A 10-YEAR MAIZE OATS ROTATION UNDER 3 TILLAGE SYSTEMS .2. PLANT-POPULATION, ROOT DISTRIBUTION AND FORAGE YIELDS

The effect of zero, minimum or full tillage on a maize-winter oats rotation grown on an imperfectly-drained silt-loam overlying clay-loam loess (Aeric Fragiaqualf) was investigated over a 10-year period. On average, maize (Zea mays L.) forage yields on zero tillage plots were 16% lower than for full-tillage plots. Differences in maize forage yields between minimum and full-tillage were much smaller with an average 3% lower yield on minimum-tillage plots. The winter oat (Avena sativa L.) crop was direct-drilled into all plots and showed little consistent variation in yield. Our objective was to investigate possible causes for the yield differences. Low plant populations were probably the primary cause of low maize forage yields on zero-tillage plots. Plant establishment was poorest on zero-tillage plots in those years when little rain fell in the 16 days immediately after sowing. In wetter years, differences in yields between tillage treatments disappeared. Delayed early plant growth on zero-till soil was also observed. This is thought to be caused by restricted early root development as a consequence of mechanical impedance in the 50-200 mm zone. Only minor differences in mean root-length density (cm root cm-3 soil) were measured 3 months after sowing. However, the proportion of soil core samples which contained roots (percent occupancy) was especially low below 600 mm depth in the zero-tilled maize, despite no obvious differences between tillage treatments in root occupancies in the topsoil. Our data suggest that zero-tilled maize explored substantially less soil than tilled plants. The rooting depth alone proved to be of little use as an indicator of rooting volume. Zero tillage may be inappropriate because of restricted root development in heavier textured soils which seem to benefit from being worked or disturbed by tillage implements.