The requirement for inorganic fertilizer nitrogen (N) by winter wheat crops in the United Kingdom is derived using the Department for Environment, Food and Rural Affairs (Defra) Fertilizer Manual. In the experimental programme described and discussed in the present paper, the appropriateness of these recommendations for winter wheat grown in Northern Ireland is examined.
Yield response to N varied in experiments conducted on two winter wheat cultivars (cvars) in Northern Ireland from 2007 to 2009. Consequently the optimum N rate (Nopt, defined as the rate of applied N where the value of the increase in yield equals the cost of the increment in fertilizer applied and beyond which additional N would not repay its cost) also varied from year to year. The band of fertilizer N rates over which margins were reduced by £20 (GBP) and £50 also varied from year to year. Changes in the N:grain price ratio affected Nopt to differing extents in the three experiments depending on the shape of the yield v. N response.
Nopt should therefore be considered as a range of N rates because (1) it varies from year to year and probably also field to field and (2) the margin of income from grain over cost of fertilizer varies little over a range of N rates because of the shape of the asymptotic response of yield to N. Alternatively, in high rainfall areas (annual rainfall >700 mm) of England, Wales and Northern Ireland, where Table C of the Fertilizer Manual (formerly RB 209) is used to determine soil nitrogen supply (SNS) index, a single N rate could be adopted at SNS indices of 2 or less (equating to soil N supplies of 100 kg/ha or less). A rate of 240 kg N/ha could be adopted based on the over-years function fitted to all results in the three experiments reported in the present paper and including treatments that vary in the splitting of N applied between the two applications and in their timing.
Grain N concentration rarely exceeded the guideline 19 mg/g for feed wheat crops identified in the Fertilizer Manual (Anon. 2010). Overall, N taken up by the crops was used efficiently, and particularly so at lower N rates. However, at low fertilizer N rates the contribution from ‘free’ soil N inflates the ‘apparent’ value of grain yield produced. The responses of yield and grain N concentration to N show that crop processes work to maximize yield at the expense of N concentration in the grain. Therefore there is less need to be concerned about identifying the optimum N rate and predicting fertilizer N requirement with a high degree of precision. Instead growers could assess and adjust the efficiency of their N use based on grain N concentration generally, rather than specifically assess whether their fertilizer N applications were close to Nopt. Essentially as grain N concentration increases, yield/kg of applied fertilizer N decreases. Thus at low grain N concentrations, yield could be increased by increasing N applications and at high grain N concentrations yield could be maintained and profitability increased by reducing N applications.