Sweet sorghum (Sorghum bicolor L. Moench) is an important bio-energy crop, and mostly it is grown for syrup and ethanol production. In sweet sorghum, sugar concentration increases at certain growth stage and drops down at maturity. As the crop matures, changes in the volume and composition of the juice influence the sugar yield, since the sugar content usually continues to increase as maturity approaches, the best harvest time in sweet sorghum is the most critical in getting higher sugar and juice yields.
A field experiment was conducted during 2009 to study the effect of different harvest time on sugar and juice yield of sweet sorghum. Sweet sorghum variety M81E was harvested at ten growth stages initially at flag leaf stage, boot, panicle emergence, anthesis and post-anthesis followed by milk, soft dough and hard dough stage and finally the maturity stages. The physiological parameters like chlorophyll SPAD readings, leaf temperature, stem temperature and FvFm ratio were measured before each harvest. Growth and yield parameters like plant height, stem girth, brix, juice yield, sugar yield, total dry biomass, grain yield and juice quality characteristics were recorded after harvest.
The results showed the effect of harvesting stage on brix, sugar yield, juice yield, juice purity, total sugars, and non-reducing sugars were significant. The results show that highest brix () was obtained when plants were harvested at post physiological maturity. On the other hand, sugar yield was significantly highest when plants were harvested at hard dough stage but the difference was comparable with early harvests either at milk stage or soft dough stage of the crop. Juice yield (kL ha-1) was highest when harvesting was done during the hard dough stage, although the magnitude of increase was comparable to those plants harvested from the milk stage and soft dough stage. Juice purity (), total sugars (wv), and non-reducing sugars (wv) were significantly highest among plants harvested at hard dough stage . Based on the results, harvesting the plants at hard dough stage gave the highest juice purity(), total sugars (wv), and non-reducing sugars(wv). High sugar yield(t ha-1) and juice yield (kL ha-1 ) are obtainable from plants harvested at milk stage to hard dough stage. Late harvest at post physiological maturity produced the highest brix ().
Plant height
Plant height (cm) of sorghum steadily increased from the youngest stage (flag leaf stage) to the oldest stage (post physiological maturity). Tallest plant of 388.9 cm was noted at late harvest or post physiological maturity but this height did not significantly vary with the heights at soft dough until the physiological maturity stages. Highest percentage increase of 1.59 was noted between the panicle emergence and anthesis or flowering stage. The lowest plant of 249 cm was noted at the youngest stage (flag leaf stage) of the plant. The highest height increase of 25.94 was noted between flag leaf stage and boot stage.
Number of leaves per plant
The periodic increase in number of leaves per plant show an upward trend up to milk stage after which the value declined with the lowest of 10.8 obtained at physiological maturity. The most number of leaves was 15.1 on the average and obtained at milk stage harvest and this value was significantly the highest among all other values obtained at various stages of harvests. The highest percentage increase of 7.03 percent was noted during the boot stage harvest. Beyond the milk stage, the number of leaves decreased at a magnitude ranging from 9.3 at soft dough stage to 11.67 at physiological maturity which indicated that the plant is way past maturity and had already reached the stage of senescence.
Number of internodes per plant
The number of internodes per sorghum plant increased from flag leaf stage (11.1) to milk stage (15.1) after which decline was noted until physiological maturity. The number of internodes per plant at milk stage did not significantly vary with those at post anthesis stage (14.8). Both values varied significantly with the number of internodes per plant obtained at all other stages of plant growth. Just like any vegetative growth stage, highest percentage increase of 19.82 in the number of internodes per plant was noted during the boot stage and decrease of 9.27 noted between milk stage to soft dough stage.
Leaf area (cm)
The leaf area (cm) of sorghum for ten harvesting stages, gradually increased from flag leaf (4141.6cm) until post anthesis stage (5074.6cm) where the maximum leaf area was obtained until it declined to the lowest value of 4017.8 at soft dough stage. The increase in leaf area at post anthesis stage did not significantly differed with those obtained when plants were harvested either at anthesis or soft dough stages. There was no longer recorded leaf areas from hard dough stage to post physiological maturity stage. The highest increase in leaf area (cm) was obtained from panicle emergence stage to anthesis stage at 11.64 which are critical reproductive stages.
3rd Internode Girth (cm)
The third internode girth steadily increased with harvests at the youngest stage of flag leaf stage until the oldest stage at physiological maturity. The 3rd internode girth value of 16.8 cm was lowest at flag leaf stage and highest (20.6) at physiological maturity. However, the 3rd internode girth values when plants were harvested at milk, soft dough, hard dough and physiological maturity did not significantly differ. The highest percentage of 3rd internode girth measurement increase in cm of 5.88 was recorded during the milk stage harvest. The slight increases from the value noted at soft dough (3.0), hard dough (0.49), and physiological maturity (1.5) stages explained why 3rd internode girth (cm) were comparable at this stages.
6th internode girth (cm)
The 6th internode girth (cm) also increases with harvesting at the youngest stage of plant growth (flag leaf stage) until the oldest (physiological maturity) where significantly the highest value of 16.4 cm was obtained. The smallest 6th internode girth of 13.9 cm was obtained at flag leaf harvest. The highest 6th internode girth increase of 6.4 was noted between the hard dough stage and physiological maturity stage harvests. From flag leaf stage, the 6th internode girth increased by 5.8 at boot stage harvest with added 1.4 and 2.7 during harvests at panicle emergence and anthesis. An additional 5.9 gain in 6th internode girth was obtained during the post anthesis harvest and another 1.9 for milk stage harvest.
9th internode girth (cm)
The 9th internode girth (cm) ranged from 12.2 for early harvest at flag leaf stage to 15.9 when harvesting was done at the latest stage of physiological maturity. The 9th internode girth (cm) of plants harvested during physiological maturity was significantly the biggest. The highest increase in 9th internode girth of 6.3 percent was noted between panicle emergence and anthesis and another 5.5 between hard dough stage to physiological maturity.
