THERE was no increase in wheat grain yield in the first crop following canola, however, both grazing and burning stubble increased grain yield in the second wheat crop after canola by an average of 0.7 and 0.8 tonnes to the hectare respectively, and by 1t/ha when both grazed and burnt.
These yield gains occurred on average each year in a long-term research study between 2009 and 2016. Yield in the second wheat crop following canola (continuous zero till rotation of canola, wheat, wheat), where stubble was retained and not grazed or burnt averaged 3.93 t/ha. However, there was a 0.7t/ha yield increase to 4.58t/ha where stubble was heavily grazed, and a 0.8 t/ha increase to 4.68t/ha where stubble was not grazed but burned before sowing and a 1t/ha increase where stubble was grazed and burnt (average yield was 4.89t/ha).
These results were presented at the recent 2017 Australian Agronomy Conference at Ballarat, Victoria. Authors of the report and study were respected CSIRO and FarmLink (farmer group) researchers Tony Swan (CSIRO), James Hunt (CSIRO), Paul Breust (FarmLink), Tony Pratt (FarmLink), Brad Rheinheimer (CSIRO), Kellie Jones (FarmLink) and John Kirkegaard (CSIRO). The Grains Research and Development Corporation funded much of the research.
The trial was undertaken near Temora in south east NSW (299 metres above sea level) on a red chromosol soil (red loam with contrasting texture through the profile). Crops were sown in mid-late April in all years with both crop phases kept in a rotation of canola-wheat-wheat. Sowing included Flexi-Coil 250-kilogram break out tines set on 305-millimetre row spacing and fitted with Agmaster® boots, with 12mm knife points and press wheels. All crop operations were on control traffic and there was zero tolerance for weeds in-crop and during the fallow.
Tony Swan said grazing and burning stubbles increased soil mineral nitrogen accumulation during the summer fallow to a much greater extent in the second wheat crop compared with the first wheat crop. Presumably this was because of both higher amounts and higher carbon-nitrogen (C:N) ratio of wheat stubble compared with canola stubble, which would lead to more nitrogen immobilisation.
Grazing stubble increased soil mineral nitrogen (compared with stubble retention) by only 13kg/ha in the first wheat crop following canola, but by 33kg/ha in the second wheat crop. Burning stubble had no impact on soil nitrogen in the first wheat crop, but increased soil mineral nitrogen by 13kg/ha in the second. Grazing either removed carbon from the system or neutralised carbon with potential immobilising power of 52kg/ha nitrogen.
In one of the eight years (2013), burning also reduced frost-induced sterility in the second wheat crop after canola, from 59 to 30 per cent, following severe frosts of -2.6, -1.8 and -3.6 degrees Celsius (screen temperatures) that occurred on October 15, 16 and 18. In 2013, grazing increased yield of the second wheat crop by 1t/ha, burning by 1.4 and combined by 1.6 t/ha.
While Mr Swan believes yield differences attributed to grazing and burning could logically also be attributed to various biotic stresses such as disease, no treatment differences were recorded with all having low level of stubble-borne diseases (yellow leaf spot, crown rot, septoria tritici). The research indicated nitrogen dynamics are principally responsible for the measured differences in yield.
Tony Swan and colleagues conclude that where full stubble retention was practised avoiding growing wheat after wheat as part of the rotation would be recommended. Reducing stubble load by grazing and/or burning will reduce the risk of yield penalties related to nitrogen immobilisation and frost damage. If farmers want to grow consecutive wheat crops and retain stubble, additional nitrogen and/or deep banding N at sowing would be recommended.
