Genetic and nutritional diversity of sweet potato (Ipomoea batatas), water spinach (Ipomoea aquatica) and water hyacinth (Echhornia crassipes) and their potential as pig feed in the Mekong Delta of Vietnam

Le Thi Men

Cantho University, Cantho City, Vietnam,
ltmen@ctu.edu.vn

Abstract

The genetic diversities of some locally available plants in the Mekong Delta of Vietnam that are commonly used as feed for pigs were estimated because of the necessity to conserve and select their valuable traits, and to track the genetic variations. Furthermore, their nutritional value was determined, as they are potentially useful sources of protein and lipid for monogastric livestock.Variations in five sweet potato (Ipomoea batatas) varieties were investigated. Protein subunits were estimated by the SDS-PAGE method, and phenotype (Ho), genetic value (H_EP), and the sum of the effective number of alleles (SENA) was also determined. The contents of crude protein (CP), ether extract (EE) and amino acids of each variety were analyzed. The total oil was extracted from the samples, then the fatty acid composition analysed.

Results showed that 'Hong Dao', 'Hsinchu', 'Duong Ngoc' and 'Lang Bi' varieties of sweet potatowere diverse, while the 'Hong Ngoc' variety was a pure genotype. The CP content of the leaves ranged from 19.8-26.8% and EE from 4.8-5.8%, while the lysine content ranged from 0.48-1.07% (DM basis), and the alpha linolenic acid (ALA) ranged from 39.7-45.5% of the total lipid. Three varieties of water spinach (Ipomoea aquatica): 'HAT' which grows in the soil, 'NUOC', which floats on the water surface, and 'DONG' which is grown in the field, and three populationsof water hyacinth(Echhornia crassipes), were analysed for the values above. The water spinach varietieswere diverse in the Ho, H_EP and SENA, while the 'NUOC' variety was the best in protein value in genetic diversity. The CP and EE contents of the water spinach ranged from 18.6- 29.5% and 6.9-7.2%, respectively. The lysine content ranged from 0.8-1.4 % and ALA from 45.1-47.3% of total lipid. Water hyacinth was also diverse in Ho and the SENA. Crude protein and EE contents of the water hyacinth ranged from 16.4-21.8% and 4.3-4.8%, respectively, and lysine and ALA ranged from 0.56-1.03% and 37.6-38.6%, respectively.

Key words: amino acids, conservation, fatty acids, genetic diversity, nutritional value
 

Introduction

Plant varieties that have adapted well to the local environment, give high yields and are of good quality are important genetic resources that should be conserved for the future. Recently wild species and landraces have been decreasing, or have disappeared as a result of economic and agricultural development, and the nutritional usefulness and diversity of these potentially valuable resources have been neglected. Green feeds give a high production of protein-rich biomass and are ideal complements for fibre-free basal diets in pig and poultry feeding systems. They satisfy the sustainability indicator and also fill a particular niche because of their capacity to decontaminate water excessively charged with organic matter (Preston 1995).

Sweet potato (Ipomoea batatas) is among the five most important food crops in developing countries in terms of total production (Horton 1988). It originated in South America, and has been widely cultivated in Vietnam (Hoang et al. 1998). Sweet potato output expanded rapidly during the last decade in various locations, including Vietnam and other tropical countries. The central region of Vietnam, which has soils of low fertility accounts formore than 50% of the total cultivated area of the country. The Mekong Delta is the third important region for sweet potato production after the Red River Delta (Ly 2000). However, its tuber yield of 1.4 tonnes/ha is the lowest in the country, where the average yield is 6.0 tonnes/ha, which can be compared with the yield of 19 tonnes/ha in Japan (Minh 1999). Besides the roots being used for humans and animals, the vines are valued as a green feed, especially for pigs and poultry, in southern Vietnam(Lam 2003). The vines can be cut throughout the year, with subsequent harvests of 20 days, and yield of the fresh vine can be up to 1.7 tonnes/ha/harvest (Man 1994). However, selecting and crossing sweet potato varieties has not been carried out yet in the Mekong Delta, although this has been done in the North of Vietnam.

