Slurry-grown duckweed (Spirodela polyrhiza) as a means to recycle nitrogen into feed for rainbow trout fry

Liquid manure from livestock production systems is a major source of nitrogen and phosphorus release from nutrient cycles and a cause of ecosystem eutrophication. Duckweeds, small aquatic plants, may be used to recover N and P from livestock slurry while producing high-quality protein feed. In order to assess N and P uptake efficiency and utility for fish feed, two duckweed species, Landoltia punctata and Spirodela polyrhiza, were grown in controlled climate chambers on two nutrient-rich media: diluted (1:10) cattle slurry and mechanically filtered household sewage. Treatments were in triplicate, each running in four cycles with fresh substrate (one week each). Spirodela polyrhiza exhibited the strongest growth (96 g fresh matter m−2 day−1) and highest protein content (306 g per kg dry matter) on diluted slurry. The weakest growth was found for L. punctata on treated sewage (52 g fresh matter m−2 day−1). Average removal of total provided and utilizable inorganic N from the media was 73.2% and 83.9% for sewage and diluted slurry, respectively. Spirodela polyrhiza grown on diluted slurry was subsequently tested as feed ingredient for rainbow trout (Oncorhynchus mykiss) fry. Two different ingredient levels of S. polyrhiza meal (6.25% and 12.5% of feed) were fed to rainbow trout fry for 4 weeks, during which fish growth, feed and nutrient utilization and gut health were assessed. Feed was accepted, but both duckweed meal treatments resulted in 5–10% poorer growth traits and feed efficiency compared to control. The intestine somatic index was not affected. This is the first time the potential of duckweed as feed for rainbow trout fry has been demonstrated. Furthermore, our experiments found considerable N and P uptake from diluted slurry by S. polyrhiza, which produced protein at a high rate per unit time and area

Dilution rates of cattle slurry affect ammonia uptake and protein production of duckweed grown in recirculating systems

Duckweed is a potential collector of nitrogen from animal liquid manure and a source of protein suitable as feed for livestock and fish. Therefore, it provides opportunities for circular economy systems. Two duckweed species, Spirodela polyrhiza and Landoltia punctata, were grown in five recirculating systems each connected to a reservoir tank filled with water and graded organic cattle slurry concentrations. Fresh and dry biomass, protein production and amino acid profiles among the nitrogen removal were evaluated. Spirodela polyrhiza showed a significantly higher fresh biomass production but L. punctata dry matter content was generally higher resulting in similar dry biomass production for both species This study shows clearly that the crude protein content, ranging between 29.3 and 37.9% of dry matter, was positively correlated to slurry and total ammonia nitrogen (TAN) concentration of the substrate, independent of the duckweed species. Total crude protein yield was in the range of 1.37–1.95 g/m2/d, following a quadratic function regarding slurry and TAN concentrations, with marginal differences between species. Biomass and crude protein yields were optimal for both duckweed species at a TAN concentration of 19 mg/l, which corresponded to a slurry dilution of 1:8. The results of this study provide important information for operation of recirculating duckweed production systems on slurry and operators should aim to keep TAN concentrations in that range for optimization of protein production in conjunction with TAN removal