In the present experiment we aimed to study, if milk synthesis is more sensitive toward deficiency in supply of amino acids in early (EL) versus late lactation (LL), and if energy yielding substrates in the form of acetate (but not glucose) can contribute to sustain milk (protein) synthesis, when amino acid supply is suboptimal. Goats were fed a basal diet deficient in energy (90% of requirements) and protein (80% of requirements), and were randomly allocated to 4 treatments in a balanced 4 x 4 Latin square design. The treatments consisted of 4-d continuous intravenous infusions of isoosmotic, isoenergetic solutions of essential amino acids (EM), sodium acetate (ACE) and glucose (GLU) with saline (SAL) as control. There was a 3-d rest period between treatments. Milk production was recorded during the last 48h of the infusion. Arterio-venous concentration differences (AVD) across each udder half (gland) were determined every 4h during the last 24h of infusion for blood acid base parameters and key plasma metabolites. In EL, and compared to the SAL treatment, gross milk yield was increased significantly by GLU and with a tendency by EM, ECM yield by ACE treatment, milk protein yield by EM and close to significantly by ACE, but not by GLU treatment. GLU reduced milk protein percentage compared to all other treatments. High milk protein yields on EM and ACE treatments were associated with higher arterial AVD for acetate and oxygen (not significant for ACE), and higher AVD also for beta-hydroxybutyrate on EM treatment compared to GLU and SAL. In LL, EM increased ECM compared to all other treatments, increased milk protein yield and percentage compared to GLU and protein yield close to significantly compared to ACE. Fat percentage and milk fat yield were also significantly or numerically lower on GLU compared to all other treatments in LL, and this was associated with lower AVD across the mammary gland for glucose, beta-hydroxybutyrate and long chain fatty acids. In conclusion, the mammary gland is sensitive toward insufficient EM supply in both EL and LL Interestingly, increased mammary supply of ACE, but not GLU, could compensate for insufficient EM supply in EL, but this was not the case in LL This suggests that acetate (or beta-hydroxybutyrate) can improve mammary amino acid utilization for protein synthesis in EL by generation of ATP from oxidation, potentially pointing to a scope for differential protein energy recommendations for ruminants across the lactation period. (C) 2012 Elsevier B.V. All rights reserved.