Growth of sugarcane under high input conditions in tropical Australia .1. Radiation use, biomass accumulation and partitioning

There is little detailed information on yield accumulation in sugarcane under high-input conditions, which can be used to quantify the key physiological parameters contributing to yield variation. Sugarcane is grown under plant and ratoon crop conditions. This study analysed canopy development, radiation interception and biomass accumulation of two contrasting cultivars of sugarcane under irrigation during the same season under plant and ratoon crop conditions. Over the 15 month season, 11 crop samplings were conducted. Biomass partitioning to stalk was also measured to determine to what extent differences in partitioning between cultivars under ratoon and plant crop conditions contribute to differential productivity. The key findings were: (1) The ratoon crop accumulated biomass more quickly than the plant crop during the first 100 days of growth due to higher stalk number, faster canopy development and greater radiation interception. For similar reasons, cultivar Q138 had higher early biomass production than cultivar Q117 in the plant crop, (2) Early differences in biomass accumulation due to crop class became negligible at about 220 days because maximum RUE of the plant crop (1.72 +/- 0.01 g MJ(-1)) was 8% higher than in the first ratoon crop (1.59 +/- 0.08 g MJ(-1)). The higher maximum RUE in the plant crop was consistent with a higher crop growth rate (35.1 +/- 2.3 versus 31.0 +/- 3.4 g m(-2) d(-1)) during the Linear phase of biomass accumulation. (3) Biomass accumulation, which ceased about 300 days after planting/ratooning and 140 days before final harvest, attained similar levels of 53-58 t ha(-1) in all four crops. (4) The plateau in biomass was associated with loss of live millable stalks, and not a cessation in the growth rate of individual stalks. The crops continued to intercept radiation while on the biomass plateau, so that average RUEs at final harvest were much lower than the maximum values. (5) There was no effect of crop class or cultivar on the fraction of biomass in the leaf and millable stalk components. This study emphasizes that maximising early radiation interception and biomass accumulation will not necessarily lead to higher yield in an environment where biomass production reaches a plateau well before final harvest. Loss of live millable stalks late in the crop cycle results in poor utilisation of intercepted radiation.