174              Vitamin-A Deficiency in the Diet of Farm Animals

yellow maize was deemed a sufficient vitamin-A supplement, any further
additions of vitamin-A concentrates to the ration being of no additional
benefit [Sheehy, 1932]. It has, however, been pointed out that a 100 lb.
pig should have the equivalent of four mg. carotene daily in its diet to satisfy
its vitamin-A requirements for maintenance and normal growth [Dunlop,
1935].

In an investigation into the minimum vitamin-A requirements of cattle,
it was noticed that animals receiving a small daily ration of cod-liver oil or
alfalfa meal for two years showed scarcely any storage, the total reserve of
the body being estimated at 200 mg. of carotene and vitamin-A. It was
also determined that the rate of depletion of this reserve was about 9 to 11
γ per kg. of body weight per day. For animals depleted till night blind-
ness had set in, the minimum daily dose to enable them to have a normal
health and growth was found to be 26 to 33 γ per day per kg. of live weight
[Guilbert and Hart, 1935]. On this basis the approximate requirement of a
1,000 lb. bullock worked out to be 11.7 to 14.85 mg. of pure β-carotene
per day. Besides the above mentioned maintenance requirement, cows
would have to secrete large amounts daily in their milk. The vitamin-A
value of the best grade of cows' milk was found to be 1,500 rat units per quart.
It was also observed that only less than five per cent of the carotene content
of the food was transmitted through the milk [Russell, 1933]. In another
experiment it was recorded that the cows' milk contained 2,500 to 1,600 inter-
national units per quart and that more than 3.5 per cent of the feed carotene
was never present in the milk [Russell et al, 1935]. A biological estimation
of the vitamin-A content of whole milk showed it to be about 454 international
units per pint [Cunningham, 1936]. When the carotene or vitamin-A inges-
tion was low, cows secreted 3.3 per cent of the intake through its milk and
when on carotene rich feed 1.3 per cent only [Garry et al, 1936].

                    STORAGE AND UTILIZATION OF VITAMIN-A

Having considered the vitamin-A requirements of animals for main-
tenance, growth and health, it is natural to suppose that the excess amount
in the feed of the animals would be stored in the system for use in produc-
tive purposes. This takes us on to the question of the animals' capacity
to store and the mechanism through which they eliminate this constituent.

It has been found that of the total amount fed to a horse about 70 to 80
per cent of the carotene and 55 to 64 per cent of the xanthophyll were excret-
ed in the faeces. An examination of the dung pigments showed that they
remained unchanged by the digestive process [Zechmeister and Tuzson,
1934]. Another experiment showed that the vitamin-A excess was mainly
excreted in the urine as a degradation product, giving it an increased iodine
value [Santos Ruis, 1935].

Quantities of vitamin-A in the feed which escaped elimination through
either of these channels should have found their way into the store of the
body, namely, the liver, which is also the seat of carotene metabolism. An