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

Experiments were carried out on steers to determine the effects of differen t 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, maiz e silage, and soybean meal containing 6 mg beta -carotene/kg DM, and were s laughtered 0, 28, 42, or 70 days after starting on the ration. Significant decreases in fat yellowness and carotenoid concentration in the fat occurre d 70 days after starting on the ration. The exception was subcutaneous fat, which had a significant reduction of carotenoid concentration after only 2 8 days. Decreases in fat yellowness coincided with and were of a similar ma gnitude 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, whic h contained 33 mg beta -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 g rowth rates of the steers over the 63 days were 1.72, 0.71, and -0.03 kg/he ad.day for FASTGN, SLOWGN, and WTLOSS, respectively. Fat yellowness and car otenoid concentration in the fat depots were lower (P < 0.05) for FASTGN th an for WTLOSS and steers slaughtered at the beginning of the experiment, wi th 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 expe riments there were negative correlations between plasma cholesterol (PChol) concentration and fat yellowness and carotenoid concentration, and positiv e correlations between these traits and plasma carotenoid concentration (PC ar). PCar: PChol ratio was a better predictor of fat yellowness and caroten oid concentration than PCar or PChol concentration alone. The conclusions w ere that reduction in fat yellowness was largely caused by a dilution of ex isting fat with whiter fat and the rate of change in fat yellowness depende d on the rate of fat deposition. The results suggest that carotenoids do no t come out of fat once deposited there but further experiments are required to confirm this suggestion.