The rate of reduction in carotenoid concentration in fat of steers fed a low carotenoid ration, and the role of increasing carcass fatness

Abstract

Experiments were carried out on steers to determine the effects of different allowances of a low carotenoid ration on fat yellowness and the interval from starting on the ration to significant decreases in fat yellowness. In Expt 1, forty 17–18-month-old steers were fed a ration of maize grain, maize silage, and soybean meal containing 6 mg β-carotene/kg DM, and were slaughtered 0, 28, 42, or 70 days after starting on the ration. Significant decreases in fat yellowness and carotenoid concentration in the fat occurred 70 days after starting on the ration. The exception was subcutaneous fat, which had a significant reduction of carotenoid concentration after only 28 days. Decreases in fat yellowness coincided with and were of a similar magnitude to increases in fat deposits. In Expt 2, 3 groups (n = 13) of 24–26-month-old steers were fed a ration of maize grain and pasture silage, which contained 33 mg β-carotene/kg DM. The ration was fed to steers for 63 days at 1 of 3 allowances with the aim of gaining weight rapidly (FASTGN), gaining weight slowly (SLOWGN), or losing weight (WTLOSS). A fourth group of steers (n = 9) was slaughtered at the beginning of the experiment. The growth rates of the steers over the 63 days were 1.72, 0.71, and –0.03 kg/head.day for FASTGN, SLOWGN, and WTLOSS, respectively. Fat yellowness and carotenoid concentration in the fat depots were lower (P < 0.05) for FASTGN than for WTLOSS and steers slaughtered at the beginning of the experiment, with no differences between the latter 2 groups. The SLOWGN were intermediate with higher fat yellowness or carotenoid concentration in some fat depots than for FASTGN and lower than for WTLOSS in other fat depots. In both experiments there were negative correlations between plasma cholesterol (PChol) concentration and fat yellowness and carotenoid concentration, and positive correlations between these traits and plasma carotenoid concentration (PCar). PCar :PChol ratio was a better predictor of fat yellowness and carotenoid concentration than PCar or PChol concentration alone. The conclusions were that reduction in fat yellowness was largely caused by a dilution of existing fat with whiter fat and the rate of change in fat yellowness depended on the rate of fat deposition. The results suggest that carotenoids do not come out of fat once deposited there but further experiments are required to confirm this suggestion.