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ASAE Journal Article

Determining Effects of Sowing Time and Nitrogen Fertilizer Rate on Brix of Sweet Sorghum Using a Gear-Type Extractor

X. Z. Sun, N. Yamana


Published in Transactions of the ASABE Vol. 55(4): 1589-1594 ( Copyright 2012 American Society of Agricultural and Biological Engineers ).

Submitted for review in December 2011 as manuscript number FPE 9545; approved for publication by the Food & Process Engineering Institute Division of ASABE in July 2012.

The authors are Xiao-Zheng Sun, ASABE Member, Visiting Researcher, Faculty of Agriculture, Tottori University, Tottori, Japan; and Lecturer, Engineering College, Northeast Agricultural University, Harbin, China; and Nobuki Yamana, Professor, Faculty of Agriculture, Tottori University, Tottori, Japan. Corresponding author: Xiao-Zheng Sun, Faculty of Agriculture, Tottori University, 4-101 Koyama-cho Minami, Tottori-shi, Tottori 680-8553, Japan; cell phone: +81-90-9951-9120; e-mail: sxz1976@hotmail.com.


Abstract. Sweet sorghum is an attractive energy crop, but the short harvest window and the poor storability of stalks and sugar juices are obstacles in the production of ethanol from sweet sorghum. The sugar content and harvest window of sweet sorghum are very important for ethanol yield and the operating rate of an ethanol production plant. To avoid sugar degradation, the sugar content of sweet sorghum juice needs to be measured immediately after juice extraction from the whole plant. A gear-type extractor suitable for the whole sweet sorghum plant was designed, fabricated, and tested for precise and convenient determination of the sugar content of sweet sorghum. The mean extraction speed of the developed extractor was 9.3 s plant -1 . The effects of sowing time and nitrogen fertilizer rate on the sugar content and harvest window were investigated. The sugar contents of sweet sorghum sown on May 1, June 1, and July 3 applying 70 kg ha -1 nitrogen fertilizer and of sweet sorghum sown on May 29 applying 70, 140, and 210 kg ha -1 nitrogen fertilizer were determined using the developed extractor. Brix exceeded 10% for around 30 and 60 days for sweet sorghum sown in May and July, respectively. The sugar content of the juice from sweet sorghum with 70 kg ha -1 N fertilizer applied was higher than that of sweet sorghum with 210 kg ha -1 N fertilizer applied. The results indicate that the later sowing and the lower nitrogen fertilizer rate could prolong the harvest window of sweet sorghum. The harvest window of sweet sorghum used as a feedstock for ethanol production was prolonged to three months by sowing twice and applying 70 kg ha -1 nitrogen fertilizer in one year in Tottori, Japan (35° 32' N, 134° 10' E).

Keywords. Extractor, Harvest window, Nitrogen fertilizer rate, Sowing time, Sugar content, Sweet sorghum.

Sweet sorghum is an efficient collector of sunlight with the C-4 photosynthate pathway. The crop growth rate of sweet sorghum (14 to 22 g m -2 d -1 ; Nitta et al., 2008) exceeds that of sugar cane (9 to 12 g m -2 d -1 ; Nose et al., 1989). Sweet sorghum can be cultivated in both temperate and tropical countries, requiring only 1/3 of the water needed for sugarcane cropping and half of the water required for corn (Mahapatra et al., 2011). Sweet sorghum yields 25 to 100 ton ha -1 green matter depending on the variety and hybrid, the location (soil, water, climate, pests, and diseases), inputs, and production practices (Eiland et al., 1989; Smith and Buxton, 1993; Turguta et al., 2005; Tew et al., 2008). The theoretical ethanol yields are 7,393 L ha -1 from sweet sorghum juice and bagasse (Kim and Day, 2011), while the ethanol yields are 4,557 L ha -1 from corn (3,977 L ha -1 ; Mascia et al., 2010) and corn stover (580 L ha -1 ; Humbird et al., 2011). Therefore, sweet sorghum is a promising alternative feedstock for ethanol production.

