Livestock plays an important role in most developing countries and it's production represents nearly 20% of the agricultural output value in Vietnam (Ly, 2001). Pigs are predominantly kept on smallholder farms in Vietnam. Pig production, with its main product-pork, is the main animal protein source for the Vietnamese, as it accounts 70% of the meat consumed. However, among the major constraints limiting the development of pig production in Vietnam is shortage of feed resources in terms of both quantity and quality.
Sweet potato (Ipomoea batatas (L) Lam) is a tropical crop with a relatively short vegetative cycle, the tubers of which are usually for both human and animal consumption (Woolfe, 1992). It is among the five most important food crops in developing countries (Horton, 1988) and is the third most important crop after rice and maize in many areas in the north of Vietnam. Since the early 1990s, when paddy production increased in productivity, Vietnam has become one of the biggest rice exporters, and the sweet potato is now used mainly as feed for animals. In fact nearly 100% of the sweet potato crop is used as animal feed in the Red River Delta, and 80% in the highland and mountain areas (Statistical Year Book, 1999).
The productive potential of certain varieties of sweet potato can reach from 24 to 36 tonnes/ha/crop of root (Morales, 1980 cited by Dominguez, 1992) and the foliage production varies from 4.3 to 6.0 tonnes dry matter per ha (Ruiz et al, 1980).
Sweet potato can be harvested twice per year, in the summer and spring-winter seasons, and both forage and tuber have been used widely as an alternative feed for livestock in tropical countries. The roots have low protein, fat and fibre content, but high nitrogen-free extractives, which thus indicates their potential value, mainly as an energy source. The vines have a low carbohydrate content but are higher in fibre and protein, and their principal value is as a source of vitamins and protein.
Traditionally, farmers in most of Northern and central Vietnam boil sweet potato roots and vines to make the feed for their pigs, a process that is time consuming. The farmers, usually women, must spend several hours every day chopping the ingredients, gathering fuel and doing the cooking. Sometimes they feed their pigs fresh sweet potato, in which the presence of trypsin inhibitors can lead to problems .
Thus research on the processing and utilization of sweet potato in diets for pigs in different seasons based on traditional diets under smallholder conditions is necessary.
The typical pig number on smallholder farms in the Northern provinces is 1-2 sows and 2-4 fattening pigs. Private intensive farms are few in number and are mainly concerned with fattening pigs, with the herd size varying from 50 to a few hundred animals. The F1, which is a crossbred between local (Mongcai) sows and exotic (usually Yorkshire) boars, is distributed widely throughout the whole country. Commercial crossbred pigs account for about 60% of the total national fattening pig population (Ly, 2002) (unpublished). The diet of pigs in smallholder farms is primarily based on rice bran, broken rice, sweet potato, cassava roots, and other available local by-products as the dominant energy source, plus some soybean or fish meal bought from local markets as supplementary protein sources. Recently, as the availability of commercial feeds has increased, some compound feeds are also mixed with the home-made diet to improve daily gain. However, nutritionally imbalanced diet for pigs are still typical on smallholder farms.
Making use of locally available low cost feed resources for pigs is very important. One traditional crop is the sweet potato, that contributes both energy from the root and protein from the vine, and which is also traditionally linked to pig production in Vietnam (Bottema, 1992 cited by Peter, 1998). Normally the growing cycle of sweet potato is completed within 100 to 150 days. In practical conditions of smallholder farms, sweet potato roots are boiled and sweet potato vines fed fresh, make up the majority of the feed for fattening pigs, although the optimum amount that can be used in the total diet is not considered. The storage of sweet potato after harvesting is also a problem which has to be solved. Without adequate storage facilities, farmers are often forced to feed their pigs large amounts of sweet potato immediately after harvesting in order to minimize losses in storage due to weevils, rats, molding and other factors, and this results in over-feeding, which sometimes causes diarrhea and low growth rates.
