Increases in dry matter production and feed quality of forage rape (Brassica napus) after subsoil manuring

Keywords

• soil structure, organic amendments, sodic soil, clay aggregation.

Take home messages

• A subsoil manuring trial was established on a brown Sodosol at Epping Forest, Tasmania in May 2015.
• Application of poultry manure at depth significantly increased feed quality and dry matter production of forage rape (Brassica napus cv. Rangi) in 2017-18 compared with both the deep ripping only and standard treatments (7.0t/ha compared with 4.2 and 4.1t/ha, respectively).
• Feed quality tests also showed 84% higher crude protein content in the manure-only treatment.
• Trial work will continue in 2019 to determine the longevity of subsoil manuring effects on crop and pasture production, changes in soil-water characteristics and the economics of application.

Background

Texture contrast soils, where shallow topsoils overlie clay, comprise 63% of soils in the Tasmanian high rainfall (>500 mm) cropping agro-ecological zone of south-eastern Australia (MacEwan et al 2010). The major constraints to root growth in these soils are physicochemical, the result of dense clay subsoils with high bulk density and low macro-porosity and water infiltration. Many texture contrast soils in south-eastern Australia are also sodic at depth and dispersive which further exacerbates subsoil constraints (Gill et al 2009, MacEwan et al 2010). Sodic soils, where sodium comprises over 6% of the cation exchange capacity (CEC) in the B horizon, are estimated to occupy 23% of the Tasmanian land area (Doyle and Habraken,1993) with most of this area being used for grazing and increasingly, broadacre cropping (Cotching et al 2001).These subsoil constraints commonly restrict root growth and are limitations to the productivity of pastures (Greenwood et al 2006) and cropping across much of the high rainfall zone of south-eastern Australia (MacEwan et al 2010).

A number of field studies in these soils, particularly in the HRZ of south west Victoria, have demonstrated the benefits of subsoil manuring (SSM) where high nutrient quality organic amendments, such as poultry manure litter, are slotted at depth into the clay subsoil. Across 11 site x season trials, Peries and Gill (2015) cite a 63% average increase in cereal yields with application of 10-20t/ha of composted poultry manure compared with nil and ripping-only treatments. While productivity gains have been attributed to enhanced nutrition (Celestina et al 2018), significant improvements in soil structure through reduced bulk density and increased macro-porosity and saturated hydraulic conductivity have been recorded (Clark et al 2009, Gill et.al. 2009). Improvements in soil structure encourage deeper root growth in the clay horizon and increases in plant available water capacity have also been well documented (Gill et.al. 2009; Peries and Gill, 2015; Armstrong et al 2017). Rainfall/soil moisture during grain fill is commonly yield limiting on these soils and access to increased soil water at depth is important for improved productivity.

This study was conducted to evaluate the effects on plant productivity and soil quality of slotting a high nutrient quality organic amendment (poultry manure) into a sodic soil in a crop and pasture rotation in northern Tasmania. The first two years of the trial examined plant growth and grain yield responses in wheat crops. This paper reports on the effects of subsoil manuring on dry matter production and feed quality of forage rape in the third year of the trial.

Methods

Site, treatments and experimental design

The trial site was established at Epping Forest, northern Tasmania (41°44’26” S, 147 21'10" E) in May 2015. The soil type is a brown Sodosol with a fine sandy loam topsoil generally 20–25cm deep overlying a bleached sandy loam A2 horizon of variable thickness (commonly 5–10cm) and beneath this a B horizon of medium clay. General soil chemical and physical properties are presented in Table 1. Common to many sodic soils used for agricultural purposes in Tasmania, the subsoil is only moderately sodic (6–15% ESP) but in addition there are high levels of exchangeable magnesium (Mg) (over 50% of the CEC). Rainfall was recorded at the Australian Bureau of Meteorology weather station at Epping Forest (Forton; 1927 - present), 3km from the trial site.

Table 1. Summary of soil chemical and physical properties at commencement of subsoil manuring trial, Epping Forest, 2015. EC, Electrical conductivity (1:5 water); OC, organic C; available N, total of nitrate-N + ammonium-N; CEC, cation exchange capacity; ESP, exchangeable sodium %; EMP, exchangeable magnesium %; BD, bulk density.

