Managing nitrogen applications to new Group 1 and 2 wheat varieties

Abstract

• Using full nitrogen (N) response curves (7 N rates), the N requirements of selected modern high yielding Group 1 and 2 varieties (e.g. Einstein, Malacca, Xi19) was examined at 9 experimental sites (2002-2005), and compared to those of older and lower yielding varieties (e.g. Avalon and Mercia) that represent the dataset used to underpin the current recommendations in Defra's 'Fertiliser Recommendations (RB209)' publication (7th edition, 2000). Chopin Alveograph analyses were done at selected sites. At a further 9 sites, alternative strategies for applying extra late N for protein were examined, including combinations of late solid and foliar urea spray applications. It is anticipated that these results will be considered in any future revision of RB209.
• As expected, the average yield of the modern varieties was about 1t/ha higher than the older varieties (9.09t/ha compared to 8.09t/ha). Using a linear plus exponential curve fitting procedure, economic optimum N rates (Nopt) were derived for each variety tested, and at different economic breakeven ratios. Nopt values for yield (3:1 ratio) ranged from zero to over 340kg/ha N and there was a significant relationship (p<0.05) that showed that the higher yielding varieties had higher Nopt values by an average of 17.3kg N/t grain yield. At these N rates, the grain protein contents were mostly between 12.5-13.5% and averaged 12.76% - this is close to the current critical level in RB209 for breadmaking wheat. It was not possible to show any consistent differences in the N uptake efficiency of different varieties.
• Overall, the fitted Nopt values for yield (3:1 ratio) for the modern varieties were higher than the current RB209 recommendations. When each recommendation was adjusted for the grain N (protein) content of the experimental crop (as recommended in RB209), the Nopt was 29kg/ha higher on average than the recommended rate. The adjusted RB209 recommendations were usually higher than the unadjusted recommendation. Thus, both the current unadjusted and adjusted RB209 recommendations were generally under-estimating the Nopt for modern varieties at these sites. If the Nopt values were based on a breakeven ratio of 6:1 (equivalent to ammonium nitrate at £145/t and grain at £70/t) rather than 3:1 (as in RB209), the average Nopt reduced by 29kg/ha N, yield at the Nopt (Yopt) reduced by 0.13t/ha and grain protein at the Nopt reduced by 0.32%.
• For the modern variety crops, 6 out of 16 (38%) needed more than 280kg/ha N to achieve 13% protein, and 4 out of 16 (25%) needed more than 300kg/ha N. The highest unadjusted recommendation in the current RB209 is 280kg/ha N. At N rates needed to achieve 13% protein, the the extra margin over N cost (above that from achieving Yopt alone) was around £100/ha, assuming a £13/t premium and a 100% success rate of crops grown for the premium. At lower premiums and success rates (e.g. £5/t and 1 in 3 success rate), the margin did not generally justify growing milling compared to feed wheats.
• Late N applications increased protein contents but foliar urea was generally more effective than solid ammonium nitrate (0.66% and 0.34% protein from 40kg/ha late N respectively). Effects of late N on yield were uncommon and small, though small yield reductions were observed due to leaf scorch from late foliar urea application.