The presence of suckers, late-formed tillers, in mature sugarcane crops reduces the sugar concentration of harvested material to the detriment of profitability. The amount of suckering varies with cultivar and season. However, the environmental stimuli promoting suckering, i.e. the number of suckers, are not understood. This paper describes the effects on suckering of increasing soil moisture, nitrogen, and the level of light penetrating the canopy. Light was manipulated by plant spacing or removal of dead leaf from mature stalks. Increased nitrogen availability late in the crop's growth cycle promoted suckering, even in a cultivar of low suckering propensity. Higher levels of nitrogen applied at the beginning of the growing season had inconsistent effects, possibly due to variation in rainfall. Increased soil moisture late in the growing season greatly increased suckering and also had a positive effect when combined with high levels of nitrogen. The effect of plant spacing on sucker number was only significant in the plant crop, and then only when expressed as sucker number per mature stalk. Removal of dead leaves had a significant effect on suckering at one site but not another. In all cases, higher plant spacing and dead leaf removal increased light levels recorded under the canopy. The quality and quantity of light required to promote suckering still remain unknown, as does the means by which the plant perceives the light stimuli. The differences in suckering between the plant and ratoon crops suggested that not all stimuli were tested in the treatments applied, or that other factors negated stimuli that were present or that suckering is inherently more prevalent in ratoon crops. All cultivars tested responded similarly to the environmental stimuli that produced a significant effect.
Introduction
Sugarcane is propagated vegetatively from stem pieces containing axillary buds. Like most grass crops, yield (biomass and sugar) comes from mature stalks arising from primary shoots from the axillary buds originally planted and from higher order shoots produced from axillary buds on these primary and other shoots (tillers) during the early stages of growth. Shoot number then declines because competition for light, moisture, and nutrients increases as the crop canopy closes (Borden, 1948, Bell and Garside, 2005, in this issue). Unlike many other grasses, sugarcane produces tillers later in the growth cycle, called suckers. Their morphology differs from tillers produced earlier in the growing cycle (Hes, 1954, Barnes, 1974, Bonnett et al., 2001). Suckers are characterised by thicker stems with shorter, broader, and thicker leaves than found on earlier-produced primary and secondary shoots.
Suckers have negative consequences in mechanically harvested sugarcane production systems as they are harvested together with mature stalks. The overall sucrose concentration of the harvested material is lowered (Berding et al., 2005, in this issue) because of the lower sucrose concentration of suckers (Ivin and Doyle, 1989, Berding et al., 2005). Reducing sucker number would improve sugar concentration and profitability for growers in industries where sugar concentration is part of the payment system.
Environmental factors that affect tiller number in sugarcane have been studied previously. That increasing nitrogen fertiliser increases secondary shoot number has been demonstrated many times (e.g., Borden, 1945, Borden, 1948, Shrivastava and Kumar, 1984). Other factors have been less well studied but have demonstrated some significant effects. Duration, intensity, and spectral composition of light affects grass tillering. Martin and Eckart (1933) demonstrated that reduced light intensity prevented secondary shoot development in sugarcane grown in large pots under three light levels. We can find no reports of manipulation of spectral composition of light and its effect on sugarcane tillering. However, red:far-red ratio has been shown to affect tillering in other grasses (Casal et al., 1987). Ludlow et al. (1990) measured the spectral composition underneath canopies of several sugarcane cultivars but could not determine if observed differences in red:far-red ratio were a cause or effect of measured differences in tillering.
Reports on environmental factors affecting suckering are rare. Borden (1948) and Salter and Bonnett (2000) reported the effect of nitrogen on suckering in single factor experiments. Manipulation of the amount of light reaching the stalk by removing dead leaves increased sucker numbers in the outside rows of sugarcane crops (Bonnett et al., 2001). Long-term observations of suckering resulted in the proposal of a hypothesis that harvest-season soil moisture, soil nitrogen, and increased light affected sucker initiation. The objective of the studies reported here was to determine whether soil moisture, soil nitrogen and light or a combination of these factors stimulate sucker development in a range of cultivars. A companion paper (Berding et al., 2005) reported on the yield and quality components of suckers in the multifactor experiment described here.
