Abstract
A fertilization strategy was developed with synthetic nitrogen (N) and phosphorus (P) supplementation of of low quality manure from small-scale farm cattle. Fertilization aimed at a balanced production of natural food organisms and the maintenance of a suitable environment for growth of Nile tilapia (Oreochromis niloticus). A series of four experiments were conducted in outdoor 5m2 static water concrete tanks, the design of each successive experiment was dependent upon the results of the preceding experiment. Experiment 1 was designed with a constant loading rate of manure but a variable rate of supplementation with synthetic N and P; Experiment 2 had a constant loading rate of manure but variable ratios of N and P; Experiment 3 had a constant loading rate of total N and P but with variable manure and synthetic N and P loading rates; and Experiment 4 had constant loading rates of manure and synthetic N and P but had variable fertilization frequencies.
Neither synthetic fertilizer nor cattle manure alone gave satisfactory fish yield in comparison to a mixture of both. Fish growth in tanks fertilized with synthetic fertilizers was limited by high pH and high un-ionized ammonia-N concentration. Cattle manure alone was also unable to produce a good fish yield because of its relatively low nutrient content compared to manure from feedlot livestock; a loading rate as high as 400 kg dry matter/ha/day which provided relatively high levels of N and P, caused anaerobic conditions at dawn and led to low phytoplankton biomass due to its high biological oxygen demand. However, a fertilization regime with a mixture of manure and synthetic N and P was balanced in terms of water quality and fish growth. Variation in fertilization frequency did not affect fish growth and yield in a short-term tank experiment. However, it is suggested that fertilization should comprise an initial high dose to trigger algal growth, followed by frequent smaller doses to ensure a continuous supply of nutrients for algae. Mortality of tilapia was mainly caused by a high concentration of un-ionized ammonia-N (>1.0 mg/1), accompanied by high pH (>10.0) and low dawn dissolved oxygen (<1.0 mg/1). Tilapia were quite resistant to low dissolved oxygen and survived by gulping oxygen from the air-water interface, even when tanks were anaerobic at dawn, provided that the un-ionized ammonia-N concentration remained low.
The best fertilization strategy obtained from the tank experiments was 50 kg dry matter cattle manure/ha/day supplemented with 2.5 kg N and 0.2 kg P/ha/day, applied twice a week.
Finally, a comparison of tilapia monoculture and carp polyculture (silver carp, Hypophthalmicthys molitrix; rohu, Labeo rohita; and mrigal, Cirrhinus mrigala ) was carried out to assess the best fertilization strategy determined from the tank under more commercially realistic conditions in earthen ponds (200m2).