The economic optimum harvest age for sugar-cane depends to a large extent on how rates of photosynthesis and respiration change with age. A simple model of the carbon balance in which a constant photosynthetic efficiency was assumed, accounted to a large extent for dry matter accumulation by cv. NCo376 grown under irrigation. Rain-fed crops of this cultivar were simulated by linking the model of the carbon balance to the CERES-Maize water balance routine. The combined model was validated with data from a comprehensive growth analysis experiment undertaken by staff of the SASA Experiment Station in the 1960's. Dry matter accumulation was explained satisfactorily without changing the coefficients for gross photosynthesis (Pg= 0.082 PAR MJ m−2), maintenance (0.242 Pg) or growth respiration (0.003×total phytomass). Leaf area index and leaf appearance data indicated that the assumptions regarding leaf area development were valid. The observed fluctuations in water content of the top 300 mm of soil were well explained by the CERES-maize water balance but the gypsum block resistance readings in deeper layers indicated the presence of water that on some occasions was not explained by the model. It was concluded that the sucrose accumulation rate on many farms may have declined because of the recent reduction in the age at harvest in the sugar industry.