While it may be reasonable to expect some inputs to increase after a profitable year, no one expected the cost of one commodity in particular to rise like it did.

Of course, I’m talking about fertiliser. When we first saw prices rise last year, few could have ever imagined them reaching their current levels.

It is arguably the biggest cost challenge that growers have ever had to face, so it was not surprising that the first session of this year’s online National Tillage Conference focused on crop nutrition strategies.

Dr Richie Hackett spoke about the importance of understanding how plants respond to nitrogen (N) and the need to determine an economical application rate for crops this year.

This will help inform growers on the level of sensible nitrogen reduction, if any. Based on Richie’s calculations, this reduction could range between 27-35kg N/ha for winter barley and wheat crops. This article explains how these levels were determined and how growers can assess their individual reduction rates.

Nitrogen response curve

Plants response to N application is not a straight line relationship, but a curve. The biggest yield responses to applied N come from the lower doses, while the higher doses return less per unit of nitrogen. Based on winter cereal research, Richie said the first 50% of an N application typically gives 70% of the crop’s yield response. As N rates increase, the law of diminishing returns kicks in, leaving the last 50% of N giving just 30% of the yield response.


Because of this, the economics of applying N change depending on where it is applied on the response curve.

Let’s look at an example based on average prices of grain at €154/t and CAN at €251/t. This equates to €0.93/kg N. In the first application, 1kg N/ha would return around 52kg of grain, or €8 for every €0.93 of N spent.

As N application rates increase and we move further up the response curve, this return decreases. At around 100kg of N/ha, Richie says 1kg N/ha would return around 30kg of grain or €4.62 for every €0.93 of N spent.

Eventually, you reach a point on the curve where every 1kg N/ha applied delivers no extra grain. Just before this point, you reach the breakeven point where €0.93 spend on N delivers €0.93 of grain. Applications beyond this result in a loss.

When worked through, this equates to a total application rate of 209kg N/ha, known as the economic optimum N rate.

On average, based on an overall rate of 209kg N/ha, every 1kg N applied will return 6kg of grain. This 6 figure is what is known as the Break Even Ratio (BER). Put simply, the BER can be calculated by dividing the N cost per kg of N by the price per kg of grain.

Today’s prices

Now let’s factor in today’s prices to calculate our BER. This example is based on grain at €220/t (€2.2/kg) and CAN at €675/t (€2.50/kg N). The BER works out at 11. Therefore, every kg of CAN applied must deliver 11.4kg of grain.

To achieve that BER of 11.4, the optimum rate of N will be lower, i.e the optimum rate will move further back on the N response curve.

Richie explains that for each unit change in the BER, N rates are adjusted by 6kg N/ha. From 6 to 11.4 (i.e 5.4) this works out as a reduction in N rates of around 32.5kg N/ha.

Tailoring this to specific crops equates to a reduction of 35kg N/ha for winter wheat and 27 kg N/ha for winter barley. This is roughly a 16% reduction in N application rates.

Richie did stress however that this is not an absolute figure due to variability in fields, crops, seasons etc.

How will this impact yield?

The question then turns to what impact this N reduction will have on yield. Richie estimates that this 16% N reduction will result in a yield loss of between 0.2-0.3t/ha for winter and spring barley and winter wheat.

Richie stressed that malting barley is an exception to this, as N rates are already significantly reduced for distilling.

How will this impact margins?

Richie explains that as long as grain prices hold up (based on €220/t), an average N rate of 175kg N/ha will still deliver an acceptable margin, as long as anticipated yields are achieved.

He added that if N application rates decrease, fertiliser should be applied as close as possible to when the crop needs it. In the case of winter crops, N uptake is at its greatest during stem extension.

He recommended sticking to a three split application strategy when applying more than 150kg N/ha, but equalise the percentage reduction in rates across all splits.