WATER USE EFFICIENCY OF COMMERCIAL SUGARCANE PRODUCTION IN MPUMULANGA

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1 WATER USE EFFICIENCY OF COMMERCIAL SUGARCANE PRODUCTION IN MPUMULANGA BP SWART South African Sugar Association Experiment Station, P/Bag X02, Mount Edgecombe, 4300 Introduction There is continued pressure on farmers to produce more tons of cane of the highest quality with the lowest input cost. Although it is true that some lower yielding varieties, under certain conditions can produce higher tons ERC (or RV) per hectare than high yielding varieties, it is found in practice that high yield has a better chance to do so. The first step to rectify a range of problems on a specific field is to address the problem of low yield. From a growers perception, most cases of poor cane growth is associated with inefficient irrigation not enough water. Over irrigation is seldom seen as a problem. The ability of soil to let excessive water drain from the profile has created the idea of; apply as much water as possible to obtain high yield. By using accurate climatic data from the Komati Mill Area, it is possible to determine the exact amount of irrigation water needed for an optimal cane yield. With efficient irrigation systems, it should also be possible to apply the correct amount of water, at the right times to meet the physiological crop water demand of the sugar cane plant. The following is a brief introduction to the Komati Mill Area to get an idea of the Onderberg s dilemma and most probably the dilemma of other areas during the current and previous seasons. Production regions These can be divided into the following areas, mainly based on soil potential (Table 1). The yield values in Table 1 are actual yield obtained by growers during the 2001 delivery season for each region. The yield range is quite big possible because of factors such as irrigation systems, drainage, salinity, sodicity, etc. These factors can have a major effect on the ability of the field to produce an optimal yield under specific climatic conditions. This however does not mean that rectifying some of these problems cannot alter the potential. Table 1. Production regions. Yield (t/ha Yield (t/ha) Area Weighted Low average High East of Komati River Lower Crocodile River Middle Strydom Block Northern Strydom Block Southern Strydom Block Projects AREA STATISTICS Table 2. Cane area and total tons for the 2001 season. Area (ha) Yield (t/ha) Total tons cane Commercial Growers Project Growers Total

2 Since the first crush of the Komati Mill in 1995, milling capacity was increased twice, to accommodate grower s demand for more hectares under cane. To meet the increased capacity requirement of the mill, contracts were signed between Miller and Growers for the production of cane on a total area of 19,332 hectares. An average yield of 110 tons/ha was used as basis for calculations. All cane fields were mapped with a GPS and a sophisticated database was established to ensure maximum efficiency. However, due to poor climate conditions since the start of the mill, overall yield steadily declined form the highest mill average of tons/ha in 1999 to an expected 96 t/ha for the current season. This is causing serious financial problems not only to the Miller but also to the whole of the farming and business community. The following tables provide an explanation of the effect of a yield loss of 17 tons cane per hectare. Table 3. Mill Capacity vs Area Potential Mill capacity 2,200,000 tons 2001 estimate 1,866,102 tons Total shortfall 333,898 tons Tons/ha shortfall 17 tons Commercial growers increased avg = 119 Project growers increased average = 99 Final Mill average tons / ha Table 3 shows the ideal scenario to enable the mill to crush at its full capacity. It will also earn the growers an extra R The Hauliers will benefit an extra R5,154,912 and cane cutter R1 million, not to mention the overall economic boost to suppliers of goods and services. The solution to the problem as explained is simple increase the average yield. The ways and means to do that is however, not so easy. Possible factors effecting yield Climatic Soils (Chemical/Physical) Irrigation Nutrition Weeds Pests and Diseases Nematodes Varieties Management of above mentioned This paper will only focus on a few of these directly linked to water use efficiencies (WUE). Most of these factors are so much inter linked that one specific factor cannot be separated from the other. Climate yield potential By using temperature and radiation values since 1998, in a simple growth model, the following trends are shown. Chart 1 CLIMATIC YIELD POTENTIAL (01/10/2001) t/ha t/ha t/ha t/ha 2000 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

3 Chart 2 CLIMATIC POTENTIAL, ACTUAL YIELD & EFFECTIVE RAIN Effective rain Climatic Potential yield Climatic Potential yield Effective Rainfall Since 1998, due to lower radiation and average temperatures, the climatic yield potential of cane steadily declined from a potential yield of 173 t/ha to 151 t/ha in In contrast to the climatic decline of 20 tons, the actual yield stayed very much the same since 1997, with the lowest expected yield of 96 t/ha for this current season. Since 1999 with a climatic potential of t/ha, only 65% of this potential could be obtained on average. For the 2000 delivery season, 70% of the climatic potential could be obtained mainly due to the high overall rainfall of 1500 mm (only 630 mm was considered to be effective). Water use efficiencies The well-known Thomson equation states: For every 100mm of Et, a cane yield of 9.6 tons/ha can be expected. The following table shows some of the values from the Canesim Model, using data from the Komati/Amanxala automatic weather station. Values of other models are also included. Year Table 4. Yield values from Canesim and Oompie models. Model Climatic Potential Obtainable F= (151) (121) (F=0.63) 102 (F=0.59) 107 (F=0.65) (F=0.70) (98) (F=0.65) Dryland Model Difference Irrigated (50) (42) The second column shows the climatic potential with an obtainable yield factor of 80% in column three. Over the past five years only 64% of the climatic potential could be obtained. (Based on the specific model simulation). 3

