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Live stock production, climate change and resource depletion |
Two experiments were conducted on a prívate farm in Binh Thuy District, Cantho City to study: (i) the yield and composition of duckweed cultivated with different levels of biodigester effluent; and (ii) the effect on duckweed composition of a “shock” treatment of transferring high quality duckweed to plain water containing no nutrients. In experiment 1, the treatments were 6 levels (0 4, 8, 12, 16 and 20%) of biodigester effluent added to fresh water in plastic containers containing duckweed. The surface of water in each container was 0.4 m2 with 20 cm depth giving a volume of 60 liters. Duckweed was inoculated at a rate of 400 g/m2. The yield of duckweed was measured over a period of 14 days by removing and weighing one third of the biomass every 48h.
There was a curvilinear response in yield, and in crude protein content of the duckweed, to level of effluent with maximum values for both at a ratio of 12% effluent and 88% water (Figures 1 and 2). Duckweed from this treatment was then transferred to fresh water and the composition studied over 5 days. The content of starch increased and that of crude protein decreased with increasing time in the fresh water (Figures 3 and 4).
It was concluded that yield and crude protein content of duckweed was optimized when the culture medium contained 12% biodigester effluent and 88% water. The “shock” treatment of transferring the best quality duckweed to nutrient-free fresh water led to decreases in crude protein and corresponding increases in starch; however, the effects were relatively small and unlikely to have significance from the points of view of duckweed as a feed for ducks or as a substrate for ethanol production.
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Figure 1. Relationship between effluent concentration and biomass yield of duckweed |
Figure 2. Relationship between effluent concentration and crude protein content of duckweed |
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Figure 3. Relationship between time exposed to nutrient-free water and starch content of duckweed |
Figure 4. Relationship between time exposed to nutrient-free water and crude protein content of duckweed |
