Growth and yield responses to amendments to the sugarcane monoculture: effects of crop, pasture and bare fallow breaks and soil fumigation on plant and ratoon crops

Yield decline has been a major issue limiting productivity improvement in the Australian sugar industry since the early 1970s and is suspected to be largely due to growing sugarcane in a long-term monoculture. In order to address this issue, rotation experiments were established in several sugarcane-growing regions in Queensland, Australia, to ascertain whether breaking the sugarcane monoculture could, at least in part, assist in overcoming yield decline. The rotation experiments involved other crop species, pasture and bare fallow for different periods of time. When cane was replanted, the growth and yield following breaks was compared with that in a sugarcane monoculture system where the soil was unamended or fumigated before replanting. Yield increases were recorded in the plant and first ratoon (R1) crops in all experiments: in response to soil fumigation (average of 42 and 18%, respectively), and breaks (average of 27 and 30%, respectively). The data indicated that the response to breaks, while smaller in the plant crop, may have greater longevity than the response to fumigation. Further, there were indications that the response to breaks could continue into later ratoons (R2 and R3). Break type had little overall effect with the average response in the plant and R1 crops being 35% for breaks in excess of 30 months. Breaks of longer duration produced larger yield responses: 17% (<12 months), 24% (18-30 months) and 28% (>30 months) in the plant crop. However, the average yield increase over a plant and three ratoon crops when one cane crop was missed (6-12 months' break) and a grain legume or maize break included was similar to 20%. Yield increases with breaks and fumigation were due to either increased stalk number, increased individual stalk weight or a combination of both. The component accounting for the majority of the variance changed between experiments, with a general trend for individual stalk weight to have more impact under better late season growing conditions and/or conditions that hampered early stalk development, while stalk number was more important under conditions of late season water stress and/or low radiation input. The results demonstrate that the long-term sugarcane monoculture is having an adverse effect on productivity. Further, breaking the sugarcane monoculture and sacrificing one sugarcane crop is likely to have minimal impact on the supply of cane to the mill. The increase in yield during other stages of the cane cycle is likely to compensate for the loss of 1 year of sugarcane, especially as the crop that is sacrificed is the last and almost always lowest-yielding ratoon.