Key Messages

2022/2023 Season

• The level of biomass and N fixation for each legume species differed greatly, with the faba bean treatments performing (by far) the best.

• The vetch and chickpeas suffered in the hot and dry summer conditions and were least productive.

• Faba bean contributed almost 250 kg/ha in above ground nitrogen to the rotation.

• Surprisingly, the observed differences in the 2023 (over-sown) wheat crop plots for each treatment were far less pronounced than expected and did not correlate directly to the biomass production of the fallow legume.

• Results for the 2023 wheat shoot nitrogen % and harvest grain protein also did not show a significant impact as a result of the legume cover crops grown beforehand.

2023/2024 Season

• The 2023/24 summer and following autumn period were extremely dry with only 22.4mm of rainfall received and as a result, the legumes in Trial 2 were irrigated twice.
• Not all plots within Trial 2 produced viable legumes but measurements taken from viable plots showed soybean to be the best performing legume in 2024 with 2.75 t/ha of above-ground biomass produced in very challenging conditions.
• Over sown wheat yields in Trial 2 yielded marginally better on the legume plots compared to the fallow (control) plots. However, there was no impact on grain quality.
• Second-year results for Trial 1 showed there was no significant legacy impact of the successfully grown 2023 summer legumes on wheat yields in 2024.
• Soil nitrogen balances were positive across all treatments in 2024, compared to mostly negative in Trial 1 in 2023 (except for the high N-contributing 2023 faba bean plots). This is likely due to the overall reduced wheat grain yields in 2024 compared to 2023.

The GRDC has invested in a summer active legumes project led by Stirlings to Coast Farmers to explore the use of legume cover crops grown over summer to produce nitrogen for winter sown crops, in replacement of the traditional chemical fallow in the high rainfall zone.

Project Background:

Since January 2021, the price of urea has increased from $256/t to $1,026 (Australian Trade and Investment Commission, 2022). This has been driven by rising global energy prices and by Russia and China imposing fertiliser export restrictions in 2021 and constraining global supply. It has been predicted that continued conflict between Russia and Ukraine will further increase global energy and fertiliser prices.

This is currently a considerable concern for growers in Western Australia. Growers in the highly productive (and high-risk) Albany Port zone are looking towards management measures that will de-risk their nitrogen strategies.

An opportunity lies in utilising often plentiful residual soil moisture at harvest to seed a legume cover crop such as cowpea, lab lab, soybeans, vetch or other small seeded legumes that can be brown-manured and incorporated prior to seeding. This is in place of the traditional summer chemical fallow.

Method:

In the summers of 2022/23 and 2023/24, one small plot trial site was established with up to 6 summer active legume species by three replications. Controls for comparison were included. In the winter of both 2023 and 2024, a cereal crop was established over these plots. The summer growing legumes were protected by planting a sacrificial crop around the perimeter of the small plot trial site. If adequate soil moisture was not available in the top 10cm then the trial was irrigated. In the first year of the project (2022/23) the small plot treatments included:

• Hairy Vetch

• Common Vetch

• Chickpea

• Soybean

• Lablab or Lucerne

• Pigeon Pea

• Chemical Fallow (farmer practice control)

• Tilled fallow (tillage effect control)

The treatments were managed in line with best practice agronomy for each species selected with the aim of maximising biomass production and nitrogen fixation. The treatments were replicated, randomised, and blocked to minimise spatial and environmental error, and to ensure a robust data set was be collected. Two controls were included in the trial, a chemical fallow (standard grower practice), and a tilled fallow, where the seeder tilled the soil without seed. This allowed observation of the tillage effect on mineralisation over the summer fallow period.

Each species was terminated at either: peak biomass, prior to seed set, or at the time of the autumn knockdown spray if the previous two parameters were yet to be met. Termination was conducted on an individual species basis, as each species differed in growth rates, growth stage triggers, and seasonal response. The species were monitored by SCF for growth stages and terminated by SCF.

Each year a comprehensive suite of measurements on soils and plants was completed to better understand the contribution of nitrogen (N) from the legume CC to the following winter cereal crops (to develop a comprehensive N budget).

Reach out to project lead Lizzie von Perger ceo@scfarmers.org.au