Take Home Messages
- Temporarily intercropped vetch in a wheat crop did not provide any benefit to the crop in 2021 or 2022.
- Over-applying urea did not result in ’haying off’ in a decile 4 year and left nitrogen for the following crop.
- In the presence of high levels of nitrogen, fewer nodules formed on vetch roots.
Aim
To determine how different methods of nitrogen management affect the performance of a crop.
Background
Nitrogen (N) supply is the biggest factor contributing to the yield gap in Australia and reducing N deficiencies in crops would increase national wheat yields by 40 per cent (Hochman et al., 2018). Nitrogen can be managed in a system in several ways, including using N fertiliser or growing a legume in the rotation. Management of the legume can be grown in a sequence with non-legume crops or grown simultaneously with non-legume crops i.e., temporary intercropping.
Nitrogen legacy benefits from vetch are valuable to a system, although the benefits can vary depending on the end use of the vetch crop; whether it is cut for hay, brown manured, grazed or harvested for grain. Furthermore, it is important to get the timing for terminating a vetch brown manure crop correct in the Mallee and strike a balance between early enough for soil water conservation and late enough for greatest biomass production, which directly influences N fixation (Ferrier et al., 2013).
Temporary intercropping can improve crop yields, weed competition, harvestability, disease management and nitrogen inputs (Bedoussac et al., 2015). However, most studies are based in organic farming situations, mixed farming systems or areas that receive higher annual rainfall than the Victorian Mallee.
This trial investigated different N inputs (fertiliser, legumes in sequence or intercropped legumes) over two years to gain a better understanding of how the different methods of nitrogen input affect crop yield and quality
Paddock Details
Location: Woomelang
Crop year rainfall (Nov–Oct): 2021: 270mm, 2022: 559mm
GSR (Apr–Oct): 2021: 217mm (decile 4), 2022: 451mm (decile 10)
Soil type: Sandy loam
Paddock history: 2020 – Oaten hay
Trial Details
Crop types: Refer to Table 1
Treatments: Refer to Table 1
Target plant density: wheat monoculture: 130 plants/m², vetch monoculture: 60 plants/m², intercropped wheat: 90 plants/m², intercropped vetch: 18 plants/m²
Seeding equipment: Knife points, press wheels, 30cm row spacing
Sowing date: 2021 – 25 May, 2022 – 18 May
Replicates: Four
Harvest date: 2021 – 6 December, 2022 – no harvest due to 100 per cent hail damage
Starting soil N 2021 69kg/ha
Trial Inputs
Fertiliser: Refer to Table 1 and Table 2
Trial managed as per best practice for weeds, pests, and diseases
Method
One replicated field trial was sown using a complete randomised block trial design over two years. In year 1 treatments included wheat with varying N rates, vetch with varying termination timings and wheat and vetch grown simultaneously with varying termination timings of the vetch (temporarily intercropped (TI)). (Table 2.) All plots were sown to barley in 2022 and urea applied in 2022 was calculated for all treatments to reach a 5t/ha barley yield potential using Yield Prophet® Lite (Table 2). Assessments included NDVI, crop biomass at flowering, nodulation counts on individual plants, soil nitrogen, plant available water (PAW) and grain yield and quality parameters.
Results & Interpretation
Year 1: 2021
Nodulation
Nodule counts on a subset of treatments (Figure 1) showed significantly fewer nodules formed on the plants in the high N plots than the plots with no N applied. There were similar vetch biomass yields (Figure 2) in the two intercropped treatments which indicated poor nodulation was caused by the vetch having access to adequate nitrogen (>100kg N/ha) (Swan et al., 2022) rather than low biomass weights. Potentially less nitrogen was fixed in these treatments.
Vetch biomass
A late and dry start to the 2021 season resulted in low vetch biomass yields across all treatments (Figure 2). Biomass was measured at each time of termination (TOT) and at both termination timings showed no response to nitrogen treatments. The intercropped treatments were ~25 per cent of the monoculture treatment at both termination timings. This most likely reflects sowing rate (intercropped treatments were at 30 per cent the sowing rate of vetch monoculture) rather than competition from the intercrop. Delaying termination of the vetch crop by three weeks more than doubled the crop biomass. However, this did not result in significantly higher PAW or soil N levels at the end of the season (Table 3). The vetch hay treatment cut on the same day as the latest termination timing had lower soil N and PAW, most likely due to removing the shoot biomass — which is where two-thirds of plant N is stored (Swan et al., 2022) — and perhaps lower PAW due to less ground cover.