Water spinach (Ipomoea aquatica) grows in marshy or wet, sandy soils or floats on water in many tropical regions of the world (Bruemmer and Roe 1979). The plant has creeping, hollow, water-filled stems and shiny green leaves, and big funnel-shaped flowers, 2-5cm long, and purple or white in colour. It is used as human food and animal feed throughout Southeast Asia (Gohl 1981). The fresh leaves and stems contain 20 to 31% CP in DM basis (Phuc 2000) that is well-balanced in essential amino acids, i.e. 1.3% lysine, 0.4% methionine and 1.1% threonine on a DM basis (Men et al 2002).

Water hyacinth (Echhornia crassipes), floats on the water surface, and is a common weed in eutrophicated water bodies due to it high productivity, which is in the range of 106 to 144 tonnes of DM per hectare per year (Casabianca et al 1992). Water hyacinthhas been investigated since the 1970's (Boyd 1970) and is currently in use on a large scale for the treatment of municipal water waste in Asia (Huub and Siemen 1998). The young, light-green leaves and stems are rich in CP (18% on a DM basis) and lysine (1.2%), and were found to be soft and palatable to the pigs in the Mekong Delta (Men et al. 2002).
 

Materials and methods

Varieties and populations

Five varieties of sweet potato, 'Hong Ngoc' (HNgoc), 'Hong Dao' (HDao), 'Hsinchu', 'Duong Ngoc' (DNgoc), and 'Lang Bi' (LBi) were collected in both Vinhlong and Cantho provinces. The traditional sweet potato varieties in the Mekong Delta are now propagated by cloning.

Three varieties of water spinach were studied: 'HAT', which is often seeded and grown in the soil, and has a fresh biomass yield 3 weeks after sowing of 10.4 tonnes/ha (Luyen 2003); 'NUOC', which grows by putting out creeping stems on the surface of water along river and canal banks or ponds (Dat 2002), and has a fresh biomass yield when fertilized with livestock manure of as much as 450 tonnes/ha/year (Hau 1998), and 'DONG', which is a wild plant that grows from seed in moist soil, and is abundant in the margins of the rice fields. Like Indian water spinach, 'DONG' grows densely near the periphery of low lands and spreads rapidly on moist soils (Tiwari and Chandra 1985).'HAT' was collected in the green vegetable belt and 'NUOC' along the river banks of Cantho City, while the 'DONG' variety was collected in rural fields in Tan Phu Thanh village, which is around 20km from Cantho City.

There is no typical variety of water hyacinth. Thus, three places where water hyacinth is prevalent were selected, based on the differences in the aquatic environment, and three populations of water hyacinth were collected at each site: Campus 2 of Cantho University, where the water is almost stagnant, and in Tan Phu Thanh village, from a canal bounded by fences of bamboo, and from ditches in the gardens

Protein electrophoresis

Thirty leaves from the different individuals of each sweet potato and water spinach variety or water hyacinth population samples were weighed to 1g. The samples were ground with liquid nitrogen, a 500 ml extraction solution (0.1M Tris-HCl pH 8.0 containing 1.5M 2-Mercaptoethanol, 0.01 M MgCl₂, 18% Sucrose and 2% SDS) was added, and then incubated at room temperature overnight. Different proteins were separated on 5% stacking gel and 12% separating gel using 40 volts for stacking gel and 80 volts for separating gel. Gel was stained with 0.2M Coomassie Brilliant Blue R250 in methanol, acetic acid, and distilled water with a ratio of 44: 6: 50. The de-stained solution included acetic acid, methanol, and distilled water in a ratio of 5: 28: 67, respectively.

Genetic diversity values

Several formulae (Huh and Ohnishi 2002; Thanh et al. 2003) were used to describe genetic diversity, as follows: all the bands visible to the naked eye were scored as present (1) or absent (0). The degree of polymorphism was quantified using Shannon's index of phenotypic diversity (Ho). If the Ho is equal to zero, this means that all individuals in a population are homologous, which implies inbreeding. In general the higher the Ho value, the more the population is phenotypically diverse.

