Introduction

Sorghum [Sorghum bicolor (L.) Moench], based on the use can be broadly classified as grain, sweet (sorgos) and fodder (high biomass). Three distinctly different types of sorghum can be used as ethanol feedstocks and they supply three different ethanol conversion systems-grains produce starch, sweet sorghum produce simple sugars, and high biomass energy sorghum for lignocellulosic conversion to ethanol (Rebecca 2009). Recently, global emphasis is focused on the development of these renewable energy sources so as to replace conventional fossil fuels due to depletion of petroleum reserves and increase in demand of petroleum products. Sweet sorghum is most important and excellent energy source for liquid fuels such as ethanol, which is used as biofuel. The improved varieties of this crop can be used as dedicated bioenergy crop, where both the sugars, and the grain, and the bagasse are used for ethanol production (Vermerris et al. 2011), also animal feed or fertilizer after composting with agro-wastes (Rao et al. 2011). Sorghum yields a better energy output/input ratio compared to other feedstocks such as sugar cane, sugar beet, maize and wheat (Almodares and Hadi 2009; Hallam et al. 2001). In order to meet this, sorghum cultivars are developed with potential to use for ethanol production along with increase in yields. Being an annual and a C4 plant with efficiency in photosynthesis, short duration dry land adaptation with food, fodder usage with high fermentable sugar content and ability to come up well in marginal soils with low input growth makes it an ideal feedstock (Almodares et al. 1994; Reddy et al. 2005; Rao et al. 2009). Thus commercial cultivation of sweet sorghum will play an important role in promoting the development of agricultural production, livestock husbandry (Fazaeli et al. 2006), and livelihood opportunities (Rao et al. 2012). In the United States, sorghum is the second most commonly used grain in ethanol production, most ethanol is produced from maize grain starch, which is enzymatically converted to glucose and then fermented. The same process is used for grain sorghum (Renewable Fuels Association 2007). Ethanol produced from sucrose extracted from sugarcane is simpler, as it eliminates the need for enzymatic degradation of starch and requires less processing as is done in Brazil. Sweet sorghum juice could certainly be used in a similar system as sugarcane provided inhibiting substances such as starch, aconitic acid must be addressed (Rao et al. 2012; Mask and Morris 1991). The presence of reducing sugars in sweet sorghum prevents crystallization and sweet sorghum cultivars have 90 % fermentation efficiency (Ratanavathi et al. 2004).

Sweet sorghum has a rapid growth rate and matures in 90–120 days (Prasad et al. 2006) and can produce a ratoon crop in subtropical environments. Ratoon cropping i.e., harvesting of the crop twice or more times from a single planting during the growing season. Ratooning does not involve sowing since it utilizes the regeneration stems, which involve the harvesting of the crop twice or more number of times from a single planting during the growing season (Duncan and Gardner 1984). This facilitates the supply of raw material i.e., sweet stalks to fulfill the need of biofuel distillers and be commercially viable. Different varieties of sorghum have different ratooning potential. This should be considered in the evaluation of promising varieties especially in areas where ratooning is practiced (Pamploma 1986). Certain sorghum varieties have been reported to ratoon well, and the ratoon crop has in some cases been reported to yield more than the planted one (KARI 1992, 2005). Ratoonability is dependent upon genotype and genotype × environment interaction (Rooney et al. 2007) which can be adopted in a given environment for sweet sorghum especially in tropical conditions due to multiple seasons of sorghum cultivation to extend the raw material supply to the distillery there by reducing the cost of feedstock production and facilitating relay cropping to maximize the returns on unit land and labour (Rao et al. 2009, 2012).

The present study was conducted to assess ratoonability of different sweet sorghum cultivars and the inter-relation between various candidate traits effecting biomass yield in main and ratoon crops.

