Aquaculture effluent as a water and nutrient source for hay production in the seasonally dry tropics

In the water-limited environment of the U. S. Virgin Islands, tank culture of Tilapia (Oreochromis niloticus) is gaining importance. A by-product of intensive fish culture is a nutrient-rich wastewater (effluent). It is not known if effluent can replace the nitrogen (N) fertilizer required by guineagrass (Panicum maximum Jacq.) managed for hay. This study was conducted from 1997 to 1999 to compare the effects of irrigated aquaculture effluent (AE) and inorganic N rates of 60 kg ha(-1) (N60) and 0 (N0) on dry matter (DM) yield, forage crude protein concentration (CP), and in vitro organic matter disappearance (IVOMD), soil pH and phosphorous (P). Aquaculture effluent was applied at 2 cm ha(-1) wk(-1) to guineagrass for an 18-wk period in 1997 and 1999 but not in 1998. Guineagrass plots were harvested at eight-wk intervals (three eight-wk harvests in 1997, 1998, and 1999). Effluent DM yield (three harvests in 1997; 14.0 Mg ha(-1)) was similar (P>0.05) to N60 (13.0 Mg ha(-1)). In 1998, effluent DM yield (three harvests; 13.7 Mg ha(-1)) was higher (P<0.05) than N60 (9.8 Mg ha(-1)) suggesting a slow release N from the effluent. In 1999, AE out-yielded N60 by more than 50%. There was a trend for higher CP (9.6%) for the AE compared to (8.6%) for the N60 in 1997. In 1999, AE was two percentage units higher (P<0.05) in CP (10.3%) compared to N60 (8.3%). In vitro organic matter disappearance in 1997 and 1999 were similar for all treatments and averaged 56% and 61%, respectively. There were no changes in soil pH (7.8) and P (22 mg kg(-1)), averaged across treatments. These results confirm that aquaculture effluent is an excellent N and water source for forages in the dry season and can replace the inorganic N requirements of guineagrass. The results should encourage the utilization of this effluent as a fertilizer rather than disposition to the environment.