Brown Manure Field Peas
The area sown to brown manure field peas has grown over the last three years. Brown manure crops can be an effective way of managing resistant weeds, plus increasing soil Nitrogen and organic matter levels. However, they need to be grown as part of a system, with a clear objective and a disciplined approach to management. The introduction of brown manure field peas and the research done by Robert Patterson and a number of Rural Management Strategies clients, evolved as a result of clients' desire to remove livestock and pastures from their production system.
When livestock, or more importantly legume pastures were removed from the system, it very quickly became apparent that continuous cropping in the absence of a legume break was unsustainable. In addition, the reliance on selective herbicides for weed control lead to a rapid build up in resistant grass weeds.
To date the system appears to have worked well, but it should be recognised that it is a system devised to allow continuous cropping to be carried out in a sustainable manner. Its place is where livestock are removed from the system; it is not seen as an improvement over a mixed farming system. Where management is willing and capable of running a productive mixed farm based on legume pastures, this is an equally profitable, lower risk, sustainable and proven business model.
Growing a brown manure crop provides the opportunity to rebuild soil Nitrogen, increase soil organic matter and gain control of resistant wild oats and ryegrass. Field peas were selected for their ability to produce dry matter and build Nitrogen over a range of soil types and seasonal conditions. Field peas are also more competitive than say lupins or vetch, against grass weeds in the absence of selective herbicides.
Like all other aspects of farming, the success of brown manure crops relies on having a clear objective plus a disciplined approach to management. Maximising the success of brown manure field peas requires the following:
• Early Sowing - Allows the peas to get out of the ground with or before the weeds. It also extends the growing season allowing the peas to produce more dry matter and fix more Nitrogen.
• Varietal Selection – Selecting a vigorous and tall, competitive variety will increase weed competition, dry matter production and hence Nitrogen fixation.
• Minimising Inputs – One of the primary aims of the system is to reduce growing costs and achieve a break in the use of commonly used chemicals, for reasons of resistance management. To do this, peas are sown with no fertiliser on high Phosphorus soils and no selective herbicides, or at most with low rates of MAP plus an application of Trifluralin incorporated by sowing. No other inputs are applied during the growing season.
• Minimising the Risk of Bacterial Blight –Bacterial Blight is the single biggest risk to the use of peas as a brown manure crop. The Bacterial Blight risk is minimised by:
o Avoiding growing field peas adjacent to (particularly downwind) of field pea stubble. o Ensuring seed is sourced from a Bacterial Blight free crop. o As Bacterial Blight is associated with physical damage of the plant, vehicles, people or livestock should not under any circumstances enter the paddock after the crop is sown.
• Well Timed Chemical Fallow – The Glyphosate spray to chemically fallow the field peas must be applied prior to Wild Oats setting seed. This is likely to be mid to late September.
• Double Knock – A follow up application of Paraquat, approximately ten days after the Glyphosate application, is essential to ensure that 100% control of grass weeds is achieved and that Glyphosate resistant weeds do not escape.
• Appropriate Management of the Field Pea Stubble – Management of the field pea stubble after the chemical fallow will depend on an individual’s sowing system. Many disc machines and tined machines with coulters will be able to sow through field pea stubble. In some cases an additional operation may be required to allow the following crop to be sown. This may be an offset disc or a speed tiller.
• Appropriate Selection of the Following Crop – In most cases canola is grown as the crop following brown manure field peas. Canola provides a second year of grass weed control, a second year break from cereal diseases, plus has a high Nitrogen requirement.
As pea straw is the mulching choice of gardeners, the residue from brown manured peas provides very good ground over the summer, reducing evaporation of any summer rainfall. This allows for earlier sowing of canola with confidence.
To date this system has provided improved weed control, increased soil Nitrogen, plus an extended disease break. This has lead to lower costs, lower risk and increased yields. However, the system is not without risks. By far the greatest risk is the build up of Bacterial Blight, which has the potential to completely wipe out badly infected field pea crops.
One of the greatest risks of Bacterial Blight build up, is likely to come from growers attempting to grow their own seed, or from those without a clear objective and plan. Seed production should be left to experienced field pea growers, who have a track record of producing Bacterial Blight free seed, which is also free of Pea Weevil infestation and Heliothis damage.