November 2013
70
Focus on water and energy
The effect of nitrogen application
levels on sweet sorghum yield
WIKUS SNIJMAN, ARC-GRAIN CROP INSTITUTE
Since 2009, the ARC-Grain Crops Institute (ARC-GCI) has been involved
with a European Union project named “Sweet Sorghum – an alternative
energy crop”.
As part of this project, the ARC-GCI endeavoured research to determine
the nitrogen application rates needed for optimum sweet sorghum
production. Nitrogen has an effect on the sugar content of the syrup and
the sugars are the components which will be fermented into bio-ethanol.
The trials
Up to this point in time, no such research had been done in South Africa
because the initial concept was that the amount of syrup is important
and not the sugar content of the syrup. However, a number of trials were
executed to investigate genotypes regarding drought and temperature
tolerance.
The nitrogen application trial started two years ago and some very
interesting facts came to light. Results in the first year proved to be
different from the common perception that the highest nitrogen levels
will produce the best results.
The 2012/2013 year appeared more true to the norm. The 2013/2014 year
will be the last year during which this interaction will be investigated. It is
foreseen that once the trials have been finalised, results obtained will aid
the industry by supplying answers into what application rates should be
used for optimum sweet sorghum production to serve as an alternative
bio-ethanol source.
For each of the two seasons, three cultivars were selected (2011/2012
–S27, S120, PX174; 2012/2013 S868, S63, S120) –
Photo 1
. The trials
were conducted at the ARC’s experimental farm at Vaalharts under flood-
irrigation practises.
The fertiliser used was ammonium sulphate which was applied at five
nitrogen levels: 0 kg/ha (as control), 30 kg/ha, 60 kg/ha, 90 kg/ha and
120 kg/ha. Plant density was 5 kg of seed per hectare. Biomass, plant
height, stalk diameter, syrup production and Brix percentage (sugar
content of syrup) were measured and documented.
Results from the first year were inconsistent, indicating that the best
yields were produced from a 60 kg/ha nitrogen application rate. Some
genotypes even produced better at 30 kg/ha. During the 2011/2012
season, the S120 genotype did the best regarding biomass production at
an application rate of 60 kg/ha.
Biomass is an important characteristic because a high biomass yield
yields more syrup. Genotype S27 contained the highest sugar levels (Brix
percentage) in the syrup and S120 yielded the most syrup.
The second year’s results were more consistent and indicated that the
highest application rate of 120 kg/ha produced the best results regarding
biomass yield, sugar content and syrup yield. During the 2012/2013
season, S868 produced the tallest stalks with an average length of
2,75 m at a 120 kg/ha nitrogen application level. At an application rate of
Photo 1: Genotype differences.
Photo 2: Plant height differences among genotypes.
Nitrogen application levels affect
sweet sorghum yields. Optimum bio-ethanol
production depends on syrup production
for first generation ethanol production and
biomass yield for second generation
ethanol production.
