The Influence of Growth Stage on Carcass Composition and Factors Associated with Marbling Development in Beef Cattle

Subcutaneous fat and intramuscular fat (marbling) both increase in beef cattle during the feeding phase, but are antagonistic in regard to their contribution to beef carcass value. The objective of this study was to determine whether cellular factors associated with marbling development change with growth stage throughout the feeding period and whether they are correlated to marbling relative to carcass composition. Twenty four (n=24) steers of known origin with the CT leptin genotype were allotted to three harvest groups. Six steers (n=6) per harvest group were harvested at the following predetermined points: 35 days (d) on feed (early-feeding period; EF), average live weight of 464 kg (mid-feeding period; MF), and 1.17 cm 12th rib subcutaneous fat thickness (late-feeding period; LF). Longissmus muscle (LM) samples were collected within 30 m postmortem and snap frozen for real time polymerase chain reaction (PCR) and western blot analysis of lipoprotein lipase, AMP activated protein kinase α (AMPKα), stearoylcoenzyme A desaturase (SCD), peroxisome proliferator-activated receptor γ (PPARγ), CCAAT/enhancer binding protein β, and myostatin. Carcass data were recorded and LM samples were collected and aged 2, 7, 14, and 21 d postmortem for Warner-Bratzler shear force (WBSF) determination. Carcass composition was estimated by dissection of the 9- 10-11 rib section and subsequent proximate analysis of the soft tissue. Intramuscular fat content of the LM increased linearly throughout the feeding period, giving additional support to marbling as an early developing tissue. Expression of AMPKα was found to be down-regulated while SCD expression was up-regulated in the LF group relative to the first two harvest groups. Additionally, SCD and PPARγ were down-regulated in the EF group relative to the latter two harvest groups. These changes in gene expression only resulted in a linear increase in PPARγ protein abundance while myostatin tended to increase both linearly and quadratically. A correlation was found between intramuscular fat and PPARγ abundance. This gives further evidence of the importance of hyperplasia in increasing marbling and targeting and increasing PPARγ expression may serve as a mechanism to increase marbling deposition. Lastly, LF steaks were more tender than MF or EF steaks; indicating improved tenderness with increased days on feed