Trials put N rates in question

It is rather fortuitous that a number of growers have been trialling variable rate nitrogen over the last few seasons, because although grain prices remain high, there appears no change with nitrogen prices either. As a result, many growers are interested to see if potential savings are possible.

Nutrient use e ciency (NUE) was the driver for these trials but they have proved useful in giving a steer on just how low nitrogen rates could go.

The farmers are using Bayer’s FieldView platform to scrutinise the results and last season’s data does pose a question over how much exibility growers have.

The results are a ‘mixed bag’. The headline result from the 2021/22 season is that the higher rates delivered the better results, even when targeted at lower biomass eld areas. But that wasn’t a universal nding.

Recording signi cant di erences

Andrew Williamson, of Upper Overton, Shropshire, recorded signi cant di erences with his trial. He does stress it is one trial in one season but it does make him ponder how low rates can be trimmed.

He typically applies N in three splits; each with a generous dose of sulphur, as he feels this improves mobilisation.

Both treatments started with a solid application of 30kg N/ha with 40kg N/ha sulphur followed by two further liquid applications of 96kg N/ha and 28kg SO3/ha on the farm standard plots, compared to two further applications of 64kg N/ha and 18kg SO3/ha on the lower rate plots.

FieldView analysis showed a signi cant yield di erence between the two treatments with the higher rates of N delivering the higher yields (see Figure 1 below).

Di erent result this season

Robert Burden of Chilbridge Farm, Dorset, saw a similar result. In the past, he too has been adjusting N rates to crop biomass and this has always paid dividends – but this season, the work has thrown up a di erent result.

He compared rates ranging from 20% below to 10% above his typical winter wheat treatment of 230kg N/ha. He applied four programmes with total volumes of 180, 200, 230 and 250kg N/ ha, the second application being the one where rate was varied.

Last season’s result surprised him, as the higher the loading of N, generally the better the result.

In the wheat trial – with three di erent elds and varieties – the higher N loading delivered the greater the yield in every case and with Robert shrewd enough to anticipate rising N prices and locking in fertiliser prices at £1/kg, the improved yield delivered the best return on investment.

But recalculate those gures on the current fertiliser price of £2/kg and that investment score shifts depending on eld and variety. In the ‘higher ground’ eld, the best return remained with the highest loading, but in others, 230kg N/ ha produced the best gross multiple of invested capital (MOIC), while in another eld the 250 and 200kg N/ha applications came out equal.

The WOSR result is probably best described as ‘neutral’ – the higher rates and resulting yield being cancelled out by increased input costs, although in the highest biomass areas the lower rates did deliver a small improvement in yield and MOIC.

Mr Burden is a little puzzled by both results, and his conclusion is that more work is needed. With the WOSR, Mr Burden suspects this is down to the drought. Research has shown that WOSR is more sensitive to drought than winter wheat. With the wheat trials, he feels a season of lower disease pressure may have had some in uence. Higher N rates usually fuel disease pressure, resulting in higher fungicide rates to preserve crop potential.

But he still made savings across some parts of the farm. In one WOSR eld satellite imagery coming in from FieldView highlighted areas of low crop biomass. Rob decided to drop the nal N application, and that paid as although yield was impaired the crop returned a respectable margin.

Figure 1

Developing precision practices

across the enterprise. He also wanted to test his theory that a slight reduction in total nitrogen is possible, and varied rates to match crop biomass based on varying crop biomass captured using FieldView satellite imagery (see Figure 2 below).

Feed wheats typically get 220kg N/ha and milling wheats a 20kg N/ha top-up. For the trial, the standard dose was reduced to 200kg/N/ha with a further variation of +/-20 based on low or high biomass.

Like Mr Williamson, he stresses it is one season, and more data is needed before any conclusions can be drawn, but through a combination of improved yield and reduced inputs the trial eld delivered a £105.86 improvement. Had he achieved a similar result across his wheat area that equates to an extra £35,887.

A similar approach was employed with two rape elds. Here the results were more variable, one eld delivering an improvement, the other a slight reverse. But still a modest gain over the two was recorded.

The data is a valuable insight for Mr Cross in re ning future agronomic practice, and he will build up his library with further trials this season. He has added variable rate seed to the 22/23 farm trials agenda with three rate bands based on historic eld performance.

Figure 2

Mixed set of results

Simon Gent of Stocksbridge, Hampshire, saw a mixed set of results. His WOSR trials also showed no real gain last season from varying fertiliser rates to re ect crop biomass.

Although the variable rate areas brought input savings it resulted in a 0.1t/ha yield loss; with rapeseed at £508/t Simon was worse o to the tune of £20/ha.

Caution advised

But caution is needed. For the purpose of the trial, the group decided to continually cut back on the higher areas of biomass and divert more nitrogen to the thinner areas.

This goes against the standard convention of loading nal N rates to higher biomass areas where the crop has the greater yield potential. Interestingly, at Stocksbridge where higher rates were used on the low biomass areas, there was a return of £91.51/ha compared to the farm standard at rate.

Some might see this as a wasted exercise, but Mr Burden doesn’t. It is another piece of data in the library that might be useful in the future. “Rarely are two seasons alike. I’m building up data over several seasons which I can use to re ne future decision making. It’s not just about improving NUE and MOIC but also our farm footprint,” he concludes. FG