Section snippets
Multifactor experiment
An experiment to test the effects of moisture, nitrogen, and light on suckering of two cultivars of sugarcane was established. This experiment has been fully described (Berding et al., 2005 in this issue). Briefly, the experiment was established south of Cairns, north Queensland (17°5′ S, 145°47′ E), and consisted of two moisture regimes, rainfed and a post-monsoonal irrigation treatment. These treatments were used to test the hypothesis that harvest season rainfall (July–October), which has
Multifactor experiment
The number of mature stalks within the core plots in the plant and ratoon crops (Fig. 1) was very similar. There was a slight decline between 180 and 380 days. There were greater differences between crops for number of suckers. Suckers appeared earlier in the development of the ratoon than the plant crop and were more abundant (Fig. 1). At the final count in the plant crop (383 DAP), all main effects, except moisture regimes, showed small but significant differences in mature stalk number (
Discussion
The experiments reported here have shown that nitrogen and moisture have a significant impact upon suckering. The results for experiments where under-canopy light levels were manipulated are however, less clear.
Conclusions
Nitrogen, when applied from May onwards, and increased soil moisture late in the crop's growth period, have been shown unequivocally to influence suckering. The cultivars tested responded similarly to these factors irrespective of their suckering propensity. The role of light remains unresolved though there is evidence that it can promote suckering. The spatially diverse and complex canopy environment makes manipulation and measurement of an altered light environment difficult. Nitrogen and
Acknowledgements
Tom Watters, Alan Zappala, Warren Galligan, and John Dallachy on whose farms this work was performed are thanked for their collaboration and excellent co-operation. This work was funded in part by the Australian Sugar Research and Development Corporation. Barry Salter was additionally supported by the Co-operative Research Centre for Sustainable Sugar Production and Robert Lawn is thanked for his advice and supervision. Fiona Joseph and farm staff at BSES Meringa, Michael Hewitt and Franco
A field experiment was conducted for two crop cycles during 2003–2005 and 2004–2006 at Indian Institute of Sugarcane Research, Lucknow to improve bud sprouting, dry matter accumulation (DMA), nutrient uptake and ratoon yield by using potassium fertilizer. Potassium (K) fertigation in standing plant cane increased the number of buds per stubble and number of stalks in ratoon cane. K content of stubble increased by 16.7% with K fertigation. The content of reducing sugars in buds at the time of ratoon initiation improved significantly with K fertigation. It improved dry matter accumulation, number of millable canes, individual cane weight, ratoon cane and sugar yields. Thus, it could be concluded that application of 66 kg K ha−1 with irrigation water in standing plant cane before harvest improved bud sprouting, dry matter accumulation and nutrient uptake in ratoon crop. Irrigation in standing plant cane increased ratoon cane (69.9 t ha−1) and sugar yields (7.6 t ha−1). This increase for ratoon cane and sugar yield was 8.7 and 5.55%, respectively over the control. Further, it increased ratoon cane yield by 15.21% (74.1 t ha−1) and sugar yields by 13.9% (8.2 t ha−1) with K fertigation over the control. Thus, K nutrition holds great promise for improving growth of ratoon cane and sugar yields.
Sugarcane has a variable propensity to produce suckers, late-formed tillers in mature crops. These suckers are low in sugar content and result in a dilution of the sucrose content of the harvested crop. Differences in the propensity to form suckers have been observed among cultivars within the same experiment and within different parts of a crop of the same cultivar. The environmental factors promoting sucker initiation, or suckering have, however, not been elucidated. A field experiment was run in plant and first ratoon crops to investigate the factors of post-monsoonal irrigation, nitrogen fertiliser level, and within-row stool spacing to test the roles of soil moisture, soil nitrogen and light in suckering. Although the effects of these treatments on mature stalk traits were significant but small, their effects on suckering were relatively large. Increased soil nitrogen and soil moisture promoted suckering and these effects were additive. Plant density, although generating measurable differences in the light levels penetrating the canopy, did not result in a consistent effect on suckering. The effect of suckers in reducing commercial cane sugar (CCS) content was greater than previously reported. CCS was reduced by 1.6 and 1.3 units in the plant and ratoon crops, respectively, for each 10% of the harvested crop mass that were suckers. Availability of excessive nitrogen, in combination with post-monsoonal rain in autumn and winter, will increase suckering and reduce profitability.
