Hoang Huong Giang, Le Viet Ly and Brian Ogle*

Transcription

1 Livestock Research for Rural Development 16 (7) 2004 Citation of this paper 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 *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 (onfarm) 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

2 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 subtropical 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

3 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 h and 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

4 and stored in a freezer at -18 o C until analysis. Urine was kept at a ph below 4 to preserve the ammonia by using 50 ml 5% H 2 SO 4 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 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). Table 1. Analyzed chemical composition of the feedstuffs, Experiment 1 and 2 Ingredients Parameter Maize meal Fish meal Soybean meal Rice bran Cassava root meal SPV SPR Sweet potato meal* Sweet potato silage* DM, % DM basis ME, MJ/kg ** Crude

5 protein Lysine Methionine Calcium Phosphorus Crude fiber NDF Ash *Consists of 50% sweet potato vines and 50% sweet potato roots in dry basis **Source: NIAH 2001 SPV: Sweet potato vines, SPR: sweet potato roots, SPM: sweet potato meal, SPS: sweet potato silage Table 2. Ingredients (% as fed) and chemical composition of diets in Experiment 1 Dietary treatment Ingredient SPO SPM50 * SPS50 ** Maize meal 48.2 Fish meal 5.0 Soybean meal 10.0 Rice bran 23.0 Cassava root 50% SP0 + 50% SPM 16% SP0 + 84% SPS meal 12.0 Ground limestone 1.3 Salt 0.5 Total Nutritive value *** DM, % DM basis ME, MJ/kg Crude protein Lysine Methionine Calcium Phosphorus Crude fiber NDF Ash *Consists of 50% SP0 and 50% SPM in dry basis **Consists of 50% SP0 and 50% SPS in dry basis ***Calculated from ingredient composition SPM: Sweet potato meal, consists of 50% sweet potato vines and 50% sweet potato roots (dry basis) SPS: Sweet potato silage, consists of 50% sweet potato

6 vines and 50% sweet potato roots (dry basis) 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). Table 3. Effect of replacing the basal diet with sweet potato meal and silage on the apparent nutrient digestibility and nitrogen retention in F1 growing pigs Diet SP0 SPM50 SPS50 SE P Digestibility, % DM 85.4 a 76.1 b 78.1 b CP 87.9 a 67.8 b 67.7 b CF 51.2 a 47.0 b 48.6 ab NDF 63.8 a 57.2 b 56.7 b OM 88.2 a 79.8 c 83.1 b Nitrogen retention:.g/day 14.9 a 9.8 b 7.5 b % of total N ingested 45.9 a 39.8 b 38.0 b abc Means within rows with different superscript letters are different (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. Table 4. Apparent nutrient digestibility (%) of sweet potato meal (SPM) and sweet potato silage (SPS) in F1 growing pigs* SP0 SPM SPS SE P DM 85.4 a 66.7 b 70.9 b CP 87.9 a 47.7 b 47.6 b CF 51.2 a 42.8 b 45.9 ab NDF 63.8 a 50.6 b 49.6 b OM 88.2 a 71.5 c 77.9 b *Calculated from the data in Table 3 abc Means within rows with different superscript letters are different (P<0.05) Discussion

7 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

8 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 sundried 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 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%

9 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 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).

10 Table 5. Ingredients (% as fed) and chemical composition of diets (% of dry matter) (15-50 kg live weight) Dietary treatment Ingredients SP0 SPM40 SPM60 SPS40 SPS60 Maize meal 48.2 Fish meal 5.0 Soybean 10.0 meal 40% 22.3% 1.3% Rice bran % SP0 SP0 SP0 SP0 Cassava 40%SPM 60% 77.7% 88.7% 12.0 meal SPM SPS SPS Ground limestone 1.3 Salt 0.5 Total DM, % Dietary treatment Nutritive value * SP0 SPM40 and SPM60 and SPS40 SPS60 ME, MJ/kg Crude protein Lysine Methionine Calcium Phosphorus Crude fiber NDF Ash *Calculated from ingredient composition SPM and SPS consist of 50% of SPV and 50% of SPR in dry basis

11 Table 6. Ingredients (% as fed) and chemical composition of diets (% of dry matter) (50-80 kg live weight) Dietary treatment Ingredients SP0 SPM40 SPM60 SPS40 SPS60 Maize meal Fish meal 2.00 Soybeanmeal 7.0 Rice bran Cassava 40% SP0 60%SPM 60% SP0 40% SPM 22.3% SP0 77.7% SPS 11.3% SP0 88.7% SPS meal Limestone 1.60 Salt 0.50 Total DM, % Dietary treatment Nutritive value * SP0 SPM40 and SPM60 and SPS40 SPS60 ME, MJ/kg Crude protein Lysine Methionine Calcium Phosphorus Crude fiber NDF Ash *Calculated from ingredient composition SPM, SPS consist of 50% of SPV and 50% of SPR in dry basis 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).

