Continuous abomasal infusion of 15N-Lys to determine metabolizable Lys supply and Lys utilization for milk protein synthesis in dairy cows

Amino acids (AA) undergo complex metabolism for protein synthesis or excretion after oxidation. We investigated the fate of metabolizable Lys via continuous abomasal infusion of ¹⁵N-Lys. Four rumen-cannulated Holstein cows (147.0 ± 63.2 d in milk, 49.4 ± 9.5 kg milk yield) received 0.6 g/d of ¹⁵N-Lys into the abomasum for 5 d. Cows were offered a common diet (16.6% crude protein; 89% of metabolizable Lys requirement) once daily for an ad libitum intake. Total feces, urine, and milk were collected 1 d before the infusion and for 5 d during the infusion. All samples collected were analyzed for total N and ¹⁵N (atom percent excess) for the mass balance of ¹⁵N. Milk samples were determined for ¹⁵N enrichment of milk protein AA. Total Lys absorbed was estimated by NASEM (2021) and calculated using ¹⁵N enrichment of milk Lys and ¹⁵N absorbed (infused – total ¹⁵N excreted in feces) assuming that net utilization of absorbed Lys by peripheral tissues other than mammary tissues is zero. The statistical analysis was conducted using the Mixed procedure of SAS with a random effect of cow. On d 5, ¹⁵N excretion in feces was 19.8% of ¹⁵N infused, and ¹⁵N secretion in milk and excretion in urine were 33.2 and 13.5% of ¹⁵N absorbed, respectively, indicating that 53.3% of absorbed ¹⁵N was retained. Total Lys absorption calculated using ¹⁵N of milk Lys was not different (200 vs. 183 g/d, SEM = 17.7, P = 0.41) from that estimated by NASEM (2021). ¹⁵N enrichment was observed in various milk protein AA except Ile, Thr, Pro, Phe, and Tyr+His. Milk Leu and Met tended to be enriched with ¹⁵N (P < 0.10). The distribution of ¹⁵N among milk AA was determined. Milk AA with the largest ¹⁵N enrichment was Lys (76.3 ± 3.2% of total ¹⁵N in milk AA) followed by Glx (10.8 ± 1.4%), Asx (4.3 ± 0.5%), Ser (2.2 ± 0.3%), Ala (1.9 ± 0.4%), Val (1.8 ± 0.5%), Leu (1.5 ± 0.3%), Gly (0.8 ± 0.2%), and Met (0.4 ± 0.1%). The results showed potential use of 15N for AA metabolism and confirmed the flexibility of mammary tissues in producing other AA from Lys to synthesize milk protein.

Key Words: milk amino acids, stable isotope, 15N enrichment.