Adequate soil moisture is required for normal development of the wheat plant at all the growth stages. Since there is very little or no rainfall during winter in the Sub Saharan Region, irrigation is required to achieve a good wheat crop. The total gross amount of water required is about 500-600 mm per ha (ie 5-6 mega litres per ha), applied as the crop requires it.

Irrigation is critical and as such it chews up to 50 percent of the per hectare cost structure. It is therefore recommended to use the irrigation economically. We encourage farmers not to under-apply water as this will result in loss of yield and affects your topline (income per ha) and bottom line (margin) stories. Equally do not over apply as this causes your cost structure to balloon and affects your bottom line (margins). In worst cases over applying water will promote disease development and ultimately affect the yield.

It is therefore imperative to schedule your irrigation, so that water is applied as and when is needed by the crop from establishment up to maturity. This is very critical! This week we will give guidelines and considerations on irrigation (sprinkler and centre-pivot) scheduling as we prepare our farmers for a possible wheat bumper harvest. A green revolution is upon us.

1. Plan ahead: evaluate available water resources in order to calculate wheat area based on proposed gross application. Irrigation equipment must be ready, with checks made on pumping unit, conveyance system, sprinkler condition and nozzle wear.

2. Start at Field capacity: Crop emergence requires a soil profile that is at field capacity down to the full potential of the rooting depth. This should be achieved by the 3–4 leaf stage, at the latest. This is important because wheat roots grow downwards at a rate of 20–30 mm/day and any dry layers within the profile will impede root growth and proliferation.

3. Establishment irrigation: Seed germinates readily in the presence of soil moisture. Establishment irrigations need to be geared to achieve a uniform and adequate stand, and this depends on planting method and uniformity of irrigation. Drilled seed normally requires one good irrigation to cause germination because of good soil-seed contact. Broadcasted seed or zero tillage fields, require frequent (2–3 day intervals) light irrigations (25mm) to effect establishment. A light irrigation is essential (4-7 days after the first irrigation) in soils that are prone to crusting to assist with emergence.

4. Ensure crown root development and tillering: At 3-4 leaf stage (14-17 days after the first germination irrigation), crown roots and the ear begin to develop and tillers start growing. Water deficit adversely affect these processes yet they play an important role in yield formation. At this stage, usually the top 100–150mm of the soil is dry and crown roots will not grow into dry soil. It is necessary to apply a light irrigation to stimulate crown roots and tillering. It is also an appropriate time to top dress the wheat with Nitrogen fertiliser.

5. Initiate an irrigation schedule early and monitor the soil and crop through to maturity: Scheduling assist the manager to monitor crop progress and thereby ensure the best treatment possible is given to the crop. Assess soil and crop conditions before and after each irrigation cycle to evaluate whether or not the irrigation is recharging the soil profile to the satisfaction of the plant needs. A soil auger is extremely useful in this regard. An auger test ahead of the line will show how deep the plant is drawing water while an auger test two positions behind the line will show how effective the irrigation application is in replenishing the soil. Well irrigated wheat has a dark green hue, soft large leaves and many tillers, whilst “stressed”wheat has a bluish hue, hard, spikey leaves which may also roll up in some varieties, and a few tillers with small ears.

6. Timing of the last irrigation: There is no point in irrigating a yellowing crop and grains are fully formed and after hard dough stage. Physiological maturity is reached when the peduncle (neck, area below the ear/spike) turns yellow. Irrigation applied during later grain-fill or during grain dry down is of no value to the crop and may even reduce the quality of the grain. Water after ripening may cause pre-harvesting sprouting (germination in the ear) leading to down grading of wheat due to reduced grain quality.

7. Keeping irrigation records: It helps to plan future irrigation practices. Useful records include;

(a) Water usage with a flow meter

(b) Energy use, either electricity units or diesel litres

(c) Dates and amounts of irrigation applied

(d) Evaporation and air temperature

(e) Labour

Methods of Irrigation

Scheduling – Sprinkler

All systems depend on one fundamental point: start the crop with the soil at field capacity to the full potential rooting depth. From there on, some systematic irrigation schedule should be imposed to obtain the best results.

1. Simple methods:

(a) Set the cycle length at the start and maintain that throughout the season. Common in deficit irrigation whereby a manager (by experience) knows the optimum length of irrigation interval for his soil type and climate. However, there are no checks made on the soil water balance, so the manager has little idea whether the crop is being adequately or inadequately irrigated, and irrigations may not coincide with important plant growth stages. As a guide the following table gives the maximum allowable cycle lengths for deficit irrigation.

(b) Schedule irrigation at particular plant growth stages. These for the Highveld are;

i. Germination and ear emergence

ii. Crown root development and tillering (3–4 weeks)

iii. Appearance of the first node and start of shoot elongation (6–7 weeks)

iv. Booting and flag leaf emergence (9 – 10 weeks)

v. Ear emergence and anthesis (Flowering) at about 12 weeks

vi. Grain filling from 13–15 weeks

Note that two most critical growth stages for wheat are crown root development and from booting (first node) to milk dough stage.

