EFFECT OF DIETARY VITAMIN-A ON PLASMA AND LIVER CAROTENOID CONCENTRATIONS AND FAT COLOR IN ANGUS AND ANGUS CROSSBRED CATTLE

The effect of dietary vitamin A supplements on plasma carotenoid (PC), liver beta-carotene, and fat carotenoid concentrations, and on fat co lour in cattle were determined in three experiments. In Experiment 1, thirteen 2-year-old Angus x Friesian steers were grazed on pasture, an d 6 of the steers were supplemented daily with 1 x 10(6) IU vitamin A for 83 days. In Experiment 2, twenty 2-year-old Angus crossbred heifer s were grazed on pasture with 5 being supplemented daily with 1 x 10(6 ) IU and 5 with 2.5 x 10(6) IU vitamin A for 31 days. Cattle in Experi ments 1 and 2 were slaughtered at the end of the experiments, and live r and fat samples were analysed for retinol and carotenoid concentrati ons respectively in Experiment 1, and liver samples were analysed for both retinol and carotenoid concentrations in Experiment 2. Experiment 3 involved ninety 3-year-old Angus steers, 10 of which were slaughter ed at the beginning of the experiment, 20 were grazed on pasture, and the remaining 60 steers were fed a diet of 70% barley and 30% pasture- silage on a feedlot either without vitamin A supplement, or with a sup plement of 1 x 10(6) or 0.5 x 10(6) IU vitamin A daily. Ten steers fro m each group were slaughtered after 62 days and the other 10 after 104 days of treatment. Daily supplements of 1 x 10(6) IU vitamin A caused a linear decrease in PC concentration of 0.13-0.20 mu g/ml per day fo r about 30 days. In Experiments 1 and 2, this represented a reduction of about 40-50% in PC concentration, equivalent to a decrease of 4.2-6 .4 mu g/ml, but in Experiment 3 where steers were fed a low carotenoid diet the decrease was only 0.8-1.0 mu g/ml. PC concentrations in Expe riments 2 and 3 were not affected by either the dose of vitamin A or t he decline in pasture carotenoid concentration over the duration of th e experiments. Vitamin A supplementation reduced the liver p-carotene concentration by 40-48%, and increased the retinol concentration. Subc utaneous fat colour and carotenoid concentration were not affected by vitamin A supplements in any experiment, possibly because PC concentra tion was still too high in Experiment 1 (5-7 mu g/ml), and because the PC concentration was already low in Experiment 3 (under 2 mu g/ml). T hese experiments suggest that for vitamin A to be effective in reducin g fat colour, the initial PC concentration may need to be less than 6- 7 mu g/ml.