Lucerne - the plant and its establishment

Lucerne - the plant

Lucerne or alfalfa (Medicago sativa) is a deep-rooted, temperate, perennial pasture legume which is well adapted to mixed farming systems in southern Western Australia.

Due to its high water use and deep roots (>two metres on suitable soils) it is able to address rising watertables and associated salinity.

Lucerne will provide additional green feed at the start and at the end of the normal winter growing season with peak production in spring and early summer.

It has the ability to respond quickly to significant summer rainfall (>10 millimetres) but requires 20-25 millimetres (mm) to produce substantial growth.

A rain-fed lucerne pasture produces between 4-8 tonnes (t) of dry matter per hectare per year (DM/ha/yr) which is similar to annual pasture, but production is spread more evenly over the year.

Once established, lucerne has good drought tolerance and is well suited to irregular rainfall patterns, but it will appear to go dormant during extended dry periods.

It grows in areas receiving as little as 325mm annual rainfall but also provides good summer production in areas up to 700mm rainfall. Out-of-season production can be used to reduce supplementary feeding requirements.

Lucerne produces high quality green feed. It has high energy — digestibility of 65-72% with a metabolisable energy of 8-11 megajoules per kilogram (MJ/kg) DM — and high protein (12-24%).

The quality of feed remains relatively constant throughout the year while it is active. Lucerne is also a source of calcium, magnesium, phosphorus and vitamins A and D.

Lucerne can be grown as a pasture phase, removed and followed by a crop phase or it can be over sown with crops (pasture cropped).

Lucerne fixes between 10 and 20kg/ha of nitrogen for every tonne of dry matter produced, increasing soil nitrogen levels for subsequent crops.

Once established, it can help manage herbicide resistant weeds with its competitiveness and tolerance of some broad-spectrum herbicides.

Effective weed management will increase the legume component and nitrogen accumulation from a lucerne based pasture. Also, grain crops following a lucerne phase produce better yields and quality than unimproved pasture.

The principles for integrating lucerne into broadacre dryland farming systems are described in the Department of Agriculture and Food, Western Australia's (DAFWA) Bulletin 4785 - Lucerne Guidelines for Western Australia. The bulletin is available in hard copy from department offices or can be emailed in PDF format on request.

In brief, lucerne can:

• provide a high quality feed for livestock and improve animal health
• reduce groundwater recharge and help to alleviate the effects of salinity
• even out seasonal livestock feed and produce fodder opportunistically out-of-season
• improve soil fertility and structure
• reduce weed burden and manage herbicide resistance for cropping.

Lucerne’s limitations are:

• the cost, and slow rate (6-12 months) of establishment
• low winter production
• requirement for rotational grazing for long-term persistence
• greater monitoring for insects and susceptibility to being over-grazed
• variable out-of-season production
• the pasture phase needs to be at least three years to overcome the high upfront costs of establishment
• can be difficult/costly to remove if going into a crop phase
• can reduce crop yields in the year following the lucerne phase due to a dry soil.

Description

• Growth habit varies with winter activity. The less winter active types have a lower set crown and a more prostrate growth habit, while more winter active types have an erect growth habit (50-70 centimetres high).
• Plants are hairless with many stems originating from the crown.
• Leaves are trifoliate with leaflet length greater than the width.
• New stems emerge from the crown following grazing. These stems may branch from the lower axillary buds as they develop.
• Flowers are compact racemes with purple florets.
• Pods are coiled spirally with two to five kidney-shaped, yellow or brown seeds.
• Seed count is 400 000/kg (very small).

Soil-climate adaptation

• Rainfall: >325mm.
• Drought tolerance: very high.
• Frost tolerance: moderate to high.
• Soil type: grows well on a wide range of well drained soils including deep loams, deep yellow and brown sands, loamy sands over clay or gravel, deep sandy duplex soils and uniform clays. It is not suited to deep pale sands and shallow soils.
• Soil fertility requirements: moderate to high.
• Soil pH (CaCl₂): 4.8 to 8.0 in the top 30 centimetres (cm) — note optimum pH >5.5.
• Aluminium tolerance: low.
• Waterlogging tolerance: low.
• Salt tolerance: moderately low (if not waterlogged).
• Nutritive value: digestibility of 65-72% whole plant, 75-80% leaves and 45-70cm stems. Metabolisable energy of 8-11 MJ/kg DM and crude protein (12-24%).