12 Table 7. Effect of level of sweet potato meal and silage on daily feed and nutrient intake of F1 growing pigs Treatment SP0 SPM40 SPM60 SPS40 SPS60 SE P No of pigs (15-50 kg) DM, kg 1.19 a 1.12 a 1.05 ab 1.13 a 0.98 b ME, MJ 16.7 a 15.0 ab 13.7 bc 15.1 ab 12.8 c CP, g 201 a 161 b 137 c 161 b 128 c (>50 kg) DM, kg 2.16 a 1.91 b 1.87 b 2.02 ab 1.86 b ME, MJ 30.0 a 25.4 bc 24.3 c 26.8 b 24.2 c CP, g 292 a 234 bc 218 c 248 b 218 c Overall, 15 kg to finishing DM, kg 1.46 a 1.35 b 1.29 bc 1.38 ab 1.23 c ME, MJ 20.5 a 18.0 bc 16.7 cd 18.4 b 16.0 d CP, g 227 a 182 b 160 c 186 b 153 c abc Mean values within rows with different superscript letters are different (P<0.05) 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).

13 Table 8. Effect of level of sweet potato meal and silage on growth performance, feed conversion ratio and back fat thickness of F1 growing pigs Dietary treatment SP0 SPM40 SPM60 SPS40 SPS60 SE P No of pigs (15-50 kg) Initial weight, kg Final weight, 59.1 a 52.1 b 47.6 c 53.0 b 44.9 c kg Days ADG, g 497 a 416 b 363 c 423 b 328 d FCR, kg DM/kg gain (>50 kg) Final weight, kg 2.39 a 2.70 b 2.90 bc 2.67 b 3.00 c a 69.8 b 63.5 c 71.4 b 60.8 c Days ADG, g 561 a 504 b 454 c 524 b 452 c FCR, kg DM/kg gain 3.86 ab 3.77 a 4.12 b 3.85 ab 4.10 b Overall, 15 kg to finishing ADG, g 515 a 441 b 389 c 451 b 363 d FCR, kg DM/kg gain Backfat thickness, cm 2.84 a 3.05 b 3.31 c 3.06 b 3.39 c a, b, c ; mean values within rows with different superscript letters are significantly different (P<0.05)

14 Economical analysis Feed cost, diet cost and economic analysis for the experimental treatments are shown in Tables 9, 10 and 11. Table 9. Feed cost (VND/kg, as fed) Feedstuff Cost Maize meal 2,500 Fish meal 8,000 Soybean meal 3,000 Rice bran 2,300 Cassava root meal 2,000 Ground limestone 100 Salt 2,000 Sweet potato vines * 100 Sweet potato roots * 600 SPM, VND/kg DM ** 2,060 SPS, VND/kg DM ** 2,010 *Price at the harvesting season **Consist of 50%SPV and 50%SPR (DM basis), and include cost of labour and of grinding SPR and SPV 1 USD = 15,000 VND Table 10. Diet cost (VND*/kg DM) Treatment Live weight of pigs kg kg SP0 3,051 2,776 SPM40 2,654 2,489 SPM60 2,456 2,346 SPS40 2,634 2,469 SPS60 2,426 2,316 *1 USD = 15,000 VND

15 Table 11. Economic analysis of experimental treatments (VND) SP0 SPM40 SPM60 SPS40 SPS60 Total feed cost/pig, 319, , , , ,374 (15-50 kg) Total feed cost/pig, 209, , , , ,500 (50-80 kg) Total feed cost/pig, 529, , , , ,874 (15-80 kg) Weight gain, kg, (Overall) Feed cost/kg weight gain 8,349 7,788 7,877 7,761 7,980 % of control diet USD = 15,000 VND 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 kg period and 2.16 kg DM/day in the kg period), they would have consumed actual (as fed) amounts of

16 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%

17 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 sundrying) 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

18 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, 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, pp: 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 Dominguez P L 1990 Ipomoea batatas. Sweet potato. Animal Feed Resources Information System. 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:

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