Determining the Maximum Drying off Periods for Sugarcane Grown in Different Regions of the South African Industry

Mild moisture stress causes sucrose to be deposited preferentially in sugarcane stalks so that sucrose yields may be improved. The rule of thumb in the industry states that a crop should be dried off for the time it would take pan evaporation to equal twice the plant available water holding capacity of the soil in which the crop is rooted (or twice TAM). Despite this useful rule farmers often dry-off their crops excessively. A systems approach was employed to verify the aptness of the rule and develop recommendations in terms of drying off days that would prevent losses in sucrose yields. Data from past experiments were analysed to describe the relationship between sucrose yields and stalk biomass of well watered and dried off cane. This analysis showed that sucrose yields are increased or not affected when stress reduces stalk biomass by up to 4%. The CANEGRO model was validated using an independent data set. Thereafter, the stalk biomass for well watered and dried off treatments were simulated over periods of up to 120 days using 23 years of weather data from various regions of the industry. Simulations included soils with TAMs of 80,114,162 and 210 mm at five sites in the industry for crops harvested annually between April and November. From these simulations it would appear that the 2 x TAM rule, which excluded possible rainfall, is a good universal rule. However, our analyses provide the appropriate adjustment by including anticipated rainfall and seasonal effects. Recommended “drying-off days” have been tabulated for five areas within the SA sugar industry.