| Livestock Research for Rural Development 10 (3) 1998 | Citation of this paper |
The research had the following objectives: to document the observed improvements in
nutritive value (protein content) and yield of duckweed when biodigester effluent, rather
than original manure, is used to fertilize duckweed ponds; to determine optimum
fertilizing level of manure and effluent for duckweed; to determine if there are
differences between manure and effluent from cows versus pigs as fertilizer for duckweed
ponds.
The treatments were:
The design was a factorial arrangement (2*2*3) and there were 4 replications in a completely randomized layout. The experiment was conducted for 24 days in the rainy season (August 1998). Plastic baskets (n=48) lined with plastic film were used as the experimental ponds. The surface of the water in each basket had an area of 0.145 m² and with 10 cm depth the volume was 14.5 litres. Each container was inoculated with 40g (250g/m2) of duckweed (Lemna spp.). The yield of duckweed was calculated by subtracting the inoculum from the total biomass production measured every 24 hours and was expressed as fresh duckweed (DW) yield g/m2/d. Manure and effluent were taken from two plug-flow tubular plastic biodigesters: one charged with cow manure the other with pig manure. Manure and effluent were added daily in quantities calculated to maintain pond nitrogen levels at approximately10, 20 and 30 mg/litre.
The results of the experiment showed that with the same input of nitrogen, plants nutrients derived from biodigester effluent supported higher concentrations of crude protein in duckweed, than nutrients from raw manure. Manure and effluent from pigs tended to support higher concentrations of crude protein in duckweed than when cows were the source of these inputs. The optimum level of nitrogen in the pond water was in the range of 20 to 30 mg/litre. Root length of duckweed was inversely related with protein content. Higher pH of the pond water in the range pH 6.4 to 7.2 was associated with duckweed of higher protein content.
About 80% of the Vietnam population is engaged in the farm sector. They are trying very hard every day to maintain the productivity of their farm plots which are getting smaller every year. Of course their initiatives are limited by poverty. It seems that farmers have themselves considered everything in a good order that fits their needs for income, marketability of the product, the climate and the available feed resources.
Social and natural changes require that people adapt to new activities. It is clear that while basing these activities on the traditional practices, it is necessary to have improvements, eliminate outdated practices and establish an improved system so that a better harmony can be reached. For the farmers, the most suitable production system would appear to be that which continues to satisfy the increasing needs of food for their own needs, sending the surplus to the market to buy other commodities required by their families.
There are many animal feed resources in the tropics such as sugar cane, cassava roots and foliage, leaves from leguminous and other multi-purpose trees, and water plants. With appropriate balancing of the locally available feed resources it is argued that it is possible to have higher levels of production per unit area compared with when only the conventional feeds are used (Preston 1998). To achieve this goal, of optimizing the use of available of feed resources in a locality to secure maximum income, the chosen feeding systems do not aim at achieving maximum daily weight gain, but more importance is given to local availability and price, and to efficiency of overall resource utilization.
Duckweed is one of the outstanding local resources in the tropics. It is probably the fastest growing of all multi-cellular plants. It grows naturally on waste water and can double its weight in 24 hours. It is unique amongst plants in that its protein content can be manipulated according to the nitrogen content of the water in which it is growing (Leng et al 1995; Rodriguez and Preston 1996a). This is important because it facilitates integration of duckweed ponds with biodigesters. It is the ideal water plant to introduce into an integrated farming system because it can use the nitrogen in the effluent coming from the biodigester to enrich its protein content to a level only slightly lower than soya bean, approaching 40% (in dry matter).
The fact that protein yields of duckweed can be as high as 10 tonnes/ha/year (Preston 1998), compared with less than one tonne per year for soya bean protein, highlights the potential value of this plant at the farm level. The critical factor appears to be the protein content, which in turn depends on the nutrient status of the medium on which it is grown. This relationship was shown by Leng et al (1995) when they analyzed duckweed (Spirodela spp) grown on sewage water. The protein content rose from 20% to almost 40% in dry matter as the N content of the water was increased from 5 to 40 mg/litre.
