Nutrition & Feed Quality, Vitamin A Importance

August 10 2005

By Peter J Lester

Factors that affect the animal's health and productivity must be always viewed from the ground up. Most problems common to our agriculture can be traced directly to poor fertility both at soil and feed level. This unique country of ours 'evolved' without the aid of animals. It was quite capable of supporting scrub and trees, but, as we were soon to learn, that it had problems supporting disease free animals. A typical example is that of 'bush sickness'. This malady is associated with a deficiency or availability of the element cobalt. Many other ailments can be traced to soil deficiencies, such as milk fever and grass tetany. Vitamin deficiencies also plague our animals. This paper deals with one of these Vitamins. Livestock producers have been led to believe that all they need to satisfy the Vitamin A requirements of their livestock is green growing plants. The justification for this stems from the fact that green plants and good quality hays contain a substance called carotene. Carotene is a precursor, or a substance that precedes a later more developed form, in this case, true vitamin A. Vitamin A appears to be a product of carotene, and in theory one molecule of carotene should produce two molecules of this vitamin. Research has shown however, that the conversion is much less than expected. Different animals have differing abilities to convert carotene; table 1 is based on present research. 


Species   mg IU  
  amount converted in %
standard  1=1,667                 100
Cattle 1 = 400               24.0
Sheep1=400- 500
Swine   1 = 500                 30.0
Horses:Growth1 = 555               33.0
Horses: Pregnancy             1 = 333                30.0 
Poultry 1 = 1,667               100.0                             
Dogs1 = 833              50.0                                                  

The amount in fresh green grass is very high relative to dietary requirements. Thus, a small quantity of fresh pasture should supply the daily requirements for cattle and other ruminants. However, there are other factors that effect this conversion. In contrast to energy, protein and minerals, both carotene and Vitamin A are easily oxidized to compounds which have no vitamin A activity. Heat moisture and especially light are potent agents in the oxidation and destruction of carotene. The carotene content of hay declines continually in storage with a rapid increase in destruction when it is exposed to light. A phosphorus deficiency can reduce this conversion in the animal from carotene to vitamin A. As can the presence of Non Protein Nitrogen (NPN) and especially NO3 (Nitrates). In fact, it is generally believed that nitrates virtually block the conversion. Further, a heavy worm infestation may interfere with the conversion, thus increasing the severity of both. This may be due to the damage worms inflict on the linings of the intestinal wall where this conversion takes place. 


In Germany in 1967 revealed that there was a 20% to 30% incidence in reproductive disorders in dairy cows. Subsequently it was revealed that they had received inadequate beta-carotene in their rations. Work by several researchers had already revealed that the corpus luteum of these cattle contained high concentrations of beta-carotene but no vitamin A. This high concentration of B-carotene in the ovaries raised the speculation that this substance may play a specific role in the reproductive functions of cattle. Cows were then fed Vitamin A at a level of 0.14 mg of carotene plus 45.5 IU/Lb body weight daily. The time period between calving and conception was reduced, compared to a control group not receiving supplementation. As vitamin A protects the cell wall, or epithelium, in cases where there is poor conversion or availability, the animals may show signs of the same. These signs may include any of the following: 

Weeping from the eye

Rough hair coat

Poor conception

Pink eye

Retained placenta

Delay in the onset of ovulation

Silent heat

Photosynthesis problems (facial eczema)

Sore teats

Lowered plasma progesterone levels

Reproductive disorders

Dry scaly skin

Abnormal embryonic growth

Placenta injury

Death of Foetus

Swollen Brisket

Breathing Difficulty

The conversion of beta-carotene to vitamin A differs in species. Animals convert b-carotene to vitamin A with varying degrees of efficiency. The conversion rate of the rat is the base standard used, with I mg of b-carotene being equal to 1,667 IU (International Units) of true vitamin A. Based on this standard, ref: Table 1.cattle are not good converters at the best of times. For safety's sake, cattle should be injected with Vitamins, A, D and E at least three times a year. Once at mating, at calving and at drying off. The amount to use should be on the bottle, and the product highest in vitamin A is the one I would recommend.