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

Abstract

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.