4 The Canesim model was used to simulate cane yield with effective rainfall as the only source of water supply. These values, for each year, deducted from the actual yield obtained, are the tons cane produced with additional irrigation and Table 5. Water use efficiencies. should reflect the effectiveness of the applied irrigation water. The values in Table 6 show the water use efficiency for the different years. Year Et Effective rain Irrigation WUE IWUE FAO (DWUE) requirement Ycane/Et (Yt-Yd)/Irr req (3.57) / 100mm 7.5 / 100mm (5.65) / 100mm 6.0 / 100mm (9.09) / 100mm 4.65 / 100mm (9.20) / 100mm 6.56 / 100mm (11.76) / 100mm 3.46 / 100mm During 1996, cane was still recovering from the effects of the 1994 drought and many fields had to be replanted. The model simulation could not have taken this into account, yet the very low dry-land water use efficiencies cannot clearly be explained. The efficiencies for 1999 to 2001 are however very high and clearly demonstrate the contribution of the storage capacity of soil and the value of a well-developed root system. On the other hand, the irrigation water use efficiencies are fairly low compared to the dryland efficiencies and indicate the lack of capacity of irrigation systems and irrigators to schedule irrigation. Case studies Water use efficiencies of two growers were calculated and are shown in Tables 6 and 7. Table 6. Study 1 Middle Strydom block (2001 delivery season). Yield Category yield t/ha Yt Yd d=56 IWUE t/100mm Low (<110) Medium ( ) High (>121) Irrigation applied per production season = 1371 mm Table 7. Study 2 Lower Crocodile river (2001 delivery season). Yield Category yield t/ha Yt-Yd d=56 IWUE t/100mm Low (<110) Medium ( ) High (>121) Irrigation applied per production season = 1810 mm Solution 1 Possible solutions (1 to 5) By utilising all available sunshine and heat units, two of the keys elements in optimal yield production are used. The aim should be to yield at least % of climatic potential. Timeliness of activities is essential and no time should be wasted after harvest to ensure a quick start of new cane growth. The contribution of heat units and radiation of yield for different parts of the season is shown in table 9. Table 9. Heat units and Radiation (See appendix 6) Solution 2 A survey on irrigation systems, conducted by ARC ILI, on behalf of SASEX, showed that most of the systems performed lower than the accepted norms. The results also pointed out that well maintained and correctly operated systems could achieve or exceed the uniformity, which are considered reasonable and acceptable. Of the systems tested in the Komati Mill area the following general recommendations were made: Check leaking sprinklers Check sprinklers that are stuck Frog traps seem insufficient and cause blockages Nozzle diameters of sprinklers vary between new and badly worn. Average operation pressure is lower than the required norm Badly worn sprinklers need to be replaced. Serious blockages occur in some dripper lines. 4

5 However, some systems have been found to be operating well and according to the required norms. Solution 3 Although many growers make use of consultants to schedule irrigation, there is a need to do it them selves. Several options are available, from high technology (neutron probes) to extremely simple devices, such as the Wetting Front Detector. Even a spade will do. Solution 4 The change from old dragline systems to sophisticated drip irrigation systems created an array of new parameters and rules. The most obvious and cost effective change from overhead irrigation to drip irrigation was the alternative cane row dripper line configuration. During the first couple of years and on certain soils, it worked well. As more farmers implemented this system, problems started to appear, mainly due to the fact that the mindset of irrigators did not changed together with the change of the system. Many irrigators use the dripper line as an alternative to flood irrigation. Large amounts of water applied, cause serious drainage problems after a couple of seasons. Poor water and soil quality is also starting to have an effect on yield and could be one of the many reasons for the yield plateau. Cane row configuration, ridging of cane rows, placement of dripper lines and pulse irrigation is factors that need to be investigated. Solution 5 The overall management of all aspects of the production chain will be the final answer to obtain optimal yields in accordance to soil potential. The difference between managers will also influence the potential of a specific soil. Each field needs to be treated separately to sustain optimal production for as long as possible with the least inputs. Conclusion The inability to increase the average cane yield within a 12 month growing cycle is the main restricting factor for growers in the Komati Mill area. In spite of limiting factors such as climatic conditions and low annual rainfall, water supply is adequate. The efficiency of both irrigation systems and water use per ton of cane needs urgent attention. Other factors such as nutrition, weed control, pest and disease should also be placed high on the priority list. References Canesim model Singels A and Bezuidenhout Oompie Model (Unpublished) Swart B and Jansen B GIS database (unpublished) Komati Mill Group Board Automatic Weather Data (Unpublished) Komati Weather Station Network Sugar Cane Yields & Variety Performance. Bundaberg District Bundaberg cane Productivity Committee, Bundaberg Sugar