Land treatment of farm effluent is becoming popular in many countries because this practice is considered less harmful to surface water and groundwater resources. However, the supply of large quantities of selective nutrients, such as nitrogen (N) and potassium (K), through effluent irrigation could affect the nutrient balance and the botanical composition of crops and pasture. The aim of the project was to investigate the influence of farm effluent irrigation on the growth and chemical composition of legume‐based pasture through a field experiment conducted at Massey University Dairy Unit, New Zealand. Three levels of effluent irrigation with N loading rates of 0,150, and 200 kg ha were applied. Effluent irrigation at these rates also provided 0, 256, and 342 kg K ha. The effluent was irrigated either in the presence or absence of calcium (Ca—as gypsum), and magnesium (Mg—as epsom salt) addition. The effluent irrigation was applied once a month and the trial continued for 6 months. The dry matter (DM) yield, botanical composition, and nutrient concentration in the pasture were measured 1 month after each effluent irrigation. The pasture DM yield increased with increasing rate of effluent application. The DM response (kg DM kgN applied) ranged from 4.1 to 7.2, slightly less than the values obtained for fertiliser N. The N and K concentrations in the pasture increased with increasing levels of effluent irrigation. Calcium and Mg fertiliser application had no effect on either pasture yield or the concentration of N or K in pasture. In the absence of Ca and Mg fertiliser application, the concentration of Ca and Mg in pasture decreased with increasing effluent irrigation. Application of Ca and Mg fertiliser increased the concentration of these elements in pasture at all levels of effluent application. Excessive uptake of K by pasture is likely to cause Ca and Mg deficiencies in grazing animals, leading to milk fever and grass staggers.