Effect of supplementation of beef steer diets with oil containing n6 and n3 fatty acids and 48h feed withdrawal treatments on animal productivity, carcass characteristics and fatty acid composition

A 2×3 factorial experiment was conducted with 72 weanling (about 180d old), crossbred steers (with Hereford, Angus and Charolais genetics and initial body weight of 280.5±5.8kg) to determine the effect of dietary supplementation with a 50/50 mixture of flax and sunflower oils at 5% of diet, to steers in three feed withdrawal (FW) treatments on production factors, carcass characteristics, adipocyte marker; peroxisome proliferator activated receptor γ (PPARγ) RNA expression and fatty acid composition of muscle (Pars costalis diaphragmatis; PCD) and subcutaneous fat (SQ from brisket). The FW treatments were no FW, a single 48h FW (FW×1) before start of fattening or 48h FW at 8week intervals from weaning to start of fattening (FW×4). Individually penned steers were fed mixed hay and rolled barley diets containing no oil (Control; CON) or 5% oil (OIL) through growing, transition and fattening phases. Interaction of dietary oil and FW treatments did not affect any of the production factors. Decreased (P=0.008) dry matter intake (DMI) of the OIL diet was noted through the growing period; thus decreasing (P=0.019) total DMI by 112kg/hd for the experiment and improved gain per unit feed (P=0.019) for the growing period. The FW×1 treatment reduced (P=0.004) average daily gain (ADG) through the transition period, but ADG tended to improve (P=0.081) during fattening and steers in FW treatments gained 1.38±0.07 versus 1.25±0.06kg/d for those in the no FW treatment. Differences due to dietary oil or the FW treatments were not observed for any of the carcass characteristics. A diet×FW interaction (P=0.001) was observed for PPARγ expression in the PCD and SQ. Adipocyte marker expression was greater in PCD of steers in the FW×4 treatment with values for OIL fed steers being greater than of CON fed steers. Feeding the OIL diet increased (P=0.001) the weight percent of C18:1t10, C18:1t11, conjugated linoleic acid and n3 fatty acids by 178, 152, 66 and 32%, respectively. The n6:n3 ratio in PCD was decreased in OIL fed steers. Results indicate that repeated FW can enhance adipocyte marker expression in muscle thus improving the potential to increase marbling fat in steers, while oil, comprised of n3 and n6 fatty acids, increased biologically active fatty acids and decreased n6:n3 fatty acid ratios of tissues, without affecting productivity or carcass characteristics of the steers.