However, the short harvest period for the highest sugar content and the poor storability of stalks and sugar juices have prevented sweet sorghum from replacing corn as a source of ethanol. An ethanol production plant needs to be operated year-round to be economically feasible, but the harvest window of sweet sorghum is only four to six weeks (Mahapatra et al., 2011). As much as 20% of the fermentable sugars were lost in sweet sorghum juices stored for three days at room temperature (Wu et al., 2010). Eiland et al. (1983) reported that chopped sweet sorghum lost 49% of its fermentable sugars after one week of storage at 25°C to 30°C. Losses of over 75% of the soluble sugars (sucrose, glucose, and fructose) were observed in untreated chopped sweet sorghum that had been stored for three months (Eckhoff et al., 1985). Eckhoff et al. (1985) reported that sulfur dioxide adequately preserved chopped sweet sorghum with no significant decrease in total fermentable sugars. However, sulfur dioxide is difficult to handle, and its widespread application is doubtful because of its corrosiveness.

Bellmer and Huhnke (2008) reported that the major factors affecting the ethanol yield of sweet sorghum included biomass yield, sugar content, juice expression efficiency, and fermentation efficiency. In this study, we focused on the effects of sowing time and nitrogen fertilizer rate on the sugar content of sweet sorghum. The harvest window depends on the sugar content of sweet sorghum. Therefore, in order to increase ethanol yield, it is very important to determine the sugar content of sweet sorghum in different growing phases.

The sugar content of sweet sorghum can be measured using a Brix refractometer, which gives the percent of total sugars in sample juice on a weight basis. The traditional methods of determining the sugar content of sweet sorghum is to measure the Brix of some internodes with pinchers and a refractometer. Although this method is very simple, the variation of the determined sugar content can be very large because the sugar contents of sweet sorghum internodes differ significantly. Sun et al. (2011) measured the Brix values of sweet sorghum from top to bottom internodes. The Brix value of the first internode from the top was 7.3%, but the value of the fifth internode was 16.8%. Therefore, the sugar content determined using this method was influenced significantly by the positions of the sweet sorghum internodes. In order to measure the Brix of sweet sorghum accurately, the juice for testing should be extracted from the whole sweet sorghum plant.

fpe9545_files/image1.gif

Figure 1. Precipitation (mm) and air temperature (°C) in 2009 compared to eight-year average at Tottori University farm.

Another method of measuring the sugar content of sweet sorghum is with a juice extractor and a refractometer. The cost of this method is high because extractors are expensive, and despite the high precision of the results, it is not convenient to measure sugar content in the field. In addition, there is currently no commonly accepted standard method for measuring the Brix of sweet sorghum. Therefore, it is necessary to develop a simple system to determine the Brix of sweet sorghum in the field.

Hartley et al. (2009) reported on detailed analysis of the mean sugar composition of two sorghum varieties and stated that more analysis is necessary to determine the sugar composition of a standing crop as it matures. Tamang et al. (2011) stated that little data are available on nitrogen fertilizer requirements for ethanol production from sweet sorghum, and the effects of nitrogen fertilizer on cellulose ethanol and total ethanol are variable. Therefore, more studies must be conducted to determine the optimal nitrogen fertilizer rate for ethanol production from sweet sorghum.

The objectives of this study were: (1) to design, fabricate, and test a gear-type extractor for use to determining the Brix of sweet sorghum samples; (2) to determine the effects of sowing time and nitrogen fertilizer rate on Brix and the harvest window of sweet sorghum; and (3) to determine the harvest window of sweet sorghum in Tottori, Japan.

Table 1. Plot summary of sweet sorghum production.

Plot

P-1

P-2

P-3

P-4

P-5

P-6

Sowing time

May 1

June 1

July 3

May 29

May 29

May 29

Plot area (m2)

20

20

225

60

60

60

N fertilizer rate

(kg ha-1)

70

70

70

70

140

210

Materials and Methods

Sweet Sorghum Production

Sweet sorghum (K9041, bred in Australia by Snow Brand Seed Co., Ltd., Sapporo, Japan) was sown in a field of the Tottori University farm (35° 32' N 134° 10' E) on May 1 (plot P-1, area = 20 m 2 ), June 1 (plot P-2, area = 20 m 2 ), and July 3 (plot P-3, area = 225 m 2 ) 2009 to determine the effect of sowing time on sugar content (table 1). The sweet sorghum of P-3 was also used to complete the performance experiments of the developed extractor, and 70 kg nitrogen fertilizer was applied per hectare. Sweet sorghum was sown in 0.75 m wide rows at a planting rate of 147,763 (measured value) plants ha -1 .

