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
A95023 Received 7 April 1995; accepted 25 October 1995between 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.