Tariq Gerardi (BCG), Abe Gibson (SCU), Mick Rose (NSW DPI) and Terry Rose (SCU)
Take Home Messages
- Vetch brown manure provided the best yield results for the following wheat crop.
- There was no yield penalty from multi-species cover crops when compared to fallows in a decile 10 GSR year.
- There was no effect of rotation treatments on any soil health parameters over a 12-month period.
Background
Cover crops, including multi-species cover crops, can be beneficial for soil health and for crop yields in temperate environments, but in low rainfall regions water use by cover crops can reduce cash crop yields (Nielsen et al., 2015). Conversely, the use of long fallows (eg. wheat-fallow-wheat) in low rainfall areas can conserve soil water and stabilise cereal or rotational crop yields over the longer term. While long fallowing is practised in the Wimmera and Mallee, continuous cropping (eg. wheat-wheat-wheat or wheat-lentil-wheat) is also practised, and the use of vetch cover crop is popular among growers for fixed nitrogen contributions and weed/disease break benefits.
Where long fallows are used on clay Vertosols in northern NSW/southern Qld, spring cover crops generally have no negative impact on soil water balances and subsequent crop yields when cover crops are removed early enough in the fallow to allow re-filling of profile water with following rainfall (Whish et al., 2009). The extra groundcover typically aids water infiltration into the soil in intense summer storms, which can offset the water transpired by cover crops (Whish et al., 2009). While rainfall infiltration rates into soils may not be an issue in the Wimmera and Mallee, it is possible cover crop residues may reduce evaporation of soil moisture to partially offset water use by the cover crops, and any nitrogen fixed by legumes can offset nitrogen fertiliser costs.
Aim
To identify and validate benefits of multi-species cover crop mixtures to a low rainfall cropping zone rotation and investigate how different cover crop termination methods affect the following year’s grain crop.
Paddock Details
Location: Watchupga
Crop year rainfall (Nov–Oct): 471mm
GSR (Apr–Oct): 304mm
Soil type: Sandy Clay Loam
Trial Details
Crop type: Scepter wheat
Treatments: Refer to Table 1
Target plant density: 130 plants/m²
Seeding equipment: Knife points, press wheels, 30cm row spacing
Sowing date: 13 May 2022 Replicates: Three
Harvest date: 7 December 2022
Trial average yield: 6.3t/ha
Trial Inputs
Fertiliser: Granulock® Supreme Z + Flutriafol (200mL/100kg) @ 60kg/ha at sowing, SOA @ 100kg/ha applied at GS13 and 220kg/ha of urea applied as a split application (GS13, GS31).
Trial managed as per best practice for pests, disease and weeds.
Method
A complete randomised block trial design was set up at Watchupga, Victoria, at the start of the 2021 growing season. The first year of the trial included rotational treatments (Table 1). The second year of the trial (2022) was bulk sown to Scepter wheat. Assessments for the 2022 growing season included biomass cuts at flowering, yield data and grain quality parameters. A soil sampling regime was set up over the 2021/22 summer fallow period at two-month intervals. Soil total carbon and nitrogen, hot water-extractable carbon, microbial biomass carbon, CO2 respiration, citrate-extractable protein and six soil enzyme assays were analysed. These tests provide an indication of overall soil health.
Results & Interpretation
Seasonal conditions
2022 was a very wet year when compared to the long-term average. Woomelang received decile 10 rainfall for the 2022 growing season. Higher than average rainfall in March and April provided good soil moisture for sowing and emergence. August, September and October combined to have 173mm higher rainfall than the long-term average for the same period (Figure 1). This provided optimal grain filling conditions.
2021 peak biomass
There was no difference recorded in peak biomass between the treatments (Table 2). This is because N wasn’t limited trial-wide at this stage (122kg N/ha).
Harvest Scepter
sown after a vetch brown manure had significantly higher yields, higher protein levels and grain income than all other treatments, and the wheat and summer cover rotations had significantly lower yields and grain protein and grain income than the vetch, 13-species mix, four-species mix and lentil rotations (Table 3). Higher protein levels in the higher yielding treatments suggest nitrogen — either fixed by the preceding rotation or stored by the preceding rotation’s residue — played a big role in the yield results for this season. This makes sense as the 2022 growing season rainfall at Watchupga reached decile 10 (Figure 1), making moisture a non-yield limiting factor and nitrogen a major yield limiting factor.
Soil health parameters
Soil health parameters, including total nitrogen and carbon, glucosidase activity and citrate-extractable protein changed significantly over time, but there was minimal effect from rotation treatment on these parameters (including other enzyme activities) (Figures 2–4). The exception was significantly higher glucosidase activity at termination of winter cover crops in plots with vetch, compared to winter fallow and wheat treatments. Generally, enzyme activities declined over the course of measurement. The increase in soil total nitrogen between two and four months after cover crop termination across all treatments (Figure 4) can be explained by the blanket addition of urea across the site.
Commercial Practice and On Farm Profitability
It is important to consider the extremely wet climatic conditions of the 2022 growing season when interpreting the outcomes of this trial. April rainfall of 71mm (Figure 1) provided ideal conditions for crop establishment and 173mm of rainfall in the last three months of the growing season (Figure 1) allowed a soft finish for grain filling. Results suggest winter mixed species cover crops can be used as an alternative to fallowing a paddock, but this requires caution. More typical Mallee growing seasons where stored moisture becomes a factor would likely produce a different outcome.
If climate forecasting improves to a point where growers can confidently predict when exceptional growing season rainfall will occur, mixed species cover crops will become a more viable replacement for long fallows. But even without a yield penalty, the question must be asked, do mixed species cover crops add any benefit to the system?
Our expectation was the additional carbon from the cover crops, and a variety of carbon inputs from the mixed species treatments, would improve soil health parameters following termination, even if these changes did not persist into the following season. However, despite the addition of around 2t/ha biomass from cover crops and vetch shoots, significant changes in soil function parameters were difficult to detect in the six- months after termination. The exception was a significant increase in glucosidase activity at termination of vetch, compared with wheat and long-fallow treatment plots. Glucosidase (and other enzyme) activities were generally higher in the winter cover crop treatments at termination, but the high spatial variability in these measures makes it difficult to detect differences that are statistically significant, particularly with only three replicates. Glucosidase is an enzyme produced by soil micro-organisms which is involved in breakdown of labile organic matter, reflecting the ability of vetch residues to stimulate microbial activity.
The trial highlighted the importance of vetch in a southern Mallee farming rotation. It’s a good demonstration of how vetch brown manure rotations lead to superior wheat yields in non-moisture limited seasons and, in such scenarios, are far more valuable than fallow rotations. Vetch brown manure rotations will also benefit if a following wheat crop falls in a drier season, as the high nitrogen fixation by vetch will reduce or eliminate the need to spread urea, especially if urea prices remain high at $1400 per tonne. The trial also demonstrated growing wheat on wheat continuously can potentially lead to yield decline.
Further long-term research into how mixed species cover crops affect farming rotations and soils is required before growers can be confident in using them.
References
Nielsen, D.C., Lyon, D.J., Hergert, G.W., Higgins, R.K. and Holman, J.D., 2015. Cover crop biomass production and water use in the Central Great Plains. Agronomy Journal, 107(6), pp. 2047–2058. Whish, J.P.M., Price, L. and Castor, P.A., 2009. Do spring cover crops rob water and so reduce wheat yields in the northern grain zone of eastern Australia? Crop and Pasture Science, 60(6), pp. 517–525.
Acknowledgements
This project is supported by Southern Cross University, through funding from the Australian Government’s National Landcare Program.
