Mitochondrial function and nutrient partitioning in high and low feed efficient multiparous Holstein dairy cows

Residual feed intake (RFI) is a metric to identify high feed efficient (negative RFI; -RFI) and low feed efficient (positive RFI; +RFI) animals within and across cohorts. The objective of this study was to quantify and determine potential differences between the most RFI divergent cows in (1) metabolites, hormones, and fatty acids in tail vessel and mammary vein blood; (2) hepatic gene expression; (3) CH₄, CO₂, O₂, and related metrics; and (4) hepatic mitochondrial function. Mid-lactation Holstein dairy cows (n = 64/trial) were enrolled in two 8-wk trials to quantify intake, BW, and milk production. The top and bottom 25% (n = 64; analyzed for all variables) and the most divergent 15% (n = 19; additional analysis of gene expression, mitochondrial function, and gas emissions and consumption) RFI subgroups were analyzed across the trials. Milk yield and milk energy output were similar, but milk lactose percentage was greater and MUN and SCS was lower in -RFI cows than +RFI cows. Tail vessel concentrations of C16:0, C16:1 cis, C17:0, C18:0, and C22:0 were (or tended to be) greater, and mammary vein C16:1 cis and C18:1 cis were greater in -RFI cows than +RFI cows. Mammary vein BUN was lower and tail vessel insulin concentration tended to be greater in -RFI cows than +RFI cows. Expression of uncoupling, mitochondrial, or tricarboxylic acid cycle genes were similar across RFI status. Mitochondrial complex IV, when quantified with complex I substrate, had lower oxygen consumption rates in -RFI cows than +RFI cows. Emissions of CH₄ and overall CO₂ were lower in -RFI cows than +RFI cows. Overall, mitochondrial function and nutrient partitioning differed in dairy cows of divergent RFI status, although differences did not appear to be due to changes in hepatic gene expression.