Abstract
Four field experiments were performed in the UK in harvest seasons 2007 and 2008. Each experiment consisted of 10 winter oilseed rape varieties grown at a low level of available nitrogen (N) and at a high level of available N intended to replicate commercial practice. A combined analysis of three of the experiments with significant yield differences between the N treatments showed a significant interaction between N availability and variety for yield. Across these three experiments the proportion of yield lost when crops were grown at low N compared with high N ranged from 0.23 to 0.35 among varieties. The proportion of yield lost at low N was negatively associated with crop N uptake. There was also an interaction between N supply and variety for N use efficiency (kg of seed dry matter/kg available N) within these three experiments. Varietal differences in yield at low N correlated most closely, and positively, with crop N uptake, final crop dry matter and seeds/m
2, but not N utilisation (kg seed/kg N uptake). Every additional kilogram of N taken up by the crop increased yield at low N by 0.020
t/ha. The amount of N taken up after flowering was the most important phase of N uptake for determining yield differences between the varieties, with every additional kilogram of N taken up after flowering associated with a yield increase of 0.016
t/ha. Each additional 1000
seeds/m
2 was associated with an additional 1.4
kg
N/ha taken up after flowering. There was no correlation between yield at low N or late N uptake and individual seed size.