Sugarcane water stress criteria for irrigation and drying off

Abstract

In Australia about 60% of sugar produced from sugarcane depends to some extent on irrigation. Regional water supplies are often limited and the pressure on irrigators to defend or improve their farming practices can be quite severe. Compared to other crops, little is known about how far water can be stretched in sugarcane without affecting sugar yield. In particular, there is little information on the response of important yield forming processes to soil water deficits. Two experiments were designed to follow the effect of increasing soil water deficit (SWD) on processes such as leaf appearance, leaf and stalk extension, leaf area development, biomass and sucrose accumulation and dry matter partitioning. The first experiment was designed to coincide with a period of high evaporative demand and the second experiment with a period of low evaporative demand. As SWD increased in the first experiment reductions occurred first in leaf and stalk extension rate, then in green leaves per stalk, then in biomass accumulation and finally in sucrose accumulation. The data indicated that irrigation can be used more sparingly since biomass accumulation (hence net photosynthesis) can tolerate greater deficits than previously recognized. Water stress resulted in marked changes in dry matter partitioning. In the second experiment sucrose yield was increased at one point to more than 3 t/ha over the yield of the control treatment. Dry matter was diverted from tops (green leaves and immature stem) and from stalk fibre to make up the additional sucrose yield. Simple crop measurements (leaf or stalk extension, green leaf number) are suggested as indicators of when to irrigate to avoid reductions in biomass accumulation or when to harvest in order to benefit most from changes in dry matter partitioning due to water stress. Suggestions are also made for improving process level growth models of sugarcane.