1. Sorghum establishment, growth and yield in response to planting density
By
M.H Shinga
(213522957)
Submitted in partial fulfilment of the degree of crop science
School of Agriculture, Earth and Environmental Sciences University of
KwaZulu-Natal
AGPS 790
November 2016

2. 1. INTRODUCTION
From many years ago, the practice of mono-cropping of maize for biogas production causing
different type of problems like decreasing the crop species diversity, enhancing pest, decrease
intensity as well as nutrient losses (Mahmood et al 2013). Sorghum (Sorghum bicolor) crop
was then introduced to overcome these problems as an alternative energy crop for biogas
production (Mahmood et al 2013). Growing this drought tolerant crop in dried areas may be
the significant approach to ensure water use efficiency in agriculture (Silungwe 2011).
Cereals are an important food source for human consumption and food security (FAO, 2015)
Sorghum bicolor is indigenous to Africa, and though commercial needs and uses may change
over time, sorghum will remain a basic stable food for many rural communities (Du Plessis
2008). Sorghum is commonly planted in areas were rainfall are limited, usually on shallow
and heavy day soils (Du Plessis 2008). In South Africa, the production of sorghum ranges
from 130 000ha to 150 000ha per year (Du Plessis 2008). The largest producers of sorghum
in South Africa are Free State and Mpumalanga provinces (Du Plessis 2008).
An increase in planting density increases the total yield in cereal and grain crops irrespective
of strong indications that individual plant productivity is decreased (DAFF 2011). It has been
shown by other researchers that there is greater competition for resources such as water,
nutrients and growth factors as plant population increases and the instant effect is seen in
foliage development which is a major determinant of yield (Sibiya 2015).
Sorghum crop can tolerate many type of stresses, including temperature, water and salt
stresses (Hadebe 2015). The Department of Agriculture and Forestry (DAFF, 2010)
recommends optimal planting time of sorghum from start of November until end of
December in South Africa, with dates falling on either side of the recommended times
regarded as early and late planting, respectively (DAFF 2011).
Plant population and row spacing are two variables that can have a significant impact on the
overall gain of sorghum producers (Sibiya 2015). New developing technology in farming
equipment and chemicals that are enhanced open new doors for using rows narrower than 76
cm or twin rows on a single bed in grain sorghum production (Sibiya 2015). Yet it has been
years the narrow rows used in grain sorghum production (Sibiya 2015).
Though optimal plant densities for grain sorghum differ from place to place, preceding
research has shown that grain yields normally increase as plant population increase (DAFF

3. 2010). At lower than optional plant densities, grain sorghum head number per plant or seed
number per head increased when compared to the recommended plant density (DAFF 2010).
Plant density can affect the morphology of the crop, conversion of radiation efficiency, time
of vegetative development, dry matter generation, seed yield and eventually, the production
economy of a crop (Al-Suharbani et al 2013). And so set plant density at optimum, which
might be clarified by both the quantity of plants per unit range and the course of action of
plants on the ground, is a key necessity for achieving higher profitability of a harvest (Al-
Suharbani et al 2013). This is because the final seed yield is determined by the number of
plants in each field area and plant density is controlled by a farmer himself and commonly
not affected by environmental change (Al-Suharbani et al 2013).
A study by Snider et al 2012 showed that plant height was unaffected by row spacing in all
treatments for which plant height data were available (Snider et al 2012). Yet stem density
was highly affected by row spacing after data were podded across all seeding rate (Snider et
al 2012). For this experiment, the stem density was lower at the 76 cm row spacing (205 000
stems/ha) than either the 38 cm (340 000 stems/ha) or 19 cm (387 000 stems/ha) row spacing,
which were statistically the same (Snider et al 2012).
Many studies focused on row spacing and plant population in sorghum have regularly not
succeeded to build up ideal line separating and plant population estimates (Leach et al 1986).
However, some past research has succeeded in evaluating alluring plant populace in sorghum.
According to Butler et al 2003, it was suggested that a plant population of 60000-80000
plants/ha is of optimum yield for sorghum production (Butter et al 2003). Early research had
concluded that narrow rows of 0.35m had a greater yield potential than wider rows of 0.7 or
1.0 m rows under favorable conditions although it was more susceptible to crop failure under
water stress (Butter et al 2003).
Noteworthy yield increments were accounted for when grain sorghum was planted in double
rows apart yield of 24% (1174 kg/ha) compared with single-line planting crosswise over two
deficiency watering system levels (76 and 152 mm of in-season watering system) and two
planting densities (148 000 and 222 000 plants/ha) (Fernandez et al 2012). Although the
expansion in yield was 26% with the larger amount of watering system and 22% with the
lower levels of watering system, there were no critical contrasts between the two planting
densities (Fernandez et al 2012).

