The effect of crop start date, crop class and cultivar on sugarcane canopy development and radiation interception

The two components of sucrose yield are the biomass and the sucrose fraction thereof. Increasing one or both of these will increase yield. Biomass could be increased by maximizing radiation interception or the efficiency of its use in photosynthesis, or both. In this paper we investigated the effect of crop start date (planting or ratooning date), crop class (plant or ratoon) and cultivar on canopy development and interception of radiation. A better understanding of this interaction may point the way to more effective crop improvement and husbandry strategies that could enhance radiation interception. A brief review of the literature and available sugarcane simulations revealed where there is a lack of sufficient understanding of canopy development processes in sugarcane. Some of the information is contradictory, while some of the modelling concepts do not reflect the current status of knowledge adequately. Fractional interception of photosynthetic active radiation (FIPAR), and leaf and shoot development was measured on well-watered crops harvested at 12 months in June and December at Mount Edgecombe, South Africa. Results showed that start date had a significant impact on radiation intercepted. Seasonal mean FIPAR for a June ratoon was 80% of that of a ratoon starting in December. The cultivar effect was less than the start date effect, the largest difference being 13% between cultivars NCo376 and CP66/1043 for the June start. Leaf appearance rates were similar for different crop classes and start dates. Leaf size distribution up the stalk depended on start date and sequence of shoot appearance (shoot rank order). The dynamic nature of leaf size profiles could be explained by source-sink concepts and the dependence of leaf area on temperature. Shoot appearance rate was directly related to the density of buds present in the ground. This explained the difference in shoot appearance rate between crop classes and row spacing. Peak shoot population density occurred at a thermal-time (base 16 °C) of 600 °C d. Evidence presented here shows that radiation interception of unstressed crops could be increased significantly by adjusting planting density to optimally match the cultivar and crop start date, at least in theory. Increases of between 10 and 15% could be expected when cultivar and planting density is optimally matched with starting date. The main shortcomings of models are (1) the separation of leaf and shoot phenology from biomass dynamics and (2) inadequate representation of intra-stool shoot variation.