Duckweed has been known for a long time as a potential food for humans and animals, and as a source of natural products. Duckweed has been used as the only source of supplementary protein for fish (PRISM 1997), chickens (Haustein et al 1990), ducks (Bui Xuan Men et al 1995) and pigs (Rodriguez and Preston 1996b).
In Vietnam, the most common farm manures are from cattle, buffaloes and pigs. Fresh or anaerobically fermented farm manures are rich sources of nutrients which can be used for growing duckweed economically. Duckweed is good for the environment because it doesn't require artificial fertilizers; on the contrary it cleans up waste by removing organic and inorganic nitrogen coming from decomposition of organic matter, contributing to the fight against eutrophication. It doesn't need fungicides and has no significant natural pests.
The development of a manure-based duckweed production system and its utilization as livestock feed is essential for sustainable livestock farming in this country. Therefore, the present research programme has been undertaken with the following objectives.
The experiment was done at the "Finca Ecologica" on the Campus of the College of Agriculture and Forestry, Thu Duc district, Ho Chi Minh city. This is a small (3,000 m²) experimental farm, established to demonstrate integrated farming systems with perennial crops, multi-purpose trees, local breeds of livestock, low-cost plastic biodigesters and duckweed ponds (www.hcm.fpt.vn/inet/~ecofarm). The area is close to sea level with ranges in temperature from 24 to 38 ºC, and relative humidity in the range 40 to 100%.
The treatments were:
The treatments were arranged as a 2*2*3 factorial and there were 4 replications in a completely randomized block design. The experiment was conducted for 24 days in the rainy season (August 1998) The data were analyzed using analysis of variance (GLM) in Minitab Version Release 10.2. The sources of variation in the ANOVA were: animal species (cow vs pig), processing (manure vs effluent), interaction (species*processing), pond N levels and error.
Plastic baskets (n=48) lined with plastic film were used as the experimental ponds. The surface of the water in each basket had an area of 0.145 m² and with 10 cm depth the volume was 14.5 litres. Each container was inoculated with 40g (250g/m2) of duckweed (Lemna spp.). The yield of duckweed was calculated by subtracting the inoculum from the total biomass production measured every 24 hours and was expressed as fresh duckweed ( DW) yield g/m2/d.
Manure and effluent were taken from two plug-flow tubular plastic biodigesters: one charged with cow manure the other with pig manure. An explanation and description of the biodigester system can be found in the paper by Bui Xuan An et al (1997). Fresh samples of manure and effluent were sampled for DM and nitrogen. The amounts of manure and of effluent to be added to the ponds were determined by the content of N found in these materials.
Samples of water and duckweed were collected at each harvest for determination of nitrogen and dry matter. Root length of the duckweed was measured by extending 10 individual plants from each sample on millimetric paper and taking the average length. Dry matter was determined by weighing before and after drying to constant weight in a micro-wave oven (Undersander et al 1993) and nitrogen by Kjeldahl (AOAC 1985) using a Tekator apparatus. The samples of duckweed were analyzed immediately after harvest because when the samples were stored in a refrigerator there were obvious physical changes in their appearance and it was considered this might affect the results.
Table 1 shows that dry matter and total nitrogen contents of manure and biodigester effluent from pigs were higher than from cows.
| Table 1. Dry matter (DM) and Nitrogen (N) content of different types of manure were used on Duckweed | ||
Type of manure |
Dry matter (%) |
Total Nitrogen (% of DM) |
Fresh cow dung (CM) |
18.80 |
1.83 |
Fresh pig dung (PM) |
24.05 |
2.40 |
Digested cow dung (CF) |
3.12 |
1.65 |
Digested pig dung (PF) |
2.30 |
2.64 |
 Figure 1: Relationship between intended and actual levels of nitrogen in pond water |
 Figure 2: Relationship between N in pond water and biomass yield |
On the whole, the intended levels of nitrogen in the pond water were achieved as demonstrated by the mean values for this parameter during the trial (Figure 1). There was, however, a poor relationship between measured levels of N in pond water and biomass yield (Figure 2).