4. Planting a crop in a pattern that reduces the spacing of plants within and between rows can
increase plant biomass and leaf area index (Fernandez et al 2012). Work by Bullock et al.
showed that reduced row spacing increased the total interception of photosynthetic active
radiation (PAR) by maize canopy and the light was distributed again in the direction of the
top of the canopy (Fernandez et al 2012).
Other researches had reported that in low plant densities the maximum production capacity of
individual plant is induced, because the competition between plants have reduced and the
canopy radiation is increased for photosynthesis and other physiological processes to occur
(Sedghi et al 2008). There are factors responsible for increasing grain yield in low planting
densities to positive responses of yield components, such as head diameter, grain number per
head and grain weight (Sedghi et al 2008). Many key yield components may impair grain
yield potential and result in lower yield under high population densities (Sedghi et al 2008).
An open canopy structures increases weed-grain sorghum competition, while narrow row
planting gives grain sorghum a competitive advantage over weeds (Fernandez et al 2012). It
was reported that crop row spacing of less than 76 cm would increase grain yield in areas
with high yield potential with little risk of reduced yield in areas with lower yield potential
(Fernandez et al 2012). Light transmittance to the soil can be reduced by reducing the
distance between rows; reducing distance would improve weed control by increasing crop
competitiveness (Fernandez et al 2012).
An increase in plant density may result in an un-uniform growth and distribution of plants
where there are few large plants and many small plants or to a symmetrical competitive
response where all plants have a decline in biomass production (Wang et al 2005). In the case
of uneven distribution, the change of size among plants occurs, generally increases when
there is competition for light because larger plants may reduce available light to smaller
plants and hence reduce their growth (Wang et al 2005). Smaller individuals might be lost
due to high density depended mortality (Wang et al 2005).
The objective of this experiment conducted is to assess the response of different sorghum
cultivars exposed in different plant densities. This is done by evaluation sorghum growth
from establishment, measure the growth and physiological parameters and final grain yield.

5. 2. MATERIALS AND METHODS
2.1 Plant Material
Three genotypes of sorghum were used, namely, Macia, Ujiba and IsiZulu (imbewu
yesiZulu). Macia is an early to medium maturing (60–65 days to floral initiation and 115–120
days to maturity), semi dwarf (1.3–1.5 m tall with thick stem), and low–tannin open–
pollinated variety. It has good drought tolerance (250–750 mm rainfall range during the
growing season), with stay green characteristics extending beyond harvest. Yield potential is
3–6 t ha-1. Ujiba is a reddish-brown seeded, tall growing (>1.5 m), high–tannin landrace
genotype. IsiZulu is a dark-brown seeded, tall growing (> 1.5 m), high–tannin landrace
genotype. For landraces, phenological, morphological and physiological information was
lacking.
Picture:
2.2 Site Description
Field trials were planted at Ukulinga Research Farm (30°24'S, 29°24'E, 805 m a.s.l) on 16th
of March 2016. The farm is situated in Mkhondeni, in Pietermaritzburg in the subtropical
hinterland of KwaZulu-Natal province. Ukulinga represents a semi-arid environment and is
characterised by clay-loam soils (USDA taxonomic system). Rain falls mostly in summer,
between September and April. Rainfall distribution varies during the growing season
(Swemmer et al., 2007) with the bulk of rain falling in November, December and early
January. Occasionally light to moderate frost occurs in winter (May – July).
2.3 Trial Layout and Design
A B C

6. The experimental design was a completely randomized block design with planting density as
the main factor and genotypes as the sub–factor laid out in randomised complete blocks with
three replicates. The planting density (80 000 plants/ha, 120 000 plants/ha and 200 000
plants/ha) represented low, medium and high planting densities for sorghum respectively.
The trials comprised three sorghum cultivars, namely: Macia, Ujiba and IsiZulu. Each plot
size was 4 m2 because it was 2 m by 2 m, with 0.5 m inter-plot spacing between the plots.
Inter-row spacing was 0.5 m. with 0.30 m. Each individual plot had five rows with the three
inner most rows as the experimental plants, and the remaining rows reserved for destructive
sampling. Seeds were sown closely and thinned to the desired crop density after
establishment.
Picture 1: This shows the field trial sorghum at Ukulinga farm with cultivar heads covered.
2.4 Data Collection
Crop data
Seedling emergence was considered as coleoptile protrusion above soil surface. Weekly
emergence was scored from sowing until establishment (100% emergence). Plant height was
measured weekly from establishment using a tape measure as distance from soil surface to
the tip of the youngest developing leaf (before floral initiation) or tip of the growing panicle
thereafter. Leaf number was counted for fully expanded. A fully formed leaf was defined as
when the leaf collar was visible without dissecting the plant. The flag-leaf was counted as the
first leaf upon full formation.

