THE SORGHUM PLANT | PLANT GROWTH STAGES
NORTHERN
SECTION 4
PLANT GROWTH
AND PHYSIOLOGY
FEBRUARY 2017
SORGHUM
1 Plant growth anD PhysIology
seCtIon 4 SORGHUM
February 2017
Plant growth and
physiology
4.1 the sorghum plant
The sorghum plant’s botanical name is Sorghum bicolor (L.) Moench, and it is a
member of the Poaceae family.
It is a perennial, tropical C4 grass capable of growing beyond physiological maturity
of the grain. The hybrids available for grain production may be either determinate
or indeterminate in setting the number of tillers for grain production. Although they
can vary, most hybrids grow to ~1 m in height. A sorghum plant produces two ranks
of single leaves in alternate positions on the stem. The leaves have overlapping
sheaths, long, broad blades and, with the exception of the lowest leaf, pointed tips.
Sorghum usually has a dominant stem and, depending on the hybrid and the plant
population, several tillers. A vigorous fibrous root system supports the plant and
provides water and nutrients for shoot growth.
In the absence of subsoil constraints, sorghum roots can extract water to a
depth of 1.8 m.
The sorghum head is a many-branched panicle, with small seeds ~4 mm in diameter.
Current hybrids have either an open or a closed panicle, and the number of seeds
per head varies. Seed weight typically ranges from 24,000 to 37,000 seeds/kg. Seed
typically contains 7–12% protein. Although colour of the seed coat varies from white
through to brown, red is the most commonly grown colour in Australia.
1
4.2 Plant growth stages
Sorghum is a perennial tropical grass with a growing season of 115–140 days. The
rate of growth depends upon temperature and moisture primarily, but it can also be
influenced by soil fertility, insect and disease damage.
2 The optimum temperature
range for growth is 12–34°C.
Growth rates are very sensitive to temperature and moisture, as well as soil fertility
and insect and disease damage. The roots grow at ~2.5 cm/day.
Sorghum has 10 recognisable growth stages that can be used to plan irrigation,
desiccation, insect scouting and insect control. Leaves are counted when the
collar (where the leaf blade and leaf sheath attach) can be seen without cutting the
plant apart.
3
1 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
2 N Moore, L Serafin, L Jenkins (2014) Grain sorghum. Summer crop production gu\
ide 2014, pp. 5–16, NSW Department of Primary
Industries, http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0005/303485/Summer-cro…\
.pdf
3 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/agriculture/
broadacre/summer-crops/sorghum/sorghum/grain-sorghum
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2 Plant growth anD PhysIology
seCtIon 4 SORGHUM
February 2017
Figure 1:
Stage0 1
Vegetative phase
Floral phase Grain \flling phase
Emergence
\blanting
Height (cm)
Days 0
-10 3 912 24 35455055 60-70 7590 95-110
10
20 35 100 120
3 leaves
fully
emerged 5 leaves
fully
emerged Head
initiation Flag leaf
visible Boot
Flowering Soft
dough Hard
dough \bhysiological
maturity
2
3 45 67 8 9
\bre-plant applicatio\nn \bost-emergent applica\ntion \bost-emergent applica\ntion Avoid spraying Avoid spraying Desiccation,
harvest aid
Sorghum growth stages.
1. Stage 0. Germination
Emergence usually occurs within 3–10 days of planting. Under warm temperatures,
adequate soil moisture, good seed vigour and normal sowing depth, the time taken
from planting to emergence is closer to 3 days. Sorghum has hypocotyl emergence,
meaning a shoot emerges from the seed and pushes through the soil surface.
Under cold soil temperatures, it takes longer for the shoot to emerge and the risk of
insect or disease attack is higher.
4
Conditions for germination include a temperature range 16–18°C (a\
nd rising),
soil moisture near field capacity, seed depth of ~5 cm and a seedling vigour
percentage >90%.
The seed provides the seedling with nutrients and food reserves. The hypocotyl
or young shoot extends from the seed, taking up water. At around day 5, a fibrous
root system begins to form from the hypocotyl. During this period, the seed must be
protected against soil insect attack. If conditions turn cool and wet, fungal diseases
such as pythium may cause losses through seedling death.
5
2. Stage 1. Three-leaf stage
The growing point is still below the soil surface. This stage will usually begin ~10 days
after emergence. Following shoot emergence, leaves will progressively unfold.
6
3. Stage 2. Five-leaf stage
This occurs ~21 days after emergence. The growing point is still below the soil
surface. At the 5-leaf stage, the root system is rapidly expanding and roots produced
at the lower nodes may push the lower leaf off the plant. The lowest (first) leaf always
has a rounded tip, in contrast to the pointed tips of later leaves.
4 N Moore, L Serafin, L Jenkins (2014) Grain sorghum. Summer crop production gu\
ide 2014, pp. 5–16, NSW Department of Primary
Industries, http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0005/303485/Summer-cro…\
.pdf
5 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www. dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
6 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
fi-fl-
3 Plant growth anD PhysIology
seCtIon 4 SORGHUM
February 2017
During this stage the yield potential is set. If water, sunlight or nutrients are limiting,
yields can be restricted. Dry matter production occurs at nearly a constant rate
between this stage and maturity.
7
4. Stage 3. Growing point differentiation
The growing point changes from vegetative (leaf-producing) to reproductive (head-
producing) ~30 days post-emergence.
