Research Review No. 97

Review of how best to respond to expensive fertiliser nitrogen for use in 2022 by Roger Sylvester-Bradley and Daniel Kindred

1. Abstract

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. Part Two of this study will be reported in January 2022 and will consider issues such as how to prioritise more expensive fertiliser N on particular fields and crops, meeting crop quality specifications, and maximising the value of recycled N from livestock and legumes.

2. Introduction

This is the first part of a two-stage study of how the arable cropping industry can best respond to the recent sharp increases in prices and availability of manufactured fertilisers. This first part just addresses use of nitrogen (N) fertilisers on the major arable crops, wheat, barley and oilseed rape. The second part of this study will be reported in January 2022, and will consider other arable crops, and other aspects of crop nutrition management that are affected by the sharp increase in prices.

2.1. Recent Trends in prices of fertilisers and crop produce

Associated with sharp increases in energy prices during 2021, and with limits on availability of natural gas, prices of manufactured N fertilisers have increased, and their availability for on-farm delivery has reduced. Farmers and farm suppliers need guidance on how best to respond to these changes; their main concerns are to know whether, and by how much, they should reduce the rates of fertiliser N that they normally apply, and by how much this will reduce their crop yields. The fertiliser experiments that have habitually been conducted on the major arable crops since the use of manufactured fertilisers began have regularly been reviewed in order to support farmers’ decision-making on fertiliser use, and recommendations have been produced and regularly revised accordingly, currently published as the AHDB Nutrient Management Guide (RB209; AHDB, 2016).

2.2. What the Nutrient Management Guide (RB209) says at present

The current version of the AHDB Nutrient Management Guide (RB209) includes Tables 4.22 & 4.28 to show how N recommendations should be adjusted for changes in prices of fertiliser or crop produce. These tables were introduced in the 8th edition of RB209 (Defra, 2010 pages 106 & 116) from experimental evidence collated up to 2008; their content has not been revised since AHDB took responsibility for RB209. The tables were produced assuming the shape of the crop response around the optimum rate of fertiliser N is the same for all cereals and a different response curve is representative of all oilseeds. These tables no longer address the current and expected range of prices for grains and fertilisers, so they are extended in Section 2, and effects on crop yields are tabulated. It is also possible, with changes in varieties, and with further evidence since these tables were included in the 9th Edition of RB209 (AHDB, 2016) that the assumption of a standard response shape for all cereals and another for all oilseeds needs to be changed. Section 3 of this report 3 therefore shows how N optima are defined and Section 4 shows how response curves from recent experiments compare with the standard response assumed in RB209 at present, and in the extended tables presented in Section 2.

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