The Australian sugar industry collects, during normal commercial operations, data on the production of sugarcane, although the nature and extent of the information varies considerably between regions. These data have become increasingly accessible to researchers who, through various analytical methods, have shown that this information can be used for a range of purposes. Examples include the development of regional models to predict CCS in response to rainfall, the estimation of genetic gain over an extended period across many regions, the development of alternative options to improve the efficiency of cane supply to the mill, the assessment of trends in productivity (including intra-regional spatial and temporal productivity trends), and the identification of attributes of the production system that affect productivity. Experience has shown that, while they are essentially empirical in nature, these analyses can be used to define problems within the industry and either initiate or complement more traditional agronomic and genetic research necessary to solve these problems. This paper reviews the different ways that industry information has been used to facilitate decision-making and their value in the identification of problems such as a chronic decline in CCS in the wet tropics. The merits of expanding this type of approach in the sugar industry more generally are also discussed.
Substantial amounts of nitrogen (N) fertiliser are necessary for commercial sugarcane production because of the large biomass produced by sugarcane crops. Since this fertiliser is a substantial input cost and has implications if N is lost to the environment, there are pressing needs to optimise the supply of N to the crops’ requirements. The complexity of the N cycle and the strong influence of climate, through its moderation of N transformation processes in the soil and its impact on N uptake by crops, make simulation-based approaches to this N management problem attractive. In this paper we describe the processes to be captured in modelling soil and plant N dynamics in sugarcane systems, and review the capability for modelling these processes. We then illustrate insights gained into improved management of N through simulation-based studies for the issues of crop residue management, irrigation management and greenhouse gas emissions. We conclude by identifying processes not currently represented in the models used for simulating N cycling in sugarcane production systems, and illustrate ways in which these can be partially overcome in the short term.
In Australian sugarcane crops, growth in terms of radiation use efficiency (RUE) can appear to slow down well before final crops are harvested, despite conditions that are considered favourable for growth. In order to assess the extent and cause of this reduced growth phenomenon (RGP), 14 experiments conducted in Australia were examined. From these experiments, 34 treatments were selected where nutrients and water were likely to be non-limiting and consequently the yields obtained were likely to be limited only by temperature and radiation.
Radiation use efficiency is defined as the measured amount of net biomass accumulated above ground per unit radiation intercepted. Accumulated biomass may have been lost through loss of stalks. In almost half of the treatments RUE significantly declined between approximately 223 and 665 days after crop start. The high incidence of RGP found in the analyses (a) challenges the previously held assumption that a constant value of RUE can be used to adequately describe biomass accumulation in sugarcane as a function of cumulative radiation interception and (b) suggests that relatively low growth rates during the later phases of the cropping season are frequently experienced throughout the Australian sugar industry. Reduced growth phenomenon resulted in actual biomass at final harvest being on average 21% less than potential yield, compared with 5% in crops that did not display a slowdown in growth.
Mean maximum RUE in the plant crop was greater than in ratoon crops, at 1.37 and 1.19 g MJ−1, respectively. As there was no effect of location, cultivar, crop duration, ratoon crop class or fumigation treatment on RUE for the periods either before or after the onset of reduced growth, it would appear that the value of RUE is predominantly influenced by crop class. Despite the differences in RUE between the two crop classes, RGP occurred at a similar frequency in both plant and ratoon crops.
In order to gain a greater mechanistic understanding of the factors likely to be associated with the onset of the RGP, the relative time of lodging, specific leaf nitrogen (SLN), number of stalks and seasonal temperature were all considered individually. Whilst declining SLN and increasing stalk loss were associated with reductions in growth during the later periods of crop growth, it is likely that lodging also contributed to the widespread slowdown in biomass accumulation, whereas photosynthetic stress, as a result of extreme temperatures, did not play a substantive role in reducing growth.
