By adopting a uniform method of presentation, the interrelationship between grain yield, %N in grain and N uptake (or protein yield) from diverse experiments and surveys can be compared readily. All three variables can be shown together by plotting yield vs %N and adding the third variable in the form of hyperbolic lines representing equal N uptake (or protein yield). This method is used in two diagrams which display 2000 values derived from 124 experiments on winter wheat and 400 values from 41 experiments on spring wheat, conducted by Rothamsted Experimental Station over 20 years. Both scatter diagrams display well‐defined boundaries which have been emphasised by linking points along these boundaries to form ‘envelope curves ’For winter wheat the maximum yield was 9.5 t ha−1, the largest N concentration about 3% and the largest N uptake 170 kg N ha−1 (about 970 kg protein ha−1). Spring wheat had a maximum grain yield of 6.3 t ha−1, the largest N concentration was 3.1% and the largest N uptake 125 kg N ha−1. Four examples of other work set within the same framework show applications of our method of presentation. Results from a recent more homogeneous series of experiments in southern England (conducted by the Agricultural Development and Advisory Service), showing clear seasonal differences, are seen to lie within the envelope curve defined by the Rothamsted experiments. Further illustrations are taken from an experiment by the Plant Breeding Institute, Cambridge, comparing varieties of different bread‐making qualities, from work of the Nebraskan plant breeding team who have produced grain with larger protein content without sacrificing yield, and from survey data for England and West Germany. Copyright © 1979 John Wiley & Sons, Ltd
