With fertiliser prices keeping pressure firmly on input decisions, attention is turning to how oilseed rape uses nitrogen – not just how much is applied. As crops move towards flowering, CPM explores how growers can improve nitrogen use efficiency where it matters most, including novel approaches.
“Now more than ever, nitrogen use efficiency is key to profit.” JACK HILL
By Charlotte Cunningham
Across much of the UK this season, oilseed rape has done something it hasn’t always managed in recent years – it’s grown off to a good start. Even establishment, decent autumn growth and a relatively kind winter have left many crops carrying more promise than growers have become accustomed to. But as the crop approaches flowering, the question is no longer simply about potential – it’s about conversion.
Because while biomass might be there, yield in OSR is ultimately decided over a relatively short window and increasingly, that window is where nitrogen strategy is being rethought.
Set against a backdrop of volatile fertiliser markets and continued uncertainty around future pricing, the emphasis has shifted decisively from application rates to efficiency. In other words, it’s not just how much nitrogen is applied, but how well the crop uses it – and crucially, when.
“From a grower’s perspective, the challenge is really trying to think around budgets for this season,” says Jack Hill of Intracrop. “It’s not just about what was planned in terms of nitrogen, but how efficiently crops can use the nitrogen that’s available. That’s where the conversation has shifted.”
That shift is being driven by both economics and necessity. While some growers secured fertiliser ahead of recent price rises, others have been left navigating higher costs and tighter margins. Even where product is already in the shed, there’s growing consideration around whether it all has to be used this season.
“Now more than ever, nitrogen use efficiency is key to profit,” adds Jack. “There are conversations now where growers are asking whether they actually have to use everything they’ve bought, or whether some of that can be held for next season.”
But improving nitrogen use efficiency in OSR isn’t simply a case of reducing rates. As ADAS’ Pete Berry explains, it’s about aligning supply with the crop’s physiological demand – and nowhere is that more critical than around flowering.
“The key thing with OSR is that it’s sink-limited,” he says. “So that means you have to set as many seeds as you can to maximise yield.”
That process hinges on a relatively narrow period, spanning the end of flowering and the weeks immediately after, when pods and seeds are being set. During this time, the crop’s ability to photosynthesise becomes the primary driver of final yield.
“The amount of photosynthesis you can get your plant to carry out during that few weeks after flowering, the more seeds it’s going to set,” explains Pete. “So getting the crop performing as well as it can at that period is absolutely vital.”
Central to that is canopy management. Historically, thick stands were often seen as desirable, but research has shown that excessive canopy size can actually work against the crop at this stage.
“We used to have these very thick flower layers which reflected a large proportion of light just at the critical time,” he continues. “What we actually want are more open canopies that allow light to penetrate down into the leaves, where the crop can photosynthesise and drive seed set.”
MODEST CANOPIES
That has led to a refinement in canopy targets, with a green area index (GAI) of around 3-4 now considered optimal. While such crops may appear modest, evidence suggests they’re better equipped to convert light into yield during seed set.
“It doesn’t look massive, but those modest-sized canopies consistently set the most seeds,” adds Pete.
Achieving that balance is largely a function of nitrogen management earlier in the season, but its implications carry through into flowering. Too much early nitrogen can push crops towards excessive biomass, increasing lodging risk and reducing light penetration. Too little, and the crop may lack the capacity to support seed development.
Alongside canopy size, ensuring the crop isn’t limited by other factors is equally important. Nutrient deficiencies, particularly sulphur and magnesium, can restrict photosynthetic capacity, while disease pressure – especially light leaf spot – can impair canopy function at precisely the wrong moment.
“The crop mustn’t be deficient of any nutrients at this stage, and it mustn’t be impaired by disease,” stresses Pete. “Anything that limits photosynthesis during that period is going to limit seed set.”
Weather also plays a role, he notes. In dry springs, poor rooting combined with limited soil moisture can restrict the crop’s ability to access nitrogen and water, reducing photosynthesis and ultimately seed numbers.
Given the importance of this window, attention is increasingly turning to whether nitrogen applications can be better aligned with crop demand later in the season. Traditionally, much of the nitrogen budget is front-loaded to build biomass, but there is growing interest in shifting some of that supply closer to flowering and pod fill.
“There aren’t that many actions you can take at flowering, but late foliar nitrogen is one of them,” says Pete. “Applied from mid-flowering to a couple of weeks after, we’ve seen average yield increases of around 0.25t/ha from foliar N rates of 30-40kgN/ha.”
That principle of better timing is something Jack believes can be taken further. “It’s not about cutting nitrogen purely to reduce costs,” he says. “It’s about reallocating it to where the crop requires it most.”
In crops carrying strong biomass, that can mean trimming back the final soil-applied nitrogen and replacing it with a later, more targeted approach. One example is Intracrop’s Nutrino Pro, a foliar-applied nitrogen source combined with sulphur, magnesium and biostimulant components, designed to deliver nutrients during pod fill.
“In OSR, we’ve seen growers replace part of that final spring nitrogen with a later application,” he explains. “That’s delivering nitrogen directly to the crop when it’s setting pods and filling seeds during a 6-8-week period.”
Typically, this approach might involve replacing around 30kg/ha of soil-applied nitrogen with a 20 l/ha application of Nutrino Pro at the end of flowering. While the product supplies a lower total nitrogen dose – around 6kgN/ha – the difference lies in how efficiently that nitrogen is used, suggests Jack.
Because the nitrogen is taken up directly through the leaf and metabolised within the plant, its utilisation is far less dependent on soil moisture and root activity – both of which can be limiting later in the season.
“Typically, soil-applied nitrogen might be 30-40% efficient in dry conditions,” says Jack. “Whereas with a foliar approach, you’re looking at closer to 90-95% efficiency.”
That improved efficiency underpins the response, with work indicating yield benefits in the region of 0.4t/ha. By extending nitrogen availability into the pod fill period, it supports the crop through what’s often a supply-limited phase, reinforcing its ability to convert photosynthesis into seed number. “You’re still feeding the crop, but you’re doing it at a point where it can use it most effectively,” he adds.
BOOSTING NUE
Alongside nitrogen timing, there’s also growing interest in approaches that support the plant’s ability to utilise nitrogen more effectively – whether through improving photosynthesis, enhancing root development or driving nitrogen assimilation within the plant.
While such approaches are gaining traction, both he and Pete are clear that they sit within a wider system, rather than acting as standalone solutions.
“There’s no silver bullet,” says Pete. “It’s about getting the fundamentals right – canopy size, nutrition, disease control – and then looking at where you can make incremental improvements.”
Looking forward, as crops move through flowering, the focus shifts from building biomass to making the most of what’s already there. And in a season where fertiliser remains both a cost and a constraint, that means thinking carefully about how nitrogen is used, not just how much is applied.
“As an industry, we know there’s still a gap between what we apply and what the crop actually uses,” says Jack. “So the opportunity is to tighten that up – whether that’s through timing, approach or supporting the plant to use nitrogen more efficiently.”
This article was taken from the latest issue of CPM. Read the article in full here.
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