Effect of second-pond dairy effluent on turnip dry matter yield, nutritive characteristics, and mineral content

Dairy farms in southern Australia generally use a 2-pond system to manage dairy shed effluent. This system consists of a deep anaerobic first pond and a shallow aerobic second pond. The liquid in the second pond contains a range of nutrients that may have agronomic benefits for forages. The effect of applying second-pond dairy effluent to a summer turnip (Brassica rapa L.) crop over 3 consecutive summer periods was measured. Effluent was applied at 6 rates, 0, 15, 30, 45, 60, and 75 mm, approximately 6–8 weeks after turnips were sown each year. Turnips were assessed for dry matter (DM) accumulation, nutritive characteristics, and mineral content. In addition, total annual production for years 1 and 2 was calculated by including the DM accumulation from annual ryegrass grown from autumn to spring each year. Concentrations of nutrients within the effluent as an average over the 3 years were 31, 454, 20, and 149 kg/ML for phosphorus (P), potassium (K), sulfur (S), and nitrogen (N), respectively. In addition, effluent also contained 152 kg/ML of calcium (Ca), 225 kg/ML of magnesium (Mg), and 529 kg/ML of sodium (Na). Soil pH was generally unaffected with effluent application, while soil EC and total soluble salt (TSS) content increased with effluent addition. In the first year, application of effluent at 15 mm and higher resulted in increases in available K; however, in subsequent years, rates of 45 mm and higher led to an increase in available K, while for the control and lower effluent rates there was a marked decline in K status. In all years there was a linear increase (P < 0.05) in leaf, root, and total DM yields with applied effluent. For leaf, responses were 19, 50, and 26 kg DM per mm applied effluent and for roots, 10, 39, and 25 kg DM per mm applied effluent for years 1, 2, and 3, respectively. In years 2 and 3, turnip leaf crude protein (CP) content increased (P < 0.05) in a linear manner at rates of 0.046 and 0.044% per mm applied effluent, respectively. There was also a linear increase (P < 0.05) in turnip root CP in years 2 and 3 of 0.033 and 0.021% per mm applied effluent, respectively. In all years there was a linear increase (P < 0.05) in leaf K content, while for root K there was a quadratic trend (P < 0.05) for year 1 and a linear increase (P < 0.05) for years 2 and 3. The results from this study indicate that the use of dairy effluent can increase DM yield and improve the nutritive value of turnips through an increase in CP content. The data also indicate that this effect can be maintained over consecutive years, which in turn may provide greater flexibility for returning effluent to farm land. While results appear to indicate that the primary responses are due to N, further work is required to determine the effects of water and other nutrients within dairy effluent.