Using a simulation model to assess potential and attainable sugar cane yield in Mauritius

Abstract

The Mauritian sugar cane ( Saccharum spp.) industry is characterised by high climatic variability both from year-to-year and across agro-climatic zones. There is a need to assess yield variation with a view to facilitate crop management as well as investment decision-making, such as irrigation schemes. The aim of this study is to assess the feasibility of using crop simulation modelling to quantify potential and attainable yield at contrasting sites. Potential yield is that limited by temperature, radiation, for a given cultivar and season length, while attainable yield has the additional constraint of water supply (rainfall and irrigation where it is applied). The sugar cane module of the Agricultural Productions Systems Simulator (APSIM-Sugar cane) had not been previously used under Mauritian conditions. Hence, the first step is to validate the model using data from growth analysis experiments with plant and ratoon crops of three contrasting commercial cultivar (R 570, M 13/56 and M 555/60), harvested at two dates (July and October) at two sites. The validation step showed that only a limited number of crop coefficients needed adjustment: those concerning canopy expansion, biomass partitioning and crop development. Linear fit of simulated and observed data were close to the 1:1 line and coefficients of determination were highly significant for LAI ( r 2=0.66), above-ground biomass ( r 2=0.84), dry weight of cane ( r 2=0.73) and sucrose yield ( r 2=0.78). There was some indication that model modification for the effects of water stress and flowering on canopy development and biomass accumulation would improve the simulations The model was used to assess the variability in potential and attainable yield at three sites (Pamplemousses, Medine and Reduit) using long-term climate data for the period 1961–1995. Simulations were conducted for rainfed and fully irrigated scenarios. The three sites contrasted in annual rainfall, solar radiation and mean temperature and hence in expected potential and attainable yield. The range in model-simulated yields of cultivar R 570 were close to the range of experimentally observed yields at the selected sites under rainfed and fully irrigated conditions. Simulated yields were more variable under rainfed than irrigated production, with variability increasing across sites as mean annual rainfall decreased. Under rainfed production, yield was poorly correlated with annual rainfall at all sites. At all sites, irrigated yields across the climatic record were highly positively correlated with mean daily solar radiation. The study showed that it is feasible to import a pre-existing model, parameterise it for local cultivars and conduct scenario analysis.