Nitrogen uptake of a perennial ryegrass-white clover pasture following strategic nitrogen fertiliser application during the cool seasons in the Western Cape province, South Africa
The effect of four fertiliser nitrogen (N) levels (0, 50, 100 or 150 kg N ha-1) applied as a single dressing in either autumn (late April), early winter (early June), late winter (mid July), early spring (late August) or late spring (early October) on fertiliser N recovery by a perennial ryegrass-white clover pasture five weeks after fertiliser N application was investigated. Herbage N yield (kg N ha-1 uptake) was determined for the pasture from herbage N yield of the perennial ryegrass fraction and the clover fraction. Increased fertiliser N rate resulted in increased perennial ryegrass N yields. Perennial ryegrass N yields tend to be lowest during winter (early as well as late winter). Pasture N yield responses were similar to the perennial ryegrass N yields. Except in 2001, higher clover N yields (kg N ha-1) were recorded when no fertiliser N was applied compared to treatments that received fertiliser N. Clover N yields did not differ in the 50-150 kg N ha-1 range and would therefore not influence pasture N yield in that range. Although season of application influenced N yields the response pattern was not consistent among the years covered by the study. Calculating the apparent N recovery rate (difference between N removed in fertilised plots and control plots) can serve as an important tool to evaluate the efficiency achieved with fertiliser N application. More research is needed to develop N removal as a management tool to be used in strategic N fertilisation programmes.