About two-thirds of Queensland’s lucerne grown for hay is irrigated. The major irrigation areas are the shires of Beaudesert, Boonah, Esk, Gatton, Inglewood, Laidley, Monto and Banana. Most of these shires are in the southern sub-coastal portion of the State and have an annual rainfall of about 700 mm, which mainly falls from October to March.
Irrigation methods
The majority of irrigated lucerne in Queensland is spray irrigated. Low-pressure, portable spray systems are still in operation on many farms, as are alternative high-pressure systems such as travelling irrigators or water winches. In recent years, a range of low-pressure travelling booms, lateral move and centre-pivot systems have become popular in the industry because of their relatively low running cost and more even application patterns in windy conditions.
The most recent development in lucerne irrigation systems is sub-surface drip irrigation (SDI). First installed within the Callide Valley in 1994, there are an increasing number of SDI systems being installed throughout Queensland, and nationally. The main advantages of sub-surface drip irrigation have been increased yields (250/o increase over spray irrigated lucerne) and reduced labour requirements. Sub-surface drip irrigation systems must be properly designed, installed and maintained for this level of performance to be achieved.
Water requirements
An understanding of lucerne water use is important in efficient lucerne irrigation management. A water requirement of 1200 mm annually is necessary for optimum growth of lucerne. The uneven rainfall patter experienced in the lucerne-growing areas of Queensland means that the annual irrigation requirement can vary from 500 mm to 1000 mm (5 to 10 ML/ha). In Queensland, regular irrigation is needed for peak yield and quality.
The high temperatures and evaporation rates, combined with the rapid growth of the semi-dormant cultivars, creates a high demand for irrigation during summer. Irrigation demand falls during winter with lower temperatures and evaporation rates. The introduction of winter-active cultivars, and the need to obtain the best possible production from the semi-dormant lines, makes irrigation management during colder weather important.
Research at Biloela over a seven-year period in the 1950s showed that, on average, it took 0.73 ML of water to produce one tonne of hay (varying from 0.41 to 0.93 ML), and this increased as the plant stand thinned. Therefore, to produce a 2.5 t/ha cut of hay around 200 mm of rain and/or irrigation is needed.
Water use by a lucerne stand is a function of temperature, wind, humidity, sunlight and surface moisture. An actively growing lucerne stand during the peak-growing season in summer may use 10 mm/day of water. If under stress the rate of water use may fall as low as 1 mm/day. Such plants show no above-ground growth and yields are significantly reduced.
Water use varies with the stage of plant growth. Following cutting it may be as low as 25% of that prior to cutting. This may continue for up to a week after cutting. Water use will increase to the pre-cutting rate by 14 to 21 days after cutting — dependent on the rate of regrowth.
The deep root system of the lucerne plant enables it to survive in deep soil types when water is limited. However, in irrigated stands around 900/o of the water extracted from the soil comes from the top metre, due to the high root concentration in this zone.
Irrigation frequency and quantity of water applied
Irrigation frequency varies considerably between farms, and between soil types, making it difficult to generalise about optimum requirements.
A survey of Lockyer lucerne growers conducted by W. Easdown (pers. comm.) during the 1979-80 season found that there were only minor differences between irrigation rates per cut applied in winter or summer. The cutting intervals, however, ranged from 6 to 7 weeks in winter to around 4 weeks in summer.
Easdown concluded from this, and other work, that poor lucerne growth in the Lockyer could most often be associated with sub-standard drainage, poor inoculation, spring planting, low winter fertiliser top dressing and insufficient irrigation.
Seedling lucerne
Following germination, the seedling lucerne puts most emphasis on the production of a root system. Over-irrigation at this time in order to promote quick vegetative growth can result in seedling deaths due to ‘wet feet’, ‘damping off’, phytophthora root rot, or a combination of all three. This problem is even worse on poorly drained soils. In seedlings, as well as mature plants, a reddish-purple discolouration of foliage is a fair indication that root problems are developing due to excessive irrigation or poor drainage.
Lucerne should be planted into a well-worked, moist seedbed. Irrigations up to the first cutting should be fairly light (15-30 mm) and applied at sensible intervals, depending on weather conditions. Lucerne seedlings can withstand a fair amount of water stress. However, if stress is apparent in the early mornings or late evenings, it is probably time to irrigate, particularly if the surface crust has dried out.
Established stands
On most lucerne properties, irrigation is carried out soon after cutting and baling. This practice suits the farm routine and ensures the newly shot crowns get a good start. During peak demands in the warmer weather, two irrigations between cuts are generally necessary to obtain top yields.
Usually, each spray irrigation is in the range of 50-100 mm. Smaller, but more frequent, applications are necessary with SDI systems (applications around 15-30 mm are used). These can vary with soil type and the time of year. Deep, alluvial, basalt soils, such as those in the Lockyer Valley, can store over 100 mm of water, available to plants, within the main root zone. Spray irrigations of 75-100 mm are used on these soils. Sandier (scrub) soils store much less water. They require much more frequent, lighter irrigations (up to 50 mm), and available soil moisture must be held at a much higher level.
In Queensland, soil moisture should not be allowed to drop much below 500/o of soil water levels available to plants. Excessive irrigation can be harmful, particularly on marginal soil types with poor internal drainage. Lucerne does not tolerate poor drainage and, where ponding of irrigation water occurs, plants die out because of waterlogging, irrespective of cultivar. Spray irrigation rates in excess of 100 mm can be wasteful on well-drained soils and detrimental to marginal soil types. A general ‘reddening’ of the lucerne plant is a good indication that waterlogging or diseases through excessive moisture are affecting plant roots.
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