Effect of nitrogen on the time-course of sucrose accumulation in sugarcane

Sugarcane is harvested commercially at ages varying from 9 to 36 months. Since high N supply can decrease the sucrose concentration in fresh millable stalks and consequently decrease the commercial value of the stalks, the opportunity exists to manipulate N supply (both from fertiliser and that mineralised from soil organic matter) to maximise economic return at different times of harvest. Accordingly, this study describes how N suppl; the time-course of sucrose accumulation in sugarcane and determines yield both on a dry weight basis (as commonly analysed by crop physiologists) and on a fresh weight basis (which is how cane is paid for commercially). Data on crop N uptake and its efficiency of utilisation are also presented. There was a trade-off between maximising sucrose yield and sucrose concentration in fresh millable stalks with different N supply and this varied with time of harvest. Sucrose concentration in fresh millable stalks, particularly during early growth, was maximised by low N supply. The lower sucrose concentration (on a fresh weight basis) with high N supply could be largely explained by a decrease in stalk dry matter content. Most of the variation in stalk sucrose yield could be explained by variation in stalk biomass irrespective of N supply. Whilst increasing N supply decreased the sucrose concentration in dry millable stalks, this effect was relatively small compared to the large positive effect of N supply on stalk biomass. It is concluded that N has a marked effect on stalk dry matter content, and hence a greater effect on the commercial measures (yield and sucrose concentration of fresh millable stalks) of sugarcane production than on the physiological measures (stalk biomass and sucrose concentration on a dry weight basis) of crop performance.