Genetic, environmental and management contributions to ratoon decline in sugarcane

Studies of ratoon decline (RD) in sugarcane (Saccharum spp.) are primarily limited to investigations of cultivar ratooning ability (RA) and selection for this trait. The relative effects of cultivar, environment, and management on RD are rarely considered. This study (i) explored the relative effects of cultivar, environment, and management practices on RD patterns, (ii) evaluated the appropriateness of linear coefficients to describe differential RD trends, and (iii) determined if there are statistically significant differences in RD (as defined by a linear coefficient) between various agronomic treatments. Six irrigated cultivar trials from South Africa and Swaziland, three rainfed cultivar trials, two cycles of a long-term burning and trashing trial, and two cultivar × nematicide trials were analyzed. Cane yields (TCANE), estimated recoverable crystal percent (ERC) and ERC yields (TERC) were fit as linear functions of ratoon number (six or more crops) for individual plots in each trial. In the irrigated dataset, variance components analysis showed that environment (trial) contributed 83%, 59%, and 76% of total variation in RD for TCANE, ERC and TERC, respectively. The effect of cultivar accounted for less variation in RD compared with the effect of the cultivar × trial interaction for all three traits. However, significant (p < 0.05) differences in RD between cultivars were observed for all three traits in most trials. The rainfed trials showed sharper declines in TCANE (6-7 tonnes cane/ha/crop) compared to the irrigated trials and also showed a higher frequency of crossover interactions (switch in ranks of cultivars between young and old ratoons). In the burning and trashing trials, the fertilizer treatment produced highly significant differences (p < 0.001) in RD for all three traits in both trials. The percentage loss in TCANE between the first and last harvested crop was 17% and 36% for the fertilized treatments in the two trials. In contrast, the non-fertilized treatments showed a 70% and 47% TCANE loss in the two trials, respectively. Trashing had a highly significant effect on RD of all three traits in one trial only. The effect of nematicide, and the cultivar × nematicide interaction on RD was not significant for any of the traits across two trials. The linear coefficient allowed for detection of significant differences in RD between agronomic treatments in different trial datasets, and its use in other sugarcane studies is encouraged. This study suggests that more emphasis be placed on environment manipulation through crop management rather than focusing on cultivar choice for crop longevity.