November 30, 1998
By Peter J Lester
Vitamin A is essential for the well-being of the epithelial tissue (cell wall), and functions in its protection, and thus its preservation against disease. Vitamin A's progenitor is carotene, a yellow substance found in plants especially carrots and from which it acquires its name.
Vitamin A belongs to a group of vitamins that are termed fat soluble, in that, it requires the presence of fat as well as activator ions or minerals for proper absorption through the digestive tract. This vitamin can be stored in the liver, and need not be replenished daily.
Vitamin 'A' is found in two forms, one called retinol [found only in foods of animal origin, including butter, and fish liver oils] and pro vitamin A, or carotene, found in plants as mentioned above.
As mentioned, vitamin A is necessary for the healthy maintenance of the protective layer of the cells of the skin, it also functions in the protection of the eyes, digestive tract and respiratory system. In the event that vitamin A becomes deficient, these cells become flat, brittle and have less resistance to infection than normal. (Vitamin A is sometimes referred to as the "anti-infection vitamin") In advanced cases of deficiency, the eye forms a dry horny film over the cornea, resulting in a characteristic type of blindness called pink eye.
Too much NPN (Non Protein Nitrogen) in the diet or too little carotene has the same effect. In the case of too much crude protein, the conversion of carotene to true vitamin A ceases or is severely restricted. This results in symptoms akin to a deficiency, even in the presence of sufficient carotene. Nitrates are particularly harsh on this vitamin, and also on vitamin E.
Carotene is rapidly destroyed by sunlight and air, and especially high temperature. Hay over one year old, regardless of colour, is usually insufficient in vitamin A activity. In contrast, ensiling preserves carotene, however, the availability of carotene in maize silage may be low.
The younger the animal, the sooner the vitamin A deficiency symptoms will occur. Mature animals may store sufficient vitamin A to sustain them for up to 6 months. When deficiencies appear, they can be corrected by
A - increasing the carotene intake,
B - by supplying vitamin A by drench or injection. Vitamin A should always be administered in conjunction with vitamins D and E, and the element selenium. These are termed synergistic or agents which enhance each other's performance.
It should also be noted that vitamin A is required for the normal functions and maintenance of nerve tissue, and for the growth of bone and the enamel of teeth.
Vitamin A deficiency signs in animals include reduced feed intake, rough hair coat, swelling of the joints and brisket, secretion and discharge of tears , pinkeye, night blindness, slow growth, diarrhoea, convulsive seizures, improper bone growth, blindness, poor conception rates, abortion, still births, blind calves, and increased susceptibility to respiratory infection.