The potassium (K) content of New Zealand ryegrass/white clover pastures can exceed the requirements for optimum pasture production (2.5-3.0%) and dairy cow requirements (0.2-0.6%) during late spring. Experiments were carried out at 13 sites in three different dairying regions over 2-3 years on different soil types to determine the effects on the cow's preference for pasture of either increasing pasture K content from applications of 0, 200, 400, and 800 kg of potassium chloride (KCl) ha -1 , or the effect of dusting sodium chloride (NaCl) at 0, 5, 10, and 20 kg ha -1 immediately before grazing. Cow preference was assessed by measurements of bite rate, grazing time, pasture dry matter (DM) disappearance, and bite size. In addition, the pasture intake of two groups of cows, one group grazing pasture with K content in the optimum range for pasture production, and one group on pasture above that range, was measured using the alkane technique. In South Taranaki on Allophanic soils, increasing A04008;

Potassium chloride (KCl) was applied at 4 rates (0, 150, 450 and 1150 kg/ha) to pasture on closed 5 ha farmlets over 3 years, and pasture and animal production and animal health measured. Friesan, Jersey and Friesan x Jersey cows were stocked at 3.2/ha on an Egmont Allophanic soil near Hawera in South Taranaki. Average soil potassium quick test (QTK) levels were 7, 8, 10 and 12 respectively from 0, 150, 450 and 1150 kg KCl/ha. There was a small significant negative linear effect of rate of KCl application on annual pasture dry matter (DM) production averaged over three years (16864 - 18359 kg/ha). The average amount of silage conserved (1369 - 2112 kg DM/ha) was consistently greater at the highest rate of KCl. Increasing rate of KCl had no significant effect on the nutritive value of grazed pasture but resulted in increased K content of silage. There was no significant effect of KCl on milksolids (MS) production (1034 -1179 kg/ha/lactation) or reproductive performance. There was a trend for the incidence of clinical metabolic disorders (8-18%) to decrease with increasing rate of KCl but this was not significant because of the low number of cows in each herd. The results from this trial suggest that soil QTK levels above the target range for optimal pasture production of 7-10 on Allophanic soils are not associated with increased dairy production and have no adverse effect on animal health. Keywords: calcium, dairy cows, hypocalcaemia, hypomagnesaemia, magnesium, milksolids, pasture, potassium, potassium chloride