Stalk-Juice Quality Traits of Sweet Sorghum as Affected by Season in Sudan

An experiment was conducted in Shambat (2012-2013) during the summer and winter seasons to study the stalk-juice quality parameters of 29 exotic and local sweet sorghum varieties as affected by season. The objectives were to provide basic data attributing to syrup and sugar (jaggery) production in the Sudan. The materials were arranged in RCB design. Data were collected on brix, pol (sucrose %), reducing sugars, total sugars, purity and pH. The single and combined analysis of variance revealed highly significant differences among varieties for all traits studied. The variety x season interaction was significant for all traits other than total sugars %, implying the need for considering the variety choice in each season. The results of juice quality parameters were mostly within the range reported in the literature. The exotic materials excelled the local ones in brix, pol%, total sugars% and purity. The exotic varieties: N100, Red-X, Hastings and N110 as well as the local Ankolib selections: AnkNyala and Ank36 were identified as having juice quality traits suitable for syrup production. The exotic varieties: BlueRibbon, KansasCollies and Brawley were identified for sugar (jaggery) production. It was concluded that interaction of variety x season and yield parameters for juice, grain and stover should be considered when determining the choice of the variety. Future research should focus on developing multi-purpose varieties capable for concurrent production of syrup/sugar, grain and stover.


Introduction
Sorghum (Sorghum bicolor (L.) Moench) is a crop of world-wide importance. It ranks fifth in importance as a grain cereal. It is the most qualified crop in meeting the demand for large quantities of high-quality fodders, especially in the drier parts of the world [1] with unique ability to produce under a wide array of harsh environmental conditions [2]. Sweet sorghum is a type of sorghum characterized by ability to accumulate sweet juice in the stalk. It was first introduced to USA more than a century ago with the primary aim of manufacturing crystalized sugar, but due to some technical difficulties the emphasis switched to syrup production before being further shifted to forage production. Recently, sweet sorghum also appeared to have great potential as an annual energy crop.
The agro-industrial potentials of sweet sorghums have been discussed by many workers. The crop offers great potential for multi-purpose uses mainly for syrup, forage and ethanol production [3]. Plant breeders in the United States, India and China have developed high-brix cultivars to increase the potential of this crop for syrup production. Excellent syrup can be made from sweet sorghum when brix of raw juice is greater than 15% [4]. Syrup yields ranging 800-1900 L/ha have been reported depending on weather conditions [3]. Sweet sorghum provides options for concurrent food, feed and sugar production and is suitable for smallholder farmer cultivation on less-favored lands. Although sugar content declines by about 25% at grain maturity, this is, however, compensated for by the added gain from grain-income [5].
In Sudan, sorghum is the most important crop grown for food and feed. Sudan is believed to be one of the major centers of sorghum domestication and diversification [6,7]. 'Ankolib' is the vernacular name for sweet sorghum. It is a highly diverse population belongs to race durra-bicolor characterized by sweet stalk just like sugar cane [8]. Developing improved grain or forage types have been the main focus of research efforts in Sudan. Attempts to investigate traits attributing to syrup production from sweet sorghum are specifically lacking. Mohammed and Mohamed [9] evaluated exotic and locally developed genotypes for forage attributes. Concurrent improvement of stem-sugar, stover and grain yield was also studied in Sudanese and exotic sorghums [10,11].
The present study aimed to evaluate stalk-juice quality parameters of sweet sorghum as affected by season in Sudan. The ultimate objectives were to provide some basic data that may contribute to syrup and/or sugar (jaggery) production in Sudan.

The experimental site
The study was conducted in the Experimental Farm of the Agriculture Research Corporation (ARC) at Shambat (latitude 15˚ 39' N; longitude 32˚31'E; altitude 380m above sea level) during the summer of 2012 and winter of 2012/2013. The soil is heavy clay, non-saline, non-sodic with pH of 7.9-8.5.

The growing seasons
The major features of the climatic conditions prevailed during the growing seasons are depicted in Fig. 1. The winter season is drier than the summer with no precipitations.

The plant materials
The plant materials consisted of 29 sorghum genotypes representing 4 groups ( Table 1). The genotypes 1-15 are exotic sweet sorghum materials. The genotypes 16-29 were locally developed by selection within local populations of Ankolib, Sudan grass and Abu Sabin. Selection was practiced for juiciness (green mid-rib) and other forage attributes.

Management and experimental design
Sowing dates were on 22/July/2012 and 6/Dec/2012 for the summer and winter sowings, respectively. The materials were arranged in RCB design with 3 replicates. The plot was composed of two ridges 5 m long. Planting was done by placing 5 seeds in holes spaced 0.7m x 0.2m. The trials were watered at 7 to 10 days interval. Nitrogen fertilizer (urea) was added at the second irrigation at a rate of 55 kg n /ha. The insecticide Sevin (Carbaryl 85 WP) was sprayed against stem borers one month after sowing. Weed population was kept to minimum by hand removal.

Juice collection and preparation
The Juice was collected from the first press of the cane produced in the whole plot when each variety reached physiological maturity of the grain. The plants were cut 5-7 cm above ground level. The panicles were removed and leaves (blade + sheath) were stripped off. The fresh cane was immediately taken to the lab where it was washed, left to dry for a while and then passed through hand driven two-roller mill. The juice received was filtered using muslin cloth. The juice was kept in tightly closed plastic bottles, labeled and stored under deepfreeze condition for future investigations.