Available soil nitrogen
The treatments targeting N unlimited yield potential (190kg/ha N applied) unsurprisingly had the most soil nitrogen post-harvest 2021, followed by the vetch monoculture treatments (Table 3). Differences between treatments remained the same at pre-sowing 2022, however a high level of mineralisation had occurred across all treatments, most likely due to above average summer rainfall.
Grain yield and grain quality
Wheat grain yield and protein levels were strongly related to N fertiliser supplied in season (Table 4) When the temporarily intercropped vetch was terminated late, there was a yield penalty compared to no temporary intercrop across all N treatments. In contrast no yield penalty was observed from the earlier terminated intercrop treatment which is potentially a result of the second time of termination providing greater competition to the wheat crop. High protein was achieved wherever high N was used, suggesting the crop was able to meet the needs of yield as well as maintaining high protein. All treatments maintained acceptable test weight and screenings (data not shown), indicating no treatments ‘hayed off’, including the high N treatment which was at greatest risk.
Year 2: 2022
Initial results from 2022 suggested no remaining treatment effects from 2021, with in-season NDVI results of 0.91–0.93 and biomass cuts at peak flowering supporting these results (p= 0.116). The trial was severely impacted by hail just as it was ripening and could not be harvested, therefore no yield or quality data was reported. Very low soil N levels were recorded in early January 2023 (24–37kg N/ha), most likely reflecting the decile 10 season and suggesting higher than predicted yields or maybe denitrification. By the end of 2022, soil N levels were lower than at the start of 2021, regardless of treatment (Figure 3).
Commercial Practice and On Farm Profitability
Temporarily intercropping vetch into a wheat crop did not negatively impact the wheat crop it was sown alongside, however it did not improve yield or leave a significant amount of soil N for the following crop. Temporarily intercropping a pulse into a cereal crop is logistically a lot simpler than taking both crops through to grain harvest, however it still has its challenges. These include selecting a sowing time that suits both crop types, and controlling weeds, as pre-emergent options become restricted and in-season management of weeds is determined by termination timing of the intercrop.
Termination timing of the vetch brown manure didn’t have huge implications in this trial due to lack of vetch biomass and high rainfall following the brown manure. Previous BCG research has shown an early termination is lower risk in the Mallee as it is important to find a balance between early termination for soil water conservation but late enough to achieve adequate biomass production, which directly influences how much N is fixed (Ferrier, 2013).
The highest N treatment in 2021 did not ‘hay off’ in a decile 4 GSR and still left adequate N available for a decile 5 crop the following year. The season however ended on a decile 10 GSR and without yield results it’s impossible to know if additional mineralisation of N occurred and whether yields of some treatments might have been higher. Research into nitrogen banks, where a base level of fertility is maintained using N fertiliser, has shown the majority of applied N in low rainfall regions — on heavy textured soils — that is not used in the year it’s applied will be available in the soil for the following crops (Hunt et al., 2020).
Without final yield results from 2022 it is impossible to accurately run gross margins or confidently say which treatment had the greatest return on investment (ROI) over the two years.
References
Bedoussac L., Journet EP., Hauggaard-Nielsen H. et al., 2015, Agronomy for Sustainable Development, ‘Ecological principles underlying the increase of productivity achieved by cereal-grain legume intercrops in organic farming. A review.’ 35, pp. 911–935.
Ferrier D., Rees H., Watson L. and Peoples M., 2013, 2013 BCG Season Research Results, ‘Vetch termination: impact on the following wheat crop’ pp. 119–125.
Dunsford K., Armstrong R., Tang C. and Hunt J., 2020, 2020 GRDC Update Papers, ‘Estimating in-crop nitrogen mineralisation in Victorian cropping systems’.
Hochman Z. and Horan H., 2018, Field Crops Research, ‘Causes of wheat yield gaps and opportunities to advance the water-limited yield frontier in Australia’ pp. 20–30.
Hunt J., Murray J. and Maddern K., 2020, 2020 BCG Season Research Results, ‘Managing N fertiliser to profitably close yield gaps’ pp. 122–128.
Swan T., Dunn M., Kirkegaard J. et al., 2022, 2022 GRDC Update Papers, ‘What is the N legacy following pulses for subsequent crops and what management option are important to optimise N fixation?
Acknowledgements
This research was funded by the Hugh Williamson Foundation. We thank the Boxall family for hosting the trial.
