GRDC speakers detail how to make most of N applications

"The million dollar question this year, and every year, is how much N do I need to feed my crop?" Ms Dixon said in her presentation at a recent GRDC forum in Kimba.

The Pinion Advisory agronomy team have developed N strategies for clients using rules of thumb and data on what N requirements varying crops require to increase yield, as well as current fertiliser and grain prices.

Calculations had shown urea was the most economic N option this year, over UAN and SOA fertilisers.

"This year we're paying the most we've ever paid for nitrogen fertiliser and hopefully we're going to be getting paid the most we ever have for our grain," Ms Dixon said.

"It creates an interesting economic dynamic and means it's important to get it right.

"The question is how do we value spending money on fertiliser and what return are we going to get? What rates should we target and where are the best returns going to be?"

Using rules of thumb of a 50 per cent uptake efficiency of total N supply, and protein assumptions of wheat at 11pc, durum at 13.5pc and barley at 10.5pc, Ms Dixon calculated that the crop N requirement to increase grain yields by 1t/ha would be 35kg of N/ha for barley (equivalent of an 80kg/ha urea rate), 40kg (90kg/ha urea) for wheat, 50kg (110kg/ha urea) for durum and 80kg (175kg/ha urea) for canola.

Using wheat as an example and including a $10/ha spreading cost, that would be a $123/ha outlay on N with urea at $1250/t, or $100/ha outlay with urea at $1000/t to increase yield by 1t/ha.

The cost to increase canola yield by 1t/ha would be $229/ha with urea at $1250/t and $185/ha with urea at $1000/t.

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"Once you know how much it costs to increase yield, you can look at how it relates to the income return you're getting from your grain using a benefit cost ratio," Ms Dixon said.

"The higher the return and BCR is, the less risky that decision is. With a $1:$1 ratio being your break even."

While there would naturally be diminishing returns the closer growers got to applying towards their maximum yield possible, Ms Dixon's working model showed with urea costing $1250/t and canola price at $1000/t, and a urea rate of 175kg/ha to increase yield by 1t/ha, the cost of application would work out to $229/ha including a $10/ha spreading cost.

This would equate to a benefit cost ratio of $4.40:1 when accounting for the extra yield, with similar equations used for durum, wheat and barley resulting in BCRs of $4.10:1, $3.70:1 and $3.50:1 respectively.

Ms Dixon said those benefit cost ratios could help growers decide which crops to target with fertiliser if they had a limited N budget.

"It has become increasingly important in recent years to scrutinise our approach to applying fertiliser and finding out where we're going to get the best bang for our buck," she said.

In comparison to previous years, Ms Dixon said wheat's BCR at $3.70:1, as of the first week of July, was marginally ahead of the 2021 BCR ($3.60:1) at the same point in the season, but down on 2020 ($4.70:1).

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Ms Dixon said calculating a fed-to yield (total N supply-to-date using a conservative N mineralisation estimate divided by crop N requirements needed for 1t/ha of grain to determine a possible yield achievable) and target yield (a realistic estimate of crop yield potential based on soil moisture, rainfall, forecasts and paddock performance data) would be a vital equation in dialing in on the optimum urea rate.

"Comparing your fed-to yield to your target yield will enable you to calculate the N required to reach that target yield," she said.

"Assess the likely return from applying that, assess the inherent risk, consider future rotations and ground truth the decision by going out and looking at your crop."

Ms Dixon said that growers who had fed to their target yield then may consider applying extra N to increase protein and achieve a higher quality grade of wheat.

Using current forward prices for ASW1, APW1 and H2, and considering a urea cost of $1250/t, Ms Dixon said applying extra urea to increase protein by 1pc and move a grade would be economically justified for moving from ASW1 to APW1, but was currently not as economically viable for moving from APW1 to H2.

Fellow GRDC presenter, and Breezy Hill Precision Ag Services consultant, Jessica Koch said map layers could provide the cherry on top when it came to making precision N decisions.

Ms Koch runs a consulting business based at Booleroo Centre, providing data processing of all types of map layers.

She said N decisions were hard to make "on-the-fly" so having some data and evidence to assist was critical and would help take the emotion out of the decision.

"Having taken N tests with a site-specific purpose, maps can help us work out where to put N rich strips and how to look for a response through our imagery.

"We can use change detection by looking at a satellite image from an earlier date compared to a later date to look at the difference and work out whether a second spread is even necessary, or whether it can be a variable rate spread based on the response from the first spread."

Ms Koch and husband Joe used N requirement calculations and mapping in 2021 to conclude that a particular paddock had a yield potential of 3t/ha and corresponding N requirement of 0kg/ha of urea.

Therefore, that crop received no N after seeding and the final yield map showed an average yield of 2.94t/ha across the paddock, despite a dry finish.

Using their software and supplied data, Ms Koch said N requirements for different paddock zones could be calculated.

"We can create N prescriptions with more spatial oomph than just basic satellite imagery," she said.

Yield and protein responses could also be analysed after harvest to work out if, and how, profitable those N decisions were.