Lucerne removal trial key findings
- Autumn removal of lucerne significantly decreased following winter crop yields, compared with removal of lucerne in spring.
- Crops sown after spring removal of lucerne yielded the same as crops grown in a continuous crop rotation.
- The yield penalty from an autumn removal of lucerne lasted for one year.
- The label rate of Grazonz® and Roundup® DST is the best chemical option to control lucerne before cropping.
- Removing lucerne in spring allows maximum opportunity to replenish the soil moisture deficit over summer, giving potential to increase crop yields.
- Autumn removal or poor spring removal will decrease plant-available water and reduce yields.
- Decaying lucerne provided more than 150 kilograms per hectare of nitrogen to the crops in the first year, reducing fertiliser costs by $100/ha.
- Spring removal of lucerne followed by wheat then canola is likely to be the most profitable rotation.
Lucerne has many benefits for dryland mixed-farming systems. It can produce a large amount of high-quality feed in summer, accumulate soil nitrogen, help manage herbicide resistance by reducing the weed seedbank and its deep root system can penetrate hostile subsoils.
But the tricky part can be getting rid of it when wanting to start the crop rotation again and knowing what will be the most profitable subsequent crop sequence.
GRDC-funded research in the high-rainfall zone of south-west Victoria has investigated the best lucerne removal methods and timing, as well as its impact on subsequent crop yields.
AgVise Services’ David Watson conducted the trial at Inverleigh as part of the Grain & Graze program. In total, 10 lucerne trials were undertaken between 2010 and 2012 on a six-year-old stand of Grasslands Kaituna lucerne. Average lucerne plant densities were 47 plants per square metre and previously the stand had been grazed and cut.
Impact of removal time
Mr Watson says the time at which lucerne is removed from a paddock can significantly impact yields in the following crop because of moisture levels retained in the soil.
The trial found if a crop is to be sown in late April to early May its yield will be higher if lucerne is removed in spring of the year before, rather than autumn of the same year.
“Crops sown after spring removal of lucerne yielded the same as the crops grown in a continuous crop rotation. However, the crop yield penalty from autumn removal only lasts for one year,” he says.
New research has found that spring removal of lucerne improves the soil's capacity to retain moisture and nutrients in preparation for the subsequent crop, giving higher yields than an autumn lucerne removal.
Soil moisture and yield
Autumn removal of lucerne does not give enough time for moisture to be accumulated in the soil for the following crop.
This can mean that crops will not have the required moisture at the right time to maximise grain number and yield. Plants with a moisture shortage can divert nitrogen in soil to grain protein and not yield.
Best crop after lucerne
Mr Watson says canola is often the first crop sown after lucerne because it has higher grain value and greater nitrogen demand than barley or wheat.
While barley is considered as a crop to follow lucerne due to its higher water use efficiency when moisture is limited, wheat can put the nitrogen fixed in soil to better use through higher grain protein, for which growers are paid.
Lucerne's deep root system can penetrate subsoils, opening them up for future crop use.
PHOTOS: David Watson
The trial found limited moisture meant the first crop after lucerne performed below expectations:
Canola performed poorly in yield and oil in the first year following lucerne. Canola following an autumn lucerne removal was usually worse than after a spring removal. While plants may establish properly, if they do not thrive it may be due to a root system unable to access moisture at depth.
Yields of barley following spring removal of lucerne were about on par with continuous cropping. However, where lucerne was removed in autumn of the year barley was sown, yield losses were up to one tonne per hectare due to lack of soil moisture at grain fill (see Table 1). The moisture shortage diverted soil nitrogen to grain protein, not yield. As a result, barley at either removal time had high grain protein, causing a downgrade to feed.
In either spring or autumn removal of lucerne, wheat performed better than barley and twice as well as canola. While autumn removal resulted in a lower yield than spring removal, it still achieved a higher protein grade than barley.
“In second-year crops, yields are unlikely to be compromised by the time of lucerne removal because of the increased time in which soils can store moisture,” Mr Watson says. “However, grain proteins and canola oil status are more likely to be lower than first-year crops.”
Table 1: Yield differences between spring and autumn lucerne removal
| Treatment in trial at Inverleigh, Victoria
| 2011 barley yield (t/ha)
| 2012 canola yield (t/ha)
| Continuous crop
| Spring 2010 removal of lucerne before 2011 crop
| Autumn 2011 removal of lucerne before crop that year
Best removal method
Established lucerne is hard to remove and if it is not done properly, leftover plants will result in ongoing soil moisture depletion and decreased yields in following crops.
Rules of thumbs on lucerne removal include the following.
Correct herbicide timing is critical because plants usually start translocation from the shoots to the roots about three weeks after defoliation.
Plants need to be actively growing and translocating. Plants will respond poorly to herbicide application if moisture or temperature stress is slowing growth.
- Do not disturb the plant with defoliation or cultivation for two to three weeks after herbicide application. This allows time for translocation.
Mr Watson says the trial tested a range of chemical and cultural removal options for lucerne.
He says growers have found the most effective treatment is Grazon® Extra and glyphosate 450. But growers need to be aware that Grazon® has a four-month plant back for wheat, barley and canola.
The trial found cultivation was ineffective. In a single-disc cultivation in mid-January, 99 per cent of plants survived while 83 per cent survived the same treatment at the start of May.
Up to 80 per cent of lucerne plants survived a two or three-disc cultivation in mid-January, while 43 per cent of plants survived two passes using an offset-disc plough.
In the trial, all other herbicide treatments using mixes of products available to growers failed to control lucerne effectively. Cultivation in combination with other herbicides only increased lucerne control by 20 to 30 per cent.
Economics of early removal
Fodder production is lost when lucerne is removed in spring, allowing for moisture accumulation and mineralisation of nitrogen in soil. Growers need to measure the effects of this fodder loss in the context of their own operation.
For autumn lucerne removal, a modelling tool, such as GrazFeed®, can be used to put a value on retaining lucerne from spring (when it could have been removed) through until autumn.
To arrive at a net value per hectare, calculate liveweight (LWT) gains, the value of the lambs to be sold and costs for animal health, supplementary feeding, crutching and mortalities.
For example, at Inverleigh:
- spring removal of lucerne took out 2.1t/ha
of summer fodder from the farm;
LWT gains were 181 grams per head per day;
lambs were valued at $1.65 per kilogram LWT; and
costs deducted included $5/head; therefore
- the value of the 2.1t/ha of lucerne was $289/ha, or $138/t.
This cost of lucerne removal in spring can be compared with net returns from cropping, which are likely to be higher.
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