Pasture response to soil phosphorus levels measured under mowing and dairy grazing

Pasture responses to increasing soil Olsen P (phosphorus) levels were measured tinder dairy grazing and mowing on an allophanic soil (Egmont brown loam) in South Taranaki, New Zealand, for 4 years. Ten farmlet paddocks were allocated to each treatment with average initial soil Olsen P (0-75 min) levels of 29, 37, 45, 56, and 68 mug P ml(-1) and duplicate mowed plots sited oil each of five paddocks. Mean annual pasture production under grazing and mowing, respectively, increased from 16.9 and 13.8 t DM ha(-1) at the lowest to 17.6 and 15.0 t DM ha(-1) at the highest Olsen P. Average spring and autumn pasture P concentrations increased from 0.33 to 0.38% under grazing and from 0,31 to 0.41% under mowing. Increasing Olsen P resulted in small non-significant increases in average spring and autumn ryegrass contents in both grazing (56 to 63%) and mowing (42 to 45%) treatments. There was no effect of Olsen P on average spring and autumn white clover contents in grazed pastures (8%) but a small increase in mown pastures (8 to 12%). Fitting lines to describe the relationship between Olsen P and the pasture parameters for 4 years grazing and mowing data showed a significant difference (P < 0.05) in slope for spring clover content only. However, there was a large variation in the slope of the fitted lines between years. These results show that although there are differences in absolute pasture production under mowing and dairy grazing, the relative responses to P are not significantly different. Therefore, mowing trials can be used to indicate relative responses in pasture production over the Olsen P range typical of New Zealand dairy farms.