Average internode girth (cm)
The average internode girth (cm) range from 14.3 cm for the harvest at the youngest or flag leaf stage and 18.3 cm upon harvesting at physiological maturity with steady increases noted when harvesting was done at all stages of plant growth. The average internode girth (cm) at post physiological maturity was significantly the biggest. The data further revealed a gain of 4.2 in the average internode girth (cm) if harvesting is done at boot stage rather than at flag leaf stage and another increase in size of internode on the average between post anthesis and milk stage.
Yield Traits
Brix ()
The brix gradually increases with harvesting done at the youngest or flag leaf stage (5.4) until soft dough stage (15.1) , decreased up to physiological maturity then peaked (16.9) with harvesting at the oldest stage of post physiological maturity. At this stage, the brix was significantly the highest. Brix increase of 29.6 was noted between flag leaf to boot stage and 21.6 between physiological and post physiological maturity. The lowest percentage increase of 2.7 was recorded at soft dough stage. It can be stated that harvesting when the plant is most mature or at post physiological maturity is the best in order to get the significantly highest brix of 16.9. According to Prasad, et al (2007) citing Bitzer et al (2006), for best ethanol production from sorghum the crop should be harvested when the sugar content is in the range of 15.5 16.5. Almodares, et al (2007) also reported that high brix value was obtained at physiological maturity which was also confirmed by this study. At physiological maturity, there is decreasing acidic invertase content while natural invertase increases, hence the high sucrose content.
Juice Yield (kL ha-1)
The juice yield (kL ha-1) steadily increases with harvesting done at flag leaf stage until the hard dough stage where significantly the highest value of 41.8 (kL ha-1) was noted. Lowest juice yield (22.3 kL ha-1) was obtained when plants were harvested at youngest stage (flag leaf stage). The percentage increase in the juice yield (kL ha-1) which is 23.6 was noted between the flagleaf stage and the boot stage and lowest percent of increase (1.8) between milk stage and soft dough stage. Beyond the hard dough stage a reduction in juice yield (kL ha-1) was noted (-18.7) up to post physiological maturity.
Stem Fresh Weight (t ha-1)
Te stem fresh weight (t h-1) increased from 42.7 t h-1 at flag leaf stage harvest to 78.8 t h-1 hard dough stage harevest, then declined considerably. The stem fresh weight of plants harvested at hard dough stage was significantly the highest among all other values obtained when harvesting was done at various stages of plants growth. The highest increase of 24.6 in stem fresh weight (t h-1) was obtained when plants were harvested at boot stage. Harvesting beyond the hard dough stage, reduces the stem fresh weight (t ha-1) by 14.6 and another 7.9. reduction was with harvest done during the post physiological stage of the plant.
Total Fresh Biomass (t ha-1)
The total fresh biomass (t ha-1) at periodic stages of harvest corresponding the to the stages of plants growth ranged from 52.7g for harvest at flag leaf stage and the highest total fresh biomass weight of 95.0 (t ha-1) was obtained when plants were harvested at hard dough stage, steadily increasing in the process. The total fresh biomass (t h-1) at hard dough stage was significantly the highest among the total fresh biomass (t ha-1) obtained by harvesting the plants at ten different stages corresponding to the different growth stages of the sorghum plant. The percentage increase in the total fresh biomass (t ha-1) from flag leaf stage boot stage was 22.0. Harvesting the sorghum beyond hard dough stage, that is, at physiological maturity, reduced the total fresh biomass (t ha-1) by 23.4 and by an added 6.6 when harvesting was done during post physiological stage of the plant.
Sugar Yield (t ha-1)
Sugar yield (t ha-1) also increased with harvests from flag leaf stage (1.21 t h-1 ) to hard dough stage (6.04 t h-1) then steadily decreased thereafter. Sugar yield at hard dough stage however did not significantly differed with those obtained at earlier stages of plant growth, that is the milk stage and the soft dough stage. In fact highest increase of 34.0 was noted in the sugar yield (t h-1) was noted between post anthesis and the milk stage and 4.86 between the milk stage and the soft dough stage. It seemed that harvesting between the milk stage to hard dough stage was best for maximum sugar yield. However, because the increase was very slight between the soft dough and hard dough stages (0.8), harvesting between milk and dough stages may already be considered. Choosing the soft dough stage as the best harvest stage for getting a reasonable sugar yield (t ha-1) was based on the fact that a 4.86 increase would mean a substantial returns whereas a 0.8 increase is not enough to risk the plant of exposure to inclement weather for an extended period of time in the field. The sugar yield (t ha-1) increased considerably before the soft dough stage from flag leaf to boot stage, the increase was 59.4 and another 36.3 percent was added at panicle emergence stage. An additional 26.6 gain in sugar yield (t ha-1) was recorded at anthesis, 29.1 at post anthesis and 34.0 during the milk stage. Harvesting sorghum plant beyond the hard dough stage result in a reduction of sugar yield (t ha-1) by 21.84 when done during the physiological maturity of the crop and another 7.6 percent when harvesting it at post physiological maturity.
Panicle Fresh Weight (t ha-1)
There was no panicle fresh weight (t ha-1) data before anthesis as this is no longer a vegetative growth trait but one associated with the reproductive growth. The data show increasing fresh panicle weight (t ha-1) from anthesis until soft dough stage and a decline in value when harvesting was done beyond this stage. The panicle fresh weight of 5.67 (t ha-1) obtained during harvest at soft dough stage was significantly the highest among the other harvest periods. The trend in panicle fresh weight (t ha-1) increase with different harvest periods with 63.4 at post anthesis stage, 30.5 at milk stage and 25.3 at soft dough stage.
