What does getting the most from your farm actually look like? Is it about achieving the highest possible yields, getting the best return from every pound spent, or finding a balance between the two? Every farm, field, and season is different, so when it comes to fertiliser, why do we take the same approach across the board? In this article, AF Fertiliser Procurement Manager James Davies explores the thinking behind Maximum Sustainable Output (MSO) to understand what your farm can realistically produce and make decisions that deliver the best possible return, both now and in the future.
"Which is the more valuable conversation to have: how your farm can achieve its highest yields, or how it can deliver its most profitable years?
"That’s where Maximising Sustainable Output (MSO) comes in. The concept first emerged around 10 years ago as a way of understanding what your farming business is genuinely capable of producing, and how to make the most profitable use of the resources available.
"It could also mean looking beyond fertiliser and reviewing everything from feed and fuel to labour, crop areas and livestock numbers.
"Ultimately, it's about finding the right balance between what you produce, what it costs to produce it, and what your farm can sustainably deliver."
"There's an obvious irony in me, as your Fertiliser Procurement Manager, discussing whether there are opportunities to reduce fertiliser usage. But my objective shouldn't simply be to sell or apply more inputs. It should be to help AF Members' farming businesses make informed decisions that protect their margins and support their long-term viability.
"If using less fertiliser, where agronomically and economically justified, makes your business more profitable and reduces your environmental impact, that's certainly worth considering. Equally, if additional nitrogen can deliver a worthwhile increase in yield and profit, why wouldn't you invest in it?
"For me, MSO is about understanding your farm's capabilities, identifying what's holding it back and making every input count. It's just as important to recognise where additional investment will deliver a return as it is to understand where spending more will achieve very little.
"The aim isn't necessarily to produce the most or use the least. It's to get the best possible outcome from the resources available."
"So, where do you start?
"Understand what each field - and, where relevant, each livestock enterprise – is realistically capable of producing.
"Take nitrogen as an example. Applying the same rate across every field might seem straightforward, but soil type, drainage, rooting depth, and water availability can all influence how well a crop responds.
"Three things are particularly important:
Understand your field’s yield potential: what is the realistic yield ceiling for each field? Soil type, soil PH levels, drainage, rooting depth, variety, drilling date, disease control, and even harvest conditions can all influence what your crop can achieve. There's little value in investing in additional nitrogen if another factor is preventing the crop from making the most of it.
Understand how crops respond to nitrogen: the nitrogen response curve is central to understanding maximum yield and economic yield. It describes how crop yield changes as nitrogen application increases, assuming other major constraints remain broadly similar, as shown in the graph below.
In other words, you get a steep early response then a diminishing marginal response, and then a yield plateau - at which point further nitrogen has little effect on grain yield.
Work out when the investment pays: the economic optimum is reached when the additional value of the crop produced is equal to the cost of the extra nitrogen, adjusted for application costs and any relevant quality premiums or penalties. Put simply, is the extra yield worth more than the fertiliser needed to produce it?
"That's the question we should be asking, rather than simply whether we could get away with applying less."
Adapted from Hirzel and Undurraga (2013), Chapter 5, Nutritional Management of Cereals Cropped Under Irrigation Conditions & the University of Kentucky Agricultural Extension.
"With nitrogen prices having risen, getting these decisions right has become increasingly important.
"As a useful benchmark, UK-produced ammonium nitrate is currently around £485/t. Speaking to your AF Team about fertiliser, fuel and other input costs can help you establish the latest market position, although your actual purchase price will depend on supplier, delivery and contract terms.
"To give you some example figures, at the approximate current cost of £485/t for UK produced nitrogen 34.5% N, indicative application costs are:
£/KgN: £1.40
100 KgN: £141/ha
200 KgN: £281/ha
"Exactly how much nitrogen you need to apply is determined by several factors, including soil type, organic matter and available nitrogen. But these figures show just how quickly nitrogen investment adds up and how vital it is to know your soil and its available nutrients."
"One of the simplest ways to look at the economics is to calculate how much additional grain you need to produce to cover the cost of extra nitrogen.
"For feed wheat, a useful calculation is: break-even yield (t/ha) = additional N cost (£/ha) ÷ grain price (£/t).
"Let's assume feed wheat is worth £180/t and ammonium nitrate costs £485/t, equivalent to approximately £1.40/kg N.
