Agri Kultuur December / Desember 2018 - Page 10

rendering them unavailable. The ability of phytic acid to bind metal ions is lost when the phosphate groups are hydrolyzed through the action of enzyme phytase. Although phytase activity has been shown to be present in ruminants, animals with a simple stomach such as fish lack this enzyme in their gastrointestinal tracts and hence cannot utilize the phytate bound phosphorus or other metal ions. Therefore, feeds are often supplemented with phosphorus in the form of mono or di- calcium phosphate. Phosphorus and calcium requirements are interdependent. Addition of microbial phytase in the diet of Nile tilapia significantly improved the growth of fish. Variations in the quantitative values reported in literature can also be expected due to differences in dietary ingredients used. Table 1: Essential amino acid requirements of young O. mossambicus and O. niloticus. Percent of dietary protein Amino acid O. mossambicus O. niloticus Arginine 2.82 4.20 Histidine 1.05 1.72 Isoleucine 2.01 3.11 Leucine 3.40 3.39 Lysine 3.78 5.12 Methionine 0.99 2.68 Phenylalanine 2.50 3.75 Threonine 2.93 3.75 Tryptophane 0.43 1.00 Valine 2.20 2.80 Duckweed: Duckweeds are small (1-15 cm) free-floating aquatic plants with worldwide distribution. They are monocotyledons belonging to the family Lemnaceae (which is derived from the Greek word ‘Limne’, meaning pond) and are AgriKultuur |AgriCulture classified as higher plants or macrophytes, although they are often mistaken for algae and some taxonomists consider them as being members of the Araceae. Duckweeds serve as nutrient pumps, reduce eutrophication effects and provide oxygen from their photosynthesising activity. Duckweeds are often seen growing in thick blanket-like mats on still nutrientrich fresh and slightly brackish waters. They do not survive in fast moving water (> 0.3 m/sec) or water unsheltered from the wind. They grow at water temperatures between 6 and 33 °C. Each frond of duckweed absorbs nutrients through the whole plant, not through a central root system, directly assimilating organic molecules such as simple carbohydrates and various amino acids. The entire body is composed of non-structural, metabolically active tissue; most photosynthesis is devoted to the production of protein and nucleic acids, making duckweeds very high in nutritional value. The nutritional content of duckweed is probably more dependent on the mineral concentrations of the growth medium than on the species or their geographic location. Water low in nutrients generally results in reduced nutritional content in duckweed. Crude fibre content is generally lower (varying between 7-10 percent) for duckweed grown in nutrient- rich water than that grown in nutrient-poor water (11-17 percent). Compared with most plants, duckweed leaves have little fibre (5 percent in cultured plants) as they do not need to support upright structures. Crude fibre content was generally lower, varying between 7-10 percent, for duckweed grown in nutrient- rich water as opposed to 11-17 percent for duckweed grown in nutrient-poor water. In general, the ash content ranges between 12-18 percent. Duckweeds are known to 10