Effect of dietary vitamin A on plasma and liver carotenoid concentrations and fat colour in Angus and Angus crossbred cattle

Abstract

Abstract The effect of dietary vitamin A supplements on plasma carotenoid (PC), liver B‐carotene, and fat carotenoid concentrations, and on fat colour in cattle were determined in three experiments. In Experiment 1, thirteen 2‐year‐old Angus × Friesian steers were grazed on pasture, and 6 of the steers were supplemented daily with 1 × 106 IU vitamin A for 83 days. In Experiment 2, twenty 2‐year‐old Angus crossbred heifers were grazed on pasture with 5 being supplemented daily with 1 × 106 IU and 5 with 2.5 × 106 IU vitamin A for 31 days. Cattle in Experiments 1 and 2 were slaughtered at the end of the experiments, and liver and fat samples were analysed for retinol and carotenoid concentrations 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 slaughtered 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 supplement of 1 × 106 or 0.5 × 106 IU vitamin A daily. Ten steers from each group were slaughtered after 62 days and the other 10 after 104 days of treatment. Daily supplements of 1 × 106 IU vitamin A caused a linear decrease in PC concentration of 0.13–0.20 μg/ml per day for 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 μg/ml, but in Experiment 3 where steers were fed a low carotenoid diet the decrease was only 0.8–1.0 μg/ml. PC concentrations in Experiments 2 and 3 were not affected by either the dose of vitamin A or the decline in pasture carotenoid concentration over the duration of the experiments. Vitamin A supplementation reduced the liver P‐carotene concentration by 40–48%, and increased the retinol concentration. Subcutaneous fat colour and carotenoid concentration were not affected by vitamin A supplements in any experiment, possibly because PC concentration was still too high in Experiment 1 (5–7 μg/ml), and because the PC concentration was already low in Experiment 3 (under 2 μg/ml). These experiments suggest that for vitamin A to be effective in reducing fat colour, the initial PC concentration may need to be less than 6–7 μg/ml.