Rotation and fumigation effects on the growth and yield of sugarcane

Rotation experiments involvi ng pasture, bare fallow or other crop treatments for different periods of time were established on long-term sugarcane land at Tully in 1993, Mackay and Ingham in 1994 and Bundaberg in 1995. Either the whole or part of each of these experiments has been returned to sugarcane, in 1996 at Bundaberg (after a 12-month break) and 1997 at Mackay, Ingham (after 9, 18, and 30-month breaks) and Tully (after a 42-month break). Continual cane plots in each experiment were re-planted at the same time, either as traditional plough-out/re-plant or following fumigation with methyl bromide. In addition, differential rates of nitrogen fertiliser were applied to the Mackay, Ingham and Tully experiments. At Mackay and Ingham, plots were split to 0, 70, 140, and 280 kg N/ha, while plots at Tully received 0 or 140 kg N/ha. Cane and sugar yields for the plant crop increased following breaks and fumigation at all sites. relative to ploughoutfre-plant. There was a general positive response to breaking the sugarcane monoculture, with smaller effects of the duration and type of break. The largest differentials in cane yield between break treatments and continual cane were: 14% Bundaberg (12-month maize crop), 51 % Ingham (30-month legume/grass pasture), 84% Mackay (30-month bare fallow), and 58% Tully (42-month legume/grass pasture). Fumigation significantly out yielded the best break at Bundaberg, but not at any other site. Applied nitrogen had no significant effect on cane yield at Mackay, slightly increased yield at Tully and strongly increased yield at Ingham. Results arc discussed in terms of the dynamics of yield accumulation in the various treatments.