Materials and Methods

A sweet sorghum ratoon trial was planted at International Crop Research Institute for Semi-Arid Tropics (ICRISAT), Patancheru research farm (latitude: 17.53°N; longitude: 78.27°E; altitude: 545 m) with ten varieties and six experimental hybrids along with four standard checks viz., SSG 59-3 (forage line); ICSV 93046 and RSSV 9 and CSH 22 SS in rainy season, 2011. The experiment was conducted in a randomized complete block design (RCBD) in three replications. The planting was done on ridges with a plant stand of about 80,000 plants ha−1. Each cultivar was planted in 4 rows of 4 m length in 12 m2 plots with a spacing of 75 × 15 cm in second week of June. A fertilizer dosage of 80N, 40P ha−1 were applied with 50 % N as basal (DAP) and the balance 50 % as urea after 35 days after emergence as top dressing for the main crop and for ratoon crop 50 % N and total P (DAP) was applied during intercultivation (about 10 days after harvest of main crop) and remaining 50 % N (urea) as top dressing 25 days after first dosage application. Hand weeding was done twice followed by hoeing and inter-cultivation for both the crops. Standard agronomic practices and crop protection measures were followed throughout the crop growth. The main crop (June–October, 2011) was entirely grown as rain-fed crop while minimal irrigation was applied (November–February, 2011) for the ratoon crop. Data was recorded at flowering stage for the traits viz., days to 50 % flowering (DF), plant height (PH) (m), number of tillers (NT), number of leaves (NL) at flowering, leaf weight (LW) (g), stalk weight (SW) (g), stripped stalk weight (SSW) (g), and Brix (%). The ratooning efficiency (RE) and tillering capacity (TC) (Duncan and Gardner 1984) was estimated. For each cultivar data was recorded in the middle two rows for the first half 2 m, and the stalks were cut 2 inch above the ground level for the plant sap and at the ground level for the ratoon. Juice extraction was done after removal of panicles along with peduncles and leaves were completely stripped by hand from each plant. The stripped stalks were squeezed using a three-roller cane press mill. Brix (%) was estimated using a hand held pocket refractometer (Atago, Japan) based on the extracted juice samples taken from each plot. Growing degree days (GDD) were calculated from daily maximum (Tmax) and minimum (Tmin) air temperature measured at 2 m height for 24 h, mid-night to mid-night, as follows:

$$ {\text{GDD}} = \left[ {{\text{Average}}\,\left( {{\text{T}}_{\hbox{max} } + {\text{T}}_{\hbox{min} } } \right)} \right] - {\text{T}}_{\text{b}} $$

where Tb, base temperature, is 10 °C.

Photothermal units (PTU) (Nuttonson 1948) are the product of degree-days and daylight hours. Daylight is defined as the period between sunrise and sunset. In this experiment we considered bright sunshine hours which are the active period for photosynthetic activity.

General linear model (GLM) was used for analysis of variance and to calculate significant differences among improved varieties (SAS Institute, Inc. 2004).

Results and Discussion

The analysis of variance (ANOVA) reveals that there is a significant difference among the twenty sweet sorghum genotypes for traits like days to 50 % flowering, tillering capacity, plant height, stalk yield, no. of leaves, leaf weight and Brix (%) (Table 1). In the ratoon crop there was no significant difference in days to 50 % flowering, probably due to the observational bias in recording flowering data due to profuse tillering. All the genotypes exhibited very high level of uniformity as indicated by low CV % except for no. of tillers in both main and ratoon crops.

Table 1 ANOVA of sweet sorghum varieties and hybrids for biomass related traits in the trials conducted at ICRISAT-Patancheru 2011
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A total of three genotypes (Table 2) viz., sokoykaba (152.9 t ha−1), D 10-A (143.3 t ha−1) and (DSV 4 × SSV 84)-1-2-1-5-1-1-2 (141.7 t ha−1) produced higher stalk yields than the best performing check CSH 22 SS (140.1 t ha−1) in the main crop. In the ratoon crop, six genotypes were high yielding than the check RSSV 9 (36.6 t ha−1). The highest stalk yield in ratoon was recorded by safed moti (56.3 t ha−1) followed by ICSA 749 × SPV 1411 (46.0 t ha−1) and ICSSH 28 (45.6 t ha−1). Stalk yields ranged from 5 to 53 % in ratoon when compared with the main crop. Similar results in decrease in stalks yield was observed in sweet sorghum variety ICSV 93046 under tropical conditions (Rao et al. 2012). Ratoon efficiency (RE) was highest in safed moti (35) and ICSA 502 × ICSV 93046 (34). The best performers of main crop, sokoykaba and D 10-A showed poor ratooning efficiency while, the genotypes ICSSH 28, ICSSH 58, ICSA 749 × SPV 1411, B-24 and ICSV 93046 showed higher stalk yields in ratoon crop due to their high RC. It is noteworthy that the higher ratoon efficiency was recorded for hybrids. In all the tested genotypes the RE ranged from 5 to 35 %. The ratoon crop flowered (61 days) 19 days earlier than the main crop (80 days). There was a sharp reduction in average plant height in ratoon crop (2.3 m) compared to main crop (3.6 m). The plant height was highest in D10-A (4.7 m), sokoykaba (4.5 m) and S2-9 (4.4 m) in the main crop and in the ratoon ICSSH 28, ICSA 749 × SPV 1411, B-24 and ICSV 93046 (2 m) were best. The ratoon crop height ranged from 32 to 82 % of the main crop’s height. There was an increase of 21–341 % in the no. of tillers in the genotypes tested in ratoon vis a vis main crop. However, no. of leaves, leaf weight and stalk weight decreased from main to ratoon crop, by about 25, 58 and 75 %, respectively. The decrease of biomass may be attributed to a significant decrease in plant height on ratooning which is vindicated by a significant correlation between plant height and stalk weight (r = 0.78**).

