a1 Biochemistry Department, Massey Agricultural College, University of New Zealand, Palmerston North
1. Reasons for the seasonal variations in the vitamin A potency of New Zealand butterfat have been investigated. Adequate carotene for the production of butterfat of maximum potency has been shown to be present in New Zealand pasture throughout the year, but during the summer period this carotene is relatively non-available to dairy cows.
2. In experiments with monozygotic twins grazing average summer pasture it has been shown that a supplement of 1 g. of tocopherol per day increased both carotene and vitamin A levels in the milk fat, the total vitamin A potency being raised from a typical midsummer minimum of 34 i.u./g. fat to a value approaching the maximum late winterspring level of about 50 i.u./g. This maximum level was unaffected by increasing the tocopherol supplement to 3 g. per day or by supplying 300 mg. of carotene in a readily available form in addition to the 3 g. tocopherol.
3. Supplementing the summer pasture with 300 mg. of carotene in oil without tocopherol was without effect on the potency of the fat, supporting the conclusion that the carotene intake was already adequate.
4. A significant correlation was found to exist between vitamin A potency and tocopherol content of New Zealand butterfat.
5. It is suggested that the low vitamin A potencies may be associated with the high percentage of clover in the summer pastures.
(Received July 02 1951)