Dairy operations generate large quantities of effluent, which are stored in constructed lagoons. Lagoons, however, have finite storage capacity and can overflow, potentially polluting land and associated water bodies. Alternative uses of effluent are, therefore, needed for a more sustainable and environment-friendly dairy production. This study assesses the effects of effluent irrigation on the retention, accumulation, and movement of phosphorus. Four grassesVBana (Pennisetum purpureum K. Schumach.), California (Brachiaria mutica [Forssk.] Stapf.), Star (Cynodon nlemfuensis Vanderyst), and Suerte (Paspalum atratum Swallen)Vwere subsurface (20Y25 cm) drip irrigated with effluent at two rates based on potential evapotranspiration (ET p ) at the site (Waianae, HI)V2.0 ET p (16 mm d j1 in winter; 23 mm d j1 in summer) and 0.5 ET p (5 mm d j1 in winter; 6 mm d j1 in summer). Treatments were arranged in an augmented completely randomized design. Most of these grasses produced large amounts of dry matter with effluent irrigation. California grass receiving the 2.0 ET p effluent application outyielded all other grass species, producing 60 Mg ha j1 year j1 . Olsen soil phosphorus (P) and soil solution P did not significantly increase despite daily irrigation for at least 2 years and when irrigated at 2.0 ET p . A relatively small amount of P was measured at deeper soil depths at the 2.0 ET p irrigation rate (131 kg ha j1 year j1 ). Calcium-phosphate precipitation was predicted by calculated phosphate potentials. Acid-extractable soil P increased, supporting the hypothesis of Ca-P precipitation.

Effluent lagoons on dairy farms can overflow and potentially pollute adjacent land and associated water bodies. An alternative solution to effluent disposal is needed by dairy operators in island environments. An attractive win-win alternative is to recycle nutrients from this resource through effluent irrigation for forage grass production that minimizes environmental pollution. This study assessed biomass production and nutrient removal by, and high application rates to, tropical grasses that were subsurface drip-irrigated with dairy effluent. Four grass species -Banagrass (Pennisetum purpureum K. Schumach.), California grass (Brachiaria mutica (Forssk.) Stapf.), Stargrass (Cynodon nlemfuensis Vanderyst) and Suerte grass (Paspalum atratum Swallen) -were subsurface (20-25 cm) dripirrigated with effluent at two rates based on potential evapotranspiration (ET p ) at the site (Waianae, Hawaii) )2AE0 ET p (16 mm d )1 in winter; 23 mm d )1 in summer) and 0AE5 ET p (5 mm d )1 in winter; 6 mm d )1 in summer). Treatments were arranged in an augmented completely randomized design. Brachiaria mutica and P. purpureum had the highest dry-matter yield (43-57 t ha )1 year )1 ) and nutrient uptake especially with the 2AE0 ET p irrigation rate (1083-1405 kg ha )1 year )1 N, 154-164 kg ha )1 year )1 P, 1992-2141 kg ha )1 year )1 K). Average removal of nutrients by the grasses was 25-94% of the applied nitrogen, 11-82% of phosphorus and 2-13% of the potassium. Average values of crude protein (90-160 g kg )1 ), neutral detergent fibre (570-620 g kg )1 ) and acid detergent fibre (320-360 g kg )1 ) were at levels acceptable for feeding to lactating cattle. Results suggest that P. purpureum and B. mutica irrigated with effluent effectively recycled nutrients in the milk production system.