HARVEST MANAGEMENT EFFECTS ON PRODUCTIVITY, DINITROGEN FIXATION, AND NITROGEN TRANSFER IN BIRDSFOOT-TREFOIL ORCHARDGRASS COMMUNITIES

Dinitrogen fixation and N transfer need to be quantified to help develop optimum harvest recommendations for legume-grass mixtures. Field research was undertaken to determine how harvest management (three-cut vs. six-cut systems) influenced the productivity, N-2 fixation, and N transfer of birdsfoot trefoil-orchardgrass communities. Four birdsfoot trefoil (Lotus corniculatus L.) cultivars were seeded in binary mixtures with orchardgrass (Dactylis glomerata L.) Dinitrogen fixation and N transfer were estimated by N-15 isotope dilution using pure stands of orchardgrass as a reference. Herbage dry matter yields were greater under the three-cut system, averaging 8.8 Mg ha(-1) compared with 6.4 Mg ha(-1) for the six-cut system (means of cultivars and years). Percentage legume N derived from fixation (%Ndfa) was only slightly greater for the three-cut system. Mean %Ndfa for the three-cut system was 98% compared with 95% for the six-cut system. Total-season yields of fixed N in birdsfoot trefoil herbage, percentage N in orchardgrass herbage derived from fixation, and total-season yields of transferred N in orchardgrass herbage generally were not affected by harvest management. We conclude that six-cut management reduced dry matter yield compared with three-cut management. Relatively little difference was observed in biological N-2 fixation or N transfer, however.