The effect of strategic use of fertilizer nitrogen in spring and/or autumn on the productivity of a perennial ryegrass/white clover sward

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 kg N ha^-1 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-low autumn N than the reverse. Mean yield responses at the first harvest to 25, 50 and 75 kg ha^-1 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 of total herbage was between 7.08 t ha^-1 DM with no N and 8.19 t ha^-1 with 75 kg ha^-1 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 0.12 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.