" The additional yield needed just to cover the fertiliser cost would look like this:
Additional nitrogen | Cost per hectare | Extra wheat needed to break even |
|---|---|---|
10 kg N/ha | £14.06 | 0.078 t/ha (78 kg/ha) |
20 kg N/ha | £28.12 | 0.156 t/ha (156 kg/ha) |
30 kg N/ha | £42.18 | 0.234 t/ha (234 kg/ha) |
40 kg N/ha | £56.24 | 0.312 t/ha (312 kg/ha) |
50 kg N/ha | £70.30 | 0.391 t/ha (391 kg/ha) |
"These figures exclude additional application costs and assume the extra grain can be sold at the stated price.
"So, what does that mean in practice?
"If you apply an extra 40 kg N/ha costing around £56/ha, it needs to generate approximately 0.31 t/ha more wheat just to cover the fertiliser cost.
"If that additional nitrogen only produces another 0.20 t/ha, you're losing money before you've even accounted for any other costs like labour and fuel.
"But if the field has potential to deliver an extra 0.50 t/ha, the same investment could generate a worthwhile return. It's a good example of why understanding the potential of individual fields is so important."
"In its 16th October 2026 update, Farmers Weekly's ex-farm price series reported feed wheat at approximately £196.82/t for November movement.
"At that price, with nitrogen costing £1.41/kg N, each additional kilogram of nitrogen needs to produce around 7.1kg of saleable wheat to cover its fertiliser cost alone.
"AHDB's standard cereal break-even ratio is 5:1, although its guidance also explains that higher ratios can justify lower economic optimum nitrogen rates. The important thing is not to treat any ratio as a fixed rule. Grain values and fertiliser costs change, so the calculation needs to reflect the prices your own farm is working with."
“Yield and nutrient use efficiency can be affected by several limiting factors, but there’s one that should be given particular consideration given the dry conditions last spring.
“Winter wheat requires larger amounts of nitrogen during April and May, when crops are building tillers, extending stems, and rapidly accumulating biomass. “Insufficient soil moisture restricts fertiliser dissolution, nitrogen movement towards roots, and crop uptake, reducing nitrogen use efficiency just when demand is increasing. Urea needs moisture so it can break down into a form that plants can absorb (hydrolysis). If rain doesn’t wash it into the soil after spreading, some nitrogen can be lost into the air as ammonia before the crop gets the chance use it. Liquid UAN isn’t immune to these moisture limitations either.
"ADAS research at Terrington in 2022 found that solid ammonium nitrate, solid urea and liquid UAN all helped crops take up nitrogen and grow by similar amounts in dry conditions. The research also showed that just 0.5mm of water was enough to encourage nitrogen uptake and crop growth for more than a month.
“Building soil organic matter is particularly valuable in dry spring conditions because it improves the soil's ability to retain moisture, cycle nitrogen and maintain nutrient availability when cereal crops are rapidly building biomass.
"If nitrogen uptake is restricted and the crop eventually reaches a point where additional nitrogen delivers very little extra yield, does it make financial sense to keep investing?
"The answer lies in understanding the difference between maximum yield and economic optimum yield.
"The agronomic maximum is the point at which additional nitrogen produces virtually no extra yield.
"The economic optimum usually occurs earlier. It's the point at which the value of the additional crop produced just covers the cost of the extra nitrogen. Beyond that point, each additional kilogram of nitrogen costs more than the grain it produces.
"This doesn't mean crops should automatically receive less nitrogen. It means we need to understand how they respond and identify where the investment makes financial sense."
"For me, the strongest argument for MSO isn't that we should automatically use less fertiliser. It's that we should invest according to what our farms can realistically produce and the return we're likely to get from that investment.
"Maximum yield, realistic yield potential, and economic optimum yield are three different things, and understanding the difference is important.
"Nitrogen can deliver substantial gains when a crop is limited by it, but those gains diminish as other constraints become more influential and the crop approaches its yield ceiling.
"With ammonium nitrate currently costing around £1.40/kg N, getting those decisions right (or wrong) can have a significant impact on farm profitability.
"But a blanket reduction in nitrogen isn't the answer. Better information about your fields, a clearer understanding of crop response, and knowing where additional nitrogen will actually pay, are key to making financially sound, evidence-based decisions.
"It's not about producing more for the sake of it. It's about making every pound work harder and getting the best possible return from every acre.
"If this has got you thinking about what MSO could mean for your farm, I'd be happy to have an initial chat about where to start."
