AbstractIn an agronomic experiment, perennial ryegrass was grown with four rates of fertiliser N + K at 21 sites in England and Wales. Herbage from two successive harvests at 14 of the sites has been examined for contents of N and S and of other mineral elements in relation to the nutrition of both the crop and ruminant animals. The highest rate of fertiliser application (125 kg N+77.5 kg K/ha harvest) resulted in contents of total N greater than 4% at five sites at the second harvest, but nitrate‐N reached levels that might be toxic to livestock (>0.35%) at only two sites. There was no evidence that supplies of S from soil and the atmosphere were inadequate for crop growth, or that the contents in the herbage were insufficient for ruminant animals, even though no S was supplied as fertiliser. On the contrary, in many samples, the content of S was so high as to constitute a potential hazard for ruminant animals through impairment of Cu availability. Values for the ratio total N: total S, which ranged from 3.2 to 15.6:1, generally increased with increasing rates of fertiliser. The concentrations of sulphate‐S in the herbage were generally high and represented between 19 and 78% of the total S. The content of Mg was only slightly affected by fertiliser N + K, but the ratio K: (Ca + Mg) increased markedly with increasing rates of fertiliser. At several sites the ratio exceeded 2.2:1, the value above which there is an increasing risk of hypomagnesaemia in ruminant animals. The sum of the concentrations of the cations Ca + Mg + K + Na in the herbage increased as the concentration of N increased, and there was a similar though less marked relationship between organic anions and total N in the herb