In order to determine the effects of nitrogen fertilizer rate on sugar content, three plots of sweet sorghum were sown on May 29, 2009, applying 70 (P-4), 140 (P-5), and 210 (P-6) kg ha -1 nitrogenous fertilizer (table 1). The areas of these three plots were 60 m 2 . Sweet sorghum was sown in 0.75 m interspaced rows at a planting rate of 147,763 plants ha -1 . No irrigation was conducted in any of the six plots.

Figure 1 shows the air temperature and precipitation in 2009 and the mean for the last eight years at the Tottori University farm. The change in air temperature in 2009 was almost the same as the mean air temperature for the last eight years. The precipitation from July to October 2009 was less than the mean precipitation of the last eight years.

Extractor Design and Fabrication

A gear-type extractor suitable for the whole plant of sweet sorghum was designed and fabricated for accurate and easy determination of the sugar content. The extractor (fig. 2a) was 0.65 m long, 0.65 m tall, and 0.40 m wide, and its total weight was 45 kg. The developed extractor included a motor (GM-SS, 100 VAC, 400 W, Mitsubishi Electric Co., Tokyo, Japan), a drive gear (six modules, 30 teeth, 30 rpm), a follower gear (six modules, 15 teeth), a sieve, and a juice collector.

The processing of the extractor is depicted in figure 2b. The whole sweet sorghum plant was fed into the extractor from the feed side. The juice was extracted by the drive gear and the follower gear. The juice was collected by the juice collector and flowed into the juice container. The sugar content of the juice was determined using a digital hand-held refractometer. The bagasse was discharged from the discharge side.

fpe9545_files/image2.gif fpe9545_files/image3.jpg

Figure 2. (a) Diagram of the developed extractor (1 = motor, 2 = belt, 3 = pulley, 4 = drive gear, 5 = follower gear, 6 = sieve, and 7 = juice collector) and (b) extracting sweet sorghum using the extractor.

Experimental Methods

Three separate experiments were completed. The first experiment determined the performance of the developed extractor. The second experiment determined the effects of sowing time on the sugar content and harvest window of sweet sorghum. The third experiment focused on the effect of nitrogen fertilizer rate on the sugar content and harvest window of sweet sorghum. Statistical analysis (e.g., Duncan’s multiple range test) was performed using SAS (version 9.1.3, SAS Institute, Inc., Cary, N.C.).

Performance of the Developed Extractor

Thirty-two plants were hand-harvested at random from the P-3 sweet sorghum field (225 m 2 ) at 110 days after sowing and were extracted using the developed extractor. The juice weight, extraction speed (s plant -1 ), and extraction rate (juice weight to plant fresh weight expressed as a percentage) of every plant were measured. The Brix value of juice was measured using a digital hand-held refractometer (model PAL-1, measurement accuracy Brix ±0.2%, Atago Co., Ltd., Tokyo, Japan). The major and minor diameters of the sweet sorghum stalks were determined by measuring the first internodes from the ground ( n = 32).

Sowing Time Experiments

Twelve plants were sampled at random from each of plots P-1, P-2, and P-3 every week after 50% of the sweet sorghum plants were heading. Eight samples were extracted with the developed extractor, and the Brix values were determined using a digital hand-held refractometer. Moisture content was determined by drying four samples at 105°C in an oven to constant mass.

Nitrogen Fertilizer Rate Experiments

Twelve plants were sampled at random from plots P-4, P-5, and P-6 every week after 50% of the sweet sorghum plants were heading. Eight samples were extracted with the developed extractor, and the Brix values were determined using a digital refractometer. Moisture content was determined by drying four samples at 105°C in an oven to constant mass.

Results and Discussion

Performance of the Developed Extractor

Table 2 presents the parameters of the experimental materials and the results of the performance test of the developed extractor. Sweet sorghum sown in July 2009 (P-3, 225 m 2 ) was used as the samples. The plant height was 2.83 ±0.39 m (mean ±SD, n = 32). The maximum diameter was 21.9 mm, and the minimum diameter was 12.3 mm. The fresh weight was 490 ±184 g plant -1 . The yields of total biomass were 72.4 ton ha -1 (fresh weight per plant × planting rate per hectare).

In general, the feedstock needs to be manually forced into the roller gap because the grip of a roller extractor is weak. The developed extractor could grip sweet sorghum powerfully and extract it because of the gear-shaped structure. Therefore, the extraction speed of the developed extractor was fast (9.3 s plant -1 ). The juice extraction rate was 8.7%, and the mean Brix of juice was 11.7% in this study (table 2).