Table 2 Mean performance of sweet sorghum genotypes for agronomic parameters during rainy and post-rainy seasons, 2011 at ICRISAT, Patancheru
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The number of PTU accumulated by ratoon crop were more for heading than for the main crop indicating greater degree of photosensitivity in the genotypes tested. The genotypes S2-9, D10-A, sokoykaba and ICSR 93034 required relatively similar PTU in both the crops probably due to their photoinsensitivity (Fig. 1). The number of GDD (Table 1) required for the genotypes for heading varied significantly in all the genotypes and wide variation was noted in the GDD in all the genotypes in both main and ratoon crops. In the main crop GDD varied between 787 and 1,572 and in the ratoon crop it ranged between 820 and 1,053 (Fig. 2) (Rao et al. 2012).

Fig. 1
figure 1

Photothermal units (PTU) accumulated by main and ratoon sweet sorghum varieties and hybrids

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Fig. 2
figure 2

Growing degree days (GDD) required for main and ratoon sweet sorghum varieties and hybrids

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Correlations of biomass yield with its related parameters in main crop and ratoon are shown in the Tables 3 and 4. The degree of correlation differed for different traits under study in both main and ratoon crop, but the direction of correlation was same for biomass related traits like DF, GDD, NT, PH, SW, NL, LW and SSW. Selection for different biomass related traits with positive correlation will aid for biomass improved yield in the breeding program. SW exhibited a significant and positive correlation with traits like GDD (r = 0.49*), NT (r = 0.58**) and PH (r = 0.66**) in the main crop and similarly the relation of SW had significant and positive correlation with GDD (0.47*), NT (r = 0.46*) and PH (r = 0.78**) in ratoon crop as well. The PH had a significant positive correlation with DF (r = 0.85**) and GDD (r = 0.86**) in the main crop whereas the correlation among them was non-significant in ratoon crop. SSW showed a significant positive correlation with GDD (r = 0.48*), PH (r = 0.67**), and LW (r = 0.71**). Similar trend was noted in ratoon crop.

Table 3 Correlation between different biomass related traits in sweet sorghum main crop during rainy season, 2011 at ICRISAT, Patancheru
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Table 4 Correlation between different biomass related traits in sweet sorghum ratoon crop during postrainy season 2011 at ICRISAT, Patancheru
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Conclusion

Sweet sorghum genotypes differed significantly in their ability to produce biomass in main and ratoon crops. So in a cropping system where ratoon crop is chosen a variety with good ratoonability is very useful. High stalk yielding genotypes do not always necessarily produce good yields in a ratoon crop. Therefore, it is always advisable to test various genotypes for ratoonability. In a ratoon crop biomass influencing traits like plant height, number of leaves, leaf weight and stalk yield decrease as compared to main crop. Though, the number of tillers increase significantly they don’t seem to be contributing to biomass, so selection for plant type with biomass in tillers would be effective to develop sweet sorghum varieties suitable for ratooning. To be able to breed sweet sorghum varieties for ratooning one of the important criterions should be to select lines with photoinsensitivity, especially for SAT areas where there is greater variation in the GDD during different crop growing seasons. Sweet sorghum varieties selected with narrow range of GDD in main and ratoon crops expected to possess high ratoonable capacity as observed in ICSSH28, ICSSH 58, ICSA 749 × SPV 1411, B-24 and ICSV 93046. Higher ratoonability could be useful selection criteria for breeding sweet sorghum materials when aiming for continuous supply of feedstock for enhancing period of distillery operation.