A tightly autumn-calving herd of British Friesian cattle, running throughout the year as a single group, received two different amounts of mineralized barley supplement per day during the winter of 1973/4. As they calved, alternate cows were allocated to one of two rates of supplementary feeding differing in the approximate ratio 2:3 which was applied according to the calendar and not specifically related to milk yield.

Average herd size for the year was 140 cows; average milk sales were 4680 kg/cow at a total supplementary concentrate use of 650 kg/cow. All cows had free access to selffeed silage reinforced with brewers' grains. Cows which received 50% more supplement from 8 September to 31 January (an extra 235 kg/cow) showed an improvement in total lactation milk yield of 248 kg/cow. Standard lactation curves were used to demonstrate a significant interaction between quantity of concentrates fed and season of production and showed that most of the improvement took place during the winter period.