AFRICA
6.6 Flowering in Sugarcane
18°E 24°E 30°E 32°S
25°S
NON-CONDUCIVE GROWTH UNITS Annual
0 - 750 750 - 1500 1500 - 2250 2250 - 3000 3000 - 3750 > 3750 Mean annual number of non-conducive growth units
Base
temperature: 10ºC SMC < 50 % PAW
Figure 6.40 Mean annual number of non-conducive growth units for southern Africa
no soil water stress (Gosnel, 1973; Moore, 1987),
a day length shorter than 12.5 hours (Ethirajan, 1987), and
daily maximum and minimum temperatures of approximately 28°C and 23°C, respectively (Heinz, 1987).
It was also found by Coleman (1968) that at temperatures below 18°C, flower initiation and/or emergence were significantly reduced. The time period during which sugarcane flowering is initiated coincides with 22 days from the time when day length shortens to less than 12.5 hours (Ethirajan, 1987). In order to calculate the commencement and duration of this time period, a number of latitudinal points along the current sugarcane growing regions (cf. Figure 3.6) were taken, and the associated day when day length shortened to below 12.5 hours calculated. These time periods coincided largely with the first three weeks of March.
From the above-mentioned factors an index was developed whereby, for the 22 day period calculated from the beginning of March, the average number of days over the 50 year period spanning 1950 to 1999 on which soil water content for the day was > 50% PAW (to account for non-stress conditions), and daily minimum temperature was above 18°C, were accumulated. The South African temperature database described in Section 3.2, and the derivation of soil water contents as described in Section 6.2 for a SaClLm textured soil with a profile thickness of 0.9 m, were used in the calculations of the above-mentioned thresholds.
6.6.2 Distribution Patterns of Sugarcane Flowering in Southern Africa
The sugarcane flowering index, mapped in Figure 6.41, indicates the east coast of southern Africa to be most prone to sugarcane flowering. Parts of the central and north coast areas of KwaZulu-Natal display an average of between 8 and 12 days out of a total possible 22 that meet the conditions to induce flowering. Areas along the east coast and extending inland of KwaZulu-Natal, some of the northern coastal parts of the Eastern Cape, central Swaziland, the lowveld of Mpumulanga and the Limpopo province show between 4 and 8 days per annum which meet the condition for flowering. Between 2 and 4 conducive days are found on the peripheries of these areas. Although the above-mentioned numbers seem low, it is important to note that they are an average over a 50 year period (1950 to 1999), and that flowering does not generally occur every year, but rather more sporadically. Therefore, it is very unlikely that
the index would indicate all 22 possible days of a year as meeting the flowering conditions.
Bearing this in mind, areas showing between 8 and 12 conducive days indicate a high risk of sugarcane flowering. This index therefore provides some indication of long-term flowering risk. Further research is needed to also quantify the inter-annual variability of the flowering index.
18°E 24°E 30°E
32°S 25°S
SUGARCANE FLOWERING INDEX
0 - 2 2 - 4 4 - 8 8 - 10 10 - 12 Mean number of conducive flower initiation days
SMC > 50% PAW Day length < 12.5 hrs Max Temp ~ 28°C Min Temp ~ 23°C
Figure 6.41 Mean annual number of days prone to the flowering of sugarcane
* * * * * * *
In this chapter a number of the factors which affect the growth and production of sugarcane, and which are related to soil water, were evaluated. These factors included soil water content, soil compaction, irrigation water demands, conducive and non-conducive growing units and, finally, sugarcane flowering. The objective of mapping of soil water content was to allow for comparisons of soil water to be made between the various soil types and depths, as well as to identify specific areas in which soil water may be a limiting factor to sugarcane growth. These maps may potentially be used in strategic planning, by extension officers or farmers in general. Mapping soil compaction included estimating the start of the infield traffic season as well as their associated length and risk. Through these maps, theoretical time periods were identified as to when the risk of soil compaction is reduced. These maps may be used as a guideline to, for example, plan planting and harvesting dates, so that the risk of soil
compaction may be minimised. Irrigation water demands were mapped to allow for comparisons to be made between the various modes of irrigation, based on a 0.9 m SaClLm soil. The objective of this was to enable users to identify which specific irrigation mode and application rate would be most suited to a particular region. Conducive growth units were mapped according to the temperature and soil water criteria, and these show up areas most suitable for sugarcane growth. The intended purpose of these maps was to identify potentially new areas in which sugarcane may be grown successfully. Non-conducive growth units depicted the areas in which the sugarcane plant is more likely to become stressed, owing to a lack of soil water and increased temperatures. Flowering in sugarcane may potentially result in losses in sugarcane yield. It is for this reason that areas that may be prone to sugarcane flowering were identified. The application of these maps may be found in strategic planning, for example, by selecting flowering resistant varieties of sugarcane in areas shown to be prone to it occurring. The chapter which follows covers the mapping of yield estimates for both dryland and irrigated sugarcane.