Interactions affecting the optimal harvest age of sugarcane in rainfed regions of South Africa

Recent changes in cultivar composition, the renewed drive to increase harvest age (HA), and the impacts of the Eldana borer (Eldana Saccharina Walker: Lepidoptera Pyralidae) in the rainfed regions of South Africa have created uncertainty about the optimal HA of sugarcane. This study aimed to (i) review the optimal HA of sugarcane in the coast and inland regions, (ii) determine the variability in optimal HA with cultivar, and (iii) investigate the interactions between cultivar eldana resistance levels and HA on productivity. A secondary objective was to summarise the results of dedicated cultivar×HA experiments to evaluate the importance of this interaction. Cultivar evaluation trial data from 1980 to 2014 were categorized by region, HA, cultivar, and cultivar eldana resistance category. Mean estimated recoverable crystal yields (TERC) and percentage internodes bored (%IB) by eldana were plotted against HA using polynomial regression to investigate effects of region, cultivar, eldana resistance, and their interactions. Additionally, three separate cultivar×HA factorial trials were analysed using linear mixed models. The HA and its associated interaction terms were highly significant (P<0.001) and generally accounted for more variation than the cultivar and crop main and interaction terms in the trial-based analyses. The combined data mining analysis showed that on average, the optimal HA in the coast and inland regions were 15 and 22 months, respectively. A cultivar×HA×region×eldana infestation level interaction was demonstrated, suggesting that a generic optimal HA for specific cultivars cannot be recommended. There was a reduction in optimal HA for newer released cultivars, suggesting possible indirect selection for early maturity in the breeding programs for these regions. Eldana susceptible cultivars were more sensitive to damage as HA increased (4.1 compared with 3.1%IB per additional growth month for susceptible vs. resistant cultivars, respectively). Eldana resistance was essential for achieving the benefits of increased HA in the coastal region. The study highlights the importance of HA to sugarcane productivity and illustrates how an experimental database can be used to inform breeding strategies and gain insights into a key management factor of industrial importance.