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

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

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).

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.

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 16 th 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

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.

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.

High Medium Low0

0.1

0.2

0.3

0.4

0.5

0.6

Isizulu Macia Ujiba

Plant density (plants/ha)

Leaf

Are

a In

dex

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.

High Medium Low0

50

100

150

200

250

300

Isizulu Macia Ujiba

Plant Density (plants/ha)

Stom

atal

con

duct

ance

( m

mol

m² s

¹)⁻

⁻

Figure 4: The physiological response (SC) of sorghum cultivars in different plant densities

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.

High Medium Low05

1015202530354045

Isizulu Macia Ujiba

Plant density (plants/ha)

Chl

orop

hyll

cont

ent i

ndex

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.

Low Medium High0

200

400

600

800

1000

1200

1400

1600

1800

Plant density (plants/ha)

Thou

sand

seed

mas

s (g)

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.

2 3 4 5 60

2

4

6

8

10

Isizulu Macia Ujiba

WAP

Leaf

num

ber

Figure 7: Relationship between sorghum cultivars and leaf number

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.

.

2 3 4 5 61

3

5

7

9

11

High Medium Low

WAP

Leaf

num

ber

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).

High Medium Low0123456789

Isizulu Macia Ujiba

Plant density (plants/ha)

Leaf

num

ber

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).

2 3 4 5 60

10

20

30

40

50

60

Isizulu Macia Ujiba

Time (weeks after planting)

Plan

t hei

ght (

cm)

Figure 10: Relationship between sorghum cultivars and plant height (cm)

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.

2 3 4 5 60

10

20

30

40

50

60

70

High Medium Low

Time (weeks after planting)

Plan

t hei

ght (

cm)

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).

High Medium Low0

5

10

15

20

25

30

35

Isizulu Macia Ujiba

Plant density (plants/ha)

Plan

t hei

ght (

cm)

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

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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