At this stage the total number of leaves has been determined and the potential head
size will soon be defined. Around 30% of the total leaf area will have developed,
correlating to ~one-third of the time taken between planting and physiological
maturity. This equates to 7–10 leaves, with the lower 1–3 leaves being possibly
lost. Following growing-point differentiation, stalk or culm growth incr\
eases rapidly.
Nutrient uptake is also rapid. This rapid growth allows sorghum plants to be very
competitive against weeds for the rest of the season.
8
5. Stage 5. Boot stage
Leaf area is now at a peak, providing maximum light interception. The head, still
enclosed in the flag leaf sheath, is almost developed to full size. The peduncle has
begun to elongate and will result in the head becoming visible from the flag leaf
sheath. Potential head size is determined. Stressful conditions from lack of moisture
or herbicide damage at this stage may stop the head completely growing from the
flag sheath. Head death can occur earlier than booting under extreme moisture stress
in combination with high temperatures. The sorghum plant will preferentially favor
carrying through the primary tiller. Head death can occur earlier than booting under
extreme moisture stress in combination with high temperatures. This failure will hinder
complete pollination at flowering.
9
6. Stage 6. Flowering
The peduncle rapidly elongates, pushing the head through the flag leaf sheath. Half-
bloom is usually defined as the stage when 50% of the heads in the paddock are
50% flowered; however, it may also relate to an individual plant. The time required
from planting to 50% flowered depends on the hybrid as well as the environment.
Modern hybrids usually take 55–80 days, which represents 50–70% of the time
between planting and physiological maturity.
Sorghum is primarily self-pollinated. Flowering begins at the top and proceeds
downward requiring 4–5 days for the whole head to flower. The time a hybrid takes
to start flowering depends largely on temperature. For example, a medium-maturity
hybrid planted in the cooler, early October period at Moree flowers in ~80 days.
However, if planted in the warmer mid-November period, it flowers in 60 days.
Severe moisture stress and/or very high temperatures during flowering can result in
pollen-blasting and poor head-fill. Pollen-blast can be an issue at temperatures >36°C.
Low night temperatures
Once pollination has occurred, seeds will begin to form, taking ~30 days to reach full
development. Visually, the seeds become rounded, up to around 4 mm in size, and
will then start to change colour. The final colour varies from white with hybrids such as
Liberty White through to red or brown in most sorghum hybrids.
10
7. Stage 7. Soft dough
Soft dough is the stage after approximately half the grain dry weight has accumulated.
Following an increase in culm weight after flowering, the culm decreases in weight
as the grain rapidly forms. This period includes the time the grain is of soft as well as
medium dough consistency.
7 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
8 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
9 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw. gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
10 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
fi-fl-
4 Plant growth anD PhysIology
seCtIon 4 SORGHUM
February 2017
As dry matter accumulation is similar across hybrids, the length of this period will
strongly influence grain yield, with the longer maturing hybrids outyielding the quicker
maturing lines. This holds as long as the plants are not subjected to moisture stress
or frost during this period.
11
8. Stage 8. Hard dough
Approximately 75% of the grain dry weight has accumulated by hard dough
stage. Nutrient uptake is essentially complete. Some lower leaves may now have
been lost.
12
9. Stage 9. Physiological maturity
The grain is said to be physiologically mature when a black spot appears at the point
where the seed attaches the plant. At this time, the seed is fully mature and will not
gain any more nutrients or moisture from the plant (Figure 2). The moisture content \
is
usually ~30% at this time.
If using desiccation, this is the optimum time to apply a registered her\
bicide to kill the
plant, preventing additional moisture use.
13
Figure 2: Ripe sorghum grain.
The length of time between physiological maturity and harvest depends on a number
of factors including whether desiccation is used and environmental conditions such
as temperature.
14
The plant has reached maximum total dry weight and maximum grain weight.
Physiological maturity can be determined by the formation of a dark spot on the grain
on the opposite side from the embryo—otherwise known as black-layer formation.
While the seeds develop, nutrients and moisture flow through a series of tubes from
the parent to the seed.
Once the grain reaches its maximum size, these tubes become permanently blocked
off. This is because the water in the seed during the soft and hard dough stages is
displaced with starch, until water can no longer move in. The seed therefore dries
out, typically from ~30% down to 10% or air-dry level. The black layer appears first in
the seeds at the top of the head.
15
11 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
12 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
13 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
14 N Moore, L Serafin, L Jenkins (2014) Grain sorghum. Summer crop production gu\
ide 2014, pp. 5–16, NSW Department of Primary
Industries, http://www.dpi.nsw.gov.au/__data/assets/pdf_file/0005/303485/Summer-cro…\
.pdf
15 NSW DPI (2005) Grain sorghum. Agfact P3.3.5, NSW Department of Primary\
Industries, http://www.dpi.nsw.gov.au/__data/assets/
pdf_file/0006/146355/grain-sorghum.pdf
i MORE INFORMATION
http://www.dpi.nsw.gov.au/__data/
assets/pdf_file/0006/146355/grain-
sorghum.pdf
http://www.dpi.nsw. gov.au/
agriculture/broadacre-crops/summer-
crops/sorghum/general/summer-
crop-production-guide
http://www.dpi.nsw.gov.au/__data/
assets/pdf_file/0007/431278/
sorghum-spray-out-timing.pdfa
fi-fl-