Leaf Dry Weight (t ha-1)
The leaf dry weight (t ha-1) data show increasing value from flag leaf stage (2.3 t ha-1) until the soft dough stage (3.65 t h-1). However, the leaf dry weight at soft dough stage did not significantly differed from those obtained at earlier stages of post anthesis (3.58 t h-1), post anthesis (3.63 t h-1), or at later hard dough stage (3.64 t h-1). Highest leaf weight increase of 26.5 was noted between flag leaf and boot stages and harvesting beyond the hard dough stage steadily decreases the leaf dry weight until the post physiological maturity with a magnitude decrease ranging from 0.27 to 2.9.
Panicle Dry Weight (t ha-1)
Panicle dry weight (t ha-1) increases with harvesting done at increasing maturity of the sorghum plant from anthesis (1.01 t h-1 ) to post physiological maturity where significantly the highest panicle dry weight of 2.64 t h-1 was recorded. The increase in panicle dry weight (t ha-1) from anthesis to post anthesis was 35.6 from a base data of 1.01 t ha-1. From post anthesis a 39.4 gain in panicle dry weight (t ha-1) was obtained with harvesting during the milk stage of the sorghum plant. Another 16.8 was realized with harvest during the soft dough stage and a reduction of 7.6 percent was recoded when harvest was done at hard dough stage. However, another gain in panicle dry weight (t ha-1) of 19.9 was noted upon harvesting at physiological maturity and an additional 6.9 with harvest at post physiological maturity. During this stage the maximum panicle dry weight of 2.64 t ha-1 was significantly the highest among the panicle dry weights obtained.
Stem dry weight (t ha-1)
Stem dry weight (t ha-1) periodically increased from 4.85 at flag leaf harvest to 14.97t ha-1 for late harvest at post physiological maturity where significantly the highest stem dry weight was obtained. The stem dry weight (t ha-1) value was 4.81 at flag leaf which increased by 56.3 at boot stage and an added 17.3 at panicle emergence. Another 19.0 gain in stem dry weight (t ha-1) was realized when harvesting was done during the anthesis stage and an additional of 11.7 , 2.5, 8.2 , 3.3 and .53 when harvesting periods were delayed during post anthesis, milk stage, soft dough stage, and dough stage, and physiological maturity stages, respectively.
Total dry biomass (t ha-1)
The total dry biomass (t ha-1) also considerably increases from flag leaf stage harvest (7.15 t ha-1) until post physiological stage, where significantly the highest total dry biomass of 17.45 t ha-1 was obtained. Slight decrease in total dry biomass was obtained at physiological maturity. The total biomass (t ha-1) increased by 46.7 when harvesting was done at boot stage, another 14.9 at panicle emergence harvest, and another 16.3 when harvesting was done at anthesis with an additional 9.9 when done at post anthesis. A slight increase in total dry biomass (t ha-1) , that is, by 2.2 was realized when harvesting was done at milk stage, but an added 6.54 percent was noted when harvesting was done at soft dough stage and another 2.4 when done at hard dough stage. A reduction of 7.5 in total dry biomass (t ha-1) was realized when plants were harvested at physiological maturity.
Grain yield (t ha-1)
Grain yield data which was the lowest at the beginning of the reproductive period of anthesis ( 0.044t ha-1) steadily increased until physiological maturity (0.230t ha-1) and declined drastically at post physiological maturity registering less than half of the harvest (0.067t ha-1). This data show that harvesting at physiological maturity was the best stage to realize the maximum grain yield (t ha-1) of sorghum. The value obtained by harvesting at this stage was significantly the highest among the grain yields (t ha-1) obtained from other harvesting periods. The incremental increase in grain yield (t ha-1) was 2.3 from anthesis to post anthesis harvests, 48.9 for milk stage harvest, 31.3 for soft dough stage harvest, 27.3 percent for hard dough stage harvest and which more than doubled (105.4) during the physiological stage harvest and which reduced by 70.09 during the post physiological stage harvest.
Harvest index (Grain) ()
The trend in the harvest index (grain ) data follows that of the grain yield in that it increases steadily from milk stage (0.427 grain ) to 1.185 (grain ) at physiological maturity after decreasing from anthesis to post anthesis. Harvesting was initiated at anthesis stage with 0.321 grain harvest index. This value decreased by 7.2 when harvesting was done during post anthesis stage. Another 43.3 gain was realized with a later harvest period which was the milk stage of the plant with additional 23.0 when harvesting was done later at soft dough stage with another 25.7 gained when harvesting was made during the hard dough stage. The highest increase of 79.3 in harvest index grain () was obtained when harvesting the plant at physiological maturity at a value of 1.185. This value was significantly the highest among the harvest index (grain) () obtained from various harvest periods, so harvesting at physiological maturity was considered the best stage for harvest index (grain) ().
Harvest index sugar) ()
The final data was the harvest index (sugar) () which shows that of the ten harvest stages, the harvest index (sugar)() steadily increases from flag leaf stage until a maximum of 36.6 harvest index (sugar ) was obtained at milk stage of sorghum growth. It was also noted that this harvest index for sugar did not vary significantly with those obtained at soft and hard dough stages. Just the same, harvesting at milk stage is already the best for harvest index (sugar ) because prolonged harvesting already resulted in reduction in harvest index (sugar ). Beyond the milk stage where the highest incremental increase of 30.7 percent was obtained, the harvest index for sugar considerably decreased until eventually only 25.2 harvest index (sugar ) was obtained. By increment, the harvest index (sugar)() at flag leaf stage was 16.9 which increased by 9.5 when harvesting was done at boot stage and an additional 20.0 when harvesting at panicle emergence and another 9.5 when harvesting at anthesis and 15. at post anthesis. Highest increase of 36.7 was realized during the milk stage harvest with a 1.4 reduction during the subsequent harvests at soft and hard dough stages. Another 15.2 reduction in harvest index (sugar)() was realized when delaying the harvest at later stage of physiological maturity and still another decrease of 16.6 with further harvest delay at post physiological maturity.
