AFRICA
7. ESTIMATES OF SUGARCANE YIELDS
7.2 Estimates of Optimal Sugarcane Season Lengths and Associated Dryland Yields in Southern Africa
7.2.3 Distribution Patterns of Dryland Sugarcane Yields Over Southern Africa for Various Season Lengths
The distribution patterns of dryland sugarcane yields for a 12 month growing season, shown in Figure 7.1, display a range of between 30 to 40 t/ha/annum in the northeastern parts of southern Africa, including the eastern parts of Mpumalanga and Limpopo, to in excess of 70 t/ha/annum in an area of the northeastern part of KwaZulu-Natal. The northeastern region of KwaZulu-Natal shows the greatest potential for dryland sugarcane production with a 12 month growing season, indicating in excess of 50 t/ha/annum for most of this region.
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SUGARCANE YIELD
12 Month Season Length 30 - 40
40 - 50 50 - 60 60 - 70 > 70 unsuitable
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha/an
Figure 7.1 Distribution patterns of estimated sugarcane yields for a 12 month season length
Figure 7.2 shows the distribution patterns of sugarcane yield for a 15 month growing season.
With the extended season length, the area which is shown to be viable for sugarcane production increases, and shifts inland to the more central higher lying parts of northern KwaZulu-Natal, Mpumalanga and Limpopo. The yields calculated for a 15 month season length are generally higher than those of the 12 month season, with the majority of areas showing between 60 and 75 t/ha being harvested. However, the map of annualised sugarcane yield for a 15 month crop, given in Figure 7.3, allows for comparisons to be made, and shows
very similar annual yields to those seen for the 12 month season length. Areas previously shown to be viable in the northeastern parts of KwaZulu-Natal, Mpumalanga and Limpopo no longer show up as viable sugarcane production areas for this particular season length. The reason for this is that the upper limit of 4 500 HUs is exceeded for these regions. This suggests that the economic benefits of having a growth season of this length would be negligible, as possible increases in sugarcane yield for the increased season length would be low.
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SUGARCANE YIELD 15 Month Season Length
unsuitable 30 - 45 45 - 60 60 - 75 > 75
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha
Figure 7.2 Distribution patterns of estimated sugarcane yields for a 15 month season length
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25°S
ANNUAL SUGARCANE YIELD
15 Month Season Length 30 - 40
40 - 50 50 - 60 60 - 70 > 70 unsuitable
t/ha/an
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
Figure 7.3 Estimates of annualised sugarcane yields for a 15 month season length
The distribution patterns of the total as well as annualised yields for an 18 month sugarcane growing season are shown in Figures 7.4 and 7.5, respectively. A shift in the areas in which sugarcane production is viable is noted in Figure 7.4, with areas further inland and south, extending into the Eastern and Western Cape indicating the potential for sugarcane production. The northeastern parts of the Eastern Cape, as well as the central and southern midlands KwaZulu-Natal and Swaziland show the greatest potential for sugarcane production at this season length, with potential yields of more than 70 t/ha over the 18 month season, or an annualised yield of in excess of 50 t/ha/annum. The central parts of Mpumalanga and Limpopo indicate, for the most part, a potential yield of between 50 and 70 t/ha over the 18 month growing season. However, the annualised yield values for these regions, shown in Figure 7.5, indicate a low annual potential yield of only between 30 and 40 t/ha/annum in those regions. In Figure 7.4, small areas in the Western Cape indicate a potential for sugarcane production. These estimated potential yields are, however, relatively low. This is confirmed in Figure 7.5 which shows the annualised potential yields, and which indicates a more conservative distribution of potential sugarcane growing areas in the Western Cape.
These low annual potential yields show that planting sugarcane in these areas may not be viable in the long term.
