The surface floating duckweed Lemna minor (Lemnaceae) is a potential ingredient to replace the application of fish-meal in the aqua-feed. The culture technique of the duckweed was standardized in outdoor tanks and then applied in the pond. Three consecutive experiments were conducted in tanks (1.2 × 0.35 × 0.3 m). In experiment 1, four different manures were used. In manure 1 (organic manure, OM) and manure 3 (2x OM), cattle manure, poultry droppings, and mustard oil cake (1:1:1) were used; in manure 2 (inorganic fertilizer, IF), urea, potash, triple superphosphate were used; manure 4 (2x OM+IF) was a combination of manure 2 and manure 3. In experiment 2, manure 1 (OM) and manure 2 (IF) were used, and manure 3 (OM+IF) was a combination of both manures. In experiment 3, OM and IF were selected. In pond (20 × 10 × 0.5 m), OM was applied. Fresh duckweed was seeded after 5 days of manure application. In experiments 1 and 3, total production was significantly (P < 0.05) higher in OM compared to other treatments. In experiment 2, there was no significant (P > 0.05) difference in production between OM and IF. In pond, relative growth rate (RGR) of duckweed ranged from 0.422 to 0.073 g/g/day and total production was 702.5 Kg/ha/month (dry weight). Protein, lipid, and ash contents were higher in duckweed cultured in OM compared to IF. The duckweed was a rich source of essential (39.20%), non-essential (53.64%), and non-proteinogenic (7.13%) amino acids. Among essential amino acids, leucine, isoleucine, and valine constituted 48.67%. Glutamic acid was 25.87% of total non-essential amino acids. Citrulline, hydroxiproline, taurine, etc. were found in the duckweed. The fatty acid composition was dominated by PUFA, 60–63% of total fatty acids, largely α-linolenic acid (LNA, 18:3n-3) at around 41 to 47% and linoleic acid (LA, 18:2n-6) at 17–18%. The nutritional value of duckweeds and their production potential in the pond conditions were evaluated. Duckweed biomass may thus be used to replace commercial fish-meal that is currently used in aquaculture.

The culture technique of greater duckweed Spirodela polyrhiza (L. Schleiden) was standardized in outdoor tanks using three different manures: manure 1-cattle manure, poultry droppings and mustard oil cake, manure 2-urea, potash and triple superphosphate and manure 3-cattle manure, urea, potash and triple superphosphate. Significantly (p<0.05) higher production was recorded in manure 1 compared to others. Manure 1 was subsequently selected for pond culture. In ponds, the production of duckweed was 2020±150 kg ha-1 month-1 dry weight basis. Protein content was significantly higher (p<0.05) in duckweed cultured in manure 1. The amino acid profile study showed the presence of essential (37.4%), non-essential (58.2%) and free (4.5%) amino acids. Leucine, isoleucine and valine contributed 51.4% of total essential amino acids. Duckweed contained 7% lipid and α-linolenic acid (36-37%) was the major fatty acid. The study showed the nutritional value of duckweed as an animal feed ingredient.

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The potential nutritional value of duckweed Lemna minor (Lemnaceae) was evaluated for common carp Cyprinus carpio fry. Fish were fed diets containing five graded levels of duckweed: 0% (LM0, control), 5% (LM5), 10% (LM10), 15% (LM15) and 20% (LM20). The final weight and specific growth rate were significantly higher in LM15 and LM20 diets fed fish compared to others. Feed conversion ratio was minimum in fish fed diet LM20. Amylase activity was significantly higher in LM0 treatment. Total protease, trypsin and chymotrypsin activities showed linear relationships with the increased level of duckweed in the diet. Protein and essential amino acids contents were significantly higher in carp fed diets LM15 and LM20 compared to others. Lipid content was significantly higher in fish fed duckweed-based diets compared to control. A direct relationship was found between the inclusion level of duckweed in the diet and n-3 long-chain polyunsaturated fatty acid (LC-PUFA) content of carp. Contents of desaturated and elongated products of dietary linolenic acid (18:3n-3) including 20:4n-3, 20:5n-3, 22:5n-3 and 22:6n-3 increased in a graded manner with increasing dietary duckweed. The monounsaturated fatty acids and n-6 PUFA contents reduced significantly in fish fed duckweed. Expression of fads2d6, elovl2, elovl5 and fas were higher in carp fed diets LM10, LM15 and LM20 compared to control fish. The inclusion of L. minor in diet enhanced the nutritional value of carp by increasing protein, lipid, amino acids and n-3 PUFA contents.

The freshwater macrophytes are abundant in tropical and subtropical climates. These macrophytes may be used as feed ingredients for fish and other animals. The nutritional value of twelve freshwater-cultured macrophytes was evaluated in the present study. Significantly higher crude protein (36.94–36.65%) and lipid (8.13–7.62%) were found in Lemna minor and Spirodela polyrhiza; ash content was significantly higher in Hydrilla verticillata, Wolffia globosa, and Pistia stratiotes (20.69–21.00%) compared with others. The highest levels of sodium, magnesium, chromium, and iron levels were recorded in P. stratiotes. H. verticillata was a rich source of copper, manganese, cobalt, and zinc; the contents of calcium, magnesium, strontium, and nickel were highest in S. polyrhiza. Selenium and potassium contents were higher in Salvinia natans and W. globosa, respectively. The n-6 and n-3 polyunsaturated fatty acids (PUFAs) contents were significantly higher in W. globosa and Ipomoea aquatica, respectively compared with others. Linoleic and α-linolenic acids were dominant n-6 and n-3 PUFAs. The highest value (4.04) of n-3/n-6 was found in I. aquatica. The ratio ranged from 0.61 to 2.46 in other macrophytes. This study reveals that macrophytes are rich sources of minerals, n-6 and n-3 PUFAs.