When successfully established, lucerne will be productive over the life of the stand. Attention to detail at establishment in order to optimise plant density is critical as recruitment is limited.

Site selection

Select a site 12 months in advance of sowing and soil test for pH and nutrient levels.

Lucerne has limited tolerance to soil acidity. Soils that have a pH_Ca of 4.8-5.2 are at the lowest range suitable for the best growth and persistence of the lucerne and rhizobia. Ideally, these pH levels need to be corrected with the use of agricultural lime.

Lucerne is best established on:

• upper and mid slope sites
• well drained duplex soils
• areas which are at risk to groundwater rise
• coloured sands, loamy sands or finer textured soils.

Characteristics of better sites are:

• pH ≥5.2 (CaCl₂) in the surface 30cm
• increasing pH with increasing depth
• well drained and not waterlogged for excessive periods
• moderate to high fertility
• low salinity levels (EC <3 deciSiemens per metre)
• contain low weed populations
• low existing herbicide residues (particularly Group B sulphonylurea herbicides)
• soils with a depth of at least two metres (m) with no limiting chemical or physical conditions in the subsoil such as poor structure.

Variety selection

All lucerne varieties are summer active but they vary in their winter activity.

Each variety is given a winter activity rating between 1 and 10, with 1 being winter dormant and 10 highly winter active.

In general, winter active varieties (>6) are better adapted to Mediterranean climatic conditions as found in southern Western Australia. This is because the more reliable rainfall occurs in the cooler months. There is a small trade-off between winter production and out-of-season production. Match a variety with your regional or paddock scale environmental conditions and the intended use pattern (Table 1).

Varieties with lower winter activity ratings have crowns lower to the ground, therefore they are more tolerant of grazing than highly winter active varieties and are likely to persist for longer. However, there are some winter active varieties available that have improved grazing tolerance. The winter activity rating also influences the feed and hay quality because the lower the winter activity the greater the ratio of leaf to stem. The leaf is more digestible than stem.

Selection based on pest and disease resistance is less important compared to intended use because improved breeding has resulted in most varieties with similar tolerances.

Site preparation and weed control

Sowing should not start until good weed control is achieved. This is because seedling lucerne is a poor competitor against weeds, effective herbicide options are limited and the seedling develops slowly and may take up to eight weeks before herbicides can be safely used.

Thorough weed control prior to sowing is essential in the establishment year. The importance of weed control increases as growing season length shortens. Annual ryegrass, capeweed, geranium, radish and turnip are major weeds when establishing lucerne. Spring and summer weeds including melons, wireweed, mintweed, dock and sorrel must be controlled before sowing as they are difficult to control in seedling lucerne and will compete for water during the critical first summer.

Lucerne is best established after a crop that has reduced weed burden. If sown after pasture, weed reduction by grazing and winter pasture topping (hay freezing) with glyphosate or spraytopping with paraquat is recommended. To control weeds emerging at the seasons break, apply a 'doubleknock' treatment of glyphosate followed by Sprayseed® within 10 days of the initial glyphosate application. Trifluralin can be incorporated to control the emergence of ryegrass and wireweed.

Sowing time

The time of sowing is not as critical as achieving good weed control. If weeds are still germinating, it is better to delay seeding except in lower rainfall districts where this is not an option as there is usually insufficient time for seedlings to establish roots to survive the summer. In the high rainfall region there is a risk of waterlogging in mid-winter, so it is better to sow later when the soil is moist but not waterlogged.

Seeding machinery

A full range of machinery types can be used from air seeders to combines; the critical factor is the correct seeding depth.

The very small lucerne seed needs to be placed accurately at 0.5-1cm deep into moist soil.

Maximise seed-soil contact by using press wheels or a rubber tyred roller.

Match sowing speed to soil type to avoid furrow fill from adjacent tynes. Beware of furrow fill from subsequent operations and heavy rainfall.

Disc seeders can be useful on lighter soil types prone to erosion and in heavy stubble scenarios, although they lack the ability to incorporate herbicides.

Sowing lucerne with combine and press wheels.

Sowing rate

Lucerne will naturally 'thin down' to a sustainable plant population in rain fed stands.

