James R Hunt1,3, Antony D Swan1, Paul D Breust2,4, Mark B Peoples1, John A Kirkegaard1
1CSIRO Agriculture & Food, PO Box 1600 Canberra ACT 2601 Australia
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 [email protected]
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 graze crop stubble residues after harvest. Recently, growers practicing no-till, controlled traffic cropping, became concerned that grazing livestock would damage soil and reduce soil water capture, crop yield and profitability. Sheep grazing on stubbles remove residue cover and compact surface soil, but there is little published research on potential impacts on subsequent crop performance. A long-term experiment was established in 2009 to quantify trade-offs between grazing stubbles, resource capture and subsequent crop performance. Here we report effects on soil mineral nitrogen (N) accumulation and grain N uptake due to stubble grazing in the seven phase years of the experiment in which wheat crops were grown. Grazing wheat and canola stubbles on average increased mineral N prior to sowing of the subsequent wheat crop by 19 kg/ha, and grain N uptake by 7 kg/ha N. This could have arisen from 1) rapid mineralisation of N in livestock excreta, and/or 2) the reduction in stubble carbon inputs to soil due to grazing lowering rates of N immobilisation. Further research is necessary to confirm the relative importance of these processes, and to explore how they could be exploited to greater advantage to manage soil N availability in mixed farming systems.