Protein turnover and energy metabolism was simultaneously measured in sows on d 7 and 17 of lactation. Sows were fed a lactation diet ad libitum (14 MJ DE/kg, 20.7% crude protein, 1.02 % total lysine). On days 7 and 17 of lactation, 24 h energy expenditure was measured by indirect calorimetry while a primed-constant infusion of L-[1-13C]leucine (1.0 mg/kg/h) was delivered intravenously over 24 h to measure
rates of protein synthesis and breakdown. Milk yield and composition were also measured. Leucine intake varied due to differences in feed intake, but leucine flux was not significantly different between days of lactation. Protein synthesis and leucine appearance from body protein breakdown, as % of flux, were higher (P<.001) on d 7 than d 17 (81.1 vs. 76.9 % and 25 vs. -10 %), respectively. The protein and leucine content of milk were also higher on d 7 than d 17 (5.3% vs. 5.1% CP and 3.0 vs. 2.0 g/kg), respectively. Negative protein breakdown on d 17 indicates utilization of leucine by the mammary gland that was neither oxidation or protein synthesis. The higher dietary intake
of energy and protein on d 17 reduced both whole body protein breakdown and protein synthesis. The higher dietary energy intake on d 17 (13.4 MJ/d) was entirely accounted for by milk energy output (13.8 MJ/d). These data indicate that sows were in greater protein deficit during early lactation than late lactation; therefore diets higher protein, or with a better balance of amino acids, are required in early lactation to reduce the loss of body protein.