1529
Effect of a limited supply of phenylalanine, threonine, and tryptophan on mammary metabolism of dairy cows
Mammary metabolism is altered by deletion of essential AA (EAA) from an abomasal infusion of a total AA mixture. The objective of this study was to examine the effect of a limited supply of Phe, Thr and Trp on mammary uptake of AA and energy substrates. Five Holstein cows (63 ± 1.6 days in milk) in 2nd lactation were used in a 5 × 5 Latin square with 10-d periods. The diet supplied 100% of net energy and 70% of metabolizable protein requirements based on NRC recommendations. Treatments were abomasal infusions of water (CTL), all AA with casein profile (TAA), TAA without Phe (No-Phe), TAA without Thr (No-Thr), and TAA without Trp (No-Trp). Mammary AA and energy substrate uptake was determined from arterio-venous differences (AV-diff) of 6 blood samples collected every 2 h on d10, with mammary plasma flow (MPF) estimated using the Fick principle (Phe+Tyr). Treatment differences were determined using contrasts, comparing each treatment to TAA. Arterial concentrations of Phe, Thr, and Trp decreased (P<0.01) with their respective deletions. The mammary gland responded to a deficiency of Phe and Thr by reducing milk protein secretion through different mechanisms. With No-Phe, Phe uptake decreased mainly through a reduction (P<0.01) of AV-diff as MPF only numerically increased. When Thr was deficient, MPF increased by 32%, and despite a large decreased (P<0.01) AV-diff, Thr uptake only numerically (13%) decreased. Mammary uptakes of acetate, β-hydroxybutyrate, glucose and lactate were not affected by treatments. A limited supply of Trp had minimal impacts on mammary metabolism.
|
Treatment |
SEM |
P-value contrast |
||||||
|
CTL |
No-Phe |
No-Thr |
No-Trp |
TAA |
|
CTL vs TAA |
No-Phe vs TAA |
No-Thr vs TAA |
Milk true protein yield, g/d |
715 |
701 |
738 |
840 |
844 |
51.0 |
0.03 |
0.02 |
0.05 |
MPF, L/h |
497 |
601 |
775 |
603 |
524 |
52.6 |
0.68 |
0.24 |
0.01 |
Mammary uptake, mmol/h |
|||||||||
Phe |
10.1 |
9.0 |
10.9 |
11.9 |
11.8 |
0.85 |
0.04 |
0.01 |
0.22 |
Thr |
12.1 |
12.6 |
12.6 |
13.6 |
14.2 |
1.06 |
0.10 |
0.20 |
0.18 |
Trp |
2.9 |
3.0 |
3.1 |
2.9 |
3.1 |
0.39 |
0.64 |
0.81 |
0.92 |
Group 1 AA-N |
48.5 |
49.0 |
51.9 |
55.9 |
56.8 |
3.77 |
0.04 |
0.06 |
0.17 |
Group 2 AA-N |
101.7 |
126.8 |
141.8 |
140.9 |
130.1 |
8.85 |
0.02 |
0.76 |
0.26 |
Mammary uptake:milk output |
|||||||||
Group 1 AA-N |
1.05 |
1.08 |
1.09 |
1.03 |
1.04 |
0.03 |
0.85 |
0.35 |
0.30 |
Group 2 AA-N |
1.13 |
1.44 |
1.55 |
1.33 |
1.22 |
0.08 |
0.41 |
0.04 |
0.01 |
Non-EAA-N |
0.78 |
0.67 |
0.62 |
0.65 |
0.69 |
0.05 |
0.14 |
0.69 |
0.21 |
Keywords: amino acids, mammary metabolism, dairy