Records from 161 lactations completed in the year ended September 1969 were obtained from a herd of British Friesians kept indoors all the year round and fed grass silage with a concentrate supplement. Seasonal variation in milk production from the herd was compared with that from cattle traditionally managed.

After allowing for variation due to parity, season of calving, and stage of lactation, there was a tendency for milk production to be stimulated during periods of feeding grass consumed in a rapidly growing state. Seasonal variation was considerably less evident in the indoor herd than in the traditionally managed samples.