Effect of second pond dairy effluent applied in spring to silage regrowth of perennial ryegrass based pasture in southern Australia. 2. Changes in nutritive characteristics and mineral content

The 2-pond effluent systems used by the majority of dairy farmers in southern Australia result in the collection of liquid effluent in an aerobic second pond. This liquid effluent contains a wide range of nutrients that have the potential to significantly affect the nutritive characteristics and mineral content of forages. The effect of applying second pond dairy effluent to perennial ryegrass based pasture following harvesting for silage over a 3-year period was measured. Effluent was applied at 6 rates: 0, 15, 30, 45, 60, and 75 mm following a spring silage harvest each year, with changes to nutritive characteristics and mineral content determined at each subsequent grazing. Analysis of the effluent applied showed that on average, over the 3 years of application, the effluent contained 31, 454, 20, and 149 kg/ML of phosphorus (P), potassium (K), sulfur, and nitrogen, respectively. In addition, the effluent contained 152 kg/ML of calcium (Ca) and 225 kg/ML of magnesium (Mg). Changes in nutritive characteristics as a result of effluent application were primarily restricted to the first grazing after application. The metabolisable energy content of herbage at the first grazing in Years 1 and 2 was unaffected by effluent application; however, in Year 3 there was a linear increase of 0.0051 MJ/kg dry matter (DM).mm of applied effluent. In contrast, effluent application had a positive linear effect on pasture crude protein (CP) content at the first grazing in all years. In Year 1, CP content increased by 0.083%/ mm of applied effluent, while for Years 2 and 3 the increases in CP content were 0.044 and 0.029%, respectively. In all years, there was a negative linear effect on water-soluble carbohydrate content at the first grazing after effluent application. For Years 1, 2, and 3, responses were -0.138, -0.066, and -0.022%/mm applied effluent, respectively. The P content of pasture was higher at the first grazing following effluent application at rates of 60 and 75 mm in each year compared with the control and the 15 mm application rate. Potassium concentrations were increased with effluent application at rates of 45, 60, and 75 mm in all 3 years in comparison with the control at the first and second grazing after application. In Year 1, Mg content of the pasture was lower for the control, 15, and 30 mm treatments than for the 2 highest application rates at the first grazing after application. This experiment has highlighted the potential of using second pond dairy effluent following pasture harvesting for silage to maintain the CP content of pasture in late spring. The high mineral content of effluent led to increases in the concentration of several these minerals and, while the ratio of K to (Ca + Mg) was substantially increased, it is unlikely that effluent application in late spring and the concomitant increases would lead to animal metabolic problems. Coupled with the observed increases in DM yield, the increase in CP content provides greater flexibility in feeding options to bridge feed shortfalls before summer forage crops becoming available on dryland dairy farms in southern Australia.