| Popis: |
The relationship between heat stress, meat quality, and residual feed intake (RFI) is unknown in growing steers. To address this issue, high RFI (HRFI) and low RFI (LRFI) individuals were compared by assessing RFI in 48 Angus-sired steers during a 70-d feeding trial conducted during July through September to identify steers with calculated RFI at least 2 SD apart. The association of RFI with indices of meat quality and expression of genes within hypothalamic and adipose tissue was then determined in LRFI and HRFI steers. While on test, feed intake was recorded daily with BW and hip heights recorded every 14 d. Ultrasound measurements of rib eye area (REA) and backfat (BF) were recorded initially and before harvest. Carcass and growth data were analyzed using a mixed model with RFI level (LRFI and HRFI) as the independent variable. The least square means for RFI were -1.2 and 0.99 kg DMI/d, respectively, for the LRFI and HRFI cohorts (P0.0001). Dry matter intake was higher for the HRFI individuals versus the LRFI steers (P0.0001) while on-test gain was not different (P0.95). Marbling score was greater in LRFI than HRFI steers (P0.05). However, there were no differences in REA (P0.53), BF (P0.65), yield grade (P0.24), or objective Hunter color measures between LRFI and HRFI steers indicating there was no consistent relationship between RFI and indices of meat quality. Hypothalamic neuropeptide Y (NPY), agouti related protein (AGRP), relaxin-3 (RLN3), melanocortin 3 receptor, and relaxin/insulin-like family peptide receptor 1 (RXFP1) mRNA were expressed 280, 185, 202, 183, and 163% greater, respectively (P0.01), while proopiomelanocortin (POMC) mRNA was expressed 42% lower in LRFI than HRFI animals (P0.05). Hypothalamic GnRH mRNA expression was 67% lower while gonadotropin inhibiting hormone (GnIH) mRNA was 209% higher in LRFI than HRFI animals (P0.01). Pituitary expression of FSHβ and LHβ correlated to hypothalamic GnRH levels (P0.05) indicating changes in gene expression within the hypothalamus had functional consequences. Leptin mRNA expression levels were not different between adipose tissue of LRFI or HRFI steers (P0.84). These data indicate that animals with superior RFI evaluated during warm conditions have higher expression of orexigenic neuropeptide genes independent of the expression of adipose-derived leptin. Furthermore, the gonadotropin axis may also influence feed efficiency under these conditions. |
