We hypothesized that the provision of rumen-inert fat (RIF) to growing cattle (9 to 13 mo of age) would affect the expression of genes involved in lipid metabolism and thereby affect the size and number of adipocytes of steers slaughtered at 30 mo of age. Thirty steers with an average initial body weight (BW) of 239 ± 25 kg were allocated to six pens, balanced for BW and genetic merit for marbling, and assigned to one of two treatment groups: control (only basal diet) or test diet (basal diet with 200 g of RIF per day, on an as-fed basis) for 5 mo. Biopsy samples of longissimus lumborum (LM) muscle were then collected for analysis of fatty acid composition and gene expression. Both groups were then fed the same basal diets during the early and late fattening phases, without RIF, until slaughter (average shrunk BW = 759 kg). Supplementation with RIF increased the longissimus thoracis (LT) intramuscular fatty acid concentration at slaughter (P = 0.087) and numerically increased the quality grade score (P = 0.106). The LM intramuscular relative mRNA expression of genes such as PPARα, ZFP423 and SREBP1, FASN, SCD, FABP4, GPAT1, and DGAT2 were downregulated (P < 0.1) following RIF supplementation. Supplementation of RIF decreased (P < 0.1) diameter and concomitantly increased intramuscular adipocytes per viewing section at slaughter. This likely was caused by promotion of triacylglycerol hydrolysis during the growing phase. Another possible explanation is that the relative mRNA expression of gene ATGL was upregulated by RIF supplementation during the growing (P < 0.1) and the fattening phases (P < 0.05), while the genes associated with fatty acid uptake (FABP4) and esterification (DGAT2) were downregulated during the growing phase and upregulated (P < 0.1) during the fattening phase. This implies that the lipid turnover rate was higher for steers during the growing than fattening phase. This study demonstrated that RIF supplementation during the growing phase induced a carryover effect on the lipogenic transcriptional regulation involved in adipocyte lipid content of intramuscular adipose tissue; increased triacylglycerol hydrolysis during the growing phase subsequently was followed by increased lipid accumulation during the fattening phases.
Keywords: adipocyte size; gene expression; imprinting; lipogenesis; rumen-inert fat.
Rumen inert fat (RIF) is a type of fat supplement that is used in the diets of beef cattle as early as 6 mo of age in calves and continues through the finishing period to improve the dietary energy density which can be used by the animal to deposit more lipid in the muscle tissue. However, for Hanwoo beef cattle, the precise time of RIF supplementation has not yet been determined. This study hypothesized that supplementing RIF at the growing phase (9 to 13 mo of age) would have a positive influence on the marbling characteristics of meat at slaughter. The growth rate and performance of steers were not improved by RIF supplementation, however, an increase in intramuscular fatty acid content was noted that was accompanied by the increased number of intramuscular adipocytes and decreased intramuscular adipocyte diameter. Supportively, upregulation of the genes associated with fatty acid uptake and esterification during the fattening phase of RIF-fed animals was noted. Overall, supplementing RIF at the growing stage could improve the lipid content of the meat which is supported by the increased lipid hydrolysis during the growing phase and followed by increased lipid accumulation during the fattening phases.
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