Following unexpected challenges at the Kilombero Sugar Company mill in South Eastern Tanzania during the 2013/2014 season, the average miller-cum-planter crop age was upped to 13.7 months in the 2014/15 season.

The increased cane age revealed a critical link between low night temperatures and sucrose levels in the harvested cane, sugar milling expert Stanley Munsamy told delegates at the recent South African Sugar Technologists’ Association conference in Durban.

A unique characteristic of the cane supply at the mill, which is majority owned by the South African headquartered Illovo sugar group, is that it has the highest long-term mean night temperatures in any cane growing area in southern Africa at 20.5° C.

In South Africa by comparison, the average night temperatures in the sugar growing areas surrounding Pongola are 15.3°C, Gledhow 15.9°C, Noodsberg 11.3°C and Komati 13.9°C.

The best cane growing area for the Illovo Group, according to Munsamy’s research, was Nakambala in Zambia where the long term mean night temperature was at 15.9°C.

“As a South African technologist who was quite used to aging cane to 24 months in some areas, I was expecting increased cane yields and sucrose content from the aged cane.  But when it came to harvesting we were very disappointed in the quality,” he said.

Munsamy, who has worked in the industry for 40 years, then launched a study to investigate the ideal age at which sugarcane should be harvested, how to optimise percentage sucrose and how the high minimum temperatures in the Kilombero cane supply region affected the sucrose content.

DATA FROM SEVEN CROP SEASONS WAS ANALYSED

“Climatic data, which included average minimum temperatures for each crop season from 2009 to 2016 – or seven crop seasons – was analysed. A weighted average for tons cane per hectare, tons sucrose per hectare and tons sucrose per hectare per year was collected for each crop season. The average cane age at harvest for each of the crop seasons was also calculated,” Munsamy said.

Further, to get to a result that showed the ideal age at which cane should be harvested for the optimal sucrose content, linear regressions were used to measure tons cane per hectare and the tons sucrose per hectare.

Linear regressions were also carried out on tons cane per hectare and harvest age and the tons sucrose per hectare and harvest age. Polynomial regressions on the relationship between tons of cane per hectare and the average minimum temperature were done as well as the tons sucrose per hectare and the average minimum temperature.

The climate at Kilombero is characterised by an average rainfall of 1 387mm a year, a mean maximum temperature of 31.6°C, a mean minimum temperature of 20.5°C and 6.26 hours of sunshine a day. Minimum temperatures did not differ from the long term mean minimum temperature by more than 1.5% between 2009 and 2016 with an exception during the 2014/15 season when a mean minimum temperature of 22.4°C was recorded.

RESULTS SHOW LINK BETWEEN SUCROSE CONTENT AND NIGHT TIME TEMPERATURES

The results of the study showed a strong inverse linear relationship between tons cane per hectare and sucrose % cane (R² = 0.87). The data also revealed a strong inverse linear relationship between average cane age and sucrose % cane (R² = 0.87).

“The relationship between average cane age and the tons cane per hectare was linear until about 12.2 months then it dropped,” said Munsamy. “The relationship between average cane age and the tons sucrose per hectare was also linear until 12 months of age before it declined rapidly.”

Both the photosynthesis and respiration equations were used to show the effect of warm night temperatures on respiration losses.

“Night time temperatures drive dark respiration when no photosynthate is being produced and losses through respiration are directly related to temperature. Previous researchers (J Glover: 1973, H Rostron: 1972) established that respiration losses are a function of cane mass and night temperature. They also established that the respiration rate of sugarcane depends on both biomass attained and ambient temperature, with larger crops having a larger maintenance respiration component,” he said.

And as biomass increased with crop age, the proportion of respiring to photosynthesising tissue increased causing a gradual decline in net productivity.

“It must be noted that it is not the age per se but the increasing cane mass that contributes to increasing respiration losses,” Munsamy said.

HIGHEST NIGHT TEMPERATURES PRODUCED LOWEST SUCROSE

The Kilombero 2014/15 season, Munsamy said, produced the highest tons of cane per hectare and experienced the highest night temperatures, but produced the lowest percentage sucrose content.

“The data showed that the ideal harvest age of the miller-cum-planter sugarcane at Kilombero is around 11.8 months if optimum levels of sucrose are to be achieved.

This is in line with the findings of researchers Lonsdale and Gosnell (1975) in the Zimbabwe Lowveld, where they discovered it was not advisable to carry over cane and that 10 to 12 months was the optimum age of harvest,” he said.

And while there was an option to limit the growth of the crop by decreasing irrigation and nitrogen fertilizer, Munsamy said the concept of decreasing cane yield would be difficult for agricultural staff to assimilate but the long term benefits of reducing harvesting and transportation costs and maximising sucrose content could be worthwhile.

Munsamy said the study had shown while the benefits of ageing cane under cool night temperatures were well established, he cautioned against such a practice in warm night time temperatures, such as those measured at Kilombero. 

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