Metabolic responses to the deficiency of Lys, Arg, Met, or His in the mammary gland of lactating goats

To investigate lactational responses to the deficiency of individual essential amino acid (EAA), two successive 3×3 Latin square experiments were conducted in the present study with 3 lactating goats each. Animals were restricted fed a pelleted diet calculated to fulfill maintenance energy and protein requirements and the other required energy and protein were supplied through abomasal infusion of corn starch and an amino acid mixture mimic the profile of ruminal microbial protein. Deficiency of individual EAA was created by deleting it from the complete amino acid mixture. Treatments in experiment 1 were abomasal infusion of complete (C), Lys deleted (-L), and Arg deleted (-A) amino acid mixture, and those in experiment 2 were abomasal infusion of complete (C), Met deleted (-M), and His deleted (-H) amino acid mixture. Deletion of individual EAA from the complete AA mixture decreased milk yield by 39–49% and about halved milk protein yield. Milk fat content and yield were not affected. Milk fat to protein ratio numerically increased by 0.2–0.4 but was statistically insignificant (P>0.05). Deletion of individual EAA from the complete AA mixture tended to increase mammary blood flow (MBF), and the increases in the -A and -M treatments relative to that of complete AA mixture infusion were statistically significant (P<0.05). Except for Arg, arterial and venous concentrations of the deleted EAA decreased about 50% and that of Lys decreased even more than 60% (P<0.05). Mammary extraction efficiencies of all the deleted EAA were numerically increased but only that of Lys and His was significantly increased (P<0.05). Deletion of individual EAA from complete AA mixture had no significant effect of their respective mammary uptake (P>0.05) but changed the uptakes of some other measured AA (P<0.05). Deletion of Lys decreased the uptake of Thr; deletion of Arg increased that of Leu; deletion of Met increased that of Phe, Tyr, and Ser; and none of the AA uptakes was affected by deletion of His. Relative to that of complete AA mixture infusion, deleting Lys, Arg, Met, or His from the complete AA mixture numerically increased their respective mammary uptake to milk protein output ratio (U/O) by 29, 209, 100, and 82%, but only the increase of Met was statistically significant (P<0.05). For the other measured AA, deletion of Arg increased the U/O of Val, Ile, and Ser; deletion of Met increased that of Leu and Cys; deletion of His increased that of Met, Leu, Gly, and Cys (P<0.05). Influence of DMI on milk protein yield could be excluded in the present study as no feed refusal was observed. It was concluded that insufficiency of the deleted EAA as precursors for milk protein synthesis in the mammary gland was not the cause of depressed milk protein yield observed in the present study. Possible explanations for AA deficiency induced decreases in milk protein yield include depressed milk protein synthesis activities somehow incurred by the sharp decreases in blood concentrations of the deleted EAA and/or decreased conversion efficiencies of AA into milk protein in the mammary gland.