Abstract :
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 × 4 Latin square
design. The treatments consisted of 4-d continuous intravenous infusions of isoosmotic,
isoenergetic solutions of essential amino acids (EAA), 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 48 h of the infusion. Arterio-venous concentration
differences (AVD) across each udder half (gland) were determined every
4 h during the last 24 h 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 EAA, ECM yield by ACE treatment, milk protein yield by
EAA 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 EAA and ACE
treatments were associated with higher arterial AVD for acetate and oxygen (not significant
for ACE), and higher AVD also for -hydroxybutyrate on EAA treatment compared to GLU
and SAL.
In LL, EAA 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, -hydroxybutyrate and long chain fatty
acids.
In conclusion, the mammary gland is sensitive toward insufficient EAA supply in both
EL and LL. Interestingly, increased mammary supply of ACE, but not GLU, could compensate
for insufficient EAA supply in EL, but this was not the case in LL. This suggests
that acetate (or -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.
