Variation of milk citrate with stage of lactation and de novo fatty acid synthesis in dairy cows

Citrate is a normal constituent of milk that affects milk-processing characteristics. It is an intermediate in the tricarboxylic acid cycle and plays an indirect role in fat synthesis by providing reducing equivalents in the form of NADPH. The objective of this study was to investigate variation in citrate with stage of lactation and de novo fatty acid synthesis, without confounding dietary effects. Twenty-four cows were fed the same diet, and milk citrate and fatty acids were determined over a 10-d period. Eight cows were in early lactation [13 +/- 1.8 d in milk ( DIM; mean +/- standard error], 8 in midlactation ( 130 +/- 4.6 DIM), and 8 in late lactation ( 283 +/- 3.4 DIM). For cows in early, mid, and late lactation, milk yield was 34.4, 34.4, and 21.4 L/ d [ standard error of difference ( SED) 1.78]; milk fat was 50.4, 40.3, and 41.4 g/ L ( 3.68); milk citrate was 11.3, 9.7, and 10.1 mmol/L (0.64); the ratio of 4-14 C: 18 - 20 C fatty acids was 0.9, 1.3, and 1.2 (0.07). Activity of the fatty acid synthase enzyme system (EC 2.3.1.85) was calculated as acetate used for chain elongation ( ACE); ACE (mol/ d) for cows in early, mid, and late lactation, was 7.3, 11.1, and 8.1 (SED 1.05). For individual cows, citrate ( mmol/ L) = 14.3 - 0.44 x ACE (r(2) = 0.58). We propose that ACE provides a more accurate indication of synthase activity than do fatty acid ratios or yields. This study confirms the hypothesis that variation in milk citrate with stage of lactation is related to de novo synthesis of fatty acids and that the relationship is independent of diet and milk yield.