Investigating the Possibility of Autumn-Sown and Determining the Most Suitable Planting Date and the Best Bolt-Resistant Cultivar of Sugar Beet in Khorasan Region

Sugar beet (Beta vulgaris L.) is the second most important sugar crop after sugarcane, which annually produces about 40% of total sugar production worldwide and is adapted to different climatic conditions (El-Hag et al., 2015). Due to global warming, autumn cultivation of sugar beet is predicted to become more priority in the future, but autumn cultivation is in danger of bolting and flowering in many areas. Excessive bolting reduces sugar content, root yield, and purity of raw syrup. In general, both early sowing and delayed sowing reduce root yield, sugar, and leaf area index and increase the percentage of impurities. Therefore, this experiment was designed and implemented with the aim of feasibility study of autumn cultivation of sugar beet and determination of the best planting date in North, Razavi, and South Khorasan provinces for three new varieties resistant to sugar beet.

The experiment was conducted as a split-plot design based on a randomized complete block design with three replications in the provinces of North Khorasan (Shirvan), Khorasan Razavi (Mashhad), and South Khorasan  (Khezri Dasht-e Bayaz) in 2019-2020. The main plots included three planting dates (2, 7, and 12 October) and the subplots included three bolt-resistant sugar beet cultivars (Giada, Merak, and Sharif). Each plot consisted of 6 rows with a length of 5 m, at a distance of 50 cm and a distance between two plants of 20 cm, and planting was done manually. To determine root yield from the middle rows of each plot by eliminating the margin, harvest was done at an area of 4 m2. A sample of root paste of each treatment was sent to the Beta Lizer laboratory of Mashhad Agricultural Research and Agricultural Services Company to determine the percentage of sugar. Other quality parameters were measured by Beta Lizer (Braunschweig method). Using the polarimetry method (Sucromat), the percentage of sugar content and white sugar yield, and other quality parameters were calculated for all experimental plots. Combined analysis of variance for different locations and mean comparison based on least significant difference (LSD) at the level of 5% probability using SAS 9.4 software was performed. Also, the graph plots were performed using Excel software.

The results of the analysis of variance showed that the interaction effects of the location and cultivar were significant on bolting percentage, root yield, sugar content, Na content, yield coefficient, and white sugar yield. The first planting date (October 2) in Mashhad region for all three cultivars led to the highest percentage of bolting (78-90%). Delay in planting date from 2 October to 12 October, the bolting percentage of cultivars was reduced, significantly. The bolting percentage in Shirvan region was less than 8%. On the third planting date (October 12) in all regions, cultivars showed also a bolting percentage of less than 10%. Giada cultivar in Mashhad region with 47.3 ton.ha-1 had the highest and Sharif cultivar in Shirvan region with 22.6 ton.ha-1 had the lowest root yield. Shirvan region had less root yield than the other two regions. The highest sugar content (18.78%) belonged to Giada cultivar in Shirvan region and the lowest sugar content (13.01%) was observed in Sharif cultivar in Mashhad region. The planting date of 12 October was significantly lower in impurities, alkalinity coefficient, and molasses compared to earlier planting dates. The first planting date had the lowest (62.3%) and the third planting date had the highest (74.2%) extraction coefficient. Giada cultivar in Shirvan region had the highest extraction coefficient (78.1%) and the lowest extraction coefficient (60.8%) belonged to Sharif cultivar in Mashhad region.

In Shirvan and Khezri regions, Giada cultivar but in Mashhad region, Merak cultivars had the highest white sugar yield. In general, the results showed that in Shirvan region, planting on 2 October and in Mashhad and Khezri regions planting on 7 October could lead to reaching maximum white sugar yield.