VITAMIN A Value of Plant Feeds Fully Accounted for by Their Carotene Content

In the preceding Yearbook of Agriculture, the writer reported experiments which showed that the health and productiveness of cattle are very dependent on the quantity of vitamin A which they receive in their rations,that these animals usually receive most of their vitamin A in the roughage, and that their health and productiveness are, therefore, commonly dependent on the kind and quality of their roughage. Recent research in the Bureau of Dairy Industry and in other scientific laboratories now throws more light on the chemistry of vitamin A and its distribution in various farm feeds, and on certain important practical considerations regarding the relation between its chemistry and color and its appearance in milk and butter.

Vitamin A appears in the tissues of animals as a nearly colorless highly complex alcoholic compound associated with the fats. Plant tissues, on the other hand, contain several closely related yellow pigments called carotenes. These pigments are hydrocarbons, and are easily converted by animals into colorless vitamin A when consumed as a part of the food. So far, colorless vitamin A has not been found as a natural constituent of plant tissues, and a number of investigations, particularly a recent careful investigation in the Bureau of Dairy Industry on alfalfa hay, have indicated that this compound does not occurin plants, and that the vitamin A activity of plant feeds is wholly accounted for by their carotene content.

The vitamin A content of feeds has been determined in the past in time-consuming experiments involving the rate of growth of rats.  Recently, however, fairly rapid and accurate direct chemical methods for the determination of carotene in plant tissues have been developed.  As the carotene content of plant tissues is a measure of their vitamin A activity, this activity can now be more quickly and accurately determined in plant tissues by carotene determinations than by the older form of feeding experiments with rats.

Carotene determinations made on the alfalfa plant show that the fresh green plant material cut in the bloom stage is a very rich source of this pigment. When this material is dried and cured for the purpose of making hay a large proportion of its carotene is destroyed, the amount of destruction depending on when the hay is cut and how it is cured. Hay cut in the bloom stage or earlier and cured without exposure to rain or to too much sunshine retains a considerable proportion of its green color and of its carotene content; hay cutin the seed stage or exposed to rain, or for many days to the sun, loses most of both color and carotene. Carotene determinations on a few farm feeds give a general view of the situation, though they are not yet numerous enough to be regarded as reliable averages. The comparatively few results reported in table 13 show that the carotene content, even of a given feed, varies considerably, but that there tend to be very large and more or less characteristic differences between different feeds. The grades of alfalfa and timothy hay given in the table are the standard United States grades which have been described in detail by the Bureau of Agricultural Economics, and the grading of which depends on color, and, in the case of alfalfa, also on leafiness. The No. 1 grade is that which has the most leaves and the most intense green color.

Cows fed on ordinary farm feeds consisting of plant materials depend on the carotene content of the feed for the vitamin A activity of their milk and butter. A part of the carotene of the feed appears as such in the milk fat; another part is converted into colorless vitamin A and appears in the butter as this compound. When the cow is fed on materials low in carotene, the carotene and colorless vitamin A of the milk fat become gradually less and less; the total vitamin A activity of the butter may be 20 times as great on feeds high in carotene as on feeds low in carotene.

The carotenes are the only yellow plant pigments which appear in milk fat in considerable amounts; hence the natural yellow color of cream and butter is due almost entirely to the carotene content. It is an important practical question how far this yellow color is a measure of the vitamin A activity of these dairy products. There are congenital differences between the colors of milk fat secreted by different breeds of cows. Guernseys and Jerseys, for instance, secrete milk fat which has a higher yellow color than that of Holsteins and Ayrshires on the same feed. Experiments have shown that those breeds which secrete the fat with the higher yellow color tend to put more carotene and less colorless vitamin A into the fat than the others, so that the higher colored milk fat of Guernseys is not likely to have any greater total vitamin A activity than the lower colored milk fat of Holsteins, as long as the two breeds are kept on the same kind of feed. The yellow color of milk fat is, therefore, not a good index of the vitamin A activity when the fat of different breeds on the same feed is compared.

But the differences in butter color which can be produced by different kinds of feed are much larger than those which occur among different breeds on the same feed.  The butter color of a given breed of cow is rarely as much as twice that of another breed on the same feed, whereas it is easy to reduce the butter color of an individual cow of any breed to less than one-tenth of the original level by changing her from good pasture to a ration of grain and U.S. No. 3 timothy hay. Changes in yellow color so caused are accompanied by roughly proportional changes in vitamin A activity. As the changes in butter color produced by feed changes, and also the accompanying changes in vitamin A activity, are so much larger than the breed differences which are not an index of vitamin A activity, the natural yellow color of the milk fat is, in general, a fairly good rough index of its vitamin A activity.