AHDB Research Review 97

This is Part One of a two-part study to help cereal and oilseed growers respond appropriately to the large increases in prices of manufactured nitrogen (N) fertilisers for use in 2022. Part One considers adjustments to the total amounts of N to be applied.
Economic optimum amounts of fertiliser N decrease as the break-even price ratio (BER) between grain and fertiliser N increase. Current RB209 recommendations were devised for a BER of 5 kg cereal grain to 1 kg N and RB209 tables for BER adjustments extend to a N price of £1.40/kg. However, recent prices of fertiliser N have reached £2.00/kg and may go higher before spring 2022, so the RB209 tables have been extended here up to £2.50/kg N. Cereal grain and rapeseed prices have also been strong of late, so the tables have also been extended to prices of £350/tonne and £700/tonne respectively.
These new tables indicate that, if grain and rapeseed prices were held at £200/tonne and £400/tonne, the optimal change in use of ammonium nitrate fertiliser, due to an increase in its price from £345 (BER=5 & 2½) to £690/tonne (BER=10 & 5), is to reduce its use by 50 kg/ha N on cereals and by 70 kg/ha N on oilseeds. The resulting changes in yield would be -0.36 t/ha and -0.25 t/ha respectively. However, recent increases in oilseed prices to ~£500/tonne have mitigated this oilseed adjustment to -50 kg/ha N.
This review then considers whether new evidence justifies a change in the way that price adjustments are currently recommended in RB209. After describing conventional methods for determining N optima from results of experiments with fertiliser N on crop yields, responses to fertiliser N in recent experiments [(i) 46 on winter wheat, (ii) 6 on winter barley, (iii) 11 on spring barley, and (iv) 22 on winter oilseed rape] are compared with the standard responses used to determine current price adjustments in RB209.
It is concluded that the extended adjustment tables are satisfactory for winter wheat and for winter oilseed rape. They are also adequate for winter and spring barley at present, but some further consideration should be given to whether adjustments for barley should be somewhat less than for wheat.

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