7. Chlorophyll content index (CCI) was measured using a SPAD-502 Plus chlorophyll meter on
the adaxial surface of the first fully expanded, fully exposed leaf weekly at midday. Stomatal
conductance (SC) was measured weekly at midday using a SC-1 leaf porometer from the
abaxial surface of the first fully expanded, fully exposed leaf.
2.5 Agronomic Practices
Soil samples were collected and analysed for fertility before land preparation. Before
planting, fallow land was mechanically ploughed, disked and rotovated. A deficit of fertilizer
requirements (Smith, 2006) as per soil analysis observation prior to planting was applied
using Gromor Accelerator® (30 g kg-1 N, 15 g kg-1 P and 15 g kg-1 K), a slow release
organic fertilizer at 14 days after sowing (DAS). Planting rows were opened by hand 25 mm
deep and seeds were hand-sown in the ground. Planting was conducted by drilling sorghum
seeds. Thereafter, at crop establishment (14 DAS), seedlings were thinned to the required
spacing. Weeding was done using hand-hoes at frequent intervals.
2.6 Data Analyses
Recorded crop parameters were subjected to analyses of variance (ANOVA) using GenStat®
17th edition (VSN International, UK). Means were separated using least significant
differences (LSD) at a probability level of 5%. Multiple comparisons between means were
conducted using Duncan’s multiple range LSDs.
3. RESULTS
The following graph shows that the leaf area index (LAI) is increasing with an increase in
plant density. Macia and Isizulu cultivars are approximately equal in all plant densities. At
low plant density, the LAI is very low.

8. Figure 3: The physiological response (LAI) of sorghum cultivars in different plant densities
The graph below shows that stomatal conductance is higher in medium plant density
compared to other densities. There is no significant difference between plant density and
stomatal conductance at p ≥ 0.05.
Figure 4: The physiological response (SC) of sorghum cultivars in different plant densities
0
0.1
0.2
0.3
0.4
0.5
0.6
High Medium Low
LeafAreaIndex
Plant density (plants/ha)
Isizulu Macia Ujiba
0
50
100
150
200
250
300
High Medium Low
Stomatalconductance(mmolm⁻²s⁻¹)
Plant Density (plants/ha)
Isizulu Macia Ujiba

9. The chlorophyll content index in the graph below shows that Macia cultivar has more
chlorophyll content in medium plant density than other cultivars. Basically, medium density
has high chlorophyll content index.
Figure 5: The physiological response (CCI) of sorghum cultivars in different plant densities
The graph below shows thousand seed mass (g) in three different plant densities used in this
study. Thousand seed mass is higher in medium plant density and low in low plant density.
Whereas, high is in between. There is a significant difference between plant density and
thousand seed mass at p ≤ 0.05.
0
5
10
15
20
25
30
35
40
45
High Medium Low
Chlorophyllcontentindex
Plant density (plants/ha)
Isizulu Macia Ujiba

10. Figure 6: Thousand seed mass in different cultivars of grain sorghum
The graph below shows that the sorghum leaf number increases as the time increases. Macia
cultivar has more number of leaves than Ujiba and Isizulu cultivars. Isizulu cultivar had few
leave numbers compared to other cultivars.
Figure 7: Relationship between sorghum cultivars and leaf number
0
200
400
600
800
1000
1200
1400
1600
1800
Low Medium High
Thousandseedmass(g)
Plant density (plants/ha)
0
2
4
6
8
10
2 3 4 5 6
Leafnumber
WAP
Isizulu Macia Ujiba

11. As mentioned above that leaf number increases with an increase in time. It also increases
with plant density because the graph in figure 8 clearly shows that high density has high
number of leaves. Yet, low plant density has low number of leaves.
.
Figure 8: The relationship between sorghum plant density and leaf number
The graph below shows the interaction of plant density and sorghum cultivars and the
response of leaf number. Leave number increases with plant density and decrease as plant
density decreases irrespective of a cultivar, therefore there is no significant difference
between cultivars and leaf numbers (p ≥ 0.05).
1
3
5
7
9
11
2 3 4 5 6
Leafnumber
WAP
High Medium Low