Treatments consisted of no ripping/no amendment (standard commercial practice), deep ripping with no amendment (control) and ripping with poultry manure (15t/ha, fresh weight). As wheat straw is commonly in surplus on-farm and has a high C/N ratio, a fourth treatment was included comprising wheat chaff added to poultry manure at a ratio of 2:1 by volume (2.1:4.9t/ha on a weight basis). This resulted in an amendment with a significantly higher C/N ratio (10.3 compared with 6.8). Plots were 3.6m wide x 20m long and there were four replicates.

Amendments were slotted at a depth of 30–35cm forming a concentrated ‘sausage’ of material. This depth was generally into the top of the clay B horizon but was not consistent due to the inherent variability in this soil type. The slotting operation used a custom-made subsoil manure ripper (see acknowledgements) with two rippers spaced at 80cm. With appropriate soil moisture conditions, ripping will generally result in significant shattering of the clay subsoil. However, due to the compactness of the clay, only narrow points could be used on the ripper tynes; consequently, there was only moderate shattering of the subsoil during the ripping operation.

Trial management and measurements

Plots in the first two years of the trial (2015 and 2016) were comprised of two 1.8m wide raised beds and wheat was grown in both years. In 2017 the raised beds were disced in and the trial area sown with forage rape (Brassica napus var napus cv. Rangi) on 13 Oct at a sowing rate of 3.5kg/ha. Basal fertiliser of DAP was applied at 80kg/ha. No herbicides or pesticides were required during growth of the forage crop.

Dry matter cuts (2m2/plot) and normalised difference vegetation index (NDVI) measurements, using a Trimble GreenSeeker handheld crop sensor, were taken on 18 Dec 2017 at BBCH crop growth stage 30 (late rosette). Subsamples were oven dried at 56°C for 48 hours. Crude protein, ash and fibre content, digestibility and an estimate of metabolisable energy were determined by near infrared reflectance at FeedTest Laboratories, Werribee. All quality data are reported as a percentage of dry matter (DM). Differences between treatment effects were analysed by ANOVA using R version 3.5.1 (2018/7/2). The least significant difference (Lsd) was calculated at P =0.05 for testing differences between treatments.

Results and discussion

Plant establishment was good and there were no visible differences between treatments in early growth. Rainfall prior to sowing and during the growing season (April-Dec) was 399mm and close to average (Decile 5). DM production was also likely influenced by rainfall distribution with over 70mm rainfall in early December.

Growth of forage rape in the manure-only plots was substantially greater than in other treatments and this was reflected in NDVI ratios at GS 30 which were significantly higher compared with both the control and standard practice (Figure 1A). The poultry manure + straw treatment was intermediate and significantly different to the other treatments. DM production showed a similar ranking with comparable values for the standard practice and control (4.1 and 4.2t/ha, respectively) and a large, highly significant (P <0.001) increase in DM with the manure-only treatment (7.0t/ha; Figure 1B).

Results from previous SSM field trials have varied from a mean grain yield increase of 63% recorded across 11 site x season trials between 2005 and 2012 in south west Victoria (Peries and Gill, 2015) to little or no grain yield increases (Celestina et al 2018). Differences in responses may relate to variation in growing season rainfall and the distribution of this rainfall (Armstrong et al 2017) with low seasonal rainfall and a sharp finish limiting the benefits of SSM, particularly in the establishment years. In the current trial, rainfall was slightly above average with sufficient soil recharge over winter.

Unlike grain yield data, there have been surprisingly few SSM field trials where biomass has been documented. However, in all trials where vegetative response has been recorded (Gill et al 2008, Gill et al 2012) this has been considerably greater than that of grain and comparable with the large increase in DM production obtained in this study. While there have been significant increases in crop yields with SSM there may be potentially greater benefits with fodder crops/pasture. In the latter, productivity gains are a measure of total biomass production compared with grain yield where, with a harvest index of approximately 50%, only half of the additional biomass is taken into consideration as an improvement in productivity. The timing of this extra biomass is also important for grazing management and with visual responses apparent in late August in other trials, SSM will likely provide opportunity for additional feed before the end of winter.

Figure 1. Effect of deep ripping and subsoil manuring on normalised difference vegetation index (A); Dry matter production (B); Crude protein content (C); Ash content (D). Means with the same letter are not significantly different (P =0.05).