Ho=-Sf_ilnf_i

Where f_i is the frequency of phenotype i. Thus, the Ho value was compared among different populations. The diversity value (H_EP) for the genetic marker was calculated from the sum of the squares of the phenotypic frequencies by the formula:

H_EP=1-f_i²

As is the case with the Ho value, the H_EP value ranges from zero to one. The minimum value (zero) indicates that a population is completely inbred, while the maximum value (1) indicates maximum genetic diversity.

The sum of the effective number of alleles (SENA) was calculated by determining the effective number of alleles for each locus, namely:

SENA= (1/f_i²-1)

Nutritional values

Six kg of sweet potato leaves and seven kg of water plants for each variety or population were used to determine their nutritional value. The DM content was determined on six replicate samples using a two-step procedure and calculation, i.e. Total DM = Partial DM × Laboratory DM (Undersander 1993). The CP and EE content of the air-dried samples (six and ten replications, respectively) was determined according to AOAC (2000). The air-dried samples of each variety were analyzed for amino acid content, and samples of the oil extract analyzed for fatty acid content and composition.

Statistical analysis

The data for DM, CP and EE contents were analyzed by ANOVA using the General Linear Model of Minitab Statistical Software version 13.2 (Ryan et al. 2000). Sources of variation were treatments and error.  The Tukey test for paired comparisons was used to separate means when the differences were significant at the 1% or 5% levels.

Results and discussion

Sweet potato

The electrophoretic protein profile of the 'HNgoc' variety was the same among individual leaves while the 'HDao' was diverse in protein bands. The 'Hsinchu' protein profile was slightly less stained than the 'DNgoc' and 'LBi' varieties. The average H_EP values (0.34) of these varieties were low (ranging from 0.00 to 0.53), and the average SENA value was 0.63 (Table 1).

Table 1. Genetic diversity values of the leaves of five varieties of sweet potato (Ipomoea batatas) in the Mekong Delta
	Varieties1)					Average
	HNgoc	HDao	Hsinchu	DNgoc	LBi
Polymorphic individual (%)	0	50.0	90.0	40.0	20.0	40.0
Polymorphic band (%)	0	50.0	75.0	38.0	25.0	37.5
Ho	0	2.53	2.41	1.61	1.15	1.54
HEP	0	0.48	0.53	0.38	0.33	0.34
SENA	0	0.92	1.14	0.60	0.50	0.63
1‘HNgoc’ = ‘Hong Ngoc’, ‘HDao’ = ‘Hong Dao’, ‘DNgoc’ = ‘Duong Ngoc’ and ‘LBi’ = ‘Lang Bi’: common varieties in the Mekong Delta; ‘Hsinchu’ variety introduced from Taiwan.

Sweet potato is regenerated mainly by vegetative propagation, so the genetic diversity parameters were low in H_EP and SENA.In practice, creeping plants such as sweet potato are propagated by using the upper part of plant, or asexual propagation, and so all individuals in a line should be genetically pure, unless mutation occurs.

Table 2 shows the nutritive values of five varieties of sweet potato leaves. The average CP on a DM basis was highest in the 'DNgoc' variety (26.8 %), second highest in 'LBi' (22.8%), and lowest in 'HNgoc' (20.8%), 'HDao' (19.8%), and 'Hsinchu' (20.7%) (P<0.01). The EE content was highest in 'HNgoc' (5.8%), while the lowest value was in 'HDao' (4.8%) (P<0.01). However, the EE contents of the three varieties 'Hsinchu' (5.6%), 'DNgoc' (5.5%), and 'LBi' (5.7%) were very close to that of the 'HNgoc' variety. Arginine (Arg), glutamic acid (Glu), isoleucine (Ileu), leucine (Leu), lysine Lys), methionine (Met), phenylalanine (Phe) and valine (Val) contents were highest in the 'DNgoc' leaves. Glutamic acid, histidine (His) and threonine (Thr) (on DM basis) were highest in 'DNgoc' (2.56%), 'Hsinchu' (0.42%), 'DNgoc' (1.19%) and 'LBi' (1.22%), respectively. As for the amino acids patterns relative to lysine, the proportion of Arg, Glu, glycine (Gly), Ileu, Leu, Phe, Thre and Val was higher in 'HNgoc' than in the other varieties. In contrast, the proportion of histidine and methionine was highest in the 'Hsinchu' (53%) and 'DNgoc' leaves (50%), respectively. The total PUFA ranged from 70.2%(Hsinchu) to 77.1%(HNgoc). Of the PUFA, the proportion of ALA was highest in 'HNgoc' (59.0%), and second highest in 'DNgoc' and 'LBi' (58.6%). LA of all sweet potato varieties was 24.5% of the total of PUFA, which was about a half of the value for ALA (41.7%). Palmitic acid contributed about 14.9% of the total fatty acids. In particular, palmitic acid contributed up to 52.7% of PFA, while six PFA, including capric acid, lauric acid etc contributed less than 50%.The C18:2 and C18:3 fatty acids were the main components of PUFA, accounting for up to 93-96%of the total PUFA.

