PSXII-20 - Differences in fat accretion rate of different depots of Angus, Brahman and Charolais feedlot steers

Abstract: Understanding accretion rate of intramuscular and subcutaneous fat in feedlot steers is crucial to ensure that carcasses are meeting market specifications and therefore optimising profitability. Intramuscular fat (marbling) and subcutaneous fat (P8 and rib) are key areas of assessment on the live animals and carcases determining quality. The objective of the present study was to determine fat accretion rate in three popular breeds of cattle with contrasting accretion rate throughout the feedlotting period. We hypothesised that Angus would have the fastest fat accretion rate being early maturing. Thirty of each Angus, Brahman and Charolais steers were entered into a feedlot (344 ± 5.3kg initial BW) and six of each breed randomly selected for slaughter at 50-day intervals for 200 days on feed (DOF). Ultrasound scanning and BW were measured at each slaughter point, measuring fat thickness at the rib and P8 site, intramuscular fat (IMF), kidney fat and the eye muscle area (EMA). Liveweight, intramuscular fat (%), P8 and rib fat (mm) all had significant interactions between DOF and breed (P < 0.001). At the start of the feeding period, Charolais were lighter than Angus and Brahman (P > 0.001). However, Charolais (663.7 ± 9.31 kg) and Angus were the heavier (680.4 ± 8.68 kg) than Brahman (539.4 ± 8.68kg) at 200 DOF (P < 0.05). The interaction for P8 and rib fat was due to all breeds having similar fat thickness at 0 DOF (P > 0.05) but Angus had greatest (21.38 ± 0.882 mm, 17.12 ± 0.549 mm), Brahman intermediate (16.73 ± 0.883 mm and 10.38 ± 0.550 mm) and Charolais thinnest (9.97 ± 0.966 mm, 7.23 ± 0.600 mm) P8 and rib fat, respectively, at 200 DOF (P < 0.05). Similarly, IMF was similar between breeds at 0 DOF (P > 0.05), but Angus had higher IMF (7.76 ± 0.287 %; P < 0.001) compared to Brahman (5.08 ± 0.287%) and Charolais (5.18 ± 0.314%), which were not different amongst them (P > 0.10). Results of the present study demonstrated the contrasting fat metabolisms of each genotype with Angus steers showing the fastest fat accretion rate for all fat depots, whereas Brahman steers showed intermediate and Charolais slowest subcutaneous fat accretion rate. However, both Brahman and Charolais steers showed similarly low IMF and kidney fat accretion rates. Fat accretion rate of each depot and whole body should be considered in mathematical simulation models of animal growth.