Productivity of white clover cultivars under intensive grazing, as affected by high nitrogen fertiliser application

Nine cultivars of white clover (Trifolium repens L.) were grown with perennial ryegrass (Lolium perenne L.) and their productivity was determined over 3 years under nitrogen (N) fertiliser regimes of 0 or 390 kg N/ha per year. The site was grazed by sheep at 3- or 6-week intervals and after each grazing N fertiliser was applied at 0 or 30 kg N/ha. Fertiliser N application increased total annual pasture production by 25–31%, with the largest proportional responses occurring in autumn and winter. White clover production decreased as a result of N application by 8, 17, and 30% in Years 1, 2, and 3 respectively. During the first 2 years this reduction in clover growth occurred only in spring, and in Year 3 it was also most marked in spring. There was no significant cultivar × N interaction for clover or total pasture production on an annual or seasonal basis. Thus, differences between cultivars in relative clover production were similar in the absence or presence of N fertiliser application. Clover cultivars varied greatly in productivity, and in the nil N treatment there was a positive correlation between clover production and total pasture production in Years 1 and 2, when clover content of pastures averaged 45 and 28% respectively. In all years, Aran and Kopu were higher yielding than all other cultivars. During winter, Kopu was 2–5 fold more productive than other cultivars. In all other seasons, Aran and Kopu were the most productive. There was a highly significant negative correlation between clover production and grass production in both N treatments and in all years. Thus, the more competitive erect-growing, large-leaved cultivars of Aran and Kopu maintained a lower associated grass component in their swards.