Milk production and milk composition responses to supplementation of Holstein-Friesian cows grazing kikuyu pasture in summer and a combination of annual ryegrass and maize silage in winter with 64 mg/d flavophospholipol were determined. There was no difference in the average milk yield over the first 100 days of lactation between the control group (19.8 kg/day) and the Flavomycin supplemented group (20.5 kg/day). Milk production over the full lactation of 300 days was also not influenced by Flavomycin addition (control: 5 525 kg; treatment: 5 627 kg). There was no difference between the control and treatment group in average butterfat percentage (3.55% vs. 3.62%), butterfat yield over 300 days (195.3 kg vs. 201.2 kg), average protein percentage (3.12% vs. 3.14%) or protein yield over 300 days (170.2 kg vs. 178.5 kg). Flavomycin addition only reduced the milk urea nitrogen (MUN) concentrations during the third week of April (treatment: 16.0 ± 0.8 mg MUN/dl; control: 18.6 ± 0.4 mg MUN/dl) and the second week of September (treatment: 16.0 ± 1.2 mg MUN/dl; control: 19.6 ± 0.9 mg M-LJN/dl). Cows grazing nitrogen-fertilized pastures displayed great variation in weekly and monthly milk urea nitrogen concentrations which frequently exceed 18 mg MUN/dl.