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Technology

Improve water use of sugarcane to maximise profit

By Marcel Heine (Agronomist - East Coast Business Unit)

Measuring the total change in soil water content due to vapour losses is relatively easy. However, it is quite difficult to determine just how much water is lost directly from the soil by evaporation (E) and how much was lost from the leaf surface after plant uptake by transpiration (T). For this reason there is a lot of data available on the losses resulting from the two processes combined into evapotranspiration (ET). The E component of ET may be regarded as a waste of water from a crop productivity point of view, but the T side is essential for plant growth as it provides the water that sugarcane needs for cooling, photosynthesis, turgor maintenance, and nutrient transport.

Sugarcane is a C4 plant that is able to maintain higher rates of photosynthesis compared to C3 plants such as wheat. The low CO2 compensation point of C4 plants means that they can maintain higher rates of photosynthesis at lower CO2 levels. Therefore, C4 plants have an advantage over C3 plants during water stress periods. It is, however, important to maximise water use efficiency of sugarcane to ensure that the best yield and Recoverable Value (RV) is achieved. How do we quantify transpiration and yield? Simply put, the transpiration ratio relates to the uptake of CO2 to the loss of water by transpiration from the leaf. The inverse of this ratio is termed water use efficiency (WUE). The WUE is the most common way to relate the effective use of water and is expressed in units such as kg biomass or yield per mm water used for ET.

WUE = Crop production/ET = tons sugarcane/100 mm

How do we control ET to maximise crop productivity? 

The transpiration part of ET is closely related to the total leaf area exposed to the sun, while evaporation is related to the amount of soil surface exposed. We can manage unproductive losses of water by the way we manage the sugarcane crop. Factors that are under direct control of the producer include weed manage-ment, compaction, variety selection, irrigation scheduling/systems, green cane harvesting, drying off, and nutrition. They can all be manipulated to maximise the use of water that is available for plant growth and yield. For example, green cane harvesting can increase yield by up to 10 t/ha, because of a reduction in evaporation due to soil cover from the crop residue blanket left behind. However, a crop residue blanket could also have negative impacts on the crop by slowing down initial growth, tillering, and radiation interception in colder areas. Most of the yield benefits attributed to green cane harvesting have been under dryland conditions where rainfall is often erratic. Under irrigated conditions the benefit seems to be small, or even negative. In research conducted by Olivier and Singels (2012) in Pongola, it was found that cane yields of a residue treated plant and ratoon crop was between 4% and 15% lower than the bare soil treatment. However, the differences were not statistically significant.

Soil compaction due to inefficient traffic control measures leads to stool damage and decreases root exploration, in addition to decreased water infiltration and more surface runoff after rainfall/irrigation events.  It is going to be important in future to introduce controlled wheel traffic systems to permanently keep heavy haulage trucks and trailers off productive areas in the field.  

From a soil fertility perspective, top- and subsoil acidity has a major impact on WUE in the higher rainfall, dryland sugarcane producing areas of KwaZulu-Natal. Aluminium is toxic to plant roots and results in poor and abnormal root development, even on an acid tolerant crop like sugarcane (Figure 1). 

Roots tend to become thick and stubby, with little development of the important fine roots.  Inefficient root systems limit yields because of poor water and nutrient uptake. Aluminium toxicity often magnifies the effects of drought stress during periods of low rainfall, and is often aggravated further where subsoils are also acidic. Subsoils under these conditions often contain large amounts of plant available water but it is not accessible to roots.

In addition, deficiencies of calcium (Ca) and magnesium often occur in acid soils and may pose a limitation to plant growth. Adequate supplies of Ca are particularly important for good root growth in subsoils containing high levels of aluminium (Miles and Farina, 2013).  Fertilizer products such as GREENSULF™ and MAXI PHOS™ that contain Ca are important to help maintain adequate root growth and photosynthesis, which could increase WUE.  Nitrate-nitrogen containing fertilizer such as GREENSULF™ has been proven to enhance the uptake of potassium, calcium, and magnesium, which could lead to an increased WUE.

Adequate lime and gypsum applications, together with the proper incorporation of these products, are needed to alleviate aluminium toxicity.  It is important to note that gypsum applications will not always aid in alleviating subsoil acidity, and it is best to have representative top and subsoil samples taken for interpretation by an agronomist.

Verwysing / Reference:

  • Olivier, F.C. & Singels, A., 2012. The effect of crop residue layers on evapotranspiration, growth and yield of irrigated sugarcane. Water SA 38, 77-85.
  • Miles, N. & Farina, M.P.W., 2013.  Soil acidity and its management in crop production.  SA Grain, July 2013.