Four varieties of white clover (small‐leaved cv. Aberystwyth S184. medium‐leaved cv. Grasslands Huia and large‐leaved cvs Linda and Olwen) were sown at 3 kg ha‐1 together with 10 kg ha‐1 perennial ryegrass cv. Talbot. Herbage productivity was measured for three harvest years, 1979‐81, over four annual rates of fertilizer N (0,120,240 and 360 kg ha‐1) and two closeness of cutting treatments (80 and 40 mm from ground level). A simulated grazing regime of six cuts per year at 3‐ to 6‐week intervals was used. Production of total herbage DM was increased by increasing N rate; mean annual DM production ranged from 783 1 ha‐1 with no N to 11701 ha‐1 at 360 kg ha‐1 N. Mean herbage response to N (kg DM per kg N applied) was 73,90 and 108 for the three successive N increments relative to no N. Mean white clover DM production was reduced from 4 14t ha‐1 with no N to 051 t ha‐1 at 360 kg ha‐1 N. The large‐leaved clover varieties were more productive than the small‐ or medium‐leaved varieties at all N rates. Close cutting increased total herbage and white clover by a mean annual 16% and 31%. respectively. White clover varieties did not interact with either N rate or closeness of cutting. It is concluded that repetitive N application over the growing season is incompatible with white clover persistence and production, even with large‐leaved clover varieties or with close cutting, two factors which improved clover performance in the experimental swards.

The productivity of a mixed sward, comprising perennial ryegrass cvs Barlano and Bastion and white clover cvs Donna and Aran, was measured under sixteen fertilizer N treatments. These involved 0.25,50 and 75 kgNha"' in spring only, in autumn only and in all combinations of spring N and autumn N. A simulated grazing regime of six cuts annually at 3-to 6-week intervals was imposed.Increasing rates of total N application increased total herbage DM regardless of application pattern. Yield response was greater with N applied in the spring, and total herbage DM was higher with high spring N-Iow autumn N than the reverse. Mean yield responses at the first harvest to 25, 50 and 75 kg ha^' N in spring were 13-6.10-8 and 11 -6 kg DM per kg N. Corresponding responses at the final harvest to N rates in the autumn were 7-2, 5 8 and 6-8 kg DM per kg N. Responses were similar at these times for treatments receiving combined spring and autumn N.Over all treatments, mean annual production oftotal herbage was between 708 tha"' DM with no N and 8-19 t ha"' with 75 kg ha^' N in both spring and autumn. Owing to drought, mean production in year 2 fell by 32% compared with year 1.White clover production fell progressively with increasing N application. Treatments with spring-applied N gave the most marked decrease.White clover was more markedly depressed than the associated grass by the drought in the second year. The mean reductions in white clover content were 0-17, 0-07 and 012 percentage units per kg applied N for spring N, autumn N and combinations. Autumn N use depressed white clover less than spring N but the yield response of grass was less.It is concluded that any applied N adversely affects white clover performance to some degree. Where management factors are unfavourable to white clover even strategic N use may not be wise. Instead, it is suggested that a 'dual-sward' approach be adopted in practice, namely, grass/ white clover swards with no N. and complementary grass swards receiving optimum applied N to give better production at times when grass/white clover swards are relatively less productive.

Fourcultivarsoff)erennial ryegrass (intermediate diploid cv. Talbol and tetraploid cv. Barlatra. and late diploid cv. Parcour and tetrapioid cv. Petra) were each sown at 10,20 and 30 kg ha"', all with 3 kg ha"' of white clover cv. Donna. Herbage productivity was measured over 3 harvest years, 1982-84. under two annual rates of fertilizer N (0 and 150 kgha'); the 150 kg ha"' rate was split equally between March and August applications.Fertilizer N increased total herbage DM production; the 3-year means for the 0 and 150 kg ha-' N rates were 8 04 and 891 t ha"', respectively. In successive years, total herbage responses to N (kg DM (kg N applied)"') were 6-6, 3 5 and 7 2 (overall mean, 5 8). Mean white clover DM production over the 3 years was reduced from 448 t ha-' at nil N to 2 82 t ha' at the 150 kg ha-' rate, a fall of 37%. Grass seed rate did not influence total herbage production or white clover performance. The two intermediate perennial ryegrass cultivars had a marginal advantage in total herbage production over the two late cultivars, but white clover content and production were higher with tetraploids than diploids.It is concluded that the value of increased herbage production from strategic use of fertilizer N has to be weighed against its depressive effect on white clover performance; application of 75 kg ha"' N in both spring and autumn was Correspondence: Dr J. Frame, Agronomy Departmeni. The Wesl of Scotland Agricultural College. Auchincruive, Ayr KA6 5HW, UK.excessively high if maintenance of a good white clover content in the sward is an objective. There is considerable flexibility in the grass:clover seed ratio in seeds mixtures. Modern highly-productive perennial ryegrass varieties do not differ substantially in compatibility with white clover but tetraploids permit better clover performance than diploids.