NITROGEN TOP-DRESSING REQUIREMENTS OF WINTER OILSEED RAPE

Forty‐one field experiments were carried out on the spring N top‐dressing requirements of winter oilseed rape in the years 1973 to 1977. The experiments were in the main rape‐growing areas of England and Scotland, and almost all were on rape crops following one or more cereals. Seed and oil yield were increased by N top‐dressing in 38 of the 41 experiments, with a mean requirement by satisfactory crops for about 230 kg ha−1 of N. Those experiments with no response to N or no response beyond 90 kg ha−1 of N were mostly low yielding and suffered from lodging, drought or bird damage. N reduced oil content by about 2% on average, but there was a considerably larger depression in 10 experiments, almost always associated with low yield, drought or lodging. N fertiliser increased the N content of the seed by about 0.5%. Timing of N application within the period mid‐February to late March had little influence on seed yield or oil content but applying all or half the N in April tended to give a lower yield. There was no advantage from splitting the N application. Copyright © 1979 John Wiley & Sons, Ltd