Two surveys were done, one in 1943–44 and one in 1958–60, of the seasonal variation in the concentration of vitamin A and carotene in milk fat from 13 different areas in Great Britain. In the 1958–60 survey α-tocopherol and vitamin D were also measured. The main purpose of the second survey was to find out if the marked swing from Shorthorns to Friesians, and the trend towards greater use of carotene-rich foods in winter feeds that occurred in the intervening 15 years, had affected the pattern of vitamin A activity. In addition, monthly samples of butter were obtained in 1958–59 from a single factory in New Zealand during a complete butter-making season (9 months) and were assayed for vitamin A, carotene, α-tocopherol and vitamin D. Three experiments were done to study the effect of breed on the vitamin A and carotene content of milk fat, and in one of them α-tocopherol and vitamin D were also determined.
The seasonal trends for vitamin A and carotene were essentially the same in both surveys. A peak occurred in May after the spring flush of grass and was followed by a decline with a second peak in the autumn, except in 1959 when both the summer and autumn were very dry. Potencies were higher in the south than in the north, where the differences between summer and winter were more marked. In winter, mean vitamin A potencies of 5·8 and 7·7 μg/g fat were found for northern and southern areas, respectively; the corresponding values for β-carotene were 2·0 and 4·5 μg/g fat. Mean summer values were 8·8 and 9·7 μg/g fat for vitamin A and 5·1 and 6·5 μg/g fat for carotene, equivalent to increases of about 52 and 26% and about 155 and 44%, respectively, over winter values. Differences in mean potency between areas are explained by length of grazing season and type of winter feed.
The concentration of vitamin A in the fat was essentially the same for Ayrshires, Friesians, Jerseys and Shorthorns but somewhat lower for Guernseys; that of carotene was higher for the Channel Island breeds, particularly Guernseys, than for the other breeds studied.
Compared with this country, concentrations of carotene were higher and of vitamin A lower in milk fat from New Zealand, probably because of the predominance of Jersey cows in New Zealand herds. The seasonal trends for these vitamins were similar in both countries, but the spring peaks were earlier, the autumn peaks later and the summer declines more pronounced in New Zealand than in this country.
Concentrations of α-tocopherol in British milk fat showed a trend similar to that found for both vitamin A and carotene and were again influenced by feeding practices. Mean values of 23 and 29 μg α-tocopherol/g fat were found in the summer for northern and southern areas, respectively; the corresponding winter values were 15 and 23 μg/g fat. Mean values of about 30 μg/g fat were found, over a 28-month period, for the milk fat of each of the 3 breeds, Ayrshire, Friesian and Jersey. No seasonal trend was observed in the New Zealand samples; the mean value for the 9 months was 32 μg/g fat (range 24–39 μg).
Vitamin D potencies were higher in summer, when there is more sunshine, than in winter. The higher summer values in the northern than in the southern part of the country may have been related to a higher intake of vitamin D in the former from hay and pasture. Summer potencies of 0·44, 0·53 and 0·62 i.u. vitamin D/g fat, respectively, were found for Friesian, Jersey and Ayrshire milk; corresponding winter values were 0·06, 0·08 and 0·9 i.u./g fat. For the New Zealand butters, summerues values were higher than in this country, about 0·7 compared with about 0·4 i.u./g fat; winter values were similar in both countries.