Physiological Traits
Fo
Changes in plant Fo is highly variable as indicated by high Fo at flag leaf, then declined until panicle emergence, then increase again at anthesis slightly lowered at post anthesis then slightly increased before it hit the lowest value of 233.0 at soft dough stage. Highest Fo was obtained at anthesis stage (373.0) but this value did not significantly differed from those obtained at flag leaf stage (272.0), milk stage (272.0) and post anthesis stage (270.0).
Fm
As to the amount of Fm, it slightly increased at boot stage, then reduced at panicle emergence, increased at anthesis to post anthesis stage then continuously declined as the plant matures to soft dough stage. Fm was significantly highest at highest at boot stage (1178). Lowest Fm (339) was obtained at soft dough stage harvest of the plant. The result indicate that as early as boot stage, sorghum plant had accumulated its maximum Fm and whatever increase noted after flowering is slight enough to cause much difference in the value of this physiological trait.
The ratio of Fv to Fm generally decreased as the plant matures to soft dough stage. FvFm was highest (0.786 ) during boot stage and decreased until anthesis. The FVFm value at boot stage was significantly the highest among the ratios of FV to Fm obtained at all other stages of plant growth. There was slight increase in the ratio of FVFm at post anthesis stage after which it declined until it reaches the lowest value of 0.313 at soft dough stage.
Leaf temperature 0C
The leaf temperature of sorghum plant generally decreased as the plant grows older except for the slight increase noted at panicle emergence stage. Highest temperature of 30.30C was noted at the youngest stage of the plant (flag leaf stage) which was significantly the highest among the values obtained at various stages of plants growth except at panicle emergence stage where the leaf temperature 0f 29.40C was significantly similar. Leaf temperature (10.20C) was lowest when the plant was oldest (soft dough stage).
Stem temperature
The data trend in stem temperature 0C was similar to that of leaf temperature0C. The highest stem temperature of 28.20C was obtained at the youngest stage of the sorghum plant, that is, at flag leaf stage and this temperature was statistically similar with the value obtained at panicle emergence (28.70C ). Stem temperature steadily decreased from panicle emergence until soft dough stage where the lowest stem temperature of 10.90C was recorded.
Chlorophyll content (SPAD value)
The chlorophyll content of sorghum plant increased when harvesting was done at the youngest stage of the plant (flag leaf stage) and peaked at panicle emergence harvest with a value of 48.6. At this stage, there must be more chlorophyll to maximize the production of starch in preparation for the shift to reproductive growth. The cholorophyll content at panicle emergence was significantly the highest among the values obtained when sorghum plants are at various stages of growth. The cholorophyll content value continuously decreased as the plant progresses in its reproductive stages until the soft dough stage where the lowest SPAD value of 38.3 was obtained.
Biochemical Traits
Extractable juice ()
There was an increasing and decreasing extractable juice as plant moves from one growth stage to another with the highest value of 54.5 obtained at panicle emergence and the lowest value of 41.8 percent when the plant was oldest (post physiological maturity). The extractable juice was statistically similar indicating not much difference in the amount obtained from the youngest (flag leaf stage) to the older stage of the plant (physiological maturity). The extractable juice at post physiological maturity was significantly the lowest.
Juice pH
A slightly erratic trend in the juice pH was noted in the data. The juice pH slightly declined when plants were harvested at boot stage, then slightly increased at panicle emergence harvest, then decreased continuously until hard dough stage harvest then peaked when plants were harvested at physiological maturity. The juice pH at this stage was 5.5 and significantly the highest among the measurements done at all other stages of growth. The lowest juice pH of 4.70 was noted during late harvest when the plants are in their post physiological maturity stage. This means that fermentation had already taken place hence the juice was already acidic.
Total sugars ( wv)
The total sugars (wv) of sorghum plant continuously increased as harvesting was delayed with lowest value of 4.66 noted among plants harvested at flag leaf stage and the highest, that is, 23.7 obtained at a later harvest, that is, hard dough stage of the plant. A decline in the total sugars was observed when harvesting was done at physiological and post physiological maturity stage of the sorghum plant. The total sugars obtained at hard dough stage was significantly the highest indicating that this stage was the best stage to harvest for total sugar (wv) content.
Reducing sugars (vw)
The amount of reducing sugars (vw) also increased when harvesting was done early (flag leaf stage) until post anthesis stage where the highest amount of reducing sugar was obtained which is 12.0. This amount though although significantly the highest, did not differ significantly with the reducing sugars (vw) recorded when plants were harvested at milk stage (11.84) and those harvested at hard dough stage (11.91). Harvesting beyond physiological maturity resulted in reduced amount of reducing sugar with the lowest of 2.26 obtained when harvesting was done when plant was oldest at post physiological maturity.
Non-reducing sugar(wv)
The amount of non-reducing sugar (wv) steadily increases with various stages of plant growth, peaked at hard dough stage and steadily decreased until the post physiological stage. The highest amount of non-reducing sugar (11.8) was obtained when plants were harvested at hard dough stage. This value was significantly the highest among the non-reducing sugars obtained from harvesting sorghum at various stages of plant growth. The lowest amount 1.86 was noted when plants were harvested at the earliest stage of plant growth which is at flag leaf stage.
Starch (ug g-1)
The starch content of sorghum plant ranged from 7.8 ugg which was obtained among plants harvested at boot stage and the highest of 53.4 ug g-1 was obtained when harvesting was done at milk stage of the plant. At this stage, plants had finished rapid vegetative growth and begin the reproductive stage, hence plant sugars are converted to starch and stored in the grain which explains the highest starch content at this stage. The starch content at this stage was significantly the highest compared to the values for harvests in all other stages of the sorghum plant. The data further show an increasing trend in the starch content ug g-1 as harvests progresses from the young stage of plant growth until milk stage. Beyond this stage the starch content considerably declined but increased substantially at the last harvest when plants are almost senescent at post physiological maturity.
