Some of the results presented here were first published in a SASTA short communication (Olivier and Singels, 2006) and an ISSCT poster (Olivier and Singels, 2007 AbstractA layer of harvest residues from the previous crop can reduce wasteful evaporation from the soil surface and thereby increase the efficiency of use of limited water resources for agricultural production. The practice of harvesting sugarcane green and leaving crop residues in the field, as opposed to burning the residue, has been re-adopted in many sugarcane industries worldwide. However, a better understanding of the dynamic impacts of residue layers on various aspects of the cropping system is required to (1) enable the formulation of sets of best management practices for specific production scenarios, and (2) promote the use of residue layers in areas where it is desirable and has not been adopted, such as irrigated sugarcane production in South Africa. The objective of this study, therefore, was to quantify the effect of 2 different types of residue layers on crop growth, cane yield and evapotranspiration of fully irrigated sugarcane. A layer of cane tops and dead leaves (Trash) and a layer of green tops (Tops) were applied to the soil surface of sugarcane crops (plant crop and first ratoon crop of variety N14) grown on lysimeters at Pongola, South Africa. Observations of crop growth (stalk population, stalk height, canopy cover), cane yield and evapotranspiration for these treatments were compared to that of a bare soil treatment. The data were also used to derive values of crop evaporation coefficients for different development phases and these were compared to FAO56 recommendations. Initial stalk population in the plant crop and radiation capture in the plant and ratoon crop were affected negatively by crop residue layers, but without significantly reducing final stalk population and cane yield. Peak stalk population occurred later in crops grown in residue layers, but peak and final stalk populations were unaffected. Evapotranspiration was reduced by both residue layers, mainly due to a slower developing canopy (reduced transpiration) and reduced evaporation from the soil, during the pre-canopy phases. Increased drainage was observed under residue layers, emphasising the importance of accurate irrigation scheduling to avoid water logging. The FAO56 methodology for calculating crop evaporation coefficient values for the initial, development and late season phases are supported by the results obtained here. Crop evaporation coefficient values were significantly reduced by residue layers. It is important that irrigation scheduling practices be adjusted to realise the potential water savings of sugarcane production systems that make use of residue layers. This study provides the information required to do that. The information could also be used to improve the ability of the crop models to accurately simulate crop growth and evapotranspiration in a residue layer cropping system.
In South Africa (SA) approximately 30% of sugarcane is grown under irrigation and there is increasing pressure to demonstrate efficient use of limited water resources. Agronomic practices such as the use of a crop residue layer, changed row spacing, growing suitable varieties and accurate irrigation scheduling could potentially increase water use efficiency (WUE) by saving water and/or increasing yield. The aim of the study was to investigate to what extent WUE of irrigated sugarcane production in SA can be improved by better agronomic practices, and to gain a better understanding of the mechanisms involved in crop response to these factors.An overhead irrigated field experiment was conducted near Komatipoort, South Africa on a shallow, well-drained, sandy clay loam over a four year period (one plant (P) and three ratoon crops (R1, R2 and R3)). Treatments consisted of factorial combinations of variety Page 2 (N14 and N26), row spacing (single rows spaced at 1.5 m and dual rows spaced at 1.8 m) and soil surface cover (bare soil and crop residue layer). Measurements included tiller population, interception of photosynthetically active radiation (FI PAR ), soil water content, and cane yield at harvest. Crop water use (CWU) was estimated using the water balance approach.This study showed that significant reductions in water use and irrigation requirements, and increases in WUE, are possible by using a crop residue layer to cover the soil. Water savings were largest in P1 (26% in CWU and 32% in irrigation requirement) but substantial savings were also achieved in the R crops (about 15%). It is essential to practice accurate irrigation scheduling to realize these savings, taking into account soil cover and cultivar effects, especially during the period of partial canopy. Although the residue layer caused small reductions in yield in the P, R1 and R2 crops (on average 9%) these were not statistically significant. The combined effect of large CWU reductions and small changes in cane yield resulted in increased WUE (on average 18%).These responses to a residue layer were achieved through a reduced rate of canopy development due to delayed emergence of tillers, causing less green canopy cover and reduced CWU, especially during the period of partial canopy cover when stalk growth has not yet commenced. CWU and FI PAR were affected much less during the subsequent period of stalk growth, thus affecting cane yield minimally, provided irrigation scheduling was adjusted.Variety N14 consistently developed a canopy more rapidly, intercepted more radiation and achieved a higher yield than N26. Row configuration had a significant impact on canopy development, seasonal FI PAR , final stalk population but did not affect cane yield or WUE. Page 3The study produced quantitative data for parameterizing crop models which will improve their reliability in irrigation management and yield prediction applications.
There are indications that high-fibre sugarcane genotypes may produce more biomass and use resources more efficiently than conventional sugarcane cultivars.The objective of this research was to gather quantitative information on resource use for selected conventional and high-fibre sugarcane genotypes and benchmark it against other bioethanol crops. Although conventional sugarcane initially grew slower than sorghum and Napier grass, it produced very high biomass (about 70 t ha -1 ) and theoretical ethanol (first-and second-generation) yields (about 27 kL ha ), outperforming all other crops except sorghum. The contribution of cellulosic ethanol to total ethanol yield varied hugely, from 89% for the high-fibre sugarcane hybrid to about 48% for conventional sugarcane, to as low as 14% for sugar beet. The highfibre sugarcane hybrid grew faster initially and produced more biomass at eight months (56 t ha -1 vs. 45 t ha -1 ) than the conventional types, but then flowered, reducing its growth rates markedly thereafter. It was also less sensitive to mild drought conditions. Results suggest that cellulosic ethanol production could be a feasible option that could be incorporated into conventional or biomass sugarcane production systems.Keywords: bioethanol crops, biomass, high-fibre sugarcane, stalk fibre composition, theoretical ethanol yield IntroductionThere is increasing interest in renewable energy, including biofuel from crops.Bioethanol can be produced from the fermentation of soluble sugars in the storage organs of feedstock crops, while 2 nd generation lignocellulose technology enables the production of ethanol from cell-wall sugars extracted from plant fibre (Ragauskas et al. 2006). This will greatly enhance the potential ethanol output from feedstock crops and address concerns regarding ethanol production from food crops in high potential production areas.Potential bioethanol crops include maize, switchgrass, Miscanthus, sugarcane, sugar beet, sorghum and poplar. Compared to other crops sugarcane has abundant potential for producing high biomass yield (Alexander, 1985) and consequently high bioethanol yields from sugars in the juice (Renouf et al. 2008) and form leaf and stalk fibre (Waclawovsky et al. 2010, de Souza et al. 2013. Energy cane that produce high biomass rather than high sucrose yield and use natural resources more efficiently, are currently in development (Tew and Cobill 2008). These genotypes 2 could possibly be used for biomass production in marginal production areas where resource levels are low, such as low rainfall areas or areas with poor soils.Very little quantitative information on radiation and water use or crop productivity is available for high-fibre sugarcane types in South Africa. Waclawovsky et al. (2006) quote commercial maximum yields of 29 t ha -1 of dry biomass and puts forward a theoretical maximum of 177 t ha -1. Alexander (1985) hypothesizes that sugarcane yields can be increased two fold by using high-fibre cane and managing water and nitrogen to maximize biomass growth an...
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