reminders, some participants fed the pigs before weighing them. Others fed them extra fish after their fishing trips. However, the quantities were minimal and all the pigs involved in the four treatments were fed the additional ration, so no particular treatments were unduly biased.

RESULTS AND DISCUSSION

Pig growth

One sample was dropped from each treatment because, in Treatment 1, a pig in household 2 gained 18.3 kg whereas the average weight gained by the other five pigs was 52.3 kg; the results were then based on five samples of each treatment. Treatments 1 and 2 both reached 11.8 kg of monthly, or 0.39 kg of daily, weight gain (Table 4), which is higher than the estimated growth rate (9 kg/month) of current practices (Table 1). The participants stated that, in past years, pig growth had always fluctuated depending on disease incidence, but that they never achieved the same growth as in the trial. Moreover, the monthly weight gains of Treatments 1 and 2 ranged from 8.8 to 13.9 kg, indicating that monthly weight gain could reach 13.9 kg under optimal conditions. Since Treatment 1 approximates traditional feeding with a balanced daily formulation, the results suggest that balanced feeding contributes to improved growth. The participants attributed the improved growth rate to the balanced diet formulation, as they normally fed their pigs sporadically with whatever feed was available on the farm.

4
Monthly, daily and percentage weight gain in relation to starting weights for the four
Prise pond�rale mensuelle, journali�re et en pourcentage du poids initial, pour chacun des quatre traitements
Aumento mensual, diario y porcentual de peso en relaci�n con el peso inicial en los cuatro tratamientos

When weight gain was calculated to adjust for the different starting weights, which were never uniform, Treatment 2 showed a higher weight gain (533 percent) than Treatment 1 (477 percent), despite the same daily or monthly weight gain (Table 4). This result would support hypothesis H₁, that sweet potato chips promote better growth than fresh roots.
Conversely, Treatments 3 and 4 achieved lower daily, monthly and percentage weight gain than Treatment 1 (Table 4), refuting hypotheses H₂ and H₃, that adding fish or fish sauce residue promotes better growth. However, this result may be inconclusive, as several pigs fed with Treatments 3 and 4 were reported to have digestive problems, described by the participants as constipation, throughout the trial period. The participants suggested that the problem stemmed from feeding fish to piglets when they were too young, since they maintain that fish are difficult for 20 kg piglets to digest. They stated that this digestive problem hindered growth because it inhibited appetite, and recommended that fish be given after the pigs reach 30 kg in weight. This explanation seems plausible since it is inherently illogical that added protein should lead to stunted growth, even if it does not enhance growth. Since the stunted growth may be attributed to the digestive problem, hypotheses H₂ and H₃ were inconclusive.
The participants also reported that during the trial there were two periods of intensive heat of over 40�C, one for 15 consecutive days and one for seven days, when they observed lack of appetite and hindered growth for all pigs. However, this problem was not reflected in the daily weight gain since all four treatments increased steadily, albeit at different rates, throughout the trial (Figure 1), which suggests that neither heat waves nor digestive problems abruptly affect growth rates.

Economic aspects

In addition to achieving a higher percentage weight gain than Treatment 1, Treatment 2 was also lower in cost, D 8 028 (US$1 = 11 670 dong) for one kilogram of weight gain (Table 5). This empirical evidence supports hypothesis H₄ (supported by Treatment 2) that the alternative diet of replacing fresh sweet potato roots with dried chips is less costly per kilogram of weight gain. The market price of pigs (not pork) is D 10 000/kg^-1, which suggests that the per kilogram profit of Treatment 2 is D 1 972. However, these feed compositions were formulated on the basis of feed available on the farm, such as sweet potatoes, bran and rice, which comprise almost 70 percent of the feed as a whole (Table 6). These feeds are not usually purchased inputs but, for the purpose of calculating the opportunity costs for the trial, market prices have been used. Maize is planted in the area but not every household produces enough to support two pigs. Unmarketable fresh small fish are abundant in season, the surplus of which may be dried for feed during the off-season. The local farmers' view of the real profit per kilogram of weight gain far exceeds D 1 972 because most of the feed is not purchased.

5
Feed cost per pig and cost per kilogram of weight gain
Co�t des aliments par porc et par kilogramme de prise pond�rale
Costo del pienso para cada cerdo y costo por kilogramo de aumento de peso

6
Average feed consumption and costs per pig for the four treatments
Consommation alimentaire moyenne et co�t d'un porc pour chacun des quatre traitements
Consumo medio de pienso y costos de un cerdo en los cuatro tratamientos

¹ Percentage of the total feed cost.
² Feed readily available on the farm and not normally purchased.

Treatments 3 and 4, however, required D 11 808 and D 10 250, respectively, for one kilogram of weight gain, which is significantly higher than that of Treatment 1. The cost of fish for Treatments 3 and 4 accounts for more than 20 percent of the total cost, which makes the weight gain economically inefficient. The combination of slower growth and higher cost per kilogram of weight gain indicates that Treatments 3 and 4 are less efficient and profitable than Treatments 1 and 2.

CONCLUSIONS

The objective of the trial was to determine the most economically efficient method of improving pig growth by replacing fresh sweet potato roots with dried chips and by adding protein to the low nutrient-density diet. The trial results showed that improvements can be made by simply balancing the traditional feed composition, without extra investment.

