The objective of this study was to evaluate the effects of jugular infusionof 2 groups of AA on essential AA availability and utilization by themammary glands. Four cows (78 ± 10 DIM) were assigned to 4 jugularinfusion treatments: saline (CON), methionine plus lysine plus histidine(MKH), isoleucine plus leucine (IL), or MKH plus IL (MKH-IL) in a4 × 4 Latin square design. Each period was 16 d in length with 8 d ofadaption followed by 8 d of jugular AA infusion. Infusion rates were10 g of methionine, 38 g of lysine, 20 g of histidine, 50 g of leucineand 22 g of isoleucine per day. On last day of each period, a ~(13)C labeledAA mix was infused into the jugular vein over a 6 h period, and bloodsamples were collected hourly and assessed for AA enrichment. Cowswere fed a basal diet consisting of 15.2% crude protein with adequaterumen degradable protein but 15% deficient in metabolizable protein.Milk production increased significantly with infused IL (P = 0.04), butdid not change in response to MKH (P = 0.44). Milk protein concentrationand yield increased for the MKH infusion (P < 0.01) whereas milkprotein yield tended to increase for IL (P = 0.08). Total plasma AA entryrates were estimated for each EAA by fitting a 6-pool, dynamic modelto observed plasma, ~(13)C AA enrichment. The blood Met entry rates forCON, MKH, IL and MKH-IL were 57, 82, 62, 90 g/d respectively aftersubtracting infused Met. MKH infusion significantly increased Met entry(P < 0.01) and IL infusion tended to increase Met entry (P = 0.06), whichwas related to greater Met intake (P = 0.09). The increase in Met entrywith MKH infusion reflected increased Met availability from the dietor increased recycling of Met in the body. Essential AA transport andmetabolism by the mammary glands will be reported.
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