Optimising the yield and economic potential of high input cropping systems in the High Rainfall Zone

Citation: Christy B, Clough A, Riffkin P, Norton R, Modwood J, O'Leary G, Stott K, Weeks A, Potter T. 2015. Managing crop inputs in a high yield potential environment - HRZ on southern Australia. State of Victoria Department of Economic Development, Jobs, Transport and Resources. Melbourne. 36 pp.

Executive Summary
The high rainfall zone (HRZ) of southern Australia has the potential to produce far higher grain yields for wheat and canola than are currently achieved. A key criteria for achieving high grain yields is for crops to have access to adequate nutrition.
Information attained from growers and consultants in interviews leads to the conclusion that for a variety of reasons farmers are targeting yields of around 5 t ha-1 (far less than their potential) and are applying a level of inputs commensurate with that yield target. Up-front input costs are high, and with fertiliser application being the highest variable cost in a cropping enterprise, there appears to be a reluctance to apply higher levels of fertiliser to close the gap between actual and potential grain yields. Data collated in this report indicates that the key barrier limiting current nutrient application rates is growers and advisers lacking confidence that using higher application rates will result in an economic return on investment. Their lack of confidence stems from the knowledge gaps around the cost and responses of applying additional nutrient inputs to wheat and canola in crops with high grain yield potential. This report identifies the gaps in current knowledge that limit the ability of growers and advisers in the HRZ to confidently predict the additional nutrient inputs required by crops and the associated economic risks.

Crop yield potential
The HRZ of southern Australia has high yield potential with grain yields for wheat estimated at 4.5 t ha-1 in W.A. to 11 t ha-1 in south eastern Victoria and 3 t ha-1 to 5 t ha-1 for canola depending on location (Zhang et al. 2006; Acuña et al. 2011; Riffkin et al. 2012; Christy et al. 2013). However, current on-farm yields are often only half to a third of these values averaging 2.7 t ha-1 for wheat and 1.4 t ha-1 for canola.
Some of this unrealised potential with current varieties is not being achieved due to nutrient inputs being lower than required to achieve maximum grain yields. Additional reasons for these relatively low yields are poorly adapted germplasm, periodic waterlogging, soil acidity, disease and sub optimal management. Currently many advisers are relatively new to cropping in the HRZ and have varying levels of knowledge and support in making recommendations. Often advisers do not feel adequately equipped to confidently assess crop demands and limitations, predict yield potential or the risks associated with high input systems in a variable climate. Consequently, recommendations are often conservative, leading to unrealised potential yields, low protein content and thus lost opportunity for growers.

Current nutrient status in the HRZ
A situation analysis of the soil nutrient status across the HRZ of southern Australia indicates a range of different nutrient deficiencies in different regions. An analysis of nutrient soil test data from the National Land and Water Resources Audit (NLWRA) shows that nutrient status for each nutrient varies, with large areas of land indicating that they are likely to be responsive to the application of phosphorus (P), potassium (K), sulphur (S) and lime (Figure 1). Additionally, the spatial pattern of where each nutrient is most limited varies considerably across the HRZ. Data collated by Incitec Pivot (soil tests 2010 for the SA and Vic HRZ) suggests that these spatial images are conservative in their prediction of potential crop nutrient response. The Incitec Pivot data showed that 50% of the soils have a pHca of less than 5.0, 40% of soils were low in K and S, and soil and tissue tests showed micronutrient deficiencies of 20% for copper (Cu) and 10% for zinc (Zn).