To maximise NUE an even balance is needed between application rates and removal of nitrogen by the plant.
The development of satellite imagery technology gives growers the option to review variations in crop canopy development from a series of images, to manage nitrogen applications, says Simon Griffin, technical manager, Soyl.
“Looking at the data, growers can see where a crop is thicker, or thinner and taking up less nitrogen, and then vary nitrogen inputs across the field accordingly.”
Yield benefits of 4-9% were seen when applying nitrogen based on changes in biomass identified by satellite imagery, but last year the company undertook a series of trials in winter wheat across the country, applying variable rates of nitrogen to one half of all fields, and the farm’s uniform rate on the other.
A soil survey was also carried out on each field.
“Using soil conductivity surveys to measure a soil’s physical characteristics showed us where clay content or soil depth changes,” says Mr Griffin. “This is important relating to N supply because a heavy clay soil supplies more than a light soil would, and if that varies across the field we can factor it in.”
All the fields showed a higher NUE for the half that had N applied variably than the half of the field that had N applied at a flat rate.
“This is as a result of matching the nitrogen applications to crop demand and growth,” says Mr Griffin. “The whole crop will get the same amount of N, just some will come from the soil, and some will come from a bag.
“Improving nitrogen efficiency requires a joined-up approach across the whole farming year based on what’s most efficient. It starts with variable seed rates related to the underlying changes in the soil texture to get an even plant population across the field – if we can get an even plant number, it sets the crop up to utilise N in the spring.”
Little and often
Applying nitrogen in smaller doses and more frequently to match the growth of the crop as it progresses through the season, may contribute to an improvement in NUE by reducing the risk of leaching and volatilisation. This is according to Dr Sarah Kendall, research scientist at ADAS, who is involved in the Yield Enhancement Network.
“The little and often approach has been something that farmers who perform well in the YEN competition often talk about but from a scientific perspective it’s a bit of a knowledge gap.
“When we look at the rate of nitrogen applied there is a weak positive association. This means for N rates above the average nitrogen rate applied in the data set, there is an improvement in yield of 6kg of grain per kg of nitrogen applied.
“When we look at the number of nitrogen applications there seems to be a stronger association with yield, and there’s some indication that potentially applying five or six nitrogen splits could be more beneficial for yield. This will probably come as a surprise to the industry that there isn’t more of a stronger association with increased nitrogen rates and yields.”
However, Dr Kendall warns growers that this might not be the case in all situations.
“It’s important to understand that growers involved in YEN are going to be applying the correct amount of nitrogen, so generally they’re doing best practice or beyond. If sub-optimal rates of nitrogen are applied then this is likely to have a negative effect on yield. By trying to fine tune the nitrogen rate, if you’re already in the right ballpark area the potential for that improving yield is probably less than fine tuning how you apply it.”
A similar relationship has been seen in oilseed rape data.
“The data is more limited, but we think there’s potential for the message to be the same and that managing canopy correctly through optimising nitrogen timings is very important if your nitrogen rate is close to where it should be.”
Getting the most out of bought-in fertilisers will be one of the most important management areas for producers in the coming years, says CF Fertilisers arable agronomist Allison Grundy.
“Increasing NUE improves profitability and protects the environment. While NUE is a good overall efficiency indicator of crop nutrient recovery, it is probably more useful to look at NfUE when making fertiliser decisions,” she says.
Nitrogen Fertiliser Use Efficiency (NFUE) is used to describe the recovery of applied nitrogen fertilisers for a clearer picture of the impact fertiliser choices are having on production efficiency.
In milling wheat trials carried out by arable research contractors Armstrong, applying zero N to trial plots produced 4.2 t/ha whereas applying the optimum N rate of 254kg N/ha produced 10.4t/ha.
“The N offtake – N in the crop at harvest – was 68kg N/ha for the crop with no fertiliser added and 257kg N/ha where the optimum amount was applied giving a final equation of 257 – 68 divided by 254 which equals 74%,” says Ms Grundy.
While season plays a large part in the recovery of nutrients, decisions made during the application period all affect the final recovery rate.
“In reviewing our own trials and ADAS literature to set the correct parameters for the CF N-Calc N application rate calculator, we’re seeing NfUEs of around 70% for winter wheat but many factors can affect this with product choice being important,” says Ms Grundy.
In the Armstrong trials, Nitram (34.5% N) outperformed straight urea in terms of NfUE across all trials conducted in two different production years and at all yield levels, adds Ms Grundy.
“Looking at the data from six trials shows an average NfUE for the AN of 74% compared to 66% for urea.
“This difference of 8% NfUE is the equivalent of an additional 16% total loss of nitrogen from urea and in crops with an application rate of 200kg/ha N this would be equivalent to a loss of 32kg/ha N.
“NfUE analysis dispels the continued sentiment that whilst urea volatilises N to the air, AN leaches to groundwater so the overall environmental impact from both products is similar.
“It’s just not the case. The N losses through volatilisation from urea are far greater than any perceived spring losses with AN through loss via the soil.”
Other nutrients such as phosphate, potash and sulphur not only affect plant health and metabolism, they also have a significant effect on NfUE, adds Ms Grundy
“If indices of P and K are not maintained deficiencies can work against N recovery and utilisation. Adequate sulphur is also required to drive N utilisation.”