Visceral organ mass, cellularity indexes and expression of genes encoding for mitochondrial respiratory chain proteins in pure and crossbred mature beef cows grazing different forage allowances of native pastures

Visceral tissues impact on the partitioning of metabolizable energy between maintenance and production. The aim of this study was to evaluate long-term effects of two herbage allowances of native pastures on mass, composition and expression of mitochondrial respiratory protein encoding genes on gastrointestinal tract (GIT) organs and liver in pure (Angus and Hereford, PU) and crossbred (reciprocal F1, CR) beef cows. Mature beef cows (n=32) were used in a complete randomized block design with a factorial arrangement of herbage allowance (2.5 vs. 4kg DM/d; LO vs. HI) and cow genotype (PU vs. CR). The experiment was conducted during three years and at the end of the third year, cows were slaughtered at 190±10d postpartum, and GIT organs and liver were dissected, weighed, and samples collected and immediately frozen. Reticulum–rumen (P=0.02) and total intestine (P=0.02) absolute mass (kg) was greater in HI than LO cows and greater in CR than PU cows, and liver (P<0.01) mass were greater in HI than LO cows. Abomasum protein content was greater (P<0.01), while omasum protein content tended (P=0.10) to be greater for HI-PU than LO-PU cows. The reticulum–rumen and abomasum lipid contents tended to be less (P<0.10) in HI than LO cows. Except for the large intestine and liver, concentration of DNA and protein:DNA ratio did not differ in GIT viscera of different cow groups. The protein:DNA ratio of the large intestine was greater (P=0.03) in HI-CR than LO-CR cows, while the hepatic protein:DNA ratio was less (P=0.04) in HI-CR than LO-CR cows while not differing of HI-PU and LO-PU cows that presented intermediate values. The small intestine expression of NDUFB8 and COX5B mRNA were greater (P<0.05) and NDUFS4 mRNA tended (P=0.06) to be greater in HI than LO cows. The expression of UQCRC1 mRNA was greater (P=0.04) and SDHA mRNA tended to be greater (P=0.08) in CR than PU cows. Hepatic NDUFB8, NDUFS4 and COX5B mRNA was greater (P<0.05) in LO-CR than HI-CR and LO-PU cows being intermediate in HI-PU cows. The CYC1 mRNA was greater (P=0.05) in LO than HI cows. These results suggest that CR cows would have greater plasticity in order to adapt their visceral mass and gene expression to sparse environments.