The effect of dairy effluent on dry matter yields, nutritive characteristics, and mineral content of summer-active regrowth forage crops in southern Australia

In southern Australia, the majority of dry land dairy farms use a 2-pond system to treat and contain dairy effluent collected at the milking platform. This effluent contains a range of nutrients that have the potential to affect forage dry matter (DM) yields, nutritive characteristics, and mineral content of forages. The effect of applying second-pond dairy effluent to the summer-active forages chicory (Chichorium intybus L. cv. Grouse), Hunter (Brassica campestris L. x Brassica napus L.), Winfred (Brassica napus L.), and Sweet Jumbo (Sorghum bicolor (L.) Moench x Sorghum sudanese (Piper) Stapf.) over two summer periods was measured. Effluent was applied at rates of 0, 40, 80, and 100 mm with application split into two equal application times. The first occurred 6-10 weeks after sowing and the second immediately after the first grazing. Forages were assessed for DM yield, nutritive characteristics, and mineral content over the two growth periods in each year. Analysis of effluent showed that on average over the 2 years, the effluent contained 146, 34, 439, and 18 kg/ML of nitrogen (N), phosphorus (P), potassium (K), and sulfur (S), respectively. Furthermore, the effluent also contained 161 kg/ML of calcium (Ca) and 222 kg/ML of magnesium (Mg). For the total growth period in Year 1, all crops showed a linear increase (P < 0.05) in DM yield to applied effluent, with response values varying for each crop. Responses were 49, 52, 29, and 51 kg DM/ha.mm applied effluent for chicory, Hunter, Sweet Jumbo, and Winfred, respectively. For Year 2, all crops also showed a linear increase (P < 0.05) in DM yield (15 kg DM/ha.mm applied effluent) with applied effluent. In Year 1, crude protein (CP) content of all crops increased (P < 0.05) in a linear manner at rates of 0.073 and 0.047% per mm applied effluent for growth periods 1 and 2. There were also linear responses (P < 0.05) in Year 2, with responses varying for each crop for each growth period. For chicory there was no effect of effluent application on CP content in either growth period, while other crops generally exhibited a linear increase with responses of up to 0.08% per mm applied effluent. The greatest changes in mineral content of herbage were those of K, Ca, and Mg. There was a linear increase (P < 0.05) in K content for all growth periods in Years 1 and 2. Magnesium content of chicory (periods 1 and 2) and Winfred (period 2) showed a linear decrease (P < 0.05) in response to effluent application in Year 1, whereas there was no effect in Year 2 for any crops. The results from this study highlight the potential of second-pond dairy effluent to increase DM yields of a range of summer-active forage crops. The data also suggest that while effluent can improve DM yields when soil moisture is limiting, when additional moisture as rainfall occurs, responses from effluent are even more pronounced. In addition, the CP content of forages can be improved when effluent is applied. The combination of increased DM yield with higher CP content provides greater flexibility in dairy cattle feeding options through the dry summer period.