During the period 1986-1988, sweet potato root production in Vietnam was 1,913,000 tonnes with a planted land area of 325,000 ha, and a mean yield of 5.9 tonnes/ha (Scott, 1992). However, since then there has been rapid population growth pressure on farm land, which has resulted in decreased land available for planting sweet potato. In 1998, sweet potato occupied only 269,000 ha and gave a total root production of 1,745,300 tonnes (Statistical Year Book, 1999), which means that the average yield was 6.5 tonnes/ha, a 10% improvement compared to the 1986-1988 period. Growth in production and availability of cereals has meant that less sweet potato is needed to supplement cereal consumption, and a rising demand for meat products has encouraged sweet potato producers to look for alternative outlets for the crop. Now nearly 100 % of the sweet potato crop is used as animal feed in the Red River Delta, and 80% in the highland and mountain areas (Statistical Year Book, 1999).
The chemical composition of the leaves, stems and tubers varies (Woolfe, 1992; NIAH, 2001; An, 2003), depending on the time of harvesting as well as on genotypic differences. The leaves have superior contents of DM and CP compared with stems (An, 2003). Crude protein content in DM of sweet potato vines ranges from 16% to 29% (Dung, 2001), and the fibre content of the leaves is lower than that in water spinach, leucaena leaves, groundnut foliage and cassava leaves (Phuc, 2000). Sweet potato vines contain 8% starch and 4% sugars in DM (Onwueme, 1978 cited by Winarno, 1982). Dominguez and Ly (1997) analysed the composition of sweet potato vines and the results were (on a dry basis): 23.4% crude fibre, 40.3% NDF, 32.8 ADF, 16.0% detergent lignin, 18.3% crude protein, 21.6% ash, 7.6% hemicellulose, 16.7% cellulose and 15.8 MJ gross energy/kg DM. Chemical composition values of sweet potato vines in Paper I and II were on average 16.9% crude protein, 20.4% crude fibre, 34.4% NDF, 10.4% ash, 0.80% lysine and 0.43% methionine (in dry basis). Sweet potato roots have a very high carbohydrate content, mainly starch, low amounts of proteins and minerals and almost no fat (Manfredini et al., 1993). The root is rich in energy as it contains 80-90% carbohydrate of the DM (Dominguez, 1992). It is reported by Duke (1983) to contain (in dry basis) 92-98% total carbohydrate, 27.7-31.5% DM, 3.6-5.4% crude protein, 0.72-1.27% fat, 2.5-3.2% fibre and 2.5-3.2% ash. The chemical composition of sweet potato roots in Paper I and II was 4.0% crude protein, 4.8% crude fibre, 13.9% NDF, 1.6% ash, 0.15% lysine and 0.05% methionine (in dry basis). The chemical composition suggests that both sweet potato roots and vines can be valuable feed resources for animals, including pigs.
Ensiling is a process of fermentation of carbohydrates by acidification, and is a suitable method for preserving feeds that are seasonally abundant for later feeding during periods of feed shortage (Chedly and Lee, 1998). There are two main phases in the ensiling process (Bjorge, 1996). The first is the aerobic phase, which occurs in the presence of oxygen when microorganisms can consume oxygen and burn up the water-soluble carbohydrates (sugars), producing carbon dioxide and heat. The length of this phase is variable, depending on ensiling conditions, and can last for a few hours or as long as several days. The second phase is the anaerobic phase, which begins when the available oxygen is used up and aerobic bacteria cease to function. The microorganisms that grow most rapidly will be predominantly lacto-bacilli species which produce lactic acid that will lower the pH of the silage. Fermentation completely ceases after 3 or 4 weeks when the pH becomes so low (<4.5) that all microbial growth is inhibited.
The primary factors affecting the success of silage fermentation thus are water-soluble carbohydrate content, buffering capacity, moisture content, type of bacteria which predominate and speed of fermentation (Bjorge, 1996). Normally a minimum of 6 to 12 per cent water-soluble carbohydrates is required for proper silage fermentation. Feedstuffs with a high buffering capacity can slow down the reduction in pH, which makes the process of fermentation less successful. The low moisture content of the feed also has a negative effect on the quality of silage, as it makes the pH value higher compared to feeds with a higher moisture content. The lactic acid should be the primary acid in good silage, at least 65 to 70% of the total silage acids (Kung and Shaver, 2001).
