Constraints to high yield and CCS in large and lodged cane crops

Previous research in northern Australia and in Hawaii has suggested that large lodged crops of sugarcane experience a marked slow-down in yield accumulation prior to the normal commercial harvest. Cane yield may be as much as 40% below that expected if growth continued at rates occurring prior to the slowdown. Various explanations for this slowdown include lodging induced-stalk death and loss, reduced radiation interception as a result of lodging, seasonal factors and increased crop age. The primary objective of this thesis was to identify constraint(s) to the production of biomass and sugar in large and lodged sugarcane crops and then interpret and explain the effects of the identified factor(s) on growth processes such as stalk dynamics, CCS and cane and sugar yield. This understanding is important to address this problem either through genetic improvement and/or crop management. Secondly, it will help practical applications of crop growth simulation models to improve agronomic management. Four different field experiments were conducted over two seasons (1997/98 and 1998/99) both in dry and wet tropical environments of northern Queensland, Australia. These field experiments were designed to quantify the separate effects of lodging, crop age and seasonal conditions on crop growth and yield accumulation. The data from the field experiments indicated that lodging was a major constraint to high yields in both the dry and wet tropical environments. There was no evidence that crop age or any seasonal factors were implicated in the slowdown of growth. Prevention of lodging increased cane yield by 11 - 15%, CCS by 3 — 12% and sugar yield by 15 - 35%, at the final harvest in August/September, depending upon the extent and frequency of the lodging events. Apart from stalk death, lodging reduced the biomass and sugar content of the live stalks by reducing both the radiation interception and RUE of the crops. The economic losses from lodging were even greater due to the dilution effects from lower CCS of dead and rat-damaged cane. There was no evidence of any 'yield plateau' or 'stalk loss' when all the stalks, viz. live, dead and rat-damaged, were taken into account. To evaluate the possible causes of the observed 'yield plateau' in the earlier studies, the growth and development of live stalks and the factors that contributed to stalk dynamics in these crops were re-examined. The crop biomass was re-estimated by taking into account the missing trash components, dead cane and any spatial stalk variations to better understand the yield slowdown in these studies. The re-analyses further strengthened the conclusions drawn from the current studies and offered plausible evidence that stalk death and stalk loss were the major factors that contributed to the 'yield plateau' in the earlier studies. The observations on the number of stalks suggested that spatial variations in the number of stalks were potentially a major factor that contributed to the 'stalk loss' in these studies. The research implications of the conclusions from this thesis are considered in terms of the immediate actions that might be taken, and then the longer-term actions needed to improve productivity. Firstly, and most importantly, the longer-term implications that need to be addressed are to implement research to identify strategies to reduce lodging through crop management or breeding of lodging resistant varieties. For economic reasons, it is likely to be difficult to use changed agronomic management under Australian conditions. However, opportunities exist to improve sugarcane selection systems to select for lodging resistance. Overcoming lodging through breeding may take several years but the effects of lodging are sufficiently large to justify the research investment in this area. In the short term, there is potential to improve the accuracy and reliability of sugarcane crop growth simulation models. Secondly, again in the short term, it is important to avoid systematic bias or sampling errors in sugarcane field experimentation through improved sampling methodology. To summarise, the field research reported in this thesis has identified and described the physiological basis of a major, and hitherto largely unrecognised, constraint to productivity in sugarcane crops in tropical Australia. The thesis has also reviewed and re-interpreted earlier research, to show that the earlier work is consistent with the main conclusions of the current research. The thesis concludes by suggesting ways in which future crop improvement research might try to overcome the effects of lodging on sugarcane productivity.