Assessing the benefits of using foliar N on oilseed rape

Abstract

The specific objectives of this project were to:
1. understand whether late foliar N gives any yield advantage over and above optimal amounts of soil applied N such as ammonium nitrate.
2. identify the optimum timing and rate of foliar N.
3. quantify the efficiency with which foliar N is used by the crop
4. develop guidelines for the best use of late foliar N.

Field experiments were set up in 2008/09 and 2009/10 near ADAS Rosemaund, Herefordshire, and near ADAS High Mowthorpe, North Yorkshire. Each of the four experiments investigated six rates of soil applied N (ammonium nitrate) ranging from 0 to 280 or 320 kg N/ha with each treatment followed by zero or 40 kg/ha of foliar N applied as Nufol 20 (20% N) at the end of flowering. Each experiment also investigated five rates of foliar N ranging from 0 to 120 kg N/ha and five timings of foliar N from mid-flowering to two weeks after the end of flowering.

Foliar N at 40 kg N/ha increased the gross output by, on average, 0.20 t/ha across all experiments (range of 0 to 0.40 t/ha). This increase in gross output was achieved despite a reduction in the percentage oil content in the seed by an average of 0.9%. The gross output response was the same regardless of whether it followed sub-optimal or super-optimal rates of soil applied N, indicating that foliar N can increase yields over-and-above that achieved from optimal amounts of soil applied N. Similar yield responses were observed for foliar N applications between mid-flowering and two weeks after the end of flowering, which may indicate that foliar N could be combined with a fungicide spray during flowering. A foliar N rate of 40 kg/ha was found to be the maximum rate that should be used. Foliar N was usually taken up with a high efficiency of 70 to 100% and the resulting increase in post-harvest N residues was modest. It is recommended that foliar N should not be applied when the temperature is above 18°C. Yield responses were quite variable and further work is needed to identify environmental and crop factors that cause this variation. If minimal crop damage from applying foliar N is assumed then it may be concluded that, across a number of fields and seasons, applying foliar N at 40 kg N/ha will generally return a profit as long as the ratio of fertiliser cost (£ per kg of elemental N) to oilseed rape price (£/kg) is less than 3.0 when foliar N costs between £0.50 and £0.75 per kg of N, or the cost:price ratio is less than 3.5 when foliar N costs £0.80 to £1.00 per kg of N.