Juice purity
The percentage juice purity increased with early harvest (flag leaf stage) until panicle emergence stage, then slightly decreased until milking stage. Juice purity then increased to 49.7 when harvesting was done at soft dough stage and almost doubled (81.3) at hard dough stage. The juice purity at this stage was significantly the highest. The juice purity decreases with later harvest until the lowest of 12.52 was obtained when plants were harvested at post physiological stage.
A field experiment was conducted during 2009 to study the effect of different harvest time on sugar and juice yield of sweet sorghum. Sweet sorghum variety M81E was harvested at ten growth stages initially at flag leaf stage, boot, panicle emergence, anthesis and post-anthesis followed by milk, soft dough and hard dough stage and finally the maturity stages. The physiological parameters like chlorophyll SPAD readings, leaf temperature, stem temperature and FvFm ratio were measured before each harvest. Growth and yield parameters like plant height, stem girth, brix, juice yield, sugar yield, total dry biomass, grain yield and juice quality characteristics were recorded after harvest.
The results showed the effect of harvesting stage on brix, sugar yield, juice yield, juice purity, total sugars, and non-reducing sugars were significant. The results show that highest brix () was obtained when plants were harvested at post physiological maturity. On the other hand, sugar yield was significantly highest when plants were harvested at hard dough stage but the difference was comparable with early harvests either at milk stage or soft dough stage of the crop. Juice yield (kL ha-1) was highest when harvesting was done during the hard dough stage, although the magnitude of increase was comparable to those plants harvested from the milk stage and soft dough stage. Juice purity (), total sugars (wv), and non-reducing sugars (wv) were significantly highest among plants harvested at hard dough stage . Based on the results, harvesting the plants at hard dough stage gave the highest juice purity(), total sugars (wv), and non-reducing sugars(wv). High sugar yield(t ha-1) and juice yield (kL ha-1 ) are obtainable from plants harvested at milk stage to hard dough stage. Late harvest at post physiological maturity produced the highest brix ().
Plant height
Plant height (cm) of sorghum steadily increased from the youngest stage (flag leaf stage) to the oldest stage (post physiological maturity). Tallest plant of 388.9 cm was noted at late harvest or post physiological maturity but this height did not significantly vary with the heights at soft dough until the physiological maturity stages. Highest percentage increase of 1.59 was noted between the panicle emergence and anthesis or flowering stage. The lowest plant of 249 cm was noted at the youngest stage (flag leaf stage) of the plant. The highest height increase of 25.94 was noted between flag leaf stage and boot stage.
Number of leaves per plant
The periodic increase in number of leaves per plant show an upward trend up to milk stage after which the value declined with the lowest of 10.8 obtained at physiological maturity. The most number of leaves was 15.1 on the average and obtained at milk stage harvest and this value was significantly the highest among all other values obtained at various stages of harvests. The highest percentage increase of 7.03 percent was noted during the boot stage harvest. Beyond the milk stage, the number of leaves decreased at a magnitude ranging from 9.3 at soft dough stage to 11.67 at physiological maturity which indicated that the plant is way past maturity and had already reached the stage of senescence.
Number of internodes per plant
The number of internodes per sorghum plant increased from flag leaf stage (11.1) to milk stage (15.1) after which decline was noted until physiological maturity. The number of internodes per plant at milk stage did not significantly vary with those at post anthesis stage (14.8). Both values varied significantly with the number of internodes per plant obtained at all other stages of plant growth. Just like any vegetative growth stage, highest percentage increase of 19.82 in the number of internodes per plant was noted during the boot stage and decrease of 9.27 noted between milk stage to soft dough stage.
Leaf area (cm)
The leaf area (cm) of sorghum for ten harvesting stages, gradually increased from flag leaf (4141.6cm) until post anthesis stage (5074.6cm) where the maximum leaf area was obtained until it declined to the lowest value of 4017.8 at soft dough stage. The increase in leaf area at post anthesis stage did not significantly differed with those obtained when plants were harvested either at anthesis or soft dough stages. There was no longer recorded leaf areas from hard dough stage to post physiological maturity stage. The highest increase in leaf area (cm) was obtained from panicle emergence stage to anthesis stage at 11.64 which are critical reproductive stages.
3rd Internode Girth (cm)
The third internode girth steadily increased with harvests at the youngest stage of flag leaf stage until the oldest stage at physiological maturity. The 3rd internode girth value of 16.8 cm was lowest at flag leaf stage and highest (20.6) at physiological maturity. However, the 3rd internode girth values when plants were harvested at milk, soft dough, hard dough and physiological maturity did not significantly differ. The highest percentage of 3rd internode girth measurement increase in cm of 5.88 was recorded during the milk stage harvest. The slight increases from the value noted at soft dough (3.0), hard dough (0.49), and physiological maturity (1.5) stages explained why 3rd internode girth (cm) were comparable at this stages.
6th internode girth (cm)
The 6th internode girth (cm) also increases with harvesting at the youngest stage of plant growth (flag leaf stage) until the oldest (physiological maturity) where significantly the highest value of 16.4 cm was obtained. The smallest 6th internode girth of 13.9 cm was obtained at flag leaf harvest. The highest 6th internode girth increase of 6.4 was noted between the hard dough stage and physiological maturity stage harvests. From flag leaf stage, the 6th internode girth increased by 5.8 at boot stage harvest with added 1.4 and 2.7 during harvests at panicle emergence and anthesis. An additional 5.9 gain in 6th internode girth was obtained during the post anthesis harvest and another 1.9 for milk stage harvest.
9th internode girth (cm)
The 9th internode girth (cm) ranged from 12.2 for early harvest at flag leaf stage to 15.9 when harvesting was done at the latest stage of physiological maturity. The 9th internode girth (cm) of plants harvested during physiological maturity was significantly the biggest. The highest increase in 9th internode girth of 6.3 percent was noted between panicle emergence and anthesis and another 5.5 between hard dough stage to physiological maturity.
