Whole-body protein and glucose metabolism in cows fed diets with varying amino acid supply under heat stress

Supplementation of Lys, Met, and His may be needed to meet lactationrequirements while dairy cows ingest lower amounts of MP under heatstress. Twelve Holstein cows (42.2 ± 10.6 kg milk/d; 83 ± 28 DIM)were used to test the effect of balancing Lys, Met, and His supplyon whole-body (WB) protein and glucose metabolism in a split-plotdesign, with parity as the main plot (multiparous n = 6; primiparous n= 6). Within each parity, 3 treatments were tested in a replicated 3 × 3Latin square with 14-d periods: 1) heat stress (HS; THI 82; 17% CP;estimated supply from d7–14 (g/d): 107 Lys, 34 Met, 37 His, and 1715MP), 2) pair feeding in thermoneutrality (PF; THI 64; 17% CP; AA andMP supply equal to HS) or 3) HS with increased AA supply (HS+AA;THI 82; 17% CP; in g/d 178, 64, 43 and 1730 of Lys, Met, His, and MP, respectively). On d 12, [¹³C] NaHCO₃ (1.96 mmol/h) and on d13, 1[¹³C]L-Leu (3.38 mmol/h) and 6,6[²H₂] glucose (24.5 mmol/h) were infusedinto a jugular vein over 4-h. Blood samples were collected from the contralateral jugular vein every 25 min during the last 2 h of the infusion tomeasure CO₂, Leu and glucose isotopic enrichments and calculate WBkinetics. The statistical model included the random effects of cow andperiod, and the fixed effects of parity, treatment, and their interactions.No parity × treatment interactions were detected. Heat stress reducedDMI and milk yield by 34% and 40% from d0 to d7. The WB rate ofappearance (Ra) of Leu was not affected by treatment, but its fractionaloxidation (FO) increased in HS by 31% relative to PF cows (P = 0.02),whereas Leu FO decreased by 29% in the HS+AA group relative to HS(P = 0.02). Leu used for protein synthesis and secreted in milk proteindecreased in HS by 13%, 16%, respectively, relative to PF cows (P <0.05), but was not affected by AA supply. The glucose WB Ra did notdiffer across treatments (P = 0.11), although glucose concentrationstended to be lower (−5%, P = 0.06) in HS relative to PF. These resultsshow that HS impairs protein metabolism in cows and that increasingLys, Met and His supply did reduce Leu, and likely other AA, oxidationduring HS conditions.