Fresh sweet potato tubers can replace 30-50% of the grain in pig diets. In addition, the young leaves of sweet potato are highly relished by livestock, are a valuable source of protein, and the foliage lasts throughout the dry season (Gohl 1981). The fresh leaves and vines generally contain 19.4 to 21.9 % CP in DM basis (Gohl 1981). The five sweet potato varieties in the Mekong Delta had CP contents that ranged from 19.8% ('HDao') to 26.8% ('DNgoc'). Thus 'DNgoc' could be a better protein source for monogastric animals than the other varieties. In particular, the Methionine content of 'DNgoc' was higher than in several soybean varieties. Moreover, the PUFA content in sweet potato leaves is very high, especially C18:3, as reported by Linh (2002). Thus, sweet potato, particularly the 'DNgoc' variety, can be considered as a good source of protein and oil for use in animal diets. The direction of the selection of sweet potato varieties, as mentioned above, should take into account not only tubers but also leaves and stems, to achieve its multiple-purpose role. In addition it is worth noting that the selection of 'DNgoc' has been done by farmers themselves, due to its high quality with respect to taste, and the quality of leaves and stems is also very good, with respect to its value as an animal feed.

Water spinach

Total protein bands, determined by SDS-PAGE electrophoresis of the 'HAT'and 'NUOC' were almost the same (Table 3), although some individual proteins were slightly different. In addition, the values of Ho (5.87) and SENA (1.99) of the 'NUOC' were higher than those of 'HAT' and 'DONG', although the genetic marker, H_EP, had almost the same values (0.62 in 'DONG' to 0.67 in 'NUOC').The genetic diversity values of 'NUOC' were the highest in Ho and SENA (Table 3).

Table 3. Genetic diversity values of the water spinach (Ipomoea aquatica) leaves of three varieties in the Mekong Delta
	Varieties1)			Average
	HAT	NUOC	DONG
Polymorphic individual (%)	35.0	83.0	50.0	56.0
Polymorphic band (%)	33.0	77.0	44.0	51.0
Ho	2.62	5.87	2.95	3.81
HEP	0.65	0.67	0.62	0.65
SENA	1.53	1.99	1.64	1.72
1) ’HAT’: the water spinach (WS) that grows on soil, ‘NUOC’: WS floats on water, and ‘DONG’: WS grows on rural fields a, b, c: Means in the same row with different letters are significantly different at p<0.01

The average CP of the water spinach on a DM basis was significantly (P<0.01) highest in 'HAT' (29.5%), followed by 'NUOC' (26.9%), and 'DONG' (18.6%)(Table 4). The EE content was significantly (P<0.01) higher in 'HAT' (7.2%) than in 'NUOC' (6.9%). The concentrations of Arg, Glu, His, Ileu, Leu, Lys, Met, Phe, Thr and Val (on a DM basis) were highest in 'HAT', though the Gly content was higher in 'NUOC'. The proportion of most amino acids relative to Lysine was higher in 'NUOC' than in the other varieties. PUFA in the three varieties were around 75% of total lipid, with the content of ALA acid ranging from 45.1-47.3 %.

