Quit when you're ahead


The challenge: to manage for cropping benefits of lucerne while avoiding soil and water degradation.

Remove lucerne early and you’ll allow time for soil water recharge and nitrogen mineralisation, which usually results in higher yield and protein in the following crop.

NSW Agriculture research on rotations using lucerne pasture found that the greater the amount of dry matter produced by the lucerne, the greater the accumulation of nitrogen (N). The length of the pasture phase was less important.

During 1999 above-ground dry matter produced at six sites around Gilgandra, northern NSW, varied from 5.5 to 12.3 t/ha. The total N fixed at these sites varied in a similar way, ranging from 115 kg N/ha where dry matter production was low to 500 kg N/ha following pastures that produced large quantities of dry matter (including N in roots and nodules).

The following points come from NSW Agriculture based on this research.

Lucerne and mineralisation

The main things to look at prior to pasture removal are soil water and nitrogen accumulation. Concerning water, it doesn’t matter if lucerne is removed just a week before sowing, so long as it rains and fills the soil profile by sowing time.

(But) it takes time for organic matter to break down and supply nitrogen to the soil (mineralisation). There must be water, warm temperatures and organic matter. Increasing the fallow length allows more time for mineralisation to occur, and therefore more N will be accumulated in the soil.

Lucerne was removed at four different times on a red duplex soil near Gilgandra in 1998-99 and 1999-2000. The amount of rain that fell over these two summer fallows ensured that soil profiles were full prior to sowing, so the timing of removal did not affect soil water at sowing.

There were, however, large differences in the amount of nitrogen mineralised over the summer fallow (see graphs).

Generally earlier removal culminated in higher soil N at sowing. In 1998-99 overall mineralisation potential was low due to low rainfall during spring and summer. Conversely in 1999-2000 high rainfall in late spring enabled nitrogen to accumulate early in the fallow.

Mineralisation ranged from 30 to 70 kg N/ha in 1998-99 and from 50 to 160 kg N/ha in 1999-2000. In both seasons, removal in September resulted in the highest levels of soil N at sowing.

In the first crop following a lucerne phase, early removal generally resulted in higher yields and proteins, but the differences were not as large as the differing levels of mineral N at sowing might suggest. It is possible that higher than normal in-crop mineralisation, especially during spring, may reduce the effect of sowing N differences.

There is a downside

A downside with early removal of lucerne is the potential for erosion on a bare fallow. To counter this, the researchers recommend retaining ground cover by using chemical removal and zero tillage rather than physical removal. Slope and the inherent increased erosion risk may also be determining factors in the timing of removal.

Of equal concern on these duplex soils is the potential for leakage of water and nutrients, particularly nitrate, out of the root zone, resulting in off-site contamination of streams and groundwater and acidification of the soil.

The challenge is to manage the system to capture the short-term benefits of lucerne for cereal cropping while at the same time avoiding soil and water degradation.

Program 3.5.1 Contact: Mr Giles Butler 02 6763 1100 email giles.butler@agric.nsw.gov.au