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An analysis of the physiological consequences of water deficit during the first half of the season on sugarcane productivity, can aid the evaluation of yield expectations under rainfed systems and the opportunities for saving irrigation water early in the season. Four field experiments were conducted at Ayr in the semi-arid tropics of Australia. irrigation was withheld at different stages of crop development to investigate timing and severity of water deficit on crop development, biomass accumulation and partitioning of biomass to millable stalk and sucrose, both during the season and at final harvest. Deficits imposed during the tillering phase, while having large impacts on leaf area, tillering and biomass accumulation, had little impact on final yield. This was primarily due to the length of time required to impose significant water deficit when the canopy is small, the comparatively small amount of biomass accumulation lost through water deficit, and the ability of the crop to produce leaves and tillers at a rapid rate during subsequent well-watered conditions. On the other hand, water deficit imposed when the canopy was well-established (leaf area index >2) had a more deleterious impact on final yield of total biomass, stalk biomass, and stalk sucrose. Reductions in millable stalk biomass, could be solely explained by reductions in total biomass. Similarly, >97% of the variation in final cane or sucrose yield could be explained by variation in stalk biomass. While there were transient effects of water deficit on stalk sucrose and dry matter concentration, significant impacts at final harvest did not occur until stalk biomass levels fell to ca. 50% of that of the well-watered control. (C) 1999 Elsevier Science B.V. All rights reserved.