Annual global production of sunflower oil is about 18 million tons. As it utilises soil nutrients efficiently due to its deep roots, the crop is usually grown in dryland systems. South Africa’s sunflower seed production was 862 000t in 2018, according to the Crop Estimates Committee.
Sunflower is produced mostly for its oil. Oil concentration, expressed as a percentage of seed dry mass, generally determines the industrial yield of the crop.
In some countries, each percentage point that oil content increases between 40% and 45% earns a 2% price premium; in contrast, buyers demand a 2% price discount for each percentage point decrease in oil content between 40% and 38%, and a larger penalty as oil percentage decreases further. Both seed yield and oil percentage are thus important for maximising gross income.
When sunflowers have access to optimal water and nutrient requirements, temperature and exposure to sunlight play key roles in determining yield components and grain quality.
Grain oil concentration is determined mainly by the amount of photosynthetically active radiation (the solar radiation that plants can use in photosynthesis) intercepted per plant during the grain-filling period.
The optimal planting date is crucial, as it ensures good seed germination as well as the timely appearance of seedlings and good root development.The effects of environmental factors on oil content vary according to the crop growth stages, as oil yield components
and grain quality are defined in different phenological stages.
Seedling emergence is a sensitive stage; abiotic stresses, such as high soil temperature (above 40°C), can cause delayed emergence, resulting in poor yield.
A recent South African study showed that sunflower seedlings exposed to higher than optimal soil temperature during establishment impaired seedling development (emergence and root growth).
It was also evident that sunflower seedlings can survive and develop at 40°C, but seedling emergence and development are inhibited at temperatures in excess of this.
Internationally, it has been found that the number of seeds per unit area (the main yield component in grains and oilseed crops) is closely related to the crop growth rate during the critical period for grain set.
It was concluded that the critical period for the seed number determination was between 24 and 30 days after anthesis (flowering), while grain weight and grain oil concentration were determined during grain filling.
This is the only stage during which the determinations of the oil yield components partially overlap. Environmental conditions during this period therefore have a highly significant effect on yield.
In order to investigate the influence of planting date on oil content as well as oil and seed yield of sunflower, 14 field trials from the sunflower national cultivar evaluation trials were evaluated at various localities with different planting dates in North West and Free State from the 2016 to 2018 growing seasons.
Obtaining varieties witicentrath a higher oil concentration appears to be an alternative for enhancing sunflower production.
Although sunflower oil concentration has been reported to be a conservative genetic component, some studies have highlighted differential responses of sunflower genotypes in different cropping conditions; greater variability of oil concentration was either linked to management and environmental conditions or to genotypic and environmental interactions.
In the 2016/2017 growing season, the moisture-free oil content for cultivars at the seven localities planted varied from 39,4% to 44,4%, with an overall mean of 42,1%. The highest oil concentration among cultivars and calculated across localities was the cultivar P 65 LL02 at 44,4% followed by SV 60064 at 43,8%.
During the 2017/2018 season, the moisture-free oil content for cultivars at various localities ranged from 36,84% to 47,34%, with an overall mean of 39,54%. The highest oil concentration among cultivars and calculated across localities was SY 3970 CL at 47,34%, followed by P 65 LL 02 at 42,94%.
Oil yield per unit area is the product of grain yield and seed oil content (t/ha), and was calculated by multiplying oil percentage with seed yield. The oil yield for cultivars at the seven localities during 2016/2017 varied from 0,87t/ ha to 1,23t/ ha, with an overall mean of 1,06t/ ha.
During the 2017/2018 season, the oil content varied from 0,85t/ ha to 1,06t/ha, with an overall mean of 0,95t/ha. PAN 7100 and P 65 LL02 (both conventional hybrids) produced the highest oil yield for 2016/2017 and 2017/2018 respectively.
Different planting dates may cause flowering and seed development to occur during periods of widely variable temperatures, radiation, day length and water availability.
These variable factors can lead to modification in oil content of the seeds. During 2016/2017, Senekal (planted 15 December 2016) and Potchefstroom (planted 8 December 2016) obtained the highest oil content of 45,3% and 44,5% respectively.
During the 2017/2018 season, Potchefstroom (planting date: 20 October 2017) had the highest oil content of 45,1%, followed by Potchefstroom (planted 4 December 2017) with 44,2%. Average oil content was 42,11% and 39,54% during the 2016/2017 and 2017/2018 growing seasons respectively and confirms that oil content is highly affected by genotypes and environment.
The results also showed that the late planting reduced the oil content of sunflower in both seasons. Late planting (last week of January) reduced oil content by 5% and 10% during 2016/2017 and 2017/2018 respectively.
Oil yield per unit area is the ultimate target in growing high-oil sunflower genotypes.
Components of oil yield (number of plants per unit area, grain number, weight per grain, grain oil concentration) and quality (fatty acid composition) are genetically determined, but they may be affected to a different extent by environmental factors, which could vary during the crop cycle.
Oil yield was affected by a delay in planting dates for both seasons. In 2016/2017, the highest oil yield was achieved at Potchefstroom (planted in the first week of December), whereas the lowest (a reduction of more than half) was also at Potchefstroom (planted 19 January).
In 2017/2018, the highest oil yield was recorded at Boskop (planted in the second week of December) and the lowest in Senekal (planted in the last week of January). Comparing the oil yield of different planting dates at one locality (Potchefstroom), the late planting date (end of January) resulted in a 62% reduction in oil yield over both seasons.
In 2016/2017, the average seed yield of 18 sunflower hybrids at seven localities was 2,5t/ ha. The highest locality mean yield (3,12t/ha) was recorded at Boskop (planted on 13 January 2017) and the lowest (1,38t/ha) at Potchefstroom (planted 19 January 2017).
The average seed yield of 19 sunflower hybrids at seven localities during 2017/2018 was 2,31t/ha. The highest locality mean yield (3,39t/ha) was recorded at Boskop (planted 12 December 2017) and the lowest (1,32 t/ha) at Senekal (planted 28 January 2018).
Similar to oil yield, late planting (last week of January) during the 2017/2018 season produced lower yields than the optimal planting date at all localities. Significant differences in seed yield were noticed among the sunflower hybrids over both seasons.
Comparing yield results at Potchefstroom over two seasons and different planting dates showed that the highest yields were achieved in the first week of December and the lowest yields in the last week of January. Yield reduction was about 50% between the optimal and late planting date over both growing seasons.
The study showed that factors such as location, planting date and hybrid may significantly modify oil content and yield of sunflower grown in North West and Free State, suggesting that these could be used as management tools to improve sunflower seed yield and increase oil content.
Significant differences were observed in oil content, oil yield and seed yield among the sunflower hybrids. Late planting reduced seed and oil yield by almost 50%, and oil content was also significantly lower.
Email Dr Safiah Ma’Ali at [email protected].