More Vitamin A in Milk

by R. E. HODGSON, H. G. WISEMAN, and W. A. TURNER

THE COW normally gets her vitamin A as carotene, which she obtains mostly from pasture and other forages. Part of the carotene is secreted in the milk unchanged and part of it is secreted as vitamin A. The total vitamin A value of milk, therefore, is due to both materials because the human body also converts some carotene into vitamin A. The carotene accounts for about 15 to 25 percent of the total vitamin A value of milk in the lower testing breeds, like the Holstein; it may account for about 30 to 40 percent in the higher testing breeds, like the Jersey. The amount of the vitamin in the milk varies according to the amount of carotene in the feed. When large amounts are fed, the value is high. When small amounts are given, it is low.

Under all conditions, the proportion of ingested carotene that is recovered in the milk by the cow averages about 5 percent or less. The reason for this poor utilization is not clear. The explanation awaits further research, which also may offer a means of increasing the vitamin A value of milk by a more efficient use of the available carotene. It is worth while to study the matter. Milk is a desirable source of vitamin A, and because it is so widely accepted as an essential food it is to the interest of all of us, particularly producers, to increase the amount of the vitamin in milk.

The greatest opportunities for doing so lie in providing uniform, adequate, year-round amounts of feed rich in carotene. Under average conditions the supply of carotene varies tremendously. In summer, when cows are on good pasture, the amount consumed is large. In winter, Particularly when only roughage of poor quality is fed, it is small. The greatest effort, therefore, should be directed toward improving the winter milk. It is true, however, that efforts at improvement are slowed by lack of economic advantages. Feeding large amounts of carotene does not of itself increase milk production, although farming practices that tend to preserve carotene in roughages also save other valuable feed nutrients.

The carotene content of pasture and other roughage varies with the species and with the stage of maturity; it generally decreases rapidly after the plants mature, particularly after bloom. To produce milk of high vitamin A value, then, the forage should be utilized when relatively immature and growing vigorously. Fertilization with nitrogen promotes vigorous growth and increases the carotene in forage.

But even in summer, when pastures are relatively immature and milk has a high vitamin A value, the amount of the vitamin in milk varies, depending on the availability and condition of pasturage. Pasture-management practices that provide a continuous supply of immature grass will effectively maintain the vitamin A in the milk at high levels.

Harvested forages put up as hay or silage provide carotene during the nonpasture periods. Partly because they usually are harvested at a later stage than pasture, they are lower in carotene. The manner in which forage is harvested and preserved greatly affects the quality of the resulting feed. Carotene is easily destroyed or lost through bleaching, oxidation, leaching, and leaf shattering. The degree of loss depends on factors like length of time the forage is left in the field, the amount of handling, weather during harvesting, the length of the storage period, in the case of hay, and the extent of fermentation that takes place in the silage. We recommend harvesting practices that favor rapid moisture evaporation with a minimum exposure of the forage to the weather, such a s windrowing shortly after cutting, cocking, using machinery that minimizes leaf shattering, avoiding overcuring, using forced-ventilation barn drying when making hay, or making the forage into silage.

Field curing, especially in places where uncertain curing weather exists, is not conducive to producing hay of high carotene value. In forced-ventilation barn drying, the forage is taken from the field when about half dry and before the leaves shatter, and placed in a mow over a system of air ducts. The drying is completed by forcing air through the mass of hay. The method offers possibilities of producing hay of higher quality and probably of higher carotene content. The extent of loss of carotene in barn drying probably depends on the length of time it takes to bring the hay to a sufficient degree of dryness so that it will keep.

A method of producing harvested forage of high carotene content is by artificial dehydration, with the use of high temperatures to evaporate rapidly the moisture in the green crop.

The quality of the newly made hay, at least as far as the color and leafiness are concerned, is a good measure of its carotene value. Hay of high carotene content loses some of it during storage under average conditions, while hay in storage for a long time has lost a large part of its carotene.

Immediately before and during the war much work was done at Beltsville and at several State experiment stations to develop practical methods of making silage from forage crops that ordinarily are made into hay. The crop is taken off the field soon after it is cut, and much carotene is saved. It is ensiled with or without a preservative to promote fermentation.

At Beltsville we made comparisons during harvesting and preservation of the carotene in alfalfa that was being put up in three ways: Silage, by the wilting method developed at our station; hay, by the forced-ventilation barn-drying method; and field-cured hay. Considerably more of the carotene was saved for feeding by making the crop into silage. In the first crop, the carotene in the barn-dried hay was quite high, compared to that of the second crop. This was because the air forced through the wilted forage to dry it was heated as it entered the duct system, and the drying was completed in 3 days as against 13 days needed to dry the second crop when the air was not heated. Also, the part of the first crop of field cured hay that had been rained on lost considerably more of its carotene than that taken in in sunny weather an important point in making hay.

We have fed silage and field-cured hay made from the same crops of alfalfa to milking cows and have determined the vitamin A value of the milk produced throughout the winter months. Our results demonstrate that the alfalfa silage when fed with corn silage and grain maintains the vitamin A value of winter milk at a high level, so that it more nearly approaches that of summer milk. Thus we have a practical method of preserving large amounts of the carotene that is present in the growing crop for use by the cow in producing high vitamin A milk throughout the winter when it is most needed. Feeding experiments in which we have compared the value of alfalfa silage with alfalfa hay in the ration of milking cows indicate that slightly greater milk production also is obtained on the silage ration. The cows like the silage and consume it in large quantities. Thus it may be an economic advantage to the farmer to make at least part of his hay crop (the first cutting) into silage.

Corn silage also can be an important source of carotene, provided the corn is harvested at the proper stage of maturity. Data collected at Beltsville indicate corn harvested when the ears are in the milk stage may be three times higher in carotene than if harvested when they are dented.

THE AUTHORS

R. E. Hodgson is assistant chief of the Bureau of Dairy Industry. During 1942-43 he conducted a survey of dairying in seven Latin-American countries and in 1939 was awarded the Borden award for outstanding research in dairy production.

H. G. Wiseman is an associate chemist in the Bureau of Dairy Industry. He is a graduate of the University of Maine.

W. A. Turner, now deceased, was also an associate chemist in the Bureau of Dairy Industry.