Experiments involving breaks to the sugarcane monoculture, soil fumigation and the application of biocides were conducted in Bundaberg, the Burdekin Valley and at Tully, three sugarcane-growing regions in Queensland, Australia. The aim was to elucidate the cause(s) of previously observed positive yield responses to breaks in the monoculture and assess persistence into a subsequent cane cycle. In all three experiments there was a positive response in the plant cane crop to fumigation of land that was under sugarcane monoculture, the response being 32, 39 and 21% for the Burdekin, Tully and Bundaberg experiments, respectively. Further, at Tully, the response was maintained into the ratoons.
Fumigation after breaks also enhanced yields but the results were variable and were generally less than the response following sugarcane monoculture. At Tully, fumigating after bare fallow, grain crop and pasture breaks enhanced cane yields by 23, 26 and 29%, respectively, while in the Burdekin, responses to fumigation were much smaller at 9% (bare fallow), 4% (grain crop) and 8% (pasture). In Bundaberg, responses ranged from nil following a long-term (70 months) grass pasture up to 35% following a short-term (12 months) grass pasture, with a general trend for the response to fumigation being larger following short-term than long-term breaks.
In the Tully experiment, biocides had variable effects on sugarcane growth and yield. Fungicide application produced as good a yield as fumigation whereas nematicides had little direct influence. However, when combined with fungicides, nematicides provided a synergistic effect in terms of shoot development. The results suggested that at the Tully site, fungi were the major detrimental biota associated with poor yields in long-term sugarcane monoculture, but nematodes had some influence once fungi were controlled.
In two additional experiments at Bundaberg and Burdekin, it was shown that if the cane stool was removed after the plant crop (Bundaberg) and second ratoon (Burdekin) and sugarcane re-planted, there were no residual effects of breaks and fumigation. Thus it appears that the positive effects of breaks and fumigation measured in the ratoons were more associated with the development of a healthy stool in the plant crop than any residual effect on soil biota.
In general the similarly positive response to fumigation and breaks indicated that a considerable part of the overall response to breaks was due to reducing the adverse effect of detrimental soil biota (largely fungi although there was an effect on nematodes). The type and duration of break was also important with long-term pasture being the most effective. However, the duration of the fumigation and break effects on soil biota only lasted for the plant crop.