TRANSFER OF FIXED NITROGEN FROM WHITE CLOVER TO ASSOCIATED GRASSES IN SWARDS GRAZED BY DAIRY-COWS, ESTIMATED USING N-15 METHODS

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses. The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha-1 yr-1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha-1 yr-1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha-1 yr-1 by N-15 dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a N-15 foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.