These growth stages occur earlier in the middleveld and lowveld and timing may vary depending on variety grown. The method ensures that plants receive water at appropriate critical times, but for a manager to maximize profits per hectare, more frequent irrigations will be required. The method does not give information on the soil water status.

2. Soil Water Budgets

Calculated from estimates of potential evaporation and crop factors. Crop factor represents the ratio of crop water use (Et) to potential evaporation (Eo), and varies through the season with crop growth stage. For a given day, the crop water use can be calculated by multiplying the daily evaporation with the appropriate crop factor. This daily crop water use can be cumulated from day to day and irrigation applied when the total crop water use equals the irrigation application of a convenient set time. This method may use either historical or current evaporation data;

(a) Historical Meteorological Data

Used in the absence of current evaporation data and the schedule can be prepared at the start of a growing season. Monthly evaporation data tables for Zimbabwean sites can be obtained from the Meteorological Department.

Therefore during this period the interval between irrigations is 15 days. A new figure has to be calculated between each irrigation and from month to month. The system does not take into account daily variation in weather conditions. The figures are averages of averages and therefore do not allow for extremes when used on a daily basis.

(b) Current Evaporation Data

Utilizes evaporation data recorded from a Class A pan located in the appropriate situation or it may use data collected by an automatic weather station. Pan location is critical for the success of this system. There are two methods of scheduling using current evaporation data.

(i) Daily evaporation is multiplied by the appropriate crop factor to give the daily crop water use which is totalled each day until it equates the irrigation application of the scheme, when irrigation is applied.

(ii) Daily evaporation is entered onto a specially prepared graph. The daily readings are accumulated until they reach pre-determined line on the graph which acts as the trigger for irrigation application. These graphs are supplied by ZCPA and are specific for three factors:

• Crop

• Region – low, middle or Highveld

• Amount of net irrigation application

Seed Co has a computer irrigation scheduling programme and is based on evaporation measurements from a Standard Class A Evaporation Pan. Farmers interested in using this should contact their local Seed Co representative.

Centre pivot-irrigation scheduling – A general guide.

Generally centre pivot irrigation is the simplest method of irrigating any crop. For efficiency, there are factors to consider when using centre pivots for water application. It is proven that a farmer gets more effective water application on a fixed centre pivot as compared to a towable pivot. This is largely due to the fact that there is run down time loss due to towing from one centre to the other.

It is advisable that when using a fixed centre pivot anything between a 10mm and 12 mm spray package is recommended. However if it is a towable centre pivot and a farmer intends to do two circles with one pivot a bigger spray package is more ideal for the pivot and this can be from 14 mm to 20 mm spray package depending on specific requirements. A bigger spray package is recommended for towable centre pivots to reduce the turnaround time of the centre pivot to avoid moisture stress in the other circle.

For easy water application, a farmer is advised to run their pivot in WET mode. The wet mode allows the operator to program the pivot to apply the exact amount of mm required at the particular stage of growth of the crop.

In instances where the pivot is run in dry mode the operator will be required to calculate the percentage on the timer which corresponds with the mm that need to be applied and in most cases errors on calculation are sometimes common and a farmer will not achieve the intended spray volume results. It is advisable then that farmers should ask their centre pivot service provider to program the machine to work in the wet mode.

Chemigation/fertigation

calibrations guide.

Calibration factors that need to be considered when using a centre pivot for chemigation and fertigation include the sizing of the dosing pump and its pumping rate. Always ensure you discuss with your pumps specialist before purchasing a dosing pump for correct dosing pump sizing for your applications as applications vary from case to case. It is also important that your fertigation or chemigation unit is as close as possible to the centre pivot inlet as possible generally not more than six metres. Below are critical factors to be considered when using a pivot for both chemigation and fertigation.

1. Length of the pivot to the edge of the effective wetted area

2. Length of the pivot to the last tower

3. Last tower travel distance in a given amount of time running at present application. This point has to be verified physically by the farmer with the pivot running in wet mode at the present application rate. Do not rely on literature or your pivot control panel as other factors such as terrain (e.g. slope/gradient) can affect your last tower run speed-so this must to be verified.

4. Targeted product application rate in kgs/litres per hectare

5. Product concentration in kgs/litres per m3 of active ingredient

6. Percentage of a full circle centre pivot that will be used during the application

 About the authors

  • 1.John Basera is Seed Co Head of Agronomy Services. He can be contacted on +263 772 413 184/ [email protected].
  • 2. Tegwe Soko is Seed Co Group Wheat and Small Grains Breeding Programme Lead. He can be contacted on +263 772 516 669/  [email protected].
  • 3. Tendayi Marekera is an irrigation consultant working for Sitelink Investments (suppliers of Senter 360 pivots and irrigation equipment). He can be contacted on +263 772 530 818/ [email protected]