It can be seen from Figure 3 that there was an interaction (P=0.001) between source of nutrients (manure or effluent) and length of growing period. Manure supported higher biomass yield in the first week but the rate of decline in yield with time was faster for manure than for effluent, thus in the third week yields were higher for the effluent. These trends were the same for manure and effluent from both cows and pigs. Increasing the application rate of the manure / effluent (which aimed to establish levels of 10, 20 and 30 mg N/litre) led to increased biomass yield with little difference between the two higher levels (Figure 4). Overall, the manure supported slightly, but significantly, higher yields than the effluent.
 Figure 3: Biomass yield of fresh duckweed fertilized with biodigester effluent or manure from cattle or pigs averaged over three consecutive 7-day periods |
 Figure 4: Biomass yield of fresh duckweed fertilized with biodigester effluent or manure from cattle or pigs at three intended levels of N in the water |
Crude protein content of duckweed was higher (P=0.001) and root length shorter (P=0.001) when the ponds were fertilized with effluent rather than manure (Figures 5 and 6). There were increases (P=0.04) in crude protein content when the manure or effluent was from pigs rather than cattle. There was also a tendency (P=0.19) for root length to be shorter with manure and effluent from pigs rather than cows. Root length was negatively correlated with protein content of duckweed (Figure 7; R²=0.84) and with biomass yield (Figure 8; R²=0.40).
 Figure 5: Effect of biodigester effluent or manure from cows and pigs on crude protein content of duckweed |
 Figure 6: Effect of biodigester effluent or manure from cows and pigs on root length of duckweed |
 Figure 7: Relationship between root length and crude protein content of duckweed |
 Figure 8: Relationship between root length and biomass yield of duckweed |
The experimental procedure, of varying input levels of manure and effluent so as to achieve nitrogen concentrations in pond water in the range from 10 to 30 mg/litre, was largely successful. However yield was poorly related with the nitrogen content of the pond water. This is in marked contrast with results reported by Leng et al (1995), Rodriguez and Preston (1996a) and Nguyen Duc Anh et al (1997) where the two elements were strongly and positively related.
The increase in protein content of duckweed when it was fertilized with biodigester effluent compared with manure, from which the effluent was derived, is similar to what was observed in cassava foliage from plants receiving similar treatments (Effluent or manure) (Le Ha Chau 1998). This effect implies that during the process of anaerobic biodigestion the nitrogenous compounds in the manure become more available to the plant (presumably the conversion from organically-bound nitrogen to NH4+), and that the effect is similar in duckweed and in cassava. By contrast, the use of effluent as opposed to manure did not increase biomass yield of duckweed but increased foliage biomass yield in cassava (Le Ha Chau 1998). There is no apparent explanation for these contrasting effects. The indications of a relationship between pH of the pond water and the crude protein content of the duckweed (r = +0.39) and pH and the root length (r = -0.32) could be interpreted as an effect of increased concentration of NH4+ ions being the determinant of the protein status of the duckweed.
The close relationship (R²=0.84) between root length of the duckweed and the protein content of the whole plant is in agreement with earlier reports by Rodriguez and Preston (1996) and Nguyen Duc Anh et al (1997) and confirms the usefulness of this simple measurement to assess the protein status of duckweed. The tendency (R²=0.40) for biomass yield to be negatively related with root length suggests that this latter parameter can also be used as an indicator of yield, albeit an approximate one, as well as quality.
 Figure 9: Relationship between pH of pond water and crude protein in duckweed |
 Figure 10: Relationship between pH of pond water and root length of duckweed |
The results of the experiment strongly indicate that:
I gratefully extend my sincere thanks to Dr. Thomas R Preston for his valuable help in this study and his dedication to the sustainable agriculture in developing countries. I also thank Lylian Rodriguez for support in applying the technology and advice on practical problems of using the biodigester. My thanks to Nguyen van Lai for advice on analytical procedures and assistance in the laboratory. I also wish to express my gratitude to Dr. La Van Kinh and especially the staff of the Department of Animal nutrition of Institute of Agricultural Science of South Vietnam for creating a pleasant environment and research facilities. The Danish Embassy in Hanoi is acknowledged for the financial support to the UTA foundation which made it possible for me to carry out this research as partial fulfillment of the requirements for the Master of Science degree in Sustainable Use of Local Resources in Livestock-based Farming Systems.
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Received 11 December 1998