The 2-pond system to treat and contain dairy effluent is commonplace on dry-land dairy farms in southern Australia. The first pond is a deep anaerobic treatment pond and the second a shallow aerobic pond where the liquid effluent is stored. This liquid effluent contains a range of nutrients that have the potential to influence forage dry matter (DM) yields, herbage nutritive characteristics, and mineral content of forages.

The effect of applying second-pond dairy effluent to a summer turnip (Brassica rapa L.) crop over two consecutive summer periods was measured. In addition to the application of effluent, N fertiliser was also applied. Effluent was applied at three rates, 0, 30, and 60 mm ~7–8 weeks after turnips were sown each year, with fertiliser N applied at either 0, 25, 50, or 75 kg N/ha in combination with effluent rates immediately before effluent application. Turnips were assessed for DM accumulation, nutritive characteristics, and mineral and nitrate-N content.

Effluent contained high concentrations of both potassium (K) (440–500 kg/ML) and sodium (Na) (637–766 kg/ML), with moderate levels of calcium (Ca) (177–180 kg/ML) and magnesium (158–213 kg/ML). Total N was higher in Year 2 (208 kg/ML) than in Year 1 (160 kg/ML), with the proportion of total N present as ammonia-N also higher in Year 2 (81%) than in Year 1 (57%).

Dry matter yield responses for leaf and root were 20 and 11 kg DM/ha per effluent mm applied in Year 1 and 19 and 13 kg DM/ha.mm applied in Year 2, respectively. Total DM yield increases were 32 and 39 kg DM/ha.mm applied for Years 1 and 2, respectively. There was no effect of N application or interaction between effluent and N application in either year. For Year 1, nutritive characteristics were relatively unaffected by either effluent or N fertiliser application, while in Year 2, leaf crude protein content increased (P < 0.05) in a linear manner at 0.058% per mm effluent applied. The K and Na content of turnip leaves increased (P < 0.05) with effluent application in both Years 1 and 2, while the Ca decreased (P < 0.05) with effluent application in Year 2.

Results from this study further emphasise the potential value of second-pond dairy effluent to increasing forage DM yield and improving the nutritive value of turnips. The data, however, question the value of using N fertiliser on its own or in combination with effluent to improve the same attributes. Dry matter yield responses to effluent were similar across both years despite contrasting climatic conditions, highlighting the ability of turnips to respond to limited moisture inputs.