Optimum N rate and timing for semi-dwarf oilseed rape

Abstract

This project compared a semi-dwarf variety (PR45D03) with a standard height variety (Excalibur) to investigate whether:
i) a semi-dwarf variety has a different economically optimum nitrogen (N) rate and different optimum N timings,
ii) 'Canopy Management' principles apply for a semi-dwarf variety,
iii) there are any differences in the N residues following harvest, and
iv) a semi-dwarf responds differently to a PGR.

Three winter oilseed rape experiments were established in each of the 2008-09 and 2009-10 growing seasons near ADAS sites High Mowthorpe (N. Yorkshire), Rosemaund (Herefordshire) and Terrington (Norfolk). The hybrid varieties PR45D03 and Excalibur were tested at six or seven N rates applied at Conventional or Canopy Management timings, with or without Folicur.

Results showed that, despite being, on average, 33 cm shorter the semi-dwarf variety required the same amount of fertiliser N to achieve optimum yield. Several of the Canopy Management principles were shown to be the same for the semi-dwarf and standard height varieties including; a crop N uptake of 50 kg N/ha to build each unit of green area index (GAI), a similar target optimum GAI at flowering and the same N uptake efficiencies. Shorter stems did not affect the amount of N required to build each unit of GAI because lower stem biomass at flowering was compensated by a greater concentration of N in the stem.

The semi-dwarf variety produced similar sized yields to Excalibur, and by harvest there were no significant differences in the amount of N taken up by the crop and the amount of N taken off in the seed between the variety types. The semi-dwarf variety had an average stem biomass of 3.75 t/ha compared with 4.52 t/ha for Excalibur, and a greater N concentration in the stem and pod wall tissue at two sites. As a result of these counteracting effects both the semi-dwarf and standard height variety left a similar amount of N in crop residues following harvest.

In the one experiment where the canopy following winter exceeded the minimum threshold GAI for using a PGR it was shown that Folicur applied at 1.0 l/ha at the green bud stage significantly increased the yield of the semi-dwarf variety. This indicates that semi-dwarfs will respond positively to PGRs when canopies are large. Folicur was shown to increase seeds/m2 by increasing the amount of light that penetrated through the flowering layer.