Irrigation of lucerne for hay	AG0230
John Bourchier, Cobram March, 1998

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Important factors for successful irrigation of lucerne

Soil characteristics

Soil wetting ability. Lucerne will produce high yields on a wide range of soil types providing that the soil is sufficiently well structured to allow at least 75 mm of water to penetrate the soil profile before excess run-off occurs. If problems with water penetration are expected one should consider improving the soil structure by treatments such as gypsum application, deep cultivation or ripping.

Drainage. Good surface drainage is essential for irrigated lucerne. A slope of 0.125% (1 in 800) or greater is recommended and should be capable of draining excess irrigation or rainfall within 8 hours. Soils with restricting clay layers that cause watertables shallower than 1m must be avoided. Lucerne will not survive for long on such soils.

Water-holding capacity. Soil texture largely determines the water storage capacity of the soil. Table 1 shows the amount of available water (available for crop use) stored in a range of soils to 1m depth (provided the soil is fully wet).

Water requirements of lucerne
Lucerne has a high requirement for water and can use 7-10 megalitres of irrigation water per hectare over a full irrigation season in addition to normal rainfall. Water use by lucerne is closely related to evaporation from an open water surface. Lucerne will use water at a rate of about 80% of the evaporation that would occur from a Class A pan evaporimeter, except for a few days after cutting the lucerne when water use is about 40% of evaporation. For example, if 75 mm of water is applied as an irrigation or falls as rain and the daily water use is 5.0 mm then the 75 mm of water applied will last 15 days provided that no further rain falls (see Table 2).

Table 1. Available water of various soils to 1 m depth.

Soil texture	Available water per 1 m depth
Light soils
Sand Sandy loam Fine sandy loam	46 mm 106 mm 148 mm
Heavy soils
Clay loam Clay	157 mm 212 mm

Irrigation method
Lucerne can be irrigated by flood or spray. Medium and heavy textured soils where slopes are flatter than 1% (1 in 100) are best flood irrigated. Steeper slopes and highly permeable soils are more suited to spray irrigation. Irrigation layouts, whether for flood or spray, should be designed professionally to suit the soil type, general farm layout, water supply levels, drainage points and possible re-use of water. Precision land grading using laser controlled graders is preferable for flood irrigation to ensure accurate and uniform slopes.
Spray irrigation systems must be designed to have the capacity to keep up with water requirements of the crop and have enough flexibility to allow choice of irrigation amount and frequency of application.

When should lucerne be irrigated?
Irrigation should be applied before symptoms of water stress are apparent in the crop in order to maintain active growth and maximise yields. Common symptoms of water stress in lucerne include:

wilting and lack of vigour growth appears a dull green colour lower leaves turn yellow and fall off flowers appear prematurely new crown shoots are slow to appear regrowth after cutting develops only slowly.

The techniques described below can be used to estimate when lucerne is due for irrigation.

Soil testing for irrigation requirement
A simple soil test can be used to determine when a soil is dry enough to irrigate. A soil sample is taken using a soil auger, from 400 mm depth in the soil and tested as follows:

Loams and clay loam soils need water if a sample of clay subsoil cannot be rolled into a rod 25 mm long by 3 mm diameter between the fingers. A sandy soil needs water if a pressed ball of soil will not hold together.

Use of evaporation data
Evaporation is recorded at many country localities and is a reliable guide for predicting the number of days it takes lucerne to use water stored in the soil. In the major irrigation areas evaporation readings are usually available through the media. These daily readings, when multiplied by 0.8, will give the approximate daily water use of lucerne. As a starting point in calculating when irrigation is due, you must either estimate the amount of water stored in the soil or calculate the amount of water applied at the last irrigation as follows:

depth of water applied = by irrigation ( mm)

megalitres applied area irrigated in hectares

x 100

The next irrigation is due when the total of daily water use (measured in mm) since the last irrigation matches the depth of water applied at the last irrigation. Rainfall of 6 mm or more should be treated as additional depth of water to the last irrigation and the next irrigation should be delayed accordingly. As a broad guide to the intervals between irrigations that might be expected during the irrigation season, table 2 shows average daily water use for three irrigation districts and the required intervals between waterings. For the purpose of calculations it is assumed that 75 mm irrigations are applied at each irrigation and that no rain falls between irrigations. In practice the calculations should allow for the actual evaporation that occurs between irrigations, for any rain that falls, and for the actual amount of irrigation water applied each time.

Tensiometers
Tensiometers are simple instruments partly buried in the soil to determine soil dryness. They are most useful in moist to wet soils, but they do not operate in over-dry soils. Some practice is required for their proper use and maximum benefit.

How much water should be applied?

