The Effect of a High Vitamin a Intake on the Blood and Milk Carotene of Holstein and Guernsey Cows

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Abstract
The decrease in milk carotene which follows the administration of shark liver oil appears to be caused by the vitamin A per se. A concomitant decrease in blood carotene also occurs when the milk carotene is lowered. The daily feeding of 2,100,000 I. U. of vitamin A as compared with 700,000 I. U. resulted in a greater decline in milk carotene; however, a further increase in dosage to 4,200,000 caused no further decline in carotene content of the milk. Approximately 4 weeks were required for the maximum lowering in carotene to develop while 7 to 10 weeks are required after the cessation of vitamin A supplement for the return of carotene secretion to normal. The administration of vitamin A supplements is followed by an increase in vitamin A level of the butter in the Holsteins similar to that noted earlier with the Guernseys. The average increase in level of vitamin A was constant at approximately 47 units per gram butterfat for 1,000,000 units fed between an intake of 700,000 and 4,200,000 I. U. The maximum level of vitamin A noted was 331 I. U. per gram. The efficiency in secretion of ingested vitamin A is about 3%. A proportionality exists between the levels of carotene in the blood and in the milk. Although the slopes of the curves for the Guernseys and the Holsteins are similar, that for the latter breed lies at a much lower level.