Published online by Cambridge University Press: 18 August 2016
Two experiments determined the effects of daily oral supplements of vitamin A (retinol) and injected preparations of vitamin A on liver vitamin A concentration and plasma carotenoid (PC) concentration. Vetade is an oily injectable vitamin A preparation, while Nov-a-Vit is an aqueous injectable vitamin A preparation. Angus crossbred heifers were used in experiment 1 and Angus and Simmental crossbred steers were used in experiment 2. All cattle grazed pasture but were given also 1 kg per head of pellets daily and vitamin A supplements were included in the pellets when required. In experiment 1, one group of heifers was given 1 × 106 i.u. vitamin A per head daily for 31 days, while a second group in another part of the paddock was not supplemented with vitamin A. A third group of heifers grazed with this latter group and was injected with 2·5 × 106 i.u. vitamin A (Vetade) three times weekly. The oral vitamin A supplement proportionately reduced plasma and liver carotenoid concentrations by 0·49 and 0·41 respectively, while increasing liver vitamin A concentration three-fold. By contrast, the Vetade failed to increase liver vitamin A concentration or to reduce liver and plasma carotenoid concentrations. In experiment 2, two groups of steers were injected twice weekly for 7 weeks with 2 × 106 i.u. vitamin A as Nov-A-Vit or as Vetade. A third treatment group, grazing with these steers, received no vitamin A injection, while a fourth group grazing in a separate area of the paddock was supplemented daily with 1 × 106 i.u. vitamin A per head in their diet. The injection of Nov-A-Vit and the oral vitamin A supplement decreased PC concentration by a similar amount but the increase in liver vitamin A concentration was larger for steers injected with Nov-A-Vit. Vetade injections failed to increase liver vitamin A concentration and there was no decrease in PC concentration. The plasma cholesterol concentration for the treatment groups varied over the duration of the experiment but could not be related to the vitamin A treatment or to the changes in PC concentration. These results indicate that vitamin A injections can be used to reduce PC concentration provided they increase liver vitamin A concentration.