Pol %:
The sucrose percentage of the juice (Pol) was estimated following Horne's dry lead acetate clarification method [12] using automatic polarimeter (AP-300).
Total sugars %: Calculated as the combined percentages of sucrose and reducing sugars in a sample.
Purity %: Measured as apparent purity using the formula: (pol x 100)/Brix pH: Recorded from 200 ml sample of juice using pH meter.

Statistical Analysis
The data collected were subjected to single analysis of variance (ANOVA) before performing the combined ANOVA following the standard procedure of analyzing RCB design ( [14]. Least Significant Difference (LSD) procedure was used to separate the means. The statistical package GenStat [15] was used to run the analysis. Tables 2 and 3 show mean squares for studied traits resulting from single and combined ANOVA, respectively. Differences among genotypes were mostly highly significant for all traits in single and combined analysis. Apart from total sugars, genotype x season (GxE) interaction was significant, specifically for reducing sugars and pH where it was highly significant.

Brix (%):
The average brix in winter and summer seasons (Table 4) were 16.81% and 16.05%, respectively. The exotic sweet sorghum varieties performed better than the local ones in brix percentage. Three exotic varieties, namely: Brawley, BlueRibbon and KansasCollies gave above 20% brix in both seasons, with overall average of 21.42%, 21.39% and 20.63%, respectively. Of the local materials, the best performing were 3 varieties belong to Ankolib group, namely: Ank36, Ank18 and AnkNyala with respective average of 18.75%, 17.99% and 17.62%. The lowest brix values were obtained by the local varieties S.70 and Ank43, averaging 11.6% and 12.34%, respectively.
Reducing Sugars%: Reducing sugars (Table 6) averaged 6.75% and 5.95% in winter and summer seasons, respectively. Among exotic materials, BlueRibbon, Brawley and KansasCollies displayed the lowest reducing sugars % in both seasons averaging 3.95%, 4.18% and 4.23%, respectively. Above average values were displayed by Hastings (6.70%), Red-X (6.70%) and N110 (6.53%). Most of the local materials, specially Abu Sabin and Sudan grass groups displayed above average values of 6.5%-10.0%. The Ankolib variety Ank18 averaged 5.18% whereas AnkNyala and Ank43 gave an average of 6.85%. Some varieties showed differential performance in reducing sugar, i.e. N100 and SugarDrip displaying 10.0% during winter vs 5.35% and 4.50% during summer, respectively. pH: The pH averaged 5.298 and 5.401 in winter and summer seasons, respectively ( Table 9). The overall average of pH was 5.349. KansasCollies showed the lowest pH (4.947). The highest pH was shown by S.126 (5.742). Apart from SugarDrip (5.378) and AGSilo200 (5.343) all of the exotic materials showed below average pH whereas the opposite is true for the local ones. Generally, most varieties displayed lower pH in winter than summer season.

Discussion
Variability: The highly significant differences among genotypes for juice quality parameters indicated that most of the variability observed were attributable to genetic differences. However, GxE was also significant, pointing to the differential performance of genotypes for all traits other than total sugars. Therefore, although some varieties have similar performance across seasons, the choice of variety should be considered in each season separately for traits other than total sugars. Similar results showing variety x season interaction for juice quality parameters in sweet sorghum were reported [16].

Identification of materials:
As far as juice quality traits are concerned, sweet sorghum varieties showing high reducing sugars relative to sucrose content with brix greater than 15% are more preferred for syrup production [16,4]. Based on the results of this study, the exotic varieties: N100, Red-X, Hastings and N110 as well as the local Ankolib varieties: AnkNyala and Ank36 could have potential for syrup production. On the other hand, varieties suitable for sugar (jaggery) production require high purity (minimum of 75%) with pol of 10%-14% and total sugars of 13% to 17% (i.e. relatively low reducing sugars) [3,18]. In this study, such requirements were more or less met by the exotic varieties: BlueRibbon, KansasCollies and Brawley. However, the latter showed moderate performance in purity (65.2%).
The varieties BlueRibbon and Red-X were reported to have good performance in the yields of juice, stover and grain ( [10]. Since the quality parameter of the juice was evaluated at the physiological maturity of the grain, the two varieties may have the potential for concurrent production of three products i.e. Jaggery or syrup, grain and stover (leaves and bagasse). Such results point to the possibility of combining high stem sugar, grain and stover yields in one cultivar, however, in Sudan, emphasis should be placed on grain for food rather than for feed, a choice which is not offered by the exotic cultivars but largely met by the local materials, specially Abu Sabin group, which unfortunately, of low sugar yield. Therefore, a local x exotic crossing project was started by the Forage Improvement Program (FIP) to transfer the desired traits from exotic to local materials by selection in the filial generation. Very promising materials in F5 generation are now accumulating with attributes matching the project objectives.

Conclusions
The study revealed some exotic and local (Ankolib) sweet sorghum varieties having juice quality parameters suitable for syrup and/or sugar (jaggery) production in the Sudan. Interaction of variety x season and yield parameters for juice, grain and stover should be considered when determining the choice of the variety. Future research should focus on developing multi-purpose varieties capable of concurrent production of syrup/sugar, grain and stover.

Conflict of Interest
The authors declare that there is no conflict of interest.