Dairy producers accumulate large quantities of
effluent in lagoons. These lagoons occasionally overflow causing the nutrients
and other contaminants in effluent to pollute the land and water bodies.
Alternative uses of effluent are urgently needed for a sustainable and
environment-friendly dairy production. This study determined the sustainable
effluent application rate and assessed the effects of effluent irrigation on
plant and soil (Cumulic Haplustoll) properties. Four grasses—bana (Pennisetum purpureum K.),
signal (Brachiaria decumbens
S.), star (Cynodon
nlemfuensis V.), and suerte
(Paspalum atratum S.)—were subsurface
drip irrigated with dairy effluent at two rates based on potential
evapotranspiration (ETp) at the site (Waianae, Hawaii)—2.0 ETp
(7-44 mm d-1) and 0.5 ETp (2-11 mm d-1).
Treatments were arranged in an augmented completely randomized design. Forage
was harvested, and soil and soil solution were collected every month. Pennisetum purpureum and P. atratum
had the highest dry matter yields (~60 and 46 mT ha-1 y-1,
respectively). Nutrient
removal of grasses was 30-279%, 14-100% and 2-18% of applied effluent N, P and
K, respectively. Extractable soil P
(86-159 mg kg-1 at 0-15 cm; 81-123 mg kg-1 at 15-30 cm)
and soil solution total P (2-9 mg L-1) did not significantly
increase after two years of irrigation. Soil pH increased from 7.4-7.8 (July
2003) to 8.1-8.9 (July 2006). Electrical conductivity (0.5-1.3 dS m-1
in July 2006) was below 4.0 dS m-1
(U.S. Salinity Laboratory's critical level for classifying saline
soils). Soil exchangeable sodium percentage (ESP) remained below 15% (critical
ESP for classifying sodic soils). Results suggest that irrigating high yielding
tropical grasses with effluent at 2.0 ETp may be acceptable.
Additional monitoring is needed to determine the longer-term impacts of
effluent application on soil and plant properties. Information generated will
allow dairy producers to reduce feed costs while minimizing pollution
associated with effluent application—thus, creating a win-win option.