Global market projections are indicating that low-protein grain will be increasingly difficult to sell, ultimately leading to greater disparity between premium and feed grain prices. Together with escalating fertiliser prices in recent years, this is driving the search for alternative means of boosting cereal protein levels. Elite legume pastures grown within crop sequences could help achieve this.

Large quantities of Western Australia’s cereal crops are not achieving grain protein levels preferred by markets due to several interacting factors. These include WA soils being highly weathered and generally low in organic matter and nutrients, a decline in forage legume density where pastures are used in cropping sequences, and growers producing more canola as a profitable break crop. This seems to be resulting in a decline in soil fertility, particularly organic matter content and nitrogen.

Continuous cropping systems, which are common in low-rainfall areas of WA, are highly dependent on synthetic nitrogen fertiliser. Its application is usually matched to crop type, soil characteristics, prevailing rainfall and crop yield potential. Improved crop varieties and agronomic management have increased yield potential considerably, but increased nitrogen fertiliser is needed to achieve these yields without compromising protein content.

Biologically fixed nitrogen from legumes in sequence with crops can significantly increase the sustainability of cropping systems and offer an alternative nitrogen source. With elite pasture varieties being developed together with new establishment techniques, a significant boost in the yield and protein levels of following cereal crops is possible – in some cases without the need to apply any nitrogen fertiliser.

As part of the national Dryland Legume Pasture Systems project, the impact of these new pasture species on subsequent cereal crops grown in dryland systems of WA has been quantified.

Regional trials

Rotational trials were conducted over four years at Ardath, Narembeen and Canna in WA. Soil types varied from coarse-textured acidic sands at Ardath to fine-textured loams at the other two sites.

The legumes that were sown differed between sites depending on the soil type and expected rainfall (Table 1). Common vetch was included as a comparison sown on the first early rain. It does not have hard seed and needs to be resown each year.

Table 1: Site characteristics, annual rainfall (spring rainfall in brackets) and the pasture legumes examined at three locations in WA.

Source: CSIRO

Wheat and some barley were grown after several different pasture legumes, either sown in summer or late autumn or naturally regenerating from the established seedbank.

Soil was sampled at the start of every growing season and analysed for nutrient levels and moisture. There was no significant difference in the soil total nitrogen between plots within sites, except for the fallow plots which were significantly higher.

Plots in Ardath were 20 metres long, while plots in Canna and Narembeen were 10m long. Each cereal plot received fertiliser nitrogen (rates varied from four to 120 kilograms of nitrogen per hectare), applied as urea at the early tillering stage (Zadoks 20 to 25), 24 hours before a significant rainfall event (more than five millimetres).

Wheat protein benefits

Protein levels for Scepter (PBR) wheat are shown for all three sites in Figures 1a, 1b and 1c. These results demonstrate how pasture legumes, grown across three markedly different districts in dryland WA, increased grain protein levels in following wheat crops. The horizontal line in each graph indicates APW1 wheat classification. Wheat grown in a continuous cereal failed to reach the protein levels of wheat after a pasture legume, in many cases even when 120kg of fertiliser nitrogen was applied per hectare to the continuous wheat.

Figure 1: (a)  Scepter wheat protein percentage in the fourth year of rotation at Ardath, where wheat yields ranged from 2.9 t/ha to 4.0t/ha; (b) second year at both Narembeen (wheat yields 3.2 to 4.3t/ha); and (c) Canna (wheat yields 2.4 to 4.2t/ha).

Source: Murdoch University and CSIRO

Efficient nitrogen-fixing pasture legumes (such as serradella, vetch and trigonella) used in rotation with cereals can allow growers to hedge on undersupplying nitrogen without significantly compromising yield and protein in high-demand wet years, while reducing the chance of oversupply in years of low demand.

Other benefits and considerations

In general, there were small differences between the nitrogen benefits of the different pasture legume species. However, pasture species selection needs to be carefully considered. For example, medics can be poor nodulators and fix small amounts of nitrogen when the nitrogen content of the soil type is already large, compared with other pasture legumes. Therefore, the wheat crops studied had lower yield and protein levels at Narembeen and Canna when grown after medic, compared with other legume species.

But there are other rotational effects to consider. For example, deep-rooted legumes that produce large amounts of biomass tend to deplete soil moisture, unlike traditional pastures that sometimes leave moisture that benefits following crops.

Some of the hard-seeded varieties used as pasture legumes (such as serradella and bladder clover) are tolerant to the herbicide imidazoline and provide another tool for weed management within a crop/pasture rotation, especially for problem weeds such as barley grass.

An important note is that the pasture legumes in this study were not grazed by sheep. It is likely that heavy grazing will redistribute nitrogen from the systems studied but may improve the economic returns and potentially offset risk within the system, as seen in related economic modelling of pasture legume effects in rotations.

Further work will continue with crop rotation trials for trigonella to evaluate its capacity to regenerate, its water-use efficiency and ongoing benefits to subsequent crops.

This research was part of the national Dryland Legume Pasture Systems project supported by the Australian Government Department of Agriculture, Water and Environment (DAWE) Rural R&D for Profit program and the Grains Research & Development Corporation, Meat & Livestock Australia and Australian Wool Innovation.

More information: Robert Harrison, robert.harrison@csiro.au; Ron Yates, ronald.yates@dpird.wa.gov.au

Read more: Cereals after pasture legumes have higher grain protein levels.