Based on the chemical composition data of sweet potato roots and vines, as well as the essential parameters required for a good silage mentioned above, sweet potato roots and vines should be suitable to be preserved by the ensiling method. Ruiz (1982) calculated that sweet potato foliage doesn't require any rapidly fermenting carbohydrate additives and that the forage is sufficiently fermentable. The root is very rich in easily available carbohydrate, which can be fermented well without any supportive additives. In Paper I the five different mixtures of sweet potato roots and vines, which contained between 30 and 70% of sweet potato vine and 30 to 70% of root (in DM basis) without any additives were calculated to contain from 32.4 to 59.6% carbohydrate, which indicates that the mixtures should be suitable for fermentation. A good sweet potato silage can be judged according to the fermentation characteristics, such as levels of organic acids, pH and NH3, smell, colour, and also changes in chemical composition. In Paper I, all of the sweet potato silages had a good smell and a typical fermentative colour. The changes in DM, CP and NDF were slight and could be considered to be acceptable during the 84 days of ensiling. The pH values also dropped quickly in the first 7 days and remained at around 3.6 from 14 days of ensiling onwards, as the lactic acid continued to increase with increasing ensiling time. Butyric acid and NH3-N were very low and changed little during ensiling in all the different mixtures of sweet potato root and vine.
Drying is an ancient method of preservation which works on the principle that microorganisms that cause the food to decay need water to grow and multiply, so it is necessary to reduce the moisture content of the crop to a level low enough to inhibit the action of the microorganisms (McDonald, 1995). Sun-drying is the simplest method that can be applied easily under farm conditions in the summer in North Vietnam. The initial moisture contents of sweet potato roots and vines in Paper I and II were 80.9% and 85.1%, respectively, and after sun-drying were reduced to around 10%. This meant that they then could be put into sealed plastic bags and preserved for at least 3-4 months. During the sun-drying process, the temperature was from 25-40 degrees C which should not have had any negative effects on the nutrient content, which is in agreement with earlier reports on the effect of the sun-drying preservation method on nutrient content in cassava leaf (Ravindran et al., 1987 and Phuc, 2000).
Energy is the body's fuel supply. The pig needs energy for all of activities such as breathing, heart action, digestion, muscular movement, as well as heat to keep the body warm. If the pigs consume more energy than necessary to carry out vital functions, the excess is stored as body fat. The main energy sources in the diet are carbohydrates. Cereal grains are widely used in swine feeding because of their very high soluble carbohydrate (60-70%) and low crude fibre contents (Tamminga and Jansman, 1993), and generally make up between 55% and 85% of compounded feeds (Machin, 1992). Another group of energy sources is the fats and oils, which are very concentrated sources of energy. The use of cereal grains for animal feed however competes with the needs of humans, so it appears that replacement of the cereal component by cheap and locally available roots and tubers is likely to give both economic and other benefits. Roots and tubers have high carbohydrate contents, and thus are rich energy sources. For example, cassava roots have a carbohydrate content of about 88% (Dominguez, 1985 and Ravindran et al., 1982 cited by Perez, 1997) and sweet potato roots 80-90% (Dominguez, 1992). NIAH (2001) recommended that diets for F1 crossbred fattening pigs (15-80 kg) should contain 13.4 MJ/kg DM of metabolizable energy (ME). Changes in ME content in the diet will affect feed intake, for example, when lower energy diets are fed intake increases in order to maintain the balance between energy intake and requirements (Tamminga and Jansman, 1993). In Experiment 2 of Paper II the sweet potato meal and silage had an ME concentration of 12.4 MJ/kg DM, which is not much lower than the recommendation for F1 pigs, and thus replacing the basal diet with the sweet potato silage or meal at 40% and 60% (DM basis) resulted in diets with an acceptable metabolizable energy content that should not have caused the pigs any consumption difficulties.