Average internode girth (cm)
The average internode girth (cm) range from 14.3 cm for the harvest at the youngest or flag leaf stage and 18.3 cm upon harvesting at physiological maturity with steady increases noted when harvesting was done at all stages of plant growth. The average internode girth (cm) at post physiological maturity was significantly the biggest. The data further revealed a gain of 4.2 in the average internode girth (cm) if harvesting is done at boot stage rather than at flag leaf stage and another increase in size of internode on the average between post anthesis and milk stage.
Yield Traits
Brix ()
The brix gradually increases with harvesting done at the youngest or flag leaf stage (5.4) until soft dough stage (15.1) , decreased up to physiological maturity then peaked (16.9) with harvesting at the oldest stage of post physiological maturity. At this stage, the brix was significantly the highest. Brix increase of 29.6 was noted between flag leaf to boot stage and 21.6 between physiological and post physiological maturity. The lowest percentage increase of 2.7 was recorded at soft dough stage. It can be stated that harvesting when the plant is most mature or at post physiological maturity is the best in order to get the significantly highest brix of 16.9. According to Prasad, et al (2007) citing Bitzer et al (2006), for best ethanol production from sorghum the crop should be harvested when the sugar content is in the range of 15.5 16.5. Almodares, et al (2007) also reported that high brix value was obtained at physiological maturity which was also confirmed by this study. At physiological maturity, there is decreasing acidic invertase content while natural invertase increases, hence the high sucrose content.
Juice Yield (kL ha-1)
The juice yield (kL ha-1) steadily increases with harvesting done at flag leaf stage until the hard dough stage where significantly the highest value of 41.8 (kL ha-1) was noted. Lowest juice yield (22.3 kL ha-1) was obtained when plants were harvested at youngest stage (flag leaf stage). The percentage increase in the juice yield (kL ha-1) which is 23.6 was noted between the flagleaf stage and the boot stage and lowest percent of increase (1.8) between milk stage and soft dough stage. Beyond the hard dough stage a reduction in juice yield (kL ha-1) was noted (-18.7) up to post physiological maturity.
Stem Fresh Weight (t ha-1)
Te stem fresh weight (t h-1) increased from 42.7 t h-1 at flag leaf stage harvest to 78.8 t h-1 hard dough stage harevest, then declined considerably. The stem fresh weight of plants harvested at hard dough stage was significantly the highest among all other values obtained when harvesting was done at various stages of plants growth. The highest increase of 24.6 in stem fresh weight (t h-1) was obtained when plants were harvested at boot stage. Harvesting beyond the hard dough stage, reduces the stem fresh weight (t ha-1) by 14.6 and another 7.9. reduction was with harvest done during the post physiological stage of the plant.
Total Fresh Biomass (t ha-1)
The total fresh biomass (t ha-1) at periodic stages of harvest corresponding the to the stages of plants growth ranged from 52.7g for harvest at flag leaf stage and the highest total fresh biomass weight of 95.0 (t ha-1) was obtained when plants were harvested at hard dough stage, steadily increasing in the process. The total fresh biomass (t h-1) at hard dough stage was significantly the highest among the total fresh biomass (t ha-1) obtained by harvesting the plants at ten different stages corresponding to the different growth stages of the sorghum plant. The percentage increase in the total fresh biomass (t ha-1) from flag leaf stage boot stage was 22.0. Harvesting the sorghum beyond hard dough stage, that is, at physiological maturity, reduced the total fresh biomass (t ha-1) by 23.4 and by an added 6.6 when harvesting was done during post physiological stage of the plant.
Sugar Yield (t ha-1)
Sugar yield (t ha-1) also increased with harvests from flag leaf stage (1.21 t h-1 ) to hard dough stage (6.04 t h-1) then steadily decreased thereafter. Sugar yield at hard dough stage however did not significantly differed with those obtained at earlier stages of plant growth, that is the milk stage and the soft dough stage. In fact highest increase of 34.0 was noted in the sugar yield (t h-1) was noted between post anthesis and the milk stage and 4.86 between the milk stage and the soft dough stage. It seemed that harvesting between the milk stage to hard dough stage was best for maximum sugar yield. However, because the increase was very slight between the soft dough and hard dough stages (0.8), harvesting between milk and dough stages may already be considered. Choosing the soft dough stage as the best harvest stage for getting a reasonable sugar yield (t ha-1) was based on the fact that a 4.86 increase would mean a substantial returns whereas a 0.8 increase is not enough to risk the plant of exposure to inclement weather for an extended period of time in the field. The sugar yield (t ha-1) increased considerably before the soft dough stage from flag leaf to boot stage, the increase was 59.4 and another 36.3 percent was added at panicle emergence stage. An additional 26.6 gain in sugar yield (t ha-1) was recorded at anthesis, 29.1 at post anthesis and 34.0 during the milk stage. Harvesting sorghum plant beyond the hard dough stage result in a reduction of sugar yield (t ha-1) by 21.84 when done during the physiological maturity of the crop and another 7.6 percent when harvesting it at post physiological maturity.
Panicle Fresh Weight (t ha-1)
There was no panicle fresh weight (t ha-1) data before anthesis as this is no longer a vegetative growth trait but one associated with the reproductive growth. The data show increasing fresh panicle weight (t ha-1) from anthesis until soft dough stage and a decline in value when harvesting was done beyond this stage. The panicle fresh weight of 5.67 (t ha-1) obtained during harvest at soft dough stage was significantly the highest among the other harvest periods. The trend in panicle fresh weight (t ha-1) increase with different harvest periods with 63.4 at post anthesis stage, 30.5 at milk stage and 25.3 at soft dough stage.
Leaf Dry Weight (t ha-1)
The leaf dry weight (t ha-1) data show increasing value from flag leaf stage (2.3 t ha-1) until the soft dough stage (3.65 t h-1). However, the leaf dry weight at soft dough stage did not significantly differed from those obtained at earlier stages of post anthesis (3.58 t h-1), post anthesis (3.63 t h-1), or at later hard dough stage (3.64 t h-1). Highest leaf weight increase of 26.5 was noted between flag leaf and boot stages and harvesting beyond the hard dough stage steadily decreases the leaf dry weight until the post physiological maturity with a magnitude decrease ranging from 0.27 to 2.9.
