The source of vitamin D in summer milk

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Abstract
Eight Shorthorn cows in four groups of two were treated as follows: (1), indoors on winter ration; (2), outdoors on the same ration; (3), on pasture; (4), indoors but given 1 cwt. daily per cow of freshly cut grass. Butters were churned and with them prophylactic tests were carried out on groups of 15 rats. Butter fats produced outdoors and indoors on the summer ration had potencies of 0.46 and 0.15 I.U. per g. respectively (3:1); figures for similar butters produced on the winter ration were 0.88 and 0.27 I.U. per g. respectively (also 3:1). The average total amount of vitamin D secreted daily by a cow was as follows: (1), 110 I.U.; (2), 313 I.U.; (3), 252 I.U.; (4), 52 I.U. The yields per kg. of milk were 8.3, 26.0, 17.0 and 5.3 I.U. respectively. Thus the direct exposure of the cow to sun- and sky-shine contributes all, and the pasture none, of the increase in the vitamin D potency of milk which takes place in the summer.