By Frank Kamuntu
Wheat undergoes various stages before it reaches maturity. Farmers need to know what these stages are if production is to be optimal, says Dr. Eric Morojele.
Knowing at which stage of development wheat is most vulnerable to certain pests, diseases or weed increase can significantly help to reduce production losses.
It will also help to ensure that wheat receives fertiliser and other soil-improving materials at the time when the crop will benefit most from it in terms of accelerated growth, quality and yield.
Scientists have identified numerous growth stages based on vegetative parts, the number of leaves unfolded and the development of stem tips.
As a farmer, you can use the following phases to monitor growth:
- Emergence and seedling growth.
- Tillering.
- Stem elongation.
- Forming heads and flowering.
- Physiological maturity.
During each phase, significant features develop that assist in marking the end or beginning of a specific developmental process.
EMERGENCE AND SEEDLING GROWTH
- This stage, which occurs when the plumule breaks through the surface of the soil, takes 8 to 12 days, depending on environmental conditions such as temperature and moisture.
- It follows germination.
- For germination to occur the seed must be viable and there should be adequate moisture and air in the soil.
- Soil moisture has to be absorbed by the seed to trigger the germination process, after which the plumule and the radicle emerge.
- The radicle develops into the root, and the plumule grows to form a stem.
- When purchased, seed should have a tag indicating a germination percentage of more than 85% and it should be treated with chemicals to protect it from soil-borne diseases.
The seed bed should be prepared well to promote germination.
It should be:
- A fine tilth to allow the plumule to push through the soil to the surface and the radicle to easily penetrate deep into the soil.
- Well-aerated to provide the air required for germination.
- Level, and the soil should be friable (loose) so that rain or irrigation water can easily enter the soil.
- Broken down by a harrow so there aren’t any big lumps.
TILLERING
- If all conditions are favourable, tillering (the development of lateral shoots) should occur 6 to 8 weeks after emergence.
- For a winter cultivar, a low temperature (-3°C) is required to stimulate the development of lateral branches.
- The low temperature retards the growth rate of the stems, making them prostrate.
- The number of tillers produced by a plant depends on the seeding rate, sowing date, nutritional minerals, and the cultivar.
- Some tillers die because of the competition for light, water and or nutrients.
- Other tillers survive and produce ears.
- Look out for pests and infestations of weeds as the temperature rises.
- The most problematic pests in wheat are Russian wheat aphids, which can be controlled by chemicals.
- The way to avoid this pest is to purchase a resistant cultivar.
- If the relative humidity is high, diseases such as downy mildew, yellow rust, stem rust and leaf rust may also develop.
- All these diseases can be controlled chemically.
- Weeds – in particular nutsedge (Mutabatabane) and wild oats – have to be controlled at an early stage.
- Other weeds are easily controlled by post-emergence herbicides.
- Many herbicides are available for the control of weeds.
STEM ELONGATION
- This stage takes 4 to 6 weeks from tillering, depending on the environment and cultivar.
- When the temperature starts to rise, prostrate tillers become erect and increase in length, developing the nodes and internodes along the stem.
- The number of lateral branches stops developing, but the leaves unfold continuously as the stem elongates.
- Head formation starts at this stage.
- Many organs develop, such as booting and flag leaf formation.
- Booting indicates that the head is forming. Flag leaf is an important stage, responsible for grain filling.
- At this stage, you need to inspect the fields to determine the extent of pest, disease and weed infestation.
- It is extremely important to apply chemicals when the infestation has reached a specific level; before it gets out of control and there may be yield losses.
- Nitrogen should be applied as top dressing at flag leaf.
- The most common nitrogenous fertilisers are lime ammonium nitrate (applied in dryland conditions) and urea (applied where irrigation is employed).
- Both should immediately be covered with soil.
- Urea is highly concentrated, so it needs water immediately after application, otherwise it burns the plants.
- Nitrogen applied at flag leaf stage contributes significantly to the protein content of the grain.
FORMING HEADS AND FLOWERING
- At this stage, the heads appear from the apex of the stem, covered by the sheath.
- Flowering and pollination occur 2 to 7 days after the heads form, after which the grain starts to develop.
- Flowering starts midway down the spike and progresses downwards and upwards.
- When the yellow anthers form around the florets, it marks the end of flowering.
- Optimum temperature and adequate soil moisture are critical at this time.
- If temperatures are too low or too high, the yield will be adversely affected.
- Extreme cold can cause flowers to become sterile.
- It takes 4 to 6 weeks for the stem to elongate.
- Once again, you have to inspect the crop for pests and diseases.
- Birds may start causing serious damage; unfortunately, nothing has been found that controls them.
- The damage may not be so severe, however, if you plant wheat over an extended area.
PHYSIOLOGICAL MATURITY
- Wheat grain starts growing immediately after flowering and reaches its maximum size within about 2 weeks.
- The maximum weight – which is determined by temperature – occurs 4 to 6 weeks after flowering.
- Low temperatures may prolong the period.
- At the start of development, the grain has the colour and consistency of milk.
- As it fills, the endosperm thickens into first a soft, and finally a hard, dough.
- Maturity is reached when the grain has accumulated its highest content of dry matter, has hardened and changed its colour to gold.
- The stage could be prolonged if there is a lot of rain, as this will delay the drying of the grain and wheat stalk.
- African bollworm can cause serious damage to grains if not controlled.
- Diseases that affect flag leaf may reduce the yield significantly during this stage of wheat development.