Panicle Dry Weight (t ha-1)
Panicle dry weight (t ha-1) increases with harvesting done at increasing maturity of the sorghum plant from anthesis (1.01 t h-1 ) to post physiological maturity where significantly the highest panicle dry weight of 2.64 t h-1 was recorded. The increase in panicle dry weight (t ha-1) from anthesis to post anthesis was 35.6 from a base data of 1.01 t ha-1. From post anthesis a 39.4 gain in panicle dry weight (t ha-1) was obtained with harvesting during the milk stage of the sorghum plant. Another 16.8 was realized with harvest during the soft dough stage and a reduction of 7.6 percent was recoded when harvest was done at hard dough stage. However, another gain in panicle dry weight (t ha-1) of 19.9 was noted upon harvesting at physiological maturity and an additional 6.9 with harvest at post physiological maturity. During this stage the maximum panicle dry weight of 2.64 t ha-1 was significantly the highest among the panicle dry weights obtained.
Stem dry weight (t ha-1)
Stem dry weight (t ha-1) periodically increased from 4.85 at flag leaf harvest to 14.97t ha-1 for late harvest at post physiological maturity where significantly the highest stem dry weight was obtained. The stem dry weight (t ha-1) value was 4.81 at flag leaf which increased by 56.3 at boot stage and an added 17.3 at panicle emergence. Another 19.0 gain in stem dry weight (t ha-1) was realized when harvesting was done during the anthesis stage and an additional of 11.7 , 2.5, 8.2 , 3.3 and .53 when harvesting periods were delayed during post anthesis, milk stage, soft dough stage, and dough stage, and physiological maturity stages, respectively.
Total dry biomass (t ha-1)
The total dry biomass (t ha-1) also considerably increases from flag leaf stage harvest (7.15 t ha-1) until post physiological stage, where significantly the highest total dry biomass of 17.45 t ha-1 was obtained. Slight decrease in total dry biomass was obtained at physiological maturity. The total biomass (t ha-1) increased by 46.7 when harvesting was done at boot stage, another 14.9 at panicle emergence harvest, and another 16.3 when harvesting was done at anthesis with an additional 9.9 when done at post anthesis. A slight increase in total dry biomass (t ha-1) , that is, by 2.2 was realized when harvesting was done at milk stage, but an added 6.54 percent was noted when harvesting was done at soft dough stage and another 2.4 when done at hard dough stage. A reduction of 7.5 in total dry biomass (t ha-1) was realized when plants were harvested at physiological maturity.
Grain yield (t ha-1)
Grain yield data which was the lowest at the beginning of the reproductive period of anthesis ( 0.044t ha-1) steadily increased until physiological maturity (0.230t ha-1) and declined drastically at post physiological maturity registering less than half of the harvest (0.067t ha-1). This data show that harvesting at physiological maturity was the best stage to realize the maximum grain yield (t ha-1) of sorghum. The value obtained by harvesting at this stage was significantly the highest among the grain yields (t ha-1) obtained from other harvesting periods. The incremental increase in grain yield (t ha-1) was 2.3 from anthesis to post anthesis harvests, 48.9 for milk stage harvest, 31.3 for soft dough stage harvest, 27.3 percent for hard dough stage harvest and which more than doubled (105.4) during the physiological stage harvest and which reduced by 70.09 during the post physiological stage harvest.
Harvest index (Grain) ()
The trend in the harvest index (grain ) data follows that of the grain yield in that it increases steadily from milk stage (0.427 grain ) to 1.185 (grain ) at physiological maturity after decreasing from anthesis to post anthesis. Harvesting was initiated at anthesis stage with 0.321 grain harvest index. This value decreased by 7.2 when harvesting was done during post anthesis stage. Another 43.3 gain was realized with a later harvest period which was the milk stage of the plant with additional 23.0 when harvesting was done later at soft dough stage with another 25.7 gained when harvesting was made during the hard dough stage. The highest increase of 79.3 in harvest index grain () was obtained when harvesting the plant at physiological maturity at a value of 1.185. This value was significantly the highest among the harvest index (grain) () obtained from various harvest periods, so harvesting at physiological maturity was considered the best stage for harvest index (grain) ().
Harvest index sugar) ()
The final data was the harvest index (sugar) () which shows that of the ten harvest stages, the harvest index (sugar)() steadily increases from flag leaf stage until a maximum of 36.6 harvest index (sugar ) was obtained at milk stage of sorghum growth. It was also noted that this harvest index for sugar did not vary significantly with those obtained at soft and hard dough stages. Just the same, harvesting at milk stage is already the best for harvest index (sugar ) because prolonged harvesting already resulted in reduction in harvest index (sugar ). Beyond the milk stage where the highest incremental increase of 30.7 percent was obtained, the harvest index for sugar considerably decreased until eventually only 25.2 harvest index (sugar ) was obtained. By increment, the harvest index (sugar)() at flag leaf stage was 16.9 which increased by 9.5 when harvesting was done at boot stage and an additional 20.0 when harvesting at panicle emergence and another 9.5 when harvesting at anthesis and 15. at post anthesis. Highest increase of 36.7 was realized during the milk stage harvest with a 1.4 reduction during the subsequent harvests at soft and hard dough stages. Another 15.2 reduction in harvest index (sugar)() was realized when delaying the harvest at later stage of physiological maturity and still another decrease of 16.6 with further harvest delay at post physiological maturity.
Physiological Traits
Fo
Changes in plant Fo is highly variable as indicated by high Fo at flag leaf, then declined until panicle emergence, then increase again at anthesis slightly lowered at post anthesis then slightly increased before it hit the lowest value of 233.0 at soft dough stage. Highest Fo was obtained at anthesis stage (373.0) but this value did not significantly differed from those obtained at flag leaf stage (272.0), milk stage (272.0) and post anthesis stage (270.0).
