High soil nitrate concentrations during autumn and winter increase suckering

The presence of suckers in sugarcane crops grown in the wet tropics of Australia has been implicated as a factor contributing to the decline of CCS in this region. The environmental stimuli that result in crops producing suckers are not known. However, it is speculated that nitrogen is likely to play a role in the process. The basis for this hypothesis is the co-incident decline in CCS with the adoption of green cane trash blanketing (GCTB). The organic matter built up from many years of GCTB is a potential SOurce of nitrogen to growing crops. The effect of increased nitrogen availability in the aurumn and winter prior to harvest on suckering was determined for a ratoon crop of Q152 in Tully. In addition to the commercial application of 150 kg of N/ha applied after ratooning, 70 kg N/ha was applied in either May, June, or July. Sucker numbers were counted every month, and soil samples were taken in order to determine whether the applied nitrogen increased soil nitrate levels. Sucker numbers increased following the nitrogen applications. Nitrogen applications were found to increase soil nitrate concentrations and hence plant available nitrogen. However, at the final sampling (17/9/99), the sucker number was similar in the control and all treatments, as a result of a flush of small suckers in the last two weeks of the experiment. This flush was due to factors other than nitrogen. This study is the first to demonstrate a particular environmental factor intluencing suckering. From this study, the implication is that nitrogen, available in autumn and winter prior to harvest, will have greatest effect on CCS and profitability for crops harvested early in the harvesting season. Other factors were responsible for increasing sucker number later in the harvesting season.