Sheep liveweight gain (LWG) can be higher under sheep/cattle mixed-grazing than under monospecific grazing. We conducted an experiment to assess the relative role of parasite dilution and complementarity of feeding niches to improved sheep LWG, under mixed-grazing. We compared LWG of ewe lambs grazing alone or with heifers, at two contrasting cattle/sheep ratios (~50/50% and 80/20% in livestock units). We assessed parasitism through strongyle eggs per gram of faeces (EPG) and foraging mechanisms through faecal nitrogen content (N). We observed that: (1) N did not significantly differ across treatments (~2%); (2) EPG was ~50% lower in mixed compared to monospecific treatments (545 and 716 vs 1278, P<0.001); and (3) LWG were higher in mixed compared to monospecific treatments (~40 g per day higher, P<0.001). Neither EPG nor LWG significantly differed between the two cattle/sheep ratios. We conclude that: (1) improved sheep LWG was mainly due to parasite dilution; and (2) benefitting from this mechanism can be obtained on a broad range of sheep/cattle ratios. As no fine-tuning of the cattle/sheep ratio is required to benefit from mixed-grazing, we consider this practice as operable, i.e. likely to be ‘easily’ put into operation, from a biological viewpoint.