An experiment was carried out for 4 weeks in early spring with 20 Friesian cows in their 6th month of lactation. Treatments were associated with grazing intensity of Persian clover (Trifolium resupinatum) such that daily pasture intakes ranged from 10·0 to 19·5 kg dry matter (DM) per cow. The marginal return to extra feeding obtained over the whole range of grazing treatments averaged only 0·4 kg milk for each additional kg DM of Persian clover eaten. This was very low considering the quality of the diet on offer (0·73 digestibility in vitro). It is suggested that a leaf fungal disease may have been implicated; this is discussed.