Generally, water spinach, with its high biomass yield, gives high incomes from harvesting and selling it in local markets (Prak Kea et al 2003). Water spinach is a vegetable with a high potential to convert efficiently the nitrogen in urea into edible biomass with a high protein content (Luyen 2003). Thus, it is interesting as a source of food and/or feed protein, because the edible portion contains a proportion of CP that is comparable with alfalfa leaves (22.3% on a DM basis) (Bruemmer and Roe 1979).

 Of the samples of water spinach in the study the 'DONG' variety had the lowest content of CP (18.6%), probably because it often grows in the seasonal rice fields after harvest, in the margins of the rice fields or in ditches. Thus, it grows in water with low levels of nutrients, resulting in low CP and amino acid contents. However, this variety is still a useful natural source of nutrients for animals and humans. The 'HAT' variety (29.5% CP) grows well and has a high nutritive value under good management, and although people usually eat the young shoots the remaining parts are still valuable for feeding to animals. However, to achieve such a high protein content (Table 4), 'HAT' should be fertilized and managed well, with pesticides and fertilizer applied. Therefore, this means that there can be problems of environmental pollution and residues on the vegetable, which can be harmful for humans and animals. The CP of the 'NUOC' (26.9%) was lower than that of the 'HAT', but all the analyzed amino acids of 'NUOC' in terms of the relative values to Lys were higher than those of the 'HAT' variety. As the 'NUOC' variety grows on the surface of canals, rivers and the lakes, it can absorb natural nutrients in the water in order to accumulate photosynthetic products. Moreover, the 'NUOC' could help to reduce water pollution, and be used to treat the waste water from the cities.

Water hyacinth

Results from protein profiles (Table 5) show that for 'WH-s', Ho had the highest value (6.26) while 'WH-c' (5.62) and 'WH-d' (4.28) had lower diversity. Although their Ho values were somewhat different, their H_EP values were almost the same, with the values ranging from 0.67 in WH-d to 0.70 in 'WH-c' and 'WH-s'. The SENA in 'WH-s' (2.36) was higher than that of 'WH-c' (2.29) and WH-d (2.04). Water hyacinth is regenerated mainly by stolon propagation. Thus, the natural population values of genetic diversity parameters (Ho ranged from 4.28 to 6.26, and SENA ranged from 2.04 to 2.36, Table 5) were similar to the values of the 'NUOC' water spinach variety (Ho=5.87, SENA=1.99, Table 3). So far, there have been no efforts to select elite water hyacinth lines from the diverse populations in the Mekong Delta, particularly from water hyacinth found in the canals, which would be valuable because of its very high CP (21.8%) and higher biomass and usefulness in reducing environmental pollution.

Table 5. Genetic diversity values of the water hyacinth (Echhornia crassipes) leaves of three populations in the Mekong Delta
	Populations1)			Average
	WH-s	WH-c	WH-d
Polymorphic individual (%)	80.0	80.0	60.0	73.0
Polymorphic band (%)	67.0	50.0	50.0	56.0
Ho	6.26	5.62	4.28	5.39
HEP	0.70	0.70	0.67	0.69
SENA	2.36	2.29	2.04	2.23
)WH-s: Water hyacinth (WH) found in the Cantho City area; WH-c: WH found on river and canal  banks;  WH-d: WH that grows in ditches around farms

The average CP of the young light green water hyacinth on a DM basis (Table 6) was significantly (P<0.01) highest in 'WH-c' (21.8 %), next highest in 'WH-d' (18.3 %), and lowest in 'WH-s' (16.4 %). The EE content was significantly (P<0.01) higher in WH-d (4.8 %) than in the other samples. The concentrations of Arg, Glu, Ileu, Leu, Lys, Met, Phe and Val on a DM basis were highest in the 'WH-c', though the contents of Gly and Thr were higher in the 'WH-d' and His in the 'WH-s'. The proportion of Arg, Gly, Ileu, Leu, Phe, Thr and Val relative to Lys was higher in 'WH-d' than in the other water hyacinth varieties. The highest content of PUFA was also ALA in water hyacinth. PUFA of the three populations of water hyacinth accounted for around 70 % of total lipid, like water spinach, and the range was 37.6 to 38.6%.