The amount of water applied at each irrigation will depend on the method of irrigation, the soil type, and the amount of water already held in the soil. A predetermined amount of water is easily applied by spray irrigation on light soils. However, on heavier soils and on flood irrigated soils one usually has little control over how much water actually goes into the soil, regardless of the method of application or the amount of water applied.

Table 2. Guide to average daily water use and irrigation intervals during the lucerne growing season in Swan Hill, Tatura and Maffra

	Average daily water use* of lucerne (mm/day)			Required interval for 75 mm irrigations if no rain falls (days)
	Swan Hill	Tatura	Maffra	Swan Hill	Tatura	Maffra
September	2.3	2.2	2.6	33	34	29
October	3.4	3.1	3.1	22	24	24
November	4.8	4.5	3.9	16	17	19
December	6.2	5.8	5.2	12	13	14
January	6.4	5.9	5.2	12	13	14
February	5.4	5.2	4.8	14	14	16
March	3.9	3.9	3.6	19	19	21
April	2.5	2.3	2.4	30	33	31

*Water use is assumed to be 80% of Class A pan evaporation

Most soils suited to lucerne will hold between 100 and 200 mm of available water within the 0 - 1.0 m depth. As most of the effective roots for water extraction are found within this depth there is generally no benefit in wetting below 1.0 m. Amounts of 75-100 mm of water per irrigation are best suited to lucerne. Excessive irrigation may result in wastage of water due to water penetrating too deeply into the profile. Irrigating too quickly with sprinklers will cause excess run-off.

Light irrigations (that is, less than 60 mm) will require an increased number of irrigations during the season; this will increase production costs and increase the danger of scald on heavier soils (see below). Although, on most soils, there is little benefit in wetting the soil deeper than 1 m, there are considerable management and economic benefits to be gained from increasing the water penetration within the top 1 m of the soil profile.At Tatura, for example, lucerne yielded 11 t/ha over four hay cuts where two irrigations per cut of hay wet the soil to 400 mm depth. It yielded 17 t/ha where two irrigations per cut of hay wet the soil to 700 mm. If shallow irrigation is suspected, the depth to which water penetrates into the soil can be checked using a 10 mm diameter spring steel probe with a 12 mm diameter pointed tip. The probe is pushed by hand into the wet soil one day after irrigation to indicate the depth to dry soil.

Suggested irrigation strategy

For high yields, lucerne is commonly irrigated twice and sometimes three times for each cut of hay. The first irrigation is applied within 6-8 days after cutting the lucerne and subsequent irrigations are applied according to the practice described earlier. A second irrigation is usually sufficient to last until the lucerne is cut again and also provides enough stored water in the soil to initiate new lucerne growth after cutting. Three irrigations per cut of hay at closer intervals may be necessary if poorly structured soils prevent water penetration.

Irrigation should be applied only when it is safe to irrigate the wettest part of the crop. Slow-drying areas may occur because of changes in soil type, or because some areas wet more deeply than others. Irrigation of flood bays should be completed within 8 hours. Irrigation times of from five to eight hours per bay are common.

Scald

Scald is the rapid death of lucerne in hot wet soils. The danger of scald is greatest when lucerne is irrigated immediately after cutting. It is most likely to occur when there is a combination of irrigation water exposed to the direct sun, high air temperatures and low leaf cover of the lucerne. Lucerne with some leafy growth is more tolerant. Generally irrigation should be delayed after cutting if temperatures exceed 36°C, especially if surface water is normally slow to drain away.

Irrigating a young stand

Spring-sown lucerne requires special care with irrigation through its first year to establish a highly productive stand. Lucerne establishes best when sown into a soil profile having a moist subsoil as a result of irrigation or winter rainfall. In these conditions the tap root will be encouraged to exploit subsoil moisture (provided no hard pans prevent roots from growing). The lucerne should in most circumstances grow for several weeks and produce a number of true trifoliate leaves before the first irrigation is required. Over-watering at an early age may damage the tap root, cause yellowing and stop growth. On the other hand the situation should be watched closely and water applied according to the principles discussed earlier in this Agriculture Note, well before the seedlings show signs of moisture stress.

Over-watering in the first year can also cause heavy weed growth, which in turn will depress lucerne yields and slow lucerne establishment. Appropriate weed control measures should be taken before sowing and during establishment to allow rapid establishment of lucerne.

Useful irrigation data

1 hectare (ha)	= 10 000 square metres
1 megalitre (ML)	= 1 000 000 litres
1 inch	= 25.4 millimetres (mm)
1 gallon	= 4.54 litres
1 megalitre will cover 1 ha to a depth of 100 mm.

Delivery of Dethridge meter wheels:

Small meter outlet (SMO) at 5 revs/min = 2.4 ML/day
Large meter outlet (LMO) at 5 revs/min = 6.0 ML/day




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