Abstract:

Heat stress is an unspecific reaction of the body in hot and moist environments. Many studies have demonstrated
that heat stress can be formed at temperature over 22 ℃ or temperature-humidity index higher than 72. In order to explore
the relationship between free amino acids in plasma and milk protein synthesis under heat stress, we selected 6 dairy
cows with similar parturition, regular diet and unlimited drink. The experiment lasted 35 days (from June 29 to August 5).
The temperature (at 10:00 and 18:00) and milk yield were recorded. The protein content in whole-day milk sample was
tested weekly by FOSS milk composition analyzer. Blood samples from jugular vein in both the first and last weeks were
also collected. The free amino acid contents in blood samples were analyzed by RP-HPLC system. Our results showed
that the milk yield decreased from 15.2 to 10 kg/d as the temperature rose from 26 to 38 ℃, while milk protein declined
from 0.47 to 0.31 kg/d. In blood, Glu, Asp, Gly and Val, known as the main amino acids for gluconeogenesis, increased
(0.01 < P < 0.05), so did Leu and Ile, known as the main amino acids for ketogenesis. The branch chain amino acids (Val, Ile
and Leu) responsible for immune response increased significantly. In conclusion, in hot and moist environment, dairy cows were
vulnerable to heat stress, which could cause reduction of milk production and milk protein synthesis, suggesting that the high level
of blood amino acids could preferentially exert other functions besides milk protein synthesis under heat stress.

Key words: heat stress, lactating dairy cows, milk, milk protein, plasma, free amino acids