Effect of rumen-protected L-tryptophan on productivity and metabolic responses in dry Holstein cows under heat stress conditions

This study aimed to investigate the potential effects of L-tryptophan on the productivity, physiological indicators, blood hematology, metabolite and free amino acid concentrations, gene expression in peripheral blood mononuclear cells, and metabolomics of dry Holstein cows under heat stress conditions. The study involved eight dry Holstein cows, with four cows per experiment, and was carried out in environmentally controlled metabolic chambers. Each experiment involved treatment using two heat stress conditions: low-temperature and low-humidity (LTLH) conditions (25 °C, 35%), and high-temperature and high-humidity (HTHH) conditions (31 °C, 80%). The cows were exposed to LTLH and HTHH conditions for 3 days during the adaptation phase, followed by 4 days of heat stress, and then 7 days of heat stress with rumen-protected L-tryptophan supplementation (ACT). In the study, dry matter intake increased, but rectal and skin temperature decreased in the HTHH group. Blood urea nitrogen and non-esterified fatty acid decreased, while glucose and ghrelin increased in the HTHH group. DL-plus allo-δ-Hydroxylysine increased in the HTHH group. In the HTHH group, the supplementation ACT resulted in increased gene expression of heat shock protein 70 in peripheral blood mononuclear cells. The blood metabolomic analysis also identified a distinct tendency in the HTHH group. These results suggest that supplementation with rumen-protected L-tryptophan improved the productivity, physiological indicators, blood parameters, and gene expression in the peripheral blood mononuclear cells of dry Holstein cows under high-temperature-humidity conditions. Overall, the L-tryptophan and acetate in ACT may alleviate stress in dry Holstein cows exposed to heat stress and has potential as an anti-stress supplement.