Effect of rate of gain during the stocker period on beef cattle skeletal muscle development, satellite cell activity, and marbling development
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It is estimated that 76% of the yearling beef cattle calf crop enters a stocker management system. Stocker systems are used to prepare cattle for finishing by adapting them to drinking out of fountain waters and eating out of a bunk. Stocker programs allow producers to utilize homegrown feedstuffs, take advantage of grazing opportunities, delay finishing, target a specific market, and promote skeletal growth in small-framed cattle. Research has shown that rate of gain during the stocker period affects tissue growth and feedlot performance. Cattle that are fed at lower rates of gain during the stocker period experience a period of compensatory gain upon entering the finishing phase. Cattle that experience lower rates of gain during the stocker period require more days on feed to reach the same carcass composition as cattle that are fed diets to achieve high rates of gain during the stocker period. To determine the effects of stocker programs on skeletal muscle development, satellite cell activity and marbling development, fall-weaned angus steers (n = 72; 259 ± 28 kg) from the Oklahoma State University cow herd were randomly assigned to 1 of 4 stocker systems. Stocker systems included: 1) grazing dormant native range (NR) supplemented with cottonseed (1.0 kg∙steer-1∙d-1) followed by season-long grazing on summer pasture (CON); 2) grazing dormant NR supplemented with corn based supplement (1% of BW) followed by short season grazing on summer pasture (CORN); 3) grazing winter wheat pasture at high stocking density (3.0 steers/ha) to produce a moderate ADG (LGWP); and 4) grazing winter wheat pasture at low stocking density (1.0 steers/ha) to produce high ADG (HGWP). Steers continued grazing until the average BW of the treatment group reached a common BW of approximately 375 kg, at which time steers were transitioned into a common finishing diet and fed to a common backfat thickness of at least 1.18 cm. At the end of the stocker and finishing phases, four steers per treatment were harvested and longissimus muscle (LM) was collected for analysis. LM fiber cross-sectional area, fiber type, Pax7+ cell density, Myf-5+ cell density, Pax7+/Myf-5+ cell density, capillary density, and nuclear density were determined by immunoflourescence on cryosections. Bovine satellite cells (BSC) from the LM of each steer were cultured, satellite cell activity, fusion capacity and gene expression were analyzed. 3T3-L1 preadipocytes were grown to confluence and subsequently exposed to conditioned media from mitotically active and differentiated BSC, to detect the effects of BSC produced factors on preadipocyte differentiation. At the conclusion of the stocker phase, LGWP and HGWP steers had greater (P < 0.01) muscle fiber cross sectional area when compared to CON and CORN steers. CON steers tended (P = 0.12) to have more Type 1 muscle fibers compared to all other treatments. CORN steers had a lower (P = 0.05) percentage of cells within the LM co-expressing Pax7 and Myf-5 when compared to all other treatments. At the conclusion of the finishing phase, relative gene expression of satellite cells after 14 d in culture were similar among treatments, except HGWP steers had greater (P = 0.03) relative abundance of Pax7 than all other treatments. LGWP steers tended (P = 0.06) have greater Pax7 expression in culture and numerically lower myonuclei when compared to all other treatments. 3T3-L1 preadipocytes exposed to conditioned media from HGWP BSC at 96 h in culture had greater (P = 0.05) PPARγ and lower (P = 0.04) FABP4 mRNA abundance when compared to preadipocytes treated with media from CON and CORN BSC, with preadipocytes treated with media from LGWP BSC being intermediate. Overall, these data suggest that rate of gain and stocker management strategy during the stocker period can impact skeletal muscle characteristics, satellite cell activity, and marbling development. Further research is required to elucidate the mechanisms behind these effects, but factors from BSC do seem to affect adipocyte differentiation.