Take home messages

• Delaying the brown manure termination timing by 4-6 weeks doubled biomass production.
• Morava and Volga produced the highest hay yield.
• The conservation of soil water from a vetch termination timing is of little use in a decile 8 summer rainfall year.

Background

Common vetch (Vicia sativa) has been grown in the Victorian Mallee for many years as a common crop rotation for farms due to its versatility. Vetch can be utilised as a disease break, a grass weed break and is sown for several end uses including grazing, hay, brown manure or harvested for seed. It is known that sowing vetch earlier will increase biomass for grazing and hay yield (GRDC 2018). It has also been reported that there is a trade-off made in brown manure timings. The earlier the termination timing, the higher the soil moisture left behind but the lower the nitrogen (N) fixed. In contrast, the later the termination timing, higher levels of soil mineral N are left behind but this is quite often coupled with lower soil moisture levels (Ferrier et al. 2012). It is not well understood how the agronomic decisions made during the season affect soil moisture and soil mineral N levels following the vetch phase, and what outcome this has on yield and quality of the subsequent crop. Agronomic decisions to consider, tested as part of this research, are grazing, variety choice and therefore maturity of vetch and termination timing.

This paper discusses the effect of agronomic decisions on vetch end uses and soil moisture and soil mineral N following the end use. The outcomes of the vetch phase are analysed here, and the trials have been sown to wheat in 2024 to assess the effect on the subsequent crop.

Method

Two replicated field trials were sown at Ouyen and Kinnabulla on 19April and 20 April 2023 using a complete randomised block trial design. Three varieties were chosen for this trial with varying phenology characteristics: Morava (late maturity), Volga (early) and Studenica (very early). The in-season treatments included grazing which was carried out just prior to canopy closure during mid-July. Two termination timings were also studied. The earlier termination timing was undertaken four months after sowing for all varieties, irrespective of growth stage, whereas the late termination timing and hay cut timing was determined by variety maturity, being terminated or cut for hay when the variety reached 50% flat pod formation. One treatment was untreated throughout the season and harvested for grain. Assessments included grazing biomass yield (data not shown), hay yield, feed test quality (data not shown), grain yield (data not shown), soil N and soil moisture. Soil sampling was carried out at a site level prior to sowing to get a baseline understanding of the sites’ soil moisture and soil mineral N. Post-harvest and pre-sowing soil tests were carried out per plot to determine soil mineral N and plant available water (PAW) to a depth of 1m and segmented into 0–10cm, 10–40cm, 40–70cm and 70–100cm depths. The two trials were analysed separately using one-way ANOVA in Genstat 22nd edition.

Results

Termination timing of brown manure treatments

There were large differences in the amount of biomass produced between the early and late brown manure termination timings. An additional 4.79tDM/ha and 3.49tDM/ha were produced at Ouyen and Kinnabulla, respectively, in the later termination timing. The only difference in variety performance across both sites was that less biomass was produced in the late termination timings of Studenica at Kinnabulla compared to Volga and Morava (Figure 1).

Figure 1. Biomass at termination timing (t DM/ha) of vetch varieties at Kinnabulla and Ouyen. Columns with different letters show significant difference within a site. The sites were analysed separately. Kinnabulla P<0.001, Lsd 1.16t DM/ha, CV 17.4%, Ouyen P<0.001, Lsd 1.29t DM/ha, CV 18.7%.

Hay yield

At Kinnabulla, Morava and Volga had the highest hay yields at 6.0tDM/ha and 5.5tDM/ha, respectively (Figure 2). At Ouyen, there were no significant hay yield differences between varieties. A large yield penalty occurred in the hay treatments that had been grazed earlier in the season at both sites (data not shown). Additionally, Volga recovered the best from grazing at both sites, with the lowest loss of 1tDM/ha when considering the combination of biomass removed at grazing and the hay cut (data not shown).

Figure 2. Hay yield (t DM/ha) of varieties at Ouyen and Kinnabulla. Columns with different letters show significant difference. The sites were analysed separately. Ouyen = NS, Kinnabulla P<0.001, Lsd 1.86t DM/ha, CV 26.7%.

