Precision N rates for dryland farming? Financial analysis of variable-rate Nitrogen (VRN) in the Riverine Plains Region

Collaboration with members of the Riverine Plains Inc. group of farms, near
Yarrawonga and Dookie, Victoria in south-eastern Australia, allowed estimates
of the financial costs and benefits of precision Site-Specific Nitrogen
Applications (also called Variable Rate Nitrogen ,VRN).
Geo-referenced wheat, barley and canola crop yields were matched with
growing season rainfalls (GSR) and EM38 survey maps (indicating water-
holding and cation exchange capacity), to compare their values for calibrating N
application rates given the high spatial and temporal variabilities of productivity
in the paddocks sampled.
Tracing the yield-rankings of 90x90m geo-referenced grid areas across several
years showed fewer than 5% of grid areas remained in the same yield quartile in
four out of five years. EM38 data were positively correlated with grain yield
only in years of low to medium GSR. A N-rate trial on high and low EM38
patches on each of two farms produced mixed results in 2017. NDVI (greenness
indications) appeared to give better results.
Farm financial risk profiles were determined by simulating long-term effects of
VRN on the equity of hypothetical farms. These profiles showed that the costs
of VRN would be met by less than one percent increase in yields, or by at least a
70% decrease in applied N. Beginning with no debt and assuming no further
benefits of VRN, a low-cost farm showed no risk of loss over random decades
drawn from the past 56 years of weather records while a high-cost farm faced
risk of loss of 41%, increasing to 44% with the additional costs of VRN.