Management of ash/impurity ratio in sugarcane: relative effects of genotypes, and N and K fertiliser rates

Abstract

In sugarcane juice or raw sugar, ash refers to soluble inorganic salts. The ratio of ash/impurity is important because high levels reduce sucrose extraction in sugar mills and the market value of raw sugar. The aim of research reported here was to examine, through a series of field experiments, the effects of genotypes and varying N and K fertiliser application rates on ash and ash/impurity levels. Ash levels were estimated via conductivity measurements, following prior studies indicating close relationships between conductivity and ash. Significant variation due to genotypes for conductivity/impurity levels was observed, with a range of about ±25% around the mean. Despite this variation, broad-sense heritability of conductivity/impurity levels on unselected genotypes on the basis of measurements in one environment were low (0.20), suggesting that selection in early phases of selection systems in breeding programs would probably be of limited value. However, it was recommended that measurements on conductivity/impurity ratio be made in multi-environment trials for genotypes being tested for commercial release, to assist in comprehensive economic evaluation. On average it was found that conductivity/impurity levels increased by 15% per 100 kg/ha of applied K, highlighting potential costs of high rates of K fertiliser. Increased N rates had a significant but small effect on reducing conductivity/impurity levels by ~10% per additional 100 kg/ha of applied N. The information reported here may be used to help develop optimal strategies for cultivar selection in sugarcane breeding programs and for developing recommendations for optimal fertiliser management, and some guidelines in relation to this are discussed.