The effects of the grazing animal (treading, defoliation, and excreta) on N2 fixation were investigated in an intensively managed legume-based dairy pasture in the Waikato region, North Island, New Zealand. Two major 12-month field experiments were conducted in a long-term (>30 years) permanent mixed pasture of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) to determine the full effect of grazing animals on legume growth and N2 fixation. Each study was commenced in spring ( 1998 and 1999), when pasture was taken out of grazing and the relevant treatments applied in a randomised block design. Subsequently, pasture was defoliated by mowing at regular intervals and the herbage dry matter yield and pasture species composition determined. N2 fixation was estimated using the 15N isotope dilution technique to give an integrated estimate of the proportion of clover N derived from atmospheric N2 (%Ndfa) at each harvest. The first experiment (initiated in October 1998) investigated the effects of urine additions on N2 fixation in pasture managed with light and moderately severe cutting treatments. Urine application (equivalent to 746 kg N ha-1) increased average annual production by 85%, but did not significantly impact on clover production, except in the light cutting treatment at days 50-100. Urine application reduced %Ndfa to 25% of the control (as measured by 15N methods), with recovery taking almost a year, thus reducing total fixed N in clover herbage from 232 to 145 kg N ha-1 yr-1. Four methods of estimating %Ndfa from 15N data were used in the urine treated plots. The use of 15N-labelled urine gave estimates of %Ndfa that were 20-30% below values calculated using conventional 15N-labelling during the first 161 days. These differences were probably due to differences in the rooting depth between ryegrass and white clover in conjunction with treatment differences in 15N distribution with depth. The 15N natural abundance technique gave highly variable estimates of %Ndfa (-56 to 24%) during the first 23 days after urine application, but thereafter estimates of %Ndfa were similar to those using 15N-labelling methods. Defoliation severity had no immediate effect on N2 fixation, but during summer and autumn %Ndfa inmoderately-severe plots was consistently higher than under light defoliation. The greater abundance and productivity of vigorous growing weed and summer-grass species recorded in moderately-severe plots during summer and autumn indicate that these species were probably regulating N2 fixation by changing soil N availability. Overall, total N fixed increased by 36% under moderately-severe defoliation compared to light defoliation. Defoliation severity had no significant effect on clover production. The second major experiment (initiated in September 1999), examined the effects of pugging intensity on pasture growth and N2 fixation. A single pugging event at stocking rates of 4.5 cows 100 m-2 for 1.5 or 2.5 hours on a wet silt loam soil initiated the experiment, and was equivalent to a moderate or severe pugging event in spring. Fixed N in clover herbage decreased significantly from 76 kg N ha-1 yr-1 in the non-pugged control, to 66 kg N ha-1 yr-1 and 36 kg N ha-1 yr-1 under moderate and severe pugging, respectively. The decrease in fixed N was mainly due to treading damage to clover plants and an associated reduction in clover production which persisted for up to 259 days under severe pugging. Detailed clover morphology measurements revealed an increase in stolon fragmentation and a decrease in plant density, which heightened the vulnerability of plants to dry soil conditions following pugging. A medium-term reduction in grass growth under pugging probably led to increased soil inorganic N and would account for an observed decline in %Ndfa that occurred during the first 3-months of the study .Although pugging caused a major disturbance and rearrangement of the soil surface layers this only had a marginal short-term effect on soil physical properties. Soil aeration was reduced for approximately 21 days and did not measurably affect N2 fixation. However, an increase in denitrification during the first 21 days after pugging suggested biological process were affected by the reduction in soil aeration. A conceptual model is proposed to describe the principal processes that affect N2 fixation by grazing animals in intensively managed clover-grass pastures. The use of a whole farm dairying model of N cycling to further investigate animal impacts in legume-based pastures highlighted the need to consider pasture management strategies and interactions between treading, defoliation and excreta. In the future, more complex models specifically dealing with legume-based pasture ecosystems will be needed to integrate detailed animal-plant-soil components and thus assist in identifying how N2 fixing efficiency can be improved.