Year round study of duckweed grown
on biodigester effluent

 

Nguyen Kim Khang

Cantho University, Cantho

 

Abstract

The research was carried out during 12 months from September 1998 to August 1999 in the experimental farm of Cantho University. The objective was to measure at monthly intervals  the yield of duckweed grown continuously for a one year cycle in a canal fertilized with effluent from a biodigester and to relate this to rainfall and sunshine hours.

 

Average monthly yields exceeded 100 g fresh biomass/m²/day with a protein content generally in the range of 35 to 37% in dry matter.  This is equivalent to a protein yield of around 600 kg/ha per month or over 7 tonnes/ha/year.  There was no apparent relationship between growth rate of the duckweed and either sunshine hours or rainfall.

 

Keywords: duckweed, biodigester, effluent, yield, sunshine hours, rainfall.

 

Introduction

The Mekong Delta with large areas of low land and extensive water resources provides good conditions for growing of water plants, especially duckweed (Lemna spp.). Duckweed has been recognized as a potential feed for animals and especially as a protein supplement for pigs (Le Thi Men et al 1994) and ducks (Bui Xuan Men et al 1995). Duckweed is good for the environment because it does not require artificial fertilizers; on the other hand, it cleans up waste by “removing organic and inorganic nitrogen contributing to the fight against eutrophication (Leng 1999)”.

 

Duckweed can grow on fresh or polluted water, so it is an ideal water plant to introduce into the integrated farming system because it can use the nitrogen in the effluent coming from biodigester to enrich its protein content. The crude protein content of duckweed depends on the nutrient content of the water upon which it grows (Leng 1999). According to Leng (1999), yield of duckweed in dry matter is reported to be in the range of 10 to 30 tonnes/ha/year giving a protein yield of from 6 to 10 tonnes/ha/year.

 

The objective of this study was to measure at monthly intervals  the yield and composition of duckweed grown continuously for a one year cycle in a pond fertilized with effluent from a biodigester and to relate this to rainfall and sunshine hours. record the yield of duckweed at months when affected by the sunshine, the rainfall.

 

Material and methods

Management and data collection

The experiment was conducted for 12 months from September 1998 to August 1999 at the experimental farm of Cantho University. The effluent came from a concrete biodigester (diameter 1.60m, length 3.73m and depth 1.75m) charged daily with  40 kg of from pigs. The effluent discharged directly into a canal pond (area 240 m²) which was divided into 4 parts. In each part, a bamboo square frame (1*1 m) was floated in which the measurements were made. Each frame was inoculated with 1 kg of fresh duckweed and after two days the total biomass was harvested and weighed. A fresh inoculum of duckweed was then added to the frame and the harvesting repeated two days later. This process was continued over a period of 10 days each month. The fresh biomass yield of duckweed was calculated by subtracting the weight of the inoculum from the total biomass production measured after 2 days. Samples of canal water and of duckweed were collected at each harvest time for determination of nitrogen and dry matter content. Dry matter was determined by weighing before and after drying to constant weight in a microwave oven (Undersand et al 1993) and nitrogen by the Kjeldahl method (AOAC 1990). Records for rainfall and sunshine hours were collected from the meteorological station of the University. The data were analyzed by ANOVA using the General Linear Model in Minitab Software (version 12).

 

Results and discussion

There were significant differences in yield of duckweed among months, with a tendency for an increase from November through to April (mean of 122 g/m2/day) and a decrease from May to December (119 g/m2/d) (Figure 1). There appeared to be no relationship between duckweed growth and sunshine hours (R² = 0.25) or rainfall (R² = 0.04).

 

There were significant differences in the protein content among months, with lower values in February and March. However, there were no differences from April to November when the crude protein ranged from 35.5 to 38.8% in dry matter. This is in agreement with the report of Nguyen Nhut Xuan Dung (1996) that the protein content of duckweed in the rainy season was higher than in other months. The capacity of the duckweed to produce protein is shown by the monthly yields of protein (Figure 2). The median value is about 600 kg/ha/month. 

 

Table 1: Meteorological data and yield and protein content of duckweed during a year cycle 1998-99
Months	Sunshine Hours	Rainfall mm	Temp (0C)	Yield (g/m2/d)	Crude protein (% in DM)
Sep. 98 Oct. 98 Nov. 98 Dec. 98 Jan. 99 Feb. 99 Mar. 99 Apr.99 May.99 Jun. 99 Jul. 99 Aug. 99	Na Na 6.42 6.33 6.17 6.18 5.2 6.5 6.4 5.76 3.72 7.72	na na 150 153 56.6 4.8 37.9 160 166 220 231 230	na na 26.5 25.3 25.9 26.1 27.7 27.3 27.2 26.4 26.4 26.6	138 111 140 120 120 115 121 142 108 105 107 66.6	30.8 35.9 32.5 30.8 30.6 28.2 27.7 35.5 36.1 35.5 38.8 36.7

 

Figure 1: Monthly variation in duckweed biomass yield and protein content

 

Figure 2: Monthly yield of protein from duckweed

Conclusion

• When it is fertilized with biodigester effluent, duckweed can produce continuously high yields of biomass with a protein content of 35 - 37% in the dry matter.
• There was no relationship between growth rate of the duckweed and either sunshine hours or rainfall

 

References

AOAC 1990  Official methods of analysis of the Association of Official Analytical Chemist (15^th Edition.), Washington, DC  1: 69-90.

Bui Hong Van, Le Thi Men, Vo Van Son and Preston T R 1997 Duckweed (Lemna spp) as protein supplement in an ensiled cassava root diet for fattening pigs. Livestock Research for Rural Development  (9) 1: http://www.cippaav.org.co/lrrd/lrrd9/1/lemen912

Bui Xuan Men, Ogle B and Preston T R 1996 Duckweed (Lemna spp) as replacement for roasted soya beans in diets of broken rice for fattening ducks on a small scale farm in the Mekong delta. Livestock Research for Rural Development  (8) 3:14-19

Leng R A 1999 Duckweed - A tiny aquatic plant with enormous potential for agriculture and environment. FAO/UTA .

 

Le Thi Men, Bui Hong Van, Mai Thi Chinh and Preston T R 1997  Effect of dietary protein level and duckweed (Lemna spp) on reproductive performance of pigs fed a diet of ensiled cassava root or cassava root meal. Livestock Research for Rural Development  (9) 1:1-5

 

Nguyen Nhut Xuan Dung 1996 Identification and evaluation of noncultivated plants used for livestock feed in the Mekong Delta of Vietnam. M.Sc.Thesis. Swedish University of Agricultural Science. Department of Animal Nutrition and Management. Uppsala 1996.

Undersander D, Mertens D R and Thiex N 1993  Forage analysis procedures.  National Forage Testing Association. Omaha   pp:154