Slow-N-release pastures: early removal key

Photo of a lucerne plant

Lucerne is a very cost-effective slow-release nitrogen fertiliser, according to John Angus of CSIRO Plant Industry. But you have to get the timing right, or you might not see the benefits for a couple of years.

Researchers have also looked at the relative benefits of other perennial grasses in terms of the soil nitrogen store with results that compare favourably with lucerne, at least initially.

Dr Angus blames nitrogen mineralisation rates (how quickly nitrogen is released from decaying lucerne root) for the sometimes poor crop yields after a lucerne pasture that has been removed late (in autumn).

Benefits of early removal

"We found that early removal of lucerne, in the spring before sowing a wheat crop, added an additional three-quarters of a kilo of nitrogen per hectare for each day of fallow, or half a kilogram per hectare per mm of fallow rainfall," says Dr Angus.

Dr Angus' research at Junee in southern NSW showed that this gave a corresponding wheat yield increase of 1.9 t/ha for a crop following an early-removed lucerne pasture. The benefit, he says, lasts for about five years of cropping in south-eastern Australia, when the nitrogen store runs out and another pasture phase is needed to replenish it.

According to Dr Angus, the same type of result would be seen across much of the eastern Australian cropping zone, a point with which research agronomist Giles Butler of NSW Agriculture in Tamworth agrees.

"Early lucerne removal has been part of rotation management for some time here in the north, not only for nitrogen accumulation but also to build up soil moisture levels," says Mr Butler.

Take soil drying into account

Even so, research from southern Queensland suggests it can take a couple of years for wheat yields to get back to yields seen under continuous cropping (with added nitrogen fertiliser).

"We think these lower yields are due mainly to lucerne's ability to really dry out the soil profile. But in that first year, protein levels are often high enough to achieve PH or DR1 quality," says Mr Butler.

The tables beiow show the yields of each of the crops in a cropping sequence following annual subclover and lucerne. In the short term, it seems that two or three years of subclover give a greater yield benefit to the following crops than two or three years of lucerne. But if you compare canola yields in 1997 (the last column in all three tables), the longer-term benefits of lucerne are starting to show up. Because of the long-lasting effects of lucerne, it is possible that the yield benefits of a first lucerne phase will extend until after the second phase

Crop yields (t/ha) after annual subclover pasture
No cropCanolaWheatTriticaleCanola
5 Years of subclover1.57
4 Years of subclover3.061.58
3 Years of subclover4.484.331.26
2 Years of subclover1.395.094.391.37
Crop yields (t/ha) after lucerne pasture
No cropCanolaWheatTriticaleCanola
5 Years of subclover1.66
4 Years of subclover3.271.89
3 Years of subclover2.654.101.91
2 Years of subclover0.224.323.801.78

The main difference for northern growers is that the warmer and wetter summer results in higher mineralisation rates. Mr Butler says it is also possible that more nitrogen can become available through the crop's growing season, particularly in spring.

"The result is that the yield penalty due to nitrogen availability in the first year may not be as severe as it is in the south if removal is late," says Mr Butler. "But it is still important to remove lucerne early to avoid yield penalties due to a lack of soil water during the crop's growing season."

The same type of yield penalties have been seen in trials near Katanning in western Australia (see below for more about this).

Although lucerne gave the longest-lasting nitrogen benefits in the east, other perennial pastures could contribute almost as well to the soil nitrogen store in the short term, without the yield penalty in the first year of cropping.

Comparing other grasses

Comparing crop yields after lucerne with those after a grassy pasture containing phalaris, cocksfoot and some volunteer subclover, showed that yields of the first year of crops grown after grass were greater than after lucerne.

"It is astonishing because there was less nitrogen fixed by perennial grasses in the previous years," says Dr Angus. "Our current speculation is that more nitrogen is available where perennial grass roots are decaying in the soil than from the more slowly decaying lucerne."

"It comes down to the carbon to nitrogen ratio of the roots, which is much higher in lucerne than in other pastures legumes, and so lucerne takes longer to decompose."

As well as higher crop yields, the beneficial effects of perennial pasture grasses for the first crop appeared as improved seeding emergence, increased seedling vigour and less take-all, but this alone could not account for the increased crop yields.

"We are not quite sure of the reason for some of these effects, but local growers have reported the same benefits, and so it's not just an accident of our trial methods or the run of seasons," says Dr Angus.

The greater benefits of lucerne over other legume pastures have been reported after the second phase of lucerne, when there was no yield penalty on the wheat crop. Dr Angus attributes this to the residual high levels of nitrogen in the soil from the last of the decaying lucerne roots from the first lucerne phase.

"Anyway, a deep soil nitrate test at the time of sowing would resolve any concerns, giving growers the option of applying extra nitrogen fertiliser in that first year," says Dr Angus.

Annuals and mineralisation

Dr Angus adds that winter cleaning of annual pastures led to the largest short-term yield response of any management system he trialed, something that is well known and generally assumed to be due to control of take-all. However, with canola yields responding as well as wheat yields in his trials, this explanation was no longer adequate and rapid mineralisation of clover residue is most likely to be a contributing factor.

"While annual pastures have their place in the cropping system, the longer-term benefits of perennial pastures for building up soil nitrogen, as well as improving soil structure and de-watering soils, mean that they are an increasingly viable option for growers in a rotational cropping system."

Program 3.4.2

Contact: Dr John Angus 02 6246 5095

See also our feature on pasture cropping into native perennials p28—29.

Region North