Table 2. Parameters of experimental materials and results ( n = 32).

Parameter

Max.

Min.

Mean ±SD[a]

Height (m)

3.27

1.73

2.83 ±0.39

Internodes

12

9

10.5 ±0.9

Major diameter (mm)

21.9

13

17.8 ±2.7

Minor diameter (mm)

20.3

12.3

16.7 ±2.4

Fresh weight (g plant-1)

806

121

490 ±184

Extraction speed (s plant-1)

22.1

5.3

9.3 ±3.3

Juice brix

13.2%

8.4%

11.7% ±1.18%

Juice weight (g plant-1)

110.2

6.7

42.5 ±25.0

Juice extraction rate (%)

-

-

8.7%

Moisture content (%)

-

-

76.0%

Total biomass yield (ton ha-1)

-

-

72.4

[a] SD = standard deviation.

The following assumptions were made in order to calculate a suitable number of samples when the developed extractor was used to determine the Brix of sweet sorghum: (1) the sweet sorghum Brix has a normal distribution; (2) because more than 30 samples were used, the standard deviation of the samples was regarded as the standard deviation of the population; (3) the confidence level was 95%; and (4) the confidence interval was ±1% (Brix value). Therefore, the necessary number of samples was calculated as follows (Hibana, 2006):

FPE9545_files\eqn\eqn1.gif FPE9545_files\eqn\eqn2.gif (5)

where SD is the standard deviation, and n is the number of samples. The results indicated that six or more samples were necessary when the gear-type extractor was used to determine the Brix of sweet sorghum.

Effect of Sowing Time on Sugar Content and Harvest Window of Sweet Sorghum

The days to 50% sweet sorghum heading were significantly affected by sowing time. The days to 50% sweet sorghum heading decreased when the sowing date was delayed (table 3). The days to 50% sweet sorghum heading were 100 in May, 84 in June, and 75 in July. The days to exceed 10% of Brix were also affected significantly by sowing time. The days to exceed 10% of Brix of sweet sorghum sown on May 1 were more than 100, but less than 90 days were required for sweet sorghum sown on June 1 and July 3. The days of Brix exceeding 10% increased when the sowing date was delayed (around 60 days for sweet sorghum sown in July, and around 30 days for sweet sorghum sown in May). Therefore, later sowing could prolong the harvest window of sweet sorghum in Tottori.

Table 3. Comparison of results for different sowing dates (N fertilizer rate = 70 kg ha -1 ).

Sowing

Date

Days to 50%

Heading

Days to Exceed

10% of Brix

Days of Brix

Exceeding 10%

May 1

100

>110

Around 30

June 1

84

<90

>30

July 3

75

<90

Around 60

Figure 3 shows the sugar content changes of sweet sorghum sown in different months. The sugar content of sweet sorghum sown in May and June had the same trend, exceeding 10% at almost the same time. The trend for sweet sorghum sown in July differed from that of May and June because most of the sweet sorghum was blown down by a typhoon on October 8. However, the sugar content of sweet sorghum sown in July remained over 10% until the end of November.

fpe9545_files/image6.gif

Figure 3. Sugar content changes of sweet sorghum sown in different months ( n = 8).

Effect of N Fertilizer Rate on Sugar Content and Harvest Window of Sweet Sorghum

The difference in N fertilizer rate did not affect the days to 50% sweet sorghum heading, which was 86 days for all three plots (P-4, P-5, and P-6). Table 4 presents the test results for sweet sorghum with different N fertilizer rates applied. The number of internodes, green matter per plant, moisture content, and yield of sweet sorghum with three different N fertilizer rates did not differ significantly at the 95% confidence level, as determined by Duncan’s multiple range test. Dry matter per plant was affected significantly by the N fertilizer rate in this study. Tamang et al. (2011) observed an N fertilizer response in total dry matter (TDM) in 2009, but not in 2008, with an N fertilizer rate ranging from 0 to 168 kg N ha -1 . Erickson et al. (2011) reported that dry matter yield was not affected by N fertilizer rate (45, 90, 135, or 180 kg N ha -1 ) at two sites differing in soil type in Florida. Jacobs and Ward (2011) indicated that dry matter yield responses to applied N fertilizer were variable, with increases observed in the first year in southern Australia; the experiments for N fertilizer rate (0, 40, 80, 120, 160, or 200 kg N ha -1 ) were conducted in two summer periods. Therefore, further research on the effects of N fertilizer rate on dry matter yield is needed.