The aerial part of water hyacinth contains a range of 14.4 % to 16.4 % CP on a DM basis (Ly et al. 2002). The young, light green leaves and leaf stalks of water hyacinth sampled in ditches contain a higher level of protein (18.6% CP) on a DM basis (Men et al. 2003). The samples in the present study had a similar average content of CP (18.3%). As for absolute values of amino acid composition (% of DM), 'WH-c' was highest in Arg, Glu, Iso, Leu, Lys, Met, Phe, and Val but lower in Thr, Gly, and His (Table 6). However, based on the values relative to Lys, (Lys=100), the value of these amino acids in parentheses were not corresponding to the absolute values of amino acids. In reality, the values relative to Lys indicate the nutritive balance in each material source; thus, the most appropriately balanced value should be selected according to kind of animal for which it is intended. The oils extracted from the water hyacinth are also a rich source of PUFA, such as LA and ALA, which are generally related to the prostaglandin metabolites and can also convert into the EPA, as in other plant lipids, such as those in spinach and sweet-potato leaves (Linh 2002).

Table 6. Nutritional value of water hyacinth (Echhornia crassipes) in the Mekong Delta1)
	Populations2)						Standard Error
	WH-s		WH-c		WH-d1)
Fresh basis, %
DM	6.0b		4.0a		7.1c		0.08
CP	1.0b		0.9a		1.3c		0.02
EE	0.3b		0.2a		0.4c		0.01
Dry matter basis
CP	16.4a		21.8c		18.3b		0.42
EE	4.4a		4.3a		4.8b		0.07
Amino acid composition, % DM
Arginine	1.11	(134)	1.35	(131)	1.30	(232)
Glutamic acid	1.54	(184)	1.81	(175)	0.44	(79)
Glycine	0.80	(96)	1.01	(97)	1.13	(202)
Histidine	0.34	(41)	0.28	(27)	0.32	(57)
Isoleucine	0.87	(105)	1.11	(108)	0.88	(157)
Leucine	1.46	(176)	1.80	(175)	1.47	(263)
Lysine	0.83	(100)	1.03	(100)	0.56	(100)
Methionine	0.14	(17)	0.36	(35)	0.25	(45)
Phenylalanine	0.91	(110)	1.13	(110)	0.88	(157)
Threonine	0.57	(69)	0.67	(65)	0.92	(164)
Valine	0.94	(113)	1.21	(117)	1.01	(180)
Fatty acids, % of lipid
C12:0 (Lauric acid)	0.0		0.2		0.0
C14:0 (Myristic acid)	0.3		0.2		0.4
C16:0 (Palmitic acid)	21.4		22.0		21.8
C16:1 (Palmioleic acid)	3.7		3.3		3.7
C18:0 (Stearic acid)	1.2		1.4		1.2
C18:1 (Oleic acid)	2.5		2.7		2.6
C18:2 (Linoleic acid)	31.5		31.1		31.4
C18:3 (Linolenic acid)	38.6		37.6		38.2
C20:0 (Arachidic acid)	0.4		0.3		0.3
C22:0 (Behenic acid)	0.4		0.2		0.0
Unsaturated Fatty Acids, % of total	76.3		75.4		76.2
Saturated Fatty Acids, % of  total	23.7		24.6		23.8
a, b, c: Means in the same row with different letters are significantly different at p<0.01

Conclusions

• The sweet potato (DNgoc), water spinach (NUOC) and water hyacinth elite lines can be recommended as good local resources for animal feeding at farm level.

• In general the sweetpotato varieties studied had less genetic diversity than the water spinach and water hyacinth varieties. Consequently, collection and conservation of the local varieties is needed to maintain the diverse balance and allow the further selection of valuable traits.

References

AOAC 2000: Official Methods of Analysis. Animal Feed. Association of official analytical chemist, Washington, DC., USA, pp.1-54.

Boyd C E 1970: Vascular aquatic plants for mineral nutrient removal from polluted waters. Economic Botany 24: 95-103.

Bruemmer J H and Roe B 1979: Protein extraction from water spinach (Ipomoea aquatica). In: Proceedings Florida State Horticultural Society 92: 140-143.

