Effect of clover cultivar on production and nitrogen fixation in clover-ryegrass swards under dairy cow grazing

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Abstract
‘Grasslands Huia’, ‘Grasslands Pitau’, ‘Grasslands Kopu’, Aran and resident white clovers and ‘Grasslands Pawera’ red clover were sown with perennial ryegrass and examined for production (over 3 years) and nitrogen (N) fixation (over 2 years) under dairy cow grazing. Total sward production was similar for all treatments, although grass-only plots were 11 and 20% lower yielding in Years 1 and 2 respectively. Pawera produced the most clover in Year 1, but the least in Years 2 and 3. In Years 1 and 2, the best-producing white clover cultivars were Aran, Kopu, and Pitau, but by Year 3, Aran was 20% higher yielding than all other cultivars. In all years, Kopu showed superior winter production, whereas Aran was highest-producing during autumn. The resident clover was slow establishing from buried seed but by 6 months its production was similar to that for Huia. Measurements of plant composition showed that there was an increase in the number of resident clover plants from buried seed in all plots over time. N fixation by Aran, Kopu, Pitau, and resident clovers averaged 108 and 280 kg N/ha per year in Years 1 and 2 respectively. Corresponding values for Huia were 82 and 224 kgN/ha per year respectively. In Year 2, Aran produced 15% more dry matter (DM) than the resident clover which was offset by a lower total N concentration (by 14%) resulting in similar amounts of fixed atmospheric N. The proportion of clover N fixed (estimated by 15N isotope dilution) was similar for all cultivars, except in summer and autumn when it tended to be highest for Kopu. This study indicates that there may be no benefit from sowing new clover cultivars into dairy pastures which had previously contained white clover, in terms of N fixation. However, the use of Aran or Kopu can give greater clover production in autumn or winter respectively, and this may be beneficial if high quality feed is required during either of these periods.