Frape et al. (1959a) recommended the addition of 800 IU vitamin A palmitate on a dry carrier per lb of feed as the minimum requirement of the young pig (up to 8 weeks of age) under the conditions of their experiment. The recent requirement of vitamin A listed in the NRC (1998) is 2,200 to 1,300 IU per kg of diet (1,000 to 591 IU per lb) for weanling to finishing pigs. Breeding animals require 4,000 IU per kg (1,818 IU per lb), while the estimated requirement for lactating gilts and sows is 2,000 IU per kg (909 IU per lb). The reason that lactating animals require less vitamin A per unit of feed than for breeding is related to allowed feed consumption. Lactating sows would consume an average of 5.3 kg of feed (11.7 lbs), while breeding animals are restricted to about 1.9 kg (4.2 lbs) of feed to prevent excessive weight gains.
These requirements are deemed sufficient to provide for optimal growth, satisfactory reproduction and prevention of deficiency signs. Suggested requirements should be designed to be adequate for these purposes under practical conditions of feeding and management as well as to allow for a certain amount of storage. The decision as to the minimum vitamin A requirement of young swine depends on whether the criterion to determine the requirement is based on growth and feed utilization or also considers liver vitamin A storage. Sheffy et al. (1954) reported that 18 mg of oral vitamin A per kg body weight was required to supply the minimum requirement to promote a trace of liver storage of vitamin A. Storage of vitamin A in the liver certainly appears to be desirable, since under conditions of little or no liver storage, stresses and diseases may precipitate vitamin A deficiency. The vitamin A reserves of the sow make it difficult to establish requirements. Braude et al. (1941) reported that mature sows fed diets without supplemental vitamin A completed three pregnancies normally; only in the fourth pregnancy did deficiency signs appear. Gilts receiving adequate vitamin A levels until 9 months of age completed two reproductive cycles without signs of vitamin A deficiencies (Selke et al., 1967).
Heaney et al. (1963) indicated that liver stores at birth were less important to newborn pigs than the vitamin A in the colostrum and milk. Thomas et al. (1947) reported that the extent of placental and mammary transfer of vitamin A in swine and dairy goats could be elevated by adding large amounts of vitamin A to the diet of pregnant sows or does. Thomas et al. (1947) indicated that feeding massive doses of vitamin A during late pregnancy increased the vitamin A content in the colostrum by 2.5 to 3 times that of the control group. Rearing sows on pasture versus drylot also increased the concentration of vitamin A in milk and colostrum (Bowland et al., 1949a; 1949b).
In establishing a satisfactory vitamin A level for practical diets, it is necessary to consider a number of factors that may alter the vitamin A requirement. Practical factors influencing vitamin A requirements are listed in Table 1. The conversion of carotenoids to vitamin A is one of the most important considerations in determining swine vitamin A requirements (see Section I).