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SUGARCANE YIELD 18 Month Season Length
unsuitable 30 - 50 50 - 70 70 - 90 > 90
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha
Figure 7.4 Distribution patterns of estimated sugarcane yields for an 18 month season length
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32°S 25°S
ANNUAL SUGARCANE YIELD
18 Month Season Length 30 - 40
40 - 50 50 - 60 60 - 70 > 70 unsuitable
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha/an
Figure 7.5 Estimates of annualised sugarcane yields for an 18 month season length
In Figures 7.6 and 7.7, areas showing the potential sugarcane production for a season length of 21 months include the inland regions of KwaZulu-Natal, coastal parts of the Eastern Cape, as well as small patches in the Western Cape, western parts of Swaziland, central
Mpumalanga and Limpopo. The central inland regions of KwaZulu-Natal and the northern parts of the Eastern Cape indicate the greatest potential for sugarcane production for a 21 month harvest season, showing patches of potential yields exceeding 100 t/ha over the 21 month season. In Figure 7.7 of the annualised potential sugarcane yields, the majority of regions indicated in KwaZulu-Natal, Swaziland and the Eastern Cape show potential annual yields of more than 50 t/ha/annum. Similar to the distribution patterns shown in Figure 7.5, a reduction in the area shown to be viable in the Western Cape is noted in Figure 7.7.
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SUGARCANE YIELD 21 Month Season Length
30 - 60 60 - 90 90 - 120 > 120 unsuitable
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha
Figure 7.6 Distribution patterns of estimated sugarcane yields for a 21 month season length
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25°S
ANNUAL SUGARCANE YIELD
21 Month Season Length 30 - 40
40 - 50 50 - 60 60 - 70 > 70 unsuitable
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha/an
Figure 7.7 Estimates of annualised sugarcane yields for a 21 month season length
The distribution patterns shown in Figures 7.8 and 7.9 of potential sugarcane yields for a 24 month growing season are limited largely to the cooler inland regions of KwaZulu-Natal, the Eastern Cape, Western Cape, Swaziland and small patches of the central regions of Mpumalanga and Limpopo. The annualised yield estimates for the areas indicated in Figure 7.9 are generally above 50 t/ha/annum, apart from areas in the Western Cape, where yields of between 30 and 40 t/ha/an are shown.
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25°S
SUGARCANE YIELD
24 Month Season Length 30 - 60 60 - 90 90 - 120 > 120 unsuitable
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha
Figure 7.8 Distribution patterns of estimated sugarcane yields for a 24 month season length
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32°S 25°S
ANNUAL SUGARCANE YIELD
24 Month Season Length 30 - 40
40 - 50 50 - 60 60 - 70 > 70 unsuitable
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
t/ha/an
Figure 7.9 Estimates of annualised sugarcane yields for a 24 month season length
Figure 7.10 shows the distribution patterns of optimal season lengths for southern Africa.
These distributions were obtained by mapping the harvest season length for which the highest yield is simulated,, given the criteria described in Section 7.2.1. From this map, shorter harvest seasons are most optimal in the northern coastal regions of KwaZulu-Natal, eastern
Mpumalanga and eastern Limpopo province. Areas showing more optimal yields with longer growing seasons include the more inland and higher altitude regions of Limpopo, Mpumalanga, Swaziland and KwaZulu-Natal regions, as well as the coastal and inland regions of the Eastern and Western Cape. The longer growing periods may be attributed to the reduced temperatures in the higher altitude regions and the cooler southern parts of South Africa. These distribution patterns are confirmed in Figure 7.11, which show the distribution patterns of dryland sugarcane harvest cycle lengths, determined by Schulze (2007f), based on algorithms developed by Smith (1998).
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OPTIMAL HARVEST SEASON
LENGTHS Unsuitable
12 Month 15 Month 18 Month 21 Month 24 Month
Season Length
Model :
ACRU - Thompson Soil : SaClLm Depth : 0.9 m Constraints:
HU (cycle) > 3300 < 4500 Tmin (Jul) < 5ºC Min An Yield >30 t/ha
Figure 7.10 Optimal season lengths for sugarcane growth over southern Africa
Figure 7.11 Length of the harvest cycle (months) over southern Africa of dryland sugarcane based on the Smith (2006) model (Schulze, 2007f)
From the distribution patterns of sugarcane yields with varying season lengths shown in Figures 7.1 to 7.9, and summarised in Figure 7.10, it was found that areas viable for sugarcane production with short growing seasons are limited to the hotter coastal and northeastern parts of KwaZulu-Natal, as well as the eastern parts of Swaziland, Mpumalanga and Limpopo. However, with an increase in season length, a shift in the areas shown to be viable for sugarcane production to inland locations at higher altitudes, as well as more southern parts of South Africa, are noted.