Using grain N as a signature for good N use

Abstract

Recent fertiliser recommendations suggest that grain nitrogen (N) analysis can be used to monitor the precision of N management for wheat crops. In this project, datasets with up to 443 experiments (including those used to revise the fertiliser recommendations) were analysed to gauge the confidence to place in this approach, and whether it should be extended to barley and oilseed rape.

Average grain N (% in dry matter) at the optimum amount of fertiliser N (assuming a 'break-even' N:grain price ratio of 5) was 2.0% (11.5% protein) for wheat, and 1.9% for barley, both winter and spring types. It was significantly greater for breadmaking than feed wheats by 0.17%, significantly less for malting than feed barleys (by 0.16% for winter and 0.05% for spring types), and significantly less after break crops than after cereal crops by 0.08% in both winter wheat and winter barley; some soil type effects were significant but inconsistent. Unexpectedly, grain N% with optimum N supplies tended to increase as optimum fertiliser N increased; this reduces the value of grain N% as a signature of good N use. Consistency in grain N% of barley with optimum N was greater than if one fixed N amount had been applied everywhere, but this did not apply for wheat or oilseed rape.

Responses in grain N% to applied N were relatively consistent in the region of the optimum N amount; a difference of 0.1 in grain N% could be taken to indicate a difference of about 30 kg/ha in N applied to wheat and spring barley, 25 kg/ha in N applied to winter barley and 50 kg/ha in N applied to oilseed rape.

It was concluded that grain N (or protein) analysis is useful as a retrospective check on the N management of feed varieties of wheat and barley, but not oilseed rape. For feed wheats it can be taken that grain protein around 11.5% signifies optimal N management, and so does grain N around 1.9% for feed barleys. Differences from these 'standards' were only 70-80% successful in
identifying crops that had been over- or under-fertilised with N. Hence comparisons on several fields or over several seasons will be needed before confident conclusions can be drawn about accuracy of N management. Grain N analysis could probably prove useful in accrediting the green-house gas emissions associated with biofuel production, but it appeared less useful in judging N management of breadmaking or malting varieties because these crops 3 generally have specifications (with attendant financial incentives) that encourage N use that is non-optimal for yield, including late urea sprays. A similar approach may be applicable outside the UK but would need different standards since grain N% in Danish experiments was 0.4% less than in the UK; this difference is not explained.

It is suggested that these conclusions should be transferred to the UK arable industry through the forthcoming HGCA publication on 'Nitrogen for winter wheat - management guidelines', and that they should inform the next revision of national fertiliser recommendations.