Semi-dwarf oilseed rape varieties are 20% more efficient in their uptake of soil N and produce larger canopies at flowering, according to the initial results of an HGCA funded project on nitrogen rates and timings.

The study, which was also funded by Growhow, investigated optimum nitrogen rates and timings for semi dwarf varieties, compared standard height Excalibur with Pioneer’s semi-dwarf PR45D03, and also looked at whether canopy management principles can be applied to the shorter types.

The work was done at three ADAS sites using seven nitrogen rates, varying from 0 to 360kg/ha N, applied at either conventional or canopy managed timings. The use of 0.5 litres/ha of Folicur (tebuconazole) in April was also investigated.

“The first finding was that both varieties produced very similar yields and had the same economically optimum nitrogen rates, at more than 200kg/ha,” reports Pete Berry of ADAS, who led the project.

“That was a surprise to some, as the official yield figures on the Recommended List would suggest otherwise, showing Excalibur with a yield advantage. But the semi-dwarf proved to be very responsive.”

The other findings, which are being investigated in further work this year, were that canopy management principles might help to increase the yield of semi-dwarfs where canopies are too big after the winter, but that the principles may need to be altered to take account of their different growth habit.

“In this trial, all the test crops had small canopies following a hard winter, so we saw the same yields with canopy managed and conventionally applied nitrogen,” says Dr Berry.

“But we also found that the semi-dwarf produced a larger canopy at flowering, with the same amount of crop nitrogen uptake, but that it was 20% more efficient at soil nitrogen uptake. So there may have to be some changes to the canopy management approach with semi-dwarfs, to maximise yields and nitrogen use efficiency.”

At two of the sites, PR45D03 required less nitrogen to produce each unit of Green Area Index (GAI), and there were indications that each unit of GAI intercepted less light, he notes.

Where Folicur was applied, the canopy of the semi-dwarf variety was reduced at two of the sites. “If there’s a likelihood that the canopy will be too big, then it would appear that Folicur has the potential to increase yields.”

Dr Berry stresses that the work was done for just one year, and that the crop canopies following winter were smaller than average.

Dick Neale, technical manager at Hutchinsons, is pleased that the HGCA funded work supports his view that there’s no need to change the optimum nitrogen rates for semi-dwarf varieties.

“The interesting thing is that they do produce a bigger canopy and that they are better at extracting nitrogen from the soil. This means that we should be able to use different nitrogen timings to manage the canopy more effectively.

“Furthermore, this trial confirms what we’ve been finding on farm. Semi-dwarfs are producing the same yields as other types – they’re holding their own.”

Mr Neale adds that the different growth habit of the varieties allows access to the crop throughout the growing season, making their agronomic management far easier.

“Where you get delayed by the weather, for example, there’s no problem with late nitrogen applications, as the plants won’t lodge. Similarly, we can use more nitrogen on them to increase yields, knowing that we won’t get any problems.”

His other observation is that they show a greater yield response to the use of plant growth regulators (PGRs).

Andy Stainthorpe of Pioneer says that previous research has also shown that the semi dwarfs have a higher root mass than standard varieties. “It might explain why they are better at using soil nitrogen. We’re learning about them all the time.”

He adds that the semi dwarf varieties are proving to be very responsive to both fertilisers and fungicides. “When they were first introduced, they were seen as the ideal low input variety. But we’re now seeing a different picture, which when put together with their easier management, makes them attractive to growers.”

Simon Kightley of NIAB agrees. “We weren’t expecting these varieties to behave in this way, as they have a low biomass. But using a lower optimum nitrogen rate on them is a mistake.”

He has conducted a pilot trial on one site, with eight varieties. “Interestingly, PR45D03 proved to be the most responsive of all. It took me by surprise.”

He believes that more information is needed before accurate advice can be given on nitrogen use. “But we do know that semi-dwarfs can be trusted on very fertile sites. The plant habit means that they won’t lodge.”