Fm
As to the amount of Fm, it slightly increased at boot stage, then reduced at panicle emergence, increased at anthesis to post anthesis stage then continuously declined as the plant matures to soft dough stage. Fm was significantly highest at highest at boot stage (1178). Lowest Fm (339) was obtained at soft dough stage harvest of the plant. The result indicate that as early as boot stage, sorghum plant had accumulated its maximum Fm and whatever increase noted after flowering is slight enough to cause much difference in the value of this physiological trait.
The ratio of Fv to Fm generally decreased as the plant matures to soft dough stage. FvFm was highest (0.786 ) during boot stage and decreased until anthesis. The FVFm value at boot stage was significantly the highest among the ratios of FV to Fm obtained at all other stages of plant growth. There was slight increase in the ratio of FVFm at post anthesis stage after which it declined until it reaches the lowest value of 0.313 at soft dough stage.
Leaf temperature 0C
The leaf temperature of sorghum plant generally decreased as the plant grows older except for the slight increase noted at panicle emergence stage. Highest temperature of 30.30C was noted at the youngest stage of the plant (flag leaf stage) which was significantly the highest among the values obtained at various stages of plants growth except at panicle emergence stage where the leaf temperature 0f 29.40C was significantly similar. Leaf temperature (10.20C) was lowest when the plant was oldest (soft dough stage).
Stem temperature
The data trend in stem temperature 0C was similar to that of leaf temperature0C. The highest stem temperature of 28.20C was obtained at the youngest stage of the sorghum plant, that is, at flag leaf stage and this temperature was statistically similar with the value obtained at panicle emergence (28.70C ). Stem temperature steadily decreased from panicle emergence until soft dough stage where the lowest stem temperature of 10.90C was recorded.
Chlorophyll content (SPAD value)
The chlorophyll content of sorghum plant increased when harvesting was done at the youngest stage of the plant (flag leaf stage) and peaked at panicle emergence harvest with a value of 48.6. At this stage, there must be more chlorophyll to maximize the production of starch in preparation for the shift to reproductive growth. The cholorophyll content at panicle emergence was significantly the highest among the values obtained when sorghum plants are at various stages of growth. The cholorophyll content value continuously decreased as the plant progresses in its reproductive stages until the soft dough stage where the lowest SPAD value of 38.3 was obtained.
Biochemical Traits
Extractable juice ()
There was an increasing and decreasing extractable juice as plant moves from one growth stage to another with the highest value of 54.5 obtained at panicle emergence and the lowest value of 41.8 percent when the plant was oldest (post physiological maturity). The extractable juice was statistically similar indicating not much difference in the amount obtained from the youngest (flag leaf stage) to the older stage of the plant (physiological maturity). The extractable juice at post physiological maturity was significantly the lowest.
Juice pH
A slightly erratic trend in the juice pH was noted in the data. The juice pH slightly declined when plants were harvested at boot stage, then slightly increased at panicle emergence harvest, then decreased continuously until hard dough stage harvest then peaked when plants were harvested at physiological maturity. The juice pH at this stage was 5.5 and significantly the highest among the measurements done at all other stages of growth. The lowest juice pH of 4.70 was noted during late harvest when the plants are in their post physiological maturity stage. This means that fermentation had already taken place hence the juice was already acidic.
Total sugars ( wv)
The total sugars (wv) of sorghum plant continuously increased as harvesting was delayed with lowest value of 4.66 noted among plants harvested at flag leaf stage and the highest, that is, 23.7 obtained at a later harvest, that is, hard dough stage of the plant. A decline in the total sugars was observed when harvesting was done at physiological and post physiological maturity stage of the sorghum plant. The total sugars obtained at hard dough stage was significantly the highest indicating that this stage was the best stage to harvest for total sugar (wv) content.
Reducing sugars (vw)
The amount of reducing sugars (vw) also increased when harvesting was done early (flag leaf stage) until post anthesis stage where the highest amount of reducing sugar was obtained which is 12.0. This amount though although significantly the highest, did not differ significantly with the reducing sugars (vw) recorded when plants were harvested at milk stage (11.84) and those harvested at hard dough stage (11.91). Harvesting beyond physiological maturity resulted in reduced amount of reducing sugar with the lowest of 2.26 obtained when harvesting was done when plant was oldest at post physiological maturity.
Non-reducing sugar(wv)
The amount of non-reducing sugar (wv) steadily increases with various stages of plant growth, peaked at hard dough stage and steadily decreased until the post physiological stage. The highest amount of non-reducing sugar (11.8) was obtained when plants were harvested at hard dough stage. This value was significantly the highest among the non-reducing sugars obtained from harvesting sorghum at various stages of plant growth. The lowest amount 1.86 was noted when plants were harvested at the earliest stage of plant growth which is at flag leaf stage.
Starch (ug g-1)
The starch content of sorghum plant ranged from 7.8 ugg which was obtained among plants harvested at boot stage and the highest of 53.4 ug g-1 was obtained when harvesting was done at milk stage of the plant. At this stage, plants had finished rapid vegetative growth and begin the reproductive stage, hence plant sugars are converted to starch and stored in the grain which explains the highest starch content at this stage. The starch content at this stage was significantly the highest compared to the values for harvests in all other stages of the sorghum plant. The data further show an increasing trend in the starch content ug g-1 as harvests progresses from the young stage of plant growth until milk stage. Beyond this stage the starch content considerably declined but increased substantially at the last harvest when plants are almost senescent at post physiological maturity.
Juice purity
The percentage juice purity increased with early harvest (flag leaf stage) until panicle emergence stage, then slightly decreased until milking stage. Juice purity then increased to 49.7 when harvesting was done at soft dough stage and almost doubled (81.3) at hard dough stage. The juice purity at this stage was significantly the highest. The juice purity decreases with later harvest until the lowest of 12.52 was obtained when plants were harvested at post physiological stage.
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