Where large increases in grain yield have been obtained, in addition to nutrient response, it has been hypothesised that organic amendments stimulate soil microbes which generate extra-cellular polysaccharides ‘cementing’ clay particles to produce stable macro-aggregates (Clark et al 2009). Root growth at depth also proliferates producing root exudates and mucilage which provide further aggregation of clay particles as well as supporting additional microbial activity (Gill et al 2009). Finally, aggregation is also enhanced by soil wetting and drying cycles with subsequent root growth extending into regions of the B horizon beyond the immediate zone of application (Gill et al 2009, Armstrong et al 2017). The outcome of this is greater root exploration in the B horizon and increased plant available water capacity and extraction of subsoil water. Of note, on a fresh weight basis in the current study there was a 160% increase in yield with the manure-only treatment; the moisture content of the manure treated plants was significantly higher (P <0.001) than from standard non-ripped plots (data not presented) presumably through greater access to moisture in the clay subsoil. To confirm this, detailed soil water and physics measurements will be conducted at the end of the 2019 season to compare with benchmark soil tests from the first year of the trial.

Ripping without amendment provided no production benefit compared with the standard of no ripping (Figure 1). While the results in this study are from three years after the ripping operation there was also no difference in wheat crop growth and grain yield between these two treatments in the first year of the trial (data not presented). Previous studies have also generally recorded few positive crop responses from ripping alone on poorly-structured clay soils (e.g. Gill et al 2008, McBeath et al 2010).

Feed quality characteristics generally showed significant improvements with both the manure-only and manure + straw treatments. For example, crude protein and ash content were respectively 84% and 50% higher with the manure-only treatment compared with the standard practice (Figure 1C and Figure 1D). Other studies have shown similar improvements in quality. Increases in grain protein in SSM trials sown with wheat have ranged from 20-39% (Gill et al. 2008, Gill et al. 2012, Peries and Gill, 2015).

Conclusions

Application of poultry manure at depth significantly increased DM production and feed quality and this study has shown the significant potential of SSM for improving forage productivity on sodic texture contrast soils. While the majority of SSM field trials have been associated with grain production there is considerable scope to improve forage crop and pasture productivity with this practice. The site at Epping Forest was sown with ryegrass/clover pasture in 2018 and trial work will continue in 2019 to determine the longevity of subsoil manuring effects on crop and pasture production, changes in soil-water characteristics and the economics of application.

References

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Cotching WE, Cooper J, Sparrow LA, McCorkell BE, Rowley W (2001) Effects of agricultural management on Sodosols in northern Tasmania. Australian Journal of Soil Research 39, 711–735.

Celestina C, Midwood J, Sherriff S, Trengove S, Hunt J, Tang CX, Sale P, Franks A (2018) Crop yield responses to surface and subsoil applications of poultry litter and inorganic fertiliser in south-eastern Australia. Crop and Pasture Science 69, 303–316.

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Gill JS, Clark GJ, Sale PW, Peries RR, Tang C (2012) Deep placement of organic amendments in dense sodic subsoil increases summer fallow efficiency and the use of deep soil water by crops. Plant and Soil 359, 57–69.

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MacEwan RJ, Crawford DM, Newton PJ, Clune TS (2010) High clay contents, dense soils and spatial variability are the principal subsoil constraints to cropping the higher rainfall land in south-eastern Australia. Australian Journal of Soil Research 48, 150–166.

McBeath TM, Grant CD, Murray RS, Chittleborough DJ (2010) Effects of subsoil amendments on soil physical properties, crop response, and soil water quality. Australian Journal of Soil Research 48, 140–149

Peries R, Gill JS (2015) Subsoil manuring in the high rainfall zone: a practice for ameliorating subsoil for improved productivity. In ‘Proceedings of the 17th Australian Agronomy Conference, Hobart’. (Australian Society of Agronomy: Melbourne)

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation (specifically Andrew and Rae Dowling; Michael and Fiona Chilvers) and the support of the GRDC, the authors would like to thank them for their continued support. The authors also thank NRM North (Project PL080) and TIA for initial seed funding to establish the trial and Bill Field and Bruce Dolbey for technical assistance. John McPhee, TIA and Tasmanian Agricultural Productivity Group are acknowledged for assistance and loan of the subsoiler, built through a CFOC Innovation Grant (INNOV-145).

Contact details

Geoff Dean  
Tasmanian Institute of Agriculture,
165 Westbury Rd, Prospect, Tasmania, 7250    
0407 872084
Geoff.Dean@utas.edu.au