Influence of addition of monensin to an alfalfa hay diet on net portal and hepatic nutrient flux in steers

Six Holstein steers (381 +/- 11 kg BW; mean +/- SE) surgically fitted with hepatic portal (n = 6), hepatic venous (n = 4), mesenteric venous, and arterial catheters were used in a replicated crossover design experiment to evaluate the influence of monensin addition to an alfalfa hay diet on net portal and hepatic nutrient flux. Steers were fed 6.5 kg of DM/d in 12 portions using automatic feeders. Diets included chopped alfalfa hay (20.4% CP) plus 418 g of DM/d of finely ground corn (8.5% CP) with monensin added (240 mg/d) for treated steers. Experimental periods lasted 21 d, with blood samples taken on the final 2 d of each period. Five sets of arterial, portal, and hepatic blood samples were collected hourly from 0900 to 1300 each day. Ruminal fluid samples were collected by stomach tube at the end of each sampling day for VFA analysis. Blood flow was determined by a primed-continuous infusion of p-aminohippurate into the mesenteric venous catheter. Addition of monensin increased (P = .04) the molar percentage of ruminal propionate and decreased ruminal isobutyrate (P = .02) and isovalerate (P = .03). Percentages of the other ruminal VFA, total ruminal VFA concentration, and pH were not affected by feeding monensin. The arterial concentrations of L-lactate (P = .02) and beta-hydroxybutyrate (P = .01) were greater with monensin; however, none of the arterial concentrations of the other metabolites was changed. Feeding of monensin also did not affect (P > .10) portal, hepatic, or hepatic arterial blood flow. Portal-arterial differences in glucose concentration and net portal flux tended (P = .10) to be greater with monensin. Net portal fluxes of isobutyrate (P = .08) and 3-methylbutyrate (P = .01) were decreased by feeding monensin, whereas net fluxes of the other blood VFA were not affected (P > .10). Net hepatic and total splanchnic flux of glucose, L-lactate, beta-hydroxybutyrate, acetate, propionate, or butyrate were not affected (P > .10) by feeding monensin. Changes in VFA net absorption from feeding monensin were not consistent with its role in increasing ruminal propionate production, and total net portal energy flux did not change. These results are not consistent with the role of monensin in improving feed efficiency directly through increased ruminal propionate production.