Antony D Swan1, James R Hunt1,3, Paul D Breust2,4, Tony Pratt2, Brad Rheinheimer1, Kellie Jones2, John A Kirkegaard1
1CSIRO Agriculture and Food, PO Box 1600 Canberra ACT 2601 Australia. firstname.lastname@example.org
2FarmLink Research, PO Box 240 Junee NSW 2663 Australia
3Present address: Department of Animal, Plant and Soil Sciences, AgriBio Centre for AgriBiosciences, 5 Ring Rd, La Trobe University, Bundoora 3086, Australia
4Present address: Southern Farming Systems, 23 High Street, Inverleigh VIC 3321
In southern Australia, the majority of farms combine a sheep enterprise with cropping to form a mixed farming business. Crops are grown in sequence with pastures, and sheep access crop stubble residues after harvest. A canola crop followed by two wheat crops (C-W-W) established using no-till seeding systems is a typical three-year crop sequence in southern New South Wales (SNSW). After harvest, growers reduce residues by grazing or burning, or retain all stubble. A long-term experiment was established in 2009 to quantify trade-offs between heavy grazing and/or burning stubble compared with retaining all stubble on soil conditions and crop performance. Here we report the effects of either grazing, burning or retaining the stubble on wheat grain yield in a C-W-W sequence over the eight years. Neither burning nor grazing increased yield in the 1st wheat crop after canola. However, both grazing and burning increased yield in the 2nd wheat crop after canola by an average of 0.7 t/ha and 0.8 t/ha respectively, and by 1.0 t/ha when applied together. This yield response was at least partly due to immobilisation of N by large amounts of high C:N ratio wheat stubble retained by the 1st wheat crop. Grazing the stubble increased soil mineral N by only 13 kg/ha in the 1st wheat crop following canola, but by 33 kg/ha in the 2nd wheat crop. Burning stubble had no impact on soil N in the first wheat crop but increased soil mineral N by 13 kg/ha in the 2nd wheat crop. In one of the eight years, burning also reduced frost-induced sterility in the 2nd wheat after canola, from 59% to 30%. Our results suggest that growers wishing to retain all stubble should avoid growing wheat after wheat, or reduce the stubble load to reduce the risk of yield penalties related to N immobilisation and frost damage.