Casabianca-Chassany M, Boonne C and Basseres A 1992: Echhornia crassipes systems on three ammonium-containing industrial effluent: production and purification. Bio-source Tech. 42: 95-101.

Dat D H 2002: Cultivated Vegetables Manual. Hanoi Publishing House, 180pp (in Vietnamese).

Göhl B 1981: Tropical feeds. FAO, Rome. Animal Production and Health Series 12, 240- 241, 254, 333-336.

Hau N T P H 1998: The yield of water spinach cultivated on water surface and used in the pregnant sow diets in the Mekong Delta. B. Sc. thesis, Cantho University (in Vietnamese).

Hoang V T, Hoanh M T, Tien T N and Ho T V 1998: The sweet potato in Vietnam. Report of the "Workshop on Sweet Potato Improvement in Asia", help in ICAR, Indian Council of Agricultural Research, Oct 24-28, pp.105-182.

Hoanh M T and Vinh N C 2003: Varieties and intensive technology for root and tube crops. Agricultural Publishing House, Hanoi, 196pp (in Vietnamese).

Horton D 1988: Underground crops. Long term trends in production of roots and tubers. Winrock International.

Huh M K and Ohnishi O 2002: Genetic diversity and genetic populations of wild radish revealed by AFLP. Breeding Science 52: 79-88.

Huub J G and Siemen V 1998: Duckweed Based Wastewater Treatment for Rational Resource Recovery and Re-use. In: Environmental Biotechnology and Cleaner Bioprocesses.

Lam V 2003: Agricultural potential of the sweet potato (Ipomoea batatas L. (LAM.)) for forage production and sweet potato vine as a feed for growing goats. MSc. thesis. Swedish University of Agricultural Sciences.

Linh N Q 2002: Fatty Acids Supply of Growing Pigs in Central Vietnam. Doctoral thesis. Faculty of Veterinary Medicine. Utrecht University. The Netherlands.

Ly Thi Luyen and Preston T R  2004: Effect of level of urea fertilizer on biomass production of water spinach (Ipomoea aquatica) grown in soil and in water.  Livestock Research for Rural Development. Vol. 16, Art. #81. Retrieved, from http://www.cipav.org.co/lrrd/lrrd16/10/luye16081.htm

Ly L V 2000: Ruminant production in Vietnam and development of forage in smallholders farm. http//: www.bangladoot.com/mainul/papers-rf/v95-57.pdf

Man L N 1994: Identification of chemical composition of some unconventional feeds. BSc. thesis. Cantho University (in Vietnamese).

Men L T, Yamasaki S, Khang N T K, Quyen N N B and Takada R 2002: Evaluation of water hyacinth diets for fattening pigs in Tan Phu Thanh village. In: Proceedings of the 2002 Annual Workshop of the JIRCAS Mekong Delta Project. Vietnam.

Minh D 1999: Vegetables textbook. Cantho University, Cantho City, 94pp (in Vietnamese).

NRC 1998: Nutrient Requirement of Swine, 10th ed. National Academy Press, Washington, D.C., 189pp.

Phuc B H N 2000: Tropical forages for growing pigs. Digestion and nutritive value. Doctoral thesis. Swedish University of Agricultural Sciences. Uppsala. Sweden

Prak Kea, Preston T R and Ly J 2003 Feed intake, digestibility and N retention of a diet of water spinach supplemented with palm oil and / or broken rice and dried fish for growing pigs.  Livestock Research for Rural Development (15) 8 Retrieved, from http://www.cipav.org.co/lrrd/lrrd15/8/kea158.htm

Preston T R 1995: Tropical Animal Feeding. A manual for research workers. FAO, Rome. Animal Production and Health 126: 305pp.

Ryan B, Joiner B L and Ryan T A 2000: Minitab statistical software release 13. Duxbury Press.

Thanh V C, Ky H, Thao T B and Hirata Y 2003: Evaluation of soybean related genetic resources in the Mekong Delta, and the identity of a new wild soybean related species Glycine mekongensis. Biosphere Conservation 5 (2): 53-61.

Tiwari N C and Chandra V 1985: Water spinach its varieties and cultivation. National Botanical Research Institute. Indian Horticulture.

Go to top