Semi-dwarf oilseed rape varieties need the same nitrogen input as conventional varieties to achieve their optimum yield, according to recent ADAS research.

Even though semi-dwarf varieties have lower biomass at mid-flowering than conventional cultivars, six trials over two seasons showed that they needed roughly the same nitrogen doses to achieve their economic optimum yield, said Sarah Clarke, crop physiologist for ADAS, at a recent Association of Applied Biologists conference near Grantham, Lincolnshire.

Standard hybrid Excalibur and semi-dwarf PR46D03 were compared in the trials, which also compared nitrogen applied at either RB209 or canopy managed timings to achieve a target green area index (GAI) of 3.5 by mid flowering, she said.

PR46D03 was roughly 40cm shorter than Excalibur, but optimum nitrogen dose wasn’t significantly different in any of the trials. At ADAS’s High Mowthorpe site, the optimum rate for both varieties was 255kg N/ha in 2009, said Dr Clarke.

“Despite being shorter PR46D03 required the same amount of nitrogen to achieve optimum yield.”

This was because its GAIs and seed yields were similar and stem biomass was only significantly lower than Excalibur in two out of the six experiments, she explained. “It appears that greater branching from the foot of the main stem often compensated for the shorter main stem in terms of biomass.”

There were no consistent differences in nitrogen uptake efficiency or N utilisation efficiency between the varieties, she said. “This indicates that semi-dwarf varieties have a similar level of root exploration to standard height varieties.”

Therefore, nitrogen rates and timings recommended for standard height varieties would be relevant for semi-dwarf varieties, she noted.

In the canopy managed treatments, yields weren’t significantly affected in either variety, but plant height was reduced.

This lack of yield difference could have been because GAI was low in spring making it difficult to produce an over large canopy by mid-flowering, said Dr Clarke.

In all the HGCA and Growhow-funded trials, overall yields were similar in both varieties, she added.

Two-year trial exposes the long and the short of it 

Yield response to increasing nitrogen rate varied significantly between varieties tested in a two-year oilseed rape trial, said Simon Kightley, oilseed expert for NIAB TAG.

Semi-dwarf hybrid PR46D03 gave the greatest performance boost with seed return increasing from just under 4t/ha with no nitrogen applied to almost 5.5t/ha with 200kg/ha applied.

Tall hybrid Excel gave the lowest response to increasing nitrogen rate, moving from just over 4.5t/ha with no nitrogen to around 5.25t/ha with 200kg/ha, said Mr Kightley.

“Low biomass varieties tend to be the most responsive to nitrogen, whereas taller, high biomass varieties tend to invest too much nitrogen in vegetative growth which increases lodging risk.”

This meant shorter, more responsive types should be grown on sites with naturally high fertility to maximise yield response and minimise lodging risks, he said.

However, much more work was required to explore these trends, which could result in a new approach to managing some oilseed rape varieties, he noted.

Generally, flowering started and finished earlier in zero nitrogen trials and maturity got progressively later as nitrogen content was increased.

There were no consistent height responses to increasing nitrogen rate, but there was more branching, he concluded.

The trials were carried out at the Cambridge site in 2008-2009 using three nitrogen rates applied to eight varieties. Nitrogen rates were zero, 100 and 200kg/ha.