In addition to using a balanced formulation of daily feed, the results showed that sweet potato chips promoted better growth than fresh roots. Dried chips not only solve the storage problems associated with fresh roots, but also increase the economic efficiency of pig raising. This practice should be encouraged in areas where the sweet potato harvest coincides with the dry season, such as in Tinh Gia district.
Owing to the digestive problems experienced by pigs under Treatments 3 and 4, the results of this part of the trial may be compromised and hence no definite conclusions on the growth rate can be drawn. In any case, the economics of Treatments 3 and 4 showed that adding fish as protein supplement is not cost-efficient since the cost per kilogram of weight gain would exceed the selling price per kilogram of meat. It is evident that these treatments cannot be recommended as an avenue for improving pig raising in Tinh Gia district.

Locally constructed basket for weighing pigs on-farm
Panier de fabrication locale utilis� pour la pes�e des porcs
Cesto de fabricaci�n local para pesar los cerdos en la explotaci�n

Pigs enter weighing cage directly from on-farm pens
Porcs p�n�trant directement de la porcherie � l'aire de
Los cerdos entran directamente en la jaula para pesarlos desde la celda de la explotaci�n

It is clear that the technical and economic conclusions of this trial have implications for other parts of Asia, for example China, the Philippines, India, some eastern islands of Indonesia (Bali and Irian Jaya) and Papua New Guinea, where there is an abundance of sweet potato but a shortage of feed sources and a rising demand for meat. Similar trials, adapted to local feed situations, can be conducted to determine the most efficient method of fattening pigs using local feed resources.
In addition to the conclusions drawn from technical and economic aspects of this trial, the method of the trial also needs to be addressed. During an extension meeting held on completion of the trial, repeated praise from local officials and community leaders was directed towards its on-farm, participatory nature. The Vice-Director of the Agriculture and Forestry Extension Center, Department of Agriculture and Rural Development of Thanh Hoa Province, stated the following:

"I am the head of the extension service of the province, but never have I seen a trial like this. The only trials I have seen were on big research stations. I told the farmers that we have scientists coming to help them and they need to follow the instructions of the trial. Even though sometimes the farmers made mistakes, most of the time they followed instructions well. It is important for farmers, extension people, and scientists to work together. In Tinh Gia, [where] there is a long tradition of feeding sweet potato to pigs, most farmers do not think about how to improve pig growth. Now perhaps they will think more about it."

It was apparent that officials and farmers all found "on-farm" an important aspect of this trial as it addressed farmers' needs, constraints and opportunities.
A follow-up trial was organized to validate the conclusion that a balanced formulation and sweet potato chips consistently achieve better growth. This ongoing validation trial consists of two treatments (Treatments 1 and 2) with 20 replications to be carried out in 20 households. For this trial, the participants are taking more responsibility and are weighing the feed daily, under the supervision of the extension service. In fact, a training session was organized to train the participants in measuring and weighing the daily feed. The original six women who participated in the earlier trial are serving as leaders among the current participants and as assistants in the trial. The following changes were made as suggested by the original participants on the grounds of their experience:

• The piglets were purchased collectively from one pig farm to ensure their uniformity.
• All piglets weighed approximately 15 kg so that they were stronger and healthier and less prone to illness.
• Larger pens and lighter roofs were constructed to allow for more movement and light.
• Fish will not be included as feed until the pigs have reached 30 kg.
• The trial is to be conducted during the autumn when the heat has subsided and the pigs are less prone to diseases.
• A veterinarian will be consulted whenever there are signs of illness.

Bibliography

Bottema, T. 1992. Rapid market appraisal, issues and experience with sweet potato in Vietnam. In G. Scott, S. Wiersema & P.I. Ferguson, eds. Product development for root and tuber crops. Vol. 1 - Asia. Lima, International Potato Center.
Heider, K.J. 1979. Grand Valley Danis: peaceful warriors. Columbia, South Carolina, USA, University of South Carolina.
Koh, F.K., Yeh, T.P. & Yen, H.D. 1976. Effect of fresh sweet potato, sweet potato chips, and sweet potato vines on raising finishing pigs. J. Chinese Society of Animal Science, 5(3-4): 55-67. (in Chinese)
Scott, G.J. 1991. Sweet potato as animal feed in developing countries: present patterns and future perspectives. Paper presented at the FAO Expert Consultation on the Use of Roots, Tubers, Plantains and Bananas in Animal Feeding, held at the International Centre for Tropical Agriculture (CIAT), Cali, Colombia, 21-25 January 1991.
Thorn, P.J. 1993. A physical and financial appraisal of a feeding system using combinations of low nutrient-dense and concentrated feeds for growing pigs in the Solomon Islands. Trop. Agric., 70: 78-82.
Wang, M.K., Yeh, T.P. & Lee, P.K. 1984. A study on the use of high protein sweet potato chips and dehydrated sweet potato vines as the main ingredients of hog feed formula. Annual Research Report of the Animal Industry Research Institute, Taiwan Sugar Corporation, Taiwan Province of China. (in Chinese)
Woolfe, J.A. 1992. Sweet potato: an untapped food resource. New York, USA, Cambridge University Press.
Wu, J.F. 1980. Energy value of sweet potato chips for young swine. J. Animal Sci., 51(6): 1261-1265.
Wu, J.F., Huang, M.D. & Chen, S.Y. 1985. Effect of dietary sweet potato chips, cassava chips and cassava pomace on nutritional value for growing pigs. J. Agricultural Association of China, New Series No. 131. (in Chinese)
Yeh, T.P., Wung, S.C., Koh, F.K., Lee, S.Y. & Wu, J.F. 1976-77. Improvements in the nutritive values of sweet potato chips by different methods of processing. Annual Research Report of the Animal Industry Research Institute, Taiwan Sugar Corporation, Taiwan Province of China. (in Chinese)