Proteins supply the building materials from which body tissues, and many body regulators are made, such as enzymes and hormones. The pig has a specific requirement for each of the essential amino acids which cannot be synthesized by the animal to meet its requirement. The quality of a protein depends on its amino acid composition, and a good quality protein should contain all the essential amino acids in proper proportions and amounts, because the real need of pigs is for amino acids rather than for protein. A feed which is imbalanced in amino acid contents, as for a low protein diets, is effective in depressing feed intake. In growing pigs, lysine is often the first limiting amino acid, and a serious lack of lysine in diets results in decreased feed intake (Tamminga and Jansman, 1993). The requirement of F1 crossbred fattening pigs for dietary protein according to NIAH (2001) is 16.5% for pigs of 15-50 kg and 13.3% for pigs of 50-80 kg (DM basis). Under farm conditions in Northern Vietnam, the main protein sources are soybean meal, groundnut cake and fish meal, which are expensive and restricted in availability. Therefore more attention should be focused on using locally available protein sources such as cassava leaf and sweet potato leaf. While sweet potato roots are a good energy source, sweet potato vines provided the protein in the sweet potato silage and meal in Paper II. The crude protein content of these in a 50:50 mixture was only 10.5% (DM basis), which is much lower than the requirements of F1 pigs, and which therefore resulted in the sweet potato silage and meal mixtures having lower protein and lysine contents than the requirement.
The digestibility of a food is closely related to its chemical composition (McDonald, 1995), and the fibre fraction has the greatest influence on its digestibility, which varies according to differences in the chemical and physical structures of the fibre components (Chen et al.,1982; Agarwall and Chauhan, 1989 and McDonald, 1995) The negative effects of dietary fibre are partly a result of a high rate of passage of the ingesta through the small intestine, limiting the time for nutrient digestion and absorption, which means reduced nutrient digestibility (Furuya and Takahashi, 1980; Gargallo and Zimmerman, 1981 and Dominguez and Ly, 1997). Dominguez and Ly (1997) showed that sweet potato vine meal has a high crude fibre content (23.4% in DM basis), and the vines used in the feeding trial (Paper II) had a similar crude fibre level (20.4%), which resulted in the diets have higher crude fibre content when the sweet potato vine meal level increased. These finding are confirmed by the results of Experiment 1 of Paper II, where replacing the basal diet with 50% of a mixture of roots and vines, both as meal or silage, resulted in diets with high fibre contents, which had negative effects on the digestibility of DM, CP, NDF and OM. In addition, Brown and Chavalimu (1985) showed that sweet potato vines have high levels of unavailable nitrogenous substances, and the presence of trypsin inhibitors and proteinase inhibitors in sweet potato roots also make proteins unavailable (Zhanga et al., 2001), which would have also negative effects on the digestibility. Yeh (1982) concluded that the quantity and quality of digestible protein and energy of sweet potato chips are much lower than that of maize meal in pig diets. Thus, the high maize content of the basal diet would also have contributed to its higher digestibility compared to the sweet potato diets in Experiment 1 of Paper II. The poorer nitrogen digestibility would also have made nitrogen retention much lower in the sweet potato diets compared to the basal diet.
The preparation of foods, such as by chopping, chaffing, crushing or grinding, and cooking have effects on the nutrient digestibility of the food. For example, cooking sweet potato did not significantly affect the utilization of energy, but increased the digestibility of the nutrients (Dominguez, 1992), and grinding cereal grains for pigs can also improve digestibility (McDonald, 1995). Phuc et al. (1997) pointed out that it appears that ensiled cassava leaf was digested better than the sun-dried leaf meal and supported higher nitrogen retention. However a later study by Phuc (2000) found that there was no difference of digestibility of sweet potato vines between two processing methods (ensiling and sun-drying). These finding are in agreement with the results in Experiment 1 of Paper II where there were no significant differences found in the digestibility of DM, CP, CF and NDF between sweet potato silage and sweet potato meal, and the sweet potato silage had higher dry matter and organic matter than sweet potato meal.
Both sweet potato silage and meal had low nutrient digestibility by growing pigs, especially the crude protein, that was only about 47% digested. These results are similar to previous studies, and for example Dominguez and Ly (1997) found that in vivo total crude protein digestibility of sweet potato vine meal was rather low, about 54%, and Diaz et al. (1999) reported a similar value (52.3%).
The protein content and quality of sweet potato roots and vines are two of the most important factors that deserve attention when sweet potato is used as feed (Dominguez, 1992). Fuller and Chamberlain (1982) (cited by Dominguez, 1992) showed that the amino acid composition of sweet potato roots and vines is quite good, but total sulfur amino acids and lysine are rather low compared to the ideal protein.