Soil moisture and soil mineral nitrogen

Soil measurements taken after harvest indicated the lowest level of plant available water (PAW) and soil mineral N was left behind in the grain harvest treatments (Table 2 and 3). Treatments with early termination timings reported the most soil N, which was unexpected given the large differences in biomass produced from the two termination timings (Figure 2). However, measurements taken closer to sowing showed a higher level of mineralisation in the late termination timing at Kinnabulla. The grain treatment remained with the least amount of soil nitrogen at Kinnabulla. Differences in plant available water close to sowing were insignificant due to higher than average summer rainfall at both sites (decile 8 at Ouyen, decile 7 at Kinnabulla).

The higher amount of soil mineral N measured after harvest at the earlier termination timing is likely owing to increased soil mineralisation (Farquharson et al. 2022). A similar amount of soil mineral N was detected in the late termination treatments and the hay treatments (Table 3). With 3–6tDM/ha of biomass removed in the hay cut treatments, it would be expected that there would be a large difference in these plots. This illustrates the time needed for the N fixed to break down into available soil mineral N. When measured prior to sowing the amount of nitrogen measured under the later termination timing had increased however not to the predicted amount, upon inspection of the vetch stubble at sowing there remains a significant amount on the surface and it is likely that additional time is still required to measure the full nitrogen mineralisation.

Table 2: The effect of end use decision on plant available water (PAW) (mm) (0-100cm).

Table 3: The effect of end use decision on soil mineral N (kg N/ha) (0-100cm).

Conclusion

Morava performed well all throughout the season in 2023 in terms of biomass production, however Volga illustrated versatility; as an early maturing line it has performed similarly to Morava (late maturing) and recovered well from grazing. Studenica’s performance was sub-optimal compared with the other varieties, which is reflective of it being a very early maturing cultivar in a longer-than-average season. Variety decision had no significant effect on PAW or soil N levels at the end of the season.

Delaying the termination timings by 4–6 weeks significantly increased biomass left behind, and, although not reflected in the immediate post-harvest soil results, it could result in an additional ~70–95kg N/ha being available for the subsequent crop (using an estimate of 20kg of N being fixed for each tonne of dry matter produced (Peoples et al. 2012)). Soil mineral N measured prior to sowing had not resulted in the predicted amount of N mineralised despite warm temperatures and rainfall over summer indicating ideal mineralisation conditions. On inspection of the trial at sowing in 2024 there was still a significant amount of vetch stubble remaining on the soil surface of the late termination timings treatments and it is likely greater time is needed for the biomass to break down and be mineralised.

The summer rainfall following the first year of the trials was above average and therefore there was no moisture conservation gain from terminating the vetch earlier. However, nitrogen is something that can be applied in season whereas water cannot so easily be supplemented. Therefore, the decision to make the trade-off between moisture and nitrogen will likely depend on a growers appetite for risk. In 2023, it was no surprise that the vetch hay generated the largest income of the treatments. At the completion of the 2024 season, we will be able to determine if any additional yield from the subsequent wheat crops has been generated as a result of the 2023 treatments, and if this leads to increased income/gross margins over the two seasons.

References

Anderson D, Allen R (2021) 2021 Hart trial results: improving vetch dry matter production

Farquharson EA, Ballard RA, Herridge DF, Ryder MH, Denton MD, Webster A, Yates RJ, Seymour NP, Deaker RJ, Hartley E, Gemmel LG, Hackney B, O’Hara GW (2022) Inoculating legumes: practice and science, Grains Research and Development Corporation, Australia. Pp 76-80.

Ferrier D, Watson L, Peoples M (2012) 2012 BCG Season Research Results: Vetch termination: finding a compromise

GRDC (2018) Vetch Grow Notes

Peoples MB, Brockwell J, Hunt JR, Swan AD, Watson L, Hayes RC, Li GD, Hackney B, Nuttall JG, Davies SL, Fillery IRP (2012) Factors affecting the potential contributions of N2 fixation by legumes in Australian pasture systems. Crop and Pasture Science 63, 759-786.

Key Contact

Angus Butterfield
73 Cumming Avenue, Birchip VIC 3487
0490837579
angus.butterfield@bcg.org.au