Introducing an appropriate target population close to the expected final plant density is recommended. If depth control is not precise or soil moisture conditions are not optimal then increase the sowing rate.

Limited success has been achieved in re-sowing where sub optimal density has occurred. Coated seed will have less seeds per kilogram so increase the sowing rate by at least 33% to achieve the same plant density as bare seed. Calibrate seeding equipment for particular seed types to ensure accuracy of seeding rate.

Inoculation

Lucerne requires Group AL inoculum. Do not rely on previous lucerne or legume history at the site for inoculation.

Lucerne seed can be:

• manually inoculated on farm using the peat inoculum (for example rhizobia and lime pellet)
• inoculated and lime pelleted by the seed supplier/inoculation service provider
• inoculated as part of a specialised coating process by the seed company
• granular inoculants can be drilled with the seed.

Lime pelleting can extend survival time of rhizobia providing more sowing time flexibility. Seeding rate may need to be adjusted with coated seed. Remember a sticker (for example Methocel) is required for peat based inoculants.

Cover-cropping

Cover-cropping refers to sowing lucerne with an annual crop such as barley or canola.

The annual crop provides an economic return offsetting establishment costs and reduces wind erosion. However, the crop will compete with the seedling lucerne for light, nutrients and water and can cause poor lucerne establishment.

For the best result use a short season species at a low rate and plant on wide rows.

Options include:

• over-sow cereals at 20-30 kg/ha; barley is the best option
• sow lucerne in every row and cereal on alternate rows
• over-sow IT canola — Imidazolinone tolerant canola varieties allow lucerne establishment under them as lucerne is also tolerant of similar herbicides and the canola crop can offset cost of lucerne establishment. Ensure the paddock is free of capeweed and silvergrass as control options for these weeds are difficult in seedling lucerne & IT canola
• beware of limited weed control options.

Nutrition

Establishing lucerne will benefit from 10-15 kg/ha of phosphorus (drilled into the furrow) and 15-20 kg/ha of potassium (topdressed) at seeding on all mildly acidic soils.

Potassium (K) is recommended on lighter soils with depleted K reserves (<80 mg/kg K).

Check trace element application history and topdress Super Copper Zinc & Molybdenum pre-seeding if trace elements have not been applied in the past 6-10 years.

Insects

The main pests of lucerne at establishment are red legged earth mite and lucerne flea.

Incidental pests include aphids, weevils and grasshoppers.

Insects are likely to invade an establishing stand of lucerne.

Implement a control program the season prior for red legged earth mite (for example Timerite®). Monitor regularly as damage can occur before mites can be seen. Use a bare earth insecticide post sowing pre-emergent to control mites.

Check closely for aphid damage in the first spring and spray as necessary. Aphid damage appears as leaf curling and stunted plants.

Small lucerne weevil, note crescent shaped chew marks in leaves

In-crop weed control

It is essential to control weeds before the establishment of lucerne because seedling lucerne is a poor competitor. In addition, many herbicides can only be used after the lucerne plant has developed the third trifoliate leaf. Development to this stage may take eight weeks or more which is much slower than the growth rate of weeds.

Before using any herbicide, consult product label regarding safe and effective use (particularly the withholding periods for grazing).

Trifuralin (Group D) can be used pre-sowing to control annual ryegrass and wireweed. Use of herbicides at the seedling stage requires the lucerne to have a minimum of one trifoliate leaf up to a minimum of three trifoliate leaves depending on the herbicide, so refer to the label.

Grass selective herbicides (Group A fops and dims) can be used to remove most grass weeds except silver grass.

Propyzamide (Group K) can also be used to control most grass weeds including silver grass and seedling wireweed.

Many broadleaf weeds can be removed using the following herbicides:

• Bromoxynil (Group C)
• Bromoxynil and diflufenican (Group C and Group F)
• Flumetsulam (Group B)
• Imazethapyr (Group B), a range of grass weeds are also controlled
• Imazamox (Group B), a range of grass weeds are also controlled.

Measure your success

Monitor your stand continually once sown so as to react quickly to problems caused by insects, weeds and nutrition, until plants are well established.

Review your target plant density (Table 3). Check for successful nodulation 4-6 weeks after sowing by digging up 5-10 plants and looking for nodules on roots. If no nodules, check again in four weeks. A sub-optimal stand density and poor nodulation should be investigated.