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Effects of diet forage-to-concentrate ratio and intake on balances of energy and nitrogen and portal-drained viscera (PDV), liver and kidney blood flow and O2 consumption were measured in seven growing beef heifers. Isonitrogenous pelleted diets containing approximately 75% alfalfa or 75% concentrate were fed daily as 12 equal meals every 2 h at two isoenergetic metabolizable energy intakes. A split-plot design was used, with 4 wk for adaptation to diet followed by 3-wk intake periods within 6-wk diet periods. Heifers consumed and digested less dry matter, energy and nitrogen when fed the 75% concentrate vs. 75% alfalfa diet at equal metabolizable energy. Heifers fed the 75% concentrate diet produced less heat energy and retained more tissue energy than when fed the 75% alfalfa diet. Blood flow for PDV, liver and kidneys increased with intake and was greater when heifers were fed the 75% alfalfa vs. 75% concentrate diet. Increased PDV and liver O2 uptake accounted for 44 and 72% of heat increment for the 75% concentrate and 75% alfalfa diets, respectively. Greater PDV uptake of O2 accounted for 72% of the decrease in tissue energy of heifers fed the 75% alfalfa vs. 75% concentrate diet at equal metabolizable energy.

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Effects of bovine somatotropin (bST) on irreversible loss rate (ILR) and oxidation rate of glucose and nonesterified fatty acids (NEFA) were examined. Nine lactating cows received bST or excipient in a single reversal design using 14-d periods. Kinetic variables were estimated by compartmental analysis of blood metabolite and expired CO2 specific activity values obtained during infusion of [U-14C]glucose or [1-14C]palmitate. With bST treatment, milk energy yield increased by 31% but feed intake was unchanged. Blood glucose concentrations were not affected by treatment or correlated with any glucose kinetic variables. In the control period, glucose ILR was 12.1 mol/d with 66.5% utilized for milk lactose synthesis and 17.4% oxidized to CO2. Treatment with bST increased glucose ILR (+1.5 mol/d) and reduced glucose oxidation (-0.4 mol/d); this accommodated the additional glucose (+1.3 mol/d) required for the increase in lactose secretion. Increases in milk energy yield with bST treatment caused cows to be in a substantial negative net energy balance (-9.8 Mcal/d). No acute lipolytic response occurred with bST treatment, but plasma NEFA were chronically elevated (+104 mumol/L) and NEFA ILR increased (+2.3 mol/d). Increased NEFA turnover was primarily used for increased oxidation to CO2 (+0.5 mol/d) and 41% increase in milk fat (equal to approximately 1.3 mol fatty acids/d). For NEFA, plasma concentrations were correlated with ILR (r = +0.80), oxidation to CO2 (r = +0.74) and net energy balance (r = -0.78). Overall, bST resulted in an exquisite coordination of metabolism to meet nutrient needs for increased synthesis of milk components.