12. Figure 9: Sorghum leaf number respond to plant density and cultivar interaction
The graph below represents the plant height vs weeks after planting. It shows a linear
relationship between plant height and time (as weeks after planting), meaning plant height
increases as the time progresses until it reaches a maximum growth. This tendency occurs
regardless of the cultivar type and therefore there is no significant difference between
cultivars and plant height (p ≥ 0.05).
Figure 10: Relationship between sorghum cultivars and plant height (cm)
0
1
2
3
4
5
6
7
8
9
High Medium Low
Leafnumber
Plant density (plants/ha)
Isizulu Macia Ujiba
0
10
20
30
40
50
60
2 3 4 5 6
Plantheight(cm)
Time (weeks after planting)
Isizulu Macia Ujiba

13. The figure below represents the plant height vs plant density. Plant height is high in medium
plant density and low in low plant density. An increase in plant height indicate the growth of
a crop.
Figure 11: The relationship between sorghum plant density and plant height (cm)
The graph below represents the plant height against the plant density and cultivars
interaction. The medium plant density shows an optimum plant height while low plant
density has low plant height (cm).
0
10
20
30
40
50
60
70
2 3 4 5 6
Plantheight(cm)
Time (weeks after planting)
High Medium Low

14. Figure 12: Sorghum plant height (cm) respond to plant density and cultivar interaction
4. Discussion
In the field trial, two weeks after planting emergence was 100 %, other seedlings were even
thinned to ensure the ideal planting density between plants. Therefore, there was no
significant difference in emergence percent among cultivars.
In too low plant population of sorghum canopy photosynthesis is negatively affected due to
less light penetration in the crop canopy and more competition for available nutrients which
adversely affect plant growth and development resulting in low yield (Sibiya 2015). On the
other hand, too high plant population lead to less light interception due to lower leaf index
and more weeds germinate and grow rapidly which also result in lower yield (Sibiya 2015).
In figure 3, the graph shows that leaf area index increases with an increase in plant density,
sometimes leaf area may increase when the nitrogen fertilizer is applied enough. Leaf area is
greater under normal plant density which is basically (200 000 plants/ha) in this study in
different sorghum cultivars. Increasing plant density has significant effect on grain sorghum.
Increasing plant density has significant effect on grain yield. Sorghum cultivars are also
different in their ability to maintain leaf area index above ground dry matter production at
different plant densities. Therefore, leaf area may also vary with cultivars.
Stomatal conductance estimates the rate of gas exchange and transpiration through the leaf
stomata as determined by the degree of stomatal aperture. Figure 4 represent the stomatal
conductance results in different cultivars in different plant densities. Stomatal conductance is
0
5
10
15
20
25
30
35
High Medium Low
Plantheight(cm)
Plant density (plants/ha)
Isizulu Macia Ujiba

15. high in medium plant density which is 120 000 plants/ha in this study. This signify that plants
in medium plant density had their stomata opened greatly which allowed carbon dioxide to
occur and hence indicating that photosynthesis and transpiration rate were potentially higher.
It is important to study chlorophyll content in plants. Long or medium term changes in
chlorophyll can be related to photosynthetic capacity, thus productivity (Anatoly et al 2005).
The graph in figure 5, shows the chlorophyll content index response to cultivars and plant
densities. Macia has high content of chlorophyll meaning that if environmental conditions
are conducive, there is the possibility of high photosynthetic rate. Basically, medium density
has high chlorophyll content index, this may lead to a high dry matter accumulation.
The graph in figure 6, shows that different plant densities had different seed mass, medium
plant density produced larger seed mass (g) than other plant densities. This data indicates that
the medium density has high yield. Therefore, the competition between plants, between
plants and weeds was not as high to decrease final yield of sorghum. The final yield may be
correlated to the overall performance of crops in the field. Crop performance may be
evaluated through measurements of growth parameters (plant height and leaf number)
including physiological response (such as leaf area index, chlorophyll content index,
photosynthetic active radiation data and stomatal conductance).
5. CONCLUSION
As per the objective, it is concluded that sorghum cultivar (Macia) was responding very well
in almost all parameters followed by Ujiba cultivar, therefore it would be very wise to use
this cultivar as a sorghum grain production cultivar in many regions. It is also important to
figure out which plant density is best produce quality and high final yield of a targeted plant.
In this study, 120 000 plants/ha plant density is the one which perform well in
correspondence to sorghum cultivars. This can be seen in crop growth and development
parameters and how are they related to final yield.

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