However, in spite of these disadvantages the whole sweet potato plant is an ideal livestock feed because the root is a good energy source and the vine is a source of protein, while both can be used in fresh and dried form or mixed and fermented into silage (Woolfe, 1992). Dehydrated sweet potato by-product meal can replace all the grain in diets of growing pigs, resulting in lower growth rate and feed efficiency but increased carcass length (Tor-Agbidye et al., 1990). With a level of 40% of sweet potato chips as a substitute for maize meal in diets for heavy pigs, it appeared that daily gain, feed efficiency and dressing percentage may be lower compared to a control diet, but were still acceptable (Manfredini et al., 1993). The use of fresh sweet potato foliage for pigs at a low level of substitution (25%) of soybean meal as a protein source in sweet potato roots-soybean diets gave similar feed conversion ratios to those obtained using sweet potato-soybean diets (Dominguez, 1992). Feeding pigs with sweet potato ensiled with chicken manure resulted in higher weight gains compared to pigs fed fresh sweet potato vines (Tinh et al., 2000). Sweet potato meal and silage in Experiment 2 of Paper II were made from a mixture of 50% vines and 50% roots (DM basis) and included in F1 fattening pig diets, as a replacement for 40% of a basal diet. This resulted in somewhat lower feed intake and daily gain, and poorer feed conversion ratio. However performance was still acceptable compared to the basal diet, and economical efficiency was improved.
As the sweet potato is a locally available feed resource the price is relatively low compared to other sources of energy, such as maize, broken rice and rice bran. The sweet potato vines are considered as waste at harvesting season and have no economic value because the vine decays quickly, which occurs within 2 or 3 days. Thus, including both parts of the sweet potato in diets for pigs resulted in substantial reductions in the cost of feed. Drying the sweet potato root not only solves the storage problems associated with fresh roots, but also increase the economic efficiency (Peter, 1998). Both sweet potato roots and vines silage reduce the feed cost per kilo of weight gain and save labour and fuel for cooking (Peter et al., 2001). Ruiz (1982) concluded that when sweet potato chips replaced 25% of the corn in pigs rations the overall production economy was improved, not only due to the lower cost of the chips but also because the carcass lean percentage was higher compared to the corn ration. Tinh et al. (2001) pointed out that fermenting sweet potato roots with 20% of chicken manure in combination with 0.5% of salt and feeding the fermented feed to fattening pigs gave the highest economic benefits. The results from Experiment 2 in Paper II confirmed that the diets including sweet potato meal or silage had lower feed cost and lower cost per kilo weight gain than the basal diet.
This study was carried out at the National Institute of Animal Husbandry, Hanoi, Vietnam, and at Catque village, Hoaiduc District, Hatay Province, North Vietnam, during 2002, with a grant from the Swedish International Development Authority (Sida/SAREC) and the Swedish University of Agricultural Sciences, Department of Animal Nutrition and Management. I am very grateful to their support of this study.
I would like to thank the National Institute of Animal Husbandry, Hanoi, Vietnam for allowing me two years study leave and helping me to carry out the studies
I would like to express my cordial and faithful gratitude to Dr. Brian Ogle, my first supervisor, the Director of the course, and Professor Le Viet Ly, the second supervisor, and the regional coordinator of the SAREC program, for their kind, support, professional guidance, and valuable advice in many different ways.
I would also like to thank Dr. Inger Ledin, the head of the Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences, for her scientific guidance and warm heart during the MSc course.
My sincere thanks to all Professors, Doctors and Lecturers who provided me with useful knowledge during the course.
My sincere thanks to Dr. Luu Trong Hieu, the regional coordinator of the SAREC program, and Dr. Nguyen Dang Vang, the Director of the National Institute of Animal Husbandry, Hanoi, Vietnam for their help and encouragement.
I would like to thank the Department of Animal Nutrition, Department of Feed Analysis and Livestock Feed Research and Trial Station, National Institute of Animal Husbandry, Hanoi, Vietnam for facilitating and helping me to carried out this study.
Special thanks to the farmers in Cat que village, Hoai duc district, Hatay province for their cooperation and allowing me to conduct this research, and to my colleagues Dr. Viet, Mrs. Len, Mr. Kien and Mr. Linh for their help in carrying out this study.
Many thanks to my classmates in the MSc course for their contributions, suggestions and friendship during the course.
Lastly my warm heart-felt gratitude is given to my parents, my husband Le Huu Hoang and my son Le Hoang Hung for their love, patience and support.
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