Table 4. Results for sweet sorghum with different N fertilizer rates. [a]

Result

N Fertilizer Rate

70 kg ha-1

140 kg ha-1

210 kg ha-1

Height (m)

3.10 ±0.10 a

3.03 ±0.14 ab

2.97 ±0.12 b

Number of internodes

12.4 ±0.2 a

12.5 ±0.4 a

12.5 ±0.4 a

Green matter (g plant-1)

619.1 ±33.6 a

591.8 ±46.3 a

567.6 ±52.9 a

Dry matter (g plant-1)

147.4 ±12.9 a

140.2 ±13.6 ab

126.8 ±13.7 b

Moisture content (%)

76.4 ±1.5 a

76.5 ±2.1 a

77.8 ±1.4 a

Yield (ton ha-1)

81.7 ±4.4 a

78.1 ±6.1 a

74.9 ±7.0 a

[a] Data are represented as means ±SD. Means in the same row followed by the same letter are not significantly different at the 95% confidence level as determined by Duncan’s multiple range test (n = 32).

Table 5 shows the sugar content changes of sweet sorghum with different N fertilizer rates applied. Erickson et al. (2011) reported that the mean Brix value of sweet sorghum was negatively related to N fertilizer rate (45 to 180 kg N ha -1 ). In this study, the sugar content of sweet sorghum decreased with increased N fertilizer rate. The data tendency for October 12 differed from that for the other days because much of the sweet sorghum was blown down by a typhoon on October 8, 9, and 10, and the sugar content of the lodged plants was lower than that of the erect plants. The sugar content of sweet sorghum with 70 kg ha -1 N fertilizer applied significantly differed from that of sweet sorghum with 210 kg ha -1 N fertilizer applied. The sugar content of sweet sorghum exceeded 10% for 25 days when 210 kg N fertilizer was applied per hectare, but for more than 45 days when 70 and 140 kg ha -1 N fertilizer were applied.

The sugar content of sweet sorghum sown on May 29 exceeded 10% from the beginning of September to the middle of October, and the sugar content of sweet sorghum sown on July 3 exceeded 10% from the beginning of October to the end of November (fig. 3 and table 5). Therefore, it is suitable to sow sweet sorghum twice in one year in Tottori. The harvest window would be prolonged to three months (September, October, and November) if sweet sorghum was sown at the end of May and at the beginning of July with 70 kg ha -1 of N fertilizer applied. The sugar content of the obtained juice would exceed 10%. The experiment results (data not shown) in 2010 confirmed these results.

Table 5. Effect of nitrogen fertilizer rate on Brix of sweet sorghum. [a]

Plot (and N

Fertilizer Rate)

Sugar Content (%)

Sept. 2

Sept. 9

Sept. 16

Sept. 23

Sept. 30

Oct. 7

Oct. 12

Oct. 19

P-4 (70 kg ha-1)

12.3 ±1.0 a

14.5 ±0.9 a

14.6 ±0.7 a

13.6 ±0.6 a

11.9 ±0.4 a

13.4 ±1.0 a

9.9 ±1.9 ab

13.2 ±1.2 a

P-5 (140 kg ha-1)

11.7 ±0.6 ab

14.2 ±0.6 a

14.1 ±0.4 a

13.6 ±0.8 a

10.6 ±0.7 b

12.1 ±1.2 a

11.1 ±0.7 a

12.7 ±0.6 a

P-6 (210 kg ha-1)

11.4 ±0.7 b

13.7 ±0.9 a

13.3 ±0.5 b

11.2 ±1.7 b

8.8 ±1.3 c

10.2 ±1.5 b

8.8 ±1.3 b

10.1 ±1.2 b

[a] Data are represented as means ±SD. Means in the same column followed by the same letter are not significantly different at the 95% confidence level as determined by Duncan’s multiple range test (n = 8).

Conclusions

A gear-type extractor suitable for the whole sweet sorghum plant was designed, built, and tested. In addition, the effects of sowing time and nitrogen fertilizer rate on the Brix and harvest window of sweet sorghum were investigated using the developed extractor. The following conclusions were drawn from this study:

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