Our objective was to evaluate the effects of phenotypic ranking on residual feed intake (RFI) on the transcription of genes 1) involved in the respiratory chain complex and 2) coding for transcriptional factors regulating mitochondrial biogenesis, across two contrasting diet types. Beef heifers (n = 86) fed a diet comprising 70:30 concentrate-corn silage [low forage (LF)] over a 82-day period were ranked on RFI. The 10 highest (feed inefficient, high-RFI) and 10 lowest (feed efficient, low-RFI) ranking animals were selected for the current study. Biopsies of the M. longissimus dorsi were harvested following initial selection (LF diet) and again following a 6 wk period while the animals were offered a high-forage (HF) grass silage-only diet. Real-time PCR was used to quantify mRNA transcripts of 17 genes associated with cellular energetic efficiency. The mRNA expression of UCP3 tended to be upregulated (2.2-fold, P = 0.06) for the high-RFI compared with the low-RFI animals. mRNA transcripts coding for the transcription factor PGC-1α was 1.7-fold higher (P = 0.01) in low compared with high-RFI animals. A phenotype × diet interaction was evident for the abundance of ANT1 mRNA transcript, with greater (P = 0.04) expression levels detected in the low-RFI phenotype during the HF period, but no difference (P = 0.50) between phenotypes during the LF period. A phenotype × diet interaction was also evident for COX II with greater expression levels detected (P = 0.04) in the low compared with the high RFI phenotype while on LF but not the HF diet (P = 0.22). These data suggest an association between cellular energetic efficiency and RFI in cattle.
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