Three experiments were carried out to investigate the dry matter production of nine white clover (Trifolium repens L.) varieties grown with the same variety of perennial ryegrass (Lolium perenne L.) and subjected to rotational grazing by sheep over a period of 10 years. The experiments differed only in the nitrogen fertilizer added: no nitrogen, 100 kg/ha per year and 200 kg/ha per year. Dry matter (DM) yields of white clover and perennial ryegrass components of the sward were recorded at approximately 6-week intervals during the growing season.

Without applied nitrogen, 10-year mean white clover annual DM yields ranged from 2·89–4·54 t/ha. Mean clover content for all varieties ranged from 31–50% for the first 9 years, declining to 20% in the 10th year. The most consistently high yielding variety, AberVantage, gave yields greater than 3·5 t/ha in every year. Similar results were observed for the variety Aran. The 10-year mean annual DM yield of the perennial ryegrass companion (variety Talbot) ranged from 4·41–5·57 t/ha depending on the white clover variety in the mixture. Variation in perennial ryegrass DM yield was of a similar range and pattern as for white clover.

In the experiment in which 100 kg N/ha per year was added, 10-year mean white clover annual DM yields ranged from 2·27–3·87 t/ha. The varieties AberVantage and Aran showed yields that fell below 3 t/ha per year in only 1 or 2 years, respectively. Mean clover content for all varieties ranged from 26–38% for the first 9 years, declining to 17·5% in the 10th year. There was no consecutive annual decline in white clover DM yields. The 10-year mean annual DM yields of the perennial ryegrass companion ranged from 6·03–7·57 t/ha.

With applied nitrogen fertilizer of 200 kg N/ha per year (Experiment 3), 10-year mean white clover annual DM yields ranged from 1·21–2·67 t/ha. Mean clover content ranged from 12–27% for the 10 years. AberVantage and Aran showed yields consistently above 2 t/ha. No general decline in white clover DM yields was observed although some varieties showed consecutive annual reductions in years 8, 9 and 10. The 10-year mean annual DM perennial ryegrass yields ranged from 7·27–8·68 t/ha.

The results show that, under rotational grazing by sheep, white clover can give reliably high yields over a 10-year period. With moderate to high applications of nitrogen fertilizer white clover yields were lower than without nitrogen but they were consistent and sufficient to make a considerable impact on forage quality. There was no evidence for a general decline in clover content until the 10th year of the experiment and no cyclical pattern of annual variation in white clover yield was observed. The implications of these results for the role of white clover in UK agriculture are discussed.