In southern Australia the standard treatment system for dairy shed effluent is a 2-pond system. Effluent from the dairy is first treated in a deep anaerobic pond where sedimentation and breakdown of organic matter occur. Treated liquid effluent then enters a shallow aerobic pond where it is stored. Relatively little information is available on forage agronomic responses to application of this second pond liquid. The effect of applying this second pond dairy effluent to perennial ryegrass based pasture was measured over a 3-year period. Effluent was applied at 6 rates: 0, 15, 30, 45, 60, and 75mm following spring silage harvest each year. Changes to herbage dry matter (DM) accumulation, and botanical composition were determined at each subsequent grazing. Over the 3 years of application, the average concentrations of phosphorus, potassium, and sulfur in the effluent were 31, 454, and 20 kg/ML, respectively. In addition, the effluent contained 149 kg/ML of nitrogen and 529 kg/ML of sodium. For all 3 years, the application of effluent resulted in a linear increase (P > 0.05) in annual herbage DM accumulation. Response rates for the 3 years were 27, 22, and 9 kg DM/mm applied effluent, respectively. The majority of this response occurred by the first grazing after application, where effluent application gave rise to linear responses (P > 0.05) in herbage DM accumulation of 17, 18, and 7 kg DM/mm for Years 1, 2, and 3, respectively. Botanical composition was not significantly affected by effluent application. This experiment has highlighted the potential of using second pond dairy effluent following pasture harvesting for silage as a means to grow additional herbage and extend the growing season. Based on the N content of effluent, herbage DM accumulation responses (Year 1, 10.4; Year 2, 11.4; Year 3, 5.1 kg DM/kg N) were similar to those expected from application of fertiliser N. It is likely that the primary response in terms of herbage accumulation is due to the N component within dairy effluent. While this study has shown positive herbage DM accumulation responses to applied effluent, further work is required to determine longer term effects of repeated effluent application, especially sodium, to soil characteristics.
