Duckweed - a potential high-protein feed resource for domestic animals and fish

Summary Duckweeds have received research attention because of their great potential to remove mineral contaminants from waste waters emanating from sewage works, intensive animal industries or from intensive irrigated crop production. Duckweeds need to be managed, protected from wind, maintained at an optimum density by judicious and regular harvesting and fertilised to balance nutrient concentrations in water to obtain optimal growth rates. When effectively managed in this way duckweeds yield 10-30 ton DM/ha/year containing up to 43% crude protein, 5% lipids and a highly digestible dry matter. Duckweeds have been fed to animals and fish to complement diets, largely to provide a protein of high biological value. Fish production can be stimulated by feeding duckweed to the extent that yields can be increased from a few hundred kilograms per hectare/year to 10 tonnes/ha/year. Mature poultry can utilise duckweed as a substitute for vegetable protein in cereal grain based diets whereas very young chickens suffered a small weight gain reduction by such substitution. Pigs can use duckweed as a protein/energy source with slightly less efficiency than soyabean meal. Little work has been done on duckweed meals as supplements to forages given to ruminants, but there appears to be considerable scope for its use as a mineral (particularly P) and N source. The protein of duckweeds requires treatment to protect it from microbial degradation in the rumen in order to provide protein directly to the animal. The combination of crop residues and fresh duckweeds in a diet for ruminants appears to provide a balance of nutrients capable of optimising rumen microbial fermentative capacity. These diets can, therefore, be potentially exploited in cattle, sheep and goat production systems particularly by small farmers in tropical developing countries.